CN201742107U - Power quality regulating device based on stored energy of super capacitor - Google Patents

Abstract

The invention relates to a power quality regulating device based on supercapacitor energy storage, which belongs to the technical field of power quality regulation. It is characterized in that the device includes a supercapacitor 5 , a bidirectional DC/DC converter 4 and a three-phase voltage type PWM rectifier 3 . Among them, the supercapacitor 5 is connected to the low-voltage side of the bidirectional DC/DC converter 4, the high-voltage side of the bidirectional DC/DC converter 4 is connected to the DC side of the three-phase voltage type PWM rectifier 3, and the AC side of the three-phase voltage type PWM rectifier 3 The side is connected in parallel with the grid system 1. In this device, the three-phase voltage-type PWM rectifier 3 can realize the decoupling control of active power and reactive power in the dq rotating coordinate system; the bidirectional DC/DC converter 4 adopts the BUCK-BOOST bidirectional DC/DC converter structure , Changing the duty ratio of the switching tube can precisely control the voltage and current at both ends. The effects and benefits of the utility model are: the device adopts a simple equipment structure to simultaneously realize active and reactive power, harmonic compensation and low voltage ride-through; the supercapacitor 5 has a long service life and reduces operating costs.

Description

Power quality control device based on supercapacitor energy storage

technical field

The utility model belongs to the technical field of power quality adjustment. It involves a new type of power quality adjustment device with supercapacitors connected to the power grid system in parallel to adjust the power quality of the power grid system, especially the three-phase voltage PWM rectifier, bidirectional DC/DC converter, Combination of supercapacitors, three-phase voltage-type PWM rectifier, bidirectional DC/DC converter and their control design when the grid system is in normal operation or when the grid system is faulty.

Background technique

With the rapid development of the national economy and science and technology, various power electronic devices are widely used, which will inevitably increase the demand for electric energy in all walks of life, and the power sector and users will pay more and more attention to power quality.

Resistive and inductive loads occupy a large proportion of industrial and domestic electricity consumption. Resistive and inductive loads must absorb reactive power to work normally, which is determined by their own nature. Asynchronous motors, transformers, and various phase-controlled devices in power systems, such as phase-controlled rectifiers, phase-controlled AC power adjustment circuits, and cycloconverters, all consume a large amount of reactive power during work. At the same time, in the power system, when the load has nonlinear characteristics, the current will contain harmonics, and the waveform will be distorted to become a non-sinusoidal wave containing harmonics. The main harmonic sources that cause harmonics in power systems are: nonlinear excitation of power transformers; harmonics caused by rotating motors; waveform distortion caused by electric arc furnaces; harmonics caused by electrified railways; harmonics generated by various power electronic devices.

The profit and loss of reactive power in the power system and the existence of harmonic currents bring harm to the power system as follows: it will increase the loss of power equipment and lines; it will cause severe fluctuations in the grid voltage and seriously affect the quality of power supply; Harmonics affect the normal operation of various electrical equipment and reduce the efficiency of power generation, transmission and power consumption equipment; Cause relay protection and automatic device malfunction. In recent years, with the continuous development of renewable energy technology, especially wind power technology, the installed capacity and power generation of wind power have increased significantly. Grid-connected operation of wind farms is an effective way to realize large-scale utilization of wind energy, but the output power of wind farms is uncontrollable and unpredictable. When the capacity of wind turbines accepted in the power system exceeds a certain proportion, the fluctuating power output by wind power will bring a series of technical problems to the grid-connected system in terms of power quality such as reactive power of the power system, harmonics and flicker of the power grid. It can be seen that the traditional static var compensator is more and more important in the safe and stable operation of the modern power system.

The biggest difference between grid-connected wind turbines and other grid-connected power generation equipment is that they cannot maintain the voltage and frequency of the grid during grid faults, which is very detrimental to the stability of the power system. When wind power accounts for a large proportion of the power grid, if the wind turbines still adopt passive protective disconnection when the voltage drops, it will increase the difficulty of fault recovery of the entire system, and may even aggravate the fault, eventually leading to the disconnection of all other units in the system. For this reason, the power grid companies of various countries have put forward strict technical requirements for the grid connection of wind farms and wind turbines based on their own actual conditions, including low voltage ride-through capability, which means that when the voltage at the grid connection point of the wind turbine drops, the wind turbine can remain connected to the grid, and even provide power to the grid. A certain amount of reactive power supports the recovery of the power grid until the power grid returns to normal, thereby "crossing" this low-voltage time (region). LVRT requirements vary from country to country. For example, the low-voltage ride-through capability of wind turbines stipulated by the American Wind Energy Association: before the voltage drops, the voltage at the grid-connected point of the wind farm is maintained at the rated level. When a short-circuit fault occurs in the power grid at 0s, the voltage drops. When the wind farm is not lower than 15% of the rated voltage, the wind farm must maintain grid-connected operation within the time range of 625ms; in addition, when the grid-connected point voltage of the wind farm returns to the rated voltage within 3s of the grid failure When more than 90% of the wind farm is connected to the grid during this process, the wind farm must maintain grid-connected operation.

The traditional parallel STATCOM relies on feedback control to maintain the voltage stability of its DC side, and at the same time realizes the throughput of reactive power on the AC side, which can meet the requirements of harmonic and reactive power dynamic compensation when the power system is in normal operation. When a short-circuit fault occurs in the wind power grid-connected system, the grid voltage drops, and practice shows that the voltage loop control fails at this time, resulting in STATCOM being unable to generate a large amount of reactive power to support the grid voltage. To ensure the normal operation of STATCOM when the power grid fails, it must be provided with a stable DC side voltage, which can be maintained by adding an additional set of power supply devices that have nothing to do with the grid voltage. It will cause redundancy and idleness of equipment.

In order to solve the above problems, the utility model adds a supercapacitor and its control system to the DC side of the traditional STATCOM device. Through the switching of the control mode, the device can meet the different requirements for reactive power during normal operation and fault operation of the power grid, improve the power quality, and effectively reduce the investment cost while improving the safety and stability of the power grid operation.

Contents of the invention

The technical problem to be solved by the utility model is: to overcome the shortcomings of the traditional STATCOM device, and to design a power quality adjustment device with an energy storage device, so that it can meet the harmonic and reactive power compensation requirements while the power grid is in normal operation. It can also quickly generate a large amount of reactive power when a short-circuit fault occurs in the grid, effectively support the grid voltage, improve the voltage quality, and enhance the safety and stability of the grid system operation. When the power grid is faulty, the device needs to be able to meet the reactive power demand of the power grid system without the help of an external power source; when the power grid is running normally, the device needs to be able to supplement the capacity of the supercapacitor, eliminating the need for additional charging devices.

The technical solution of the utility model is: a power quality adjustment device based on supercapacitor energy storage, characterized in that: the device includes a supercapacitor 5, a bidirectional DC/DC converter 4, a three-phase voltage type PWM rectifier 3; wherein the supercapacitor 5 is connected to the low-voltage side of the bidirectional DC/DC converter 4, the high-voltage side of the bidirectional DC/DC converter 4 is connected to the DC side of the three-phase voltage type PWM rectifier 3, and the AC side of the three-phase voltage type PWM rectifier 3 is connected in parallel Grid system 1.

The power quality adjustment device is characterized in that: the three-phase voltage type PWM rectifier 3 is composed of the following structure: the drains of the power switch tubes G3, G4, G5 are connected together to the positive pole of the DC side capacitor C2, and the power switch tubes G3, G4, The source of G5 is respectively connected to the drains of the power switch tubes G6, G7 and G8, and the sources of the power switch tubes G6, G7 and G8 are connected together to the negative terminal of the DC side capacitor C2. The freewheeling diodes D3, D4, D5, D6, D7, and D8 are connected in antiparallel between the drain and the source of each power switch tube respectively. Under the DQ rotating coordinate system, the rectifier can realize the precise control of the decoupling of active power (DC side voltage) and reactive power in parallel, and can flexibly control the three-phase bridge arm switching tubes by changing the conduction sequence and time. The phase-voltage PWM rectifier 3 works in different modes (rectification mode, inverter mode).

The power quality adjustment device is characterized in that: bidirectional DC/DC converter 4 is composed of energy storage inductor L1, power switch tubes G1, G2, freewheeling diodes D1, D2 and output filter capacitor C1, and the positive stage of the low-voltage side power supply passes through the energy storage The inductor L1 is connected to the drain of the power switch G1 and the source of the power switch G2, the source of the power switch G1 is connected to the negative stage of the converter, and the drain of the power switch G2 is connected to the positive stage of the high-voltage side power supply , the filter capacitor C1 is connected in parallel to both ends of the high voltage side, and the freewheeling diodes D1 and D2 are connected in antiparallel between the drain and source stages of the power switch tubes G1 and G2. By controlling the conduction time of the two switch tubes in the bidirectional DC/DC converter 4 , the voltage and current at the high voltage side and the low voltage side of the converter can be precisely controlled.

Since the supercapacitor 5 has the characteristics of long cycle life, low failure rate and fast response speed, the operation and maintenance costs are greatly reduced.

The operation control strategy analysis of this device is as follows:

(1) When the grid system is running normally

The three-phase voltage-type PWM rectifier can realize decoupling control of active power and reactive power in parallel under DQ rotation coordinates. When the grid system is running normally, the three-phase voltage PWM rectifier operates in the rectification state to maintain the stability of its DC side voltage, and at the same time control the AC side to exchange reactive power with the grid system in real time according to the demand: when the power factor of the grid system does not meet the requirements When the three-phase voltage type PWM rectifier can be controlled to send corresponding reactive power to maintain a constant grid power factor; when the harmonic current of the grid system does not meet the requirements, the three-phase voltage type PWM rectifier can be controlled to send out corresponding compensation current to offset original harmonic current. Since the three-phase voltage-type PWM rectifier can control the stability of its DC side voltage, at this time, the supercapacitor can be charged and controlled through a bidirectional DC/DC converter, and the method of constant current and then constant voltage is generally adopted. When the supercapacitor is fully charged, the supercapacitor can be float-charged by controlling the voltage on the low voltage side of the bidirectional DC/DC converter to be constant. When the power grid system is operating normally, it is necessary to keep the supercapacitor fully charged at all times to meet the response requirements in case of sudden short-circuit faults in the power grid.

(2) When a short-circuit fault occurs in the grid system

When a short-circuit fault occurs in the grid system, the grid voltage will drop sharply, which will cause the three-phase voltage PWM rectifier to fail to maintain a constant DC side voltage, and the previous control loop will fail. At this time, the supercapacitor is discharged, and the bidirectional DC/DC converter works in a constant voltage output state that maintains a constant high voltage side voltage, that is, maintains the stability of the DC side voltage of the three-phase voltage type PWM rectifier. The three-phase voltage-type PWM rectifier turns to run in the inverter state. According to the real-time voltage situation of the power grid, it generates reactive power to support the PCC point voltage, improves the voltage quality, and maintains the shortest 625ms to realize low-voltage ride-through.

The supercapacitor in this device generally adopts an electric double layer capacitor. In the process of use, a plurality of single supercapacitors are generally connected in series and parallel to form a supercapacitor bank to increase the capacity and withstand voltage level of the supercapacitor, so that its performance can meet the actual use requirements.

Effect and benefit of the present utility model are:

(1) The power quality adjustment device can exchange reactive power with the grid system according to the operation of the grid system, achieve the purpose of reactive power and harmonic compensation, and comprehensively improve the power quality of the power system.

(2) This power quality adjustment device can maintain the stability of the voltage on the DC side of the three-phase voltage-type PWM rectifier when a short-circuit fault occurs in the grid system, so that it can generate a large amount of reactive power to support the grid voltage, improve voltage quality, and increase wind power. The failure operation capability of the grid-connected system enhances its safety and stability.

(3) In this power quality adjustment device, the energy storage device on the DC side uses a supercapacitor, and is equipped with a corresponding bidirectional DC/DC converter as its charge and discharge controller. Among them, the bidirectional DC/DC converter is not only used to control the stability of the DC voltage of the three-phase voltage PWM rectifier when the power grid is faulty, but also can be used to control the charging of the supercapacitor and supplement the energy of the supercapacitor when the power grid is in normal operation. Therefore, once the supercapacitor is installed, it will be controlled by the device to charge and discharge energy in a timely manner. Since its energy is ultimately sourced from the grid, no additional charging device is required.

(4) Due to the advantages of high power density, fast power response speed and long cycle life, supercapacitors can effectively reduce operation and maintenance costs.

(5) This power quality adjustment device integrates the functions of harmonic compensation, reactive power compensation, and low voltage ride-through. While meeting the needs of the power grid system for power quality adjustment devices, it effectively reduces equipment redundancy and idle.

Description of drawings

Figure 1 is a structural diagram of a power quality adjustment device based on supercapacitor energy storage connected to the power grid.

In the figure: 1 grid system; 2 load; 3 three-phase voltage type PWM rectifier; 4 bidirectional DC/DC converter; 5 supercapacitor.

Fig. 2 is a control block diagram of a three-phase voltage-type PWM rectifier when the power quality adjustment device based on supercapacitor energy storage is in the normal operation state of the power grid.

Fig. 3 is a control block diagram of a three-phase voltage-type PWM rectifier when a power quality adjustment device based on supercapacitor energy storage is in a grid fault state.

Fig. 4 is a structural schematic diagram of a bidirectional DC/DC converter in a power quality adjustment device based on supercapacitor energy storage.

Fig. 5 is a structural schematic diagram of a three-phase voltage-type PWM rectifier in a power quality adjustment device based on supercapacitor energy storage.

Detailed ways

The specific embodiment of the utility model is described in detail below in conjunction with technical scheme and accompanying drawing.

As shown in FIG. 1 , the power quality adjustment device based on supercapacitor energy storage of the present invention is connected to the power grid system 1 through parallel connection. The composition of the device includes: a three-phase voltage type PWM rectifier 3 , a bidirectional DC/DC converter 4 , and a supercapacitor 5 . The supercapacitor 5 is connected to the low-voltage side of the bidirectional DC/DC converter 4, the high-voltage side of the bidirectional DC/DC converter 4 is connected to the DC side of the three-phase voltage type PWM rectifier 3, and the AC side of the three-phase voltage type PWM rectifier 3 is connected to Connected to the power grid system 1 in parallel.

When the power grid system 1 is in normal operation, the three-phase voltage-type PWM rectifier 3 operates in the rectification state. According to the analysis of its mathematical model, it can be known that in the dq rotating coordinate system (rotating at a synchronous speed), its active power and reactive power can be decoupled parallel control. Among them, the voltage (active power) control loop is used to control the stability of the DC side voltage, and the reactive power control loop is used to generate corresponding reactive power and compensation current according to the reactive power and harmonic compensation requirements of the grid system 1 . When the grid voltage is oriented along the q-axis, the q-axis component of the grid current represents the active component, and the d-axis component represents the reactive component. The q-axis component of the grid current is used to adjust the stability of the DC side voltage, and its target value is obtained through the PI link after the difference between the measured value of the DC side voltage and the target value of the DC side voltage. The d-axis component of the grid current is used to regulate the generation of reactive power and harmonic current, so its target value is obtained through the theory of instantaneous reactive power according to the requirements of reactive power and harmonic compensation. After obtaining the target values of the current d and q axis components, the variable can adopt the classic decoupling control algorithm of the three-phase voltage type PWM rectifier 3 to obtain the control signal, and use the control signal to drive the three bridge arms of the three-phase voltage type PWM rectifier 3 The switching tube can realize precise control of the DC side voltage, AC side reactive power, and harmonic current. At this time, the control block diagram of the three-phase voltage-type PWM rectifier 3 is shown in FIG. 2 . Since the three-phase voltage type PWM rectifier 3 maintains the stability of the DC side voltage, the three-phase voltage type PWM rectifier 3 is equivalent to a constant voltage source compared to the bidirectional DC/DC converter 4, and can automatically extract active power from the power grid system 1 according to demand power. At this time, the bidirectional DC/DC converter 4 can precisely control the low-voltage side current and the low-voltage side voltage by adding the closed-loop control of the PI link, so as to realize the charging control of the supercapacitor 5 with a constant current first and then a constant voltage.

When a short-circuit fault occurs in the grid system 1, the voltage control loop of the three-phase voltage-type PWM rectifier 3 cannot maintain the stability of its DC side voltage due to the sudden drop of the grid voltage, resulting in the failure of the reactive loop control. At this time, the bidirectional DC/DC converter 4 should be controlled to operate in the boost state, and the output voltage of the high voltage side should be maintained constant through the closed-loop control of the high voltage side voltage, that is, the constant voltage of the DC side of the three-phase voltage type PWM rectifier 3 is ensured. . At this time, the three-phase voltage-type PWM rectifier 3 is controlled to run in the inverter state, and dq decoupling control under synchronous speed rotation is also adopted. Among them, the target value of the current q-axis component (active component) is set to 0, and the target value of the current d-axis component (reactive component) is set according to the demand for reactive power due to the grid voltage drop. After setting the current d , the target value of the q component, use the classic decoupling control algorithm of the three-phase voltage PWM rectifier 3 to obtain the control signal, and use the control signal to drive the three bridge arm switch tubes of the three-phase voltage PWM rectifier 3 to realize a large number of wireless The output of power plays a role in supporting the grid voltage and improves the voltage quality. At this time, the control block diagram of the three-phase voltage type PWM rectifier 3 is shown in Figure 3

In the utility model, the bidirectional DC/DC converter 4 respectively plays the role of the supercapacitor 5 charging controller and maintaining the voltage stability of the DC side of the three-phase voltage type PWM rectifier 3 when the grid system 1 is in normal operation and when a short circuit fault occurs. These two working states require the converter to be able to control the bidirectional flow of energy. The bidirectional DC/DC converter 4 in the utility model adopts a BUCK-BOOST type bidirectional DC/DC converter structure, as shown in Figure 4: bidirectional The DC/DC converter 4 is composed of an energy storage inductor L1, power switch tubes G1, G2, freewheeling diodes D1, D2 and an output filter capacitor C1. The positive end of the low-voltage side power supply is connected to the drain of the power switch G1 and the source of the power switch G2 through the energy storage inductor L1, the source of the power switch G1 is connected to the negative stage of the converter, and the drain of the power switch G2 Connect to the positive end of the high-voltage side power supply, filter capacitor C1 is connected in parallel to both ends of the high-voltage side, and the freewheeling diodes D1 and D2 are anti-parallel connected between the drain and source stages of the power switch tubes G1 and G2. Through the on-off control of the power switch tubes G1 and G2, the bidirectional flow of energy from the low-voltage side to the high-voltage side or from the high-voltage side to the low-voltage side can be realized. Now the working mode of the converter is analyzed as follows (assuming that the high-voltage side and The low voltage side is connected to the energy source):

(1) Mode 1, the power switch tube G1 is turned on, the power switch tube G2 is turned off, the low-voltage side power supply charges the energy storage inductor L1 through G1, and the inductor current increases.

(2) Mode 2, the power switch tube G1 is turned off, and the power switch tube G2 is turned on. At this time, because the current of the inductor L1 cannot change suddenly, the current still flows from the low voltage side to the high voltage side through the freewheeling diode D2. At this time, from the low voltage The current from the side to the high voltage side gradually decreases.

(3) Mode 3. At this time, the power switch tube G1 is still turned off, and the power switch tube G2 is turned on. When the current in the inductor from the low-voltage side to the high-voltage side gradually decreases to zero, the high-voltage side power supply passes through the power switch G2 reversely charges the inductor L1, and the reverse current flowing through the inductor L1 gradually increases.

(4) Mode 4, the power switch tube G1 is turned on, and the power switch tube G2 is turned off. At this time, because the current of the inductor L1 cannot change suddenly, the reverse current in the inductor L1 continues to flow in the reverse direction along the freewheeling diode D1. At this time The reverse current in the inductor L1 decreases gradually, and when the reverse current in the inductor L1 decreases to 0, the mode 1 is re-entered, thereby starting the next control cycle.

From the above analysis, it can be seen that in a cycle, by controlling the on-off of the power switch tubes G1 and G2, the bidirectional flow of energy between the low-voltage side and the high-voltage side can be realized. When the energy is greater than the energy flowing from the high-voltage side to the low-voltage side, the converter operates in the step-up chopping state, otherwise the converter operates in the step-down chopping state. The amount of energy flowing in two directions in each cycle depends on the on-off time of the power switch tubes G1 and G2, and the energy of the low-voltage side power supply and the high-voltage side power supply. By adding closed-loop control to the converter, not only can the flow direction of energy be controlled, but also the voltage and current of the low-voltage side and the voltage and current of the high-voltage side can be precisely controlled, which lays the foundation for the use of the converter in the utility model technical foundation.

The structure of the three-phase voltage type PWM rectifier 3 is (as shown in Figure 5): the drains of the power switch tubes G3, G4, and G5 are connected together to the positive pole of the DC side capacitor C2, and the drains of the power switch tubes G3, G4, and G5 are connected together. The sources are respectively connected to the drains of the power switch tubes G6, G7, G8, and the sources of the power switch tubes G6, G7, G8 are connected together to the negative pole of the DC side capacitor C2. The freewheeling diodes D3, D4, D5, D6, D7, and D8 are connected in antiparallel between the drain and the source of each power switch tube respectively. L2, L3, and L4 are filter inductors on the AC side, and the filtered output is connected to the three-phase AC grid system 1 . The flexible switching between the rectification state and the inversion state of the converter and the precise control of the voltage on the DC side and the reactive power on the AC side can be realized by on-off control of the above switching tubes.

The power switch tube of the above-mentioned bidirectional DC/DC converter 4 and three-phase voltage type PWM rectifier 3 can be an insulated gate bipolar transistor (IGBT). characteristics such as reduction. In the actual use process, IPM power module can be used. On the basis of integrating IGBT and its anti-parallel diode, IPM also has drive, logic, control, detection and protection circuits, which not only reduces the size of the device, but also shortens the development time. , It also enhances the reliability of the device and adapts to the development direction of today's power devices.

Claims (3)

1. A power quality regulator based on supercapacitor energy storage, characterized in that: the device comprises supercapacitor (5), bidirectional DC/DC converter (4), three-phase voltage type PWM rectifier (3); wherein, The supercapacitor (5) is connected to the low-voltage side of the bidirectional DC/DC converter (4), and the high-voltage side of the bidirectional DC/DC converter (4) is connected to the DC side of the three-phase voltage PWM rectifier (3), and the three-phase voltage The AC side of the type PWM rectifier (3) is connected to the grid system (1) in parallel. 2. A kind of electric energy quality adjustment device based on supercapacitor energy storage according to claim 1, is characterized in that: three-phase voltage type PWM rectifier (3) composition structure is: the drain electrode of power switch tube G3, G4, G5 Connected to the positive pole of the DC side capacitor C2, the sources of the power switch tubes G3, G4, and G5 are respectively connected to the drains of the power switch tubes G6, G7, and G8, and the sources of the power switch tubes G6, G7, and G8 Connected together to the negative pole of the DC side capacitor C2, freewheeling diodes D3, D4, D5, D6, D7, D8 are connected in anti-parallel between the drain and source of each power switch tube. 3. A kind of power quality regulating device based on supercapacitor energy storage according to claim 1, characterized in that: the bidirectional DC/DC converter (4) consists of energy storage inductance L1, power switch tubes G1, G2, freewheeling Composed of diodes D1, D2 and output filter capacitor C1, the positive stage of the low-voltage side power supply is connected to the drain stage of the power switch G1 and the source stage of the power switch G2 through the energy storage inductor L1, and the source of the power switch G1 is connected to the converter The negative stage of the power switch tube G2 is connected to the positive stage of the high-voltage side power supply, the filter capacitor C1 is connected in parallel to both ends of the high-voltage side, and the freewheeling diodes D1 and D2 are anti-parallel connected to the drain-source of the power switch tube G1 and G2 between levels.

Images