Grid-connected system point of common coupling voltage balance control method and system

Abstract

This invention discloses a method and system for controlling the voltage balance at the point of common coupling (PCC) of a grid-connected system, belonging to the field of electrical engineering. The grid-connected system includes one current-controlled grid-connected inverter and one voltage-controlled grid-connected inverter. The control method includes steps such as sampling, power calculation and PCC voltage estimation, power control, and negative-sequence adaptive voltage control. When a voltage imbalance occurs at the PCC of the grid-connected system, the voltage-controlled inverter is activated to output negative-sequence adaptive control. By adjusting the negative-sequence component of the output voltage, the voltage-controlled inverter compensates for the unbalanced voltage at the PCC. The final compensation effect is equivalent to a negative-sequence voltage source connected in series with the voltage-controlled inverter, which improves the voltage quality at the PCC and facilitates the stable grid connection of other inverters in the system.

Description

Technical Field

[0001] This invention belongs to the field of electrical engineering, and in particular relates to a method and system for voltage balance control at the common coupling point of a grid-connected system. Background Technology

[0002] With the rapid development of new energy sources, a large number of distributed power sources are connected to the three-phase power grid at the common coupling point of the system through grid-connected inverters. Since large-scale new energy power generation is generally located in remote areas far from load centers, its connection point typically presents a high-impedance, weak grid condition. Traditional current-controlled inverters are prone to resonance and instability when connected. While voltage-controlled inverters are suitable for operation in weak grids, large-scale adoption of voltage-controlled inverters increases system costs. Multi-inverter grid-connected systems employing a hybrid current-control and voltage-control mode can achieve stable system operation while maximizing the utilization rate of new energy sources.

[0003] In hybrid grid-connected systems, factors such as grid voltage fluctuations, nonlinear loads, and unbalanced loads degrade the voltage quality at the system's point of common coupling (PCC). Voltage imbalance is a common problem that directly affects the grid-connected power quality of current-controlled grid-connected inverters. Voltage-controlled inverters can actively support the system voltage and fulfill their grid configuration function. Therefore, researching their control strategies for improving voltage imbalance at the PCC of grid-connected systems is of great significance.

[0004] Currently, several papers have studied the voltage balancing method at the common coupling point, for example:

[0005] 1. Chinese invention patent document (publication number CN103368191B) published on March 25, 2015, entitled "A Method for Compensating Voltage Imbalance in Parallel Connection of Multiple Inverters in a Microgrid". This method adds a negative sequence reactive conductance droop control loop to the traditional power droop control to achieve three-phase voltage imbalance compensation. However, this method cannot completely compensate for unbalanced voltage, and the line impedance affects the compensation effect.

[0006] 2. Chinese invention patent document (publication number CN111917126B), published on October 8, 2021, entitled "DFIG Unbalanced Grid Voltage Compensation Method Based on Phase-Locked Loop Self-Synchronization Control," compensates for the grid voltage imbalance component by adding a direct resonator. However, the resonator affects the steady-state characteristics of the inverter, causing phase lag in the open-loop system, which is detrimental to system stability. Furthermore, it does not consider the influence of other inverters in the system.

[0007] 3. An article titled "An Improved Voltage Imbalance Compensation Strategy for Grid-Connected Inverters," published in *Electrical Drive*, 2022, No. 19, pp. 19-24. This compensation strategy, based on traditional droop control, adds a positive-sequence reactive power droop compensation circuit and a negative-sequence voltage feedforward compensation circuit to the power loop to reduce voltage imbalance. However, the compensation effect of this method is affected by the line impedance characteristics, and an excessively large imbalance compensation coefficient can lead to system instability.

[0008] Based on the above literature, the existing technology has the following shortcomings:

[0009] 1. Existing voltage imbalance compensation methods based on droop control power loop additional control are affected by line impedance and cannot completely compensate for voltage imbalance at the point of common coupling. It is necessary to study a common coupling voltage imbalance compensation method with high adaptability to line impedance.

[0010] 2. Existing voltage imbalance compensation methods based on voltage-controlled inverters do not consider the adaptability when there are other control types of inverters in the system. It is necessary to study a common coupling point voltage balance control strategy applicable to hybrid grid-connected systems.

[0011] 3. Existing voltage imbalance compensation methods based on resonant controllers affect the steady-state output impedance characteristics of inverters. It is necessary to study a common coupling point voltage balance control strategy that adapts to system state during startup and shutdown. Summary of the Invention

[0012] The technical problem to be solved by this invention is that the voltage imbalance compensation control strategy at the common coupling point affects the steady-state characteristics of the inverter and is easily affected by line impedance. The invention estimates the magnitude of the negative sequence voltage at the common coupling point by detecting the output voltage and current of the inverter, and adaptively adjusts the output negative sequence voltage of the inverter to reduce the three-phase voltage imbalance at the common coupling point caused by the negative sequence voltage, thereby improving the voltage quality at the common coupling point and facilitating the stable grid connection of other new energy equipment.

[0013] The objective of this invention is achieved by providing a voltage balance control method for the common coupling point of a grid-connected system. The grid-connected system includes one #1 grid-connected inverter operating in current control mode and one #2 grid-connected inverter operating in voltage control mode. The steps of the voltage balance control method are as follows:

[0014] S1, based on the sampled AC side capacitor voltage of grid-connected inverter #2. , , And the grid-connected current of the No. 2 grid-connected inverter , , Power calculations and negative sequence voltage estimation at the point of common coupling were performed to obtain the output active power of the No. 2 grid-connected inverter. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point ;

[0015] S2, based on the active power output of grid-connected inverter #2. The reactive power output of the No. 2 grid-connected inverter Power control is performed to obtain the output angular frequency of the No. 2 grid-connected inverter. and the positive sequence voltage control command value of grid-connected inverter #2 ;

[0016] S3, Estimate the amplitude based on the negative sequence voltage at the common coupling point. The negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control start-up judgment is performed to obtain the negative sequence voltage adaptive control start-up status value of the No. 2 grid-connected inverter. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point ;

[0017] S4, adaptively control the pre-operation state value based on the negative sequence voltage of the #2 grid-connected inverter. Negative sequence voltage index at common coupling point The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control pre-run was performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 ;

[0018] S5, based on the negative sequence of inverter #2. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point 2# Grid-connected inverter negative sequence voltage adaptive control start-up status value The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 Negative sequence adaptive voltage value control is performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter ;

[0019] S6, adaptively control the start-up status value based on the negative sequence voltage of the #2 grid-connected inverter. Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value Output angular frequency of #2 grid-connected inverter and the positive sequence voltage control command value of grid-connected inverter #2 The voltage control signal for the bridge arm of the No. 2 grid-connected inverter is obtained through positive and negative sequence voltage synthesis control. , , Then, the control signal for the power device is obtained through SPWM modulation. After controlling the power device to turn on and off, the system returns to S1 to perform the next round of voltage balance control.

[0020] Preferably, the initial settings of the No. 2 grid-connected inverter operating in voltage control mode are as follows:

[0021] Set the negative sequence voltage adaptive control start-up status value for grid-connected inverter #2. The initial value is 0, and the negative sequence voltage adaptive control shutdown timer value is set for the #2 grid-connected inverter. The initial value is 0, and the pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter is set. The initial value is 0, and the negative sequence voltage adaptive control operating status value of the No. 2 grid-connected inverter is set. The initial value is 0, and the adaptive control variation coefficient of the negative sequence voltage of the No. 2 grid-connected inverter is set. The initial value is 0.

[0022] Preferably, the specific process of power calculation and negative sequence voltage estimation at the common coupling point described in S1 is as follows:

[0023] Sample the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The capacitor voltage of grid-connected inverter #2 was calculated. Axial components , Output current of #2 grid-connected inverter Axial components , The calculation formulas are as follows:

[0024]

[0025]

[0026]

[0027]

[0028] Calculate the output active power of grid-connected inverter #2. The reactive power output of the No. 2 grid-connected inverter Negative sequence voltage estimate at the point of common coupling The calculation formulas are as follows:

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035] in, For the Laplace operator, The imaginary unit, The rated frequency of the three-phase power grid. The damping coefficient of the second-order voltage signal filter. This is the rated resistance value of the branch impedance of grid-connected inverter #2. The rated reactance value of the branch impedance of the #2 grid-connected inverter;

[0036] Based on the negative sequence voltage estimate at the common coupling point The estimated amplitude of the negative sequence voltage at the common coupling point was calculated. , , This represents the modulus of a complex number.

[0037] Preferably, the power control process described in S2 is as follows:

[0038] Based on the active power output of the #2 grid-connected inverter The reactive power output of the No. 2 grid-connected inverter The low-frequency component of the active power of the No. 2 grid-connected inverter was calculated. Low-frequency component of reactive power of grid-connected inverter #2 The calculation formulas are as follows:

[0039]

[0040]

[0041] in, For the Laplace operator, The rated frequency of the three-phase power grid. The damping coefficient of the second-order notch filter is the active power. The time constant of the active power first-order low-pass filter is... The damping coefficient of the second-order notch filter for reactive power is... The time constant of the first-order low-pass filter for reactive power;

[0042] Based on the low-frequency component of the active power of the No. 2 grid-connected inverter The output angular frequency of the No. 2 grid-connected inverter is obtained through the active power-frequency droop control equation. Then, by integration, the output reference angle of the No. 2 grid-connected inverter is obtained. The active-frequency droop control equation and integral form are as follows:

[0043]

[0044]

[0045] in, The active power command is given to the No. 2 grid-connected inverter. The rated angular frequency of the No. 2 grid-connected inverter. The active power-frequency droop factor for the No. 2 grid-connected inverter is given. For the Laplace operator;

[0046] Based on the low-frequency component of reactive power of the No. 2 grid-connected inverter The positive sequence voltage amplitude of the No. 2 grid-connected inverter is obtained through the reactive power-voltage droop control equation. The reactive power-voltage droop control equation is as follows:

[0047]

[0048] in, Give the reactive power command to the No. 2 grid-connected inverter. This refers to the rated output voltage amplitude of the No. 2 grid-connected inverter. The reactive power-voltage droop coefficient for grid-connected inverter #2;

[0049] Based on the output reference angle of grid-connected inverter #2 and the positive sequence voltage amplitude of the No. 2 grid-connected inverter output The positive sequence voltage control command value of grid-connected inverter #2 was calculated. The formula for its calculation is:

[0050]

[0051] in, It is a natural constant.

[0052] Preferably, the specific process of the negative sequence voltage adaptive control start-up judgment in S3 is as follows:

[0053] Based on the negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Estimated amplitude of negative sequence voltage at common coupling point Determine the start-up state value of the negative sequence voltage adaptive control for grid-connected inverter #2. :

[0054]

[0055] in, This is the upper limit reference value for the negative sequence voltage of the #2 grid-connected inverter. This is the lower reference limit for the negative sequence voltage of the No. 2 grid-connected inverter;

[0056] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The calculated negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter was obtained. And the negative sequence voltage adaptive control shutdown state value of the No. 2 grid-connected inverter The calculation formulas are as follows:

[0057]

[0058]

[0059] in, For timing coefficients, The reset coefficient is... The calculation cycle for one round of control, For symbolic functions, the expression is:

[0060]

[0061] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0062]

[0063] in, To initiate the determination of the lower threshold parameter, To initiate the judgment of the proportional coefficient, To initiate the judgment, the upper threshold parameter must be determined, and the following conditions must be met:

[0064]

[0065] Amplitude estimated based on negative sequence voltage at common coupling point Calculate the negative sequence voltage index value at the common coupling point. The formula for its calculation is:

[0066] .

[0067] Preferably, the specific process of the negative sequence voltage adaptive control pre-run in S4 is as follows:

[0068] Based on the pre-operation state value of the voltage adaptive control of the No. 2 grid-connected inverter. The negative sequence of the No. 2 grid-connected inverter was calculated.d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance The calculation formulas are as follows:

[0069]

[0070]

[0071] in, For grid-connected inverter #2, negative sequence d Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence q Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance. For time;

[0072] Based on the negative sequence voltage index value of the common coupling point Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The disturbance response of the negative sequence voltage index at the common coupling point was calculated. Then, the negative sequence voltage adaptive control operating state value of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0073]

[0074]

[0075] in, The negative sequence voltage index at the common coupling point is the damping coefficient of the second-order filter. For the Laplace operator, To determine the threshold parameter during operation, To determine the scaling factor, The threshold parameter is used for pre-run judgment, and it satisfies... , For symbolic functions, the expression is:

[0076] .

[0077] Preferably, the negative sequence adaptive voltage value control process described in S5 is as follows:

[0078] According to the negative sequence of the No. 2 grid-connected inverter d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The calculation formulas are as follows:

[0079]

[0080]

[0081] in, For time, For integration variables, For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance;

[0082] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0083]

[0084] in The integral coefficients of the variable gain inertial element, The calculation cycle for one round of control;

[0085] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control operation status value Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The negative sequence of the No. 2 grid-connected inverter was calculated. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The formula for its calculation is:

[0086]

[0087]

[0088] in, The integral coefficient for voltage-value adaptive control. For the Laplace operator.

[0089] Preferably, the specific process of positive and negative sequence voltage synthesis control in S6 is as follows:

[0090] Based on the negative sequence voltage adaptive control pre-operation state value of the No. 2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The calculation formulas are as follows:

[0091]

[0092]

[0093] in, and For piecewise linear functions, their expressions are as follows:

[0094]

[0095]

[0096] In the formula, To set boundary parameters, and satisfy ;

[0097] Based on the output angular frequency of grid-connected inverter #2 Negative sequence inverter #2 d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value First, calculate the negative sequence voltage control reference angle for grid-connected inverter #2. Then, the negative sequence voltage control command value of the No. 2 grid-connected inverter is calculated. The calculation formulas are as follows:

[0098]

[0099]

[0100] in, The time constant of the low-pass filter for frequency signals. For the Laplace operator, The imaginary unit;

[0101] According to the positive sequence voltage control command value of grid-connected inverter #2 and the negative sequence voltage control command value of grid-connected inverter #2 The voltage control signal of the bridge arm of the No. 2 grid-connected inverter was calculated. , , The control signal for the power device is then obtained through SPWM modulation, and the power device is turned on and off through the drive circuit.

[0102] The voltage control signal of the No. 2 grid-connected inverter bridge arm , , The formula for calculation is:

[0103]

[0104]

[0105]

[0106] in, A function to extract the real part of a complex number. This is a function for extracting the imaginary part of a complex number.

[0107] This invention provides a voltage balance control system for the common coupling point of a grid-connected system, comprising:

[0108] Used to set the start-up status value of the negative sequence voltage adaptive control for grid-connected inverter #2. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Operating status values ​​of negative sequence voltage adaptive control for #2 grid-connected inverter And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter An initialization module for initial values;

[0109] Used to obtain the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The sampling module;

[0110] This is used to obtain the output active power of grid-connected inverter #2 through power calculation and negative sequence voltage estimation at the point of common coupling. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point The estimation module;

[0111] Used to obtain the output angular frequency of grid-connected inverter #2 through power control. and the positive sequence voltage control command value of grid-connected inverter #2 The control module;

[0112] Used to determine the start-up status value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter through the negative sequence voltage adaptive control start-up judgment. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point The judgment module;

[0113] Used to obtain the negative sequence voltage of grid-connected inverter #2 through negative sequence voltage adaptive control pre-operation. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 The processing module;

[0114] Used to obtain the negative sequence voltage value control of grid-connected inverter #2. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The control module;

[0115] Used to obtain the bridge arm voltage control signal of the No. 2 grid-connected inverter through positive and negative sequence voltage synthesis control. , , The control module;

[0116] The module and microprocessor are programmed or configured to perform the steps of the grid-connected system common coupling point voltage balance control method, and a microprocessor and memory are also included.

[0117] The present invention provides a computer-readable storage medium storing a computer program programmed or configured to perform the grid-connected system common coupling point voltage balance control method.

[0118] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0119] 1. The common coupling point voltage balance method of inverter voltage adaptive control described in this invention estimates the magnitude of the negative sequence voltage at the common coupling point by detecting the output voltage and current of the inverter, and adaptively adjusts the output negative sequence voltage of the inverter, thereby improving the voltage balance at the common coupling point.

[0120] 2. The present invention adopts a voltage imbalance compensation control strategy that adaptively starts and stops the system state. The compensation control is only started when an imbalance occurs at the common coupling point. The final compensation effect is equivalent to the inverter being connected in series with a negative sequence voltage source, and does not affect the steady-state characteristics of the voltage-controlled inverter.

[0121] 3. The inverter voltage adaptive control described in this invention performs disturbances near the negative sequence base frequency and adaptively adjusts the negative sequence voltage according to the disturbance response, which is suitable for hybrid grid-connected systems with complex grid impedance characteristics. Attached Figure Description

[0122] Figure 1 This is a schematic diagram of the main circuit structure of the grid-connected system according to an embodiment of the present invention.

[0123] Figure 2 This is a control flowchart of the control method of the present invention.

[0124] Figure 3 The three-phase voltage at the common coupling point of the grid-connected system before and after the application of the control method of this invention. , , The waveform diagram.

[0125] Figure 4 To estimate the amplitude of the negative sequence voltage at the common coupling point before and after using the control method of this invention. The time-domain curve.

[0126] Figure 5 To determine the negative sequence of the No. 2 grid-connected inverter before and after the application of the control method of this invention. d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The time-domain curve.

[0127] Figure 6 To determine the active power output of the No. 1 grid-connected inverter before and after using the control method of this invention. The waveform diagram. Detailed Implementation

[0128] The following is a detailed description of this embodiment with reference to the accompanying drawings.

[0129] The grid-connected system using this method includes one #1 grid-connected inverter operating in current control mode and one #2 grid-connected inverter operating in voltage control mode. Figure 1 This is a circuit diagram of the grid-connected system used in this invention. Figure 1 As can be seen, the grid-connected system includes grid-connected inverter #1, grid-connected inverter #2, grid impedance, and a three-phase grid. The grid impedance includes grid inductance. and grid resistance The outputs of grid-connected inverter #1 and grid-connected inverter #2 are connected in parallel at a common coupling point and sequentially connected through the grid inductor. and grid resistance Connected to a three-phase power grid. The No. 1 and No. 2 grid-connected inverters have the same topology, both including a DC-side power supply, a three-phase bridge inverter circuit, an LC filter, and branch impedances connected in series. The LC filter includes a filter inductor, a filter capacitor, and a damping resistor, and the branch impedance includes a branch inductor and a branch resistance. Figure 1 superior, , , , These are the DC-side power supply, filter inductor, filter capacitor, damping resistor, branch inductor, and branch resistor of the No. 1 grid-connected inverter. , , and These are the DC-side power supply, filter inductor, filter capacitor, damping resistor, branch inductor, and branch resistor of the #2 grid-connected inverter.

[0130] In this embodiment of the invention, the power of grid-connected inverter #1 is 100kW, the power of grid-connected inverter #2 is 20kW, and the rated phase voltage of the three-phase power grid is denoted as... , , . , , , , , , , , , , , , , .

[0131] This invention provides a method for voltage balance control at the common coupling point of a grid-connected system, wherein the grid-connected system includes one #1 grid-connected inverter operating in current control mode and one #2 grid-connected inverter operating in voltage control mode. Figure 2 This is a control flowchart of the control method of the present invention. Figure 2 It can be seen that the voltage balance control method is applied in each control cycle of the No. 2 grid-connected inverter. A round of voltage balance control based on inverter negative sequence voltage adaptive control is performed on the No. 2 grid-connected inverter. The steps of the first round of voltage balance control are as follows:

[0132] Step 1: Sample the AC side capacitor voltage of grid-connected inverter #2. , , And the grid-connected current of the No. 2 grid-connected inverter , , And based on the AC side capacitor voltage of the #2 grid-connected inverter , , And the grid-connected current of the No. 2 grid-connected inverter , , Power calculations and negative sequence voltage estimation at the point of common coupling were performed to obtain the output active power of the No. 2 grid-connected inverter. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point .

[0133] In this embodiment of the invention, the specific process of power calculation and negative sequence voltage estimation at the common coupling point is as follows:

[0134] Sample the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The capacitor voltage of grid-connected inverter #2 was calculated. Axial components , Output current of #2 grid-connected inverter Axial components , The calculation formulas are as follows:

[0135]

[0136]

[0137]

[0138]

[0139] Calculate the output active power of grid-connected inverter #2. The reactive power output of the No. 2 grid-connected inverter Negative sequence voltage estimate at the point of common coupling The calculation formulas are as follows:

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146] in, For the Laplace operator, The imaginary unit, The rated frequency of the three-phase power grid. The damping coefficient of the second-order voltage signal filter. This is the rated resistance value of the branch impedance of grid-connected inverter #2. The rated reactance value of the branch impedance of the #2 grid-connected inverter;

[0147] Based on the negative sequence voltage estimate at the common coupling point The estimated amplitude of the negative sequence voltage at the common coupling point was calculated. , , This represents the modulus of a complex number.

[0148] Step 2, based on the active power output of grid-connected inverter #2 The reactive power output of the No. 2 grid-connected inverter Power control is performed to obtain the output angular frequency of the No. 2 grid-connected inverter. and the positive sequence voltage control command value of grid-connected inverter #2 .

[0149] In this invention, the specific process of power control is as follows:

[0150] Based on the active power output of the #2 grid-connected inverter The reactive power output of the No. 2 grid-connected inverter The low-frequency component of the active power of the No. 2 grid-connected inverter was calculated. Low-frequency component of reactive power of grid-connected inverter #2 The calculation formulas are as follows:

[0151]

[0152]

[0153] in, For the Laplace operator, The rated frequency of the three-phase power grid. The damping coefficient of the second-order notch filter is the active power. The time constant of the active power first-order low-pass filter is... The damping coefficient of the second-order notch filter for reactive power is... The time constant of the first-order low-pass filter for reactive power;

[0154] Based on the low-frequency component of the active power of the No. 2 grid-connected inverter The output angular frequency of the No. 2 grid-connected inverter is obtained through the active power-frequency droop control equation. Then, by integration, the output reference angle of the No. 2 grid-connected inverter is obtained. The active-frequency droop control equation and integral form are as follows:

[0155]

[0156]

[0157] in, The active power command is given to the No. 2 grid-connected inverter. The rated angular frequency of the No. 2 grid-connected inverter. The active power-frequency droop factor for the No. 2 grid-connected inverter is given. For the Laplace operator;

[0158] Based on the low-frequency component of reactive power of the No. 2 grid-connected inverter The positive sequence voltage amplitude of the No. 2 grid-connected inverter is obtained through the reactive power-voltage droop control equation. The reactive power-voltage droop control equation is as follows:

[0159]

[0160] in, Give the reactive power command to the No. 2 grid-connected inverter. This refers to the rated output voltage amplitude of the No. 2 grid-connected inverter. The reactive power-voltage droop coefficient for grid-connected inverter #2;

[0161] Based on the output reference angle of grid-connected inverter #2 and the positive sequence voltage amplitude of the No. 2 grid-connected inverter output The positive sequence voltage control command value of grid-connected inverter #2 was calculated. The formula for its calculation is:

[0162]

[0163] in, It is a natural constant.

[0164] Step 3: Estimate the amplitude based on the negative sequence voltage at the common coupling point. The negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control start-up judgment is performed to obtain the negative sequence voltage adaptive control start-up status value of the No. 2 grid-connected inverter. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point .

[0165] In this embodiment of the invention, the specific process of determining the start-up of the negative sequence voltage adaptive control is as follows:

[0166] Based on the negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Estimated amplitude of negative sequence voltage at common coupling point Determine the start-up state value of the negative sequence voltage adaptive control for grid-connected inverter #2. :

[0167]

[0168] in, This is the upper limit reference value for the negative sequence voltage of the #2 grid-connected inverter. This is the lower reference limit for the negative sequence voltage of the No. 2 grid-connected inverter;

[0169] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The calculated negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter was obtained. And the negative sequence voltage adaptive control shutdown state value of the No. 2 grid-connected inverter The calculation formulas are as follows:

[0170]

[0171]

[0172] in, For timing coefficients, The reset coefficient is... The calculation cycle for one round of control, For symbolic functions, the expression is:

[0173]

[0174] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0175]

[0176] in, To initiate the determination of the lower threshold parameter, To initiate the judgment of the proportional coefficient, To initiate the judgment, the upper threshold parameter must be determined, and the following conditions must be met:

[0177]

[0178] Amplitude estimated based on negative sequence voltage at common coupling point Calculate the negative sequence voltage index value at the common coupling point. The formula for its calculation is:

[0179] .

[0180] Step 4: Adaptively control the pre-operation state value based on the negative sequence voltage of the #2 grid-connected inverter. Negative sequence voltage index at common coupling point The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control pre-run was performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 .

[0181] In this embodiment of the invention, the specific process of the negative sequence voltage adaptive control pre-run is as follows:

[0182] Based on the pre-operation state value of the voltage adaptive control of the No. 2 grid-connected inverter. The negative sequence of the No. 2 grid-connected inverter was calculated. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance The calculation formulas are as follows:

[0183]

[0184]

[0185] in, For grid-connected inverter #2, negative sequence d Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence q Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance. For time;

[0186] Based on the negative sequence voltage index value of the common coupling point Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The disturbance response of the negative sequence voltage index at the common coupling point was calculated. Then, the negative sequence voltage adaptive control operating state value of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0187]

[0188]

[0189] in, The negative sequence voltage index at the common coupling point is the damping coefficient of the second-order filter. For the Laplace operator, To determine the threshold parameter during operation, To determine the scaling factor, The threshold parameter is used for pre-run judgment, and it satisfies... , For symbolic functions, the expression is:

[0190] .

[0191] Step 5, according to the negative sequence of the #2 grid-connected inverter d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point 2# Grid-connected inverter negative sequence voltage adaptive control start-up status value The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 Negative sequence adaptive voltage value control is performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter .

[0192] In this embodiment of the invention, the process of negative sequence adaptive voltage value control is as follows:

[0193] According to the negative sequence of the No. 2 grid-connected inverter d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The calculation formulas are as follows:

[0194]

[0195]

[0196] in, For time, For integration variables, For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance;

[0197] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter was calculated. The formula for its calculation is:

[0198]

[0199] in The integral coefficients of the variable gain inertial element, The calculation cycle for one round of control;

[0200] Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control operation status value Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The negative sequence of the No. 2 grid-connected inverter was calculated. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The formula for its calculation is:

[0201]

[0202]

[0203] in, The integral coefficient for voltage-value adaptive control. For the Laplace operator.

[0204] Step 6: Adaptive control start-up status value based on negative sequence voltage of #2 grid-connected inverter. Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value Output angular frequency of #2 grid-connected inverter and the positive sequence voltage control command value of grid-connected inverter #2 The voltage control signal for the bridge arm of the No. 2 grid-connected inverter is obtained through positive and negative sequence voltage synthesis control. , , Then, the control signal for the power device is obtained through SPWM modulation. After controlling the power device to turn on and off, the process returns to step 1 for the next round of voltage balance control.

[0205] In this embodiment of the invention, the process of positive and negative sequence voltage synthesis control is as follows:

[0206] Based on the negative sequence voltage adaptive control pre-operation state value of the No. 2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The calculation formulas are as follows:

[0207]

[0208]

[0209] in, and For piecewise linear functions, their expressions are as follows:

[0210]

[0211]

[0212] In the formula, To set boundary parameters, and satisfy ;

[0213] Based on the output angular frequency of grid-connected inverter #2 Negative sequence inverter #2 d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value First, calculate the negative sequence voltage control reference angle for grid-connected inverter #2. Then, the negative sequence voltage control command value of the No. 2 grid-connected inverter is calculated. The calculation formulas are as follows:

[0214]

[0215]

[0216] in, The time constant of the low-pass filter for frequency signals. For the Laplace operator, The imaginary unit;

[0217] According to the positive sequence voltage control command value of grid-connected inverter #2 and the negative sequence voltage control command value of grid-connected inverter #2 The voltage control signal of the bridge arm of the No. 2 grid-connected inverter was calculated. , , The control signal for the power device is then obtained through SPWM modulation, and the power device is turned on and off through the drive circuit.

[0218] The voltage control signal of the No. 2 grid-connected inverter bridge arm , , The formula for calculation is:

[0219]

[0220]

[0221]

[0222] in, A function to extract the real part of a complex number. This is a function for extracting the imaginary part of a complex number.

[0223] In this embodiment of the invention, the adaptive control startup state value of the negative sequence voltage of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control is... The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control operating status value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. All values ​​are initially set to 0 during the first round of control.

[0224] In an embodiment of the present invention, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , .

[0225] To demonstrate the beneficial effects of the present invention, a simulation was performed. Figure 3 , Figure 4 , Figure 5 and Figure 6 The simulation results are shown in the figure. The simulation process is described as follows:

[0226] At the start of the simulation, inverter #2 was configured to use power control for grid connection only, without negative sequence voltage adaptive control. Inverter #1 was configured to use PQ control for grid connection, with an active power input of 100kW and a reactive power input of 0kW. Inverters #1 and #2 were connected to a balanced three-phase grid at their common coupling point. At 5 seconds, the grid voltage increased by a negative sequence voltage equal to 15% of the rated voltage. At 10 seconds, inverter #2 began grid connection control using the method of this invention. At 15 seconds, the grid voltage returned to balance, and the simulation ended at 20 seconds. During the simulation, the three-phase voltage at the common coupling point of the grid-connected system was sampled. , , And obtain the active power output of the No. 1 grid-connected inverter. This is to verify the effectiveness of the control method of the present invention.

[0227] Figure 3 The three-phase voltage at the common coupling point of the grid-connected system before and after the application of the control method of this invention. , , The waveform diagram shows that during the 5-10 second period, the #2 grid-connected inverter did not use the control method of this invention, and the three-phase grid voltage was unbalanced. A negative sequence component existed in the three-phase voltage at the common coupling point of the grid-connected system, resulting in voltage imbalance. After the #2 grid-connected inverter adopted the control method of this invention at the 10-second mark, the negative sequence component of the three-phase voltage at the common coupling point of the grid-connected system was suppressed, and the voltage imbalance was significantly reduced.

[0228] Figure 4 To estimate the amplitude of the negative sequence voltage at the common coupling point before and after using the control method of this invention. The time-domain curve shows that, at time 10 seconds, after the control method of this invention is applied to the No. 2 grid-connected inverter, the amplitude is estimated based on the negative sequence voltage at the common coupling point. Adaptive start-up negative sequence voltage control The voltage drops rapidly, and voltage compensation is completed after 11 seconds. Maintaining a low, constant level demonstrates the effectiveness of the control method of the present invention for negative sequence voltage compensation.

[0229] Figure 5 To determine the negative sequence of the No. 2 grid-connected inverter before and after the application of the control method of this invention. d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The time-domain curve shows that after voltage compensation is completed at time 11 seconds. and That is, locking the output value, the compensation result is equivalent to connecting a negative sequence voltage source in series in the voltage-controlled inverter branch. After 15 seconds, the grid voltage returns to balance, and after 1 second of adjustment, the negative sequence voltage of the No. 2 grid-connected inverter... d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The voltage becomes 0, and negative sequence voltage compensation is no longer performed, indicating that the control method of the present invention has little impact on the steady-state characteristics of the system.

[0230] Figure 6 To determine the active power output of the No. 1 grid-connected inverter before and after using the control method of this invention. The waveform diagram shows that when the three-phase grid voltage becomes unbalanced at 5 seconds, the output active power of the current-controlled grid-connected inverter begins to fluctuate. At 10 seconds, after the #2 grid-connected inverter adopts the control method of this invention, the output active power of the #1 grid-connected inverter... The reduced fluctuations are beneficial to the stable operation of the grid connection.

[0231] In summary, Figure 3 , Figure 4 , Figure 5 , Figure 6 This clearly demonstrates the effectiveness of the method proposed in this invention.

[0232] The present invention also provides a voltage balance control system for the common coupling point of a grid-connected system, comprising:

[0233] Used to set the start-up status value of the negative sequence voltage adaptive control for grid-connected inverter #2. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Operating status values ​​of negative sequence voltage adaptive control for #2 grid-connected inverter And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter An initialization module for initial values;

[0234] Used to obtain the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The sampling module;

[0235] This is used to obtain the output active power of grid-connected inverter #2 through power calculation and negative sequence voltage estimation at the point of common coupling. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point The estimation module;

[0236] Used to obtain the output angular frequency of grid-connected inverter #2 through power control. and the positive sequence voltage control command value of grid-connected inverter #2 The control module;

[0237] Used to determine the start-up status value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter through the negative sequence voltage adaptive control start-up judgment. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point The judgment module;

[0238] Used to obtain the negative sequence voltage of grid-connected inverter #2 through negative sequence voltage adaptive control pre-operation. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 The processing module;

[0239] Used to obtain the negative sequence voltage value control of grid-connected inverter #2. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The control module;

[0240] Used to obtain the bridge arm voltage control signal of the No. 2 grid-connected inverter through positive and negative sequence voltage synthesis control. , , The control module;

[0241] The module and microprocessor are programmed or configured to perform the steps of the grid-connected system common coupling point voltage balance control method, and a microprocessor and memory are also included.

[0242] The present invention also provides a computer-readable storage medium storing a computer program programmed or configured to perform the grid-connected system common coupling point voltage balance control method.

[0243] The circuit topology, control strategy, and method of the present invention described above can be viewed as a hardware embodiment of the circuit topology alone, or as a software embodiment containing only the control strategy and method, or as a combined hardware and software implementation based on the control strategy and method of the circuit topology and modules. Furthermore, the control strategy and method of the present invention can be implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code, appearing in the form of a computer program product; and can be implemented using various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0244] Furthermore, the embodiments of the present invention are described in conjunction with flowcharts and / or block diagrams, and should be understood to mean that each block of the flowcharts and / or block diagrams, and combinations of blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing device to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing device, generate instructions for implementing the flowcharts of the present invention. Figure 1 One or more processes and / or boxes Figure 1 The means specifying the functions in one or more boxes. These computer program instructions may also be stored in a computer-readable storage medium capable of directing a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means implemented in the process of the invention. Figure 1 One or more processes and / or boxes Figure 1 The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process of the present invention. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0245] Therefore, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any changes and modifications made by those skilled in the art based on the specific embodiments of the present invention and the above circumstances should be considered as equivalent solutions of this application and should fall within the protection scope of the present invention.

Claims

1. A method for balancing the voltage at the point of common coupling of a grid-connected system comprising a first grid-connected inverter operating in current control mode and a second grid-connected inverter operating in voltage control mode, the method comprising: determining a voltage difference between the voltage at the point of common coupling and a reference voltage; and adjusting the voltage at the point of common coupling by adjusting the voltage at the output of the second grid-connected inverter. The steps of the voltage balance control method are as follows: S1, based on the sampled AC side capacitor voltage of grid-connected inverter #2. , , And the grid-connected current of the No. 2 grid-connected inverter , , Power calculations and negative sequence voltage estimation at the point of common coupling were performed to obtain the output active power of the No. 2 grid-connected inverter. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point ; S2, based on the active power output of grid-connected inverter #2. The reactive power output of the No. 2 grid-connected inverter Power control is performed to obtain the output angular frequency of the No. 2 grid-connected inverter. and the positive sequence voltage control command value of grid-connected inverter #2 ; S3, Estimate the amplitude based on the negative sequence voltage at the common coupling point. The negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control start-up judgment is performed to obtain the negative sequence voltage adaptive control start-up status value of the No. 2 grid-connected inverter. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point ; S4, adaptively control the pre-operation state value based on the negative sequence voltage of the #2 grid-connected inverter. Negative sequence voltage index at common coupling point The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage adaptive control pre-run was performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 ; S5, based on the negative sequence of inverter #2. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point 2# Grid-connected inverter negative sequence voltage adaptive control start-up status value The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 Negative sequence adaptive voltage value control is performed to obtain the negative sequence voltage of the No. 2 grid-connected inverter. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter ; S6, adaptively control the start-up status value based on the negative sequence voltage of the #2 grid-connected inverter. Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value Output angular frequency of #2 grid-connected inverter and the positive sequence voltage control command value of grid-connected inverter #2 The voltage control signal for the bridge arm of the No. 2 grid-connected inverter is obtained through positive and negative sequence voltage synthesis control. , , Then, the control signal for the power device is obtained through SPWM modulation. After controlling the power device to turn on and off, the system returns to S1 to perform the next round of voltage balance control.

2. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The initial settings for the No. 2 grid-connected inverter operating in voltage control mode are as follows: Set the negative sequence voltage adaptive control start-up status value for grid-connected inverter #2. The initial value is 0, and the negative sequence voltage adaptive control shutdown timer value is set for the #2 grid-connected inverter. The initial value is 0, and the pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter is set. The initial value is 0, and the negative sequence voltage adaptive control operating status value of the No. 2 grid-connected inverter is set. The initial value is 0, and the adaptive control variation coefficient of the negative sequence voltage of the No. 2 grid-connected inverter is set. The initial value is 0.

3. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The specific process of power calculation and negative sequence voltage estimation at the common coupling point described in S1 is as follows: Sample the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The capacitor voltage of grid-connected inverter #2 was calculated. Axial components , Output current of #2 grid-connected inverter Axial components , The calculation formulas are as follows: Calculate the output active power of grid-connected inverter #2. The reactive power output of the No. 2 grid-connected inverter Negative sequence voltage estimate at the point of common coupling The calculation formulas are as follows: in, For the Laplace operator, The imaginary unit, The rated frequency of the three-phase power grid. The damping coefficient of the second-order filter for voltage signals. This is the rated resistance value of the branch impedance of grid-connected inverter #2. The rated reactance value of the branch impedance of the #2 grid-connected inverter; Based on the negative sequence voltage estimate at the common coupling point The estimated amplitude of the negative sequence voltage at the common coupling point was calculated. , , This represents the modulus of a complex number.

4. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The specific process of power control described in S2 is as follows: Based on the active power output of the #2 grid-connected inverter The reactive power output of the No. 2 grid-connected inverter The low-frequency component of the active power of the No. 2 grid-connected inverter was calculated. Low-frequency component of reactive power of grid-connected inverter #2 The calculation formulas are as follows: in, For the Laplace operator, The rated frequency of the three-phase power grid. The damping coefficient of the second-order notch filter is the active power. The time constant of the active power first-order low-pass filter is... The damping coefficient of the second-order notch filter for reactive power is... The time constant of the first-order low-pass filter for reactive power; Based on the low-frequency component of the active power of the No. 2 grid-connected inverter The output angular frequency of the No. 2 grid-connected inverter is obtained through the active power-frequency droop control equation. Then, by integration, the output reference angle of the No. 2 grid-connected inverter is obtained. The active-frequency droop control equation and integral form are as follows: in, The active power command is given to the No. 2 grid-connected inverter. The rated angular frequency of inverter #2 is the rated angular frequency. The active power-frequency droop factor for the No. 2 grid-connected inverter is given. For the Laplace operator; Based on the low-frequency component of reactive power of the No. 2 grid-connected inverter The positive sequence voltage amplitude of the No. 2 grid-connected inverter is obtained through the reactive power-voltage droop control equation. The reactive power-voltage droop control equation is as follows: in, The reactive power command is given to the No. 2 grid-connected inverter. This refers to the rated output voltage amplitude of the No. 2 grid-connected inverter. The reactive power-voltage droop coefficient for grid-connected inverter #2; Based on the output reference angle of grid-connected inverter #2 and the positive sequence voltage amplitude of the No. 2 grid-connected inverter output The positive sequence voltage control command value of grid-connected inverter #2 was calculated. The formula for its calculation is: in, It is a natural constant.

5. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The specific process for determining the start-up of the negative sequence voltage adaptive control described in S3 is as follows: Based on the negative sequence voltage adaptive control startup state value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. Estimated amplitude of negative sequence voltage at common coupling point Determine the start-up state value of the negative sequence voltage adaptive control for grid-connected inverter #2. : in, This is the upper limit reference value for the negative sequence voltage of the #2 grid-connected inverter. This is the lower reference limit for the negative sequence voltage of the No. 2 grid-connected inverter; Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. The negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The calculated negative sequence voltage adaptive control shutdown timing value of the No. 2 grid-connected inverter was obtained. And the negative sequence voltage adaptive control shutdown state value of the No. 2 grid-connected inverter The calculation formulas are as follows: in, For timing coefficients, The reset coefficient is... The calculation cycle for one round of control, For symbolic functions, the expression is: Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The pre-operation state value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter was calculated. The formula for its calculation is: in, To initiate the determination of the lower threshold parameter, To initiate the judgment of the proportional coefficient, To initiate the judgment, the upper threshold parameter must be determined, and the following conditions must be met: Amplitude estimated based on negative sequence voltage at common coupling point Calculate the negative sequence voltage index value at the common coupling point. The formula for its calculation is: 。 6. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The specific process of the negative sequence voltage adaptive control pre-run described in S4 is as follows: Based on the pre-operation state value of the voltage adaptive control of the No. 2 grid-connected inverter. The negative sequence of the No. 2 grid-connected inverter was calculated. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance The calculation formulas are as follows: in, For grid-connected inverter #2, negative sequence d Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence q Amplitude of shaft voltage disturbance For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance. For time; Based on the negative sequence voltage index value of the common coupling point Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter The operating status values ​​of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The disturbance response of the negative sequence voltage index at the common coupling point was calculated. Then, the negative sequence voltage adaptive control operating state value of the No. 2 grid-connected inverter was calculated. The formula for its calculation is: in, The negative sequence voltage index at the common coupling point is the damping coefficient of the second-order filter. For the Laplace operator, To determine the threshold parameter during operation, To determine the scaling factor, The threshold parameter is used for pre-run judgment, and it satisfies... , For symbolic functions, the expression is: 。 7. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The negative sequence adaptive voltage value control process described in S5 is as follows: According to the negative sequence of the No. 2 grid-connected inverter d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The calculation formulas are as follows: in, For time, For integration variables, For grid-connected inverter #2, negative sequence dq Frequency of shaft voltage disturbance; Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control starts up with the following status value. Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter obtained from the previous round of voltage balance control. The negative sequence voltage control variation coefficient of the No. 2 grid-connected inverter was calculated. The formula for its calculation is: in The integral coefficients of the variable gain inertial element, The calculation cycle for one round of control; Based on the negative sequence voltage of the No. 2 grid-connected inverter, adaptive control operation status value Negative sequence inverter #2 d Shaft voltage control coefficient Negative sequence inverter #2 q Shaft voltage control coefficient And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The negative sequence of the No. 2 grid-connected inverter was calculated. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The formula for its calculation is: in, The integral coefficient for voltage-value adaptive control. For the Laplace operator.

8. The grid-connected system common coupling point voltage balance control method according to claim 1, characterized in that, The specific process of positive and negative sequence voltage synthesis control described in S6 is as follows: Based on the negative sequence voltage adaptive control pre-operation state value of the No. 2 grid-connected inverter Negative sequence inverter #2 d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Negative sequence inverter #2 d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value The negative sequence of the No. 2 grid-connected inverter was calculated. d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value The calculation formulas are as follows: in, and For piecewise linear functions, their expressions are as follows: In the formula, To set boundary parameters, and satisfy ; Based on the output angular frequency of grid-connected inverter #2 Negative sequence inverter #2 d Shaft voltage control command value Negative sequence inverter #2 q Shaft voltage control command value First, calculate the negative sequence voltage control reference angle for grid-connected inverter #2. Then, the negative sequence voltage control command value of the No. 2 grid-connected inverter is calculated. The calculation formulas are as follows: in, The time constant of the low-pass filter for frequency signals. For the Laplace operator, The imaginary unit; According to the positive sequence voltage control command value of grid-connected inverter #2 and the negative sequence voltage control command value of grid-connected inverter #2 The voltage control signal of the bridge arm of the No. 2 grid-connected inverter was calculated. , , The control signal for the power device is then obtained through SPWM modulation, and the power device is turned on and off through the drive circuit. The voltage control signal of the No. 2 grid-connected inverter bridge arm , , The formula for calculation is: in, A function to extract the real part of a complex number. This is a function for extracting the imaginary part of a complex number.

9. A voltage balance control system for the common coupling point of a grid-connected system, characterized in that, include: Used to set the start-up status value of the negative sequence voltage adaptive control for grid-connected inverter #2. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Operating status values ​​of negative sequence voltage adaptive control for #2 grid-connected inverter And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter An initialization module for initial values; Used to obtain the AC side capacitor voltage of grid-connected inverter #2 , , And the grid-connected current of the No. 2 grid-connected inverter , , The sampling module; This is used to obtain the output active power of grid-connected inverter #2 through power calculation and negative sequence voltage estimation at the point of common coupling. The reactive power output of the No. 2 grid-connected inverter Estimated amplitude of negative sequence voltage at common coupling point The estimation module; Used to obtain the output angular frequency of grid-connected inverter #2 through power control. and the positive sequence voltage control command value of grid-connected inverter #2 The control module; Used to determine the start-up status value of the negative sequence voltage adaptive control of the No. 2 grid-connected inverter through the negative sequence voltage adaptive control start-up judgment. 2# Grid-connected inverter negative sequence voltage adaptive control shutdown timing value #2 Grid-connected Inverter Negative Sequence Voltage Adaptive Control Shutdown Status Value Pre-operation state value of negative sequence voltage adaptive control for #2 grid-connected inverter Negative sequence voltage index at common coupling point The judgment module; Used to obtain the negative sequence voltage of grid-connected inverter #2 through negative sequence voltage adaptive control pre-operation. d shaft voltage disturbance Negative sequence inverter #2 q shaft voltage disturbance Disturbance response of negative sequence voltage index at common coupling point Operating status values ​​of negative sequence voltage adaptive control for grid-connected inverter #2 The processing module; Used to obtain the negative sequence voltage value control of grid-connected inverter #2. d Axis adaptive voltage value Negative sequence inverter #2 q Axis adaptive voltage value And the negative sequence voltage adaptive control variation coefficient of the No. 2 grid-connected inverter The control module; Used to obtain the bridge arm voltage control signal of the No. 2 grid-connected inverter through positive and negative sequence voltage synthesis control. , , The control module; The module and microprocessor are programmed or configured to perform the steps of the grid-connected system common coupling point voltage balance control method according to any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that is programmed or configured to perform the grid-connected system common coupling point voltage balance control method according to any one of claims 1 to 8.

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