Direct current neutral line stability control method and control system of NPC three-level three-phase four-wire system PCS

Abstract

The invention provides an NPC three-level three-phase four-wire PCS DC neutral line stability control method and control system, which is applied to the technical field of energy storage systems, and comprises a DC voltage control module, the DC voltage control module comprises an outer loop control system and an inner loop control system, the outer loop control system is used for carrying out voltage stabilization control on DC bus voltage, and the inner loop control system is used for carrying out voltage stabilization control on DC bus voltage; the inner loop control system is used for carrying out voltage-sharing control on the direct-current middle line voltage; an outer loop control process: calculating a difference value between the direct-current bus voltage and the reference voltage to obtain a direct-current error signal; a voltage stabilizing signal is obtained through the outer loop control system; and an inner loop control process: calculating a difference value between the upper discrete voltage and the lower discrete voltage to obtain a midpoint voltage, and obtaining a voltage-sharing signal after the midpoint voltage passes through an inner loop control system. In the outer loop control and the inner loop control, a second-order low-pass filter is respectively used for compensation so as to guarantee the stability of the DC bus voltage and realize the voltage-sharing control of the DC middle line voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage systems, and in particular relates to a DC neutral line stabilization control method and a control system for an NPC three-level three-phase four-wire PCS. Background Art

[0002] With the rapid development of the new energy storage industry, three-level power modular PCS based on diode embedding has become the mainstream solution, but most of them adopt three-phase three-wire system, and three-phase four-wire system is rarely used. In actual applications, it is often necessary to combine grid-connected and off-grid needs, especially in Africa and Europe. It is necessary to provide three-phase four-wire PCS to meet the energy storage needs of three-phase and single-phase loads at the same time.

[0003] like Figure 1 In the three-level three-phase four-wire PCS of type I shown, the upper and lower groups of discrete DC capacitors in the circuit are connected in series to form a high-voltage DC bus, which needs to be controlled to stabilize it. Because the DC side circuit is composed of discrete capacitors, and the neutral point is connected to the N line on the AC side, considering the factors such as the fact that the instantaneous active power distribution cannot be absolutely symmetrical and the current flowing through the N line, it is inevitable that the midpoint will be offset, causing the voltage on both sides of the upper and lower DC capacitors to be unbalanced, which is a key factor affecting the performance of the three-level NPC type PCS. Therefore, it is necessary to add voltage balancing control to balance the terminal voltage of the upper and lower DC capacitors.

[0004] In the prior art, in the DC voltage control process, PI controller or PID controller is mostly used for compensation and regulation. However, in a specific energy storage system, the compensation effect of PI controller or PID controller is not good. Summary of the invention

[0005] In view of the above problems in the prior art, the purpose of the present invention is to provide a DC neutral line stabilization control method for an NPC three-level three-phase four-wire PCS, in which a second-order low-pass filter is used for compensation in the outer loop control and the inner loop control respectively to ensure the stability of the DC bus voltage and realize the equalization control of the DC neutral line voltage.

[0006] A DC neutral line stabilization control method for an NPC three-level three-phase four-wire PCS, wherein the DC bus voltage is stabilized by an outer loop control and the DC neutral line voltage is balanced by an inner loop control; The outer loop control process specifically includes the following steps: Calculate the DC bus voltage and reference voltage The DC error signal is obtained by taking the difference between ; DC error signal After passing through the outer loop control system, the voltage stabilization signal is obtained , the regulated voltage signal The calculation formula is: ,in, is the open-loop transfer function of the DC voltage regulator controller of the outer loop control system, and its expression is: ; in, For compensation network controller 1, its transfer function is: , is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, ; The inner loop control process specifically includes the following steps: Calculate the upper discrete voltage and the lower discrete voltage The midpoint voltage is obtained by taking the difference between ; Midpoint voltage After passing through the inner loop control system, the pressure equalization signal is obtained , balanced signal The calculation formula is ,in, is the transfer function of the voltage equalization controller in the inner loop control system, and its expression is: ; in, To compensate the network controller II, The transfer function is: ; is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0007] Preferably, the construction process of the DC voltage regulator controller includes the following steps: The circuit balance equation of the DC bus voltage is established based on the instantaneous power balance principle:

[0008] Where R is the equivalent resistance of the AC side filter network, is the equivalent inductance on the PCS converter side, is the equivalent inductance on the grid side, is the instantaneous current of phase A, is the instantaneous voltage of phase A, is the instantaneous current of phase B, is the instantaneous voltage of phase B, is the instantaneous current of phase C, is the instantaneous voltage of phase C; According to the instantaneous reactive power theory, the circuit equations are established:

[0009] in, is the instantaneous fundamental active current on the AC side, It is the instantaneous reactive power and harmonic current of the AC side of the PCS converter; is the effective value of the phase voltage on the AC side, is the equivalent bus capacitance on the DC side, is the DC side equivalent bus capacitance The current; is the voltage of the equivalent bus capacitor on the DC side; When only reactive current is considered, the balanced equation of DC bus voltage is obtained by substituting equation (2.2.2) into equation (2.2.1):

[0010] The DC bus voltage balance equation is modeled with small signals to obtain the following equation:

[0011] Considering the DC part to be equal and ignoring the high-order terms, the small signal and LAPLACE transformation models are obtained:

[0012] The outer loop control system is constructed according to formula (2.2.7), and the open-loop transfer function of the DC voltage regulator controller of the outer loop control system is: .

[0013] Preferably, the construction process of the voltage balancing controller includes the following steps: Establish the KCL equations for the DC bus voltage:

[0014] in, is the terminal voltage drop of the upper discrete DC capacitor group, is the terminal voltage drop of the discrete DC capacitor group, is the instantaneous current of the upper discrete capacitor group, is the instantaneous current of the lower discrete capacitor group, is the equivalent capacitance of the upper capacitor group, is the equivalent capacitance of the lower capacitor group; Set the upper capacitor equivalent capacitance and the lower capacitor equivalent capacitance equal, simplify equation (2.2.12) to obtain:

[0015]

[0016] Perform small signal modeling on equation (2.2.14):

[0017] The inner loop control system is constructed according to formula (2.2.14), and the transfer function of the pressure equalization controller of the inner loop control system is: .

[0018] The second object of the present invention is to propose a DC neutral line stability control system of an NPC three-level three-phase four-wire PCS, which is used to implement the above-mentioned DC neutral line stability control method of the NPC three-level three-phase four-wire PCS. The stability control system includes a DC voltage control module, and the DC voltage control module is configured with an outer loop control system and an inner loop control system. The outer loop control system is used to stabilize the DC bus voltage, and the inner loop control system is used to balance the DC neutral line voltage.

[0019] Preferably, the outer loop control system includes a DC voltage regulator controller, and the expression of the open loop transfer function of the DC voltage regulator controller is: , To compensate the network controller, The transfer function is: ,in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0020] Preferably, the DC voltage regulator controller is provided with a regulating signal , used to input the DC error signal Perform normalization.

[0021] Preferably, the inner loop control system includes a voltage equalization controller, and the expression of the open loop transfer function of the voltage equalization controller is: , To compensate the network controller II, The transfer function is: ;in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0022] Preferably, the voltage equalization controller is provided with a regulating signal , used for the midpoint voltage of the input Perform normalization.

[0023] The beneficial effects of the present invention are as follows: the DC neutral line stabilization control method and control system of the NPC three-level three-phase four-wire PCS configure an outer loop control system and an inner loop control system in a DC voltage control module, a second-order low-pass filter is set in the outer loop control system for compensation, and the DC bus voltage is stabilized and controlled by the outer loop control system; a second-order low-pass filter is set in the inner loop control system for compensation, and the DC neutral line voltage is balanced and controlled by the inner loop control system, so that the DC voltage control module can output a stable DC bus voltage and a balanced DC neutral line voltage, which is beneficial to ensuring the accuracy of subsequent operation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1 The present invention relates to a circuit diagram of a type I three-level three-phase four-wire PCS; Figure 2 is a block diagram of the control system of the present invention; Figure 3 is a block diagram of the outer loop control system of the present invention; Figure 4 is a frequency domain characteristic diagram of the outer loop control system of the present invention before compensation; Figure 5 is a frequency domain characteristic diagram after compensation of the outer loop control system of the present invention; Figure 6 is a block diagram of the inner loop control system of the present invention; Figure 7 is a frequency domain characteristic diagram of the inner loop control system of the present invention before compensation; Figure 8 is a frequency domain characteristic diagram of the inner loop control system of the present invention after being compensated by the PI controller; Fig. 9 It is a frequency domain characteristic diagram of the inner loop control system of the present invention after being compensated by a second-order low-pass filter. DETAILED DESCRIPTION

[0025] Embodiment 1 A DC neutral line stability control method for NPC three-level three-phase four-wire PCS is based on Figure 1The three-level three-phase four-wire PCS of type I shown performs stable control on its DC voltage, performs voltage stabilization control on the DC bus voltage through an outer loop control, and performs voltage balancing control on the DC neutral line voltage through an inner loop control.

[0026] The outer loop control process of the DC bus voltage includes the following steps: like Figure 2 As shown, calculate the DC bus voltage and reference voltage The DC error signal is obtained by taking the difference between .

[0027] like Figure 3 As shown, the DC error signal After passing through the outer loop control system, the voltage stabilization signal is obtained , voltage regulation signal The calculation formula is: ,in, is the open-loop transfer function of the DC voltage regulator controller of the outer loop control system, and its expression is: .in, For compensation network controller 1, its transfer function is: .in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0028] In addition, in order to improve the voltage stabilization effect, a regulation signal is set in the DC voltage regulator controller , used to input the DC error signal Perform normalization.

[0029] Specifically, the construction process of the DC voltage regulator controller of the outer loop control system is as follows: Step 1: Establish the circuit balance equation of DC bus voltage. Figure 1 For the three-level three-phase four-wire PCS of type I shown in the figure, ignoring the influence of the AC filter capacitor absorption branch and ripple, the balance equation is obtained based on the instantaneous power balance principle of the PCS converter, as follows:

[0030] Where R is the equivalent resistance of the AC side filter network, is the equivalent inductance on the PCS converter side, is the equivalent inductance on the grid side, is the instantaneous current of phase A, is the instantaneous voltage of phase A, is the instantaneous current of phase B, is the instantaneous voltage of phase B, is the instantaneous current of phase C, is the instantaneous voltage of phase C.

[0031] Step 2: Based on the instantaneous reactive power theory , we get the following system of equations:

[0032] In formula (2.2.2), is the instantaneous fundamental active current on the AC side, It is the instantaneous reactive power and harmonic current of the AC side of the PCS converter; is the effective value of the phase voltage on the AC side, is the equivalent bus capacitance on the DC side, is the DC side equivalent bus capacitance The current; is the voltage of the equivalent bus capacitor on the DC side.

[0033] Step 3: For the simplicity of analysis, only reactive current is considered. Substituting formula (2.2.2) into (2.2.1) yields the balanced equation of DC bus voltage:

[0034] Step 4: Perform small signal modeling on the DC bus voltage balance equation to perturb the variables in equation (2.2.3) and obtain the following equation:

[0035] Step 5: Considering the DC part to be equal and ignoring the high-order terms, we can get the small signal and LAPLACE transformation model:

[0036]

[0037] Step 6: According to formula (2.2.7), we can get Figure 3 The outer loop control system block diagram is shown in FIG. is the compensation network controller 1. At this time, the open-loop transfer function of the DC voltage regulator controller of the outer loop control system is:

[0038] First, assume , and set the parameter data to draw the frequency domain characteristic diagram of the open-loop transfer function. In this embodiment, the AC side phase voltage effective value is set The voltage is 220V, the equivalent resistance R in the AC side filter network is 0.1ohm, and the instantaneous fundamental active current on the AC side is is 144A, 0.00022H, DC bus capacitance is 0.00846F, the voltage of the DC bus capacitor is 720V, so we get Figure 4 Shown is the frequency domain characteristic diagram of the open-loop transfer function before compensation.

[0039]

[0040] refer to Figure 4 The amplitude-frequency characteristic curve crosses 0dB at -20dB / dec, and the bandwidth is 21.9HZ at this time. However, the high-frequency attenuation is slow and close to linear attenuation after 1kHZ, so it is necessary to add a -40dB / dec incoming line attenuation. Since the low-frequency band basically does not need attenuation, but the high-frequency band needs to be attenuated, a second-order low-pass filter is used to compensate the network controller. The transfer function is:

[0041] Among them, the second-order low-pass filter is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, which depends on the system's low-frequency gain and stability margin, frequency bandwidth and expected phase margin and high-frequency attenuation performance.

[0042] In this embodiment, the gain is set -1500, cutoff frequency for rad / s and damping ratio is 0.78, and the simulation verifies the frequency domain characteristics after compensation. Figure 5 shown.

[0043] refer to Figure 5 The amplitude-frequency characteristic curve crosses 0dB at -40dB / dec; and the corresponding amplitude is less than 0dB at a phase of -180 degrees, and the control system is stable; at this time, the bandwidth is 700HZ, with good low and medium frequency characteristics; the high-frequency amplitude-frequency curve decays rapidly at a rate close to -40dB / dec, and the steady-state performance is good; moreover, the phase margin is 75 degrees, and the dynamic performance is good.

[0044]

[0045] Therefore, a compensation network controller is set in the DC voltage regulator controller. , can obtain better DC voltage control performance.

[0046] On the other hand, the inner loop control process of the DC neutral voltage includes the following steps: like Figure 2 As shown, the upper discrete voltage is calculated and the lower discrete voltage The midpoint voltage is obtained by taking the difference between .

[0047] like Figure 6 As shown, the midpoint voltage After passing through the inner loop control system, the pressure equalization signal is obtained , balanced signal The calculation formula is ,in, is the transfer function of the voltage equalization controller in the inner loop control system, and its expression is:

[0048] in, For compensation network controller 2, its transfer function is: .in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0049] In addition, in order to improve the voltage balancing effect, a regulation signal is set in the voltage balancing controller , used for the midpoint voltage of the input Perform normalization.

[0050] Specifically, the construction process of the voltage balancing controller of the inner loop control system is as follows: Step 1: Establish the KCL equations for the DC bus voltage:

[0051] Formula (2.2.12) is based on Figure 1 The KCL equations for the DC bus voltage established by the type I three-level three-phase four-wire PCS shown.

[0052] In formula (2.2.12), is the terminal voltage drop of the upper discrete DC capacitor group, is the terminal voltage drop of the discrete DC capacitor group, is the instantaneous current of the upper discrete capacitor group, is the instantaneous current of the lower discrete capacitor group, is the equivalent capacitance of the upper capacitor group, is the equivalent capacitance of the lower capacitor group.

[0053] Step 2: Considering that the equivalent capacitance of the upper and lower capacitor groups is usually designed to be equal, the formula (2.2.12) is simplified to:

[0054] Then we get formula (2.2.14):

[0055] It can be seen from formula (2.2.14) that fluctuations in the midpoint current will inevitably cause an imbalance in the upper and lower DC voltages on the DC side.

[0056] Step 3: Perform small signal modeling analysis on equation (2.2.14) and obtain the frequency domain formula by introducing small signals of variable current and voltage fluctuations:

[0057] Step 4: According to formula (2.2.15), we can get Figure 6 The inner loop control system block diagram is shown in Figure 1, where is the compensation network controller 2. At this time, the transfer function of the voltage balance controller in the inner loop control system is:

[0058] At this point, assume , and set parameter data to draw the frequency domain characteristic diagram of the open-loop transfer function. In this embodiment, the DC bus capacitor is set for F. Thus we get Figure 7 Shown is the frequency domain characteristic diagram of the transfer function before compensation.

[0059]

[0060] refer to Figure 7 The system bandwidth is 19.8HZ, and the high-frequency part attenuates at a rate of -20dB / dec. Considering that the zero-sequence current flows through the N line of the three-phase four-wire system, it will also flow through the DC side. Therefore, in order to ensure sufficient control accuracy of the DC voltage balancing, the bandwidth of the control system after compensation should be less than 150HZ and greater than 50HZ.

[0061] First, consider using a PI controller for compensation correction. The transfer function of the PI controller is: ,in, is the proportional gain, is the integral gain. In a typical design of a PI controller, , , into the transfer function of the DC voltage-balanced control system to obtain the following formula:

[0062] According to formula (2.2.16), the corresponding frequency domain characteristic diagram of the transfer function is obtained Figure 8 ,like Figure 8 As shown, it is obvious that the correction effect of the PI controller is very limited. When the phase is -180 degrees, the corresponding amplitude is approximately greater than 0dB, and the control system is unstable. Therefore, the compensation network controller is set For a second-order filter:

[0063] The parameter gain of the second-order low-pass filter is , Cutoff frequency and damping ratio , respectively depend on the system's low-frequency gain and stability margin, frequency bandwidth and expected phase margin and high-frequency attenuation performance.

[0064] In this embodiment, the gain is set 3, cut-off frequency for HZ(rad / s) and Damping Ratio is 0.9; the simulation verifies the frequency domain characteristics after compensation as follows Fig. 9 shown.

[0065] refer to Fig. 9 The amplitude-frequency characteristic curve crosses 0dB at -20dB / dec; and the corresponding amplitude at the phase of -180 degrees is about -11.9dB, which is less than 0dB, and the control system is stable; at this time, the bandwidth is 47.5HZ, with good low and medium frequency characteristics; the high-frequency amplitude-frequency curve decays rapidly at a rate close to -40dB / dec, and the steady-state performance is good; moreover, the phase margin is 48 degrees, and the dynamic performance is good.

[0066]

[0067] Therefore, a compensation network controller is set in the voltage balancing controller. By performing second-order calibration, the DC voltage regulation control part obtains better overall performance.

[0068] Embodiment 2 A DC neutral line stability control system for an NPC three-level three-phase four-wire PCS is used to implement the DC neutral line stability control method for an NPC three-level three-phase four-wire PCS described in Embodiment 1. Specifically, in the example Figure 2The DC voltage control module (DC Voltage Controller) shown is configured with an outer loop control system and an inner loop control system. The outer loop control system is used to perform voltage stabilization control on the DC bus voltage, and the inner loop control system is used to perform voltage balancing control on the DC neutral line voltage.

[0069] like Figure 2 As shown, the DC bus voltage and reference voltage The difference signal between the two is the DC error signal , DC error signal After entering the DC voltage control module, the outer loop control system performs voltage stabilization processing to obtain a voltage stabilization signal , voltage regulation signal After passing through the conversion module in the DC voltage control module, the reference current is obtained And output for subsequent processing based on current signal.

[0070] like Figure 3 As shown, the open-loop transfer function of the DC voltage regulator controller of the outer loop control system is ;in, To compensate the network controller, The transfer function is: ,in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0071] DC error signal After entering the outer loop control system for voltage stabilization processing, the voltage stabilization signal is output , whose expression is: .

[0072] like Figure 2 As shown, the upper discrete voltage and the lower discrete voltage The difference signal between the midpoint voltage , midpoint voltage After entering the DC voltage control module, the inner loop control system performs voltage equalization processing to obtain a voltage equalization signal And output for subsequent PMW control.

[0073] like Figure 6 As shown, the transfer function of the voltage balance controller in the inner loop control system is ;in, To compensate the network controller II, The transfer function is: ;in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0074] Midpoint voltage After entering the inner loop control system and performing voltage equalization processing, the balanced voltage is output , whose expression is: .

[0075] The DC bus voltage is stabilized and controlled through the outer loop control system, and the DC neutral voltage is balanced and controlled through the inner loop control system, so that the DC voltage control module can output a stable DC bus voltage and a balanced DC neutral voltage, which is beneficial to ensure the accuracy of subsequent operation results.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A DC neutral line stabilization control method for an NPC three-level three-phase four-wire PCS, characterized in that: The DC bus voltage is stabilized by the outer loop control, and the DC neutral voltage is balanced by the inner loop control. The outer loop control process specifically includes the following steps: Calculate the DC bus voltage and reference voltage The DC error signal is obtained by taking the difference between ; DC error signal After passing through the outer loop control system, the voltage stabilization signal is obtained , the regulated voltage signal The calculation formula is: ,in, is the open-loop transfer function of the DC voltage regulator controller of the outer loop control system, and its expression is: ; in, For compensation network controller 1, its transfer function is: , is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, ; The inner loop control process specifically includes the following steps: Calculate the upper discrete voltage and the lower discrete voltage The midpoint voltage is obtained by taking the difference between ; Midpoint voltage After passing through the inner loop control system, the pressure equalization signal is obtained , balanced signal The calculation formula is ,in, is the transfer function of the voltage equalization controller in the inner loop control system, and its expression is: ; in, To compensate the network controller II, The transfer function is: ; is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

2. The DC neutral line stabilization control method of the NPC three-level three-phase four-wire PCS according to claim 1 is characterized in that: The construction process of the DC voltage regulator controller includes the following steps: The circuit balance equation of the DC bus voltage is established based on the instantaneous power balance principle: Where R is the equivalent resistance of the AC side filter network, is the equivalent inductance on the PCS converter side, is the equivalent inductance on the grid side, is the instantaneous current of phase A, is the instantaneous voltage of phase A, is the instantaneous current of phase B, is the instantaneous voltage of phase B, is the instantaneous current of phase C, is the instantaneous voltage of phase C; According to the instantaneous reactive power theory, the circuit equations are established: ,in, is the instantaneous fundamental active current on the AC side, It is the instantaneous reactive power and harmonic current of the AC side of the PCS converter; is the effective value of the AC side phase voltage, is the equivalent bus capacitance on the DC side, is the DC side equivalent bus capacitance The current; is the voltage of the equivalent bus capacitor on the DC side; When only reactive current is considered, the balanced equation of DC bus voltage is obtained by substituting equation (2.2.2) into equation (2.2.1): , the DC bus voltage balance equation is modeled by small signal, and the following equation is obtained: Considering the DC part to be equal and ignoring the high-order terms, the small signal and LAPLACE transformation models are obtained: The outer loop control system is constructed according to formula (2.2.7), and the open-loop transfer function of the DC voltage regulator controller of the outer loop control system is: 。 3. The DC neutral line stabilization control method of the NPC three-level three-phase four-wire PCS according to claim 1 is characterized in that: The construction process of the voltage equalization controller includes the following steps: Establish the KCL equations for the DC bus voltage: in, is the terminal voltage drop of the upper discrete DC capacitor group, is the terminal voltage drop of the discrete DC capacitor group, is the instantaneous current of the upper discrete capacitor group, is the instantaneous current of the lower discrete capacitor group, is the equivalent capacitance of the upper capacitor group, is the equivalent capacitance of the lower capacitor group; Set the upper capacitor equivalent capacitance and the lower capacitor equivalent capacitance equal, simplify equation (2.2.12) to obtain: Perform small signal modeling on equation (2.2.14): The inner loop control system is constructed according to formula (2.2.15), and the transfer function of the pressure equalization controller of the inner loop control system is: 。 4. A DC neutral line stability control system of an NPC three-level three-phase four-wire PCS, characterized in that: A DC neutral line stabilization control method for implementing an NPC three-level three-phase four-wire PCS according to any one of claims 1 to 3, wherein the stabilization control system comprises a DC voltage control module, wherein the DC voltage control module is provided with an outer loop control system and an inner loop control system, wherein the outer loop control system is used for stabilizing the DC bus voltage, and the inner loop control system is used for equalizing the DC neutral line voltage.

5. The DC neutral line stability control system of the NPC three-level three-phase four-wire PCS according to claim 4 is characterized in that: The outer loop control system includes a DC voltage regulator controller, and the expression of the open loop transfer function of the DC voltage regulator controller is: , To compensate the network controller, The transfer function is: ,in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

6. The DC neutral line stabilization control method of the NPC three-level three-phase four-wire PCS according to claim 5 is characterized in that: The DC voltage regulator controller is provided with a regulating signal , used to input the DC error signal Perform normalization.

7. The DC neutral line stability control system of the NPC three-level three-phase four-wire PCS according to claim 4 is characterized in that: The inner loop control system includes a voltage equalization controller, and the expression of the open loop transfer function of the voltage equalization controller is: , To compensate the network controller II, The transfer function is: ;in, is the gain of compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

8. The DC neutral line stabilization control method of the NPC three-level three-phase four-wire PCS according to claim 7 is characterized in that: The voltage equalization controller is provided with a regulation signal , used for the midpoint voltage of the input Perform normalization.

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