Zero-sequence circulating current rapid generation method for interphase voltage sharing of chained angle joint STATCOM (Static Synchronous Compensator)

Abstract

The invention relates to a zero-sequence circulating current rapid generation method for interphase voltage sharing of a chain type angle joint STATCOM (Static Synchronous Compensator). A chain type angle joint STATCOM commutation chain is mainly formed by cascading n power unit modules PU1-PUn; the power unit module adopts an H-bridge circuit, and the direct current side voltage of the power unit module is udcx (x is 1-n); collecting the bus voltage of the STATCOM grid-connected point, and calculating the effective value amplitude U of the line voltage; acquiring a bus voltage synchronization angle omega t by a phase-locked loop, and calculating sine and cosine values sin omega t and cos omega t of the bus voltage synchronization angle omega t; the method has the advantages that the zero-sequence circulating current required by the chained angle joint STATCOM when the interphase direct-current side voltage is balanced can be quickly and accurately calculated, the program code operation amount is small, and the program space is saved; when external power grid interference is dealt with and unbalanced current is output by the zero-sequence circulating current, the zero-sequence circulating current can exert a good voltage-sharing effect; the zero-sequence circulating current is directly superposed on a chained angle joint STATCOM phase current instruction, and the SVG angle internal phase current can be directly used as feedback for control, so that the control is more intuitive, easy to realize and high in compatibility.

Description

Technical Field

[0001] The invention relates to the field of flexible alternating current transmission, and in particular to a method for quickly generating zero-sequence circulating current for phase-to-phase voltage balancing of a chain-type delta-connected STATCOM. Background Art

[0002] Chain STATCOM is one of the most effective power quality solutions at present. Its commutation chain is divided into two fundamental wave topologies: star connection and delta connection. Compared with chain star connection STATCOM, chain delta connection STATCOM can not only compensate reactive power and suppress harmonics, but also has strong negative sequence current compensation capability.

[0003] Under the ideal condition that the grid voltage is balanced and the losses of each power submodule in the commutation chain are exactly the same, the active power among the three commutation chains of the chain-type delta-connected STATCOM is evenly distributed, the DC side voltage of each submodule is the same, and the chain-type delta-connected STATCOM can operate normally.

[0004] However, in actual applications, when the grid voltage is unbalanced or unbalanced current is compensated, the active power absorbed or output by the three converter chains will be different. At this time, if an effective phase-to-phase voltage equalization control strategy is not adopted, the difference in the average voltage of the DC side of the converter chain will gradually increase, which will lead to an imbalance in the DC side voltage of each power submodule, resulting in overvoltage or undervoltage faults, and ultimately affecting the normal operation of STATCOM.

[0005] The balance control of the DC side voltage of the chain-type delta-connected STATCOM power unit can be divided into three levels, namely, global DC side voltage control, phase-to-phase voltage balancing control of the commutation chain, and DC side voltage control of the power unit. The first and third levels have a single control target, and the controller or control algorithm design is relatively easy; as an intermediate control link, the phase-to-phase voltage balancing control of the commutation chain plays a key role in dealing with system voltage imbalance or compensating for negative sequence current. Since it involves the active power flow between the three chains, its control algorithm is more complicated. Summary of the invention

[0006] The purpose of the present invention is to provide a method for quickly generating zero-sequence circulating current for phase-to-phase voltage balancing of a chain-type delta-connected STATCOM, which can quickly and accurately obtain the zero-sequence fundamental circulating current required for phase-to-phase DC side voltage balancing control of the chain-type delta-connected STATCOM.

[0007] To achieve the above object, the present invention is implemented through the following technical solutions:

[0008] A method for rapidly generating zero-sequence circulating current for phase-to-phase equalization of chained delta-connected STATCOM is proposed. The effective value amplitude U of the line voltage at the grid-connected point of STATCOM and the synchronous angle ωt of the line voltage at the grid-connected point Uab are obtained, and the sine value sinωt of the synchronous angle ωt and the cosine value cosωt of the synchronous angle ωt are calculated. The contents are as follows:

[0009] S1, PI control is performed on the average voltage of the DC side of any two-phase commutation chain, and the output is U dc_PI1 , U dc_PI2

[0010] S2, obtain the negative sequence current command DQ axis component in the STATCOM angle;

[0011] S3, the PI closed-loop control output of the DC side voltage average value of the commutation chain, combined with the sine value sinωt and cosine value cosωt of the synchronization angle of the grid-connected point line voltage Uab, is used to obtain the zero-sequence circulating current i 01 ;

[0012] S4, the negative sequence current command DQ axis component in the STATCOM angle is combined with the sine value sinωt and cosine value cosωt of the synchronization angle of the grid-connected point line voltage Uab to obtain the zero sequence circulating current i 02 ;

[0013] S5, zero sequence circulating current i 01 and i 02 The final zero-sequence circulating current i0 is obtained by superposition, and the zero-sequence circulating current i0 is the fundamental current.

[0014] In step S1, the specific steps are as follows:

[0015] S11, collecting the DC side voltage of each power unit module of any two-phase commutation chain, and obtaining the average DC side voltage of each phase commutation chain;

[0016] S12, setting the target value of the DC side voltage of the power unit module, and using the average value of the DC side voltage of the two commutation chains as feedback for PI closed-loop control.

[0017] In step S2, the specific steps are as follows:

[0018] S21, collecting the load current, taking the grid-connected point line voltage synchronization angle ωt as a reference, performing a negative-sequence Park transformation on the load current, and obtaining the negative-sequence DQ component of the load line current;

[0019] S22, convert the load line current negative sequence DQ component into the in-angle phase current negative sequence DQ component, and invert it to obtain the in-angle negative sequence current command DQ axis component, which are I ndref ,I nqref .

[0020] The commutation chain is a chain-type delta-connected STATCOM commutation chain, which is composed of a cascade of H-bridge power units. There are three commutation chains in total, adopting a triangle topology.

[0021] In S3, the zero-sequence circulating current i 01 The formula is as follows:

[0022]

[0023] In S3, the zero-sequence circulating current i 02 The formula is as follows:

[0024] i 02 =I nqref *sinωt-I ndref *cosωt ②

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. It can quickly and accurately calculate the zero-sequence circulating current required by the chain-type delta-connected STATCOM to balance the phase-to-phase DC side voltage, with small program code calculation amount and save program space;

[0027] 2. When dealing with external grid interference and unbalanced current output, the zero-sequence circulating current can play a good voltage balancing effect;

[0028] 3. The zero-sequence circulating current is directly superimposed on the phase current command of the chain-type delta-connected STATCOM, and can be directly controlled by using the phase current in the SVG angle as feedback, which is more intuitive, easy to implement and highly compatible. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the topological structure diagram of the chain-type delta-connected STATCOM commutation chain.

[0030] Figure 2 This is the PI closed-loop control block diagram of the average voltage on the DC side of the commutation chain.

[0031] Figure 3 Generate a block diagram for zero-sequence circulating current. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0033] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[0034] [Example 1]

[0035] The topology of the chain-type delta-connected STATCOM commutation chain is shown in Figure 1 The commutation chain is mainly composed of n power unit modules PU1~PUn cascaded; the power unit module adopts an H-bridge circuit, and its DC side voltage is udcx (x is 1~n).

[0036] The bus voltage at the STATCOM grid connection point is collected and the effective value of the line voltage amplitude U is calculated; the bus voltage synchronization angle ωt is obtained by the phase-locked loop, and its sine and cosine quantities sinωt and cosωt are calculated.

[0037] Collect the DC side voltage of each power unit module of any two-phase commutation chain. Take the AB phase commutation chain and the BC phase commutation chain as examples, and calculate the average DC side voltage of the AB phase commutation chain and the BC phase commutation chain as U dcave_ab and U dcave_bc .

[0038] Given the target voltage value U of the DC side of the power unit dc_ref , the average voltage of the DC side of the AB phase and BC phase commutation chain unit is used as feedback for PI closed-loop control. After the limiting link, the outputs are U dc_PI1 、Udc_PI2,see Figure 2 .

[0039] Collect load current i loada 、i loadb 、i loadc , based on the grid-connected point line voltage synchronization angle ωt, the load current is transformed by negative-sequence Park, and the negative-sequence DQ component of the load line current is obtained. After mean filtering, I ndload ,I nqload , the calculation formula is as follows:

[0040]

[0041] In formula ①, I ndload is the d-axis component of the load negative sequence current, I nqload is the q-axis component of the load negative sequence current, i loada 、i loadb 、i loadc is the load current;

[0042] Convert the negative sequence DQ component of the load line current into the negative sequence DQ component of the phase current within the angle, and invert it to obtain the negative sequence current command DQ axis component I within the angle nd_ref ,I nq_ref , the calculation formula is as follows:

[0043]

[0044] In formula ②, I nd_ref is the negative sequence d-axis current command, Inq_ref is the negative sequence q-axis current command;

[0045] The average voltage of the DC side of the commutation chain is output by PI closed-loop control. dc_PI1 , U dc_PI2 Combined with the sine and cosine quantities sinωt and cosωt of the line voltage synchronization angle at the grid connection point, the zero-sequence circulating current i is obtained. 01 , the calculation formula is as follows:

[0046]

[0047] The zero-sequence circulating current i can be obtained by combining the negative-sequence current command DQ axis components Ind_ref and Inq_ref with the sine and cosine quantities sinωt and cosωt of the grid line voltage synchronization angle. 02 , the calculation formula is as follows:

[0048] i 02 =I nqref *sinωt-I ndref *cosωt ④

[0049] See Figure 3 According to formula ③ and formula ④, the final zero-sequence circulating current i0 is calculated as follows:

[0050] i0=i 01 +i 02 ⑤

[0051] The present invention can quickly and accurately calculate the zero-sequence circulating current required by the chain-type delta-connected STATCOM when balancing the phase-to-phase DC side voltage, with a small amount of program code calculation and saved program space; the zero-sequence circulating current can play a good voltage-equalizing effect when dealing with external power grid interference and self-output unbalanced current; the zero-sequence circulating current is directly superimposed on the phase current instruction of the chain-type delta-connected STATCOM, and can be directly controlled by taking the phase current in the SVG angle as feedback, so the control is more intuitive, easy to implement and has strong compatibility.

Claims

1. A method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM, characterized in that: Obtain the effective value amplitude U of the line voltage at the grid connection point of STATCOM and the synchronization angle ωt of the line voltage at the grid connection point Uab, and calculate the sine value sinωt of the synchronization angle ωt and the cosine value cosωt of the synchronization angle ωt. The contents are as follows: S1, PI control is performed on the average voltage of the DC side of any two-phase commutation chain, and the output is U dc_PI1 , U dc_PI2 S2, obtain the negative sequence current command DQ axis component in the STATCOM angle; S3, the PI closed-loop control output of the DC side voltage average value of the commutation chain, combined with the sine value sinωt and cosine value cosωt of the synchronization angle of the grid-connected point line voltage Uab, is used to obtain the zero-sequence circulating current i 01 ; S4, the negative sequence current command DQ axis component in the STATCOM angle is combined with the sine value sinωt and cosine value cosωt of the synchronization angle of the grid-connected point line voltage Uab to obtain the zero sequence circulating current i 02 ; S5, zero sequence circulating current i 01 and i 02 The final zero-sequence circulating current i0 is obtained by superposition, and the zero-sequence circulating current i0 is the fundamental current.

2. A method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM according to claim 1, characterized in that: In step S1, the specific steps are as follows: S11, collecting the DC side voltage of each power unit module of any two-phase commutation chain, and obtaining the average DC side voltage of each phase commutation chain; S12, setting the target value of the DC side voltage of the power unit module, and using the average value of the DC side voltage of the two commutation chains as feedback for PI closed-loop control.

3. A method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM according to claim 1, characterized in that: In step S2, the specific steps are as follows: S21, collecting the load current, taking the grid-connected point line voltage synchronization angle ωt as a reference, performing a negative-sequence Park transformation on the load current, and obtaining the negative-sequence DQ component of the load line current; S22, convert the load line current negative sequence DQ component into the in-angle phase current negative sequence DQ component, and invert it to obtain the in-angle negative sequence current command DQ axis component, which are I ndref ,I nqref .

4. A method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM according to claim 1, characterized in that: The commutation chain is a commutation chain of a chain-type delta-connected STATCOM, which is composed of a cascade of H-bridge power units, has three commutation chains in total, and adopts a triangle topology structure.

5. The method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM according to claim 1, characterized in that: In S3, the zero-sequence circulating current i 01 The formula is as follows:

6. A method for rapidly generating zero-sequence circulating current for phase-to-phase voltage balancing of a cascaded delta-connected STATCOM according to claim 1, characterized in that: In S3, the zero-sequence circulating current i 02 The formula is as follows: i 02 =I nqref *sinωt-I ndref *cosωt ②。

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