Control system for static VAR generator in asymmetrical and transient power grid

Abstract

A control system for asymmetric, transient static VAR generators (SVRs) in power grids includes a control system body and a main control unit. The main control unit includes subunits for constant voltage, transient control, reactive power control, reactive power-consistent voltage control, harmonic control, negative-sequence voltage control, and remote control. The present invention processes the asymmetric and transient voltage signals sampled by the control system body, filters out harmonic components, and extracts three-phase positive-sequence voltage signals as a reference signal. The control system body then samples reactive current, negative-sequence voltage, and voltage over-limit based on the positive-sequence signals. Furthermore, the control system controls the SVG power devices to emit trigger pulses based on different control strategy modules of the SVG power compensation device, thereby improving control efficiency and increasing the signal processing speed of the entire control system body. This ensures that the control system body controls the SVG dynamic VAR compensation device at a high compensation rate, thereby achieving good dynamic compensation results during power grid transients.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power application, in particular to a control system for an asymmetrical, instantaneous power grid static reactive generator. Background Art

[0002] In power supply systems, asymmetric loads (asymmetric loads refer to loads with initial phase or amplitude differences, or both, in three-phase AC power systems) can cause three-phase asymmetry in grid voltage, leading to unstable grid operation, increased equipment losses, reduced efficiency and lifespan of power-consuming equipment, and even potential safety hazards. For example, single-phase electrified railways and AC welding equipment, without power compensation devices, can generate significant negative-sequence currents. These negative-sequence currents can cause malfunctions in many protective and automatic systems that rely on negative-sequence currents as triggering elements. Furthermore, because the phase sequence of negative-sequence and positive-sequence currents is opposite, the additional electromotive force generated by rotating motors can cause vibration and additional losses. Furthermore, negative-sequence currents can also affect the power output capacity of generating equipment. Furthermore, when transmitting power over long distances from hydropower, thermal power, or offshore wind power via long cables, the reactance and resistance of the transmission lines can lead to reactive power losses and voltage drops, compromising transmission efficiency and stability. Furthermore, the transient no-load operation of large-capacity transformers can cause transient grid fluctuations. All of the above require dynamic reactive power compensation via power compensation devices.

[0003] In existing technology, power compensation devices typically perform dynamic reactive power compensation using SVCs (static VAR compensators, a device used for dynamic reactive power compensation in power systems, improving grid voltage stability and power quality by rapidly regulating reactive power) or SVGs (static VAR generators, also used for dynamic reactive power compensation in power systems. These devices utilize power semiconductor bridge converters for dynamic reactive power compensation, capable of emitting or absorbing reactive power, and their output can be varied to control specific parameters in the power system). Compared to SVCs, SVGs offer advantages in dynamic reactive power compensation, such as faster response, reduced harmonic content, stronger reactive power regulation capabilities, safe operation, a smaller footprint, and excellent low-voltage characteristics, leading to their widespread application. However, due to technical limitations, existing SVG dynamic reactive power compensation equipment suffers from inaccurate sampling of the reference DC signal voltage collected by the sensor in its control system when the three-phase grid voltage is asymmetrical, affecting the compensation effect on the power supply line. Furthermore, for the SVG, the imbalance in the three-phase voltage can cause one phase to be unable to input the control system's signal terminal due to excessively large sampled DC signal voltage. In severe cases, this can lead to control system instability and adversely affect the compensation effect on the power supply line. Summary of the Invention

[0004] In order to overcome the drawbacks of existing SVG-based power compensation devices due to technical limitations as described in the background, the present invention provides a control system for use in conjunction with the SVG power compensation device, processing the front-stage signal of the control system body, and sampling the reactive current, negative sequence voltage, voltage over-limit, etc. based on the positive sequence signal. The different control strategy modules of the SVG power compensation device are used to control the SVG power devices to emit trigger pulses, thereby improving control efficiency and ensuring the speed at which the control system body controls the compensation of the SVG dynamic reactive compensation equipment, so as to achieve good dynamic compensation effects during the transient state of the power grid, thereby achieving a control system for asymmetric and transient power grid static reactive generators.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] The control system for asymmetric and transient power grid static VAR generator includes a control system body, a control signal output end of the control system body and a control signal input end of an SVG power compensation device are connected via a wire, a sensor of the control system body and a three-phase power line are connected via a wire, a control strategy of the basic control software unit of the control system body itself is defined as a bottom-level control algorithm based on the constant current of the SVG power compensation device to ensure that the voltage source converter module of the SVG power compensation device can work normally, and an auxiliary control unit of the control system body itself includes a power grid overvoltage control subunit and a power grid undervoltage control subunit; the characteristic is that when the control system body is working, the sensor samples the power grid current. The voltage signal is processed by passive filtering, and the positive sequence three-phase balanced voltage is extracted as the basic voltage for the control system body to control the operation of the SVG power compensation device; the control system body also has a main control unit, which includes a constant voltage control subunit, a transient control subunit, a reactive power control subunit, a reactive power and voltage control subunit, a harmonic control subunit, a negative sequence voltage control subunit, and a remote control subunit; when the constant voltage control subunit is working, it controls the working mode of the voltage regulator based on the combination of voltage deviation and current adjustment rate, and the set voltage deviation is determined by its own reference voltage and the voltage feedback of the controlled power line collected by the sensor; when the transient control subunit is working, When the grid fault signal collected by the sensor causes the voltage at the access point to drop sharply but not less than 0.2pu, the SVG power compensation device directly outputs the rated capacitive current to support the grid voltage. When the voltage recovers to 0.8pu, the SVG power compensation device resumes the normal steady-state control mode. When the reactive power control subunit is working, the control system body detects the load current, calculates the load reactive current, and then makes a difference between the target reactive current set by the control system body and the load reactive current and takes a negative value, that is, the reactive current given value of the inner loop control of the SVG power compensation device itself is obtained. When the reactive power and voltage control subunit is working, when the voltage at the sensor detection point is within the allowable range, the SVG power compensation device resumes the normal steady-state control mode. The device compensates for reactive power at the detection point. Once the voltage at the detection point exceeds the limit, the SVG power compensation device reduces its own reactive power characteristics to control the voltage within the allowable range. When the harmonic control subunit is operating, the SVG power compensation device suppresses load harmonics on the basis of reactive power compensation, and only requires the SVG power compensation device to output a constant harmonic current. The negative-sequence voltage control subunit compensates for voltage imbalance in the substation by delta-connecting the SVG power compensation device. The remote control subunit enables the SVG power compensation device to receive external control instructions, change the SVG control target, and realize automatic scheduling of reactive power or voltage, so as to achieve optimal reactive power distribution for the entire system.

[0007] Furthermore, in the constant voltage control subunit, the voltage regulation rate is achieved by using a current feedback method.

[0008] Furthermore, in the transient control subunit, when the grid voltage is higher than 1.15 pu, the SVG power compensation device directly outputs the rated inductive current to reduce the grid voltage. When the grid voltage returns to normal, the SVG power compensation device returns to the conventional steady-state control mode.

[0009] Furthermore, in the reactive power control subunit, when the SVG power compensation device adopts power factor control, the control system body calculates the current active power of the SVG power compensation device and calculates the required reactive power according to the target power factor. The reactive power is used as the reactive power control target, and subsequent control is consistent with the reactive power control.

[0010] Furthermore, the harmonic control subunit can simultaneously achieve the dual goals of compensating reactive current and harmonic current.

[0011] Furthermore, in the negative-sequence voltage control subunit, its working process is: collecting the grid line voltage, calculating its negative-sequence voltage, subtracting the negative-sequence voltage from 0, and the deviation passes through the PI regulator as the negative-sequence reactive current instruction within the SVG power compensation device.

[0012] Furthermore, the remote control subunit includes the following working modes: reactive power control of the SVG power compensation device body, voltage control of the SVG power compensation device access point, and reactive power control of the SVG power compensation device access point. According to the above control working modes, a trigger pulse of the SVG power compensation device body is issued to control the conduction of the IGBT of the SVG power compensation device body, thereby realizing the reactive output corresponding to the control strategy to the load.

[0013] Compared with the prior art, the present invention has the following advantages: it works in conjunction with the SVG power compensation device to process the pre-stage signal of the control system. Specifically, the asymmetric and transient voltage signals sampled by the control system are processed, harmonic components are filtered out, and the three-phase positive-sequence voltage signal is extracted as a reference signal. The control system then samples reactive current, negative-sequence voltage, and voltage over-limit based on this positive-sequence signal. Furthermore, the SVG power compensation device's different control strategy modules are used to control the SVG's power devices to emit trigger pulses, thereby improving control efficiency. Furthermore, the signal processing speed of the entire control system is increased, ensuring the speed at which the control system controls the SVG dynamic reactive compensation device, enabling good dynamic compensation during grid transients and preventing inaccurate reference voltages from affecting the compensation effect of the SVG dynamic reactive compensation device. Based on the foregoing, the present invention has promising application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0015] Figure 1 It is a schematic block diagram of the application of the constant voltage control subunit of the present invention.

[0016] Figure 2 Schematic diagram of the U-I characteristic curve of the constant voltage controlled SVG of the present invention.

[0017] Figure 3 Schematic diagram of the transient control characteristic curve of SVG in the present invention.

[0018] Figure 4 Schematic diagram of the output waveform of the SVG during transient control of the present invention.

[0019] Figure 5 A schematic block diagram of the application of the reactive power control subunit of the present invention.

[0020] Figure 6 Schematic block diagram of the application of the harmonic control subunit of the present invention.

[0021] Figure 7 Schematic block diagram of the application of the negative sequence voltage control subunit of the present invention. DETAILED DESCRIPTION

[0022] A control system for an asymmetric, transient static VAR generator (SVP) in an asymmetric, transient power grid includes a control system body, wherein a control signal output terminal of the control system body and a control signal input terminal of an SVG power compensation device are connected via a wire, and sensors of the control system body and three-phase power lines are connected via a wire. The control strategy of the basic control software unit of the control system body is defined as a low-level control algorithm based on the constant current of the SVG power compensation device to ensure the normal operation of the voltage source converter module of the SVG power compensation device. The auxiliary control unit of the control system body includes a power grid overvoltage control subunit and a power grid undervoltage control subunit. When the control system body is in operation, the sensor samples the power grid voltage signal, performs passive filtering on the signal, and extracts the positive sequence three-phase balanced voltage as the basic voltage for the control system body to control the operation of the SVG power compensation device. The control system body also includes a main control unit, which includes a constant voltage control subunit, a transient control subunit, a reactive power control subunit, a reactive power-consistent voltage control subunit, a harmonic control subunit, a negative sequence voltage control subunit, and a remote control subunit. In the application of constant voltage control subunits, SVG power compensation devices generally use constant voltage control subunits as a strategy to stabilize the voltage of the power supply system. Specifically, the control system body uses a voltage regulator based on a combination of voltage deviation and current regulation rate as a device for regulating the output voltage and current. The voltage deviation is determined by the reference voltage set by the constant voltage control subunit itself and the controlled voltage feedback output to the power load. The regulation rate is achieved by current feedback. Its block diagram is shown as follows: Figure 1The voltage regulator operates as follows: it compares the control variable measured by the sensor with its own reference voltage value to obtain a voltage deviation. It then multiplies the voltage deviation by the calibration value of the feedback current from the SVG power compensation device and the adjustment rate. The corrected deviation passes through the SVG power compensation device's PI controller and serves as the setpoint for the SVG power compensation device's inner loop reactive current control, thereby achieving system voltage stabilization. The U-I characteristic curve of the constant voltage control SVG is shown in Figure 1. Figure 2 shown.

[0023] In the transient control subunit application, when a grid fault causes the voltage at the power line access point to drop sharply but not below 0.2 pu, the SVG power compensation device directly outputs its rated capacitive current to support the grid voltage. When the voltage returns to 0.8 pu, the SVG power compensation device resumes its steady-state control strategy. Similarly, when the grid voltage exceeds 1.15 pu, the SVG power compensation device directly outputs its rated inductive current to reduce the grid voltage. When it returns to normal, the SVG power compensation device resumes its steady-state control strategy. Specifically, due to the SVG power compensation device's extremely fast response speed, when the power supply system experiences a voltage sag or swell, the SVG power compensation device can quickly support (or suppress) the power supply system voltage, improving the transient stability of the power supply system. Figure 3 is the SVG transient control characteristic curve; Figure 4 Output waveform for SVG transient control.

[0024] In the reactive power control (power factor control) subunit application, the SVG power compensation device's own control subsystem detects the load current (usually obtained by subtracting the grid current from the SVG power compensation device's output current), calculates the reactive current of the load, and takes the negative of the target reactive current and the subtraction from the load reactive current to obtain the reactive current setpoint for the SVG power compensation device's own inner loop control. The control block diagram is shown below. Figure 5 When the SVG power compensation device uses power factor control, its control subsystem calculates the current power supply system's active power data and, based on its own set target power factor, the required load reactive power data. This reactive data serves as the load reactive control target, and subsequent control is consistent with load reactive control. In the reactive power and voltage control subunit application, when the voltage at a transmission line test point is within the allowable range, the SVG power compensation device compensates for the reactive power data at that test point. If the voltage at that test point exceeds the limit, the SVG power compensation device reduces some of its reactive power characteristics to control the voltage within the allowable range.

[0025] In the application of harmonic control subunit, the working principle of the active filter medium voltage SVG power compensation device using direct current control is as follows: Figure 6 As shown. From the figure, we can get formula (1), that is, the power supply current is the load current and compensation current The phasor sum; assuming the load current Contains fundamental positive sequence current (including fundamental positive sequence reactive current and fundamental positive sequence active current ), fundamental negative sequence current and harmonic currents As shown in formula (2).

[0026]

[0027] To make the power supply current If the fundamental positive sequence reactive power and fundamental negative sequence current are not contained in the current, it is necessary to control the output current of the SVG power compensation device. Satisfy equation (3); thus, the power supply current only contains fundamental positive sequence active power and harmonic current, as shown in equation (4).

[0028]

[0029] Therefore, in order to achieve the compensation purpose, the key is to control the output current of the SVG power compensation device Satisfies equation (3). From the description of the working principle of the SVG power compensation device, it can be seen that if the SVG power compensation device is to suppress load harmonics on the basis of reactive power compensation, it is only necessary to make the SVG power compensation device output the corresponding harmonic current. Therefore, in this sense, the SVG power compensation device can simultaneously achieve the dual goals of compensating reactive current and harmonic current. Specifically, the input signal of the SVG power compensation device can be power, voltage, current, etc., and the line active power is generally selected as the input signal of the SVG damping controller.

[0030] In the application of negative sequence voltage control subunit, the imbalance of output voltage of some substations can be compensated by using delta connection SVG power compensation device, such as Figure 7 As shown in the figure, assuming that the voltage on the power supply side Us is unbalanced, the SVG power compensation device generates a negative sequence current , forming a negative sequence voltage drop on the line impedance, thereby making the access point voltage reach balance; the specific control block diagram is as follows Figure 1 The process is as follows: collecting the grid line voltage, calculating its negative sequence voltage, and subtracting the negative sequence voltage from 0. This deviation passes through the PI regulator and serves as the in-angle negative sequence reactive current instruction for the SVG power compensation device.

[0031] In the application of remote control subunits, the so-called remote control means that the SVG power compensation device can receive external control instructions, change the control target of the SVG power compensation device, realize automatic scheduling of reactive power or voltage, and optimize the reactive power distribution of the entire power supply system. Specifically, the remote control system controls the SVG power compensation device in different ways depending on the access system, which generally includes the following methods. (1) Reactive power control of the SVG power compensation device itself: that is, the remote control system regards the SVG power compensation device as a reactive source, and the SVG power compensation device controls its own reactive power within the reactive power target given by the remote control system. (2) Voltage control of the SVG power compensation device access point: The SVG power compensation device serves as the main voltage regulation device in the area. The remote control system gives the SVG power compensation device the current target voltage value according to the requirements of the control system itself. The SVG power compensation device controls the voltage of the power supply area near the given target through the voltage control strategy. (3) Reactive power control at the access point of the SVG power compensation device: The SVG power compensation device serves as the main reactive power regulation device in the area. The remote control system sets the reactive power of the area controlled by the SVG power compensation device according to the requirements of the control system itself. The SVG power compensation device controls the reactive power of the area near the given target through the reactive power control strategy. In actual situations, if a voltage surge or sag occurs during remote control, the SVG power compensation device will be separated from remote control and enter the transient control strategy, unless there are special requirements that do not allow the SVG power compensation device to perform transient control. According to the above control strategy,

[0032] Send out a trigger pulse to the SVG power compensation device, control the IGBT of the SVG power compensation device to turn on, and realize the reactive power output corresponding to the control strategy to the load.

[0033] In summary, the present invention cooperates with the SVG power compensation device to process the front-stage signal of the control system body. Specifically, the asymmetric and transient voltage signals sampled by the control system body are processed, the harmonic components therein are filtered out, and the three-phase positive-sequence voltage signal is extracted as a reference signal. The control system body then samples the reactive current, negative-sequence voltage, voltage over-limit, etc. based on this positive-sequence signal. Furthermore, according to different control strategy modules of the SVG power compensation device, the SVG power device is controlled to emit a trigger pulse, thereby improving control efficiency. On the other hand, the speed of signal processing of the entire control system body is improved, and the speed of the control system body controlling the compensation of the SVG dynamic reactive compensation device is ensured, so that a good dynamic compensation effect can be obtained during the transient process of the power grid, and the compensation effect of the SVG dynamic reactive compensation device is prevented from being affected by the inaccurate reference voltage.

[0034] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, from all points of view, the embodiments should be regarded as illustrative and non-restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing description, and it is intended that all changes that come within the meaning and range of equivalents of the claims be included in the present invention. Any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0035] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in the embodiments can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A control system for an asymmetric, transient power grid static VAR generator, comprising a control system body, wherein a control signal output terminal of the control system body and a control signal input terminal of an SVG power compensation device are connected via a wire, and sensors of the control system body and three-phase power lines are connected via a wire. The control strategy of the basic control software unit of the control system body is defined as a bottom-level control algorithm based on maintaining a constant current in the SVG power compensation device to ensure the normal operation of the voltage source converter module of the SVG power compensation device. The auxiliary control unit of the control system body includes a power grid overvoltage control subunit and a power grid undervoltage control subunit; characterized in that: When the control system is working, the sensor samples the grid voltage signal, performs passive filtering on the signal, and extracts the positive sequence three-phase balanced voltage as the basic voltage for the control system to control the SVG power compensation device. The control system also has a main control unit, which includes a constant voltage control subunit, a transient control subunit, a reactive power control subunit, a reactive power and voltage control subunit, a harmonic control subunit, a negative sequence voltage control subunit, and a remote control subunit. When the constant voltage control subunit is working, it controls the working mode of the voltage regulator based on the combination of voltage deviation and current adjustment rate. The set voltage deviation is determined by its own reference voltage and the voltage feedback of the controlled power line collected by the sensor. When the transient control subunit is working, when the grid fault signal collected by the sensor causes the access When the voltage at a point drops suddenly but does not fall below 0.2 pu, the SVG power compensation device directly outputs the rated capacitive current to support the grid voltage. When the voltage recovers to 0.8 pu, the SVG power compensation device resumes conventional steady-state control. When the reactive power control subunit is operating, the control system detects the load current, calculates the load reactive current, and then subtracts the target reactive current set by the control system from the load reactive current, taking the negative value to obtain the reactive current setpoint for the inner loop control of the SVG power compensation device. When the reactive power and voltage control subunit is operating, when the voltage at the sensor detection point is within the allowable range, the SVG power compensation device compensates for the reactive power at the detection point. Once the voltage at the detection point exceeds the limit, the SVG power compensation device reduces its own reactive power characteristics to control the voltage within the allowable range. When the harmonic control subunit is working, the SVG power compensation device suppresses the load harmonics on the basis of reactive power compensation, and only needs the SVG power compensation device to output a constant harmonic current; The negative-sequence voltage control subunit compensates for the voltage imbalance problem in the substation by delta-connecting the SVG power compensation device. The remote control subunit is an SVG power compensation device that can receive external control instructions, change the SVG control target, and realize automatic scheduling of reactive power or voltage, so as to achieve optimal reactive power distribution of the entire system.

2. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: In the constant voltage control subunit, the voltage regulation rate is realized by current feedback method.

3. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: In the transient control subunit, when the grid voltage exceeds 1.15pu, the SVG power compensation device directly outputs the rated inductive current to reduce the grid voltage. When the grid voltage returns to normal, the SVG power compensation device resumes the conventional steady-state control mode.

4. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: In the reactive power control subunit, when the SVG power compensation device adopts power factor control, the control system body calculates the current active power of the SVG power compensation device and calculates the required reactive power based on the target power factor. This reactive power is used as the reactive power control target, and subsequent control is consistent with reactive power control.

5. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: In the harmonic control subunit, the dual goals of compensating reactive current and harmonic current can be achieved simultaneously.

6. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: In the negative-sequence voltage control subunit, its working process is as follows: collecting the grid line voltage, calculating its negative-sequence voltage, and subtracting the negative-sequence voltage from 0. This deviation passes through the PI regulator and serves as the in-angle negative-sequence reactive current instruction of the SVG power compensation device.

7. The control system for an asymmetric, transient power grid static VAR generator according to claim 1, characterized in that: The remote control subunit includes the following working modes: reactive power control of the SVG power compensation device body, voltage control of the SVG power compensation device access point, and reactive power control of the SVG power compensation device access point. According to the above working modes, a trigger pulse of the SVG power compensation device body is issued to control the conduction of the IGBT of the SVG power compensation device body, thereby realizing the reactive power output corresponding to the control strategy to the load.

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