A DC neutral line stability control method and control system for an NPC three-level three-phase four-wire PCS

Abstract

The present invention provides a DC midline stability control method and control system for an NPC three-level three-phase four-wire PCS, which is applied to the technical field of energy storage systems. The method includes a DC voltage control module, which includes an outer-loop control system and an inner-loop control system. 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 midline voltage. Outer-loop control process: Calculate the difference between the DC bus voltage and the reference voltage to obtain a DC error signal; Obtain a voltage stabilization signal through the outer-loop control system. Inner-loop control process: Calculate the difference between the voltage drop across the upper discrete DC capacitor bank and the voltage drop across the lower discrete DC capacitor bank to obtain the midpoint voltage, and obtain an equalization signal after passing through the inner-loop control system. Second-order low-pass filters are respectively used for compensation in the outer-loop control and the inner-loop control to ensure the stability of the DC bus voltage and achieve the equalization control of the DC midline voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage systems, and particularly relates to a DC midline stability control method and 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 clamping has become the mainstream solution, but most of them adopt three-phase three-wire systems and few adopt three-phase four-wire systems; in practical applications, it is often necessary to combine grid-connected and off-grid requirements, especially for application requirements in Africa and Europe abroad, where there is a need for energy storage to simultaneously carry three-phase and single-phase loads, etc., and a three-phase four-wire PCS needs to be provided.

[0003] Such as Figure 1 the three-phase four-wire PCS of the I-type three-level shown, its upper and lower two groups of discrete DC capacitors are connected in series to form a high-voltage DC bus, and it is necessary to control the high-voltage DC bus to make it stable. Also, because the DC-side circuit is composed of discrete capacitors, and the neutral point is led out and connected to the N line of the AC side, considering factors such as the instantaneous active power distribution cannot be absolutely symmetric and the current flowing through the N line, it will inevitably cause the midpoint to shift, thereby causing the voltages 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, a voltage equalization control needs to be added to balance the terminal voltages of the upper and lower DC capacitors.

[0004] In the prior art, during the DC voltage control process, mostly PI controllers or PID controllers are used for compensation and regulation. However, in a specific energy storage system, the compensation effect of the 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 midline stability control method for an NPC three-level three-phase four-wire PCS, which uses a second-order low-pass filter for compensation in the outer-loop control and the inner-loop control respectively to ensure the stability of the DC bus voltage and achieve the voltage equalization control of the DC midline voltage.

[0006] A DC midline stability control method for an NPC three-level three-phase four-wire PCS performs voltage stabilization control on the DC bus voltage through outer-loop control and voltage equalization control on the DC midline voltage through inner-loop control;

[0007] The outer-loop control process specifically includes the following steps:

[0008] Calculate the difference between the DC bus voltage and the reference voltage to obtain the DC error signal ;

[0009] DC error signal After passing through the outer loop control system, a regulated voltage signal is obtained , and the regulated voltage signal The calculation formula is: , where is the open-loop transfer function of the DC voltage regulator controller of the outer loop control system, and the expression is: ;

[0010] where is compensation network controller 1, and its transfer function is: , is the gain of compensation network controller 1, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0011] The specific process of the inner loop control includes the following steps:

[0012] Calculate the difference between the upper discrete voltage and the lower discrete voltage to obtain the midpoint voltage ;

[0013] The midpoint voltage After passing through the inner loop control system, an equalization signal is obtained. The calculation formula of the equalization signal is , where is the transfer function of the voltage equalization controller in the inner loop control system, and the expression is: ;

[0014] where is compensation network controller 2, The transfer function of is: ; is the gain of compensation network controller 2, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0015] Preferably, the construction process of the DC voltage regulator controller includes the following steps:

[0016] Establish a circuit balance equation of the DC bus voltage according to the principle of instantaneous power balance:

[0017]

[0018] where R is the equivalent resistance of the AC side filter network, is the equivalent inductor on the PCS converter side, is the equivalent inductor 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;

[0019] Establish a circuit equation set according to the instantaneous reactive power theory:

[0020] ; where, is the instantaneous fundamental active current on the AC side, is the instantaneous reactive power and harmonic currents of each order on the AC side of the PCS converter; is the effective value of the phase voltage on the AC side, is the equivalent bus capacitor on the DC side, is the current of the equivalent bus capacitor on the DC side ; is the voltage of the equivalent bus capacitor on the DC side;

[0021] When only considering the reactive current, substitute Equation (2.2.2) into Equation (2.2.1) to obtain the balance equation of the DC bus voltage:

[0022] ;

[0023] Perform small-signal modeling on the balance equation of the DC bus voltage to obtain the following equation:

[0024]

[0025] Considering the equality of the DC part and neglecting the higher-order terms, obtain the small-signal and Laplace transform model:

[0026]

[0027] Construct an outer-loop control system according to Equation (2.2.7), and the open-loop transfer function of the DC voltage regulator controller of the outer-loop control system:

[0028] .

[0029] Preferably, the construction process of the voltage equalization controller includes the following steps:

[0030] Establish the KCL equation set of the DC bus voltage:

[0031]

[0032] Among them, is the terminal voltage drop of the upper discrete DC capacitor bank, is the terminal voltage drop of the lower discrete DC capacitor bank, is the instantaneous current of the upper discrete capacitor bank, is the instantaneous current of the lower discrete capacitor bank, is the equivalent capacitance of the upper capacitor bank, is the equivalent capacitance of the lower capacitor bank;

[0033] Set the equivalent capacitance of the upper capacitor and the equivalent capacitance of the lower capacitor to be equal, and simplify Equation (2.2.12) to obtain:

[0034]

[0035]

[0036] Perform small-signal modeling on Equation (2.2.14):

[0037]

[0038] Perform LAPLACE transform on the above equation to obtain the frequency-domain formula:

[0039]

[0040] Construct an inner-loop control system according to Equation (2.2.15), and the transfer function of the voltage equalization controller of the inner-loop control system is:

[0041] .

[0042] The second object of the present invention is to propose a DC neutral line stability control system for an NPC three-level three-phase four-wire PCS, which is used to implement the DC neutral line stability control method of the above-mentioned NPC three-level three-phase four-wire PCS. The stability control system includes a DC voltage control module, and an outer-loop control system and an inner-loop control system are configured in the DC voltage control module. 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 equalization control on the DC neutral line voltage.

[0043] Preferably, the outer-loop control system includes a DC voltage stabilizer controller, and the expression of the open-loop transfer function of the DC voltage stabilizer controller is: , is the compensation network controller one, The transfer function of is: , among which, is the gain of the compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0044] Preferably, an adjustment signal is provided in the DC regulated controller for normalizing the input DC error signal .

[0045] 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: , is the compensation network controller II, and the transfer function of is: where is the gain of the compensation network controller II, represents the cut-off frequency, represents the damping ratio, represents the complex frequency,

[0046] Preferably, an adjustment signal is provided in the voltage equalization controller for normalizing the input midpoint voltage .

[0047] The beneficial effects of the present invention are as follows: The DC midline stability 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 the DC voltage control module, set a second-order low-pass filter for compensation in the outer loop control system, and perform voltage stabilization control on the DC bus voltage through the outer loop control system; set a second-order low-pass filter for compensation in the inner loop control system, and perform voltage equalization control on the DC midline voltage through the inner loop control system, so that the DC voltage control module can output a stable DC bus voltage and an equalized DC midline voltage, which is beneficial to ensuring the accuracy of subsequent operation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation to the present invention. In the drawings:

[0049] Figure 1 is the circuit diagram of the I-type three-level three-phase four-wire PCS related to the present invention;

[0050] Figure 2 is the block diagram of the control system of the present invention;

[0051] Figure 3 is the block diagram of the outer loop control system of the present invention;

[0052] Figure 4 is the frequency-domain characteristic diagram of the outer-loop control system of the present invention before compensation;

[0053] Figure 5 is the frequency-domain characteristic diagram of the outer-loop control system of the present invention after compensation;

[0054] Figure 6 is the block diagram of the inner-loop control system of the present invention;

[0055] Figure 7 is the frequency-domain characteristic diagram of the inner-loop control system of the present invention before compensation;

[0056] Figure 8 is the frequency-domain characteristic diagram of the inner-loop control system of the present invention after being compensated by a PI controller;

[0057] Figure 9 is the 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 implementation manners

[0058] Example 1

[0059] A DC midline stability control method for an NPC three-level three-phase four-wire PCS, based on the three-phase four-wire PCS of type I three-level as shown in Figure 1 , performs stable control on its DC voltage, performs voltage regulation control on the DC bus voltage through outer-loop control, and performs equal-voltage control on the DC midline voltage through inner-loop control.

[0060] Among them, the outer-loop control process for the DC bus voltage includes the following steps:

[0061] As shown in Figure 2 , calculate the difference between the DC bus voltage and the reference voltage to obtain the DC error signal .

[0062] As shown in Figure 3 , the DC error signal obtains the voltage regulation signal after passing through the outer-loop control system. The calculation formula for the voltage regulation signal is: , where is the open-loop transfer function of the DC voltage regulator of the outer-loop control system, and the expression is: . Among them, is the compensation network controller 1, and its transfer function is: . Among them, is the gain of the compensation network controller 1, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0063] In addition, in order to improve the voltage regulation effect, a regulation signal is set in the DC voltage regulator controller, which is used to normalize the input DC error signal.

[0064] Specifically, the construction process of the DC voltage regulator controller in the outer loop control system is as follows:

[0065] The first step: Establish the circuit balance equation of the DC bus voltage. Based on the Figure 1 type-I three-level three-phase four-wire PCS shown in the figure, ignoring the influence of the AC filter capacitor absorption branch and ripple, the balance equation is obtained from the instantaneous power balance principle of the PCS converter as follows:

[0066]

[0067] 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.

[0068] The second step: According to the instantaneous reactive power theory , the following system of equations is obtained:

[0069] ;

[0070] In formula (2.2.2), is the instantaneous fundamental active current on the AC side, is the instantaneous reactive power and harmonic currents of each order on the AC side of the PCS converter; is the effective value of the phase voltage on the AC side, is the equivalent bus capacitor on the DC side, is the current of the equivalent bus capacitor on the DC side ; is the voltage of the equivalent bus capacitor on the DC side.

[0071] The third step: For the simplicity of analyzing the problem, only the reactive current is considered. Substituting formula (2.2.2) into (2.2.1) gives the balance equation of the DC bus voltage: ;

[0072] Step 4: Perform small-signal modeling on the DC bus voltage balance equation to perturb the variables in Equation (2.2.3), obtaining the following equation:

[0073]

[0074] Step 5: Considering the equality of the DC part and neglecting the higher-order terms, obtain the small-signal and Laplace transform models:

[0075]

[0076]

[0077] Step 6: According to Equation (2.2.7), obtain the outer-loop control system block diagram as shown in Figure 3 , where 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:

[0078]

[0079] First, assume , and set the parameter data to plot the frequency-domain characteristic diagram of the open-loop transfer function. In this embodiment, set the effective value of the AC side phase voltage to 220V, the equivalent resistance R in the AC side filter network to 0.1 ohm, the instantaneous fundamental active current on the AC side to 144A, to 0.00022H, the DC side bus capacitor to 0.00846F, and the voltage of the DC side bus capacitor to 720V, thereby obtaining the frequency-domain characteristic diagram of the open-loop transfer function before compensation as shown in Figure 4 .

[0080]

[0081] Referring to Figure 4 , the amplitude-frequency characteristic curve crosses 0dB at -20dB / dec. At this time, the bandwidth is 21.9HZ, but the high-frequency attenuation is slow and it approaches linear attenuation after 1kHz. It is necessary to add a -40dB / dec incoming line attenuation. Since its low-frequency segment basically does not need attenuation while the high-frequency segment needs to strengthen attenuation, a second-order low-pass filter is adopted. The transfer function of the compensation network controller 1 is:

[0082]

[0083] Among them, the is the gain of compensation network controller 1, represents the cut-off frequency, represents the damping ratio, which respectively depend on the low-frequency gain and stability margin, frequency bandwidth, and expected phase margin and high-frequency attenuation performance of the system.

[0084] In this embodiment, the gain is set to -1500, the cut-off frequency is rad / s, and the damping ratio is 0.78. The simulation verifies that the frequency-domain characteristic effect after compensation is as Figure 5 shown.

[0085] Referring to Figure 5 , the amplitude-frequency characteristic curve crosses 0 dB at -40 dB / dec; and the corresponding amplitude is less than 0 dB at a phase of -180 degrees, and the control system is stable; at this time, the bandwidth is 700 HZ, and it has good low- and mid-frequency characteristics; the amplitude-frequency curve in the high-frequency band decays rapidly at a rate close to -40 dB / dec, and the steady-state performance is good; moreover, the phase margin is 75 degrees, and the dynamic performance is good.

[0086]

[0087] Therefore, by setting compensation network controller 1 in the DC voltage stabilizer controller, better DC voltage control performance can be obtained.

[0088] On the other hand, the inner-loop control process of the DC midline voltage includes the following steps:

[0089] As Figure 2 shown, calculate the difference between the upper discrete voltage and the lower discrete voltage to obtain the midpoint voltage .

[0090] As Figure 6 shown, the midpoint voltage passes through the inner-loop control system to obtain the equalization signal . The calculation formula of the equalization signal is . Among them, is the transfer function of the voltage equalization controller in the inner-loop control system, and the expression is:

[0091]

[0092] Among them, is compensation network controller 2, and its transfer function is: . Among them, is the gain of the compensation network controller 2, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0093] In addition, in order to improve the voltage sharing effect, a regulation signal is set in the voltage equalization controller for normalizing the input midpoint voltage .

[0094] Specifically, the construction process of the voltage equalization controller of the inner loop control system is as follows:

[0095] Step 1: Establish the KCL equations for the DC bus voltage:

[0096]

[0097] Equation (2.2.12) is the KCL equations for the DC bus voltage established based on the type-I three-level three-phase four-wire PCS as shown in Figure 1 .

[0098] In Equation (2.2.12), is the voltage drop across the upper discrete DC capacitor bank, is the voltage drop across the lower discrete DC capacitor bank, is the instantaneous current of the upper discrete capacitor bank, is the instantaneous current of the lower discrete capacitor bank, is the equivalent capacitance of the upper capacitor bank, is the equivalent capacitance of the lower capacitor bank.

[0099] Step 2: Considering that the equivalent capacitances of the upper and lower capacitor banks are usually designed to be equal, Equation (2.2.12) is simplified to:

[0100]

[0101] Furthermore, Equation (2.2.14) is obtained:

[0102]

[0103] It can be seen from Equation (2.2.14) that the fluctuation of the midpoint current will inevitably cause the imbalance of the upper and lower DC voltages on the DC side.

[0104] Step 3: Perform small-signal modeling analysis on Equation (2.2.14). By introducing small-signals of variable current and voltage fluctuations, the frequency-domain formula is obtained:

[0105]

[0106] Perform the Laplace transform on the above equation to obtain the frequency-domain formula:

[0107]

[0108] Step 4: Obtain the inner-loop control system block diagram as shown in Figure 6 , where is the compensation network controller II. At this time, the transfer function of the voltage equalization controller in the inner-loop control system is:

[0109]

[0110] At this time, first assume , and set the parameter data to plot the frequency-domain characteristics of the open-loop transfer function. In this embodiment, set the DC-side bus capacitor to be F. Thus, the frequency-domain characteristics diagram of the transfer function before compensation as shown in Figure 7 is obtained.

[0111]

[0112] Referring to Figure 7 , its system bandwidth is 19.8HZ, and the high-frequency part decays at a rate of -20dB / dec; considering that the zero-sequence current flowing through the N line of the three-phase four-wire system will also flow through the DC side, therefore, in order to ensure sufficient control accuracy of DC voltage equalization, the designed bandwidth of the compensated control system should be less than 150HZ and greater than 50HZ.

[0113] First, try to consider compensating and correcting through a PI controller. The transfer function of the PI controller is , where is the proportional gain, and is the integral gain. In the typical design of the PI controller, set , , and substitute it into the transfer function of the DC voltage equalization control system to obtain the following equation:

[0114]

[0115] According to Equation (2.2.16), plot the frequency-domain characteristics diagram of the corresponding transfer function to obtain Figure 8 , as shown in Figure 8 . Obviously, the correction effect of the PI controller is very limited. When the phase is -180 degrees, the corresponding amplitude is about greater than 0dB, and the control system becomes unstable. Therefore, set the compensation network controller II as a second-order filter:

[0116]

[0117] Among them, the parameters of the second-order low-pass filter, namely the gain , the cut-off frequency and the damping ratio , respectively depend on the low-frequency gain and stability margin, the frequency bandwidth, and the desired phase margin and high-frequency attenuation performance of the system.

[0118] In this embodiment, the gain is set to 3, the cut-off frequency is HZ (rad / s), and the damping ratio is 0.9; the frequency-domain characteristic effect after simulation verification and compensation is as Figure 9 shown.

[0119] Referring to Figure 9 , the amplitude-frequency characteristic curve crosses 0 dB at -20 dB / dec; and when the phase is -180 degrees, the corresponding amplitude is about -11.9 dB, which is less than 0 dB, and the control system is stable; at this time, the bandwidth is 47.5 HZ, and it has good low- and medium-frequency characteristics; in the high-frequency band, the amplitude-frequency curve decays rapidly at a rate close to -40 dB / dec, and the steady-state performance is good; moreover, the phase margin is 48 degrees, and the dynamic performance is good.

[0120] Therefore, a compensation network controller II is set in the voltage equalization controller to perform second-order calibration, so that the DC voltage stabilization control part obtains better comprehensive performance.

[0121] Embodiment II

[0122] A DC midline stability control system for an NPC three-level three-phase four-wire PCS is used to implement the DC midline stability control method of the NPC three-level three-phase four-wire PCS described in Embodiment I. Specifically, an outer-loop control system and an inner-loop control system are configured in the DC voltage control module (DC Voltage Controller) as shown in Figure 2 . 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 equalization control on the DC midline voltage.

[0123] As shown in Figure 2 , the difference signal between the DC bus voltage and the reference voltage is the DC error signal . After the DC error signal enters the DC voltage control module, it is subjected to voltage stabilization processing by the outer-loop control system to obtain a voltage stabilization signal . The voltage stabilization signal passes through the conversion module in the DC voltage control module to obtain a reference current And output it for subsequent processing based on the current signal.

[0124] As Figure 3 shown, the open-loop transfer function of the DC voltage stabilizer controller in the outer-loop control system is ; where is the compensation network controller one, The transfer function of is: , where is the gain of the compensation network controller one, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0125] The DC error signal enters the outer-loop control system for voltage stabilization processing and outputs the voltage stabilization signal , and its expression is: .

[0126] As Figure 2 shown, the difference signal between the upper discrete voltage and the lower discrete voltage is the midpoint voltage , and the midpoint voltage enters the DC voltage control module and is subjected to voltage equalization processing by the inner-loop control system to obtain the equalization signal and output it for subsequent PWM control.

[0127] As Figure 6 shown, the transfer function of the voltage equalization controller in the inner-loop control system is ; where is the compensation network controller two, The transfer function of is: ; where is the gain of the compensation network controller two, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, .

[0128] The midpoint voltage enters the inner-loop control system for voltage equalization processing and outputs the equalization voltage , and its expression is: .

[0129] The DC bus voltage is regulated by the outer-loop control system, and the DC midline voltage is equalized by the inner-loop control system, enabling the DC voltage control module to output a stable DC bus voltage and an equalized DC midline voltage, which is beneficial to ensuring the accuracy of subsequent operation results.

[0130] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within 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, ; is the effective value of the AC side phase voltage, is the equivalent resistance of the AC side filter network, is the instantaneous fundamental active current on the AC side, is the equivalent bus capacitance on the DC side, is the DC side equivalent bus capacitance The voltage, is the equivalent inductance of the PCS converter on the AC side, is the equivalent inductance of the grid side on the AC side; The inner loop control process specifically includes the following steps: Calculate the terminal voltage drop of the discrete equivalent capacitor and the terminal voltage drop of the discrete equivalent capacitor The midpoint voltage is obtained by taking the difference between ; Midpoint voltage After the inner loop control system, the balanced 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 2, 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 of the PCS converter on the AC side, is the equivalent inductance of the AC grid, 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, is the instantaneous reactive power and harmonic current of the PCS converter side on the AC side; 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 DC side equivalent bus capacitance Voltage; When only reactive current is considered, substituting equation (2.2.2) into equation (2.2.1) yields the DC bus voltage equilibrium equation: ； The DC bus voltage balance equation is modeled with small signals to obtain the following equation: 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 equivalent capacitor, is the terminal voltage drop of the discrete equivalent capacitor, The equivalent capacitance of the upper discrete The instantaneous current, The discrete equivalent capacitance The instantaneous current, is the upper discrete equivalent capacitance, is the lower discrete equivalent capacitance; Set the upper discrete equivalent capacitor And the discrete equivalent capacitance Equal, simplifying formula (2.2.12) yields: Perform small signal modeling on equation (2.2.14): Perform LAPLACE transformation on the above formula to obtain the frequency domain formula: According to formula (2.2.15), an inner loop control system is constructed, and the transfer function of the voltage balancing 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: The system is used to implement the DC neutral line stability control method of the NPC three-level three-phase four-wire PCS according to any one of claims 1 to 3, 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.

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, ; is the effective value of the AC side phase voltage, is the equivalent resistance of the AC side filter network, is the instantaneous fundamental active current on the AC side, is the equivalent bus capacitance on the DC side, is the DC side equivalent bus capacitance The voltage, is the equivalent inductance of the PCS converter on the AC side, is the grid-side equivalent inductance on the AC side.

6. The DC neutral line stability control system 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 2, represents the cut-off frequency, represents the damping ratio, represents the complex frequency, ; is the equivalent bus capacitance on the DC side.

8. The DC neutral line stability control system 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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