CN105518958B - DC power grid current differential protection method and system - Google Patents

Abstract

The invention discloses a current differential protection method and system for a DC power grid. The method comprises the following steps: a sampling value obtaining step: obtaining an electrode voltage sampling value and an electrode current sampling value in a local terminal and a remote terminal of the DC line; a fault component extraction step: respectively calculating voltage values of fault components according to the voltage sampling values of the local terminal and the remote terminal; respectively calculating the pole current values of the fault components according to pole current sampling values of the local terminal and the remote terminal; b, calculation of a Bergeron model: calculating fault component pole voltage values and fault component pole current values of the local terminal and the remote terminal calculated in the fault component extraction step based on a Bergeron model to obtain fault component pole current values at selected points on a DC line between the local terminal and the remote terminal; current differential protection determination: and if the current values of the fault component poles at the selected points of the local terminal and the remote terminal, which are obtained in the calculation step of the Bergeron model, meet the preset current differential protection criterion, judging the internal fault. The invention adopts the Bergeron model, thereby eliminating the interference of the distributed charging current without long time extension and further greatly improving the calculation speed of the invention.

Description

DC power grid current differential protection method and system

technical field

The present application relates to a DC grid current differential protection method and a system thereof.

Background technique

In existing HVDC systems, protection based on traveling wave fronts, usually based on local measurements, is used as primary protection, and classical current differential protection is used as backup protection. However, their disadvantages are: the primary protection has poor sensitivity to high-resistance faults, and may malfunction in the LCC DC grid; while the backup protection has a very slow action speed.

In existing two-terminal HVDC systems, the primary protection for transmission lines is mainly based on the rate of change and amplitude of the directional traveling wave front. This type of protection has the obvious advantage that it uses only local measurements and has a very fast action on metal faults.

But one disadvantage of this type of protection is its very low (poor) sensitivity to high resistance faults. Typically a fault resistance of >200 Ohm may cause actuation failure, since the amplitude of the wavefront is largely dependent on the fault resistance. Therefore, high-resistance faults have to be cleared by back-up current differential protection whose action speed is very slow (eg >0.5s). This is obviously unreasonable.

Furthermore, the protection is based on the physical characteristics of smoothing reactors in HVDC systems, which slow down current changes. In some types of DC grid systems (for example, some types of series MTDC systems), traveling waves caused by external faults will not flow through smoothing reactors, and the above-mentioned traveling wave-based HVDC protection will not operate or malfunction . In the worst case, if an external DC fault occurs on a line with a higher voltage level, then its traveling wavefront will be even larger than that caused by the internal fault. This will bring big troubles to the existing HVDC protection based on traveling waves.

Figure 1 is a graph showing traveling wave fronts for internal and external DC faults in an LCC DC grid.

As shown in Figure 1, the rate of change of the wavefront from internal and external faults is absolutely the same at the beginning. At the same time, the traveling wave front of an external fault is even much larger than that of an internal fault because of the higher voltage level of the external line.

In a traditional traveling wave protection device, as shown in Figure 2, three different measurements will start to determine whether the wave has sufficient amplitude for a specified time. The first measurement calculates the wave difference between just before the wavefront and just after 10 samples (0.2ms). The second and third measurements calculate the wave difference between just before the wavefront and just after 25 and 35 samples (0.5ms and 0.7ms). If all three measurements are greater than the threshold, a line fault has been detected.

Given that the wave front of the external fault in Fig. 1 is even larger than that of the internal fault, and the rate of change of both the external and internal faults has the same rate, the existing HVDC main protection will misfire in the LCCDC grid in this case. verb: move. In other words, existing HVDC traveling wave protection cannot be directly used in LCC DC grids.

In existing HVDC systems, the backup protection usually used for transmission lines is line current differential protection. Classical current differential protection algorithms are used in this type of protection. This type of protection operates when the main protection (traveling wave protection) fails (eg high resistance fault).

Typical criteria for current differential protection are shown below,

|I _DL -I _{DL_FOS} |＞max(120A, 0.1×|I _DL +I _{DL_FOS} |/2)

where I _DL is the current on the local side and I _{DL_FOS} is the current on the remote side.

Another typical criterion for current differential protection is shown below,

||I _DL |-|I _{DL_FOS} ||＞90A

Usually, the sensitivity of current differential protection is quite good if set properly. But it moves too slowly. Its action time is usually hundreds of milliseconds or even seconds. The main reason is that the fault transient and charging current will greatly affect the protection algorithm. Therefore, long delays are necessary to ensure reliability.

Both primary and backup protection can be affected by high impedance faults.

1) Impact on traveling wave protection

The existing traveling wave criterion is:

|W _COMM |＝|Z _COM I _COM -U _COM |＞350kV

|W _POLE |＝|Z _DIF I _DIF -U _DIF |＞210kV

Among them, Z _COM is the common-mode wave impedance, Z _DIF is the differential-mode wave impedance, W _POLE is the pole wave, and W _COMM is the ground wave.

I _COM is the common-mode current, U _COM is the common-mode voltage, I _DIF is the differential-mode current, and U _DIF is the differential-mode voltage.

This protection uses the rate of change of ground waves to detect wave fronts.

|dW _COMM /dt|＞396kV/ms

When the line is connected to the ground through a large impedance, the DC voltage drops at a small rate of change, which leads to the malfunction of the existing protection based on traveling waves.

If the traveling wave protection malfunctions, the control and protection system will delay to eliminate the fault.

2) Impact on voltage change rate and low voltage protection

The criterion for the rate of voltage change is:

D _UT =dU _dl /dt<-396kV/ms&U _dl <200kV, where U _dl is the line voltage and D _UT is the corresponding rate of change.

When the line is connected to the ground through a large impedance, a small DC voltage drop will cause the voltage change rate protection to malfunction.

3) Impact on current differential protection

Typical criteria for current differential protection are shown below:

|I _DL -I _{DL_FOS} |＞max(I _SET , k×|I _DL +I _{DL_FOS} |/2)

Among them, I _SET is a fixed current setting value, usually set to 120A, and k is a ratio coefficient, usually set to 0.1.

In order to ensure operation under the condition of a large impedance fault, the setting values I _SET and k are usually set to small values. Therefore the delay time must be set long enough to avoid malfunction caused by the capacitor charging current.

If the fast protection (traveling wave protection) malfunctions, the backup protection will delay working. And the delay time is too long to ensure the stable operation of the power system.

Contents of the invention

Therefore, one aspect of the present invention provides a DC grid current differential protection method, comprising the following steps:

The sampling value obtaining step: obtaining the sampled value of the pole voltage and the sampled value of the pole current of the local terminal and the remote terminal of the DC line;

The fault component extraction step: calculating the pole voltage value of the fault component according to the pole voltage sampling values of the local terminal and the remote terminal respectively; and calculating the pole current value of the fault component respectively according to the pole current sampling values of the local terminal and the remote terminal;

Bergeron model calculation step: By calculating the fault component pole voltage value and fault component pole current value of the local terminal and remote terminal calculated in the fault component extraction step based on the Bergeron model, the DC between the local terminal and the remote terminal is obtained Pole current value of the fault component at a selected point on the line;

Judgment step of current differential protection: If the pole current values of the fault components at the selected points of the local terminal and the remote terminal obtained in the Bergeron model calculation step satisfy the preset current differential protection criterion, an internal fault is determined.

Preferably, the DC grid is bipolar, and the DC line includes a positive DC line and a negative DC line, the local terminals include a positive local terminal and a negative local terminal, the remote terminals include a positive remote terminal and a negative remote terminal, and the positive DC line is electrically connected to the positive The local terminal and the positive remote terminal, and the negative DC line electrically connects the negative local terminal and the negative remote terminal, the distance from the selected point to the positive local terminal is the same as the distance from the selected point to the negative local terminal, and from the selected point to The distance from the positive remote terminal is the same as the distance from the selected point to the negative remote terminal; also includes:

Pole mode transformation step: obtaining each modulus of the local terminal and the remote terminal by performing pole mode transformation on said fault component pole voltage value of each of the positive local terminal, the positive remote terminal, the negative local terminal and the negative remote terminal and obtaining each of the local terminal and the remote terminal by pole-mode transforming said fault component pole current value for each of the positive local terminal, the positive remote terminal, the negative local terminal, and the negative remote terminal A modulus fault component modulus current value;

Bergeron model calculation steps also include:

By calculating the fault component mode voltage value and fault component mode current value of each modulus of the local terminal and the remote terminal based on the Bergeron model, the fault component mode traveling wave voltage value of each modulus of the local terminal and the remote terminal is obtained respectively ;

Converting the fault component mode traveling wave voltage values of the local terminal and the remote terminal into fault component mode traveling wave current values of the local terminal and the remote terminal respectively;

determining fault component mode current values for the local terminal and the remote terminal at selected points on the DC line based on the fault component mode traveling wave current values for the local terminal and the remote terminal, respectively;

The fault of each of the positive local terminal and the negative local terminal at a selected point on the DC line is obtained by modulo-polar transformation of the fault component modulo current value of each modulus of the local terminal at the selected point Component pole current values, and by performing modulo-pole transformation on the fault component modulo current value of each modulus of the remote terminal at the selected point, the fault component poles of the positive remote terminal and the negative remote terminal at the selected point are obtained current value.

Conveniently, the pole voltage sampling values include: u _LP (t), that is, the voltage sampling value of the positive local terminal; u _LN (t), that is, the voltage sampling value of the negative local terminal; u _RP (t), that is, the voltage sampling value of the positive remote terminal Voltage sampling value; u _RN (t), that is, the voltage sampling value of the negative remote terminal; wherein t refers to time;

Pole current sampling values include: i _LP (t), the current sampling value of the positive local terminal; i _LN (t), the current sampling value of the negative local terminal; i _RP (t), the current sampling value of the positive remote terminal ; i _RN (t), the current sampling value of the negative remote terminal;

The fault component pole voltage value includes: Δu _LP (t), that is, the fault component voltage value of the positive local terminal corresponding to u _LP (t); Δu _LN (t), that is, the negative local terminal corresponding to u _LN (t). The fault component voltage value of the terminal; Δu _RP (t), that is, the fault component voltage value of the positive remote terminal corresponding to u _RP (t); Δu _RN (t), that is, the negative remote terminal corresponding to u _RN (t) Terminal fault component voltage value;

The fault component pole current value includes: Δi _LP (t), that is, the fault component current value of the positive local terminal corresponding to i _LP (t); Δi _LN (t), that is, the negative local terminal corresponding to i _LN (t). The fault component current value of the terminal; Δi _RP (t), that is, the fault component current value of the positive remote terminal corresponding to i _RP (t); Δi _RN (t), that is, the negative remote terminal corresponding to i _RN (t) Terminal fault component current value;

The fault component mode voltage value includes: Δu _L0 (t), which is the common mode voltage value of the fault component of the local terminal; Δu _L1 (t), which is the differential mode voltage value of the fault component of the local terminal; Δu _R0 (t), which is the remote terminal The common-mode voltage value of the fault component of ; Δu _R1 (t), that is, the differential-mode voltage value of the fault component of the remote terminal;

The fault component mode current value includes: Δi _L0 (t), which is the fault component common mode current value of the local terminal; Δi _L1 (t), which is the fault component differential mode current value of the local terminal; Δi _R0 (t), which is the remote terminal Δi _R1 (t), that is, the differential mode current value of the fault component of the remote terminal;

The fault component traveling wave voltage value includes: Δu _L0+ (t), which is the common mode forward traveling wave voltage value of the fault component of the local terminal; Δu _L0- (t), which is the common mode reverse traveling wave voltage value of the fault component of the local terminal ; Δu _L1+ (t), that is, the fault component differential mode forward traveling wave voltage value of the local terminal; ΔΔu _L0- (t), that is, the fault component differential mode reverse traveling wave voltage value of the local terminal; Δu _R0+ (t), That is, the common-mode forward traveling wave voltage value of the fault component of the remote terminal; Δu _R0- (t), that is, the common-mode reverse traveling wave voltage value of the fault component of the remote terminal; Δu _R1+ (t), the differential mode fault component of the remote terminal Forward traveling wave voltage value; Δu _R1- (t), remote terminal fault component differential mode reverse traveling wave voltage value;

The fault component traveling wave current value includes: Δi _L0+ (t), which is the common mode forward traveling wave current value of the fault component of the local terminal; Δi _L0- (t), which is the common mode reverse traveling wave current value of the fault component of the local terminal ; Δi _L1+ (t), that is, the differential mode forward traveling wave current value of the fault component of the local terminal; Δi _L1- (t), that is, the reverse traveling wave current value of the fault component differential mode of the local terminal; Δi _R0+ (t), That is, the common-mode forward traveling wave current value of the fault component of the remote terminal; Δi _R0- (t), that is, the common-mode reverse traveling wave current value of the fault component of the remote terminal; Δi _R1+ (t), that is, the fault component difference of the remote terminal Δi _R1- (t), that is, the fault component differential mode reverse traveling wave current value of the remote terminal;

The fault component mode current value at the selected point includes: Δi _L0 (x, t), which is the fault component common mode current value at the selected point of the local terminal; Δi _L1 (x, t), which is the selected point of the local terminal The fault component differential mode current value at the fixed point; Δi _R0 (x, t), which is the fault component common mode current value at the selected point of the remote terminal; Δi _R1 (x, t), which is the selected point of the remote terminal The fault component differential mode current value at , where x is the selected point;

The fault component pole current value at the selected point includes: Δi _LP (x, t), which is the fault component pole current value at the selected point of the positive local terminal; Δi _LN (x, t), which is the negative local terminal The fault component pole current value at the selected point; Δi _RP (x, t), the fault component pole current value at the selected point of the positive remote terminal; Δi _RN (x, t), the selected negative remote terminal The fault component pole current value at the point.

Conveniently, in the fault component extraction step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following manner:

Where T represents a preset time delay;

In the pole mode conversion step, the fault component mode voltage value and the fault component mode current value are calculated in the following manner:

The calculation steps in the Bergeron model include:

The following method is used to calculate the fault component mode traveling wave voltage value:

Among them, Z _C0 is the common-mode wave impedance; Z _C1 is the differential-mode wave impedance;

Calculate the fault component mode traveling wave current value in the following way:

Calculate the fault component modulo current value at the selected point in the following way:

Among them, v ₀ is the traveling speed of the common-mode traveling wave of the fault component, and v ₁ is the traveling speed of the differential-mode traveling wave of the fault component;

The fault component pole current value at the selected point is calculated as follows:

Conveniently, the determination steps of the current differential protection include:

If |Δi _LP (x, t)+Δi _RP (x, t)|＞I _res is satisfied, the state is determined to be positive internal fault, if |Δi _LN (x, t)+Δi _RN (x, t)| >I _res , then it is determined that the state is a negative internal fault, where I _res is the preset threshold;

Otherwise, the differential protection will not be activated.

Preferably, the DC grid is unipolar:

Bergeron model calculation steps also include:

Based on the Bergeron model, calculate the polar voltage value of the fault component and the mode current value of the fault component of the local terminal and the remote terminal, and obtain the polar traveling wave voltage value of the fault component of the local terminal and the remote terminal respectively;

Converting the fault component polar traveling wave voltage values of the local terminal and the remote terminal into the fault component modulus traveling wave current values of the local terminal and the remote terminal;

Based on the fault component polar traveling wave current values of the local terminal and the remote terminal, the fault component pole current values of the local terminal and the remote terminal at selected points on the DC line are determined.

Conveniently, in the fault component extraction step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following manner:

Among them, T represents the preset time delay, Δi _L (t) is the pole current value of the fault component of the local terminal, Δi _R (t) is the pole current value of the fault component of the remote terminal, and Δu _L (t) is the fault component of the local terminal Pole voltage value, Δu _R (t) is the fault component pole voltage value of the remote terminal, i _L (t) is the current sampling value of the local terminal, i _R (t) is the current sampling value of the remote terminal, u _L (t) is the voltage sampling value of the local terminal, u _R (t) is the voltage sampling value of the remote terminal, and t refers to time;

The calculation steps in the Bergeron model include:

The following method is used to calculate the fault component polar traveling wave voltage value:

where Z _C is the wave impedance, Δu _L+ (t) is the positive traveling wave voltage value of the fault component pole of the local terminal; Δu _L- (t) is the reverse traveling wave voltage value of the fault component pole of the local terminal; Δu _R+ (t ) is the positive traveling wave voltage value of the fault component pole of the remote terminal; Δu _R- (t) is the reverse traveling wave voltage value of the fault component pole of the remote terminal;

Calculate the pole traveling wave current value of the fault component in the following way:

Among them, Δi _L+ (t) is the polar forward traveling wave current value of the fault component of the local terminal; Δi _L- (t) is the polar reverse traveling wave current value of the fault component of the local terminal; Δi _R+ (t) is the value of the remote terminal Pole forward traveling wave current value of the fault component; Δi _R- (t) is the reverse traveling wave current value of the fault component pole of the remote terminal;

The fault component pole current value at the selected location is calculated as follows:

where Δi _L (x, t) is the pole current value of the fault component at the selected point of the local terminal; Δi _R (x, t) is the pole current value of the fault component at the selected point of the remote terminal, and v is the fault component row The speed at which the wave travels.

Conveniently, the step of judging the current differential protection includes:

If |Δi _L (x, t)+Δi _R (x, t)|>I _res is satisfied, the state is determined to be an internal fault, where I _res is a preset threshold.

Conveniently, the step of judging the current differential protection further includes:

If the status is judged as an internal fault, a failsafe command is sent to activate the differential protection; otherwise, the differential protection will not be activated.

Another aspect of the present invention provides a computer program comprising computer program code adapted to perform all the steps of any of the above aspects when run on a computer.

Still another aspect of the present invention provides a computer program recorded in a computer readable medium according to the above.

Another aspect of the present invention provides a DC grid current differential protection system, including the following modules:

Sampled value obtaining module: obtain the pole voltage sampling value and pole current sampling value in the local terminal and the remote terminal of the DC line;

Fault component extraction module: respectively calculate the pole voltage value of the fault component according to the pole voltage sampling values of the local terminal and the remote terminal; and calculate the pole current value of the fault component respectively according to the pole current value of the local terminal and the remote terminal;

Bergeron model calculation module: by calculating the fault component pole voltage value and fault component pole current value of the local terminal and the remote terminal calculated in the fault component extraction step based on the Bergeron model, the distance between the local terminal and the remote terminal is obtained. The pole current value of the fault component at a selected point on the DC line of ;

Current differential protection judging module: If the fault component pole current values at the selected points of the local terminal and the remote terminal obtained in the Bergeron model calculation module meet the preset current differential protection criterion, then judge the internal fault.

Preferably, the DC grid is bipolar and the DC line includes a positive DC line and a negative DC line, the local terminal includes a positive local terminal and a negative local terminal, the remote terminal includes a positive remote terminal and a negative remote terminal, the positive DC line is electrically connected to the positive local terminal and positive remote terminal, the negative DC line electrically connects the negative local terminal and the negative remote terminal, the distance from the selected point to the positive local terminal is the same as the distance from the selected point to the negative local terminal, and from the selected point to the positive remote terminal The distance is the same as the distance from the selected point to the negative remote terminal, plus:

Pole mode conversion module: by performing pole mode conversion on the fault component pole voltage value of each of the positive local terminal, the positive remote terminal, the negative local terminal and the negative remote terminal, each mode of the local terminal and the remote terminal is obtained. and by performing a pole-mode transformation on said fault component pole current value of each of the positive local terminal, the positive remote terminal, the negative local terminal, and the negative remote terminal, the values of the local terminal and the remote terminal are obtained The fault component modulus current value of each modulus;

The Bergeron model calculation module also includes:

By calculating the fault component mode voltage value and fault component mode current value of each modulus of the local terminal and the remote terminal based on the Bergeron model, the fault component mode traveling wave voltage of each modulus of the local terminal and the remote terminal is obtained respectively value;

Converting the fault component mode traveling wave voltage values of the local terminal and the remote terminal into fault component mode traveling wave current values of the local terminal and the remote terminal respectively;

determining fault component mode current values for the local terminal and the remote terminal at selected points on the DC line based on the fault component mode traveling wave current values for the local terminal and the remote terminal, respectively;

The fault of each of the positive local terminal and the negative local terminal at a selected point on the DC line is obtained by modulo-polar transformation of the fault component modulo current value of each modulus of the local terminal at the selected point Component pole current values, and by performing modulo-pole transformation on the fault component modulo current value of each modulus of the remote terminal at the selected point, the fault component poles of the positive remote terminal and the negative remote terminal at the selected point are obtained current value.

Conveniently, the pole voltage sampling values include: u _LP (t), that is, the voltage sampling value of the positive local terminal; u _LN (t), that is, the voltage sampling value of the negative local terminal; u _RP (t), that is, the voltage sampling value of the positive remote terminal Voltage sampling value; u _RN (t), that is, the voltage sampling value of the negative remote terminal; wherein t refers to time;

Pole current sampling values include: i _LP (t), the current sampling value of the positive local terminal; i _LN (t), the current sampling value of the negative local terminal; i _RP (t), the current sampling value of the positive remote terminal ; i _RN (t), the current sampling value of the negative remote terminal;

The fault component pole voltage value includes: Δu _LP (t), that is, the fault component voltage value of the positive local terminal corresponding to u _LP (t); Δu _LN (t), that is, the negative local terminal corresponding to u _LN (t). The fault component voltage value of the terminal; Δu _RP (t), that is, the fault component voltage value of the positive remote terminal corresponding to u _RP (t); Δu _RN (t), that is, the negative remote terminal corresponding to u _RN (t) Terminal fault component voltage value;

The fault component pole current value includes: Δi _LP (t), that is, the fault component current value of the positive local terminal corresponding to i _LP (t); Δi _LN (t), that is, the negative local terminal corresponding to i _LN (t). The fault component current value of the terminal; Δi _RP (t), that is, the fault component current value of the positive remote terminal corresponding to i _RP (t); Δi _RN (t), that is, the negative remote terminal corresponding to i _RN (t) Terminal fault component current value;

The fault component mode voltage value includes: Δu _L0 (t), which is the common mode voltage value of the fault component of the local terminal; Δu _L1 (t), which is the differential mode voltage value of the fault component of the local terminal; Δu _R0 (t), which is the remote terminal The common-mode voltage value of the fault component of ; Δu _R1 (t), that is, the differential-mode voltage value of the fault component of the remote terminal;

The fault component mode current value includes: Δi _L0 (t), which is the fault component common mode current value of the local terminal; Δi _L1 (t), which is the fault component differential mode current value of the local terminal; Δi _R0 (t), which is the remote terminal Δi _R1 (t), that is, the differential mode current value of the fault component of the remote terminal;

The fault component traveling wave voltage value includes: Δu _L0+ (t), which is the common mode forward traveling wave voltage value of the fault component of the local terminal; Δu _L0- (t), which is the common mode reverse traveling wave voltage value of the fault component of the local terminal ; Δu _L1+ (t), that is, the fault component differential mode forward traveling wave voltage value of the local terminal; Δu _L0- (t), that is, the fault component differential mode reverse traveling wave voltage value of the local terminal; Δu _R0+ (t), That is, the common-mode forward traveling wave voltage value of the fault component of the remote terminal; Δu _R0- (t), that is, the common-mode reverse traveling wave voltage value of the fault component of the remote terminal; Δu _R1+ (t), that is, the fault component difference of the remote terminal Δu _R1- (t), that is, the fault component differential mode reverse traveling wave voltage value of the remote terminal;

The fault component traveling wave current value includes: Δi _L0+ (t), which is the common mode forward traveling wave current value of the fault component of the local terminal; Δi _L0- (t), which is the common mode reverse traveling wave current value of the fault component of the local terminal ; Δi _L1+ (t), that is, the differential mode forward traveling wave current value of the fault component of the local terminal; Δi _L1- (t), that is, the reverse traveling wave current value of the fault component differential mode of the local terminal; Δi _R0+ (t), That is, the common-mode forward traveling wave current value of the fault component of the remote terminal; Δi _R0- (t), that is, the common-mode reverse traveling wave current value of the fault component of the remote terminal; Δi _R1+ (t), that is, the fault component difference of the remote terminal Δi _R1- (t), that is, the fault component differential mode reverse traveling wave current value of the remote terminal;

The fault component mode current value at the selected point includes: Δi _L0 (x, t), which is the fault component common mode current value at the selected point of the local terminal; Δi _L1 (x, t), which is the selected point of the local terminal The fault component differential mode current value at the fixed point; Δi _R0 (x, t), which is the fault component common mode current value at the selected point of the remote terminal; Δi _R1 (x, t), which is the selected point of the remote terminal The fault component differential mode current value at , where x is the selected point;

The fault component pole current value at the selected point includes: Δi _LP (x, t), which is the fault component pole current value at the selected point of the positive local terminal; Δi _LN (x, t), which is the selected point of the negative local terminal The fault component pole current value at the fixed point; Δi _RP (x, t), which is the fault component pole current value at the selected point of the positive remote terminal; Δi _RN (x, t), which is the selected point of the negative remote terminal The fault component pole current value at .

Conveniently, in the fault component extraction module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following manner:

Wherein, T represents the preset time delay;

In the pole mode conversion module, the fault component mode voltage value and the fault component mode current value are calculated in the following way:

In the Bergeron model calculation module includes:

The following method is used to calculate the fault component mode traveling wave voltage value:

Among them, Z _C0 is the common-mode wave impedance; Z _C1 is the differential-mode wave impedance;

Calculate the fault component mode traveling wave current value in the following way:

Calculate the fault component modulo current value at the selected point in the following way:

Among them, v ₀ is the traveling speed of the common-mode traveling wave of the fault component, and v ₁ is the traveling speed of the differential-mode traveling wave of the fault component;

Calculate the fault component pole current value at the selected point in the following way:

Conveniently, the current differential protection judging module includes:

If |Δi _LP (x, t)+Δi _RP (x, t)|＞I _res is satisfied, the state is determined to be positive internal fault, if |Δi _LN (x, t)+Δi _RN (x, t)| >I _res , then the status is determined as a negative internal fault, where I _res represents the preset threshold;

Otherwise, the differential protection will not be activated.

Preferably, the DC grid is unipolar:

The Bergeron model calculation module also includes:

Based on the Bergeron model, calculate the polar voltage value of the fault component and the mode current value of the fault component of the local terminal and the remote terminal, and obtain the polar traveling wave voltage value of the fault component of the local terminal and the remote terminal respectively;

Converting the fault component polar traveling wave voltage values of the local terminal and the remote terminal into the fault component modulus traveling wave current values of the local terminal and the remote terminal;

The fault component pole current values of the local terminal and the remote terminal at selected points on the DC line are determined from the fault component pole traveling wave current values of the local terminal and the remote terminal.

Conveniently, in the fault component extraction module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following manner:

Where T represents the preset time delay, Δi _L (t) is the current value of the fault component pole of the local terminal, Δi _R (t) is the current value of the fault component pole of the remote terminal, Δu _L (t) is the fault component pole current value of the local terminal Voltage value, Δu _R (t) is the fault component pole voltage value of the remote terminal, i _L (t) is the current sampling value of the local terminal, i _R (t) is the current sampling value of the remote terminal, u _L (t) is The voltage sampling value of the local terminal, u _R (t) is the voltage sampling value of the remote terminal, and t refers to time;

In the Bergeron model calculation module includes:

The following method is used to calculate the fault component polar traveling wave voltage value:

Among them, Z _C is the wave impedance, Δu _L+ (t) is the positive traveling wave voltage value of the fault component of the local terminal; Δu _L- (t) is the reverse traveling wave voltage value of the fault component of the local terminal; Δu _R+ ( t) is the polar forward traveling wave voltage value of the fault component of the remote terminal; Δu _R- (t) is the reverse traveling wave voltage value of the fault component polarity of the remote terminal;

Calculate the pole traveling wave current value of the fault component in the following way:

Among them, Δi _L+ (t) is the polar forward traveling wave current value of the fault component of the local terminal; Δi _L- (t) is the polar reverse traveling wave current value of the fault component of the local terminal; Δi _R+ (t) is the value of the remote terminal Pole forward traveling wave current value of the fault component; Δi _R- (t) is the reverse traveling wave current value of the fault component pole of the remote terminal;

Calculate the fault component pole current value at the selected location as follows:

where Δi _L (x, t) is the pole current value of the fault component at the selected point of the local terminal; Δi _R (x, t) is the pole current value of the fault component at the selected point of the remote terminal, and v is the fault component row The speed at which the wave travels.

Conveniently, the current differential protection judging module includes:

If |Δi _L (x, t)+Δi _R (x, t)|>I _res is satisfied, the state is determined to be an internal fault, where I _res is a preset threshold.

Conveniently, the current differential protection judging module further includes:

If the state is judged to be an internal fault, a failsafe command is sent to activate the differential protection, otherwise the differential protection will not be activated.

The Bergeron model is based on distribution parameters and the telegraph equation (wave equation). Therefore, the present invention adopts the Bergeron model, so no long time extension is required to eliminate the disturbance of the distributed charging current, thereby greatly improving the calculation speed of the present invention.

At the same time, the invention uses the fault component to remove the influence of the load current on the differential protection, thereby improving the sensitivity.

Description of drawings

Figure 1 shows a graph of traveling wave fronts for internal and external DC faults in an LCC DC grid;

Fig. 2 shows the measurement schematic diagram of traditional traveling wave protection device;

FIG. 3 shows a flowchart illustrating a method for DC grid current differential protection according to the present invention;

Fig. 4 schematically shows fault component distribution grid;

Figure 5 shows the state when an internal fault occurs in the line;

Figure 6 shows the state when an external fault occurs in the line;

Figure 7 shows the simulation model;

Figure 8 shows the simulation results;

Fig. 9 shows a structural block diagram of a DC grid current differential protection system;

Figure 10 shows a unipolar HVDC system.

Detailed ways

Hereinafter, the present invention will be described in more detail through specific embodiments in conjunction with the accompanying drawings.

FIG. 3 shows a flowchart illustrating a method for DC grid current differential protection according to the present invention, the method includes the following steps:

Step S301: Obtain the pole voltage sampling value and pole current sampling value in the local terminal and the remote terminal of the DC line;

Step S302: Calculate the pole voltage value of the fault component according to the pole voltage sampling values of the local terminal and the remote terminal respectively; and calculate the pole current value of the fault component according to the pole current sampling values of the local terminal and the remote terminal respectively;

Step S303: Based on the Bergeron model (Bergeron model), by calculating the fault component pole voltage value and the fault component pole current value of the local terminal and the remote terminal calculated in step S302, obtain the DC line between the local terminal and the remote terminal Pole current value of the fault component at the selected point;

Step S304: If the pole current values of the fault component at the selected points of the local terminal and the remote terminal obtained in step S303 satisfy the preset current differential protection criterion, then determine an internal fault.

The Bergeron model is based on distribution parameters and the telegraph equation (wave equation). The method therefore needs to inherently and accurately account for the charging current distributed during fault transients in theory.

Therefore, the present invention adopts the Bergeron model, which does not require a long time extension to eliminate the disturbance of the distributed charging current, thereby greatly improving the calculation speed in the present invention.

At the same time, in step S302, the pole voltage sampled value is converted into a fault component pole voltage value, and the pole current sampled value is converted into a fault component pole current value. Therefore, in this step, the fault component pole voltage value is separated from the pole voltage sampled value, and the fault component pole current value is separated from the pole current sampled value. In this case, when a fault occurs in the grid, the grid may be divided into a fault-free network and a fault component network, so that the fault component pole voltage value and the fault component pole current value are the pole voltage/current values in the fault component network. In subsequent steps S303 and S304 , pole-mode transformation is performed on the pole voltage value of the fault component and the pole current value of the fault component and applied to the Bergeron model. That is, the present invention provides current differential protection based on fault components. Therefore, the present invention uses the fault component to remove the influence of the load current on the differential protection, thereby increasing the sensitivity.

In a preferred embodiment of the invention, in particular, the DC grid is bipolar, and the DC line includes a positive DC line and a negative DC line, the local terminals include a positive local terminal and a negative local terminal, and the remote terminals include a positive remote terminal and The negative remote terminal, the positive DC line electrically connects the positive local terminal and the positive remote terminal, and the negative DC line electrically connects the negative local terminal and the negative remote terminal, the distance from the selected point to the positive local terminal and the distance from the selected point to the negative local terminal The distance from the selected point to the positive remote terminal is the same as the distance from the selected point to the negative remote terminal, including:

Pole mode conversion step: by performing pole mode conversion on the fault component pole voltage value of each of the positive local terminal, the positive remote terminal, the negative local terminal and the negative remote terminal, each mode of the local terminal and the remote terminal is obtained. The fault component modulo voltage value of the quantity, and by performing pole mode transformation on the fault component pole current value of each of the positive local terminal, the positive remote terminal, the negative local terminal and the negative remote terminal, the local terminal and the remote terminal are obtained The fault component modulus current value of each modulus;

Step S303 also includes:

By calculating the fault component mode voltage value and fault component mode current value of each modulus of the local terminal and the remote terminal based on the Bergeron model, the fault component mode traveling wave voltage of each modulus of the local terminal and the remote terminal is obtained respectively value;

Converting the fault component mode traveling wave voltage values of the local terminal and the remote terminal into fault component mode traveling wave current values of the local terminal and the remote terminal respectively;

determining fault component mode current values for the local terminal and the remote terminal at selected points on the DC line based on the fault component mode traveling wave current values for the local terminal and the remote terminal, respectively;

The fault component polarity of each of the positive local terminal and the negative local terminal at a selected point on the DC line is obtained by modulo-polar transformation of the fault component modulo current value of each modulus of the local terminal at the selected point The current value, by performing modulo-polar transformation on the fault component modulo current value of each modulus of the remote terminal at the selected point, obtains the fault component pole current value of the positive remote terminal and the negative remote terminal at the selected point.

This embodiment applies the fault component modulo voltage and fault component modulo current subjected to modulo-polar transformation to the Bergeron model, thereby specifically realizing the Bergeron model based on the fault component of the bipolar DC grid.

In one embodiment:

Pole voltage sampling values include: u _LP (t), namely the voltage sampling value of the positive local terminal; u _LN (t), namely the voltage sampling value of the negative local terminal; u _RP (t), namely the voltage sampling value of the positive remote terminal ; u _RN (t), the voltage sampling value of the negative remote terminal; where t refers to the time;

Pole current sampling values include: i _LP (t), the current sampling value of the positive local terminal; i _LN (t), the current sampling value of the negative local terminal; i _RP (t), the current sampling value of the positive remote terminal ; i _RN (t), the current sampling value of the negative remote terminal;

The fault component pole voltage value includes: Δu _LP (t), that is, the fault component voltage value of the positive local terminal corresponding to u _LP (t); Δu _LN (t), that is, the fault component voltage value of the negative local terminal corresponding to u _LN (t). Fault component voltage value; Δu _RP (t), that is, the fault component voltage value of the positive remote terminal corresponding to u _RP (t); Δu _RN (t), that is, the fault component of the negative remote terminal corresponding to u _RN (t) Voltage value;

The fault component pole current value includes: Δi _LP (t), that is, the fault component current value of the positive local terminal corresponding to i _LP (t); Δi _LN (t), that is, the fault component current value of the negative local terminal corresponding to i _LN (t). Fault component current value; Δi _RP (t), that is, the fault component current value of the positive remote terminal corresponding to i _RP (t); Δi _RN (t), that is, the fault component of the negative remote terminal corresponding to i _RN (t) current value;

The fault component mode voltage value includes: Δu _L0 (t), which is the common mode voltage value of the fault component of the local terminal; Δu _L1 (t), which is the differential mode voltage value of the fault component of the local terminal; Δu _R0 (t), which is the remote terminal The common-mode voltage value of the fault component of ; Δu _R1 (t), that is, the differential-mode voltage value of the fault component of the remote terminal;

The fault component mode current value includes: Δi _L0 (t), which is the fault component common mode current value of the local terminal; Δi _L1 (t), which is the fault component differential mode current value of the local terminal; Δi _R0 (t), which is the remote terminal Δi _R1 (t), that is, the differential mode current value of the fault component of the remote terminal;

The fault component traveling wave voltage value includes: Δu _L0+ (t), which is the common mode forward traveling wave voltage value of the fault component of the local terminal; Δu _L0- (t), which is the common mode reverse traveling wave voltage value of the fault component of the local terminal ; Δu _L1+ (t), that is, the fault component differential mode forward traveling wave voltage value of the local terminal; Δu _L0- (t), that is, the fault component differential mode reverse traveling wave voltage value of the local terminal; Δu _R0+ (t), That is, the common-mode forward traveling wave voltage value of the fault component of the remote terminal; Δu _R0- (t), that is, the common-mode reverse traveling wave voltage value of the fault component of the remote terminal; Δu _R1+ (t), that is, the fault component difference of the remote terminal Δu _R1- (t), that is, the fault component differential mode reverse traveling wave voltage value of the remote terminal;

The fault component traveling wave current value includes: Δi _L0+ (t), which is the common mode forward traveling wave current value of the fault component of the local terminal; Δi _L0- (t), which is the common mode reverse traveling wave current value of the fault component of the local terminal ; Δi _L1+ (t), that is, the differential mode forward traveling wave current value of the fault component of the local terminal; Δi _L1- (t), that is, the reverse traveling wave current value of the fault component differential mode of the local terminal; Δi _R0+ (t), That is, the common-mode forward traveling wave current value of the fault component of the remote terminal; Δi _R0- (t), that is, the common-mode reverse traveling wave current value of the fault component of the remote terminal; Δi _R1+ (t), that is, the fault component difference of the remote terminal Δi _R1- (t), that is, the fault component differential mode reverse traveling wave current value of the remote terminal;

The fault component mode current value at the selected point includes: Δi _L0 (x, t), which is the fault component common mode current value at the selected point of the local terminal; Δi _L1 (x, t), which is the selected Δi _R0 (x, t), that is, the fault component common-mode current value at the selected point of the remote terminal; Δi _R1 (x, t), that is, the selected point of the remote terminal The differential mode current value of the fault component;

The fault component pole current value at the selected point includes: Δi _LP (x, t), which is the fault component pole current value at the selected point of the positive local terminal; Δi _LN (x, t), which is the selected point of the negative local terminal The fault component pole current value at the fixed point; Δi _RP (x, t), which is the fault component pole current value at the selected point of the positive remote terminal; Δi _RN (x, t), which is the selected point of the negative remote terminal The fault component pole current value at .

This embodiment is calculated separately for the positive pole and the negative pole, thereby realizing the differential protection for the two poles respectively.

In one embodiment:

In step S302, the pole voltage value of the fault component and the pole voltage value of the fault component are calculated in the following manner:

Among them, T represents the time delay;

In the pole mode conversion step, the fault component mode voltage value and the fault component mode current value are calculated in the following manner:

In step S303 include:

The following method is used to calculate the fault component mode traveling wave voltage value:

Among them, Z _C0 is the common-mode wave impedance; Z _C1 is the differential-mode wave impedance;

Calculate the fault component mode traveling wave current value in the following way:

Calculate the fault component modulo current value at the selected point in the following way:

Among them, v ₀ is the traveling speed of the common-mode traveling wave of the fault component, and v ₁ is the traveling speed of the differential-mode traveling wave of the fault component;

Calculate the fault component pole current value at the selected point in the following way:

In one embodiment:

In step S304 include:

If |Δi _LP (x, t)+Δi _RP (x, t)|＞I _res is met, the state is determined to be a positive internal fault; if |Δi _LN (x, t)+Δi _RN (x, t)| >I _res , then the status is determined as a negative internal fault, where I _res represents the preset threshold;

Otherwise, the differential protection will not be activated.

In one embodiment, the DC grid is unipolar:

Step S303 also includes:

Based on the Bergeron model, calculate the polar voltage value of the fault component and the mode current value of the fault component of the local terminal and the remote terminal, and obtain the polar traveling wave voltage value of the fault component of the local terminal and the remote terminal respectively;

Converting the fault component polar traveling wave voltage values of the local terminal and the remote terminal into the fault component modulus traveling wave current values of the local terminal and the remote terminal;

Based on the fault component pole traveling wave current values of the local terminal and the remote terminal, the fault component pole current values of the local terminal and the remote terminal at selected points on the DC line are determined.

In particular this embodiment implements a Bergeron model based on the fault component of a unipolar DC grid.

In one embodiment, in step S302, the pole voltage value of the fault component and the pole current value of the fault component are calculated in the following manner:

Among them, T represents the time delay, Δi _L (t) is the pole current value of the fault component of the local terminal, Δi _R (t) is the pole current value of the fault component of the remote terminal, and Δu _L (t) is the pole voltage of the fault component of the local terminal Δu _R (t) is the fault component pole voltage value of the remote terminal, i _L (t) is the current sampling value of the local terminal, i _R (t) is the current sampling value of the remote terminal, u _L (t) is the local The voltage sampling value of the terminal, u _R (t) is the voltage sampling value of the remote terminal, and t refers to time;

Step S303 includes:

The following method is used to calculate the fault component polar traveling wave voltage value:

Among them, Z _C is the wave impedance, Δu _L+ (t) is the positive traveling wave voltage value of the fault component of the local terminal; Δu _L- (t) is the reverse traveling wave voltage value of the fault component of the local terminal; Δu _R+ ( t) is the polar forward traveling wave voltage value of the fault component of the remote terminal; Δu _R- (t) is the reverse traveling wave voltage value of the fault component polarity of the remote terminal;

Calculate the pole traveling wave current value of the fault component in the following way:

Among them, Δi _L+ (t) is the polar forward traveling wave current value of the fault component of the local terminal; Δi _L- (t) is the polar reverse traveling wave current value of the fault component of the local terminal; Δi _R+ (t) is the value of the remote terminal Pole forward traveling wave current value of the fault component; Δi _R- (t) is the reverse traveling wave current value of the fault component pole of the remote terminal;

The fault component pole current value at the selected location is calculated as follows:

where Δi _L (x, t) is the fault component pole current value at the selected point of the local terminal; Δi _R (x, t) is the fault component pole current value at the selected point of the remote terminal, v is the fault The travel velocity of the component traveling wave.

In one embodiment, step S304 includes:

If |Δi _L (x, t)+Δi _R (x, t)|>I _res is satisfied, it is determined that the state is an internal fault.

In one embodiment, step S304 also includes:

If the state is judged to be an internal fault, a failsafe command is sent to activate the differential protection, otherwise the differential protection will not be activated.

Bipolar DC grid

In a preferred embodiment of the invention, with a schematically illustrated fault component distributed grid as shown in Figure 4, the current differential protection method of the invention seeks to calculate the Δi _LP (x , t) and Δi _RP (x, t) to make a positive fault decision, and simultaneously calculate Δi _LN (x, t) and Δi _RN (x, t) at a selected point x at a specific time t , in order to make a negative fault judgment. The local side 41 and the remote side 42 can communicate through communication lines, so that the local side 41 can obtain all parameter information of the local side 41 and the remote side 42 . In particular, Δi _LP (x,t), Δi _RP (x,t), Δi _LN (x,t) and Δi _RN (x,t) can be calculated as follows:

Fault Component Current and Voltage Calculations

The fault component is calculated by the following formula (1):

Wherein, T is a time delay, and T can be set to, for example, 10 ms or 100 ms according to requirements.

polar mode transformation

After obtaining the fault component current value and voltage value Δi _LP (t), Δi _LN (t), Δu _LP (t), Δu _LN (t), Δi _RP (t), Δi _RN (t) through formula (1), After Δu _RP (t) and Δu _RN (t), the next step is to do polar mode transformation to convert polar quantities into moduli. The pole-mode transformation matrices for voltage and current are given in equation (2).

Calculation of Differential Current Based on Bergeron Model

In this step, based on the Bergeron model (traveling wave propagation equation), the fault components traveling wave common and differential modes at a selected point x on the line to be protected will be calculated using the measured values respectively from the two terminals current value.

(1) Calculate the fault component mode traveling wave voltage value

Equation 3 can be used to calculate the common-mode forward voltage traveling wave Δu _L0+ and reverse voltage traveling wave Δu _L0- of the local side fault component, and the differential mode forward voltage traveling wave Δu _L1+ and reverse voltage traveling wave of the local side fault component Δu _L1- , the common-mode forward voltage traveling wave Δu _R0+ and reverse voltage traveling wave Δu _R0- of the remote side fault component, and the differential mode forward voltage traveling wave Δu _R1+ and reverse voltage traveling wave Δu of the remote side fault component _R1- .

Where Z _C0 is the common-mode wave impedance and Z _C1 is the differential-mode wave impedance.

(2) Calculate the fault component mode traveling wave current value

Equation 4 can be used to calculate the common-mode forward current traveling wave Δi _L0+ and reverse current traveling wave Δi _L0- of the local side fault component, and the differential mode forward current traveling wave Δi _L1+ and reverse current traveling wave of the local side fault component Δi _L1- , the common-mode forward current traveling wave Δi _R0+ and reverse current traveling wave Δi _R0- of the remote side fault component, and the differential mode forward current traveling wave Δi _R1+ and reverse current traveling wave Δi of the remote side fault component _R1- .

(3) The fault component modulus current value at the selected position

Based on the principle of traveling waves, the fault component differential and common mode currents of the local terminal and the remote terminal at a selected point x can be calculated using Equation 5 below, where the local terminal at the selected point x is calculated from the measured value of the local terminal The fault component differential mode and common mode currents of the remote terminal are calculated from the measured values of the remote terminal at the selected point x. The fault component differential mode and common mode current of:

(4) Modulus transformation

In this step, the positive and negative currents at selected points are calculated using modulo-polar transformation for both the local and remote terminals. The transformation matrix is shown in Equation 6 below:

Criteria for activating current differential protection:

If the following formula 7 is satisfied:

|Δi _LP (x, t)+Δi _RP (Lx, t)|＞I _res (7)

Then the state is judged as "positive internal fault", so that a fault protection command is sent and the control of the differential protection is activated.

If the following formula (8) is satisfied:

|Δi _LN (x, t)+Δi _RN (Lx, t)|＞I _res (8)

Then the state is judged as "negative pole internal fault", so that a fault protection command is sent and the control of the differential protection is activated.

Unipolar DC Grid:

In a preferred embodiment of the present invention, as shown in Figure 10, Δu _L (t) and Δi _L (t) are the fault component voltage and current of the local terminal, Δu _R (t) and Δi _R (t) is the remote terminal fault component voltage and current,

Δi _L (x, t) is the current at point x calculated using local measurements,

Δi _R (x, t) is the current at point x calculated using remote measurements,

As shown in the figure above, based on the Bergeron model (telegraph equation, traveling wave equation), the fault component current at the selected point "x" will be calculated separately using the measured values of the two terminals.

For current components calculated using local measurements, x is the distance between any chosen point along the line and the local terminal. For example, if the selected point is a remote terminal, then the distance x is the length L of the line;

For current components calculated using remote measurements, x is the distance between any chosen point along the line and the remote terminal. For example, if the selected point is a remote terminal, then the distance x is zero.

In the following sections, the calculation steps will be described in detail.

Fault Component Current and Voltage Calculation

The way to calculate the failure component is:

In Equation 9, u(t) and i(t) are the measured pole voltage and current samples. Δu(t) and Δi(t) are the corresponding fault component voltage and current values. T is a time delay, which can be set to, for example, 10 ms or 100 ms as needed. According to this method, both poles and fault component voltage and current values at both ends can be calculated as in formula 10.

Calculation of Differential Current Based on Bergeron Model

In this step, based on the Bergeron model (traveling wave propagation equation), the traveling wave current at a selected point x on the line to be protected will be calculated separately using the measured values from the two terminals.

Calculation of traveling wave voltage components

Equation 11 can be used to calculate forward voltage traveling wave Δu _L+ and reverse voltage traveling wave Δu _L- .

Traveling wave current component calculation

Then, Equation 12 can be used to calculate the forward current traveling wave Δi _L+ and the reverse current traveling wave Δi _L- .

Calculation of traveling wave current components at selected points

Based on the principle of traveling waves, the current at a selected point x can be calculated using Equation 13.

where v is the traveling speed of the traveling wave;

t is time;

x is any point along the line, it can be an intermediate point, an end point, a starting point or any other point;

Δi _L (x, t) is the fault component current at the selected point x calculated using measurements at the local terminal;

Δi _R (x, t) is the fault component current at a selected point x calculated using measurements from the remote terminal;

Differential current calculation

As shown in Equation 14, the differential current is calculated from the pole current and compared with the threshold. If the differential current is greater than the inhibit current, it means an internal fault. Otherwise it means external failure. The criteria for detecting internal faults are shown below.

|Δi _L (x, t)+Δi _R (x, t)|＞I _res (14)

performance analysis

Classic Differential Protection in HVDC Lines

The following shows the criteria for a typical classical differential protection:

|I _Local +I _Remote |＞I _Set (9)

Among them, I _Local is the local terminal current, and I _Remote is the remote terminal current.

Fig. 5 shows the state when an internal fault occurs in the line. For internal faults, there are

|I _Local +I _Remote |＝I _F +I _C (10)

where _IF is the fault current through the faulted branch as shown in Figure 5, and _IC is the current flowing through the capacitance distributed along the line, which is usually much higher than zero, especially for long-length transmission lines. Comparing with equation (9), we can observe that the protection principle works correctly.

However, for external faults, the capacitor current will cause problems. Fig. 6 shows the state when an external fault occurs in the line. For external faults, there are

|I _Local +I _Remote |＝I _C (11)

From formula (11), we know that in order to avoid malfunction under external fault, the setting value I _Set must be higher than I _C . Since I _C only exists temporarily after a fault, another way to avoid spurious operation is to keep I _Set at the standard value, but use a long time delay to wait until the transient disappears.

Usually in practical applications, in order not to impair the sensitivity of the protection criterion under high impedance faults, the second method, ie a long time delay (0.5s-1.5s), is used. But then, the response slowed down.

this invention

The invention is based on the traveling wave component calculated using the Bergeron model, which already takes into account the distributed capacitance of the line.

From this, it is possible to calculate the exact differential current, which excludes the charging current of the distributed capacitors:

- When an internal fault occurs, the calculated differential current is the fault current I _F flowing through the faulted branch, for example, |Δi _LP (x, t)+Δi _RP (x, t)|= _IF ;

- When an external fault occurs, the differential current calculated by the present invention is zero, eg |Δi _LP (x,t)+Δi _RP (x,t)|=0.

This allows the invention to be independent of capacitance distributed along the line, thereby ensuring speed of operation.

fast motion speed

Speed of movement is very important for protection; it is one of the most important requirements for protection. When a fault occurs, system stability and personnel safety are threatened, and rapid isolation is very beneficial to system stability and personnel safety. Two other important requirements for protection include stability and sensitivity. A good protection principle must achieve these three advantages: fast action speed, stability and sensitivity.

Due to capacitive currents, classical differential protection cannot act fast, but waits until the transient phase has passed, thus limiting its speed. Unlike classical differential protection, the present invention is not affected by capacitance distributed along the line, so it can achieve faster operating speed. And also because it is not affected by capacitive current, it can use lower current threshold and achieve higher sensitivity.

Considering the communication time depending on the length of the line and the communication route, the action speed of the present invention is less than 15ms in most cases, while the action time of the classic differential protection is 0.5s-1.5s. The algorithm of the present invention can be used as the main protection for the above-mentioned LCC DC power grid, and can be used as a backup protection for other types of DC power grids, and can obtain a higher action speed than the classical differential protection; and it can also be used It is used as the main protection for short lines, where the communication time of short lines is shorter than other types of DC grid systems, or point-to-point HVDC systems.

Good sensitivity to high resistance faults

The present invention has good sensitivity to high-resistance faults because it is based on fault components and removes the influence of load current on differential protection, which reduces the sensitivity of classical differential protection.

wide adaptability

In this section, the adaptability will be analyzed from two aspects: working principle and motion speed.

Working Principle Relative Adaptability

It can be seen from the above analysis that the differential protection principle of the present invention is only related to the line parameters, and it uses the line parameters to calculate the current at the point "x", and it has no special requirements on the topology and control of the DC system.

Relative adaptability of movement speed

From this we understand that the action speed of the present invention is less than 15ms in most cases.

Therefore, we can choose to configure the protection of the present invention as main protection or backup protection according to the requirements for the action speed of different DC systems.

For example, for point-to-point DC lines or DC grids based on LCC technology, and point-to-point DC lines with VSC technology, the present invention can be used as both primary protection and backup protection.

For the DC power grid based on VSC technology, the present invention can be used as backup protection, because the requirement for the action speed is quite high, usually within 5ms. If the length of the transmission line is short, the time delay caused by communication can be reduced, so that the present invention can also be used as main protection.

It should be noted that when configured as back-up protection, the present invention performs much better than existing differential current-based back-up protection, whose operating time is typically longer than a few hundred milliseconds.

resonance effect

There is a distributed line capacitance along the line, which is large because the HVDC lines are very long. When a fault occurs, there will be a large voltage and current oscillation ("resonance"), which will seriously affect some traditional protection principles, such as traditional current differential protection, low-voltage protection, etc.

While the directional element of the present invention is based on the Bergeron model, this model inherently takes "resonance" into account and is not affected by "resonance".

simulation

simulation model

Figure 7 shows the simulation model, the ±800kV 4-terminal series MTDC consists of two rectifier stations (R1 and R2) and two inverter stations (I1 and I2). The total length of the transmission line is 2000km, including two branch lines (500km each) and a main line (1000km). Each inverter station has a configuration with a 12-pulse valve bank. Each rectifier converter will have a nominal DC voltage across 400kV, each inverter converter will have a nominal DC voltage across 373kV, and the HV DC line will have a ground voltage of approximately 400kV (for R1 and I1) or approximately 800kV (for R2 and I2).

The present invention protects that the relays 71 and 72 are located at the two terminals of the +800kV transmission line shown in the figure above. And the internal fault is at the end of the +800kV line, and the external fault is on the +400kV line. And in this case the pole-to-pole wave impedance Z _C is 264Ω.

Fig. 8 shows the simulation results, the internal fault occurs at 2s, and the external fault occurs at 4s. In Figure 8,

- the actual current flowing through the faulty branch at "IF";

- "Idif Bergeron" is the differential current calculated by the protection principle based on the Bergeron model;

- "Idif classic" is the differential current calculated by the classic differential protection.

Internal failure analysis

As shown in Figure 8, an internal fault occurs at 2s with a fault resistance of 3000 ohms.

It should be noted that "Idif Bergeron" and "IF" are not exactly the same when an internal fault occurs, because the principle based on the Bergeron model uses the fault component to calculate the differential current, and the fault component in this simulation only exists 50ms, so the two currents differ 50ms after the fault starts. But during the period from the fault start (2s) to 2.05s, the differential current calculated according to the principle based on the Bergeron model is close enough to the actual fault current "IF".

It can also be obtained from Fig. 8 that the differential current calculated by the classic protection is also close to the actual fault current "IF", but the waveform is large.

External failure analysis

As shown in Figure 8, the external fault occurs at 4s.

When an external fault occurs, theoretically there should be no fault current. However the differential current calculated according to the classical differential protection ("Idif classical") is quite large, even higher than the differential current when an internal fault occurs within 2s. So we can observe that during the transient the classical differential current protection cannot distinguish between external and internal faults, it has to wait before the transient disappears.

"Idif Bergeron" shows the differential current calculated by the present invention. It can be observed from Fig. 8 that the calculated differential current after an external fault occurs is very small, much smaller than that calculated under an internal fault. That is, it can effectively distinguish external faults from internal faults.

In summary, the simulation results show that the differential protection based on the Bergeron model is less affected by the distributed capacitance of the line compared to the classical differential protection.

Figure 9 shows a structural model diagram of the DC grid current differential protection system, including the following modules:

The sampled value obtaining module 901 is used to obtain the sampled value of the pole voltage and the sampled value of the pole current of the local terminal and the remote terminal of the DC power grid;

The fault component extraction module 902 is used to calculate the pole voltage value of the fault component respectively according to the pole voltage sampling values of the local terminal and the remote terminal; and calculate the pole current value of the fault component respectively according to the pole current sampling values of the local terminal and the remote terminal;

The pole mode conversion module is used for obtaining the mode voltage value of the fault component by performing pole mode conversion on the pole voltage value of the fault component in the local terminal and the remote terminal respectively, and by performing pole mode conversion on the pole voltage value of the fault component in the local terminal and the remote terminal The current and voltage values are subjected to pole-mode transformation to obtain the fault component mode current values respectively;

The Bergeron model calculation module 903 is used to calculate the fault component mode voltage value and the fault component mode current value in the local terminal and the remote terminal based on the Bergeron model, and respectively obtain the local terminal and the remote terminal at a selected point. The fault component pole current value;

Current differential protection determination module 904, including determining an internal fault if the fault component pole current value in the local terminal and remote terminal at the selected point satisfies the preset current differential protection criterion, and then sending a fault protection command to activate differential protection, otherwise the differential protection will not be activated.

The above-mentioned embodiments are only used to describe several examples of the present invention. Although these embodiments have been described in detail, they should not be construed as limiting the protection scope of the present invention. It should be noted that without departing from the technical idea of the present invention, those skilled in the art may make several modifications and/or improvements, all of which fall within the protection scope of the present invention. Therefore, the protection scope of the present invention depends on the appended claims.

Claims (19)

1. a kind of DC power network currents differential protecting method, includes the following steps：

Sampled value obtains step：Obtain the pole tension sampled value in the local terminal and remote terminal of DC circuits and electrode current sampling Value；

Fault component extraction step：Fault component is calculated according to the pole tension sampled value of local terminal and remote terminal respectively Pole tension value；And fault component electrode current is calculated according to the electrode current sampled value of local terminal and remote terminal respectively Value；

Bei Jielong models calculate step：By calculating the local calculated in the fault component extraction step based on Bei Jielong models Terminal and the fault component pole tension value of remote terminal and fault component electrode current value are obtained between local terminal and remote terminal DC circuits on Chosen Point at fault component electrode current value；

Current differential protection determination step：If what is obtained in the Bei Jielong models calculating step is in local terminal and remotely whole Fault component electrode current value at the Chosen Point at end meets predetermined current differential protection criterion, then judges internal fault.

2. according to the method described in claim 1, wherein DC power grids are bipolar, and the DC circuits include anode DC circuits With cathode DC circuits, the local terminal includes anode local terminal and cathode local terminal, and the remote terminal includes anode Remote terminal and cathode remote terminal, the anode DC circuits are electrically connected the anode local terminal and the anode remotely eventually End, and the cathode DC circuits are electrically connected the cathode local terminal and the cathode remote terminal, from the Chosen Point to The distance of the anode local terminal is identical with from the Chosen Point to the distance of the cathode local terminal, and from the choosing The distance for pinpointing the anode remote terminal is identical with from the Chosen Point to the distance of the cathode remote terminal, also wraps It includes：

Pole modular transformation step：By to the anode local terminal, the anode remote terminal, the cathode local terminal and institute The each fault component pole tension value stated in cathode remote terminal carries out pole modular transformation, obtain local terminal and it is long-range eventually The fault component mode voltage value of each modulus in end；And by the anode local terminal, the anode remote terminal, Each fault component electrode current value in the cathode local terminal and the cathode remote terminal carries out pole modular transformation, Obtain the fault component mould current value of local terminal and each modulus in remote terminal；

The Bei Jielong models calculate step and further include：

Pass through the fault component mode voltage value based on Bei Jielong models calculating local terminal and each modulus of remote terminal and event Hinder component mould current value, obtain the fault component line wave voltage value of each modulus of local terminal and remote terminal respectively；

The fault component line wave voltage value of local terminal and remote terminal is converted into local terminal and long-range end respectively The fault component line wave current value at end；

Respectively according to local terminal and the fault component line wave current value of remote terminal, the choosing on the DC circuits is determined The fault component mould current value of local terminal and remote terminal at fixed point；

Mould pole-change is carried out by the fault component mould current value of each modulus to the local terminal at Chosen Point, is obtained Each fault component electrode current value in the anode local terminal and cathode local terminal at Chosen Point on the DC circuits, And mould pole-change is carried out by the fault component mould current value of each modulus to the remote terminal at Chosen Point, it obtains The fault component electrode current value of anode remote terminal and cathode remote terminal at Chosen Point.

3. according to the method described in claim 2, wherein described pole tension sampled value includes：u_LP(t), i.e. anode local terminal Voltage sample value；u_LN(t), i.e. the voltage sample value of cathode local terminal；u_RP(t), i.e. the voltage sample of anode remote terminal Value；u_RN(t), i.e. the voltage sample value of cathode remote terminal；Wherein t refers to the time；

The electrode current sampled value includes：i_LP(t), i.e. the current sampling data of anode local terminal；i_LN(t), i.e., cathode is local eventually The current sampling data at end；i_RP(t), i.e. the current sampling data of anode remote terminal；i_RN(t), i.e. the electric current of cathode remote terminal is adopted Sample value；

The fault component pole tension value includes：Δu_LP(t), i.e., and u_LP(t) fault component of corresponding anode local terminal Voltage value；Δu_LN(t), i.e., and u_LN(t) the fault component voltage value of corresponding cathode local terminal；Δu_RP(t), i.e., and u_RP (t) the fault component voltage value of corresponding anode remote terminal；Δu_RN(t), i.e., and u_RN(t) corresponding cathode is remotely whole The fault component voltage value at end；

The fault component electrode current value includes：Δi_LP(t), i.e., and i_LP(t) fault component of corresponding anode local terminal Current value；Δi_LN(t), i.e., and i_LN(t) the fault component current value of corresponding cathode local terminal；Δi_RP(t), i.e., and i_RP (t) the fault component current value of corresponding anode remote terminal；Δi_RN(t), i.e., and i_RN(t) corresponding cathode is remotely whole The fault component current value at end；

The fault component mode voltage value includes：Δu_L0(t), i.e. the fault component common-mode voltage value of local terminal；Δu_L1(t), That is the fault component differential mode voltage value of local terminal；Δu_R0(t), i.e. the fault component common-mode voltage value of remote terminal；Δu_R1 (t), i.e. the fault component differential mode voltage value of remote terminal；

The fault component mould current value includes：Δi_L0(t), i.e. the fault component common-mode current value of local terminal；Δi_L1(t), That is the fault component differential-mode current value of local terminal；Δi_R0(t), i.e. the fault component common-mode current value of remote terminal；Δi_R1 (t), i.e. the fault component differential-mode current value of remote terminal；

The fault component traveling wave voltage value includes：Δu_L0+(t), i.e. the fault component common mode direct wave voltage of local terminal Value；Δu_L0-(t), i.e. the fault component common mode backward-travelling wave voltage value of local terminal；Δu_L1+(t), i.e. the failure of local terminal Component differential mode direct wave voltage value；Δu_L0-(t), i.e. the fault component differential mode backward-travelling wave voltage value of local terminal；Δu_R0+ (t), i.e. the fault component common mode direct wave voltage value of remote terminal；Δu_R0-(t), i.e. the fault component common mode of remote terminal Backward-travelling wave voltage value；Δu_R1+(t), i.e. the fault component differential mode direct wave voltage value of remote terminal；Δu_R1-(t), i.e., far The fault component differential mode backward-travelling wave voltage value of journey terminal；

The fault component travelling wave current value includes：Δi_L0+(t), i.e. the fault component common mode direct wave electric current of local terminal Value；Δi_L0-(t), i.e. the fault component common mode backward-travelling wave current value of local terminal；Δi_L1+(t), i.e. the failure of remote terminal Component differential mode direct wave current value；Δi_L1-(t), i.e. the fault component differential mode backward-travelling wave current value of local terminal；Δi_R0+ (t), i.e. the fault component common mode direct wave current value of remote terminal；Δi_R0-(t), i.e. the fault component common mode of remote terminal Backward-travelling wave current value；Δi_R1+(t), i.e. the fault component differential mode direct wave current value of remote terminal；Δi_R1-(t), i.e., far The fault component differential mode backward-travelling wave current value of journey terminal；

The fault component mould current value in Chosen Point includes：Δi_L0Failure point at the Chosen Point of (x, t), i.e. local terminal Measure common-mode current value；Δi_L1Fault component differential-mode current value at the Chosen Point of (x, t), i.e. local terminal；Δi_R0(x, t), i.e., Fault component common-mode current value at the Chosen Point of remote terminal；Δi_R1Failure point at the Chosen Point of (x, t), i.e. remote terminal Differential-mode current value is measured, wherein x is Chosen Point；

Fault component electrode current value at the Chosen Point includes：Δi_LP(x, t), i.e., at the Chosen Point of anode local terminal Fault component electrode current value；Δi_LN(x, t), i.e., the fault component electrode current value at the Chosen Point of cathode local terminal；Δi_RP (x, t), i.e., the fault component electrode current value at the Chosen Point of anode remote terminal；Δi_RN(x, t), i.e., in cathode remote terminal Chosen Point at fault component electrode current value.

4. it according to the method described in claim 3, wherein, in the fault component extraction step, calculates in the following ways The fault component pole tension value and the fault component pole tension value：

Wherein, T is predetermined time delay；

In the pole modular transformation step, the fault component mode voltage value and the fault component mould are calculated in the following ways Current value：

Step is calculated in the Bei Jielong models to include：

The fault component line wave voltage value is calculated in the following ways：

Wherein, Z_C0It is common mode wave impedance；Z_C1It is differential mode wave impedance；

The fault component line wave current value is calculated in the following ways：

The fault component mould current value at Chosen Point is calculated in the following ways：

Wherein, v₀It is the gait of march of fault component common mode traveling wave, v₁It is the gait of march of fault component differential mode traveling wave；

The fault component electrode current value at Chosen Point is calculated in the following ways：

5. according to the method described in claim 3, wherein, the current differential protection determination step includes：

If meet | Δ i_LP(x, t)+Δ i_RP(x, t) |>I_res, then judge state for anode internal fault；If meet | Δ i_LN (x, t)+Δ i_RN(x, t) |>I_res, then state is judged for cathode internal fault, wherein, I_resRepresent predetermined threshold value；

Otherwise, differential protection will not be activated.

6. according to the method described in claim 1, wherein described DC power grids are monopoles：

The Bei Jielong models calculate step and further include：

By being based on Bei Jielong models, the fault component pole tension value and the failure of local terminal and remote terminal are calculated Component mould current value obtains the fault component pole traveling wave voltage value of local terminal and remote terminal respectively；

The fault component pole traveling wave voltage value of local terminal and remote terminal is converted into local terminal and long-range end respectively The fault component line wave current value at end；

According to local terminal and the fault component pole travelling wave current value of remote terminal, the Chosen Point on the DC circuits is determined The local terminal at place and the fault component electrode current value of remote terminal.

7. according to the method described in claim 6, institute is wherein calculated in the following ways in the fault component extraction step State fault component pole tension value and fault component pole tension value：

Wherein T represents predetermined time delay, Δ i_L(t) be local terminal fault component electrode current value, Δ i_R(t) it is long-range end The fault component electrode current value at end, Δ u_L(t) be local terminal fault component pole tension value, Δ u_R(t) it is remote terminal event Hinder component pole tension value, i_L(t) be local terminal current sampling data, i_R(t) be remote terminal current sampling data, u_L(t) it is The voltage sample value of local terminal, u_R(t) be remote terminal voltage sample value, and t refers to the time；

Step is calculated in the Bei Jielong models to include：

Fault component pole traveling wave voltage value is calculated in the following ways：

Wherein, Z_CIt is wave impedance, Δ u_L+(t) be local terminal fault component pole direct wave voltage value；Δu_L-(t) it is local The fault component pole backward-travelling wave voltage value of terminal；Δu_R+(t) be remote terminal fault component pole direct wave voltage value；Δ u_R-(t) be remote terminal fault component pole backward-travelling wave voltage value；

The fault component pole travelling wave current value is calculated in the following ways：

Wherein, Δ i_L+(t) be local terminal fault component pole direct wave current value；Δi_L-(t) be local terminal failure Component pole backward-travelling wave current value；Δi_R+(t) be remote terminal fault component pole direct wave current value；Δi_R-(t) it is remote The fault component pole backward-travelling wave current value of journey terminal；

The fault component electrode current value at selected location is calculated in the following ways：

Wherein Δ i_L(x, t) is the fault component electrode current value at the Chosen Point of local terminal；Δi_R(x, t) is remote terminal Fault component electrode current value at Chosen Point, v are the gait of march of fault component traveling wave.

8. according to the method described in claim 7, wherein, include in the current differential protection determination step：

If meet | Δ i_L(x, t)+Δ i_R(x, t) | ＞ I_res, then state is judged for internal fault, wherein I_resIt is predetermined threshold value.

9. the method described in any one in claim 1-8, wherein the current differential protection determination step also wraps It includes：

If state is determined into internal fault, error protection order is sent to activate differential protection, otherwise will not activate difference Dynamic protection.

10. a kind of computer including suitable for performing method as described in one of claim 1 to 8 when running on computers The computer-readable medium of program code.

11. a kind of DC power network currents differential protective system, including with lower module：

Sampled value obtains module：Obtain the local terminal of DC circuits and pole tension sampled value and the electrode current sampling of remote terminal Value；

Fault component extraction module：Fault component is calculated according to the pole tension sampled value of local terminal and remote terminal respectively Pole tension value；And fault component electrode current value is calculated according to the electrode current value of local terminal and remote terminal respectively；

Bei Jielong model computation modules：Based on Bei Jielong models, calculated in the fault component extraction module by calculating Local terminal and remote terminal the fault component pole tension value and the fault component electrode current value, obtain local terminal The fault component electrode current value at the Chosen Point on DC circuits between remote terminal；

Current differential protection determination module：If the local terminal that is obtained in Bei Jielong model computation modules and remote terminal The fault component electrode current value at Chosen Point meets predetermined current differential protection criterion, then judges internal fault.

12. system according to claim 11, wherein, the DC power grids are that bipolar and described DC circuits include anode DC circuits and cathode DC circuits, the local terminal include anode local terminal and cathode local terminal, the remote terminal packet Anode remote terminal and cathode remote terminal are included, the anode DC circuits are electrically connected the anode local terminal and the anode is remote Journey terminal, and the cathode DC circuits are electrically connected the cathode local terminal and the cathode remote terminal, are selected from described The distance of point to the anode local terminal is identical with from the Chosen Point to the distance of the cathode local terminal, from the choosing The distance for pinpointing the anode remote terminal is identical with from the Chosen Point to the distance of the cathode remote terminal, also wraps It includes：

Pole modular transformation module：By to the anode local terminal, the anode remote terminal, the cathode local terminal and institute The each fault component pole tension value stated in cathode remote terminal carries out pole modular transformation, obtain local terminal and it is long-range eventually The fault component mode voltage value of each modulus in end and by the anode local terminal, the anode remote terminal, Each fault component electrode current value in the cathode local terminal and the cathode remote terminal carries out pole modular transformation, Obtain the fault component mould current value of each modulus of local terminal and remote terminal；

The Bei Jielong model computation modules further include：

By being based on Bei Jielong models, the fault component mode voltage value of each modulus of local terminal and remote terminal is calculated With the fault component mould current value, the fault component line wave to each modulus of local terminal and remote terminal is obtained respectively Voltage value；

The fault component line wave voltage value of local terminal and remote terminal is converted into local terminal and long-range end respectively The fault component line wave current value at end；

Respectively according to local terminal and the fault component line wave current value of remote terminal, the Chosen Point on DC circuits is determined The local terminal at place and the fault component mould current value of remote terminal；

Mould pole-change is carried out by the fault component mould current value of each modulus to the local terminal at Chosen Point, is obtained Each fault component electrode current value in anode local terminal and cathode local terminal at Chosen Point on DC circuits and Mould pole-change is carried out by the fault component mould current value of each modulus to the remote terminal at Chosen Point, is obtained selected The fault component electrode current value of anode remote terminal and cathode remote terminal at point.

13. system according to claim 12, wherein, the pole tension sampled value includes：u_LP(t), i.e., anode is local eventually The voltage sample value at end；u_LN(t), i.e. the voltage sample value of cathode local terminal；u_RP(t), i.e. the voltage of anode remote terminal is adopted Sample value；u_RN(t), i.e. the voltage sample value of cathode remote terminal；Wherein t refers to the time；

The electrode current sampled value includes：i_LP(t), i.e. the current sampling data of anode local terminal；i_LN(t), i.e., cathode is local eventually The current sampling data at end；i_RP(t), i.e. the current sampling data of anode remote terminal；i_RN(t), i.e. the electric current of cathode remote terminal is adopted Sample value；

The fault component pole tension value includes：Δu_LP(t), i.e., and u_LP(t) fault component of corresponding anode local terminal Voltage value；Δu_LN(t), i.e., and u_LN(t) the fault component voltage value of corresponding cathode local terminal；Δu_RP(t), i.e., and u_RP (t) the fault component voltage value of corresponding anode remote terminal；Δu_RN(t), i.e., and u_RN(t) corresponding cathode is remotely whole The fault component voltage value at end；

The fault component electrode current value includes：Δi_LP(t), i.e., and i_LP(t) fault component of corresponding anode local terminal Current value；Δi_LN(t), i.e., and i_LN(t) the fault component current value of corresponding cathode local terminal；Δi_RP(t), i.e., and i_RP (t) the fault component current value of corresponding anode remote terminal；Δi_RN(t), i.e., and i_RN(t) corresponding cathode is remotely whole The fault component current value at end；

The fault component mode voltage value includes：Δu_L0(t), i.e. the fault component common-mode voltage value of local terminal；Δu_L1(t), That is the fault component differential mode voltage value of local terminal；Δu_R0(t), i.e. the fault component common-mode voltage value of remote terminal；Δu_R1 (t), i.e. the fault component differential mode voltage value of remote terminal；

The fault component mould current value includes：Δi_L0(t), i.e. the fault component common-mode current value of local terminal；Δi_L1(t), That is the fault component differential-mode current value of local terminal；Δi_R0(t), i.e. the fault component common-mode current value of remote terminal；Δi_R1 (t), i.e. the fault component differential-mode current value of remote terminal；

The fault component traveling wave voltage value includes：Δu_L0+(t), i.e. the fault component common mode direct wave voltage of local terminal Value；Δu_L0-(t), i.e. the fault component common mode backward-travelling wave voltage value of local terminal；Δu_L1+(t), i.e. the failure of local terminal Component differential mode direct wave voltage value；Δu_L0-(t), i.e. the fault component differential mode backward-travelling wave voltage value of local terminal；Δu_R0+ (t), i.e. the fault component common mode direct wave voltage value of remote terminal；Δu_R0-(t), i.e. the fault component common mode of remote terminal Backward-travelling wave voltage value；Δu_R1+(t), i.e. the fault component differential mode direct wave voltage value of remote terminal；Δu_R1-(t), i.e., far The fault component differential mode backward-travelling wave voltage value of journey terminal；

The fault component travelling wave current value includes：Δi_L0+(t), i.e. the fault component common mode direct wave electric current of local terminal Value；Δi_L0-(t), i.e. the fault component common mode backward-travelling wave current value of local terminal；Δi_L1+(t), i.e. the failure of remote terminal Component differential mode direct wave current value；Δi_L1-(t), i.e. the fault component differential mode backward-travelling wave current value of local terminal；Δi_R0+ (t), i.e. the fault component common mode direct wave current value of remote terminal；Δi_R0-(t), i.e. the fault component common mode of remote terminal Backward-travelling wave current value；Δi_R1+(t), i.e. the fault component differential mode direct wave current value of remote terminal；Δi_R1-(t), i.e., far The fault component differential mode backward-travelling wave current value of journey terminal；

The fault component mould current value in Chosen Point includes：Δi_L0Failure point at the Chosen Point of (x, t), i.e. local terminal Measure common-mode current value；Δi_L1Fault component differential-mode current value at the Chosen Point of (x, t), i.e. local terminal；Δi_R0(x, t), i.e., Fault component common-mode current value at the Chosen Point of remote terminal；Δi_R1Failure point at the Chosen Point of (x, t), i.e. remote terminal Differential-mode current value is measured, wherein x is Chosen Point；

The fault component electrode current value at Chosen Point includes：Δi_LPAt (x, t), the i.e. Chosen Point of anode local terminal Fault component electrode current value；Δi_LNFault component electrode current value at (x, t), the i.e. Chosen Point of cathode local terminal；Δi_RP Fault component electrode current value at (x, t), the i.e. Chosen Point of anode remote terminal；Δi_RNThe choosing of (x, t), i.e. cathode remote terminal Fault component electrode current value at fixed point.

14. system according to claim 13 wherein in the fault component extraction module, calculates in the following ways The fault component pole tension value and the fault component pole tension value：

Wherein, T represents predetermined time delay；

In the pole modular transformation module, the fault component mode voltage value and the fault component mould are calculated in the following ways Current value：

Include in the Bei Jielong model computation modules：

The fault component line wave voltage value is calculated in the following ways：

Wherein Z_C0It is common mode wave impedance；Z_C1It is differential mode wave impedance；

The fault component line wave current value is calculated in the following ways：

The fault component mould current value at Chosen Point is calculated in the following ways：

Wherein, v₀It is the gait of march of fault component common mode traveling wave, v₁It is the gait of march of fault component differential mode traveling wave；

The fault component electrode current value at Chosen Point is calculated in the following ways：

15. system according to claim 13, wherein the current differential protection determination module includes：

If meet | Δ i_LP(x, t)+Δ i_RP(x, t) | ＞ I_res, then judge state for anode internal fault；If meet | Δ i_LN(x, t)+Δ i_RN(x, t) |>I_res, then state is judged for cathode internal fault, wherein, I_resRepresent predetermined threshold value；

Otherwise, differential protection will not be activated.

16. system according to claim 11, wherein, the DC power grids are monopoles：

The Bei Jielong model computation modules further include：

By being based on Bei Jielong models, the fault component pole tension value and the failure of local terminal and remote terminal are calculated Component mould current value obtains the fault component pole traveling wave voltage value of local terminal and remote terminal respectively；

The fault component pole traveling wave voltage value of local terminal and remote terminal is converted into local terminal and long-range end respectively The fault component line wave current value at end；

The Chosen Point on the DC circuits is determined according to the fault component pole travelling wave current value of local terminal and remote terminal The local terminal at place and the fault component electrode current value of remote terminal.

17. system according to claim 16, wherein, in the fault component extraction model, count in the following ways Calculate the fault component pole tension value and the fault component pole tension value：

Wherein T represents predetermined time delay, Δ i_L(t) be local terminal fault component electrode current value, Δ i_R(t) it is long-range end The fault component electrode current value at end, Δ u_L(t) be local terminal fault component pole tension value, Δ u_R(t) it is remote terminal event Hinder component pole tension value, i_L(t) be local terminal current sampling data, i_R(t) be remote terminal current sampling data, u_L(t) it is The voltage sample value of local terminal, u_R(t) be remote terminal voltage sample value, and t refers to the time；

Include in the Bei Jielong model computation modules：

The fault component pole traveling wave voltage value is calculated in the following ways：

Wherein, Z_CIt is wave impedance, Δ u_L+(t) be local terminal fault component pole direct wave voltage value；Δu_L-(t) it is local The fault component pole backward-travelling wave voltage value of terminal；Δu_R+(t) be remote terminal fault component pole direct wave voltage value；Δ u_R-(t) be remote terminal fault component pole backward-travelling wave voltage value；

The fault component pole travelling wave current value is calculated in the following ways：

Wherein, Δ i_L+(t) be local terminal fault component pole direct wave current value；Δi_L-(t) be local terminal failure Component pole backward-travelling wave current value；Δi_R+(t) be remote terminal fault component pole direct wave current value；Δi_R-(t) it is remote The fault component pole backward-travelling wave current value of journey terminal；

The fault component electrode current value at selected location is calculated in the following ways：

Wherein Δ i_L(x, t) is the fault component electrode current value at the Chosen Point of local terminal；Δi_R(x, t) is remote terminal Fault component electrode current value at Chosen Point, v are the gait of march of fault component traveling wave.

18. system according to claim 17, wherein, include in the current differential protection determination module：

If meet | Δ i_L(x, t)+Δ i_R(x, t) | ＞ I_res, then state is judged for internal fault, wherein I_resIt is predetermined threshold value.

19. according to the system described in any one in claim 11-18, wherein, the current differential protection determination module is also Including：

If state is determined into internal fault, error protection order is sent to activate differential protection, otherwise will not activate difference Dynamic protection.