A composite medium voltage power quality conditioner topology and control method

Abstract

The present invention belongs to the technical field of power electronic converters, and discloses a composite medium-voltage power quality regulator topology structure and control method. The medium-voltage chain STATCOM is composed of cascaded H-bridge converters, each of which includes multiple H-bridge submodules, and each submodule includes two IGBT bridge arms and a DC capacitor. The three-phase inverter circuit includes three IGBT bridge arms and a DC side capacitor. The topology is connected to the power grid through a connecting reactor and is connected to the three-phase inverter circuit through an LC filter. The DC side of the three-phase inverter circuit can be connected to an energy storage device to provide active power. In terms of control method, the present invention uses a three-phase inverter circuit control device to realize an active power filtering function, which can compensate up to 50 harmonics. The composite regulator can simultaneously perform reactive power compensation, harmonic compensation and unbalance compensation, adapt to complex operating conditions, and show good adjustment flexibility. It is particularly suitable for scenes with serious power quality problems such as reactive power, harmonics and three-phase unbalance in medium-voltage distribution networks.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic converters, and in particular relates to a composite medium-voltage power quality conditioner topology structure and a control method. Background Art

[0002] The static synchronous compensator (STATCOM) realizes the reactive power compensation function by tracking the command current. It has two main purposes: improving the system power factor and stabilizing the access point voltage. When the device is used to stabilize the access point voltage, the acquisition of reactive current command is relatively simple. Generally, it is obtained by detecting the difference between the current voltage and the reference voltage, and then calculating the reactive command current through the proportional integral controller; when the device is used to improve the system power factor, STATCOM needs to detect the reactive current from the load current. In recent years, with the increasing severity of harmonic problems and users' higher and higher requirements for power quality, more and more users have begun to hope that STATCOM devices should have certain harmonic and unbalance compensation functions while compensating for reactive power.

[0003] The common method to make STATCOM have the ability to compensate reactive power, harmonics and imbalance is to detect the reactive current, harmonic current and unbalanced current of the load respectively and control STATCOM to generate opposite reactive current, harmonic current and unbalanced current. However, for medium voltage STATCOM, a cascade multi-level structure is usually adopted, and the switching frequency of each submodule is low. When performing harmonic compensation, the compensation ability for high-order harmonics is limited; and because the harmonic modulation voltage is introduced into the modulation voltage, it will have an adverse effect on the voltage-sharing control of the DC capacitor of the submodule.

[0004] A three-phase inverter circuit is introduced at the neutral point of the medium-voltage STATCOM, and harmonic and three-phase imbalance compensation is performed through a high-switching frequency three-phase inverter circuit, which can fully compensate for high-order harmonics and unbalanced components. The frequency division control method is adopted so that the modulation voltage of the medium-voltage STATCOM only contains the fundamental frequency voltage, and the modulation voltage of the three-phase inverter circuit only contains the harmonic voltage, making the voltage equalization control of the sub-module DC capacitor easier to achieve.

[0005] Through the above analysis, the problems and defects of the prior art are as follows:

[0006] When the medium voltage STATCOM performs reactive power, harmonic and three-phase imbalance compensation simultaneously, the compensation capability for higher harmonics is limited and the voltage equalization control of the DC capacitors of the submodules is not easy to achieve. Summary of the invention

[0007] In view of the problems existing in the prior art, the present invention provides a composite medium-voltage power quality conditioner topology structure and a control method.

[0008] The present invention is implemented in this way: a medium-voltage chain STATCOM with a low switching frequency is connected to a medium-voltage 10kV AC feeder via a connecting reactor L1 to realize the compensation function of nonlinear load reactive current; a three-phase inverter circuit with a high switching frequency is connected to the medium-voltage chain STATCOM via an LC filter, and a method of specifying subharmonic current control in a multi-synchronous rotating coordinate system is used to realize the compensation function of nonlinear load harmonic current and unbalanced current; the medium-voltage chain STATCOM and the three-phase inverter circuit jointly control the current of the composite medium-voltage power quality conditioner, but control its fundamental frequency current, harmonic current and unbalanced current respectively, thereby realizing the frequency division control of the current. The present invention adopts a low-switching-frequency medium-voltage chain STATCOM topology combined with a high-switching-frequency three-phase inverter topology, has a simple structure, is easy to control, can simultaneously compensate for reactive power, harmonics and unbalanced current, and has good comprehensive compensation capabilities.

[0009] The composite medium voltage power quality conditioner topology structure includes: three connected reactors L1, a medium voltage chain STATCOM, a three-phase LC filter, a three-phase inverter circuit and a DC port;

[0010] One end of the connecting reactor L1 is connected to the medium voltage 10kV feeder, and the other end is connected to the medium voltage chain STATCOM;

[0011] The medium voltage chain STATCOM is composed of three cascaded H-bridge converters HM (M = A, B, C, representing the three phases A, B, and C);

[0012] The cascaded H-bridge converter is composed of n H-bridge submodules HM1 to HMn, where n can be selected according to actual design requirements;

[0013] The H-bridge submodule consists of two bridge arms containing two IGBTs (a total of 4 IGBTs) and a DC side capacitor C dc constitute;

[0014] In the cascaded H-bridge converter of each phase, one side of the AC side of HM1 is connected to the medium voltage 10kV feeder through the connecting reactor L1, and the other side is connected to the AC side of HM2; one side of the AC side of HM2 is connected to the AC side of HM1, and the other side is connected to the AC side of HM3, and so on for HM3 to HMn-1; one side of the AC side of HJn is connected to the AC side of HJn-1, and the other side is connected to the three-phase LC filter;

[0015] The three-phase LC filter is connected to the medium voltage chain STATCOM on one side and to the three-phase inverter circuit on the other side;

[0016] The three-phase inverter circuit is composed of 6 IGBT switch tubes (Q1-Q6) and their parasitic diodes (D1-D6); the switch tubes Q1 and Q2 form an A-phase bridge arm, the source of Q1 is connected to the drain of Q2, the positive electrode of diode D1, and the negative electrode of diode D2, the drain of Q1 is connected to the negative electrode of D1 and the positive electrode of the DC side, and the source of Q2 is connected to the positive electrode of D2 and the negative electrode of the DC side; the switch tubes Q3 and Q4 form a B-phase bridge arm, the source of Q3 is connected to the positive electrode of Q4 The drain of Q5 is connected to the positive electrode of diode D3 and the negative electrode of diode D4, the drain of Q3 is connected to the negative electrode of D3 and the positive electrode of the DC side, and the source of Q4 is connected to the positive electrode of D4 and the negative electrode of the DC side; the switch tubes Q5 and Q6 form a C-phase bridge arm, the source of Q5 is connected to the drain of Q6, the positive electrode of diode D5 and the negative electrode of diode D6, the drain of Q5 is connected to the negative electrode of D5 and the positive electrode of the DC side, and the source of Q6 is connected to the positive electrode of D6 and the negative electrode of the DC side;

[0017] The DC side U of the three-phase inverter circuit DC It can be connected to photovoltaic power generation systems, battery energy storage equipment, etc. to provide a certain amount of active power for the system;

[0018] The medium voltage chain STATCOM detects the reactive current of the nonlinear load and compensates it;

[0019] The medium voltage chain STATCOM control unit combines the three-phase voltage of the medium voltage 10kV feeder, the three-phase current of the nonlinear load, the three-phase current of the medium voltage chain STATCOM, and the DC side voltage of each H-bridge submodule of the medium voltage chain STATCOM, adopts a phase-locked loop controller, three-layer DC voltage equalization control, and AC current control, and generates a PWM signal through carrier phase shift modulation to control each switch tube of the medium voltage chain STATCOM;

[0020] The three-phase inverter circuit detects harmonic current and unbalanced current of nonlinear load and compensates for them;

[0021] The three-phase inverter circuit control unit generates a PWM signal by combining the three-phase current of the nonlinear load and the three-phase current of the medium-voltage chain STATCOM, and adopts a method of specifying secondary current control in a multi-synchronous rotating coordinate system to control the six switch tubes of the three-phase inverter circuit;

[0022] Further, the medium voltage chain STATCOM control unit comprises:

[0023] A phase-locked loop controller is used to calculate the real-time phase of the medium voltage 10kV feeder voltage through a synchronous selected reference frame phase-locked loop (SRF-PLL);

[0024] The first Park controller is used to convert the three-phase current of the medium voltage chain STATCOM in the three-phase stationary coordinate system into the current in the two-phase synchronous rotating coordinate system;

[0025] The first proportional integral controller is used to realize the global voltage balancing control function, by adding the values ​​obtained by proportional calculation and integral calculation of the total DC voltage average value of the medium voltage chain STATCOM, and the sum is used as the reference value of the active current in the two-phase synchronous rotating coordinate system;

[0026] The second proportional integral controller is used to realize active current control, by subtracting the active current reference value from the active current actual value, and the difference is subjected to proportional calculation and integral calculation, and the summed value is used as the active voltage reference value of the part of the medium voltage chain STATCOM;

[0027] A third proportional-integral controller is used to realize reactive current control by subtracting the reactive current reference value from the reactive current actual value, performing proportional calculation and integral calculation on the difference, and adding the summed values ​​as the reactive voltage reference value of the partial medium voltage chain STATCOM;

[0028] A first anti-Park controller is used to convert a part of the medium voltage chain STATCOM modulation voltage in a two-phase synchronous rotating coordinate system into a three-phase modulation voltage in a three-phase stationary coordinate system;

[0029] The fourth proportional-integral controller is used to realize the inter-phase voltage balancing control function, by subtracting the reference value of the average value of the DC voltage of each phase submodule from the actual value of the average value of the DC voltage of each phase submodule, and the difference is subjected to proportional calculation and integral calculation. The added value is passed through other links to finally generate a zero-sequence voltage to realize inter-phase voltage balancing;

[0030] The fifth proportional-integral controller is used to realize the intra-phase voltage equalization control function, by subtracting the actual value of the average DC voltage of each phase submodule from the actual value of each submodule of each phase, and the difference is subjected to proportional calculation and integral calculation, and the added value is subjected to other links to finally realize the intra-phase voltage equalization;

[0031] A first PWM generator, whose input is a medium voltage chain STATCOM modulation voltage, and whose output is a PWM driving signal of the medium voltage chain STATCOM;

[0032] Furthermore, the three-phase inverter circuit control unit includes:

[0033] The second Park controller is used to convert the sum of the nonlinear load current and the three-phase current of the medium-voltage chain STATCOM into a second harmonic current component in a two-phase synchronous rotating coordinate system through a two-phase synchronous rotating coordinate system with an angular velocity of 2ω;

[0034] A first low-pass filter is used to perform low-pass filtering on the second harmonic current component on the two-phase synchronous rotating coordinate system;

[0035] a sixth proportional-integral controller, for subtracting the reference value of the second harmonic current component from the actual value of the second harmonic current component, performing proportional calculation and integral calculation on the difference, and adding the summed values ​​as the value of the second harmonic modulation voltage on the d-axis of the two-phase synchronous rotating coordinate system;

[0036] A seventh proportional-integral controller is used for subtracting the reference value of the second harmonic current component from the actual value of the second harmonic current component, and the difference is subjected to proportional calculation and integral calculation, and the added value is used as the value of the second harmonic modulation voltage on the q-axis of the two-phase synchronous rotating coordinate system;

[0037] The second anti-Park controller is used to convert the second harmonic modulation voltage on the two-phase synchronous rotating coordinate system into a three-phase second harmonic modulation voltage on the three-phase stationary coordinate system;

[0038] The controllers for other subharmonics can be recursively deduced in turn;

[0039] A second PWM generator, the input of which is a modulation voltage of a three-phase harmonic, and the output of which is a PWM driving signal of a three-phase inverter circuit;

[0040] Furthermore, the control method of the medium voltage chain STATCOM comprises the following steps:

[0041] (1) Grid 10kV AC feeder output voltage u sa 、u sb 、u sc The phase information ωt of the base frequency is obtained through the synchronous rotating coordinate system phase-locked loop (SRF-PLL); the output voltage and current of the 10kV AC feeder of the power grid are transformed by PARK to obtain the voltage u in the two-phase synchronous rotating coordinate system sd 、u sq and current i sd 、i sq ;

[0042] (2) Perform PARK transformation on the three-phase current of the nonlinear load and the three-phase current of the medium-voltage chain STATCOM, and obtain the current i in the two-phase synchronous rotating coordinate system. Ld 、i Lq and i 1d 、i 1q ;

[0043] (3) The first-level global voltage balancing control of the medium-voltage chain STATCOM is as follows: The DC side voltages of all cascaded H-bridge converters of each phase of the medium-voltage chain STATCOM are summed to obtain the total DC side voltage u of each phase. dcA ,u dcB,u dcC , sum the total DC side voltage of each phase and divide it by 1 / 3n to get the actual voltage value u of the first layer global voltage control dc ; The voltage reference value u of the first layer global voltage equalization control dc * It can be determined according to the actual design, and the voltage reference value u of the global voltage equalization control dc * Subtract the actual voltage value u of the global voltage balancing control dc The difference is input into the first-layer global voltage balancing control proportional integral controller, which outputs the d-axis current reference value i of the medium-voltage chain STATCOM 1d *. The first level of global voltage balancing control of the medium voltage chain STATCOM is the DC voltage outer loop control of the medium voltage chain STATCOM;

[0044] (4) The second-layer interphase voltage balancing control of the medium-voltage chain STATCOM is as follows: The total DC side voltage u of each phase is dcA ,u dcB ,u dcC Divide by 1 / n to get the average DC voltage u of each phase dca ,u dcb ,u dcc , the actual voltage value u of the global voltage balancing control dc The average DC voltage u of each phase dca ,u dcb ,u dcc The difference is input into the corresponding second-layer phase-to-phase voltage control proportional-integral controller. The output values ​​of the three second-layer phase-to-phase voltage control proportional-integral controllers are respectively related to -sinωt and sign(i 1q ), -sin(ωt-2π / 3) and sign(i 1q ), -sin(ωt+2π / 3) and sign(i 1q ) are multiplied, and the sum of the three products is added to obtain the zero-sequence voltage u0. By superimposing the zero-sequence voltage on the modulated voltage of each phase, the active power between phases is redistributed, and finally the voltage between phases is balanced;

[0045] (5) The third-layer intra-phase voltage equalization control of the medium-voltage chain STATCOM is as follows: the total DC side voltage u of each phase is dcA ,u dcB ,u dcC Divide by 1 / n to get the average DC voltage u of each phase dca ,u dcb ,u dcc The average DC voltage of each phase u dca ,u dcb ,u dcc Respectively with each phase each H bridge module DC side capacitor voltage u dcA1-n ,u dcB1-n,u dcC1-n Subtract, and the difference is input into the corresponding third-layer phase-to-phase voltage control proportional-integral controller. The output of the proportional-integral controller is multiplied by sinωt, sin(ωt-2π / 3), and sin(ωt+2π / 3) respectively to obtain the reactive vector u that needs to be superimposed on each H-bridge submodule of each phase. a2m 、u b2m 、u c2m , which in turn redistributes the active power within the phase and ultimately achieves voltage balance within the phase;

[0046] (6) The current inner loop control of the medium voltage chain STATCOM is as follows: The d-axis current reference value i of the medium voltage chain STATCOM is set to 1d *Compared to the actual d-axis current i of the medium voltage chain STATCOM 1d Subtract, and the difference is input into the current loop proportional integral controller, angular velocity ω, connecting reactor L1 and q-axis current i of medium voltage chain STATCOM 1q The product of minus the output of the current loop proportional integral controller is added to the grid 10kV AC feeder output voltage feedforward u sd , get the d-axis voltage reference value; the q-axis current reference value i of the medium voltage chain STATCOM 1q *Equal to the actual value of the nonlinear load current i Lq , subtracted from the actual q-axis current of the medium voltage chain STATCOM, and the difference is input into the current loop proportional integral controller, and the grid 10kV AC feeder output voltage is fed forward u sq Subtract the angular velocity ω, the d-axis current i of the connecting reactor L1 and the medium voltage chain STATCOM 1d The product is then subtracted from the output of the current loop proportional-integral controller to obtain the q-axis voltage reference value;

[0047] (7) Modulation voltage u output by the inner current loop a1 ,u b1 ,u c1 The zero-sequence voltage u0 output by the phase-to-phase voltage balancing control and the reactive power vector u output by the phase-to-phase voltage balancing control are respectively a2m 、u b2m 、u c2m Add together to get the final modulation voltage u a ,u b ,u c , a PWM wave is generated through carrier phase shift modulation to control each H-bridge module of the medium voltage chain STATCOM.

[0048] Furthermore, the control method of the three-phase inverter circuit comprises the following steps:

[0049] (1) Due to i 1a 、i 1b 、i1c and i La 、i Lb 、i Lc In the opposite direction, ideally, it is hoped that the harmonics compensated by the three-phase inverter circuit are the same as the harmonic currents generated by the nonlinear load, that is, i 1a 、i 1b 、i 1c with i La 、i Lb 、i Lc The sum of the harmonic components is 0. 1a 、i 1b 、i 1c with i La 、i Lb 、i Lc Add them together, and the sum is transformed into a multi-synchronous rotating coordinate system;

[0050] (2) Taking the third harmonic compensation as an example: 1a 、i 1b 、i 1c with i La 、i Lb 、i Lc The sum is transformed to a two-phase synchronous rotating coordinate system with an angular velocity of 3ω through the "equal amplitude" transformation rule to extract the third harmonic component, convert the third harmonic component into a DC quantity, and pass it through a low-pass filter with a cutoff frequency of 50Hz to obtain the actual third harmonic DC quantity;

[0051] (3) The reference value of the compensated third harmonic current is set to 0, and the actual third harmonic DC current is subtracted from the reference value. The difference is input into the third harmonic proportional integral controller. The output value of the third harmonic proportional integral controller is inversely transformed through a two-phase synchronous rotating coordinate system with an angular velocity of 3ω to obtain a modulation voltage of the third harmonic.

[0052] (4) A similar method can be used to achieve harmonic compensation of the 2nd to kth harmonics of the nonlinear load current, where k can be up to 50th;

[0053] (5) The imbalance compensation in the nonlinear load can be achieved by compensating the negative sequence fundamental. The modulation voltages of each harmonic and the negative sequence fundamental are added together to obtain the total modulation voltage for the final three-phase inverter circuit compensation, and the PWM wave is generated through SPWM modulation to control the three-phase inverter circuit.

[0054] Another object of the present invention is to provide a computer device, which includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the composite medium-voltage power quality conditioner topology control method.

[0055] Another object of the present invention is to provide a computer-readable storage medium storing a computer program, which, when executed by a processor, enables the processor to perform the steps of the composite medium-voltage power quality conditioner topology control method.

[0056] Another object of the present invention is to provide an information data processing terminal, which is used to implement the composite medium-voltage power quality conditioner topology device.

[0057] Another object of the present invention is to provide a control method for the medium-voltage chain STATCOM of the composite medium-voltage power quality conditioner topology, the control method for the medium-voltage chain STATCOM comprising the following steps:

[0058] (a) The real-time voltage phase of the medium voltage 10 kV feeder output voltage is obtained through a synchronous rotating coordinate system phase-locked loop;

[0059] (b) Performing Park transformation on the three-phase current of the medium voltage chain STATCOM to obtain the current in the synchronous rotating coordinate system; performing Park transformation on the three-phase current of the nonlinear load to obtain the current in the synchronous rotating coordinate system, and using the q-axis current in the synchronous rotating coordinate system as the q-axis current reference value of the medium voltage chain STATCOM;

[0060] (c) Calculate the sum of the DC side voltages of all H-bridge circuits of the medium voltage chain STATCOM and calculate the average value thereof. After global voltage balancing control, obtain the d-axis current reference value of the medium voltage chain STATCOM;

[0061] (d) calculating the d-axis and q-axis partial voltage reference values ​​according to the current reference value and the actual current value, performing an inverse PARK transformation, and generating a partial modulation voltage;

[0062] (e) Calculate the sum of the DC side voltages of each phase H-bridge circuit of the medium voltage chain STATCOM and find its average value. After phase-to-phase voltage equalization control, the zero-sequence voltage u0 is obtained;

[0063] (f) Calculate the sum of the DC side voltages of each phase H-bridge circuit of the medium voltage chain STATCOM and calculate its average value. After the intra-phase voltage equalization control, the modulation voltage of the intra-phase voltage equalization control is obtained; add the modulation voltages of each control to obtain the total modulation voltage, and control each H-bridge sub-module of the medium voltage chain STATCOM through the PWM generator.

[0064] The calculation of the total DC voltage average value further includes multiplying the sum of the DC side voltages of the H bridges of all sub-modules of the medium voltage chain STATCOM by 1 / 3n, and comparing it with a preset DC voltage reference value to generate a reference value of the d-axis current.

[0065] The calculation of the partial voltage reference values ​​of the d-axis and the q-axis further includes using a current loop proportional-integral controller to adjust the voltage error, and combining the angular velocity and the reactor parameters to obtain the partial voltage reference values ​​of the d-axis and the q-axis.

[0066] Another object of the present invention is to provide a control method for a three-phase inverter circuit control unit based on the above-mentioned composite medium voltage power quality conditioner topology, comprising the following steps:

[0067] (a) The three-phase currents of the medium voltage chain STATCOM and the nonlinear load are added, and the sum is input into a multi-synchronous rotating coordinate system for Park transformation;

[0068] (b) performing low-pass filtering on the output values ​​in each synchronously rotating coordinate system to obtain the actual values ​​of each harmonic and negative sequence fundamental error in the synchronously rotating coordinate system;

[0069] (c) subtracting the reference value in the synchronous rotating coordinate system from the actual value in the synchronous rotating coordinate system, and inputting the difference into a proportional-integral controller;

[0070] (d) performing an inverse Park transform on the output of each harmonic and negative-sequence fundamental proportional-integral controller to obtain a modulation voltage of each harmonic and negative-sequence fundamental; adding the modulation voltages of each harmonic and negative-sequence fundamental to generate a total modulation voltage, and controlling the three-phase inverter circuit through a PWM generator.

[0071] In combination with the above technical solutions and the technical problems solved, please analyze the advantages and positive effects of the technical solutions to be protected by the present invention from the following aspects:

[0072] First, the present invention provides a composite medium-voltage power quality regulator topology and control method, which can compensate for the reactive current of nonlinear loads through medium-voltage chain STATCOM with low switching frequency, and compensate for the harmonic current and unbalanced current of nonlinear loads through high switching frequency three-phase inverter circuit. The medium-voltage chain STATCOM and the three-phase inverter circuit realize the frequency division control of the device current, which is conducive to the effective compensation of high-order harmonics, and the voltage balancing control of the medium-voltage chain STATCOM does not need to introduce harmonics, and the voltage balancing control is easier to achieve. The present invention adopts the medium-voltage chain STATCOM topology with low switching frequency and the three-phase inverter topology with high switching frequency. It has a simple structure, is relatively easy to control and has high flexibility. It is very suitable for medium-voltage distribution networks with serious power quality problems such as reactive power, harmonics and three-phase imbalance.

[0073] Second, the significant technical advances brought about by the composite medium voltage power quality conditioner control method proposed in the present invention include:

[0074] 1) Improving the ability to compensate for high-order harmonics: This method effectively improves the ability to compensate for high-order harmonics by controlling the three-phase inverter circuit to compensate for the harmonic current.

[0075] 2) Improved efficiency: This method can maintain a low switching frequency for the medium voltage chain STATCOM through frequency division control while effectively compensating for high-order harmonics, which is conducive to improving efficiency.

[0076] 3) Reduced difficulty of voltage equalization control: This method enables the device to compensate for reactive power and harmonics while maintaining only fundamental modulation voltage in the chain STATCOM through frequency division control, thus reducing the difficulty of the chain STATCOM.

[0077] 4) Improve system response speed: This method can quickly respond to grid changes and provide faster regulation capabilities through a synchronous rotating coordinate system phase-locked loop and efficient Park transform and inverse transform algorithms.

[0078] 5) Improved stability of the device: This method can effectively stabilize the DC voltage of the medium-voltage chain STATCOM submodule by controlling the three-layer voltage balancing of the medium-voltage chain STATCOM, thereby improving the stability of the device.

[0079] 6) System integration and cost optimization: This method integrates multiple control links of the composite medium-voltage power quality conditioner, making the system more compact and reducing hardware costs and maintenance difficulties.

[0080] In summary, this control method not only improves the device's compensation capability and efficiency for high-order harmonics and reduces the difficulty of voltage equalization control, but also improves the system's response speed, improves the device's stability, and optimizes the system's cost and maintenance requirements.

[0081] Third, as auxiliary evidence of the inventiveness of the claims of the present invention, it is also reflected in the following important aspects:

[0082] The technical solution of the present invention fills the technical gap in the industry at home and abroad: the present invention realizes compensation for reactive current of nonlinear load through medium-voltage chain STATCOM with low switching frequency; the three-phase inverter circuit with high switching frequency realizes compensation for harmonic current and unbalanced current of nonlinear load; the medium-voltage chain STATCOM and the three-phase inverter circuit realize frequency division control of device current, which is conducive to effective compensation for high-order harmonics, and the voltage balancing control of the medium-voltage chain STATCOM does not need to introduce harmonics, and the voltage balancing control is easier to achieve. The present invention adopts a combination of medium-voltage chain STATCOM topology with low switching frequency and three-phase inverter topology with high switching frequency, which can simultaneously compensate for reactive current, harmonic current and unbalanced current of nonlinear load, and provides a new solution for cascade active power filters.

[0083] The technical solution of the present invention solves the technical problem that people have been eager to solve but have never succeeded: when most of the medium-voltage chain STATCOMs currently perform harmonic compensation, the switching frequency of the chain STATCOM submodules is often low, and the compensation capacity for high-order harmonics is limited; and when performing harmonic compensation, the harmonic modulation voltage is introduced into the modulation voltage, which is not conducive to the voltage-sharing control of the chain STATCOM submodules. How to improve the compensation capacity of the medium-voltage chain STATCOM for high-order harmonics is a technical difficulty that needs to be solved when the medium-voltage chain STATCOM compensates for harmonics. The present invention combines the medium-voltage chain STATCOM with a three-phase inverter circuit with a high switching frequency, and through frequency division control, allows the medium-voltage chain STATCOM to only perform reactive power compensation, and allows the three-phase inverter circuit to perform harmonic compensation and unbalance compensation. In comparison, the present invention has a stronger compensation capacity for high-order harmonics, and the voltage-sharing control of the chain STATCO submodule is easier to achieve, providing a new idea for comprehensive compensation of the medium-voltage chain STATCOM.

[0084] Fourth, the technical problems of the prior art solved by the present invention and the significant technical progress achieved are mainly reflected in the following aspects:

[0085] Existing technical problems solved:

[0086] 1. Single power quality regulation function: Traditional power quality regulation devices can only achieve a single function, such as reactive power compensation or harmonic suppression, and cannot meet the regulation requirements of multiple power quality issues at the same time. The present invention combines the functions of medium-voltage chain STATCOM and three-phase inverter circuit to achieve comprehensive regulation of reactive current, harmonic current and unbalanced current, thereby improving the comprehensive regulation capability of power quality.

[0087] 2. Insufficient regulation accuracy and response speed: Traditional power quality regulation devices often have deficiencies in regulation accuracy and response speed, and are difficult to adapt to rapidly changing power quality issues. The present invention improves regulation accuracy and response speed by adopting advanced control algorithms and modulation technologies, making power quality regulation more accurate and timely.

[0088] 3. System complexity and cost issues: Traditional power quality regulation systems are often complex in structure and expensive, which is not conducive to popularization and application. The present invention reduces system complexity and cost by optimizing the topology structure and simplifying the control system, making power quality regulation technology easier to implement and popularize.

[0089] Significant technological advances achieved:

[0090] 1. Multifunctional integration: The present invention realizes the integration of multiple power quality regulation functions such as reactive power compensation, harmonic suppression and unbalance compensation, so that a single device can solve multiple power quality problems at the same time, improving the practicality and efficiency of the device.

[0091] 2. High-precision and rapid adjustment: By adopting advanced control algorithms and modulation technologies, the present invention achieves high-precision and rapid adjustment of power quality, effectively responds to rapidly changing power quality issues, and improves the stability and reliability of power grid operation.

[0092] 3. System optimization and cost reduction: The present invention simplifies the system complexity and reduces the cost by optimizing the topology structure and control system, making the power quality regulation technology more economical and practical, and facilitating its application in a wider range.

[0093] The present invention solves the problems in the prior art and brings about significant technological progress, thus providing a new solution and idea for the development and application of power quality regulation technology, and has important practical value and promotion significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Figure 1 It is a circuit topology diagram of a composite medium voltage power quality conditioner provided by an embodiment of the present invention;

[0095] Figure 2 It is an overall control block diagram of a medium voltage chain STATCOM provided by an embodiment of the present invention;

[0096] Figure 3 It is a global voltage balancing control block diagram of a medium voltage chain STATCOM provided in an embodiment of the present invention;

[0097] Figure 4 It is a block diagram of the phase-to-phase voltage balancing control of a medium-voltage chain-type STATCOM provided in an embodiment of the present invention;

[0098] Figure 5 It is a block diagram of the intra-phase voltage equalization control of a medium-voltage chain-type STATCOM provided in an embodiment of the present invention;

[0099] Figure 6 is an overall control block diagram of a three-phase inverter circuit provided by an embodiment of the present invention;

[0100] Figure 7 It is the grid-connected current waveform before and after the composite medium-voltage power quality conditioner device provided by the embodiment of the present invention is connected;

[0101] Figure 8 is the sum of the DC side voltages of each phase submodule of the medium voltage chain STATCOM provided in an embodiment of the present invention;

[0102] Fig. 9It is the power factor waveform of the grid connection point before and after the composite medium voltage power quality conditioner device provided by the embodiment of the present invention is connected.

[0103] In the figure: 1. Medium voltage 10kV feeder; 2. Non-linear load; 3. Connection reactor L1; 4. Medium voltage chain STATCOM; 5. LC filter; 6. Three-phase inverter circuit; 7. DC port. DETAILED DESCRIPTION

[0104] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0105] Example 1: Implementation and operation of medium voltage chain STATCOM

[0106] An industrial plant often encounters voltage fluctuations, harmonics and three-phase imbalance problems, which affect the stable operation of the production line. To solve this problem, a medium-voltage chain STATCOM based on a composite medium-voltage power quality conditioner topology was installed.

[0107] Connecting reactor L1: connected to the 10kV medium voltage feeder to limit high frequency current and improve voltage quality.

[0108] Medium voltage chain STATCOM: It consists of three cascaded H-bridge converters, each of which contains several H-bridge sub-modules to achieve fast and dynamic reactive power compensation.

[0109] Three-phase LC filter: used to filter out high-frequency harmonics generated by the operation of three-phase inverter circuits and protect the connected power grid.

[0110] Three-phase inverter circuit: consists of six IGBT switching tubes, which are used to further process and stabilize the output voltage.

[0111] DC port: connected to the photovoltaic power generation system to provide additional active power support for STATCOM.

[0112] 1. Start the system: turn on the power and set the operating parameters through the human-machine interface.

[0113] 2. Voltage phase acquisition: Use the phase-locked loop controller to obtain the real-time phase of the feeder voltage to ensure the synchronization of the control signal.

[0114] 3. Current processing: Perform Park transformation on the current of STATCOM and the current of nonlinear load to obtain the current in the synchronous rotating coordinate system and optimize reactive power compensation.

[0115] 4. Voltage regulation: By calculating and controlling the DC voltage of each phase H-bridge, global and local voltage equalization control is achieved.

[0116] 5. PWM signal generation: Generate PWM signals according to the control algorithm, drive the IGBT switch tube of each H-bridge, and adjust the output voltage to compensate for the reactive power, harmonics and unbalanced components of the power grid.

[0117] 6. Monitoring and adjustment: The system continuously monitors the grid status and equipment performance, and automatically adjusts the control strategy to cope with changes in grid load.

[0118] Significantly improve the stability and power quality of the power grid.

[0119] Fast dynamic response, capable of instantly compensating reactive power and filtering harmonics.

[0120] Provides flexible control strategy and simple operation interface.

[0121] Embodiment 2: Control operation of three-phase inverter circuit

[0122] A large shopping mall was facing power quality issues, especially during high load hours. The inverter circuit portion of this hybrid medium voltage power quality conditioner was installed to improve power quality and reduce energy consumption.

[0123] #Device configuration:

[0124] Medium voltage chain STATCOM: Same as above, providing basic reactive power and harmonic compensation.

[0125] Three-phase inverter circuit: Same as above, further optimize power quality through advanced control strategy.

[0126] Monitoring system: monitors the grid status and equipment performance in real time to ensure that the inverter circuit correctly performs the compensation task.

[0127] 1. Current synthesis: The output current of the medium voltage chain STATCOM is added to the nonlinear load current and input to the Park transformation.

[0128] 2. Harmonic identification and compensation: Low-pass filter the synthetic current, identify harmonic currents of different orders, and calculate the modulation voltage required for compensation.

[0129] 3. Control signal generation

[0130] :According to the harmonic compensation requirements, PWM control signals are generated to accurately control the IGBT switches of the inverter circuit to eliminate grid harmonics.

[0131] 4. Performance monitoring: Continuously monitor the power quality of the shopping mall to ensure that harmonics and voltage fluctuations are within acceptable ranges.

[0132] Effectively reduce the energy consumption of shopping malls and improve electricity efficiency.

[0133] By reducing harmonics, the service life of electrical equipment is extended.

[0134] Improve the power stability of the shopping mall's internal environment and enhance consumer experience.

[0135] Through these two specific embodiments, it can be seen that the composite medium-voltage power quality conditioner can not only provide basic reactive power compensation and harmonic wave filtering, but also adapt to various power grid environments and requirements, and has broad application prospects.

[0136] like Figure 1 As shown, the composite medium voltage power quality conditioner topology provided by the embodiment of the present invention includes: three connected reactors L1, a medium voltage chain STATCOM, a three-phase LC filter, a three-phase inverter circuit and a DC port;

[0137] One end of the connection reactor L1 is connected to the medium voltage 10kV feeder, and the other end is connected to the medium voltage chain STATCOM; the medium voltage chain STATCOM is composed of three cascaded H-bridge converters HM (M = A, B, C, representing A, B, C three phases); the cascaded H-bridge converter is composed of n H-bridge submodules HM1~HMn, where n can be selected according to actual design requirements; the H-bridge submodule is composed of two bridge arms containing two IGBTs (a total of 4 IGBTs) and a DC side capacitor C dc Composition; in the cascaded H-bridge converter of each phase, one side of the AC side of HM1 is connected to the medium voltage 10kV feeder through the connecting reactor L1, and the other side is connected to the AC side of HM2; one side of the AC side of HM2 is connected to the AC side of HM1, and the other side is connected to the AC side of HM3, and so on for the connection mode of HM3 to HMn-1; one side of the AC side of HJn is connected to the AC side of HJn-1, and the other side is connected to the three-phase LC filter;

[0138] One side of the three-phase LC filter is connected to the medium voltage chain STATCOM, and the other side is connected to the three-phase inverter circuit;

[0139] The three-phase inverter circuit is composed of 6 IGBT switch tubes (Q1~Q6) and their parasitic diodes (D1~D6); the switch tubes Q1 and Q2 form the A-phase bridge arm, the source of Q1 is connected to the drain of Q2, the positive electrode of diode D1, and the negative electrode of diode D2, the drain of Q1 is connected to the negative electrode of D1 and the positive electrode of the DC side, and the source of Q2 is connected to the positive electrode of D2 and the negative electrode of the DC side; the switch tubes Q3 and Q4 form the B-phase bridge arm, the source of Q3 is connected to the positive electrode of Q4 The drain, the positive electrode of the diode D3, and the negative electrode of the diode D4 are connected, the drain of Q3 is connected to the negative electrode of D3 and the positive electrode of the DC side, and the source of Q4 is connected to the positive electrode of D4 and the negative electrode of the DC side; the switch tubes Q5 and Q6 form a C-phase bridge arm, the source of Q5 is connected to the drain of Q6, the positive electrode of the diode D5, and the negative electrode of the diode D6, the drain of Q5 is connected to the negative electrode of D5 and the positive electrode of the DC side, and the source of Q6 is connected to the positive electrode of D6 and the negative electrode of the DC side;

[0140] The DC side U of the three-phase inverter circuit DC It can be connected to photovoltaic power generation systems, battery energy storage equipment, etc. to provide a certain amount of active power for the system;

[0141] In the main circuit topology, the three-phase voltage of the medium voltage 10kV feeder is: sa 、u sb 、u sc , the three-phase current is: i sa 、i sb 、i sc ; The three-phase current of the nonlinear load is: i La 、i Lb 、i Lc ; The three-phase current of the medium voltage chain STATCOM is: i 1a 、i 1b 、i 1c ; The DC voltage of each submodule of the medium voltage chain STATCOM is u dcMn (M=A, B, C, representing the three phases A, B, and C); the three-phase current on the three-phase inverter circuit side is: i 2a 、i 2b 、i 2c ; The DC voltage of the DC port is U DC .

[0142] When the composite medium-voltage power quality conditioner topology structure provided by the embodiment of the present invention is in operation, the reactive current of the non-linear load is compensated by the medium-voltage chain STATCOM circuit and the harmonic current of the non-linear load is compensated by the three-phase inverter circuit, and the functions of reactive compensation, harmonic compensation and unbalance compensation are realized simultaneously through frequency division control.

[0143] Figure 2 The overall control block diagram of the medium voltage chain STATCOM provided by the present invention includes the following steps:

[0144] The three-phase current of the nonlinear load and the three-phase current of the medium-voltage chain STATCOM are transformed by PARK to obtain the current i in the two-phase synchronous rotating coordinate system. Ld 、i Lq and i 1d 、i 1q ;

[0145] The d-axis current reference value i of the medium voltage chain STATCOM is 1d *Compared to the actual d-axis current i of the medium voltage chain STATCOM 1d Subtract, and the difference is input into the current loop proportional integral controller, angular velocity ω, connecting reactor L1 and q-axis current i of medium voltage chain STATCOM 1q The product of minus the output of the current loop proportional integral controller is added to the grid 10kV AC feeder output voltage feedforward u sd , get the d-axis voltage reference value;

[0146] The q-axis current reference value i of the medium voltage chain STATCOM is 1q *Equal to the actual value of the nonlinear load current i Lq , subtracted from the actual q-axis current of the medium voltage chain STATCOM, and the difference is input into the current loop proportional integral controller, and the grid 10kV AC feeder output voltage is fed forward u sq Subtract the angular velocity ω, the d-axis current i of the connecting reactor L1 and the medium voltage chain STATCOM 1d The product is then subtracted from the output of the current loop proportional-integral controller to obtain the q-axis voltage reference value.

[0147] Figure 3 The global voltage balancing control block diagram of the medium voltage chain STATCOM provided by the present invention includes the following steps:

[0148] The total DC side voltage u of each phase is obtained by summing the DC side voltages of all cascaded H-bridge converters of each phase of the medium voltage chain STATCOM. dcA ,u dcB ,u dcC ;

[0149] Sum the total DC side voltage of each phase and divide it by 1 / 3n to get the actual voltage value u of the first layer global voltage equalization control dc , the voltage reference value u of the first layer global voltage equalization control dc *It can be determined according to the actual design;

[0150] The voltage reference value u of the global voltage balancing control dc * Subtract the actual voltage value u of the global voltage balancing control dcThe difference is input into the first-layer global voltage balancing control proportional integral controller, which outputs the d-axis current reference value i of the medium-voltage chain STATCOM 1d *. The first level of global voltage balancing control of the medium voltage chain STATCOM is the DC voltage outer loop control of the medium voltage chain STATCOM.

[0151] Figure 4 The block diagram of the phase-to-phase voltage equalization control of the medium-voltage chain STATCOM provided by the present invention includes the following steps:

[0152] The total DC link voltage u of each phase dcA ,u dcB ,u dcC Divide by 1 / n to get the average DC voltage u of each phase dca ,u dcb ,u dcc ;

[0153] Actual voltage value u of global voltage balancing control dc The average DC voltage u of each phase dca ,u dcb ,u dcc Subtract them, and input the difference into the corresponding second-layer phase-to-phase voltage-balanced control proportional-integral controller;

[0154] The output values ​​of the three second-layer phase-to-phase voltage control proportional-integral controllers are respectively related to -sinωt and sign(i 1q ), -sin(ωt-2π / 3) and sign(i 1q ), -sin(ωt+2π / 3) and sign(i 1q ) are multiplied, and the sum of the three products is added to obtain the zero-sequence voltage u0. By superimposing the zero-sequence voltage on the modulated voltage of each phase, the active power between phases is redistributed, and finally the voltage between phases is balanced.

[0155] Figure 5 The block diagram of the medium voltage chain STATCOM phase equalization control provided by the present invention includes the following steps:

[0156] The total DC link voltage u of each phase dcA ,u dcB ,u dcC Divide by 1 / n to get the average DC voltage u of each phase dca ,u dcb ,u dcc ;

[0157] The average DC voltage of each phase u dca ,u dcb ,u dcc Respectively with each phase each H bridge module DC side capacitor voltage u dcA1-n ,udcB1-n ,u dcC1-n Subtract them, and input the difference into the corresponding third-layer intra-phase voltage equalization control proportional-integral controller;

[0158] The output of the proportional-integral controller is multiplied by sinωt, sin(ωt-2π / 3), and sin(ωt+2π / 3) respectively to obtain the reactive vector u that needs to be superimposed on each H-bridge submodule of each phase. a2m 、u b2m 、u c2m , which in turn redistributes the active power within the phase and ultimately achieves voltage balance within the phase.

[0159] Figure 6 The overall control block diagram of the three-phase inverter circuit provided by the present invention includes the following steps:

[0160] will i 1a 、i 1b 、i 1c with i La 、i Lb 、i Lc Add them together, and the sum is transformed into a multi-synchronous rotating coordinate system;

[0161] Take the third harmonic compensation as an example: 1a 、i 1b 、i 1c with i La 、i Lb 、i Lc The sum is transformed to a two-phase synchronous rotating coordinate system with an angular velocity of 3ω through the "equal amplitude" transformation rule to extract the third harmonic component, convert the third harmonic component into a DC quantity, and pass it through a low-pass filter with a cutoff frequency of 50Hz to obtain the actual third harmonic DC quantity;

[0162] The reference value of the compensated third harmonic current is set to 0, and the actual third harmonic DC current is subtracted. The difference is input into the third harmonic proportional integral controller. The output value of the third harmonic proportional integral controller is inversely transformed through a two-phase synchronous rotating coordinate system with an angular velocity of 3ω to obtain the modulation voltage of the third harmonic.

[0163] A similar method can achieve harmonic compensation of the 2nd to kth harmonics of the nonlinear load current, where k can be up to 50th. The imbalance compensation in the nonlinear load current can be achieved by compensating the negative sequence fundamental. The modulation voltages of each harmonic and the negative sequence fundamental are added together to obtain the total modulation voltage of the final three-phase inverter circuit, and the PWM wave is generated through SPWM modulation to control the three-phase inverter circuit.

[0164] The composite medium-voltage power quality conditioner topology control method provided by the application embodiment of the present invention is applied to a computer device, wherein the computer device includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the three-port power router topology control method with high-reliability power supply.

[0165] The composite medium voltage power quality conditioner topology control method provided in the application embodiment of the present invention is applied to an information data processing terminal, and the information data processing terminal is used to implement the three-port power router topology device with high-reliability power supply.

[0166] The present invention builds a composite medium-voltage power quality conditioner simulation model on the MATLAB / Simulink simulation experiment platform. The initial phase of the medium-voltage 10kV feeder is 0, and the frequency is 50Hz; the nonlinear load is set as follows: the active current amplitude is 70A, the reactive current amplitude is 70A, the harmonic current amplitude is 25A, and the unbalanced current of phase A is 10A; the reactor L1 is connected to 3mH; the medium-voltage chain STATCOM uses 12 H-bridge submodules per phase, each H-bridge submodule has a switching frequency of 470Hz, a DC voltage set value of 800V, and a DC side capacitor of 5.6mF; the switching frequency of the three-phase three-port H-bridge inverter circuit is 25.6kHz, the filter inductance L2 of the LC filter is 1mH, the filter capacitor C2 is 2.5μF, and the DC side of the three-phase inverter circuit passes through the filter capacitor C DC =1mF connected to 800V DC voltage. The simulation results are as follows Figure 7 to Figure 9 shown.

[0167] Figure 7 This is the grid-connected current waveform before and after the composite medium-voltage power quality conditioner device proposed in the present invention is connected. In the initial stage, the medium-voltage 10kV feeder is connected to the nonlinear load, the composite medium-voltage power quality conditioner device is not connected, the grid-connected current harmonic content is large, the three-phase current imbalance is 8.31%, and the current THD = 8.2%; at time t0, the composite medium-voltage power quality conditioner device is connected to the system, the grid-connected current is close to sine, the three-phase current imbalance is close to 0, and the current THD = 3.82%;

[0168] Figure 8 It is the composite medium-voltage power quality conditioner device proposed by the present invention. The waveform of the DC side voltage and of each phase submodule of the medium-voltage chain STATCOM is that in the initial stage, the DC voltage of each submodule maintains the initial value of 800V, and the DC side voltage and of each phase submodule are 9600V; at time t0, the composite medium-voltage power quality conditioner device is connected to the system, and it can be seen that the DC side voltage of the medium-voltage chain STATCOM submodule can still maintain stability, and the DC side voltage and of each phase submodule are basically maintained at about 9600V;

[0169] Fig. 9 This is the power waveform of the grid connection point before and after the composite medium voltage power quality conditioner device proposed by the present invention is connected. In the initial stage, the medium voltage 10kV feeder is connected to the nonlinear load, the composite medium voltage power quality conditioner device is not connected, and the grid connection point power factor is lower than 1, about 0.85; at time t0, the composite medium voltage power quality conditioner device is connected to the system, and the grid connection point power factor is basically close to 1.

[0170] The present invention provides a composite medium voltage power quality conditioner topology structure and control method, the topology structure comprising: three connected reactors L1, a medium voltage chain STATCOM, a three-phase LC filter, a three-phase inverter circuit and a DC port;

[0171] The present invention compensates the reactive current of the nonlinear load by a medium-voltage chain STATCOM with a low switching frequency, and compensates the harmonic current and asymmetric current of the nonlinear load by a three-phase bridge inverter circuit with a high switching frequency; the DC port of the three-phase inverter circuit is connected to an energy storage device to provide a certain amount of active power for the device.

[0172] The medium-voltage chain STATCOM and three-phase inverter circuit realize frequency division control of the device current, which is conducive to the effective compensation of high-order harmonics and asymmetric components. In addition, the voltage balancing control of the medium-voltage chain STATCOM does not require the introduction of harmonics, and the voltage balancing control is easier to implement.

[0173] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A composite medium voltage power quality conditioner topology, characterized in that: include: Three connected reactors L1, whose main function is to suppress high-order harmonics and ensure the sinusoidal nature of the grid-connected current; A medium voltage chain STATCOM, whose main function is to realize the reactive power compensation function for nonlinear loads; a three-phase LC filter, whose main function is to filter the high-order harmonics of the three-phase inverter circuit with high switching frequency; a three-phase inverter circuit, whose main function is to realize the harmonic compensation and unbalance compensation functions for nonlinear loads; a DC port, whose main function is to provide active power to the composite medium voltage power quality conditioner; One end of the connecting reactor L1 is connected to the medium voltage 10kV feeder, and the other end is connected to the medium voltage chain STATCOM; the medium voltage chain STATCOM includes three cascaded H-bridge converters HM, each cascaded H-bridge converter is composed of a plurality of H-bridge sub-modules HM1-HMn; one end of the three-phase LC filter is connected to the medium voltage chain STATCOM, and the other end is connected to the three-phase inverter circuit; the three-phase inverter circuit is composed of 6 IGBT switch tubes and their parasitic diodes, and is connected to the DC port; The control method of the composite medium voltage power quality conditioner comprises the following steps: (a) Use a synchronous rotating coordinate system phase-locked loop to obtain the real-time phase of the medium voltage 10kV feeder output voltage; (b) transforming the three-phase current of the medium voltage chain STATCOM into the current in the synchronous rotating coordinate system through Park transformation, and transforming the three-phase current of the nonlinear load into the current in the synchronous rotating coordinate system to generate the q-axis current reference value of the medium voltage chain STATCOM; (c) Calculate the average value of the sum of the DC side voltages of all H-bridges of the medium voltage chain STATCOM, and obtain the d-axis current reference value through global voltage sharing control; (d) calculating partial voltage reference values ​​of the d-axis and q-axis according to the difference between the d-axis and q-axis current reference values ​​and the actual current values, and performing an inverse PARK transformation to generate a partial modulation voltage; (e) calculating the average value of the sum of the DC side voltages of each phase H-bridge circuit of the medium voltage chain STATCOM, and obtaining the modulation voltage through inter-phase voltage balancing control and intra-phase voltage balancing control; (f) outputting the obtained modulation voltage through a PWM generator to control each H-bridge submodule of the medium voltage chain STATCOM; The method of the three-phase inverter circuit of the composite medium voltage power quality conditioner topology comprises the following steps: (a) The sum of the three-phase currents of the medium voltage chain STATCOM and the nonlinear load is input into a multi-synchronous rotating coordinate system for Park transformation; (b) performing low-pass filtering on the output in the synchronously rotating coordinate system to obtain the actual value of each harmonic error; (c) inputting the difference between the harmonic error reference value and the actual value in the synchronous rotating coordinate system into a proportional-integral controller; (d) performing an inverse Park transform on the output of the proportional-integral controller to generate a modulation voltage of each harmonic; (e) The modulation voltages of all harmonics and the negative sequence fundamental are added together to generate a total modulation voltage, and the operation of the three-phase inverter circuit is controlled by a PWM generator.

2. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The H-bridge submodule includes two bridge arms containing two IGBTs, each bridge arm has a total of 4 IGBTs, and a DC side capacitor Cdc. One side of the AC side of each H-bridge submodule HMn is connected to the medium voltage 10kV feeder through the connecting inductor L1, and the other side is connected to the AC side of the next H-bridge submodule until HMn.

3. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The DC port UDC of the three-phase inverter circuit is connected to a photovoltaic power generation system or a battery energy storage device to provide active power to the system, while detecting and compensating for the harmonic current of the nonlinear load.

4. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The control unit of the medium voltage chain STATCOM includes a phase-locked loop controller, a three-layer DC voltage equalization control, and an AC current control, which are used to generate a PWM signal to control each switch tube of the medium voltage chain STATCOM.

5. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The three-phase inverter circuit control unit generates a PWM signal by adopting a method of specifying secondary current control in a multi-synchronous rotating coordinate system to control six switch tubes of the three-phase inverter circuit.

6. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The medium voltage chain STATCOM control unit further includes a proportional integral controller for realizing inter-phase voltage balancing control function and intra-phase voltage balancing control function, generating a zero-sequence voltage through proportional calculation and integral calculation to realize inter-phase and intra-phase voltage balancing control.

7. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: Calculating the total DC voltage average value includes multiplying the sum of the DC side voltages of the H bridges of all submodules of the medium voltage chain STATCOM by 1 / 3n, and comparing the average value with a preset DC voltage reference value to generate a reference value of the d-axis current.

8. The composite medium voltage power quality conditioner topology structure according to claim 1 is characterized in that: The calculation of the d-axis and q-axis partial voltage reference values ​​further includes using a current loop proportional integral controller to adjust the calculation based on the voltage error, and using the angular velocity and the inductor parameters to obtain the d-axis and q-axis partial voltage reference values.

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