Control circuit, device and method for photovoltaic inverter with reference current regulation

Abstract

A control circuit for a photovoltaic inverter with reference current regulation is characterized in that: the control circuit of the inverter includes a power limiting unit, an active reference current generating unit, a reactive current injection unit, and a coordinate transformation unit; wherein, the power limiting unit provides reactive and active reference voltages to the control circuit, and generates reactive and active reference currents based on the voltages; the active reference current generating unit selects and generates a new active reference current according to the fault condition of a decrease in the output voltage of the photovoltaic power generation system; the reactive current injection unit provides compensation for the reactive reference current according to the fault condition of a decrease or increase in the output voltage of the photovoltaic power generation system; the coordinate transformation unit performs coordinate transformation on the compensated reactive reference current and the new active reference current, and outputs it to the PWM unit to realize the control of the inverter by the PWM unit and realize high and low voltage ride-through.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection, and more specifically, to a control circuit, device, and method for a photovoltaic power generation inverter with reference current regulation. Background Technology

[0002] A photovoltaic (PV) inverter converts the direct current (DC) power output from solar cells into AC power, which is then used to charge batteries. These batteries then supply power to AC loads on the grid. With the widespread application of PV power generation systems and various distributed renewable energy power plants, PV grid-connected inverters have become an indispensable and crucial piece of equipment in PV power generation technology.

[0003] In photovoltaic (PV) grid-connected systems powered by PV inverters, the system's low-voltage ride-through (LVRT) and high-voltage ride-through (HVRT) capabilities are two crucial parameters. These parameters characterize the PV inverter's ability to operate grid-connected PV systems under various power generation conditions. Specifically, if the inverter lacks excellent voltage ride-through capability, sudden grid disconnection or reconnection of PV generators may lead to a deterioration in the power system's operating status and pose a risk of serious accidents. Conversely, if the inverter possesses excellent voltage ride-through performance, the normal operation of the power system can be largely guaranteed during PV generator grid connection or disconnection.

[0004] In the existing technology, there are also relevant studies on the voltage ride-through capability of inverters. For example, for high voltage ride-through, control methods such as virtual impedance and rotor current hysteresis can effectively improve the electromagnetic transient characteristics of the doubly fed motor itself caused by the sudden rise in grid voltage.

[0005] However, existing photovoltaic power generation system models and related calculation processes for voltage ride-through capability are quite complex, requiring the consideration and calculation of high-frequency harmonics to ensure sufficiently accurate results. This results in lengthy simulation times, making it difficult to meet the high-speed simulation requirements for large-scale photovoltaic power plants integrated into the power system. Furthermore, current technologies typically compensate for voltage ride-through capability by installing reactive power compensation devices within the photovoltaic power plant. This approach ignores the reactive power output capability of the photovoltaic inverter itself, failing to effectively utilize its reactive power output and resulting in a degree of waste.

[0006] To address the above problems, this invention provides a novel control circuit, device, and method for a photovoltaic power generation inverter with reference current regulation. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a control circuit, device, and method for a photovoltaic power generation inverter with reference current regulation. This method can determine abnormal conditions such as increases and decreases in the output voltage of a photovoltaic power generation system, and compensate for active and reactive reference currents respectively based on the determination results.

[0008] The present invention adopts the following technical solution.

[0009] The first aspect of this invention relates to a control circuit for a photovoltaic power generation inverter with reference current regulation. The control circuit of the inverter includes a power limiting unit, an active reference current generating unit, a reactive current injection unit, and a coordinate transformation unit. The power limiting unit provides a reactive reference voltage and an active reference voltage to the control circuit, and generates a reactive reference current and an active reference current based on the voltage. The active reference current generating unit selects and generates a new active reference current based on a fault condition of reduced output voltage of the photovoltaic power generation system. The reactive current injection unit compensates for the reactive reference current based on a fault condition of reduced or increased output voltage of the photovoltaic power generation system. The coordinate transformation unit performs coordinate transformation on the compensated reactive reference current and the new active reference current, and outputs the results to a PWM unit to enable the PWM unit to control the inverter and achieve high and low voltage ride-through.

[0010] Preferably, when the active power reference current generating unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, a new active power reference current is generated; when the active power reference current generating unit determines that the output voltage of the photovoltaic power generation system is within or above the steady-state voltage range, a new active power reference current is not generated.

[0011] Preferably, the new active reference current is

[0012]

[0013] Among them, I ref For active reference current, V t_scaled For V t Standard inverter voltage in the range of 0.4 pu to 0.9 pu, V t pu represents the output voltage of the photovoltaic inverter, where pu is the per-unit value.

[0014] Preferably, when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is within the steady-state voltage range, it does not provide compensation for the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, it provides a first compensation current for the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is higher than the steady-state voltage range, it provides a second compensation current for the reactive reference current.

[0015] Preferably, the first compensation current is I. q_l_inject =(V ref -V t C LV ;

[0016] Among them, V ref For a fixed reference voltage, C LV This is the low voltage compensation factor, and its value is a constant.

[0017] Preferably, the second compensation current is I q_h_inject =C hv ·(V olim -V t ) / V t Among them, C HV This is a high-voltage compensation factor, the value of which is determined based on the voltage of the distribution network to which the photovoltaic power generation system is connected; V olim The threshold voltage is 1.1 pu.

[0018] Preferably, the power limiting unit includes a first and a second coupler, a first and a second PI controller, and a first and a second divider; wherein, the first coupler couples the active reference power and the active output power, and is connected to the first N / D through the first PI controller; the second coupler couples the reactive reference power and the reactive output power, and is connected to the second N / D through the second PI controller; the first and second N / D receive the output voltage and generate the reactive reference current and the active reference current, respectively.

[0019] Preferably, the coordinate transformation unit receives the reactive reference current, the active reference current, and the phase-locked loop phase of the inverter, and generates a three-phase control current to be input into the PWM unit.

[0020] A second aspect of the present invention relates to a control device for a photovoltaic inverter with reference current regulation, the device being implemented using the control circuit for a photovoltaic inverter with reference current regulation described in the first aspect of the present invention.

[0021] A third aspect of the present invention relates to a control method for a photovoltaic power generation inverter with reference current regulation. The method is implemented using the control circuit of the photovoltaic power generation inverter with reference current regulation described in the first aspect of the present invention. The method includes the following steps: Step 1, judging the magnitude of the output voltage of the photovoltaic power generation system to obtain the operating state of the inverter; Step 2, based on the operating state of the inverter, using an active reference current generation unit or a reactive current injection unit to control the inverter to achieve high and low voltage ride-through and stable operation of the inverter.

[0022] The beneficial effects of this invention are that, compared with the prior art, the control circuit, device, and method for a photovoltaic power generation inverter with reference current regulation according to this invention can judge abnormal situations such as the rise and fall of output voltage in the photovoltaic power generation system, and realize compensation for active and reactive reference currents respectively based on the judgment results. The calculation process of this invention is simple, the calculation results are accurate, and it can provide reasonable reactive and active reference current compensation when the inverter experiences high-voltage and low-voltage fault ride-through, thereby ensuring the stable operation of the power system.

[0023] The beneficial effects of the present invention also include:

[0024] 1. This invention can utilize a photovoltaic inverter to absorb reactive power in the power grid and simultaneously inject reactive inductive current into the power grid, thereby achieving system stability during voltage faults.

[0025] 2. The model of this invention can significantly shorten the simulation time without adversely affecting the simulation results. This invention can improve the hazard index during power system faults by adjusting the high and low voltage ride-through capabilities of the inverter, thereby enhancing the safety, reliability, and flexibility of photovoltaic power generation system operation. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a grid-connected circuit structure for a photovoltaic power generation inverter in the prior art;

[0027] Figure 2 This is a schematic diagram of the power transistor control circuit in a photovoltaic inverter in the prior art.

[0028] Figure 3 This is a schematic diagram of a simplified model of a photovoltaic inverter connected to the grid in the control circuit of a photovoltaic inverter with reference current regulation according to the present invention.

[0029] Figure 4 This is a schematic diagram showing the change of active reference current with the output voltage of a photovoltaic inverter in the control circuit of a photovoltaic inverter with reference current regulation according to the present invention.

[0030] Figure 5 This is a schematic diagram showing the difference between the threshold voltage and the output voltage as a function of power in the control circuit of a photovoltaic power generation inverter with reference current regulation according to the present invention.

[0031] Figure 6 This is a schematic diagram comparing the low voltage ride-through capability of a photovoltaic inverter with reference current regulation according to the present invention with that of existing systems.

[0032] Figure 7This is a schematic diagram comparing the high voltage ride-through capability of the control circuit of the photovoltaic inverter with reference current regulation according to the present invention with that of the prior art system. Detailed Implementation

[0033] The present application will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and should not be construed as limiting the scope of protection of the present application.

[0034] Figure 1 This is a schematic diagram of the circuit structure for grid-connected photovoltaic power generation inverters in the prior art. Figure 2 This is a schematic diagram of the power transistor control circuit in a photovoltaic inverter in the prior art. Figure 1-2 As shown, the first aspect of this invention relates to a control circuit for a photovoltaic power generation inverter with reference current regulation. Specifically, the control circuit of the inverter includes a power limiting unit, an active power reference current generation unit, a reactive power injection unit, and a coordinate transformation unit. The power limiting unit provides the control circuit with a reactive power reference voltage and an active power reference voltage, and generates a reactive power reference current and an active power reference current based on the voltage. The active power reference current generation unit selects and generates a new active power reference current based on a fault condition of reduced output voltage of the photovoltaic power generation system. The reactive power injection unit compensates for the reactive power reference current based on a fault condition of reduced or increased output voltage of the photovoltaic power generation system. The coordinate transformation unit performs coordinate transformation on the compensated reactive power reference current and the new active power reference current, and outputs the result to the PWM unit to enable the PWM unit to control the inverter and achieve high and low voltage ride-through.

[0035] Specifically, the photovoltaic inverter grid-connected circuit in this invention can adopt a structure commonly used in the prior art, for example... Figure 1 The system circuits and Figure 2 The control circuit structure is described. Of course, in this invention, to shorten the simulation process, improve the accuracy of the calculation results, and reduce the computational load, a simplified model can be adopted to a certain extent.

[0036] Figure 3 This is a schematic diagram of a simplified model of a photovoltaic inverter connected to the grid in the control circuit of a photovoltaic inverter with reference current regulation according to the present invention. Figure 3 right Figure 1 and Figure 2The connection method of existing photovoltaic inverters has been improved. This improvement simplifies the control process of the power transistors to output relatively fixed reference currents Ipvaref, Ipvbref, and Ipvcref. When the duty cycles of the multiple power transistors are fixed, the magnitude of the three-phase reference currents also remains essentially constant. Furthermore, when the turn-on and turn-off times of the power transistors change, the three-phase reference currents can also change based on the adjustment of the control circuit. Therefore, this method effectively omits the power transistors and PWM unit, simplifying the average current output by the power transistors over a certain period of time into a reference current, thus making the simulation of the overall system output power easier.

[0037] This simulation method does not decrease accuracy and also overcomes the problem that the actual output signals of the power transistor and PWM unit are not accurate enough.

[0038] Preferably, when the active power reference current generating unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, a new active power reference current is generated; when the active power reference current generating unit determines that the output voltage of the photovoltaic power generation system is within or above the steady-state voltage range, a new active power reference current is not generated.

[0039] In this invention, the operating state of the system where the inverter is located can be divided into three different situations: steady state, high voltage fault, and low voltage fault, and compensation is provided according to the three different situations.

[0040] When a power grid experiences a voltage dip, the inverter current increases sharply, potentially damaging the power switching transistors and causing low-voltage ride-through failure. However, traditional low-voltage ride-through capabilities focus primarily on power output, with less consideration for current quality and overcurrent issues. Therefore, the key technology for achieving low-voltage ride-through capability through transient control modes lies in controlling the output current during the low-voltage ride-through period. This involves limiting the active reference current output and injecting additional reactive current to provide sufficient reactive power to the system for voltage recovery at the endpoints.

[0041] In existing technologies, a freeze controller module is typically used to freeze the current output when a low-voltage fault occurs. During a low-voltage fault, the freeze controller outputs a high-level signal to activate the device's current freeze mode, limiting the current output. In this case, the freeze controller limits the active current Idmax to between 0 and... Meanwhile, the freeze controller will limit the reactive current Iqmax to the positive and negative maximum current I. max between.

[0042] However, if the value of Idref is too large, it will lead to the value of Iqref being too small, making it difficult to limit the active current to a small range. In this case, if a large reactive current continues to be injected into the circuit, it may cause the total power to exceed the limit. Therefore, to address this problem, this invention introduces a new active reference current. This active reference current can prevent the active reference current from continuing to increase when Vt is too large, and instead maintain it in a relatively constant state.

[0043] Figure 4 This is a schematic diagram illustrating the variation of the active reference current with the output voltage of a photovoltaic inverter with reference current regulation, according to the present invention. Figure 4 As shown, the active reference current I dref It exhibits a piecewise linear output state, and its value is a limit upper limit value; it changes with the voltage Vt.

[0044] When 0.4pu <V t When <0.9 pu, set V t_scaled =V t At this time, I dref_l =I dref When V t When ≤0.4pu, V t_scaled =V t =0, at this time the active reference current is 0; when 0.9pu <V t When the voltage is less than 1 pu, since the voltage is within the normal range, the standard inverter voltage in the control circuit is 1, which is the normal operating state, and there is no limitation on the active power reference current. This is achieved by using V... t Transform into V t_scaled able to obtain when V t The expression for the active reference current when it varies from 0 to 1. Preferably, after the above three active current limiting methods, the new active reference current is:

[0045]

[0046] Among them, I dref This is the active reference current.

[0047] V t_scaled For V t Standard inverter voltage in the range of 0.4 pu to 0.9 pu, V t pu represents the output voltage of the photovoltaic inverter, where pu is the per-unit value.

[0048] Preferably, when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is within the steady-state voltage range, it does not provide compensation for the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, it provides a first compensation current for the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is higher than the steady-state voltage range, it provides a second compensation current for the reactive reference current.

[0049] Preferably, the first compensation current is I. q_l_inject =(V ref -V t C LV Among them, V ref The reference voltage is a fixed value, and CLV is the low voltage compensation factor, which is a constant.

[0050] Specifically, the reason why this formula is used to calculate the compensation current is that during a low-voltage fault, a corresponding capacitive reactive current can be injected according to the degree of voltage drop, with the aim of improving the reactive power output of the inverter and achieving low-voltage ride-through.

[0051] Preferably, the second compensation current is I q_h_inject =C hv ·(V olim -V t ) / V t Among them, C HV This is a high-voltage compensation factor, the value of which is determined based on the voltage of the distribution network to which the photovoltaic power generation system is connected; V olim The threshold voltage is 1.1 pu.

[0052] Figure 5 This is a schematic diagram illustrating the variation of the voltage difference between the reference current and the output voltage as a function of power in the control circuit of a photovoltaic inverter with reference current regulation according to the present invention. Figure 5 As shown, during a high-voltage fault, an appropriate inductive reactive current can be injected according to the degree of voltage surge. The purpose is to enable the inverter to absorb reactive power from the grid and achieve high-voltage ride-through.

[0053] Preferably, the power limiting unit includes a first and a second coupler, a first and a second PI controller, and a first and a second N / D converter, wherein the N / D converter is a divider that divides the input quantity at the N terminal by the input quantity at the D terminal. After power coupling, the input is passed to the N terminal of the PI controller and divided by the input V at the D terminal. pvA reference current is obtained; wherein, the first coupler couples the active reference power and the active output power, and is connected to the first N / D through the first PI controller; the second coupler couples the reactive reference power and the reactive output power, and is connected to the second N / D through the second PI controller; the first and second N / D receive the output voltage and generate the reactive reference current and the active reference current respectively.

[0054] The power limiting unit in this invention is similar to the power limiting unit in the prior art. Both can obtain the active reference current and reactive reference current by taking the values ​​of reference reactive power, reference active power, and parameters such as the voltage Vpv at the photovoltaic access control point of the system.

[0055] Preferably, the coordinate transformation unit receives the reactive reference current, the active reference current, and the phase-locked loop phase of the inverter, and generates a three-phase control current to be input into the PWM unit.

[0056] Figure 6 This is a schematic diagram comparing the low-voltage ride-through capability of a photovoltaic inverter with reference current regulation according to the present invention with that of existing systems. Figure 6 As shown, using the method in this invention, the reactive power reference power of the low voltage fault is set to 0 Mvar, which means that the inverter is operating in a unity power factor state. When the system reaches steady state, a three-phase fault is set to occur at the photovoltaic access point at 2s, and the fault is cleared after 0.5s.

[0057] After adopting the compensated reference current of this invention, the system is more stable during voltage dips, which is beneficial for the further implementation of the control strategy. When the system switches to the low-voltage fault transient control mode, the terminal voltage at the access point drops to 0.6 pu, while when the system continues in steady-state operation mode, the photovoltaic access point voltage remains below 1.2 pu. For the system before the improvement, a momentary increase in active power occurs when the system returns to steady-state operation, affecting the stable operation of the system. This conclusion fully verifies that the improved control strategy has low-voltage ride-through capability for photovoltaic power generation systems.

[0058] Figure 7 This is a schematic diagram comparing the high-voltage ride-through capability of a photovoltaic inverter with reference current regulation according to the present invention with that of a prior art system. Figure 7 As shown, the reactive power reference power in the system is set to 0 Mvar, meaning the inverter operates at unity power factor. After the system reaches steady state, the voltage at the photovoltaic access point spikes after 2 seconds, and the fault is cleared after 0.5 seconds. It is evident that by using the method of this invention to improve the high-voltage fault ride-through capability, the voltage rise at the photovoltaic access point is reduced compared to before, from approximately 1.15 pu to 1.114 pu, ensuring stable system operation under high-voltage fault transient conditions.

[0059] It is evident that the improved model not only significantly shortens the simulation time during the simulation process, but also possesses high and low voltage ride-through capabilities. Furthermore, by adjusting relevant parameters, it can effectively alter the system's regulation capabilities, thereby mitigating the damage caused by system faults to varying degrees and enhancing the reliability, safety, and flexibility of photovoltaic power generation system operation.

[0060] A second aspect of the present invention relates to a control device for a photovoltaic inverter with reference current regulation, which is implemented using a control circuit for a photovoltaic inverter with reference current regulation as described in the first aspect of the present invention.

[0061] It is understood that the control device for the photovoltaic inverter with reference current regulation includes hardware structures and / or software modules corresponding to the execution of each function in order to achieve the various functions provided in the embodiments of this application. Those skilled in the art should readily recognize that, based on the algorithm steps of the examples described herein, this application can be implemented in the form of integrated circuits, etc. The specific implementation of a particular function depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0062] A third aspect of the present invention relates to a control method for a photovoltaic inverter with reference current regulation. The method is implemented using the control circuit of the photovoltaic inverter with reference current regulation described in the first aspect of the present invention. The method includes the following steps: Step 1, judging the magnitude of the output voltage of the photovoltaic power generation system to obtain the operating state of the inverter; Step 2, based on the operating state of the inverter, using an active reference current generating unit or a reactive current injection unit to control the inverter to achieve high and low voltage ride-through and stable operation of the inverter.

[0063] The applicant of this invention has provided a detailed description of the embodiments of the invention in conjunction with the accompanying drawings. However, those skilled in the art should understand that the above embodiments are merely preferred embodiments of the invention. The detailed description is only intended to help readers better understand the spirit of the invention and is not intended to limit the scope of protection of the invention. On the contrary, any improvements or modifications made based on the inventive spirit of the invention should fall within the scope of protection of the invention.

Claims

1. A control circuit for a photovoltaic power generation inverter with reference current regulation, characterized in that: The control circuit of the inverter includes a power limiting unit, an active reference current generating unit, a reactive current injection unit, and a coordinate transformation unit. The power limiting unit provides the control circuit with a reactive reference voltage and an active reference voltage, and generates a reactive reference current and an active reference current based on the voltage. The active reference current generating unit selects to generate a new active reference current based on the fault condition of reduced output voltage of the photovoltaic power generation system. The reactive current injection unit provides compensation for the reactive reference current based on the fault conditions of the photovoltaic power generation system's output voltage decreasing or increasing. When the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is within the steady-state voltage range, it does not provide compensation for the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, it provides a first compensation current to the reactive reference current; when the reactive current injection unit determines that the output voltage of the photovoltaic power generation system is higher than the steady-state voltage range, it provides a second compensation current to the reactive reference current. The first compensation current is ; in, For a fixed reference voltage, The output voltage of the photovoltaic inverter is [missing information]. This is the low voltage compensation factor, and its value is a constant. The second compensation current is ; in, This is a high-voltage compensation factor, and its value is determined based on the voltage of the distribution network to which the photovoltaic power generation system is connected. The threshold voltage is 1.1 pu; The coordinate transformation unit performs coordinate transformation on the compensated reactive reference current and the new active reference current, and outputs the result to the PWM unit to enable the PWM unit to control the inverter and achieve high and low voltage ride-through.

2. The control circuit for a photovoltaic inverter with reference current regulation as described in claim 1, characterized in that: When the active reference current generating unit determines that the output voltage of the photovoltaic power generation system is lower than the steady-state voltage range, it generates the new active reference current. When the active reference current generating unit determines that the output voltage of the photovoltaic power generation system is within or above the steady-state voltage range, it does not generate a new active reference current.

3. The control circuit for a photovoltaic inverter with reference current regulation according to claim 2, characterized in that: The new active reference current is in, The active reference current is... For V t Standard inverter voltage in the range of 0.4 pu to 0.9 pu The output voltage of the photovoltaic inverter is [missing information]. pu is the per-unit value.

4. The control circuit for a photovoltaic inverter with reference current regulation as described in claim 1, characterized in that: The power limiting unit includes a first and a second coupler, a first and a second PI controller, and a first and a second divider; The first coupler couples the active reference power and the active output power, and is connected to the first N / D converter via the first PI controller. The second coupler couples the reactive reference power and the reactive output power, and connects to the second N / D controller via the second PI controller. The first N / D and the second N / D receive the output voltage and generate the reactive reference current and the active reference current, respectively.

5. The control circuit for a photovoltaic inverter with reference current regulation according to claim 4, characterized in that: The coordinate transformation unit receives the reactive reference current, the active reference current, and the phase-locked loop phase of the inverter, and generates a three-phase control current to be input into the PWM unit.

6. A control device for a photovoltaic power generation inverter with reference current regulation, characterized in that: The device is implemented using the control circuit of a photovoltaic power generation inverter with reference current regulation as described in any one of claims 1-5.

7. A control method for a photovoltaic power generation inverter with reference current regulation, characterized in that: The method is implemented using a control circuit for a photovoltaic power generation inverter with reference current regulation as described in any one of claims 1-5; and the method includes the following steps: Step 1: Determine the magnitude of the output voltage of the photovoltaic power generation system to obtain the operating status of the inverter; Step 2: Based on the operating state of the inverter, the inverter is controlled by an active power reference current generation unit or a reactive power injection unit to achieve high and low voltage ride-through and stable operation of the inverter.

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