Topology and control method for improving negative sequence compensation capability of star-shaped STATCOM (Static Synchronous Compensator)

Abstract

The invention provides a topology and control method for improving the negative sequence compensation capability of a star-shaped STATCOM, and relates to the technical field of electric energy quality adjustment of an electric power system. The topology comprises a medium-voltage cascade STATCOM, a low-voltage three-phase four-wire converter and an isolation transformer, the primary side of the isolation transformer is in delta connection, and the secondary side of the isolation transformer is in star connection; the primary side of the isolation transformer is connected to a medium-voltage power grid, the secondary side of the isolation transformer is connected with the three-phase input end of the medium-voltage cascade type STATCOM through the three-phase inductor, the three-phase output end of the medium-voltage cascade type STATCOM is connected with the three-phase input end of the low-voltage three-phase four-wire converter, and the output end of the low-voltage three-phase four-wire converter is connected with a neutral point of the secondary side of the isolation transformer. According to the invention, the negative sequence compensation capability and the inter-phase unbalanced power compensation range of the cascaded chain type multi-level grid-connected converter can be improved, the hardware cost is greatly reduced, and the safe and stable operation of a power system is ensured.

Description

Technical Field

[0001] This invention relates to the field of power system power quality regulation technology, specifically to a topology and control method for improving the negative sequence compensation capability of a star-type STATCOM (Static Synchronous Compensator). Background Technology

[0002] With the large-scale grid connection of new energy sources and the widespread application of asymmetrical loads, the negative sequence component in the power grid has increased significantly, leading to problems such as power output fluctuations of grid-connected converters, transformer overheating, and increased motor vibration and noise, which seriously threaten the safe and stable operation of the power system.

[0003] Cascaded multilevel grid-connected converters have become one of the mainstream topologies for medium and high voltage grid-connected applications due to their advantages such as high modularity, low output harmonic content, and ease of expansion to medium and high voltage levels. However, existing cascaded multilevel grid-connected converters have the following key problems in negative sequence compensation applications: 1. The problem of DC side voltage imbalance is prominent: During the negative sequence compensation process, the power undertaken by each phase H-bridge module of the converter is different, which can easily lead to the deviation of DC side capacitor voltage of each module. If it is not controlled, it will exceed the withstand voltage range of power devices, limit the negative sequence compensation capacity, or even cause device damage. 2. It is difficult to balance hardware cost and reliability: Some existing solutions can increase capacity by adding additional reactive power compensation devices to assist negative sequence compensation, but additional isolation transformers and control units are required, which increases the system size and cost. Moreover, the complexity of multi-device collaborative control is high and the reliability is reduced.

[0004] Therefore, there is an urgent need for a cascaded multilevel grid-connected converter topology and supporting control strategy that can effectively suppress DC voltage imbalance and improve negative sequence compensation capacity and response speed without significantly increasing hardware costs. Summary of the Invention

[0005] The purpose of this invention is to provide a topology and control method for improving the negative sequence compensation capability of star-type STATCOM, which can enhance the negative sequence compensation capability and phase-to-phase unbalanced power compensation range of cascaded multilevel grid-connected converters, and greatly reduce hardware costs, thereby ensuring the safe and stable operation of the power system.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a topology for improving the negative sequence compensation capability of a star-connected STATCOM, comprising a medium-voltage cascaded STATCOM, a low-voltage three-phase four-wire converter, and an isolation transformer, wherein the STATCOM is a static synchronous compensator, the primary side of the isolation transformer is delta-connected, and the secondary side is star-connected. The primary side of the isolation transformer is connected to the medium-voltage power grid. The secondary side of the isolation transformer is connected to the three-phase input terminal of the medium-voltage cascaded STATCOM through a three-phase inductor. The three-phase output terminal of the medium-voltage cascaded STATCOM is connected to the three-phase input terminal of the low-voltage three-phase four-wire converter. The output terminal of the low-voltage three-phase four-wire converter is connected to the neutral point of the secondary side of the isolation transformer.

[0007] According to the present invention, a topology for improving the negative sequence compensation capability of a star-type STATCOM is provided. Each phase of the medium-voltage cascaded STATCOM includes N H-bridge power modules cascaded sequentially. Each H-bridge power module includes four switching transistors S0 and an energy storage capacitor C0. Among the four switching transistors S0, two switching transistors S0 form the first bridge arm, and the other two switching transistors S0 form the second bridge arm. The first bridge arm, the second bridge arm, and the energy storage capacitor C0 are connected in parallel. The midpoint of the first bridge arm of the first H-bridge power module of each phase is the input terminal of that phase. The midpoint of the second bridge arm of the Nth H-bridge power module of each phase is the output terminal of that phase. The midpoint of the second bridge arm of the kth H-bridge power module of each phase is connected to the midpoint of the first bridge arm of the (k+1)th H-bridge power module, where 1 ≤ k < N, and k is an integer. According to the present invention, a topology for improving the negative sequence compensation capability of a star-type STATCOM is provided. The low-voltage three-phase four-wire converter includes a three-phase H-bridge inverter circuit, which includes eight switching transistors S1 and a DC-side capacitor C. dc Of the eight switching transistors S1, every two switching transistors S1 form a bridge arm, denoted as the third bridge arm, fourth bridge arm, fifth bridge arm, and sixth bridge arm; the third bridge arm, fourth bridge arm, fifth bridge arm, sixth bridge arm, and DC-side capacitor C dc Parallel connection; the midpoints of the third, fourth, and fifth bridge arms are the three-phase input terminals of the low-voltage three-phase four-wire converter, and the midpoint of the sixth bridge arm is the output terminal of the low-voltage three-phase four-wire converter. Secondly, the present invention provides a control method for a topology based on the first aspect to improve the negative sequence compensation capability of a star-type STATCOM, comprising: Medium-voltage cascaded STATCOMs employ global DC voltage control and phase-to-phase voltage equalization control, obtaining three-phase modulated voltages through global DC voltage control. The offset signal ΔU required for the i-th H-bridge power module in each phase is obtained through phase-to-phase voltage equalization control. ai ΔU bi ΔU ci Then the offset signal ΔU ai ΔU bi ΔU ci With three-phase modulation voltage The sum of 1 / N times is used to generate the drive signal for the switch S0 of the i-th H-bridge power module in each phase, which is then modulated by a phase-shifted carrier sinusoidal pulse. The low-voltage three-phase four-wire converter adopts phase-to-phase voltage equalization control. The low-voltage three-phase four-wire converter provides zero-sequence current to the medium-voltage cascaded STATCOM. The drive signals of the switching transistor S1 of the third, fourth, and fifth bridge arms are obtained through phase-to-phase voltage equalization control. The drive signal of the switching transistor S1 of the sixth bridge arm is obtained by pulse width modulation from zero potential.

[0008] According to the control method of the present invention for improving the negative sequence compensation capability of a star-type STATCOM topology, the global DC voltage control adopts a dual-loop control of positive and negative sequence voltage and current, including global voltage control, positive sequence decoupling control, negative sequence decoupling control and output control.

[0009] According to the present invention, a control method for improving the negative sequence compensation capability of a star-type STATCOM is provided. The global voltage control includes: adjusting the three-phase DC voltage reference value U of the medium-voltage cascaded STATCOM. dc_ref Subtract the three-phase DC voltage U dc_a U dc_b U dc_c The difference is obtained by averaging the values, and this difference is used by the PI controller to adjust the output of the reference d-axis positive sequence current command. ; Positive-sequence decoupling control includes: controlling the positive-sequence voltage e along the d-axis of the system. d + Plus ωLi q + Subtract the reference d-axis positive sequence current command. With the sampling d-axis positive sequence current i d + The difference is used to adjust the output signal through a PI controller to obtain the d-axis positive sequence modulated voltage; where L is the three-phase inductance and ω is the system angular frequency; the system q-axis positive sequence voltage e q + minus ωLi d + Subtract the reference q-axis positive sequence current command. With the sampling q-axis positive sequence current i q + The difference is adjusted by the PI controller to obtain the q-axis positive sequence modulation voltage; the d-axis positive sequence modulation voltage and the q-axis positive sequence modulation voltage are then subjected to inverse Parker transformation to obtain the three-phase positive sequence modulation voltage. Negative-sequence decoupling control includes: reducing the system's d-axis negative-sequence voltage e d - minus ωLi q - Subtract the reference d-axis negative sequence current command. With the sampled d-axis negative sequence current i d- The difference is used to adjust the output signal of the PI controller to obtain the d-axis negative sequence modulated voltage; the system q-axis negative sequence voltage e q - Plus ωLi d - Subtract the reference q-axis negative sequence current command With the sampling q-axis negative sequence current i q - The difference is adjusted by the PI controller to obtain the q-axis negative sequence modulation voltage; the d-axis negative sequence modulation voltage and the q-axis negative sequence modulation voltage are then subjected to inverse Parker transformation to obtain the three-phase negative sequence modulation voltage. Output control includes: superimposing the three-phase positive-sequence modulation voltage and the three-phase negative-sequence modulation voltage to obtain the three-phase modulation voltage. .

[0010] According to the present invention, a control method for improving the negative sequence compensation capability of a star-type STATCOM topology includes phase voltage equalization control comprising: averaging the DC side voltage U of each phase... dca_ave U dcb_ave U dcc_ave As a reference value, it is compared with the DC-side voltage U of the i-th H-bridge power module in each phase. dcai U dcbi U dcci The difference is adjusted by the PI controller to output a signal, which is then compared with the corresponding output current. i a , i b , i c Multiplying the current directions yields the offset signal ΔU required for the i-th H-bridge power module in each phase. ai ΔU bi ΔU ci , i=1,2,…,N; where, output current i a 、i b 、i c The expression for the direction of the current is: .

[0011] According to the present invention, a control method for improving the negative sequence compensation capability of a star-type STATCOM topology includes phase-to-phase voltage equalization control of cascaded devices and DC voltage control of the converter.

[0012] According to the control method of the present invention for improving the negative sequence compensation capability of a star-type STATCOM, the inter-phase DC voltage equalization control of cascaded devices includes: equalizing the three-phase DC voltage U of the medium-voltage cascaded STATCOM. dc_ave With three-phase DC voltage U dc_a U dc_b U dc_c The difference is used as the power deviation signal after being adjusted by the PI controller. The zero-sequence modulation signal is calculated to obtain the zero-sequence voltage modulation signal.

[0013] According to the control method of the present invention for improving the negative sequence compensation capability of a star-type STATCOM topology, the DC voltage control of the converter adopts active vector modulation, including: controlling the DC voltage reference value U of the low-voltage three-phase four-wire converter. dcT_ref With DC voltage U dcT The difference is adjusted by the PI controller to output a signal, which is then compared with the input current I. abcT The result of the multiplication operation is added to the zero-sequence voltage modulation signal, and then pulse width modulation is applied to obtain the drive signals for the switching transistor S1 of the third, fourth, and fifth bridge arms.

[0014] Compared with the prior art, the present invention has at least the following technical effects: 1. Greatly enhances the negative sequence compensation capability of cascaded grid-connected converters. By injecting the required zero sequence current through a low-voltage three-phase four-wire converter, precise phase-to-phase power balance regulation is achieved, greatly improving the power compensation range of the unbalanced DC side between the three phases, maintaining the stability of the three-phase DC voltage, and enhancing the negative sequence compensation capacity. 2. Significantly reduced hardware costs. Phase-to-phase power balance is achieved by injecting zero-sequence voltage into a low-voltage three-phase four-wire converter, eliminating the need for substantial increases in hardware costs. Furthermore, compared to traditional methods using complex equipment or symmetrical component methods on the medium-voltage side, the requirements for DC voltage levels can be reduced. This allows for a lower voltage level for the DC power supply, thereby reducing equipment costs and improving system economy. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] In the attached diagram: Figure 1 The circuit schematic diagram shows the topology for improving the negative sequence compensation capability of a star-type STATCOM according to the present invention. Figure 2 This is a block diagram illustrating the control principle of the cascaded STATCOM in this invention. Figure 3 This is a block diagram illustrating the control principle of the low-voltage three-phase four-wire converter of the present invention. Figure 4 This is a simulation diagram of the negative order compensation effect of the topology of this invention; Figure 5 This is a simulation diagram of the phase equalization effect of zero-sequence injection according to the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0018] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0019] Please see Figure 1 This invention provides a topology for improving the negative sequence compensation capability of a star-connected STATCOM, comprising a medium-voltage (e.g., rated voltage of 10kV) cascaded STATCOM with a low switching frequency (e.g., operating switching frequency below 20kHz), a low-voltage (e.g., rated voltage of 380V / 220V) three-phase four-wire converter with a high switching frequency (e.g., operating switching frequency above 30kHz), and an isolation transformer. The primary side of the isolation transformer is delta-connected, thus preventing zero-sequence current from being generated on the primary side and avoiding interference to the primary side. The secondary side of the isolation transformer is star-connected. The isolation transformer is used to achieve electrical isolation and improve the voltage level used by the topology.

[0020] The primary side of the isolation transformer is connected to the medium-voltage power grid, and the secondary side of the isolation transformer is connected to the three-phase input terminal of the medium-voltage cascaded STATCOM through a three-phase inductor L. The three-phase output terminal of the medium-voltage cascaded STATCOM is connected to the three-phase input terminal of the low-voltage three-phase four-wire converter, and the output terminal of the low-voltage three-phase four-wire converter is connected to the neutral point of the secondary side of the isolation transformer to construct a zero-sequence loop. With the help of zero-sequence current injection, the DC voltage balance of the medium-voltage cascaded STATCOM is achieved.

[0021] Specifically, each phase of a medium-voltage cascaded STATCOM includes N H-bridge power modules cascaded sequentially. Each H-bridge power module includes four switching transistors S0 and an energy storage capacitor C0. Two of the four switching transistors S0 form the first bridge arm, and the other two form the second bridge arm. The first bridge arm, the second bridge arm, and the energy storage capacitor C0 are connected in parallel. The midpoint of the first bridge arm of the first H-bridge power module in each phase is the input terminal of that phase. The midpoint of the second bridge arm of the Nth H-bridge power module in each phase is the output terminal of that phase. The midpoint of the second bridge arm of the kth H-bridge power module in each phase is connected to the midpoint of the first bridge arm of the (k+1)th H-bridge power module, where 1 ≤ k < N, and k is an integer.

[0022] The low-voltage three-phase four-wire converter includes a three-phase H-bridge inverter circuit, which consists of eight switching transistors S1 and a DC-side capacitor C. dc Of the eight switching transistors S1, every two switching transistors S1 form a bridge arm, denoted as the third bridge arm, fourth bridge arm, fifth bridge arm, and sixth bridge arm; the third bridge arm, fourth bridge arm, fifth bridge arm, sixth bridge arm, and DC-side capacitor C dc Parallel connection; the midpoints of the third, fourth, and fifth bridge arms are the three-phase input terminals of the low-voltage three-phase four-wire converter, and the midpoint of the sixth bridge arm is the output terminal of the low-voltage three-phase four-wire converter.

[0023] It should be noted that the topology of this invention is particularly suitable for medium and high voltage distribution networks, new energy grid connection, and industrial high-power load scenarios. By improving the topology structure and control logic, it significantly enhances the compensation capability of cascaded multilevel grid-connected converters for negative sequence components of the power grid, improves the interphase unbalanced power compensation range of cascaded multilevel grid-connected converters, and solves the power quality problems caused by voltage and current imbalance in the power grid.

[0024] Another embodiment of the present invention provides a control method for a topology that improves the negative sequence compensation capability of a star-type STATCOM based on the foregoing embodiments, comprising: a medium-voltage cascaded STATCOM employing global DC voltage control and intra-phase voltage equalization control, and obtaining a three-phase modulated voltage through global DC voltage control. The offset signal ΔU required for the i-th H-bridge power module in each phase is obtained through phase-to-phase voltage equalization control. ai ΔU bi ΔU ci Then the offset signal ΔU ai ΔU bi ΔU ci With three-phase modulation voltage The sum of 1 / N times is used to generate the drive signal for the switch S0 of the i-th H-bridge power module in each phase, which is then modulated by a phase-shifted carrier sinusoidal pulse. The low-voltage three-phase four-wire converter adopts phase-to-phase voltage equalization control. The low-voltage three-phase four-wire converter provides zero-sequence current to the medium-voltage cascaded STATCOM. The drive signals of the switching transistors S1 of the third, fourth, and fifth bridge arms are obtained through phase-to-phase voltage equalization control. The drive signal g0 of the switching transistor S1 of the sixth bridge arm is obtained by pulse width modulation from zero potential.

[0025] As can be seen, the control method of the present invention adopts a composite control strategy that combines medium-voltage cascaded STATCOM global DC voltage control, intra-phase voltage equalization control, and inter-phase voltage equalization control injected through zero-sequence current of low-voltage three-phase four-wire converter.

[0026] like Figure 2 As shown, the global DC voltage control adopts a dual-loop control of positive and negative sequence voltage and current, including global voltage control, positive sequence decoupling control, negative sequence decoupling control, and output control. Figure 2 (Not shown in the image).

[0027] Global voltage control includes: controlling the three-phase DC voltage reference value U of the medium-voltage cascaded STATCOM. dc_ref Subtract the three-phase DC voltage U dc_a U dc_b U dc_c The difference is obtained by averaging the values, and this difference is used by the PI controller to adjust the output of the reference d-axis positive sequence current command. .

[0028] Positive-sequence decoupling control includes: controlling the positive-sequence voltage e along the d-axis of the system. d + Plus ωLi q + Subtract the reference d-axis positive sequence current command. With the sampling d-axis positive sequence current i d + The difference is used to adjust the output signal through a PI controller to obtain the d-axis positive sequence modulated voltage; where L is the three-phase inductance and ω is the system angular frequency; the system q-axis positive sequence voltage e q + minus ωLi d + Subtract the reference q-axis positive sequence current command. With the sampling q-axis positive sequence current i q + The difference is adjusted by the PI controller to obtain the q-axis positive sequence modulation voltage; the d-axis positive sequence modulation voltage and the q-axis positive sequence modulation voltage are then subjected to the inverse Park transformation (inverse Park transformation, i.e. dq0 / abc transformation) to obtain the three-phase positive sequence modulation voltage.

[0029] Negative-sequence decoupling control includes: reducing the system's d-axis negative-sequence voltage ed - minus ωLi q - Subtract the reference d-axis negative sequence current command. With the sampled d-axis negative sequence current i d - The difference is used to adjust the output signal of the PI controller to obtain the d-axis negative sequence modulated voltage; the system q-axis negative sequence voltage e q - Plus ωLi d - Subtract the reference q-axis negative sequence current command With the sampling q-axis negative sequence current i q - The difference is adjusted by the PI controller to obtain the q-axis negative sequence modulation voltage; the d-axis negative sequence modulation voltage and the q-axis negative sequence modulation voltage are then subjected to inverse Parker transformation to obtain the three-phase negative sequence modulation voltage.

[0030] Output control includes: superimposing the three-phase positive-sequence modulation voltage and the three-phase negative-sequence modulation voltage to obtain the three-phase modulation voltage. .

[0031] Specifically, the phase voltage balancing control of a medium-voltage cascaded STATCOM essentially involves further distributing the active power of each H-bridge power module in the three phases. The specific process is as follows: The average value U of the DC side voltage of each phase dca_ave U dcb_ave U dcc_ave As a reference value, it is compared with the DC-side voltage U of the i-th H-bridge power module in each phase. dcai U dcbi U dcci The difference is adjusted by the PI controller to output a signal, which is then compared with the corresponding output current. i a , i b , i c Multiplying the current directions (using a multiplier) yields the offset signal ΔU required for the i-th H-bridge power module in each phase. ai ΔU bi ΔU ci , i=1,2,…,N.

[0032] It should be noted that multiplying the output current by its instantaneous value yields the offset signal required by each H-bridge power module. However, improper control parameters can cause sudden changes in the instantaneous value of the output current, easily leading to system oscillations. Therefore, this invention only introduces the product of the actual direction of the output current and the voltage difference, thus avoiding the uncertainties caused by drastic changes in current amplitude. Wherein, the output current... i a 、i b 、i c The expression for the direction of the current is: .

[0033] By using the offset signal ΔU ai ΔU bi ΔU ci With three-phase modulation voltage The sum of 1 / N times is used to generate the drive signal g for the switch S0 of the i-th H-bridge power module in each phase, which is then modulated by phase-shifted carrier sinusoidal pulse modulation (CPS-SPWM modulation). i-abc This enables balanced voltage control within each phase.

[0034] Specifically, the phase-to-phase voltage equalization control of zero-sequence current injection in a low-voltage three-phase four-wire converter aims to equalize the three-phase DC voltage U of the medium-voltage cascaded STATCOM. dc_a U dc_b U dc_c To maintain balance and improve its negative sequence compensation capability. Traditional converters mostly use a three-phase three-wire system, lacking a zero-sequence current loop and having a small adjustable power range. Therefore, this invention connects a three-phase four-wire converter to the neutral point of the secondary side of an isolation transformer, injecting zero-sequence current into a medium-voltage cascaded STATCOM to redistribute the active power among the three phases, thereby achieving a three-phase DC voltage U dc_a U dc_b U dc_c It achieves a balance and greatly improves the adjustable power range of interphase unbalanced power.

[0035] like Figure 3 As shown, the phase-to-phase voltage equalization control includes converter DC voltage control and phase-to-phase DC voltage equalization control of cascaded devices injected with zero sequence through the converter.

[0036] Among them, the interphase DC voltage equalization control of cascaded devices includes: averaging the three-phase DC voltage U of the medium-voltage cascaded STATCOM. dc_ave With three-phase DC voltage U dc_a U dc_b U dc_c The difference is used as the power deviation signal after being adjusted by the PI controller. The zero-sequence modulation signal is calculated to obtain the zero-sequence voltage modulation signal.

[0037] The converter DC voltage control employs active vector modulation, including: controlling the DC voltage reference value U of the low-voltage three-phase four-wire converter. dcT_ref With DC voltage U dcT The difference is adjusted by the PI controller to output a signal, which is then compared with the input current I. abcT The result of the multiplication operation, plus the zero-sequence voltage modulation signal, is then subjected to pulse width modulation (PWM modulation) to obtain the drive signal g2 for the switching transistor S1 of the third, fourth, and fifth bridge arms, in order to maintain its own DC voltage U. dcT Stability.

[0038] It should be noted that when the system contains negative sequence voltage, let the system voltage u a u b u c Output current of medium-voltage cascaded STATCOM i a , i b , i c for: (1); Among them, U p U n These represent the magnitudes of the positive and negative sequence components of the system voltage, respectively; I p I n These represent the magnitudes of the positive-sequence and negative-sequence components of the output current, respectively; with the angle of the positive-sequence voltage as the reference direction, i.e., the initial phase of the positive-sequence voltage is 0, θ n This indicates the initial phase of the negative sequence voltage output by a medium-voltage cascaded STATCOM; These represent the initial phases of the positive-sequence and negative-sequence output currents of a medium-voltage cascaded STATCOM, respectively. ω Let t be the system angular frequency and t be time.

[0039] The output power P is obtained A P B P C for: (2); It can be seen that compensating for the negative sequence component of the system will cause the output power of each phase to be unbalanced.

[0040] And the three-phase unbalanced power is: (3); Uneven output power across phases leads to unequal three-phase DC voltages in the cascaded multilevel grid-connected converter, weakening its negative-sequence compensation capability. To achieve balanced output power across phases, a low-voltage three-phase four-wire converter is used to inject zero-sequence current into the medium-voltage cascaded STATCOM through the neutral point. The method of balancing the three-phase output power, where I z The zero-sequence current amplitude, This represents the initial phase of the zero-sequence current.

[0041] After zero-sequence current injection, the power deviation is: , (4), ; It can be seen that the injection of zero-sequence components does not affect the overall active power transmission, but it can change the distribution of active power among the three-phase converter chains. Compared with the zero-sequence voltage injection method, the present invention has a wider range of adjustable active power.

[0042] make According to the abc-αβ coordinate transformation formula: (5); The power deviation in the αβ coordinate system is obtained as follows: (6); The zero-sequence current modulation signal is obtained as follows: ; in: .

[0043] In this zero-sequence loop, the zero-sequence voltage amplitude U z With zero-sequence current amplitude I z satisfy Then the zero-sequence voltage modulation signal can be derived as follows: u z =i z · 2 πjωL .

[0044] Please see Figure 4To verify the negative sequence compensation capability of the topology of this invention, a topology simulation model was built using MATLAB / Simulink software. An unbalanced load was connected at 0.5s, and the change in the negative sequence current of the system was observed under the presence of the unbalanced load. It can be seen that due to the existence of this topology, the negative sequence current generated by the unbalanced load can be compensated, making the negative sequence current of the system almost zero. This verifies the effectiveness of this invention in improving the negative sequence compensation capability of cascaded multilevel grid-connected converters.

[0045] Please see Figure 5 To verify the phase-to-phase voltage equalization control effect of the invention, the aforementioned topology simulation model was used again to observe the changes in the three-phase DC voltage of the medium-voltage cascaded STATCOM under unbalanced load conditions. It can be seen that the three-phase DC voltage of the medium-voltage cascaded STATCOM diverges when an unbalanced load is connected, but quickly stabilizes under the control method of this invention, demonstrating the effectiveness of the control method in maintaining the stability of the three-phase DC voltage of the medium-voltage cascaded STATCOM.

[0046] In summary, to overcome the deficiency of insufficient negative sequence compensation capability caused by DC side voltage imbalance in existing cascaded multilevel grid-connected converters, this invention provides an integrated solution of "improved topology + novel phase-to-phase voltage equalization control strategy," namely, a power quality regulation system based on a medium-voltage cascaded STATCOM and a low-voltage three-phase four-wire converter. It adds only a low-cost equalization module to the original cascaded topology, injects zero-sequence current using the low-voltage three-phase four-wire converter to achieve phase-to-phase power balance, and simultaneously reduces the DC voltage level, improving system economy and power quality regulation effect.

[0047] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A topology for improving negative sequence compensation capability of a star-type STATCOM, characterized in that, The low-voltage three-phase four-wire converter is connected to the three-phase input end of the medium-voltage cascaded STATCOM through a three-phase inductor, and the output end of the low-voltage three-phase four-wire converter is connected to the neutral point of the secondary side of the isolation transformer. The medium-voltage cascaded STATCOM includes N H-bridge power modules connected in cascade, each H-bridge power module including four switching tubes S0 and an energy storage capacitor C0, two of the four switching tubes S0 forming a first bridge arm, the other two of the four switching tubes S0 forming a second bridge arm, and the first bridge arm, the second bridge arm and the energy storage capacitor C0 being connected in parallel; the midpoint of the first bridge arm of the first H-bridge power module of each phase being the input end of the phase, the midpoint of the second bridge arm of the Nth H-bridge power module of each phase being the output end of the phase, and the midpoint of the second bridge arm of the kth H-bridge power module of each phase being connected to the midpoint of the first bridge arm of the (k+1)th H-bridge power module, 1≤k 2. The topology for improving negative sequence compensation capability of a boost star type STATCOM according to claim 1, characterized in that, The low-voltage three-phase four-wire converter adopts inter-phase voltage balancing control, and provides zero sequence current for the medium-voltage cascaded STATCOM; the drive signal of the switching tube S1 of the third bridge arm, the fourth bridge arm and the fifth bridge arm is obtained through the inter-phase voltage balancing control; and the drive signal of the switching tube S1 of the sixth bridge arm is obtained by pulse width modulation from zero potential.

3. The topology for improving negative sequence compensation capability of a star-connected STATCOM according to claim 2, characterized in that, The low-voltage three-phase four-wire inverter comprises a three-phase H-bridge inverter circuit, the three-phase H-bridge inverter circuit comprising eight switching tubes S1 and a DC side capacitor C dc ; among the eight switching tubes S1, every two switching tubes S1 constitute a bridge arm, denoted as a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm; The third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the DC side capacitor C dc Parallel; the midpoint of the third bridge arm, the fourth bridge arm and the fifth bridge arm is the three-phase input end of the low-voltage three-phase four-wire converter, and the midpoint of the sixth bridge arm is the output end of the low-voltage three-phase four-wire converter.

4. A control method of the topology for improving the negative sequence compensation capability of a star-type STATCOM according to claim 3, characterized in that, The global DC voltage control adopts positive and negative sequence voltage and current double-loop control, and includes global voltage control, positive sequence decoupling control, negative sequence decoupling control and output control. The medium-voltage cascaded STATCOM employs global DC voltage control and phase-to-phase voltage equalization control, obtaining three-phase modulated voltage through the global DC voltage control. The offset signal ΔU required for the i-th H-bridge power module in each phase is obtained through the intra-phase voltage equalization control. ai ΔU bi ΔU ci Then the offset signal ΔU ai ΔU bi ΔU ci With three-phase modulation voltage The sum of 1 / N times is used to generate the drive signal for the switch S0 of the i-th H-bridge power module in each phase, which is then modulated by a phase-shifted carrier sinusoidal pulse. ωLi 5. The control method of the topology for improving negative sequence compensation capability of the star-type STATCOM according to claim 4, characterized in that, ωLi 6. The control method of the topology for improving negative sequence compensation capability of the star-type STATCOM according to claim 5, characterized in that, The global voltage control includes: adjusting the three-phase DC voltage reference value U of the medium-voltage cascaded STATCOM. dc_ref Subtract the three-phase DC voltage U dc_a U dc_b U dc_c The difference is obtained by averaging the values, and this difference is used by the PI controller to adjust the output of the reference d-axis positive sequence current command. ; The positive sequence decoupling control comprises: multiplying the system d-axis positive sequence voltage e d + Plus ωLi q + Subtracting the difference between the reference d-axis positive sequence current instruction and the sampled d-axis positive sequence current i d + by a PI controller to obtain a signal, and obtaining a d-axis positive sequence modulation voltage; wherein L is a three-phase inductance, and ω is a system angular frequency; multiplying the system q-axis positive sequence voltage e q + Subtracting ωLi d + Subtracting the difference between the reference q-axis positive sequence current instruction and the sampled q-axis positive sequence current i q + by a PI controller to obtain a signal, and obtaining a q-axis positive sequence modulation voltage; and performing inverse Park transformation on the d-axis positive sequence modulation voltage and the q-axis positive sequence modulation voltage to obtain three-phase positive sequence modulation voltages. The negative sequence decoupling control comprises: adding the system d-axis negative sequence voltage e d - Subtracting The inter-phase voltage balancing control includes cascaded device inter-phase DC voltage balancing control and converter DC voltage control. q - Subtracting the reference d-axis negative sequence current instruction from the sampled d-axis negative sequence current i d - The difference is adjusted by a PI controller to output a signal to obtain a d-axis negative sequence modulation voltage; adding the system q-axis negative sequence voltage e q - ​ d - Subtracting the reference q-axis negative sequence current instruction from the sampled q-axis negative sequence current i q - The difference is adjusted by a PI controller to output a signal to obtain a q-axis negative sequence modulation voltage; and performing inverse Park transformation on the d-axis negative sequence modulation voltage and the q-axis negative sequence modulation voltage to obtain three-phase negative sequence modulation voltages;​ The output control comprises: superimposing the three-phase positive sequence modulation voltage and the three-phase negative sequence modulation voltage to obtain a three-phase modulation voltage .

7. The control method of the topology for improving negative sequence compensation capability of the star-type STATCOM according to claim 6, characterized in that, The in-phase voltage balancing control comprises: taking the average value U dca_ave 、 dcb_ave 、 dcc_ave of each phase DC side voltage as a reference value, respectively, and the difference between the DC side voltage U dcai 、 dcbi 、 dcci of the i-th H-bridge power module of each phase and the reference value is adjusted by a PI controller to output a signal, and then multiplied by the current direction of the output current i a 、 i b 、 i c corresponding to each phase to obtain the offset signal ΔU ai 、 bi 、 ci required by the i-th H-bridge power module of each phase, i=1,2,…,N; wherein the output current i a 、i b 、i c The expression of the current direction is: 。 8. The control method of the topology for improving negative sequence compensation capability of a star-type STATCOM according to claim 7, characterized in that, ​ 9. The control method of the topology for improving negative sequence compensation capability of a star-type STATCOM according to claim 8, characterized in that, The inter-phase DC voltage equalization control of the cascade device comprises: averaging the three-phase DC voltage of the medium-voltage cascade STATCOM to obtain U dc_ave The difference between the three-phase DC voltage U dc_a , U dc_b , U dc_c is adjusted by a PI controller to output a signal as a power deviation The zero sequence modulation signal is calculated to obtain a zero sequence voltage modulation signal.

10. The control method of the topology for improving negative sequence compensation capability of a star-type STATCOM according to claim 9, characterized in that, The converter DC voltage control adopts active vector modulation, comprising: taking the DC voltage reference value U dcT_ref of the low-voltage three-phase four-wire converter as an input of a PI controller, and taking the difference between the DC voltage U dcT and the output signal of the PI controller as an input of a zero sequence voltage modulation module, and taking the input current I abcT as an input of a current loop PI controller; multiplying the operation result of the input current I abcT and the output signal of the zero sequence voltage modulation module, and adding the zero sequence voltage modulation signal to obtain a driving signal of the switch S1 of the third bridge arm, the fourth bridge arm and the fifth bridge arm through pulse width modulation.

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