Photovoltaic system and parallel operation circulation suppression method thereof

Abstract

The embodiment of the invention discloses a photovoltaic system and a parallel operation circulating current suppression method thereof, the photovoltaic system comprises a plurality of power converters connected in parallel, each power converter comprises an inversion module, an output filter, a detection circuit and a controller, and the output filter comprises a variable inductor and / or a variable capacitor. The variable inductor comprises a first winding connected with the first switching circuit in series, and the controller can control the first switching circuit to be switched on or switched off according to the current value, detected by the detection circuit, flowing through the output filter so as to change the inductance value of the variable inductor. The variable capacitor is connected in series with the second switching circuit between the neutral line and the alternating current bus, and the controller can control the second switching circuit to be switched on or switched off according to the current value which is detected by the detection circuit and flows through the output filter so as to change the state that the variable capacitor is connected to the alternating current bus. By changing the inductance value and / or the capacitance value, the position of the resonant frequency point of the output filter can be changed, so that the parallel operation circulating current is inhibited.

Description

[0001] This application claims the priority of the Chinese patent application filed with the State Intellectual Property Office of China on September 26, 2023, with application number 202311259686.1 and application name “A photovoltaic system and its parallel circulating current suppression method”, all contents of which are incorporated by reference in this application. Technical Field

[0002] The present application relates to the field of power electronics technology, and in particular to a photovoltaic system and a method for suppressing parallel circulating current thereof. Background Art

[0003] In photovoltaic systems such as photovoltaic power stations, energy storage power stations or photovoltaic storage power stations, multiple parallel power conversion devices are generally included, and each power conversion device is composed of an inverter or energy storage converter connected in series with an output filter. With the increase in the types of inverters or energy storage converters and the increasing complexity of photovoltaic system application scenarios, in photovoltaic systems, different types of inverters or energy storage converters are combined in parallel, and even inverters and energy storage converters are combined in parallel. In other words, the parallel scenarios on the AC grid side are becoming more and more complicated. The parallel combination will have the risk of high-frequency parallel circulation, which will increase the current and affect the reliability of passive components such as capacitors and inductors in the filter. In severe cases, it may even damage passive components such as inductors and capacitors. Especially in the photovoltaic storage fusion scenario, when the inverter is shut down at night, the outermost capacitor of the external port will be hung on the grid side. When the energy storage converter is working, a resonance scenario is likely to occur, resulting in an increase in the circulation. Therefore, how to suppress the parallel circulation and improve system reliability is a technical problem that needs to be solved in this field. Summary of the invention

[0004] The embodiments of the present application provide a photovoltaic system and a method for suppressing parallel circulating current thereof, so as to solve the problem that the parallel circulating current affects the reliability of the system.

[0005] In a first aspect, an embodiment of the present application provides a photovoltaic system, including a plurality of parallel power converters, each power converter including an inverter module, an output filter, a detection circuit and a controller. The input end of the inverter module is connected to a DC source through a DC bus, the output end of the inverter module is connected to an AC bus, and the AC bus is connected to a power grid. The output filter includes a variable inductor and / or a variable capacitor. The inverter module is used to convert the input DC power into AC power and output it to the AC bus. The inverter module can adopt a three-phase four-wire inverter circuit or an inverter circuit of other structures, for example, a neutral point clamped (NPC) inverter circuit can be adopted, and the inverter circuit can specifically include two bus capacitors connected in series between the DC bus and three switch bridge arms connected in parallel between the DC bus, each switch bridge arm can include four switch tubes arranged in series and two clamping diodes arranged in series, wherein the connection point of the middle two switch tubes is used as the midpoint of the bridge arm for connecting to one of the three phases (phase A, phase B and phase C) in the AC bus, and the midpoint of the two clamping diodes is used to connect to the neutral line (N line). The output filter is connected to the AC bus and is used to filter the AC power transmitted by the AC bus so that the AC power transmitted to the power grid meets the waveform requirements. The detection circuit is used to detect the current value flowing through the output filter. The controller is used to change the inductance of the variable inductor and / or the total capacitance value in the output filter according to the current value flowing through the output filter, so as to change the resonant frequency point position of the output filter and keep it away from the switching frequency, thereby suppressing the parallel machine circulating current and avoiding affecting the system reliability.

[0006] In an embodiment of the present application, a variable inductor is connected in series with an AC bus, and the variable inductor includes a first winding wound on a magnetic core, and the first winding is connected in series with a first switch circuit, that is, the input end and the output end of the first winding are respectively connected to the signal input end and the signal output end of the first switch circuit, and the on-off state of the first winding can be switched by controlling the on-off state of the first switch circuit to change the inductance of the variable inductor. A controller is connected to the control end of the first switch circuit, and when the controller determines that the current value output by the detection circuit exceeds the set threshold range, it determines that the resonant frequency generated by the output filter needs to be adjusted, and the controller can send a corresponding control signal to the first switch circuit. When the controller controls the first switch circuit to switch from open to closed, the first winding is turned on to generate an induced current, which will offset the original magnetic flux in the magnetic circuit of the magnetic core, and the inductance of the variable inductor is reduced accordingly, and the frequency point position of the resonant frequency of the output filter is increased; when the controller controls the first switch circuit to switch from closed to open, the first winding is disconnected and no induced current is generated, the magnetic flux in the magnetic circuit of the magnetic core increases, the inductance of the variable inductor is increased accordingly, and the frequency point position of the resonant frequency of the output filter is reduced.

[0007] In the embodiment of the present application, the variable capacitor is connected in series with the second switch circuit and then connected between the AC bus and the neutral line. A group of variable capacitors may include three sub-capacitors connected in parallel, each of which is connected between one of the three phases (phase A, phase B, and phase C) of the AC bus and the N line. The second switch circuit may be arranged between the N line and the sub-capacitor, or between the three phases and the sub-capacitor. By controlling the on-off state of the second switch circuit, the variable capacitor may be connected to or disconnected from the AC bus. The controller is connected to the control end of the second switch circuit. When the controller determines that the current value output by the detection circuit exceeds the set threshold range, it determines that the resonant frequency generated by the output filter needs to be adjusted. The controller can send a corresponding control signal to the second switch circuit. When the controller controls the second switch circuit to switch from open to closed, the variable capacitor is connected to the AC bus. At this time, the number of capacitors connected in parallel in the output filter increases, the total capacitance value of the output filter increases accordingly, and the frequency point position of the resonant frequency of the output filter decreases; when the controller controls the second switch circuit to switch from closed to open, the variable capacitor is cut out of the AC bus. At this time, the number of capacitors connected in parallel in the output filter decreases, the total capacitance value of the output filter decreases accordingly, and the frequency point position of the resonant frequency of the output filter increases.

[0008] In some embodiments of the present application, a current sensor or a current transformer may be provided inside the output filter, and the detection circuit may determine the current value flowing through the output filter by detecting the current value flowing through the current sensor or the current transformer. The current sensor or the current transformer may be provided at the front side of any one of the three-phase inductors in the variable inductor, or the current sensor or the current transformer may also be provided between any one of the three sub-capacitors in the variable capacitor and the corresponding phase.

[0009] In some other embodiments of the present application, the variable inductor may also include a second winding, which is different from the first winding, and the second winding is disconnected between the input and output ends, that is, the second winding will not be closed to generate an induced current, but will offset the original magnetic flux in the magnetic circuit of the magnetic core to change the inductance of the variable inductor. However, the original magnetic flux in the magnetic circuit of the magnetic core of the variable inductor will generate an induced current in the second winding and be detected. Therefore, the second winding can replace the current sensor or current transformer set in the output filter, and the detection circuit can determine the current value flowing through the output filter by detecting the induced current value between the input and output ends of the second winding. The detection is achieved by adding an additional second winding to the variable inductor, which can reduce the cost of the entire detection circuit.

[0010] In other embodiments of the present application, the second winding and the first winding can be integrated into the same winding in the variable inductor, that is, a first winding realizes the function of the second winding, and the first switch circuit is connected between the input and output ends of the first winding, and the input and output ends of the first winding are connected to the detection circuit. When the first switch circuit is in an open state, the first winding is used as the second winding, that is, the detection circuit can determine the current value flowing through the output filter by detecting the induced current value between the input and output ends of the first winding. When it is determined that the inductance needs to be reduced, the controller can control the first switch circuit to switch to a closed state, and the first winding is turned on to generate an induced current to offset the original magnetic flux in the magnetic circuit of the magnetic core. The inductance of the variable inductor is reduced accordingly, and the frequency point position of the resonant frequency of the output filter is increased. In the variable inductor, the detection is realized by reusing the first winding as the second winding, which can further reduce the cost of the entire detection circuit.

[0011] In an embodiment of the present application, the output filter may specifically include a multi-stage inductor connected in series to the AC bus, the first-stage inductor in the multi-stage inductor is connected to the inverter module, the last-stage inductor is connected to the power grid, and at least one inductor in the multi-stage inductor except the first-stage inductor can be set as a variable inductor.

[0012] In an embodiment of the present application, the output filter also includes multiple groups of capacitors connected to the AC bus, the second-stage inductor in the multi-stage inductor is connected in series to the AC bus between the first-stage inductor and the power grid, the first group of capacitors in the multiple groups of capacitors is connected to the AC bus between the first-stage inductor and the second-stage inductor, and other groups of capacitors can be connected between the second-stage inductor and the power grid. At least one group of capacitors among the multiple groups of capacitors except the first group of capacitors can be set as a variable capacitor.

[0013] In an embodiment of the present application, the first switch circuit may include one or more switch devices, or the first switch circuit may also include one or more switch devices and one or more passive devices. The second switch circuit may include one or more switch devices, or the second switch circuit may also include one or more switch devices connected in series or in parallel and one or more passive devices, and the passive device may be a resistor. The switch devices in the first switch circuit and the second switch circuit may be one or more of various types of switch devices such as mechanical switches, relays, circuit breakers, contactors, metal oxide semiconductor field effect transistors (metaloxide semiconductor field effect transistors, MOSFET), bipolar junction transistors (bipolar junction transistors, BJT), insulated gate bipolar transistors (insulated gate bipolar transistors, IGBT), etc.

[0014] In the embodiment of the present application, the DC source can be a photovoltaic panel or an energy storage device. When the input end of the inverter module is connected to the photovoltaic panel, the power converter can be a photovoltaic inverter; when the input end of the inverter module is connected to the energy storage device, the power converter can be an energy storage converter. In addition, the inverter module can be directly connected to the DC source, or the inverter module can be connected to the DC source through a DC-DC conversion circuit.

[0015] In the second aspect, the embodiment of the present application also provides a method for suppressing the parallel circulating current of a photovoltaic system, including: a detection circuit detects the current value flowing through the output filter; a controller controls the first switch circuit to be turned on or off in response to the current value flowing through the output filter to change the inductance of the variable inductor; and / or, the controller controls the second switch circuit to be turned on or off in response to the current value flowing through the output filter to change the state of the variable capacitor connected to the AC bus. By changing the inductance of the variable inductor and / or changing the total capacitance value in the output filter, the resonant frequency point position of the output filter can be changed to keep it away from the switching frequency, thereby suppressing the parallel circulating current and avoiding affecting the system reliability.

[0016] In some embodiments of the present application, a current sensor or a current transformer may be provided inside the output filter, and the detection circuit may determine the current value flowing through the output filter by detecting the current value flowing through the current sensor or the current transformer.

[0017] In other embodiments of the present application, a second winding may be provided in the variable inductor, and the input and output ends of the second winding may be disconnected. The second winding may replace the current sensor or current transformer provided in the output filter, and the detection circuit may determine the current value flowing through the output filter by detecting the induced current value between the input and output ends of the second winding. The detection may be implemented by adding an additional second winding to the variable inductor, which may reduce the overall detection cost.

[0018] The technical effects that can be achieved by any possible design in the second aspect can be referred to the technical effects that can be achieved by any possible design in the first aspect, and will not be repeated here. These or other aspects of the present application will be more concise and easy to understand in the description of the following embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the framework structure of a photovoltaic system;

[0020] Figure 2 A schematic diagram of a circuit structure of a photovoltaic system;

[0021] Figure 3 A schematic diagram of a circuit structure of a photovoltaic system provided in an embodiment of the present application;

[0022] Figure 4 Another schematic diagram of a circuit structure of a photovoltaic system provided in an embodiment of the present application;

[0023] Figure 5 Another schematic diagram of a circuit structure of a photovoltaic system provided in an embodiment of the present application;

[0024] Figure 6 Another schematic diagram of a circuit structure of a photovoltaic system provided in an embodiment of the present application;

[0025] Figure 7 A schematic diagram of a structure of a variable inductor in a photovoltaic system provided in an embodiment of the present application;

[0026] Figure 8 Another structural schematic diagram of a variable inductor in a photovoltaic system provided by an embodiment of the present application;

[0027] Fig. 9 Another structural schematic diagram of a variable inductor in a photovoltaic system provided by an embodiment of the present application;

[0028] Fig.10 A schematic diagram of a flow chart of a method for suppressing circulating current in parallel in a photovoltaic system provided in an embodiment of the present application;

[0029] Fig.11a A schematic diagram of a circuit structure of a switch circuit in a photovoltaic system provided in an embodiment of the present application;

[0030] Fig.11b Another circuit structure schematic diagram of a switch circuit in a photovoltaic system provided in an embodiment of the present application.

[0031] Reference numerals:

[0032] 110-photovoltaic array; 120-power converter; 130-grid; 140-external load; 150-energy storage device; 160-DC bus; 170-AC bus; 01-inverter module; 02-output filter; 03-detection circuit; 04-controller; Lm-variable inductor; L1-first stage inductor; L2-second stage inductor; Cm-variable capacitor; C1-first group of capacitors; C2-second group of capacitors; Cn-Nth group of capacitors; K1-first switching circuit, K2-second switching circuit; X1-first winding; X2-second winding. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings. The specific operating method in the method embodiment can also be applied to the device embodiment or the system embodiment. It should be noted that in the description of the present application, "at least one" refers to one or more, wherein multiple refers to two or more. In view of this, "multiple" can also be understood as "at least two" in the embodiment of the present invention. "And / or" describes the association relationship of the associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / ", unless otherwise specified, generally indicates that the related objects before and after are in an "or" relationship. In addition, it should be understood that in the description of the present application, words such as "first" and "second" are only used to distinguish the purpose of description, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order.

[0034] It should be pointed out that in the embodiments of the present application, "connection" can be understood as electrical connection, and the connection between two electrical components can be a direct or indirect connection between the two electrical components. For example, the connection between A and B can be either a direct connection between A and B, or an indirect connection between A and B through one or more other electrical components, such as A and B connection, or A and C direct connection, C and B direct connection, and A and B are connected through C. In some scenarios, "connection" can also be understood as coupling, such as electromagnetic coupling between two inductors. In short, the connection between A and B enables the transmission of electrical energy between A and B.

[0035] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. First, the application scenarios of the photovoltaic system provided in the embodiments of the present application are introduced. The photovoltaic system provided in the embodiments of the present application can be applied to scenarios such as photovoltaic power stations, energy storage power stations, or photovoltaic storage power stations.

[0036] Figure 1 The schematic diagram of the framework structure of a photovoltaic system is shown as an example. Figure 1The photovoltaic system includes a photovoltaic array 110, a power converter 120, an energy storage device 150, and a power grid 130 / external load 140. The power converter 120 may integrate a DC / DC conversion circuit and a DC / AC conversion circuit, the energy storage device 150 may be located between the DC / DC conversion circuit and the DC / AC conversion circuit, the electric energy generated by the photovoltaic array 110 may be charged by the energy storage device 150 after being stepped up or stepped down by the DC / DC conversion circuit, and the electric energy generated by the photovoltaic array 110 may also be transmitted to the power grid 130 / external load 140 through the DC / DC conversion circuit and the DC / AC conversion circuit. When the electric energy generated by the photovoltaic array 110 is insufficient to supply power to the power grid 130 / external load 140, the electric energy stored in the energy storage device 150 may be transmitted to the power grid 130 / external load 140 through the DC / AC conversion circuit in the power converter 120. On the other hand, the energy storage device 150 can also receive power from the power grid 130 . The alternating current output by the power grid 130 is converted into direct current after passing through a DC / AC conversion circuit and transmitted to the energy storage device 150 , thereby charging the energy storage device 150 .

[0037] Figure 2 The schematic diagram of the circuit structure of a photovoltaic system is shown as an example. Figure 2In practical applications, a photovoltaic system generally includes multiple parallel power converters 120. A power converter 120 can be connected to a group of photovoltaic panels in the photovoltaic array 110, and is used to convert the direct current output by the photovoltaic panels and transmit it to the power grid 130. A power converter 120 can also be connected to the energy storage device 150, and is used to convert the direct current output by the energy storage device 150 and transmit it to the power grid 130. Therefore, the photovoltaic array 110 and the energy storage device 150 can be collectively referred to as a direct current source. Each power converter 120 includes an inverter module 01 and an output filter 02; wherein the input end of the inverter module 01 is connected to the direct current source through the direct current bus 160, and the output end of the inverter module 01 is connected to the alternating current bus 170, and the alternating current bus 170 is connected to the power grid 130. The inverter module 01 is used to convert the direct current input by the photovoltaic panel (or the energy storage device 150) into alternating current and output it to the alternating current bus 170. The inverter module 01 can adopt a three-phase four-wire inverter circuit, for example, a neutral point clamped (NPC) inverter circuit, the inverter circuit can specifically include two bus capacitors connected in series between the DC bus 160 and three switch bridge arms connected in parallel between the DC bus 160, each switch bridge arm can include four switch tubes arranged in series and two clamping diodes arranged in series, wherein the connection point of the middle two switch tubes is used as the bridge arm midpoint for connecting to one of the three phases (A phase, B phase and C phase) in the AC bus 170, and the midpoint of the two clamping diodes is used for connecting to the neutral line (N line). The output filter 02 is connected to the AC bus 170, and is used to filter the AC power transmitted by the AC bus 170, so that the AC power transmitted to the power grid 130 meets the waveform requirements. When the DC source connected to the inverter module 01 is a photovoltaic panel, the power converter 120 can be specifically a photovoltaic inverter, and the inverter module 01 can be specifically an inverter circuit of the photovoltaic inverter; when the DC source connected to the inverter module 01 is an energy storage device 150, the power converter 120 can be specifically an energy storage converter, and the inverter module 01 can be specifically an inverter circuit of the energy storage converter. The output filter 02 can have a variety of structures, for example, an LCL filter can be used, which can be specifically composed of a first-stage inductor L1, a first group of capacitors C1, a second-stage inductor L2, and a second group of capacitors C2 connected in sequence to the AC bus 170; wherein the first group of capacitors C1 mainly plays a filtering role, and the second group of capacitors C2 mainly plays a role in suppressing conducted interference and radiated interference, and one end of each group of capacitors can be connected to the AC bus 170, and the other end can be connected to the N line. When multiple power converters 120 are connected in parallel (also called parallel) on the AC bus 170, Figure 2In the example of two power converters 120 connected in parallel, the inverter module 01 of the power converter 120 becomes an excitation source, which excites the output filter 02 to generate a parallel circulation current, which increases the current stress of the capacitor and inductor in the output filter 02, reduces the reliability of the inductor and capacitor, and even damages the inductor and capacitor in severe cases. Therefore, how to suppress the parallel circulation current to improve system reliability is a technical problem that needs to be solved in this field.

[0038] Since the impedance characteristic of the output filter 02 is a key factor in generating the parallel machine circulating current, and the inductance value (also called inductance) of the inductor or the capacitance value of the capacitor in the output filter 02 will affect the magnitude of the resonant frequency of the output filter 02, if the frequency point of the resonant frequency is adjusted to keep it away from the switching frequency of the inverter module 01, the magnitude of the parallel machine circulating current will be suppressed. Based on this, the photovoltaic system and the parallel machine circulating current suppression method provided by the embodiment of the present application, by setting a variable inductor and / or a variable capacitor in the output filter 02, and monitoring the current flowing through the output filter 02, and selecting to change the inductance of the variable inductor and / or the number of capacitors connected to the AC bus, can change the frequency point position of the resonant frequency of the output filter 02, keep it away from the switching frequency, thereby suppressing the parallel machine circulating current and avoiding affecting the system reliability.

[0039] Figure 3 A schematic diagram of a circuit structure of a photovoltaic system provided in an embodiment of the present application is shown as an example. Figure 4 Another circuit structure diagram of a photovoltaic system provided by an embodiment of the present application is exemplarily shown. Figure 5 Another circuit structure diagram of a photovoltaic system provided by an embodiment of the present application is exemplarily shown. Figure 6 Another circuit structure schematic diagram of a photovoltaic system provided in an embodiment of the present application is shown by way of example.

[0040] Reference Figures 3 to 6 In the embodiment of the present application, the photovoltaic system 100 includes a plurality of parallel power converters 120, each of which includes an inverter module 01, an output filter 02, a detection circuit 03 and a controller 04. Figures 3 to 6In the figure, a power converter 120 is used as an example for explanation. The input end of the inverter module 01 is connected to the DC source through the DC bus 160, and the output end of the inverter module 01 is connected to the AC bus 170, and the AC bus 170 is connected to the power grid 130. The inverter module 01 can adopt a three-phase four-wire inverter circuit or an inverter circuit of other structures, for example, a neutral point clamped (NPC) inverter circuit can be adopted, and the inverter circuit can specifically include two bus capacitors connected in series between the DC bus 160 and three switch bridge arms connected in parallel between the DC bus 160, each switch bridge arm can include four switch tubes arranged in series and two clamping diodes arranged in series, wherein the connection point of the middle two switch tubes is used as the bridge arm midpoint for connecting to one of the three phases (A phase, B phase and C phase) in the AC bus 170, and the midpoint of the two clamping diodes is used for connecting to the neutral line (N line). The following is an example of the inverter module 01 adopting a three-phase four-wire inverter circuit structure. The output filter 02 includes a variable inductor Lm and / or a variable capacitor Cm. The inverter module 01 is used to convert the input direct current into alternating current and output it to the AC bus 170. The output filter 02 is connected to the AC bus 170 and is used to filter the alternating current transmitted by the AC bus 170 so that the alternating current transmitted to the power grid 130 meets the waveform requirements. The detection circuit 03 is used to detect the current value flowing through the output filter 02. The controller 04 is used to change the inductance of the variable inductor Lm and / or the total capacitance value in the output filter 02 according to the current value flowing through the output filter 02, so as to change the resonant frequency point position of the output filter to keep it away from the switching frequency, thereby suppressing the parallel machine circulation and avoiding affecting the system reliability. The DC source can be a photovoltaic panel or an energy storage device. When the input end of the inverter module 01 is connected to the photovoltaic panel, the power converter 120 can be a photovoltaic inverter, and the inverter module 01 can be an inverter circuit of the photovoltaic inverter. When the input end of the inverter module 01 is connected to the energy storage device, the power converter 120 can be an energy storage converter, and the inverter module 01 can be an inverter circuit of the energy storage converter. In addition, the inverter module 01 can be directly connected to the DC source, or the inverter module 01 can be connected to the DC source through a DC-DC conversion circuit, which is not limited here.

[0041] In the embodiment of the present application, the variable inductor Lm is connected in series with the AC bus 170, and the variable inductor Lm includes a first winding X1 wound on a magnetic core, and the first winding X1 is connected in series with the first switch circuit K1, that is, the input end and the output end of the first winding X1 are respectively connected to the signal input end and the signal output end of the first switch circuit K1, and the on-off state of the first winding X1 can be switched by controlling the on-off state of the first switch circuit K1 to change the inductance of the variable inductor Lm. The first switch circuit K1 may include one or more switch devices, or the first switch circuit K1 may also include one or more switch devices and one or more passive devices. The controller 04 is connected to the control end of the first switch circuit K1. When the controller 04 determines that the current value output by the detection circuit 03 exceeds the set threshold range, it determines that the resonant frequency generated by the output filter 02 needs to be adjusted. Then, the controller 04 can send a corresponding control signal to the first switch circuit K1. When the controller 04 controls the first switch circuit K1 to switch from disconnection to closing, the first winding X1 is turned on to generate an induced current, which will offset the original magnetic flux in the magnetic circuit of the magnetic core, and the inductance of the variable inductor Lm will be reduced accordingly, and the frequency point position of the resonant frequency of the output filter 02 will increase; when the controller 04 controls the first switch circuit K1 to switch from closing to disconnection, the first winding X1 is disconnected and no induced current will be generated, the magnetic flux in the magnetic circuit of the magnetic core will increase, the inductance of the variable inductor Lm will increase accordingly, and the frequency point position of the resonant frequency of the output filter 02 will decrease.

[0042] In the embodiment of the present application, the variable capacitor Cm is connected in series with the second switch circuit K2 and then connected between the AC bus 170 and the N line. A group of variable capacitors Cm may include three sub-capacitors connected in parallel, each of which is connected between one of the three phases (phase A, phase B, and phase C) of the AC bus 170 and the N line. The second switch circuit K2 may be arranged between the N line and the sub-capacitor, or between the three phases and the sub-capacitor. By controlling the on-off state of the second switch circuit K2, the variable capacitor Cm can be connected to or cut out of the AC bus 170. The second switch circuit K2 may include one or more switch devices, or the second switch circuit K2 may also include one or more switch devices, and one or more passive devices. The controller 04 is connected to the control end of the second switch circuit K2. When the controller 04 determines that the current value output by the detection circuit 03 exceeds the set threshold range, it determines that the resonant frequency generated by the output filter 02 needs to be adjusted. Then, the controller 04 can send a corresponding control signal to the second switch circuit K2. When the controller 04 controls the second switch circuit K2 to switch from disconnection to closing, the variable capacitor Cm is connected to the AC bus 170. At this time, the number of capacitors connected in parallel in the output filter 02 increases, the total capacitance value of the output filter 02 increases accordingly, and the frequency point position of the resonant frequency of the output filter 02 decreases; when the controller 04 controls the second switch circuit K2 to switch from closing to disconnection, the variable capacitor Cm is cut out of the AC bus 170. At this time, the number of capacitors connected in parallel in the output filter 02 decreases, the total capacitance value of the output filter 02 decreases accordingly, and the frequency point position of the resonant frequency of the output filter 02 increases.

[0043] In the embodiment of the present application, the controller 04 can be any one of a microcontroller unit (MCU), a central processing unit (CPU), a general-purpose processor, a digital signal processing (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc., or it can be any one or more combinations of other programmable logic devices, transistor logic devices, and hardware components.

[0044] In the embodiment of the present application, the switch device in the first switch circuit K1 and the second switch circuit K2 can be one or more of various types of switch devices such as a mechanical switch, a relay, a circuit breaker, a contactor, a metal oxide semiconductor field effect transistor (metal oxide semiconductor field effect transistor, MOSFET), a bipolar junction transistor (bipolar junction transistor, BJT), an insulated gate bipolar transistor (insulated gate bipolar transistor, IGBT), etc., which are not listed one by one in the embodiment of the present application. The switch device may include a first electrode, a second electrode and a control electrode, wherein the control electrode is used to control the conduction or disconnection of the switch device. When the switch device is turned on, current can be transmitted between the first electrode and the second electrode of the switch device, and when the switch device is disconnected, current cannot be transmitted between the first electrode and the second electrode of the switch device. Taking MOSFET as an example, the control electrode of the switch device is the gate, the first electrode of the switch device can be the source of the switch device, the second electrode can be the drain of the switch device, or the first electrode can be the drain of the switch device, and the second electrode can be the source of the switch device. The switch device in the embodiment of the present application can be in a conducting state when driven by a signal with a first level, and in a disconnected state when driven by a signal with a second level. The first level can be a high level, and the second level can be a low level. Alternatively, the first level is a low level, and the second level is a high level.

[0045] In some other embodiments of the present application, the first switch circuit K1 and the second switch circuit K2 may further include a passive device connected in series or in parallel with the switch device, and the passive device may be, for example, a resistor. Fig.11a In a switch circuit where a switch device is connected in series with a passive device such as a resistor, when the switch device is closed, the resistor can play a buffering role to prevent excessive pressure difference when the switch device is switched on and off, thereby preventing the switch device from being burned; when the switch device is opened, the resistor can limit the breaking current. Fig.11b In a switching circuit in which a switching device is connected in parallel with a passive device such as a resistor, when the switching device is closed, the resistor is bypassed; when the switching device is opened, the switching device is equivalent to a capacitor and the resistor is connected in series, which plays a role in increasing damping in the LC resonant circuit of the output filter 02 and suppressing the parallel machine ring current resonance.

[0046] Reference Figure 3In some embodiments of the present application, the output filter 02 may only include the variable inductor Lm. When the number of capacitor groups connected to the output filter 02 and the capacitance value are fixed, the controller 04 changes the inductance of the variable inductor Lm by controlling the on-off state of the first switch circuit K1 to change the frequency point position of the resonant frequency of the output filter 02 and keep it away from the switching frequency, thereby suppressing the parallel circulation current and avoiding affecting the system reliability.

[0047] Reference Figure 4 In some other embodiments of the present application, the output filter 02 may only include the variable capacitor Cm. When the inductance of the output filter 02 is fixed, the controller 04 controls the on-off state of the second switch circuit K2 to change the number of connected groups and the capacitance value of the variable capacitor Cm, so as to change the frequency point position of the resonant frequency of the output filter 02 and keep it away from the switching frequency, thereby suppressing the parallel circulation current and avoiding affecting the system reliability.

[0048] Reference Figure 5 In some other embodiments of the present application, the output filter 02 may include a variable inductor Lm and a variable capacitor Cm at the same time. The controller 04 may change the inductance of the variable inductor Lm by controlling only the on-off state of the first switch circuit K1 while keeping the number of capacitor groups and the capacitance value connected to the output filter 02 fixed, so as to change the frequency point position of the resonant frequency of the output filter 02, so as to keep it away from the switching frequency, thereby suppressing the parallel machine circulation current and avoiding affecting the system reliability. The controller 04 may also change the number of groups connected and the capacitance value of the variable capacitor Cm by controlling only the on-off state of the second switch circuit K2 while keeping the inductance of the output filter 02 fixed, so as to change the frequency point position of the resonant frequency of the output filter 02, so as to keep it away from the switching frequency, thereby suppressing the parallel machine circulation current and avoiding affecting the system reliability. The controller 04 can also change the frequency point position of the resonant frequency of the output filter 02 by simultaneously controlling the on / off states of the first switch circuit K1 and the second switch circuit K2, changing the inductance of the variable inductor Lm, and changing the number of connected groups and the capacitance value of the variable capacitor Cm, so as to keep it away from the switching frequency, thereby suppressing the parallel circulating current and avoiding affecting the system reliability.

[0049] Furthermore, in the embodiment of the present application, the number of the variable inductors Lm included in the output filter 02 may be one or more, and similarly, the number of the variable capacitors Cm included in the output filter 02 may be one or more groups. Figure 3 and Figure 5 The example in which the output filter 02 includes a variable inductor Lm is used for explanation. Figure 3 and Figure 4 The following description is only given by taking the output filter 02 including the variable capacitors Cm as one group as an example. Figure 6The following description is made by taking the output filter 02 as an example, the number of which is N-1 sets of variable capacitors Cm.

[0050] Reference Figures 3 to 6 In the embodiment of the present application, the output filter 02 can have a variety of structures, for example, an LCL filter can be used. The output filter 02 can specifically include a multi-stage inductor connected in series to the AC bus 170, the first-stage inductor L1 of the multi-stage inductor is connected to the inverter module 01, and the last-stage inductor is connected to the power grid 130. At least one inductor other than the first-stage inductor L1 of the multi-stage inductor can be set to a variable inductor Lm. Figures 3 to 6 In the example, the output filter 02 includes two-stage inductance, namely the first-stage inductance L1 and the second-stage inductance L2. The second-stage inductance L2 is connected in series to the AC bus 170 between the first-stage inductance L1 and the power grid 130. Figure 3 , Figure 5 and Figure 6 The second-stage inductor L2 is set as a variable inductor Lm for explanation. In addition, the output filter 02 generally also includes multiple groups of capacitors connected to the AC bus 170, wherein one end of each group of capacitors is connected to the AC bus 170, and the other end of each group of capacitors is connected to the N line. The first group of capacitors C1 in the multiple groups of capacitors is connected to the AC bus 170 between the first-stage inductor L1 and the second-stage inductor L2, and the other groups of capacitors can be connected to the AC bus 170 between the second-stage inductor L2 and the power grid 130. At least one group of capacitors other than the first group of capacitors C1 in the multiple groups of capacitors can be set as a variable capacitor Cm. Figures 3 to 5 In the example, the output filter 02 includes two groups of capacitors, namely the first group of capacitors C1 and the second group of capacitors C2, where: Figure 4 and Figure 5 The following description is made by taking the second group of capacitors C2 as variable capacitors Cm as an example. Figure 6 In the description, an example is taken in which the output filter 02 includes N groups of capacitors, wherein the second group of capacitors C2 to the Nth group of capacitors are set as variable capacitors Cm.

[0051] Reference Figure 7 In the embodiment of the present application, each level of inductance in the output filter 02 can be a three-phase integrated inductor, that is, each level of inductance includes a magnetic core, and a magnetic core has three branches arranged relative to phase A, phase B and phase C, and each branch is respectively wound with a winding for generating an induced current. One or more first windings X1 can be additionally wound on the magnetic core of the variable inductor Lm, and the input end and the output end of each first winding X1 are respectively connected through a first switch circuit K1. By controlling the on-off state of the first switch circuit K1, the on-off state of the first winding X1 can be switched to change the inductance of the variable inductor Lm. Figure 7In the example, two additional first windings X1 are provided. By adding an additional first winding X1 to the variable inductor Cm, the inductance of the variable inductor Cm can be adjusted at a relatively low cost.

[0052] In some embodiments of the present application, a current sensor or a current transformer may be provided inside the output filter 02, and the detection circuit 03 may determine the current value flowing through the output filter 02 by detecting the current value flowing through the current sensor or the current transformer. Figure 3 , Figure 5 and Figure 6 , the current sensor or current transformer can be set in front of any phase of the three-phase inductance in the variable inductance Lm (the position shown by the circle). Figure 4 The current sensor or current transformer can also be arranged between any one of the three sub-capacitors in the variable capacitor Cm and a corresponding phase.

[0053] Reference Figure 8 In some embodiments of the present application, the variable inductor 02 may also include a second winding X2. The second winding X2 is different from the first winding X1. The input end and the output end of the second winding X2 are disconnected, that is, the second winding X2 will not be closed to generate an induced current, and the original magnetic flux in the magnetic circuit of the magnetic core will be offset to change the inductance of the variable inductor Lm. However, the original magnetic flux in the magnetic circuit of the magnetic core of the variable inductor Lm will generate an induced current in the second winding X2 and be detected. Therefore, the second winding X2 can replace the current sensor or current transformer set in the output filter 02. The detection circuit 03 can determine the current value flowing through the output filter 02 by detecting the induced current value between the input end and the output end of the second winding X2. The detection is realized by adding an additional second winding X2 to the variable inductor Cm, which can reduce the cost of the entire detection circuit 03.

[0054] Reference Fig. 9In some other embodiments of the present application, in the variable inductor 02, the second winding X2 and the first winding X1 can be integrated into the same winding, that is, one first winding X1 realizes the function of the second winding X2, and the first switch circuit K1 is connected between the input end and the output end of the first winding X1, and the input end and the output end of the first winding X1 are connected to the detection circuit 03. When the first switch circuit K1 is in the disconnected state, the first winding X1 is used as the second winding X2, that is, the detection circuit 03 can determine the current value flowing through the output filter 02 by detecting the induced current value between the input end and the output end of the first winding X1. When it is determined that the inductance needs to be reduced, the controller 04 can control the first switch circuit K1 to switch to the closed state, and the first winding X1 is turned on to generate an induced current to offset the original magnetic flux in the magnetic circuit of the magnetic core, and the inductance of the variable inductor Lm is reduced accordingly, and the frequency point position of the resonant frequency of the output filter 02 is increased. By implementing detection in a manner of reusing the first winding X1 as the second winding X2 in the variable inductor Cm, the cost of the entire detection circuit 03 can be further reduced.

[0055] Fig.10 The flowchart of the method for suppressing the parallel circulating current of the photovoltaic system provided in the embodiment of the present application is exemplarily shown.

[0056] Based on the same inventive concept, the embodiment of the present application also provides a method for suppressing the circulating current of the photovoltaic system. The specific structure of the photovoltaic system is described above and will not be described in detail here. Fig.10 The parallel machine circulating current suppression method provided in the embodiment of the present application includes:

[0057] S1. The detection circuit detects the current value flowing through the output filter.

[0058] In some embodiments of the present application, a current sensor or a current transformer may be provided inside the output filter, and the detection circuit may determine the current value flowing through the output filter by detecting the current value flowing through the current sensor or the current transformer.

[0059] In other embodiments of the present application, a second winding may be provided in the variable inductor, and the input and output ends of the second winding may be disconnected. The second winding may replace the current sensor or current transformer provided in the output filter, and the detection circuit may determine the current value flowing through the output filter by detecting the induced current value between the input and output ends of the second winding. The detection may be implemented by adding an additional second winding to the variable inductor, which may reduce the overall detection cost.

[0060] S2. The controller controls the first switch circuit to turn on or off in response to the current value flowing through the output filter to change the inductance of the variable inductor; and / or the controller controls the second switch circuit to turn on or off in response to the current value flowing through the output filter to change the state of the variable capacitor connected to the AC bus. By changing the inductance of the variable inductor and / or changing the total capacitance value in the output filter, the resonant frequency point position of the output filter can be changed to keep it away from the switching frequency, thereby suppressing the parallel machine circulating current and avoiding affecting the system reliability.

[0061] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A photovoltaic system, characterized in that: The photovoltaic system includes a plurality of parallel-connected power converters, each of which includes an inverter module, an output filter, a detection circuit and a controller; The inverter module is used to convert the input direct current into alternating current and output it to the AC bus, and connect to the power grid through the AC bus; The output filter is connected to the AC bus and is used to filter the AC power transmitted by the AC bus; the output filter includes a variable inductor and / or a variable capacitor, the variable inductor is connected in series with the AC bus, the variable inductor includes a first winding connected in series with a first switch circuit, and the variable capacitor is connected in series with a second switch circuit and then connected between the AC bus and the neutral line; The detection circuit is used to detect the current value flowing through the output filter; The controller is used to control the first switch circuit to be turned on or off in response to the current value flowing through the output filter to change the inductance of the variable inductor; and / or, in response to the current value flowing through the output filter, control the second switch circuit to be turned on or off to change whether the variable capacitor is connected to the AC bus.

2. The photovoltaic system according to claim 1, characterized in that: The variable inductor further includes a second winding, the input end and the output end of the second winding are disconnected, and the detection circuit is used to detect the induced current value generated between the input end and the output end of the second winding.

3. The photovoltaic system according to claim 1 or 2, characterized in that: The second winding is the same winding as the first winding.

4. The photovoltaic system according to claim 1, characterized in that: It also includes a current sensor or a current transformer arranged inside the output filter, and the detection circuit is used to detect the current value flowing through the current sensor or the current transformer.

5. The photovoltaic system according to any one of claims 1 to 4, characterized in that: The output filter comprises a multi-stage inductor connected in series to the AC bus, a first-stage inductor of the multi-stage inductor is connected to the inverter module, and at least one inductor of the multi-stage inductor other than the first-stage inductor is the variable inductor.

6. The photovoltaic system according to claim 5, characterized in that: The output filter also includes multiple groups of capacitors connected to the AC bus, the second-stage inductor of the multiple-stage inductor is connected in series to the AC bus between the first-stage inductor and the power grid, the first-stage capacitor of the multiple groups of capacitors is connected to the AC bus between the first-stage inductor and the second-stage inductor, and at least one group of capacitors of the multiple groups of capacitors other than the first group of capacitors is the variable capacitor.

7. The photovoltaic system according to any one of claims 1 to 6, characterized in that: The first switch circuit includes a switch device, or the first switch circuit includes a switch device and a passive device; The second switch circuit includes a switch device, or the second switch circuit includes a switch device and a passive device connected in series or in parallel.

8. The photovoltaic system according to claim 7, characterized in that: The switch device is a relay, a circuit breaker or a contactor, and the passive device is a resistor.

9. The photovoltaic system according to any one of claims 1 to 8, characterized in that: The power converter is a photovoltaic inverter or an energy storage converter.

10. A method for suppressing the parallel circulation current of a photovoltaic system, characterized in that: The photovoltaic system includes a plurality of power converters connected in parallel, each of which includes an inverter module, an output filter, a detection circuit and a controller; the inverter module is used to convert the input direct current into alternating current and output it to the AC bus, and connect the power grid through the AC bus; the output filter is connected to the AC bus, and is used to filter the AC power transmitted by the AC bus; the output filter includes a variable inductor and / or a variable capacitor, the variable inductor is connected in series with the AC bus, the variable inductor includes a first winding connected in series with a first switch circuit, and the variable capacitor is connected in series with a second switch circuit and then connected between the AC bus and the center line; The parallel machine circulating current suppression method comprises: The detection circuit detects the current value flowing through the output filter; The controller controls the first switch circuit to be turned on or off in response to the current value flowing through the output filter to change the inductance of the variable inductor; and / or, the controller controls the second switch circuit to be turned on or off in response to the current value flowing through the output filter to change whether the variable capacitor is connected to the AC bus.

11. The method for suppressing the circulating current of parallel machines according to claim 10, characterized in that: The variable inductor further includes a second winding, wherein the input end and the output end of the second winding are disconnected; The detection circuit detects the current value flowing through the output filter, including: the detection circuit detects the induced current value generated between the input end and the output end of the second winding.

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