阅读说明：本技术 在电网不平衡情况下星形级联statcom的控制方法 (Control method of star cascade STATCOM under power grid imbalance condition ) 是由 王书征 张少文 李先允 刘海涛 刘钊 于 2021-08-20 设计创作，主要内容包括：本发明公开了在电网不平衡情况下星形级联STATCOM的控制方法,其特征在于,针对接入不平衡电网的星形级联STATCOM系统,对其进行平均有功功率平衡控制、直流侧电压控制和参考电流跟踪控制；利用等效三相星形连接电源的原理,使各相的有功功率为零；将各相平均有功功率调整为直流电压控制器确定的目标值,可对三个直流侧电压独立控制；令变换器的中性点电压等于等效电源的对应点电压,将STATCOM解耦为三个单相系统；通过矢量分析方法,计算出星形STATCOM在不平衡电网下的有效运行范围。本发明在有效运行范围内进行控制,提高了系统运行的安全性和稳定性,能够同时适用于不平衡负载和不平衡电压两种情况。(The invention discloses a control method of a star cascade STATCOM under the condition of unbalanced power grid, which is characterized in that aiming at a star cascade STATCOM system accessed into an unbalanced power grid, average active power balance control, direct current side voltage control and reference current tracking control are carried out on the star cascade STATCOM system; the active power of each phase is zero by utilizing the principle of an equivalent three-phase star connection power supply; the average active power of each phase is adjusted to a target value determined by a direct current voltage controller, and the three direct current side voltages can be independently controlled; enabling the neutral point voltage of the converter to be equal to the corresponding point voltage of the equivalent power supply, and decoupling the STATCOM into three single-phase systems; and calculating the effective operation range of the star-shaped STATCOM under the unbalanced power grid through a vector analysis method. The invention is controlled in an effective operation range, improves the safety and stability of system operation, and can be simultaneously suitable for two conditions of unbalanced load and unbalanced voltage.)

1. The control method of the star cascade STATCOM under the condition of power grid imbalance is characterized by comprising the following steps of 1, constructing an equivalent three-phase star connection power supply, calculating zero sequence voltage, enabling active power of each phase to be zero, and stabilizing direct-current side voltage; step 2, taking the power loss of the converter into consideration, controlling active power, and calculating improved zero sequence voltage; step 3, tracking and controlling the current, using the line voltage of the equivalent power supply as an input source voltage and serving as a corresponding converter voltage reference required by each phase of branch; step 4, designing a star cascade STATCOM controller; and 5, analyzing the effective operation range of the star cascade STATCOM in the unbalanced state.

2. The control method according to claim 1, wherein in step 1, an equivalent three-phase star connection power supply is constructed, and the specific method for calculating the zero sequence voltage is as follows: step 1.1, determining the amplitude and the phase of zero sequence voltage; and 1.2, determining the three-phase voltage of the equivalent star connection power supply.

3. The control method according to claim 1, wherein in step 2, the active power is controlled, and the specific method for calculating the improved zero-sequence voltage is as follows: step 2.1, establishing a direct current side voltage controller consisting of three PI controls; and 2.2, calculating the amplitude and the phase of the improved zero sequence voltage.

4. The control method according to claim 1, wherein in step 3, the specific method for designing the current tracking control is as follows: the neutral point of the equivalent power supply and the neutral point of the cascade converter are actually connected, so that the star-connected STATCOM system can be decoupled into three single-phase systems, and the current controller can be designed in a split-phase mode according to basic equations of circuits of all phases.

5. The control method according to claim 1, wherein in step 4, the specific method for designing the controller is as follows: step 4.1, phasor calculation, namely calculating the line voltage of the equivalent power supply by using a phasor representation method, and determining the control of reference current and direct-current link voltage; step 4.2, current control is carried out, the STATCOM current is fed back, closed-loop current control is achieved, and a dead-beat controller is adopted to track the reference current; and 4.3, modulating, and generating a gate electrode signal according to the obtained reference voltage.

6. The control method according to claim 1, wherein in the step 5, the specific method for analyzing the effective operating range of the star cascade STATCOM in the unbalanced state is as follows: step 5.1, analyzing the effective operation range of the star-shaped STATCOM in an unbalanced state in the power distribution system; and 5.2, analyzing the effective operation range of the star STATCOM in the unbalanced state in the power transmission system.

Technical Field

The invention relates to the technical field of power electronics, in particular to a control method of a star cascade STATCOM under the condition of power grid imbalance.

Background

The static synchronous compensation device (STATCOM) based on the voltage source converter has the advantages of improving the utilization rate of a power transmission network, stabilizing a power system, adjusting line voltage and the like. The control algorithm of the formed cascade STATCOM device is mostly suitable for the balance working condition of a power grid and can bear the influence of a small amount of negative sequence voltage. However, in actual conditions, when the grid voltage is unbalanced to a large extent, such as a single-phase drop fault, the harmonic content of the output current of the system is increased, and the voltage on the direct current side is unbalanced to a large extent.

The accurate adjustment of the active power of each phase direct current side of the cascade STATCOM is the key for enabling the cascade STATCOM to continuously and stably operate under the unbalanced working condition of a power grid, however, the existing controller is difficult to accurately adjust the unbalanced current of the cascade STATCOM, and the design difficulty of the controller is increased due to interphase coupling.

Disclosure of Invention

In view of the above-mentioned deficiencies of the prior art, the present invention provides a method for controlling a star cascade STATCOM under a grid imbalance condition. The method has the advantages of improving the safety and stability of system operation and being suitable for two conditions of unbalanced load and unbalanced voltage.

The technical solution for realizing the purpose of the invention is as follows: a control method of a star cascade STATCOM under the condition of power grid imbalance comprises the following steps:

step 1, constructing an equivalent three-phase star connection power supply, and calculating zero sequence voltage to enable active power of each phase to be zero so as to stabilize direct-current side voltage;

step 2, calculating power loss of the converter, controlling active power, and calculating improved zero sequence voltage;

step 3, tracking and controlling the current, and using the obtained line voltage of the equivalent power supply as an input source voltage as a corresponding converter voltage reference required by each phase branch;

step 4, designing a star cascade STATCOM controller;

and 5, analyzing the effective operation range of the star cascade STATCOM in an unbalanced state.

In the step 1, an equivalent three-phase star connection power supply is constructed, and the specific method for calculating the zero sequence voltage comprises the following steps:

step 1.1, determining the amplitude and the phase of the zero sequence voltage, wherein the specific formula is as follows:

(andabnormal sign)

(Andthe same number) wherein:

in the formula, voltage [ U_sa1，U_sb1，U_sc1]^TRepresenting a group of special zero-sequence component-free grid voltage phasors, the amplitudes of the A phase and the B phase are respectively U_sa1mAnd U_sb1mIn phases of respectivelyAnd andrespectively the phases of the reference current of the A-phase branch and the B-phase branch;

step 1.2, determining the three-phase voltage of the equivalent star connection power supply, wherein the specific formula is as follows:

in the formula of U_sam，U_sbm，U_scmAre the amplitudes of the three-phase voltages respectively,the phases of three-phase voltages are respectively;

in step 2, the active power is controlled, and the specific method for calculating the improved zero sequence voltage is as follows:

step 2.1, establishing a direct current side voltage controller consisting of three PI controls, wherein the output of the PI controls is the reference active power in each phase, and the output of the PI controls is not directly used as the input of a current control loop;

step 2.2, calculating the amplitude and phase of the improved zero sequence voltage, wherein the specific formula is as follows:

(andabnormal sign)

(Andthe same number) wherein:

in the formula, P_raAnd P_rbReference active power, I, for phase A and B, respectively_ramAnd I_rbmRespectively the amplitude of the reference current of the A-phase branch and the amplitude of the reference current of the B-phase branch;

in step 3, the specific method for tracking and controlling the current comprises the following steps: the neutral point of the equivalent power supply and the neutral point of the cascaded converters are actually connected. Thus, the star-connected STATCOM system can be decoupled into three single-phase systems. The current tracking controller can be designed according to the following equation:

u_tc＝3u₀-u_ta-u_tb

in the formula u_saAnd u_sbIs the line voltage of the equivalent power supply u_taAnd u_tbIs the converter output voltage i_aAnd i_bIs the STATCOM current, u₀Is the zero-sequence component of the equivalent supply voltage, u_t0Is the zero sequence component of the converter voltage.

In step 4, the specific method for designing the star cascade STATCOM controller is as follows:

step 4.1, phasor calculation, namely calculating the line voltage of the equivalent power supply by using a phasor representation method,control of the reference current and the dc link voltage is determined. Converting the obtained phasor into instantaneous value, and converting the instantaneous line voltage u_sa、u_sbAnd u_scZero sequence voltage u₀And a reference current i_ra、i_rbAnd i_rcAs input for the next stage;

and 4.2, current control, namely feeding back the STATCOM current to realize closed-loop current control, and tracking the reference current by adopting a dead-beat controller. The output of the current controller is a reference voltage normalized with respect to the corresponding dc side voltage;

and 4.3, modulating, and generating a gate signal according to the obtained reference voltage. And a phase-shifting PWM scheme based on a carrier wave is adopted to realize Pulse Width Modulation (PWM) of the cascade converter.

In step 5, the specific method for analyzing the effective operating range of the star cascade STATCOM in the unbalanced state is as follows:

step 5.1, analyzing the effective operation range of the star-shaped STATCOM in an unbalanced state in a power distribution system, and calculating the maximum amplitude of the unbalance and the three-phase voltage phasor, wherein the specific formula is as follows:

U_max＝max(U_sa,U_sb,U_sc)

wherein:

in the formula I_r1Is the amplitude of the positive sequence component of the reference current, I_r2Is the magnitude of the negative sequence component, U_pIs the nominal amplitude of the line voltage;

and 5.2, analyzing the effective operating range of the star STATCOM in an unbalanced state in the power transmission system, wherein for instantaneous faults, the controller designed by the invention does not need to take special measures to enable the converter to work in the effective operating range.

Compared with the prior art, the method has the advantages that the STATCOM is decoupled into three single-phase systems, so that the calculated amount of the controller is reduced; under the condition of unbalance, the reference current tracking and zero active power keeping of each phase can be realized simultaneously; the operation range of the system under the unbalanced condition is ensured, and the reliability of the system is improved; the method can be applied to the two conditions of unbalanced load and unbalanced voltage.

Drawings

Fig. 1 is a schematic diagram of a three-phase star-connected cascaded STATCOM system.

Fig. 2 is a schematic diagram of an equivalent connection between a STATCOM and an equivalent power supply according to the present invention.

FIG. 3 is a schematic diagram of a DC-side controller according to the present invention.

Fig. 4 is a control block diagram of the star cascade STATCOM of the present invention.

FIG. 5 is a waveform diagram of the load current simulation of the present invention.

Fig. 6 is a current simulation waveform diagram of the STATCOM of the present invention.

FIG. 7 is a simulated waveform diagram of the DC side voltage of the present invention.

Fig. 8 is a zero sequence voltage simulation waveform diagram of the present invention.

Fig. 9 is a simulation waveform diagram of the grid voltage of the present invention.

Fig. 10 is a three-phase active power simulation waveform diagram without zero sequence voltage injection.

Fig. 11 is a simulation waveform diagram of active power injected with zero sequence voltage according to the present invention.

Detailed Description

The technical scheme of the invention is further explained in detail by combining the attached drawings:

fig. 1 is a star-connected cascaded STATCOM system. The neutral point of the converter is floating. The neutral point of the power transformer and the load is not grounded if it is star-connected. Inductance of the power system is L_SSTATCOM interface inductorAnd L, the equivalent resistor R is used for representing the power loss of the STATCOM, and when the STATCOM is applied to a power transmission system, the STATCOM generates reactive current to regulate the voltage of a power grid. When applied to a power distribution system, a STATCOM is used to compensate for reactive and unbalanced currents generated on the load side.

Fig. 2 is a schematic diagram of an equivalent connection between a STATCOM and an equivalent power supply according to the present invention. When selecting the line voltage of the equivalent power supply, the line voltage of the connection point should remain unchanged. To meet this limitation, there are various possible three-phase line voltages, which can be expressed in phasor form as follows:

wherein the voltage [ U ]_sa1，U_sb1，U_sc1]^TRepresenting a particular set of grid voltage phasors without zero sequence components, may be calculated using the phase voltages measured at the connection points.

Obviously, U₀Is a zero sequence component, which can be expressed as follows:

therefore, the line voltage [ U ] is determined_sa，U_sb，U_sc]^TI.e. to determine the appropriate zero sequence voltage U₀。

The active power generated by the line voltage and the reference current of each phase of the equivalent power supply should be zero so as to satisfy the constraint that the average active power of each branch is zero. The average active power per phase can be expressed as:

and combining the constraint and vector analysis that the active power is zero to obtain the amplitude and the phase of the zero-sequence voltage.

The above analysis is performed assuming no converter power loss. However, in practical installations, a small amount of active power is required to compensate for the power loss of each phase to maintain the voltage of the dc link. The dc-side voltage control also helps to suppress transient overvoltages of the dc capacitor.

FIG. 3 is a schematic diagram of a DC-side controller according to the present invention. Establishing a direct current side voltage controller consisting of three PI controls, wherein the output of the PI controls is reference active power in each phase, the output of the PI controls is not directly used as the input of a current control loop, and the reference active power sum of the converter is obtained by the following method:

P_r＝P_ra+P_rb+P_rc

the amplitude of the active current is:

the zero sequence component U still needs to be determined₀. Generated reference active power P_ra、P_rbAnd P_rcFor calculating U₀Is input. The active power of each phase should be controlled at a target value P_ra、P_rbAnd P_rcRather than maintaining zero active power. The formula can be rewritten as:

U₀the amplitude and phase of (c) can be analytically calculated in a similar way.

Thus, U₀Can be expressed as

Where ω t is the synchronous rotation angle generated by the Phase Locked Loop (PLL). The three instantaneous voltages of the equivalent power supply can be expressed as:

the neutral point of the equivalent power supply and the neutral point of the cascaded converters are actually connected. Thus, the star-connected STATCOM system can be decoupled into three single-phase systems. Thus, the current tracking controller can be designed according to the following equation:

u_tc＝3u₀-u_ta-u_tb

by making u_t0Is equal to u₀The star cascade STATCOM is decoupled into three single phase systems. For each phase leg, the current controller may simply use the obtained line voltage of the equivalent power supply as the input source voltage and then give the corresponding converter voltage reference required for that phase leg.

Fig. 4 is a control block diagram of the star cascade STATCOM of the present invention, and the controller is divided into three parts according to its functions, phasor calculation, current control and PWM modulation.

Example 1

In order to verify the effectiveness of the invention, an experimental simulation model is built in PSCAD/EMTDC.

The actual parameters of the system are as follows: positive sequence active component I of load current_Lp1500A, positive sequence reactive component I of load current_Lq3000A, rated for line output voltage U_p8160V, reference DC side voltage U_dcrWhen the modulation ratio is 1.0, the converter can output the maximum voltage U at 10610V_max10610V, so U_max/U_p1.3, when I_r2/I_r1<At 32%, the converter will operate in the linear modulation range. When t is 0.06s, the negative sequence component I_L2Switching from 0 to 900A.

The simulation results are shown in fig. 5-11. From the load current, STATCOM current and dc voltage waveforms in fig. 5-7, the converter can operate in the linear modulation range; according to the zero sequence voltage and the voltage waveform of the power grid shown in fig. 8 and 9, the unbalanced fault occurs about 0.5 second, and at this time, the unbalanced load current can be completely compensated; even if instantaneous fault occurs, the voltage at the direct current side can be well controlled under the unbalanced compensation current, and the dynamic response capability is good. Comparing the three-phase active power waveforms before and after injecting the zero-sequence voltage as shown in fig. 10 and 11, it can be known that the selected equivalent power voltage U is used_sa、U_sbAnd U_scIn time, zero active power in each branch can be guaranteed. It can be seen that the stability range of the invention is analyzed accurately, and the control mode can reach the control target.

The above embodiments are merely illustrative of the technical ideas of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like based on the technical ideas of the present invention should be included in the scope of the present invention.