Multi-stage DC current fluctuation suppression method based on PI regulator

Abstract

The present invention provides a multi-stage DC current fluctuation suppression method based on a PI regulator, comprising the following steps: S1. determining the topological structure of a DC system based on a modular multilevel converter, S2. judging whether an AC asymmetric fault occurs in the DC system based on the modular multilevel converter, and if so, dividing the AC asymmetric fault into three stages according to time nodes, S3. determining the current input reference value of the PI regulator in each stage of the AC asymmetric fault in step S2; S4. the PI regulator performs adjustment processing according to the current input reference values ​​of different stages of the input, and outputs a corresponding control signal to the modular multilevel converter to suppress the DC current fluctuation of the DC system based on the modular multilevel converter; making full use of the superiority of the PI regulator in tracking the DC component, thereby effectively suppressing the current fluctuation and ensuring the safe and stable operation of the non-fault side in the DC system.

Description

Technical Field

[0001] The invention relates to a current fluctuation suppression method, in particular to a multi-stage direct current fluctuation suppression method based on a PI regulator. Background Art

[0002] As a new generation of power transmission technology, flexible DC transmission based on modular multilevel converter (MMC) has become a strong support in the field of power transmission due to its irreplaceable role in the fields of new energy grid connection, asynchronous interconnection of AC power grids, and power supply to isolated offshore islands. When an asymmetric fault occurs in the AC system connected to the MMC, the fault will propagate from the DC side to the non-fault side, causing fluctuations in the DC current, which in turn affects the safe and stable operation of the non-fault side.

[0003] However, there is no effective means in the prior art to effectively solve the above problems. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a multi-stage DC current fluctuation suppression method based on a PI regulator, which makes full use of the superiority of the PI regulator in tracking the DC component, divides the stages after the fault occurs into a fault occurrence stage, a stabilization stage after the fault disappears, and a completely stable stage after the fault disappears, and calculates the input reference value of the PI regulator for the first two stages respectively, and shuts down the PI regulator in the last stage, thereby effectively suppressing the current fluctuation and ensuring the safe and stable operation of the non-fault side in the DC system.

[0005] The present invention provides a multi-stage direct current fluctuation suppression method based on a PI regulator, comprising the following steps:

[0006] S1. Determine a topology of a DC system based on a modular multilevel converter, wherein the topology includes an output transformer and a modular multilevel converter, wherein the primary side of the output transformer is connected to the output end of the modular multilevel converter, and the secondary side of the output transformer is connected to the AC system;

[0007] S2. Determine whether an AC asymmetric fault occurs in the DC system based on the modular multilevel converter. If so, divide the AC asymmetric fault into three stages according to the time nodes, including the AC asymmetric fault occurrence stage, the DC current stabilization stage after the AC asymmetric fault disappears, and the DC current complete stability stage after the AC asymmetric fault disappears;

[0008] S3. Determine the current input reference value of the PI regulator at each stage of the AC asymmetric fault in step S2;

[0009] S4. The PI regulator performs regulation processing according to the current input reference values ​​at different stages of the input, and outputs corresponding control signals to the modular multilevel converter to suppress the DC current fluctuation of the DC system based on the modular multilevel converter.

[0010] Further, in step S3, the A-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage is determined by the following formula:

[0011]

[0012] in, is the A-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage, U is the voltage amplitude on the secondary side of the output transformer in the DC system during steady-state operation, a is the positive sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, b is the negative sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, I + represents the amplitude of the phase current at the AC output of the modular multilevel converter, δ is the angle at which the primary voltage of the output transformer in the DC system leads the secondary voltage, is the phase angle of the negative sequence component relative to the positive sequence component of the output transformer in the DC system after an AC fault occurs, θ is the initial phase of the phase current at the AC output end of the modular multilevel converter, U dc is the DC side voltage in the DC system, and n is the transformation ratio of the output transformer.

[0013] Further, in step S3, the B-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage is determined by the following formula:

[0014]

[0015] in, is the B-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage, U is the voltage amplitude on the secondary side of the output transformer in the DC system during steady-state operation, a is the positive sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, b is the negative sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, I + represents the amplitude of the phase current at the AC output of the modular multilevel converter, δ is the angle at which the primary voltage of the output transformer in the DC system leads the secondary voltage, is the phase angle of the negative sequence component relative to the positive sequence component of the output transformer in the DC system after an AC fault occurs, θ is the initial phase of the phase current at the AC output end of the modular multilevel converter, U dc is the DC side voltage in the DC system, and n is the transformation ratio of the output transformer.

[0016] Further, in step S3, the C-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage is determined by the following formula:

[0017]

[0018] in, is the C-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage, U is the voltage amplitude on the secondary side of the output transformer in the DC system during steady-state operation, a is the positive sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, b is the negative sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, I + represents the amplitude of the phase current at the AC output of the modular multilevel converter, δ is the angle at which the primary voltage of the output transformer in the DC system leads the secondary voltage, is the phase angle of the negative sequence component relative to the positive sequence component of the output transformer in the DC system after an AC fault occurs, θ is the initial phase of the phase current at the AC output end of the modular multilevel converter, U dc is the DC side voltage in the DC system, and n is the transformation ratio of the output transformer.

[0019] Further, in step S3, the A, B and C phase current reference values ​​input to the PI regulator in the DC current stabilization stage after the AC asymmetric fault disappears are determined by the following formula:

[0020]

[0021] in, and are the A, B and C phase current reference values ​​input to the PI regulator during the DC current stabilization stage after the AC asymmetric fault disappears, U dc is the DC side voltage in the DC system, P s2_ref It is the given active power reference value input to the modular multilevel converter by the AC system.

[0022] Beneficial effects of the present invention: Through the present invention, the superiority of the PI regulator in tracking the DC component is fully utilized, and the stage after the fault occurs is divided into a fault occurrence stage, a stabilization stage after the fault disappears, and a completely stable stage after the fault disappears. The input reference value of the PI regulator is calculated for the first two stages respectively, and the PI regulator is shut down in the last stage, thereby effectively suppressing the current fluctuation and ensuring the safe and stable operation of the non-fault side in the DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0024] Figure 1 It is a flow chart of the present invention.

[0025] Figure 2 It is a schematic diagram of the topological structure of the present invention.

[0026] Figure 3 The waveform diagram of the DC current in the case of an AC asymmetric fault (single-phase fault) according to the present invention and the prior art.

[0027] Figure 4 This is a waveform diagram of the DC current during an AC asymmetric fault (two-phase fault) of the present invention and the new technology. DETAILED DESCRIPTION

[0028] The present invention is further described below with reference to the accompanying drawings:

[0029] The present invention provides a multi-stage direct current fluctuation suppression method based on a PI regulator, comprising the following steps:

[0030] S1. Determine a topology of a DC system based on a modular multilevel converter, wherein the topology includes an output transformer and a modular multilevel converter, wherein a primary side of the output transformer is connected to an output end of the modular multilevel converter, and a secondary side of the output transformer is connected to an AC system; Figure 1 In the example, the topology is set to a double-terminal topology, that is, it has two modular multilevel converters (MCC1 and MCC2) and two output transformers (T1 and T2), wherein the two modular multilevel converters are electrically connected. When electric energy is transmitted, the electric energy of the AC system 1 is transmitted to the modular multilevel converter MCC1 through the transformer T1, and then is processed by the modular multilevel converter MCC2 and transmitted to the AC system 2 through the transformer T2. Of course, the electric energy of the AC system 2 can be transmitted back to the AC system 1, thereby forming a double-terminal topology. In the following specific example, the fault is set at the secondary side of the transformer T2 (i.e., the PCC point) as an example;

[0031] S2. Determine whether an AC asymmetric fault occurs in the DC system based on the modular multilevel converter. If so, divide the AC asymmetric fault into three stages according to the time nodes, including the AC asymmetric fault occurrence stage, the DC current stabilization stage after the AC asymmetric fault disappears, and the DC current complete stability stage after the AC asymmetric fault disappears;

[0032] S3. Determine the current input reference value of the PI regulator at each stage of the AC asymmetric fault in step S2;

[0033] S4. The PI regulator performs regulation processing according to the current input reference value of different input stages, and outputs corresponding control signals to the modular multilevel converter to suppress the DC current fluctuation of the DC system based on the modular multilevel converter. How the PI regulator outputs the corresponding control signal according to the current input reference value can be achieved by the existing technology, which will not be elaborated here. Through the above method, the superiority of the PI regulator in tracking the DC component is fully utilized, and the stages after the fault occurs are divided into the fault occurrence stage, the stabilization stage after the fault disappears, and the complete stability stage after the fault disappears. The input reference values ​​of the PI regulator are calculated for the first two stages respectively, and the PI regulator is shut down in the last stage, thereby effectively suppressing the current fluctuation and ensuring the safe and stable operation of the non-fault side of the DC system.

[0034] In this embodiment, in step S3, the A, B and C phase current reference values ​​input to the PI regulator during the AC asymmetric fault occurrence phase are determined by the following formula:

[0035]

[0036]

[0037]

[0038] in, It is the A-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage. It is the B-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage. is the C-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage, U is the voltage amplitude on the secondary side of the output transformer in the DC system during steady-state operation, a is the positive sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, b is the negative sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, I + represents the amplitude of the phase current at the AC output of the modular multilevel converter, δ is the angle at which the primary voltage of the output transformer in the DC system leads the secondary voltage, is the phase angle of the negative sequence component relative to the positive sequence component of the output transformer in the DC system after an AC fault occurs, θ is the initial phase of the phase current at the AC output end of the modular multilevel converter, U dc is the DC side voltage in the DC system, and n is the transformation ratio of the output transformer.

[0039] Among them, the specific solution process of the A, B and C phase current reference values ​​input to the PI regulator during the AC asymmetric fault occurrence stage is as follows:

[0040] When a fault occurs on the secondary side of the output transformer, the voltage here (at PCC) is determined by the following formula:

[0041] Among them, u sa 、u sb and u sc Respectively represent the phase voltages of phase A, phase B and phase C at PCC, They represent the positive-sequence component and negative-sequence component of the A-phase voltage at PCC respectively; They represent the positive-sequence component and negative-sequence component of the B-phase voltage at PCC respectively; They represent the positive sequence component and negative sequence component of the C phase voltage at the PCC respectively; at this time, the voltage at the AC output end of the modular multilevel converter is determined by the following formula:

[0042]

[0043] Among them, u va 、u vb and u vc Respectively represent the A-phase, B-phase and C-phase AC output voltages at the output end of the modular multilevel converter; They represent the positive-sequence component and negative-sequence component of the A-phase AC outlet voltage of the modular multilevel converter respectively; They represent the positive sequence component and negative sequence component of the B-phase AC outlet voltage of the modular multilevel converter respectively; They represent the positive-sequence component and negative-sequence component of the C-phase AC output voltage of the modular multilevel converter respectively; ω is the fundamental angular frequency;

[0044] Assuming that the negative sequence fault current of the AC output port of the modular multilevel converter is completely consistent, the current of the AC output port of the modular multilevel converter is determined by the following formula:

[0045]

[0046] Among them, i va 、i vb and i vc They represent the A-phase, B-phase and C-phase AC output currents of the modular multilevel converter respectively; I + Indicates the amplitude of the AC output phase current of the modular multilevel converter;

[0047] The average transmission power of the modular multilevel converter is determined by the following formula:

[0048]

[0049] Where j = a, b, c, representing phases A, B, and C respectively, P af , P bf and Pcf Represent the average transmission power of phase A, phase B and phase C respectively; and They represent the average transmission power generated by the positive sequence voltage at the A phase AC outlet and the average transmission power generated by the negative sequence voltage at the A phase AC outlet respectively; and They represent the average transmission power generated by the positive sequence voltage at the B-phase AC outlet and the average transmission power generated by the negative sequence voltage at the B-phase AC outlet respectively; and They represent the average transmission power generated by the positive sequence voltage at the C-phase AC outlet and the average transmission power generated by the negative sequence voltage at the C-phase AC outlet respectively;

[0050] At this time, the DC current I of the jth bridge arm of the modular multilevel converter is dcj Determined by the following formula:

[0051]

[0052] Among them, I dca ,I dcb and I dcc Represents the bridge arm DC current of phase A, phase B and phase C respectively; P af , P bf and P cf Represent the total average transmission power of phase A, phase B and phase C respectively; and They represent the average transmission power generated by the positive sequence voltage at the A phase AC outlet and the average transmission power generated by the negative sequence voltage at the A phase AC outlet respectively; and They represent the average transmission power generated by the positive sequence voltage at the B-phase AC outlet and the average transmission power generated by the negative sequence voltage at the B-phase AC outlet respectively; and They represent the average transmission power generated by the positive sequence voltage of the C phase AC outlet and the average transmission power generated by the negative sequence voltage of the C phase AC outlet respectively; after rearranging formula (5), it becomes the input current reference value input to the PI regulator.

[0053] In this embodiment, in step S3, the A, B and C phase current reference values ​​input to the PI regulator during the DC current stabilization stage after the AC asymmetric fault disappears are determined by the following formula:

[0054]

[0055] in, and are the A, B and C phase current reference values ​​input to the PI regulator during the DC current stabilization stage after the AC asymmetric fault disappears, Udc is the DC side voltage in the DC system, P s2_ref is the given active power reference value input from the AC system to the modular multilevel converter; of course, there is a third stage, that is, the DC current is completely stable after the AC asymmetric fault disappears. In this stage, the PI regulator exits the control of the modular multilevel converter, that is, the PI regulator requires neither input signal nor output signal, that is, the input and output of the PI regulator are both 0.

[0056] The present invention is further described in detail below with an example:

[0057] The fault time is set at 0.65s, the fault duration is 0.1s, and the system is in normal operation before 0.65s;

[0058] from Figure 3 It can be seen from the figure: the original fault waveform when the proposed control is not added, the DC current simulation waveform when the traditional double frequency control is added and the proposed multi-stage control is added. By comparing the three waveforms, it can be seen that after the fault occurs, the DC current shows obvious double frequency fluctuations; after adding the traditional control and the control strategy of this application, the fluctuations of the DC current are weakened, and the waveform becomes smoother; compared with the addition of the traditional double frequency circulating current suppression strategy, after adding the multi-stage control strategy of this application, the fluctuation amplitude of the DC current is weakened during the fault period (0.65s-0.75s) and the stabilization process after the fault disappears (0.75s-0.85s), and the value during the fault period is basically maintained at a small change around -1kA, which proves that the proposed multi-stage DC current fluctuation suppression method based on PI regulator has excellent effects on DC current fluctuation suppression during the fault period and the fault disappearance stabilization stage.

[0059] from Figure 4 It can be seen that after the fault occurs, the control strategy of the present application (represented as the proposed control in the attached figure) plays a significant role in suppressing the DC current, and its suppression effect is better than the traditional double frequency circulating current suppression. After adding the traditional double frequency circulating current control, the maximum change amplitude of the DC current is 0.72kA and the fluctuation is still relatively obvious. After adding the control strategy of the present application, the DC current almost becomes a straight line without fluctuations. In addition, in the stabilization stage of 0.75-0.8s after the fault disappears, the fluctuation degree of the DC current is also weakened.

[0060] 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 multi-stage DC current fluctuation suppression method based on PI regulator, characterized in that: The following steps are involved: S1. Determine a topology of a DC system based on a modular multilevel converter, wherein the topology includes an output transformer and a modular multilevel converter, wherein the primary side of the output transformer is connected to the output end of the modular multilevel converter, and the secondary side of the output transformer is connected to the AC system; S2. Determine whether an AC asymmetric fault occurs in the DC system based on the modular multilevel converter. If so, divide the AC asymmetric fault into three stages according to the time nodes, including the AC asymmetric fault occurrence stage, the DC current stabilization stage after the AC asymmetric fault disappears, and the DC current complete stability stage after the AC asymmetric fault disappears; S3. Determine the current input reference value of the PI regulator at each stage of the AC asymmetric fault in step S2; S4. The PI regulator performs regulation processing according to the current input reference values ​​at different stages of the input, and outputs a corresponding control signal to the modular multilevel converter to suppress the DC current fluctuation of the DC system based on the modular multilevel converter; In step S3, the A-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence phase is determined by the following formula: in, is the A-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage, U is the voltage amplitude on the secondary side of the output transformer in the DC system during steady-state operation, a is the positive sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, b is the negative sequence component amplitude of the secondary side voltage of the output transformer in the DC system during AC asymmetric fault, I + represents the amplitude of the phase current at the AC output of the modular multilevel converter, δ is the angle at which the primary voltage of the output transformer in the DC system leads the secondary voltage, is the phase angle of the negative sequence component relative to the positive sequence component of the output transformer in the DC system after an AC fault occurs, θ is the initial phase of the phase current at the AC output end of the modular multilevel converter, U dc is the DC side voltage in the DC system, n is the transformation ratio of the output transformer; In step S3, the B-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence phase is determined by the following formula: in, It is the B-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage; In step S3, the C-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence phase is determined by the following formula: in, It is the C-phase current reference value input to the PI regulator during the AC asymmetric fault occurrence stage; In step S3, the A, B and C phase current reference values ​​input to the PI regulator during the DC current stabilization phase after the AC asymmetric fault disappears are determined by the following formula: in, and are the A, B and C phase current reference values ​​input to the PI regulator in the DC current stabilization stage after the AC asymmetric fault disappears, respectively. s2_ref It is the given active power reference value input to the modular multilevel converter by the AC system.

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