CN106998074B - Control method and system for modular multi-level flexible direct current converter station - Google Patents

Abstract

The invention provides a control method and system for a modularized multilevel flexible DC converter station. The control method includes the steps of: after three-phase AC information obtained by sampling passes through a virtual synchronous motor controller and a circulating current controller, respectively, The induced electromotive force and voltage regulation voltage are obtained; the three-phase voltage reference value is obtained according to the induced electromotive force and the voltage regulation voltage; the three-phase voltage reference value is modulated by PWM, and the switching signal is output to realize the control of the modular multi-level converter station. The control method proposed by the invention can automatically adjust the output active power and reactive power according to the frequency and voltage variation of the AC system, thereby effectively reducing the frequency and voltage fluctuations of the AC system.

Description

A control method and system for a modular multi-level flexible DC converter station

technical field

The invention relates to the technical field of flexible direct current transmission, in particular to a virtual synchronous motor control method and system for a modular multilevel flexible direct current converter station.

Background technique

Faced with the worldwide threat of fossil energy crisis, global climate change and environmental degradation, countries around the world have fully realized that the development and utilization of energy must transition from traditional fossil energy to green, renewable and clean energy. Wind power and photovoltaic power generation, as important renewable energy sources, have become the main forms of energy supply. The penetration rate of distributed energy has increased, the installed ratio of traditional synchronous generators has decreased, and the rotating reserve capacity and moment of inertia of the power system have been relatively reduced. In addition, due to the combined effect of distributed energy fluctuations and intermittency, the system has experienced grid-source coordination, etc. Issues affecting the safe and stable operation of the power grid.

DC transmission technology has obvious advantages in reactive power demand, system stability, large-scale access to clean energy, and large-capacity long-distance power transmission. Power flow reversal and system control have significant advantages, making it one of the best means of renewable energy access, and also providing a new technical solution for the construction of future power transmission networks.

However, the traditional control method of HVDC flexible transmission is mainly based on vector control technology. Based on this control strategy, the flexible HVDC transmission system can realize dynamic voltage support, independent decoupling of active and reactive power, and provide stability for the transmission of active power. reliable way. In the traditional flexible HVDC transmission system, the spatial orientation vector decoupling is easily affected by the change and mismatch of system control parameters. In addition, the grid-connected converter under the traditional vector control strategy is difficult to provide appropriate damping in the low frequency band, which is easy to cause the subsynchronous resonance of the AC system and the large-scale disconnection of the grid-connected converter station, which seriously threatens the voltage and frequency of the power system. Especially during the transient fault of the power system, the grid-connected converter station cannot provide a large enough inertia, which can easily cause the system frequency instability.

Therefore, there is a need for a control technology that can not only improve the delivery of renewable energy but also improve the operational safety, stability and reliability of the power system.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a control method and system for a modular multi-level flexible DC converter station. The control method includes the steps of: sampling three-phase AC information obtained by virtual synchronization respectively. After the motor controller and the circulating current controller, the induced electromotive force and voltage regulation voltage are obtained; according to the induced electromotive force and the voltage regulation voltage, the three-phase voltage reference value is obtained; the three-phase voltage reference value is modulated by PWM, and the switching signal is output to realize the modularization of multi-phase voltage. Control of level converter stations.

The three-phase AC information obtained by sampling is respectively passed through the virtual synchronous motor controller and the circulating current controller to obtain the induced electromotive force and voltage regulation voltage, including: The motion equation of the rotor of the mechanical motion unit in the virtual synchronous motor controller is as follows:

where J is the moment of inertia of the rotor, T _m is the mechanical torque, T _e is the electromagnetic torque, D _p is the damping coefficient, ω* is the rated frequency, and ω is the actual frequency of the power grid;

The electrical equations for the stator windings of the electromagnetic unit in the virtual synchronous motor controller are as follows:

Where: u _abc is the output voltage of the stator winding, i _abc is the output current of the stator winding, e is the induced electromotive force, R _s is the winding loss, and L _s is the synchronous reactance.

The formula for calculating the induced electromotive force is as follows:

为额定频率ω*与电网实际频率ω的差值；In the formula: e is the induced electromotive force, θ is the electrical angle, M _f is the mutual inductance between the stator and the rotor, if _is the rotor winding current,

is the difference between the rated frequency ω* and the actual frequency ω of the grid;

和

为三相的相位关系，其定义如下所示：

and

is the phase relationship of the three phases, which is defined as follows:

The damping coefficient is the relationship between active power and frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the active power P, and Δθ is the electrical angle change.

The expression for output active power is as follows:

为i_abc和

两者的内积、

为额定频率^ω*与电网实际频率ω的差值、M_f为定子和转子间互感、i_f为转子绕组电流。in,

for i _abc and

The inner product of the two,

is the difference between the rated frequency ^ω * and the actual frequency ω of the grid, M _f is the mutual inductance between the stator and the rotor, and if _is the rotor winding current.

The control system includes: a control module, the three-phase AC information obtained by sampling is respectively passed through a virtual synchronous motor controller and a circulating current controller to obtain induced electromotive force and voltage regulation voltage; a voltage synthesizer is used for receiving induced electromotive force and voltage regulation commands after , outputs the three-phase voltage reference value; the modulation module modulates the three-phase voltage reference value through PWM, and outputs the switch signal to realize the control of the modular multi-level converter station.

The virtual synchronous motor controller includes a mechanical motion unit and an electromagnetic motion unit;

The equation of motion of the rotor in the mechanical motion unit is as follows:

where J is the moment of inertia of the rotor, T _m is the mechanical torque, T _e is the electromagnetic torque, D _p is the damping coefficient, ω* is the rated frequency, and ω is the actual frequency of the power grid;

The electrical equations for the stator windings in the electromagnetic unit are as follows:

Where: u _abc is the output voltage of the stator winding, i _abc is the output current of the stator winding, e is the induced electromotive force, R _s is the winding loss, and L _s is the synchronous reactance.

The formula for calculating the induced electromotive force is as follows:

为额定频率ω*与电网实际频率ω的差值；where e is the induced electromotive force, θ is the electrical angle, M _f is the mutual inductance between the stator and the rotor, if _is the rotor winding current,

is the difference between the rated frequency ω* and the actual frequency ω of the grid;

和

为三相的相位关系，其定义如下所示：

and

is the phase relationship of the three phases, which is defined as follows:

The damping coefficient is the relationship between active power and frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the active power P, and Δθ is the electrical angle change.

The expression for output active power is as follows:

为i_abc和

两者的内积、

为额定频率ω*与电网实际频率ω的差值、M_f为定子和转子间互感、i_f为转子绕组电流。in,

for i _abc and

The inner product of the two,

is the difference between the rated frequency ω* and the actual frequency ω of the grid, M _f is the mutual inductance between the stator and the rotor, and if _is the rotor winding current.

Compared with the closest prior art, the technical solution provided by the present invention has the following beneficial effects:

1. The control method proposed by the present invention introduces the swing equation and electromagnetic equation of the traditional synchronous generator into the controller, and the converter station simulates the traditional synchronous generator in terms of external characteristics and operating mechanism, so that the converter station has the function of voltage regulation and frequency regulation , provides a certain voltage and frequency support for the AC system, and improves the stability and reliability of the system.

2. The control method proposed by the present invention can automatically adjust the output active power and reactive power according to the frequency and voltage variation of the AC system, thereby effectively reducing the frequency and voltage fluctuations of the AC system.

3. The present invention provides inertia and damping for the system, so that the controllability of the entire flexible DC system has better robustness, and realizes large-scale grid connection of distributed energy while improving the stable operation capability of the system.

Description of drawings

1 is a block diagram of an overall control method of the present invention;

FIG. 2 is a block diagram of the control strategy of the virtual synchronous motor controller of the present invention.

Detailed ways

The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings.

The purpose of the present invention is to design a control method of a modularized multilevel converter station. The control system of the flexible DC converter station consists of five parts: a sampling module, a control module, a voltage synthesizer and a modulation module.

The sampling module provides three-phase voltage and current input information for the control module including the virtual synchronous machine controller and the circulating current controller, and the output voltage of the control module is synthesized in the voltage synthesizer as the input of the modulation module, so as to achieve the modular multi-electrical system. The purpose of the output voltage control of the flat converter station.

The control method proposed by the invention introduces the swing equation and electromagnetic equation of the traditional synchronous generator into the controller, and the converter station simulates the traditional synchronous generator in terms of external characteristics and operation mechanism, so that the converter station has the function of voltage regulation and frequency regulation, which is an AC generator. The system provides a certain voltage and frequency support to improve the stability and reliability of the system.

The virtual synchronous motor control method of the modularized multi-level converter proposed by the present invention: equalizes the external characteristics of the flexible DC converter station with the mathematical model, motion equation, electromagnetic equation (transient or steady state) and working characteristics of the synchronous motor effect.

In terms of control strategy, it is necessary to learn from the control methods of flexible DC converters and traditional synchronous motors. The control algorithm of the virtual synchronous motor needs to use variables such as voltage and power, so to ensure the stability and accuracy of the output of the converter station, the voltage and current double-loop control of the converter station is used in the control structure. For traditional synchronous motors, the prime mover and the governor are responsible for providing mechanical power and regulating the output frequency, and the excitation control system is responsible for regulating the excitation voltage.

In the virtual synchronous motor, it is replaced by some units with similar functions. Among them, the excitation regulator simulates the control function of the excitation control system in the synchronous motor, and the power and frequency regulators simulate the control function of the governor. The control system collects the output voltage and current of the converter valve, obtains the mechanical power command of the virtual synchronous motor from the power and frequency regulator, and obtains the excitation voltage command of the virtual synchronous motor from the excitation regulator. The reference value of the system frequency and voltage, and then through the dual-loop control system and the modulation system, the drive signal is generated to control the opening and closing of the switch tube, thereby completing the entire closed-loop system.

The core of the control method proposed by the present invention is the design of the virtual synchronous motor controller, while the circulating current controller and the PWM modulation have no essential difference from the vector control of the traditional flexible DC converter, which are not the core parts of the present invention and will not be discussed in detail.

The virtual synchronous motor controller needs to realize the function of automatically adjusting the output active power and reactive power according to the frequency and voltage variation of the AC system, and providing the frequency and voltage support for the AC system. Therefore, the traditional synchronous generator control method can be handled. .

As shown in Figure 2, using the PI controller and combining the formulas (3) and (6), the virtual synchronous motor control strategy block diagram proposed by the present invention can be obtained, including: an active power frequency regulation function module and a reactive power voltage regulation function module, The converter can adjust the output power according to the needs of the AC system, and provide frequency and voltage support for the system.

Taking the classical second-order synchronous generator model as the main model, in order to make the converter better simulate and realize the performance of the synchronous motor, the relationship between the mechanical motion and the electromagnetic motion of the synchronous motor should be considered at the same time.

Due to the existence of the moment of inertia and damping coefficient, the mechanical motion of the synchronous generator can make the converter station have inertia in the dynamic relationship between power and frequency, and damp the power oscillation. The rotor motion equation is as follows:

where J is the moment of inertia of the rotor; T _m and T _e are the mechanical torque and electromagnetic torque of the generator, respectively; D _p is the damping coefficient; ω* and ω are the rated (reference) angular frequency and the actual angular frequency of the grid, respectively.

The grid frequency obtained by the solution in the above formula realizes the active power regulation capability of the converter, and the governor is simulated in the virtual synchronous machine control.

The electromagnetic part of the synchronous generator is modeled on the basis of the stator winding electrical equation, namely

where u _abc and i _abc are the three-phase output voltage and current of the stator, respectively; e is the three-phase induced electromotive force; R _s and L _s are the stator armature winding loss and synchronous reactance, respectively. The modeling only focuses on the relationship between the voltage and current of the stator, which is relatively simple, but does not consider its inherent electromagnetic characteristics.

In order to make the control model algorithm better simulate the synchronous generator from the mechanism, considering the flux linkage between the rotor and the stator, it can be deduced that

where θ is the electrical angle obtained by integrating the angular frequency ω*; M _f is the mutual inductance between the stator and the rotor; i _f is the rotor winding current;

和

为ABC三相的相位关系，其定义如下所示：

and

is the phase relationship of the three phases of ABC, and its definition is as follows:

At the same time, for the output active power P and reactive power Q of the converter, the inner product is used to derive the power expressions, as shown below:

为i_abc和

两者的内积，

为i_abc和

两者的内积。in,

for i _abc and

The inner product of the two,

for i _abc and

the inner product of the two.

The damping coefficient in the formula (1) essentially represents the relationship between the active power and the frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the calculated value P, and Δθ is the electrical angle change. The setting of the damping coefficient can be set according to the system, for example, the power changes 100%, and the frequency fluctuates 0.5%.

Note that in steady state, the damping factor setting should generally not be selected so that the frequency changes by more than 0.5Hz. In addition, since the proposed frequency control has no delay link, the value of the moment of inertia should be set to a small value, which is generally determined according to the equation J=D _p t _c , where t _c is the time constant, which is related to the system and generally takes the value For a few milliseconds to tens of milliseconds.

In addition to the reactive power voltage regulation control function of the control method based on formula (7), it is also necessary to obtain reactive power input parameters through reactive power voltage droop control, so that it can track the voltage of the AC grid well. Similarly, the reactive droop coefficient reflects the relationship between reactive power and voltage, and its definition is as follows:

Among them, ΔV represents the difference between the AC voltage set value V _set and the measured value V _g , that is, the offset of the AC voltage to the reference value, and ΔQ represents the difference between the reactive power setting value Q _set and the calculated value Q, that is, The amount of change in reactive power.

The reactive power droop coefficient is set according to the system demand value, and the reactive power can be calculated according to formula (7), and acts as negative feedback, reactive power setting value, and reactive power value of droop control together to form reactive power regulation. part, thereby simulating the field regulator function.

Equation (3) is the relational expression for controlling the output voltage of the converter. In general, the current of the excitation winding is a DC current with little change, and the latter item of the equation can be ignored.

It is worth noting that: the inverter station and the rectifier station can be controlled at the same time by using this design method. It should be noted that the change of the power flow direction will lead to the change of the formula symbol, and when using this method to control the virtual synchronous machine of the rectifier station, it is recommended to add an additional setting. DC voltage control to provide reliable DC power for the inverter station.

Based on the same inventive concept, the present invention also provides a system for reducing the risk of DC commutation failure in hierarchical access, which will be described below.

The system provided by the present invention may include:

The control module is used to output the induced electromotive force and voltage regulation voltage after receiving the three-phase sampling information; the voltage synthesizer is used to output the three-phase voltage reference value after receiving the induced electromotive force and the voltage regulation command; the modulation module, the three-phase voltage reference value After the PWM modulation module, the switch signal is output to realize the control of the modular multi-level converter station.

The control module includes: a virtual synchronous machine controller and a circulating current controller; after receiving the three-phase sampling information, the virtual synchronous machine controller and the circulating current controller output the induced electromotive force and the voltage regulation voltage respectively. The virtual synchronous motor controller includes a mechanical motion unit and an electromagnetic motion unit.

The equation of motion of the rotor in the mechanical motion unit is as follows:

In the formula: J is the moment of inertia of the rotor, T _m is the mechanical torque, T _e is the electromagnetic torque, D _p is the damping coefficient, ω* is the rated frequency, and ω is the actual frequency of the power grid.

The electrical equations for the stator windings in the electromagnetic unit are as follows:

Where: u _abc is the output voltage of the stator winding, i _abc is the output current of the stator winding, e is the induced electromotive force, R _s is the winding loss, and L _s is the synchronous reactance.

The flux linkage between the rotor and stator windings is as follows:

为所述额定频率ω*与电网实际频率ω的差值；where e is the induced electromotive force, θ is the electrical angle, M _f is the mutual inductance between the stator and the rotor, if _is the rotor winding current,

is the difference between the rated frequency ω* and the actual frequency ω of the grid;

和

为三相的相位关系，其定义如下所示：

and

is the phase relationship of the three phases, which is defined as follows:

The damping coefficient is the relationship between active power and frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the active power P, and Δθ is the electrical angle change.

The expression for output active power is as follows:

为i_abc和

两者的内积、

为额定频率ω*与电网实际频率ω的差值、M_f为定子和转子间互感、i_f为转子绕组电流。in,

for i _abc and

The inner product of the two,

is the difference between the rated frequency ω* and the actual frequency ω of the grid, M _f is the mutual inductance between the stator and the rotor, and if _is the rotor winding current.

As will be appreciated by those skilled in the art, the embodiments of the present application may be provided as a method, a system, or a computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) having computer-usable program code embodied therein.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the present application. It will be understood that each flow and/or block in the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to the processor of a general purpose computer, special purpose computer, embedded processor or other programmable data processing device to produce a machine such that the instructions executed by the processor of the computer or other programmable data processing device produce Means for implementing the functions specified in a flow or flow of a flowchart and/or a block or blocks of a block diagram.

These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory result in an article of manufacture comprising instruction means, the instructions The apparatus implements the functions specified in the flow or flow of the flowcharts and/or the block or blocks of the block diagrams.

These computer program instructions can also be loaded on a computer or other programmable data processing device to cause a series of operational steps to be performed on the computer or other programmable device to produce a computer-implemented process such that The instructions provide steps for implementing the functions specified in the flow or blocks of the flowcharts and/or the block or blocks of the block diagrams.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Those of ordinary skill in the art can still modify or equivalently replace the specific embodiments of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that depart from the spirit and scope of the present invention are all within the protection scope of the claims of the present invention for which the application is pending.

Claims (2)

1. A control method for a modular multi-level flexible DC converter station, wherein the control method comprises the steps: The three-phase AC information obtained by sampling is respectively passed through the virtual synchronous motor controller and the circulating current controller to obtain the induced electromotive force and voltage regulation; obtaining a three-phase voltage reference value according to the induced electromotive force and the voltage regulation voltage; The three-phase voltage reference value is modulated by PWM, and a switch signal is output to realize the control of the modular multi-level converter station; After the three-phase AC information obtained by the sampling is respectively passed through the virtual synchronous motor controller and the circulating current controller, the induced electromotive force and the voltage regulation voltage are obtained, including: The motion equation of the mechanical motion unit rotor in the virtual synchronous motor controller is as follows:

where J is the moment of inertia of the rotor, T _m is the mechanical torque, T _e is the electromagnetic torque, D _p is the damping coefficient, ω* is the rated frequency, and ω is the actual frequency of the power grid; The electrical equation of the stator winding of the electromagnetic unit in the virtual synchronous motor controller is as follows:

where u _abc is the output terminal voltage of the stator winding, i _abc is the output terminal current of the stator winding, e is the induced electromotive force, R _s is the winding loss, and L _s is the synchronous reactance; The calculation formula of the induced electromotive force is as follows:

In the formula: e is the induced electromotive force, θ is the electrical angle, M _f is the mutual inductance between the stator and the rotor, if _is the rotor winding current,

is the difference between the rated frequency ω* and the actual frequency ω of the power grid;

and

is the phase relationship of the three phases, which is defined as follows:

The damping coefficient is the relationship between active power and frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the active power P, and Δθ is the electrical angle change; The expression for the active power is as follows:

Among them, P is the active power, θ is the electrical angle,

is the output current i _abc of the stator winding and the phase relationship

The inner product of the two,

is the difference between the rated frequency ω* and the actual frequency ω of the grid, M _f is the mutual inductance between the stator and the rotor, and if _is the rotor winding current. 2. A control system for a modular multi-level flexible DC converter station, wherein the control system comprises: The control module obtains the induced electromotive force and the voltage regulation voltage after the three-phase AC information obtained by sampling passes through the virtual synchronous motor controller and the circulating current controller respectively; a voltage synthesizer for outputting a three-phase voltage reference value after receiving the induced electromotive force and the voltage regulation command; a modulation module, which modulates the three-phase voltage reference value through PWM, outputs a switch signal, and realizes the control of the modular multi-level converter station; The virtual synchronous motor controller includes a mechanical motion unit and an electromagnetic motion unit; The motion equation of the rotor in the mechanical motion unit is as follows:

where J is the moment of inertia of the rotor, T _m is the mechanical torque, T _e is the electromagnetic torque, D _p is the damping coefficient, ω* is the rated frequency, and ω is the actual frequency of the power grid; The electrical equations of the stator windings in the electromagnetic motion unit are as follows:

where u _abc is the output terminal voltage of the stator winding, i _abc is the output terminal current of the stator winding, e is the induced electromotive force, R _s is the winding loss, and L _s is the synchronous reactance; The calculation formula of the induced electromotive force is as follows:

where e is the induced electromotive force, θ is the electrical angle, M _f is the mutual inductance between the stator and the rotor, if _is the rotor winding current,

is the difference between the rated frequency ω* and the actual frequency ω of the power grid;

and

is the phase relationship of the three phases, which is defined as follows:

The damping coefficient is the relationship between active power and frequency change, and its definition is as follows:

Among them, ΔP is the difference between the active power set value P _set and the active power P, and Δθ is the electrical angle change; The expression for the active power is as follows:

Among them, P is the active power, θ is the electrical angle,

for the output current i _abc of the stator winding and the phase relationship

The inner product of the two,

is the difference between the rated frequency ω* and the actual frequency ω of the grid, M _f is the mutual inductance between the stator and the rotor, and if _is the rotor winding current.

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