VSG negative sequence harmonic current control method under asymmetric fault of power grid

Abstract

The invention discloses a VSG negative sequence harmonic current control method under an asymmetric fault of a power grid, and the method comprises the steps: analyzing an instantaneous expression of VSG output power under an asymmetric working condition, and obtaining a quantitative relation between power frequency doubling fluctuation and positive and negative sequence voltage and current; introducing a direct current part of VSG reactive power output into a reactive power control ring, and constructing reactive power control ring control; positive sequence voltage current and negative sequence current are independently controlled in a positive and negative sequence separation mode; and unifying the positive sequence modulation signal and the negative sequence modulation signal to obtain a total PWM modulation wave signal, and setting a reference value for a negative sequence current loop, so that the VSG output current is balanced. According to the invention, the influence of power oscillation on the control effect of the VSG can be improved, the negative-sequence current harmonic waves can be suppressed, and the electric energy quality of the VSG is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of power grid power generation, and in particular to a method for controlling VSG negative-sequence harmonic current under asymmetric fault of the power grid. Background Art

[0002] As the power system ushered in a major transformation, the energy architecture gradually changed to be dominated by new energy, so the grid-connected converter controlled by virtual synchronous generator (VSG) gradually became an indispensable part of the new power system. It simulates the droop and swing characteristics of traditional synchronous motors, has voltage and frequency regulation characteristics, and gives virtual inertia and damping to the power system.

[0003] When an asymmetric fault occurs in the power grid, the VSG grid-connected point will contain negative-sequence voltage and negative-sequence current, and excessive negative-sequence current harmonics will have a serious impact on the power system; at the same time, when the positive and negative sequence voltages and currents are coupled, the VSG output power will produce double-frequency oscillations. Under the action of the power loop, the double-frequency oscillations are transmitted in the voltage and current inner loop control structure, seriously deteriorating the control effect and reducing the power quality of the VSG output.

[0004] Therefore, a method for controlling negative-sequence harmonic current of VSG under asymmetric grid fault is needed, which can improve the influence of power oscillation on the control effect of VSG and suppress negative-sequence current harmonics, thereby greatly improving the power quality of VSG. Summary of the invention

[0005] In view of this, the purpose of the present invention is to overcome the defects in the prior art and provide a VSG negative-sequence harmonic current control method under asymmetric grid faults, which can improve the influence of power oscillation on the VSG control effect, and at the same time can suppress negative-sequence current harmonics, thereby greatly improving the power quality of VSG.

[0006] The VSG negative sequence harmonic current control method under asymmetric power grid fault of the present invention comprises:

[0007] The instantaneous expression of VSG output power under asymmetric working conditions is analyzed, and the quantitative relationship between power double frequency fluctuation and positive and negative sequence voltage and current is obtained;

[0008] Introduce the DC part of VSG reactive output into reactive control loop to build reactive control loop control;

[0009] The positive sequence voltage and current and the negative sequence current are controlled separately by separating the positive and negative sequences;

[0010] The positive sequence modulation signal and the negative sequence modulation signal are unified to obtain the total PWM modulation wave signal, and a reference value is set for the negative sequence current loop to balance the VSG output current.

[0011] Furthermore, the quantitative relationship between the power double frequency fluctuation and the positive and negative sequence voltage and current is obtained, including:

[0012] According to the instantaneous power theory, the relationship between VSG instantaneous power output and positive and negative sequence current is obtained:

[0013]

[0014] Among them, P is the active power output by VSG, Q is the reactive power output by VSG, i d and i q are the output current of VSG in dq coordinate system, u d and u q is the output voltage of VSG in the dq coordinate system;

[0015] Considering the asymmetric fault of the power grid, the VSG output power consists of a DC component and a double frequency fluctuation, so it has the following form:

[0016]

[0017] Among them, P0 is the DC part of VSG active output, P c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG active double frequency oscillation, P s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the active double frequency oscillation; Q0 is the DC part of VSG reactive output, Q c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG reactive double frequency oscillation, Q s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the reactive double frequency oscillation, ω is the VSG output angular frequency;

[0018] According to the expression of instantaneous power theory in the dq coordinate system, the following matrix can be obtained:

[0019]

[0020] Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

[0021] Furthermore, in the reactive power control loop control, the reactive power control loop control equation is:

[0022] (Q ref -Q0)·k q +E0=E m ;

[0023] Among them, Q0 is the DC part of the reactive power output by VSG, Q ref is the reactive output reference value of VSG in the controller, k q is the droop coefficient of the VSG reactive power control loop, E0 is the voltage reference value in the reactive power control loop, and E m It is the reactive loop output.

[0024] Furthermore, the expression of the DC part Q0 of the reactive power output by the VSG is:

[0025] Q0=1.5(u qp i dp -u dp i qp +u qn i dn -u dn i qn );

[0026] Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

[0027] Furthermore, the positive sequence voltage and current are controlled separately, including:

[0028] The original voltage and current double inner loop of VSG is modified to positive sequence voltage and current double inner loop control, and the control equation is:

[0029]

[0030] in, and are respectively the positive sequence current inner loop reference values ​​of the positive sequence voltage loop output on the d-axis and q-axis, k vp and k vi are the proportional coefficient and integral coefficient of the voltage loop PI controller, k cp and k ci is the proportional coefficient and integral coefficient of the current loop PI controller, ω0 is the rated angular frequency of the power grid, Cf is the filter capacitor, L f is the filter inductor, E m is the reactive power control loop output, u dp and u qp is the component of the positive sequence voltage at the grid-connected point of the converter in the dq coordinate system; m dp and m qp It is the dq-axis component of the positive-sequence modulation wave signal output by the positive-sequence current loop.

[0031] Furthermore, the negative sequence current is controlled separately, specifically including:

[0032] The negative sequence current loop is used to control the negative sequence current separately. The negative sequence current loop equation is:

[0033]

[0034] Among them, i dn and i qn are the outputs of the negative sequence current d-axis and q-axis, k cpn and k cin are the proportional coefficient and integral coefficient of the negative sequence current loop PI controller, ω0 is the rated angular frequency of the power grid, L f is the filter inductor, and is the dq axis reference value of the negative sequence current, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

[0035] Furthermore, the total PWM modulation wave signal is:

[0036]

[0037] Among them, m d and m q is the component of the VSG modulated wave signal on the dq axis, m dp and m qp is the dq axis component of the positive sequence modulation wave signal output by the positive sequence current loop, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

[0038] Furthermore, the reference value of the negative sequence current loop setting is:

[0039]

[0040] in, and is the dq axis reference value of the negative sequence current.

[0041] The beneficial effects of the present invention are as follows: a method for controlling negative-sequence harmonic current of a VSG under an asymmetric power grid fault disclosed by the present invention obtains a quantitative relationship between each power content composition and the positive and negative sequence current and voltage according to the instantaneous power theory, thereby improving the reactive power control loop, eliminating the influence of the reactive power double frequency fluctuation on the inner loop control, and optimizing the control effect to a great extent; at the same time, the positive and negative sequence voltage and current are separated to achieve separate control. Compared with traditional control, this method realizes separate control of the negative sequence current output by the VSG through the negative sequence current loop, thereby completely eliminating the negative sequence harmonic current output by the VSG under an asymmetric power grid fault, improving the imbalance of the power system, and improving the output power quality of the VSG. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0043] Figure 1 It is a schematic diagram of the negative sequence harmonic current control method of the present invention;

[0044] Figure 2 The control topology diagram of the grid-type converter of the present invention;

[0045] Figure 3 It is a schematic diagram of the improved reactive loop control of the present invention;

[0046] Figure 4 This is a schematic diagram of VSG control after positive and negative sequence separation of the present invention. DETAILED DESCRIPTION

[0047] The present invention is further described below in conjunction with the accompanying drawings, as shown in the drawings:

[0048] This embodiment discloses a method for controlling VSG negative sequence harmonic current under a grid asymmetric fault, including the following steps:

[0049] S1. Analyze the instantaneous expression of VSG output power under asymmetric working conditions and obtain the quantitative relationship between power double frequency fluctuation and positive and negative sequence voltage and current;

[0050] S2. Introduce the DC part of the VSG reactive output into the reactive control loop to build a reactive control loop control;

[0051] S3. Separately control the positive sequence voltage and current and the negative sequence current by separating the positive and negative sequences;

[0052] S4. Unify the positive sequence modulation signal and the negative sequence modulation signal to obtain the total PWM modulation wave signal, set the reference value for the negative sequence current loop, so that the VSG output current is balanced.

[0053] In this embodiment, in step S1, according to Figure 2The electrical topology shown in the figure, combined with the instantaneous power theory, gives the relationship between the VSG instantaneous power output and the positive and negative sequence currents as follows:

[0054]

[0055] Among them, P is the active power output by VSG, Q is the reactive power output by VSG, i d and i q are the output current of VSG in the dq coordinate system, u d and u q is the output voltage of VSG in the dq coordinate system;

[0056] Then, considering the asymmetric fault of the power grid, the VSG output power can be divided into DC output and double frequency AC oscillation, so its expression can be divided into the following three parts:

[0057]

[0058] Among them, P0 is the DC part of VSG active output, P c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG active double frequency oscillation, P s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the active double frequency oscillation; Q0 is the DC part of VSG reactive output, Q c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG reactive double frequency oscillation, Q s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the reactive double frequency oscillation, ω is the VSG output angular frequency;

[0059] Then, the quantitative relationship between output power and positive and negative sequence voltage and current is analyzed through instantaneous power theory. According to the expression of instantaneous power theory in the dq coordinate system, the following matrix can be obtained:

[0060]

[0061] Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

[0062] In this embodiment, in step S2, the DC part of the VSG reactive output is introduced into the reactive control loop, thereby improving the reactive loop control and reducing the influence of the reactive power double frequency fluctuation on the VSG control effect.

[0063] According to the expression of the DC output part Q0 of the reactive component in step S1, combined with Figure 3 The control of the improved reactive loop shown in the figure can obtain the reactive control loop control output equation as follows:

[0064] (Q ref -Q0)·k q +E0=E m ;

[0065] Among them, Q0 is the DC part of the reactive power output by VSG, Q ref is the reactive output reference value of VSG in the controller, k q is the droop coefficient of the VSG reactive power control loop, E0 is the voltage reference value in the reactive power control loop, and E m It is the reactive loop output.

[0066] The expression of the DC part Q0 of the reactive power output by VSG is:

[0067] Q0=1.5(u qp i dp -u dp i qp +u qn i dn -u dn i qn );

[0068] Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

[0069] In this embodiment, in step S3, the original voltage and current double inner loop of the VSG is modified to a positive sequence voltage and current double inner loop control to achieve separate control of the positive sequence voltage and current, such as Figure 4 As shown, the control equation is as follows:

[0070]

[0071] in, and are respectively the positive sequence current inner loop reference values ​​of the positive sequence voltage loop output on the d-axis and q-axis, k vp and k vi are the proportional coefficient and integral coefficient of the voltage loop PI controller, k cp and k ci is the proportional coefficient and integral coefficient of the current loop PI controller, ω0 is the rated angular frequency of the power grid, C f is the filter capacitor, L f is the filter inductor, E m is the reactive power control loop output, u dp and u qp is the component of the positive sequence voltage at the grid-connected point of the converter in the dq coordinate system; m dp and m qp It is the dq-axis component of the positive-sequence modulation wave signal output by the positive-sequence current loop.

[0072] In this embodiment, the Figure 4 The negative sequence current loop control shown in FIG. 1 uses the negative sequence current loop to control the negative sequence current separately. The negative sequence current loop equation is:

[0073]

[0074] Among them, i dn and i qn are the outputs of the negative sequence current d-axis and q-axis, k cpn and k cin are the proportional coefficient and integral coefficient of the negative sequence current loop PI controller, ω0 is the rated angular frequency of the power grid, L f is the filter inductor, and is the dq axis reference value of the negative sequence current, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

[0075] In this embodiment, the total PWM modulation wave signal obtained by combining the positive and negative sequence modulation signals obtained in the above steps is:

[0076]

[0077] Among them, m d and m q is the component of the VSG modulated wave signal on the dq axis, m dp and m qp is the dq axis component of the positive sequence modulation wave signal output by the positive sequence current loop, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

[0078] In order to completely eliminate the negative sequence current harmonics of VSG under the asymmetric fault of the power grid, the reference value of the negative sequence current loop is taken as follows:

[0079]

[0080] in, and is the dq-axis reference value of the negative sequence current.

[0081] The present invention comprehensively considers the impact of asymmetric faults on the transient characteristics of the VSG. According to the instantaneous power theory in the three-phase circuit, the quantitative relationship expression between different power components and the positive and negative sequence currents is clarified, and the reactive power control loop is optimized based on the relationship, eliminating the impact of reactive double frequency fluctuations on the inner loop control effect; then the positive sequence voltage and current and the negative sequence current are controlled separately, and the reference value of the negative sequence current is set to 0, thereby eliminating the impact of the negative sequence harmonic current on the power system.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid, characterized by: include: The instantaneous expression of VSG output power under asymmetric working conditions is analyzed, and the quantitative relationship between power double frequency fluctuation and positive and negative sequence voltage and current is obtained; Introduce the DC part of VSG reactive output into reactive control loop to build reactive control loop control; The positive sequence voltage and current and the negative sequence current are controlled separately by separating the positive and negative sequences; The positive sequence modulation signal and the negative sequence modulation signal are unified to obtain the total PWM modulation wave signal, and a reference value is set for the negative sequence current loop to balance the VSG output current.

2. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: The quantitative relationship between power double frequency fluctuation and positive and negative sequence voltage and current is obtained, including: According to the instantaneous power theory, the relationship between VSG instantaneous power output and positive and negative sequence current is obtained: Among them, P is the active power output by VSG, Q is the reactive power output by VSG, i d and i q are the output current of VSG in dq coordinate system, u d and u q is the output voltage of VSG in the dq coordinate system; Considering the asymmetric fault of the power grid, the VSG output power consists of a DC component and a double frequency fluctuation, so it has the following form: Among them, P0 is the DC part of VSG active output, P c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG active double frequency oscillation, P s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the active double frequency oscillation; Q0 is the DC part of VSG reactive output, Q c2 is the amplitude of the coupled part of positive sequence current and negative sequence voltage in the VSG reactive double frequency oscillation, Q s2 is the amplitude of the coupled part of positive sequence voltage and negative sequence current in the reactive double frequency oscillation, ω is the VSG output angular frequency; According to the expression of instantaneous power theory in the dq coordinate system, the following matrix can be obtained: Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

3. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: In the reactive control loop control, the reactive control loop control equation is: (Q ref -Q0)·k q +E0=E m ; Among them, Q0 is the DC part of the reactive power output by VSG, Q ref is the reactive output reference value of VSG in the controller, k q is the droop coefficient of the VSG reactive power control loop, E0 is the voltage reference value in the reactive power control loop, and E m It is the reactive loop output.

4. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 3 is characterized in that: The expression of the DC part Q0 of the reactive power output by VSG is: Q0=1.5(u qp and dp -at dp and qp +u qn and dn -at dn and qn ); Among them, u dp is the d-axis component of the positive sequence voltage, u qp is the q-axis component of the positive sequence voltage, u dn is the d-axis component of the negative sequence voltage, u qn is the q-axis component of the negative sequence voltage; i dp is the d-axis component of the positive sequence current, i qp is the q-axis component of the positive sequence current, i dn is the d-axis component of the negative sequence current, i qn is the q-axis component of the negative sequence current.

5. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: Separate control of positive sequence voltage and current, including: The original voltage and current double inner loop of VSG is modified to positive sequence voltage and current double inner loop control, and the control equation is: in, and are respectively the positive sequence current inner loop reference values ​​of the positive sequence voltage loop output on the d-axis and q-axis, k vp and k vi are the proportional coefficient and integral coefficient of the voltage loop PI controller, k cp and k ci is the proportional coefficient and integral coefficient of the current loop PI controller, ω0 is the rated angular frequency of the power grid, C f is the filter capacitor, L f is the filter inductor, E m is the reactive power control loop output, u dp and u qp is the component of the positive sequence voltage at the grid-connected point of the converter in the dq coordinate system; m dp and m qp It is the dq-axis component of the positive-sequence modulation wave signal output by the positive-sequence current loop.

6. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: The negative sequence current is controlled separately, including: The negative sequence current loop is used to control the negative sequence current separately. The negative sequence current loop equation is: Among them, i dn and i qn are the outputs of the negative sequence current d-axis and q-axis, k cpn and k cin are the proportional coefficient and integral coefficient of the negative sequence current loop PI controller, ω0 is the rated angular frequency of the power grid, L f is the filter inductor, and is the dq axis reference value of the negative sequence current, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

7. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: The total PWM modulation wave signal is: Among them, m d and m q is the component of the VSG modulated wave signal on the dq axis, m dp and m qp is the dq axis component of the positive sequence modulation wave signal output by the positive sequence current loop, m dn and m qn It is the dq-axis component of the negative-sequence modulation wave signal output by the negative-sequence current loop.

8. The method for controlling VSG negative sequence harmonic current under asymmetric fault of power grid according to claim 1, characterized in that: The reference value for the negative sequence current loop setting is: in, and is the dq-axis reference value of the negative sequence current.

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