Inverter equivalent modeling method and device, storage medium and program product

Abstract

The invention belongs to the technical field of power electronic device control, and particularly relates to an inverter equivalent modeling method and equipment, a storage medium and a program product. According to the method, the difference between the reference value of the component of the output voltage of the inverter on the d axis and the actual value of the component of the output voltage of the inverter on the d axis is obtained, and the result obtained by adjusting and controlling the difference value obtained after the difference is obtained serves as the equivalent duty ratio; the reference value of the reactive power injected into the power grid by the inverter and the actual value of the reactive power injected into the power grid by the inverter are subtracted, and a result obtained by adjusting and controlling the difference value obtained after subtracting is used as the phase difference between the three-phase output voltage of the inverter and the voltage of the power grid; and obtaining a component of the equivalent duty ratio on the three-phase static coordinate system according to the equivalent duty ratio and the phase difference, and performing equivalent modeling on the inverter by using the component. The problem that an existing inverter mathematical model is not suitable for conducting simulation calculation on part of low-frequency simulation software is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic device control, and in particular relates to an inverter equivalent modeling method, equipment, storage medium and program product. Background Art

[0003] Most of my country's photovoltaic power generation uses grid-connected inverters in voltage source current control mode. In order to monitor the stability of the power grid and its recovery ability under fault disturbances, a mathematical model of inverter operation is often established, and the actual operation of the inverter is simulated by simulation software to complete the monitoring. Figure 1 The topology diagram of the three-phase voltage source grid-connected inverter is shown in Figure 1. Figure 1 All components are in an ideal state (the inductor and capacitor are ideal components, and the hysteresis time of the switch tube is ignored). a 、e b 、e c is the grid-side phase voltage, i a 、i b 、i c is the grid-connected current, S a , S b , S c is the switching function of the bridge arm, u a 、u b 、u c is the grid voltage; when the switch is on, the corresponding function value is 1, and when it is off, it is 0; u dc is the DC bus voltage, I dc It can be equivalent to the inverter feedback current, I pv is the output current of the photovoltaic array, C is the equivalent capacitance on the DC side, and ω is the fundamental angular frequency of the power grid.

[0004] When the inverter is in working state, the vector relationship between the amplitude and phase of the output voltage and output current is as follows: Figure 2 As shown, according to Kirchhoff's voltage and current laws, the mathematical model of the inverter is as follows:

[0005]

[0006] in,

[0007]

[0008]

[0009] However, when the mathematical model is applied to some low-frequency simulation software (such as iSimu), the output real-time performance of the existing mathematical model is poor due to the long sampling period (i.e., long calculation period). Therefore, the existing mathematical model is not suitable for simulation calculations on these low-frequency simulation software. Summary of the invention

[0010] The object of the present invention is to provide an inverter equivalent modeling method, device, storage medium and program product to solve the problem that the existing inverter mathematical model is not suitable for simulation calculation on some low-frequency simulation software.

[0011] In order to solve the above technical problems, the present invention provides an inverter equivalent modeling method, which includes:

[0012] 1) Subtracting the reference value of the component of the inverter output voltage on the d-axis from the actual value of the component of the inverter output voltage on the d-axis, adjusting and controlling the difference obtained after the difference, and using the result of the adjustment and control as the equivalent duty cycle; subtracting the reference value of the reactive power injected into the grid by the inverter from the actual value of the reactive power injected into the grid by the inverter, adjusting and controlling the difference obtained after the difference, and using the result of the adjustment and control as the phase difference between the three-phase output voltage of the inverter and the grid voltage;

[0013] 2) according to the equivalent duty cycle and the phase difference between the three-phase output voltage of the inverter and the grid voltage, the component of the equivalent duty cycle in the three-phase stationary coordinate system is obtained;

[0014] 3) According to the relationship between the components of the equivalent duty cycle in the three-phase stationary coordinate system and the inverter output voltage, the inverter is equivalently modeled.

[0015] Furthermore, according to the equivalent duty cycle and the phase difference between the inverter three-phase output voltage and the grid voltage, the specific calculation method of the component of the equivalent duty cycle in the three-phase stationary coordinate system is obtained as follows:

[0016]

[0017] Among them, S ar , S br , S cr is the component of the equivalent duty cycle in the three-phase stationary coordinate system, g is the phase difference between the three-phase output voltage of the inverter and the grid voltage, d is the equivalent duty cycle, ω is the fundamental angular frequency of the grid, and t is the time.

[0018] Furthermore, the reference value of the component of the inverter output voltage on the d-axis is obtained in the following manner: dual closed-loop control of the inverter DC side voltage outer loop and the inverter DC side current inner loop is performed to obtain a d-axis feedforward value; the grid fundamental angular frequency, the AC side inductance value and the actual value of the grid-connected current component on the d-axis are multiplied to obtain a product value; the d-axis feedforward value is subtracted from the product value, the difference obtained after the difference is added to the actual value of the grid voltage component on the d-axis, and the value obtained after the addition is used as the reference value of the inverter output voltage component on the d-axis.

[0019] Furthermore, a reference value of reactive power injected into the power grid by the inverter is 0.

[0020] Furthermore, the reference value of the inverter DC side voltage in the inverter DC side voltage outer loop is the inverter DC side voltage value obtained by tracking the maximum active power injected into the grid by the inverter using a maximum power point tracking method.

[0021] To solve the above technical problem, the present invention also provides a computer device, including a memory, a processor and a computer program stored in the memory, wherein the processor executes the computer program to implement the steps of the inverter equivalent modeling method as described above.

[0022] In order to solve the above technical problem, the present invention also provides a computer-readable storage medium, on which a computer program is stored, and when the computer program is executed by a processor, the steps of the inverter equivalent modeling method as described above are implemented.

[0023] In order to solve the above technical problem, the present invention also provides a computer program product, including a computer program, which implements the steps of the inverter equivalent modeling method as described above when executed by a processor.

[0024] The beneficial effects are as follows: the present invention is an improved invention, which no longer performs equivalent modeling of the inverter according to the relationship between the switching function and the inverter output three-phase AC voltage as in the prior art, but performs equivalent modeling of the inverter according to the relationship between the component of the equivalent duty cycle in the three-phase stationary coordinate system and the inverter output three-phase AC voltage; wherein the equivalent duty cycle is calculated by a control strategy, and the specific calculation process is: subtracting the reference value of the inverter output voltage component on the d-axis from the actual value of the inverter output voltage component on the d-axis, and performing the subtraction on the reference value of the inverter output voltage component on the d-axis. The obtained difference is regulated and controlled, and the result of the regulation and control is used as the equivalent duty cycle; the calculation method of the component of the equivalent duty cycle in the three-phase stationary coordinate system is specifically as follows: the reference value of the reactive power injected by the inverter into the power grid is subtracted from the actual value of the reactive power injected by the inverter into the power grid, the difference obtained after the difference is regulated and controlled, and the result of the regulation and control is used as the phase difference between the three-phase output voltage of the inverter and the power grid voltage; according to the equivalent duty cycle and the phase difference between the three-phase output voltage of the inverter and the power grid voltage, the component of the equivalent duty cycle in the three-phase stationary coordinate system is obtained. Through simulation verification, some low-frequency simulation software that cannot be simulated using the existing inverter mathematical model can be simulated on these low-frequency simulation software using the inverter equivalent mathematical model established by the present invention, and the simulation output results prove that the simulation model of the present invention is effective in low-frequency simulation software. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a topological diagram of a three-phase voltage source grid-connected inverter according to an embodiment of the present invention;

[0026] Figure 2 is an inverter quantity vector relationship diagram of an embodiment of the present invention;

[0027] Figure 3 is a schematic diagram of an inverter control strategy according to an embodiment of the present invention;

[0028] Figure 4 is an effect diagram of the model under the simulated real-time radiation intensity condition of an embodiment of the present invention;

[0029] Figure 5 It is a diagram showing the effect of the model under the condition of sudden radiation intensity simulation of an embodiment of the present invention. DETAILED DESCRIPTION

[0030] An inverter equivalent modeling method, device, storage medium and program product of an embodiment of the present invention performs equivalent modeling on the inverter according to the relationship between the component of the equivalent duty cycle in the three-phase stationary coordinate system and the three-phase AC voltage output by the inverter. The calculation method of the component of the equivalent duty cycle in the three-phase stationary coordinate system is specifically as follows: subtract the reference value of the component of the inverter output voltage on the d-axis from the actual value of the component of the inverter output voltage on the d-axis, adjust and control the difference obtained after the difference, and use the result of the adjustment control as the equivalent duty cycle; subtract the reference value of the reactive power injected by the inverter into the grid from the actual value of the reactive power injected by the inverter into the grid, adjust and control the difference obtained after the difference, and use the result of the adjustment control as the phase difference between the three-phase output voltage of the inverter and the grid voltage; obtain the component of the equivalent duty cycle in the three-phase stationary coordinate system according to the equivalent duty cycle and the phase difference between the three-phase output voltage of the inverter and the grid voltage.

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0032] Method Example:

[0033] An inverter equivalent modeling method according to an embodiment of the present invention is as follows: Figure 1 The topology diagram of the three-phase voltage source grid-connected inverter of the embodiment of the present invention shown includes a capacitor, a bridge inverter circuit, an equivalent inductor and a resistor. The capacitor is arranged in parallel at both ends of the power source (the power source in this embodiment is photovoltaic); the DC side of the bridge inverter circuit is connected in parallel with the capacitor, and the AC side of the bridge inverter circuit is connected to the AC grid through the equivalent inductor and resistor arranged in series.

[0034] Assumptions Figure 1 All components are in an ideal state (the inductor and capacitor are ideal components, and the hysteresis time of the switch tube is ignored). a 、e b 、e c is the grid-side phase voltage, i a 、i b 、i c is the grid-connected current, S a , S b , S c is the switching function of the bridge arm, u a 、u b 、u c is the inverter output voltage; when the switch is turned on, the corresponding switch function value is 1, and when it is turned off, the switch function value is 0; u dc is the DC bus voltage, I dc It can be equivalent to the inverter feedback current, I pvis the output current of the photovoltaic array, C is the equivalent capacitance on the DC side, and ω is the fundamental angular frequency of the power grid.

[0035] In the embodiment of the present invention, the component S of the equivalent duty cycle in the three-phase stationary coordinate system is ar , S br , S cr The inverter outputs three-phase AC voltage u a 、u b 、u c The inverter is equivalently modeled based on the relationship between the inverter and the load. Under low-frequency simulation conditions, the fast switching of the switch tube is achieved to reduce the error generated by the system. The specific steps include:

[0036] Step 1: The inverter in the photovoltaic power generation grid-connected system can be regarded as a converter that converts DC power into AC power. Because the AC signal output by the AC side of the inverter is not convenient for studying its control strategy, the mathematical model of the three-phase stationary coordinate system is transformed into the dq rotating coordinate system using the classic Park transformation, that is:

[0037]

[0038] Among them, u d 、u q are the components of the inverter output voltage on the d-axis and q-axis respectively, i d 、i q are the components of the grid-connected current on the d-axis and q-axis, e d 、e q are the components of the grid voltage on the d-axis and q-axis respectively, and u0 is the voltage of the zero axis (0 axis) perpendicular to the dq plane.

[0039] Step 2: From the instantaneous power theory, we can get the active power P injected into the grid by the inverter at this time: t And reactive power Q t They are:

[0040]

[0041] Among them, the PQ control of the embodiment of the present invention adopts a classic double closed-loop control structure, namely, voltage outer loop control and current inner loop control. The voltage outer loop control is used to determine the reference value of the current, while the current inner loop control is used to quickly track the reference value of the d-axis component of the inverter output voltage. There are two common voltage outer loop control methods: the first is to decouple the active power and reactive power, which is applicable to the two-stage inverter system; the second is to decouple the DC bus voltage and reactive power, which is applicable to the single-stage inverter system. The equivalent mathematical model of the embodiment of the present invention is developed on the basis of the second voltage outer loop control method. When the existing inverter equivalent modeling method adopts the second voltage outer loop control method, the result obtained after PWM modulation is used as the input of the inverter. If the sampling period is long, some low-frequency simulation software cannot use the existing inverter mathematical model for simulation. The present invention omits the PWM modulation process, and directly outputs an equivalent duty cycle through the inverter control strategy to control the inverter. Simulation can also be performed under low-frequency conditions, and the simulation result is highly accurate.

[0042] Step 3: When the PV array outputs to the DC side of the inverter, if the DC power on the DC side is unbalanced with the active power output on the AC side of the inverter, the DC side bus voltage of the inverter will be unstable. In order to keep the DC side bus voltage at the set value, the active current needs to be optimized and controlled. Figure 3 As shown, the embodiment of the present invention optimizes the active current by using a dual closed-loop control method of an inverter DC side voltage outer loop and an inverter DC side current inner loop, specifically including the following steps:

[0043] (1) The value obtained by the maximum power point tracking method is used as the DC bus voltage set value, and the typical PI voltage outer loop control is used as follows:

[0044]

[0045] Among them, u dcref is the given value of DC bus voltage (i.e. the reference value of the DC side voltage of the inverter), i dref is the reference value of the grid current component on the d-axis.

[0046] Among them, the maximum power tracking method is a perturbation observation method; the simulation environment is that the photovoltaic power station must always be in the maximum output state, that is, the maximum power. The perturbation observation method can be used to obtain the maximum power point under the current light intensity, including the maximum power point voltage and the maximum power point power. The DC-DC converter is not passed here, which is equivalent to the output of each photovoltaic array passing through the junction box and the distribution cabinet, and then directly output to the inverter. However, due to the occasional instability of light, voltage mutations may occur, so there is a large capacitor in front of the inverter to prevent such mutations. The inverter has a conversion efficiency parameter, and 100% conversion is the most ideal. There is a multiple of conversion efficiency between the power before and after the inverter, which is approximately equal, so the tracked maximum power point voltage can be used as the given value of the DC bus voltage (capacitor voltage).

[0047] (2) From formula (5), we can see that i d ,i q Mutually coupled, and also with the mutually coupled ωLi d and ωLi q Therefore, the current feedforward decoupling control method is adopted to reduce the control difficulty of the inverter, and the PI current inner loop control is introduced. According to the output result of the PI control (that is, the d-axis feedforward value), the reference value of the inverter output voltage component on the d-axis is calculated. The specific calculation method is:

[0048]

[0049] Among them, u dref is the reference value of the inverter output voltage component on the d-axis.

[0050] (3) The signal output by the PI control is used as the reference value of the component of the inverter output voltage on the d-axis. After the PI control is performed again, the output reference value is used as the equivalent duty cycle signal. The specific PI control is as follows:

[0051]

[0052] Wherein, d is the equivalent duty cycle. The regulating controllers in this embodiment all adopt PI controllers. As other methods, other controllers, such as PID controllers, can be selected according to actual needs.

[0053] Step 4: The reactive component branch controls reactive power. In an ideal photovoltaic power generation system, the expected value of reactive power is 0. The expected value is used as the reference value of reactive power, such as Figure 3 As shown, the control is as follows:

[0054]

[0055] Among them, Q refis the reference value of reactive power, Q is the actual value of reactive power, and g is the phase difference between the three-phase output voltage of the inverter and the grid voltage.

[0056] Step 5: The reactive power in the inverter system generally comes from the power distribution oscillation between different frequency components caused by the periodic opening and closing of the switching devices of the power electronic equipment, which causes the voltage and current to be deformed (making them non-sinusoidal). Then the output signal g of the controller can be used as the difference with the grid phase angle, and then combined with the equivalent duty cycle to achieve the effect of balancing the phase difference between the inverter and the grid. According to the results of steps 3 and 4, the corrected formula can be obtained as follows:

[0057]

[0058] Step 6: Under ideal conditions, when the three-phase symmetrical circuit is running, u a 、u b and u c The sum of the three phases is 0, i a 、i b 、i c The sum of the three phases is 0, and e a 、e b 、e c The sum of the three phases is 0, and the mathematical model of the inverter in the three-phase stationary coordinate system is:

[0059]

[0060] According to formulas (11) to (13), the equivalent mathematical model of the inverter in the three-phase stationary coordinate system is:

[0061]

[0062] According to Kirchhoff's law, the inverter schematic diagram can derive the following formula:

[0063] I dc =S ar i a +S br i b +S cr i c (15)

[0064]

[0065] The present invention uses the relationship between the components of the equivalent duty cycle in the three-phase stationary coordinate system and the three-phase AC voltage output by the inverter to perform equivalent modeling on the inverter;

[0066] In this embodiment, a 500kW single-stage centralized photovoltaic inverter simulation system is built on the iSimu real-time simulation platform. The sampling period is 1ms. Each string is composed of 20 270W photovoltaic panels in series, and the photovoltaic array is composed of 108 strings in parallel. The output DC power is converted into AC power by the inverter.

[0067] The real-time radiation intensity from 9:35 to 10:05 am was simulated. The radiation intensity, the DC power output of the photovoltaic array and the AC power output of the inverter were Figure 4 As shown in the figure, the trend monitoring of the iSimu simulation platform is mainly done by setting upper and lower limits, and then normalizing and presenting the effect of the corresponding parameters in the form of percentage. Since DC power and AC power are similar, the lines in the figure appear to overlap.

[0068] like Figure 5 As shown in the figure, the effect of the model under the sudden change of radiation intensity is simulated. The radiation intensity changes from 400W / m 2 Up to 600W / m 2 Then rise to 800W / m 2 Then it dropped to 700W / m 2 The simulation results are shown in Table 1. At the same time, the photovoltaic array algorithm is written in Matlab. The output power under the same radiation intensity is compared with the results shown in Table 1. Because the inverter efficiency of the inverter is 100% under ideal conditions, the output power is approximately equal to the output power of the photovoltaic array, and the photovoltaic array algorithm is easier to implement in Matlab, so the photovoltaic array output power is used for comparison here.

[0069] Radiant intensity (W / m2) 400 600 800 700 iSimu simulation results (W) 165868 280651 418156 342895 Matlab simulation results (W) 162598 275744 412659 337294 Relative error (%) 0.020111 0.017795 0.013321 0.016334

[0070] Table 1

[0071] Through simulation, it can be seen that as the radiation intensity changes, the simulation model can respond relatively quickly, and by comparing the calculation results of the photovoltaic array algorithm written in Matlab under the same radiation intensity, the relative error is less than 5%, which proves the effectiveness of the model. Through simulation verification, some low-frequency simulation software that cannot use the existing inverter mathematical model for simulation can use the inverter equivalent mathematical model established by the present invention for simulation, and the accuracy of the simulation output results is high.

[0072] Computer equipment example:

[0073] A computer device of the present invention includes a memory, a processor and an internal bus, and the processor and the memory communicate and exchange data with each other through the internal bus. The memory includes at least one software function module stored in the memory, and the processor executes various functional applications and data processing by running the computer program and module stored in the memory to implement an inverter equivalent modeling method introduced in the method embodiment of the present invention. The principle, implementation process, and achievable effects of the method have been fully introduced in the method embodiment and will not be repeated here.

[0074] Computer readable storage medium embodiment:

[0075] A computer-readable storage medium of the present invention stores a computer program thereon, and when the computer program is executed by a processor, an inverter equivalent modeling method described in the method embodiment of the present invention is implemented. The principle, implementation process, and effect that can be achieved by the method have been fully introduced in the method embodiment and will not be repeated here.

[0076] Computer program product embodiment:

[0077] A computer program product of the present invention includes a computer program, and when the computer program is executed by a processor, an inverter equivalent modeling method described in the method embodiment of the present invention is implemented. The principle, implementation process, and effect that can be achieved by the method have been fully introduced in the method embodiment and will not be repeated here.

Claims

1. An inverter equivalent modeling method, characterized in that: The method includes: 1) Subtracting the reference value of the component of the inverter output voltage on the d-axis from the actual value of the component of the inverter output voltage on the d-axis, adjusting and controlling the difference obtained after the difference, and using the result of the adjustment and control as the equivalent duty cycle; subtracting the reference value of the reactive power injected into the grid by the inverter from the actual value of the reactive power injected into the grid by the inverter, adjusting and controlling the difference obtained after the difference, and using the result of the adjustment and control as the phase difference between the three-phase output voltage of the inverter and the grid voltage; 2) according to the equivalent duty cycle and the phase difference between the three-phase output voltage of the inverter and the grid voltage, the component of the equivalent duty cycle in the three-phase stationary coordinate system is obtained; 3) According to the relationship between the components of the equivalent duty cycle in the three-phase stationary coordinate system and the inverter output voltage, the inverter is equivalently modeled.

2. The inverter equivalent modeling method according to claim 1, characterized in that: According to the equivalent duty cycle and the phase difference between the inverter three-phase output voltage and the grid voltage, the specific calculation method of the component of the equivalent duty cycle in the three-phase stationary coordinate system is obtained as follows: Among them, S ar , S br , S cr is the component of the equivalent duty cycle in the three-phase stationary coordinate system, g is the phase difference between the three-phase output voltage of the inverter and the grid voltage, d is the equivalent duty cycle, ω is the fundamental angular frequency of the grid, and t is the time.

3. The inverter equivalent modeling method according to claim 1, characterized in that: The reference value of the component of the inverter output voltage on the d-axis is obtained in the following manner: a dual closed-loop control of the inverter DC side voltage outer loop and the inverter DC side current inner loop is performed to obtain a d-axis feedforward value; the grid fundamental angular frequency, the AC side inductance value and the actual value of the grid-connected current component on the d-axis are multiplied to obtain a product value; the d-axis feedforward value is subtracted from the product value, the difference obtained after the difference is added to the actual value of the grid voltage component on the d-axis, and the value obtained after the addition is used as the reference value of the inverter output voltage component on the d-axis.

4. The inverter equivalent modeling method according to claim 1, characterized in that: The reference value of the reactive power injected into the grid by the inverter is 0.

5. The inverter equivalent modeling method according to claim 3, characterized in that: The reference value of the inverter DC side voltage in the inverter DC side voltage outer loop is the inverter DC side voltage value obtained by tracking the maximum active power injected into the power grid by the inverter using the maximum power point tracking method.

6. A computer device comprising a memory, a processor and a computer program stored in the memory, characterized in that: The processor executes the computer program to implement the steps of the inverter equivalent modeling method according to any one of claims 1 to 5.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the inverter equivalent modeling method according to any one of claims 1 to 5 are implemented.

8. A computer program product, comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the inverter equivalent modeling method according to any one of claims 1 to 5 are implemented.

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