Grid-connected inverter system to prevent potential induced degradation of photovoltaic modules

Abstract

The present invention relates to a grid-connected inverter system for preventing potential induced attenuation of photovoltaic components, which includes a photovoltaic inverter connected to a photovoltaic component, an isolation transformer connected to the photovoltaic inverter on the low-voltage side, the center point of the low-voltage side of the isolation transformer is not grounded, and the photovoltaic inverter includes a grounding circuit for grounding the output positive pole or output negative pole / bus positive pole or bus negative pole of the photovoltaic component. The grounding circuit includes a positive grounding circuit connected to the output positive pole / bus positive pole of the photovoltaic component, and a negative grounding circuit connected to the output negative pole / bus negative pole of the photovoltaic component; the positive grounding circuit includes a positive switch and a positive resistor connected in series with the positive switch; the negative grounding circuit includes a negative switch and a negative resistor connected in series with the negative switch. The present invention has a simple circuit structure and a reliable solution, which effectively prevents the occurrence of the PID effect of the photovoltaic component, reduces the construction complexity, reduces the labor input, and reduces the operation and maintenance costs.

Description

Technical Field

[0001] The invention belongs to the technical field of photovoltaic power generation, and in particular relates to a grid-connected inverter system for preventing potential-induced attenuation of photovoltaic components. Background Art

[0002] Photovoltaic grid-connected inverter technology is relatively mature at present. From the perspective of the number of grid phases, it can be divided into three-phase grid-connected inverters and single-phase grid-connected inverters. From the perspective of grid-connected isolation, it can be divided into isolated and non-isolated grid-connected inverters. In the field of photovoltaic ground power stations, photovoltaic systems almost all use inverter systems with box-type transformers to realize photovoltaic components to feed DC power to the high-voltage grid. Three-phase grid-connected inverters almost all use boost to boost the DC bus, and the DC bus passes through a three-level or multi-level inverter topology to realize the current conversion from DC to AC. The inherent modulation technology of the three-level or multi-level converter makes the half bus and the ground equipotential. Since the photovoltaic component PV- and the half bus BUS- are connected at the same potential, this makes the component PV- and the ground bear a -0.5Vbus voltage. Since the frame of the photovoltaic module is grounded, the voltage difference between the module PV- and the module frame is -0.5Vbus. This potential difference causes the migration of semiconductor carriers inside the module, resulting in module passivation, which in turn leads to a decrease in the power generation of the module. This effect is referred to as the potential induced degradation (PID) effect.

[0003] There are many ways to solve the PID effect. From the perspective of the inverter system, there are preventive solutions and repair solutions.

[0004] The repair solution is: according to the type of components, the inverter system generates electricity during the day. When the component is a P-type component, the component PV- and the earth PE are subjected to a -0.5Vbus voltage, and a +0.5Vbus voltage is added between PV- and PE at night to achieve the PID repair function of the component. When the component is an N-type component, the PV+ and PE are subjected to +0.5Vbus, and a -0.5Vbus is added between PV+ and PE at night to achieve the PID repair function of the N-type component. The PID repair function is based on the reversibility of the PID effect of the PV component to achieve its repair function. However, its disadvantage is that the component cannot be 100% completely repaired, and the PID effect always exists when the component generates electricity during the day.

[0005] The existing solution for PID prevention function is as follows: In the photovoltaic grid-connected inverter system, there are the following components: 1. Photovoltaic modules, 2. Photovoltaic inverter, 3. Isolation transformer. The unstable DC generated by the photoelectric effect of the photovoltaic module is converted into relatively stable AC through the photovoltaic inverter and fed to the grid through the transformer. Since the equivalent midpoint of the isolation transformer and the remote earth are equivalent potentials during the photovoltaic inverter grid-connected power generation process, -0.5Vbus is generated between PV- and PE, resulting in PID effect. Therefore, on the low-voltage side of the isolation transformer, a virtual midpoint is created by means of resistors, inductors, capacitors, etc., and a voltage controller between the virtual midpoint and PE generates a 0.5 times DC bus voltage value between the two, so that the voltage between PV- and PE is greater than or equal to 0, to prevent the occurrence of PID effect. Similarly, a similar PID prevention effect is achieved for N-type modules. The current PID effect prevention solution is to virtualize the midpoint on the low-voltage side of a box transformer by means of resistors, capacitors, inductors, etc., and generate a corresponding 0.5 times bus voltage loaded between the virtual midpoint and the earth through a voltage controller. One photovoltaic power station sub-array corresponds to one isolated box-type transformer, which uses a set of midpoint virtual circuits and voltage controllers to achieve PID prevention function. The existing PID prevention scheme can effectively prevent the PID effect, but it requires the development of additional PID prevention equipment boxes and the connection of the low-voltage side line of the box-type transformer, which increases the complexity and labor input of system construction. Summary of the invention

[0006] The purpose of the present invention is to provide a grid-connected inverter system for preventing potential induced degradation of photovoltaic components, which has a simple structure, a reliable solution, reduces the complexity of on-site construction, reduces system labor input, and reduces operation and maintenance costs.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A grid-connected inverter system for preventing potential-induced attenuation of photovoltaic components comprises a photovoltaic inverter connected to the photovoltaic component and an isolation transformer connected to the photovoltaic inverter on the low-voltage side, wherein the center point of the low-voltage side of the isolation transformer is not grounded, and the photovoltaic inverter comprises a grounding circuit for grounding the output positive pole or output negative pole / bus positive pole or bus negative pole of the photovoltaic component.

[0009] The grounding circuit includes a positive grounding circuit connected to the output positive electrode of the photovoltaic module / the positive electrode of the busbar, and a negative grounding circuit connected to the output negative electrode of the photovoltaic module / the negative electrode of the busbar;

[0010] The positive grounding circuit includes a positive switch connected between the output positive pole of the photovoltaic component / the positive pole of the busbar and the ground, and a positive resistor connected in series with the positive switch; the negative grounding circuit includes a negative switch connected between the output negative pole of the photovoltaic component / the negative pole of the busbar and the ground, and a negative resistor connected in series with the negative switch.

[0011] The positive electrode grounding circuit further includes a positive electrode fuse connected in series with the positive electrode switch, and the negative electrode grounding circuit further includes a negative electrode fuse connected in series with the negative electrode switch.

[0012] The photovoltaic inverter further includes a first type controller for controlling the on and off of the positive switch or the negative switch.

[0013] The photovoltaic inverter also includes a first type of sampling circuit for detecting the voltage across the positive resistor or the negative resistor. The first type of sampling circuit is connected to the first type of controller. The first type of controller controls the on and off of the positive switch or the negative switch according to the voltage across the positive resistor or the negative resistor and the type of the photovoltaic component.

[0014] When the photovoltaic component is an N-type component, the first type of controller controls the positive switch to be turned on and the output positive pole / bus positive pole of the photovoltaic component is grounded, and controls the negative pole switch to be turned off; when the photovoltaic component is a P-type component, the first type of controller controls the negative pole switch to be turned on and the output negative pole / bus negative pole of the photovoltaic component is grounded, and controls the positive pole switch to be turned off.

[0015] The positive switch and the negative switch are relays or electronic switches.

[0016] The photovoltaic inverter also includes a second type of sampling circuit for detecting the voltage between the low voltage side of the isolation transformer and the ground or the voltage between the negative pole of the bus and the ground, and a second type of controller for determining whether a live wire is short-circuited to the ground based on the voltage between the low voltage side of the isolation transformer and the ground or the voltage between the negative pole of the bus and the ground, and controlling the on and off of the positive switch and the negative switch accordingly, so as to protect the system.

[0017] The second type of sampling circuit includes a high voltage differential resistor or a voltage sensor.

[0018] Due to the application of the above technical scheme, the present invention has the following advantages compared with the prior art: the present invention is a new PID prevention scheme, which is mainly integrated and implemented inside the grid-connected inverter, has a simple circuit structure and a reliable scheme, and effectively prevents the occurrence of the PID effect of photovoltaic modules. At the same time, it reduces the complexity of on-site construction of power stations, reduces the system's labor input, and reduces operation and maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Attached Figure 1 The photovoltaic system diagram is a grid-connected inverter system using the first embodiment of the present invention.

[0020] Attached Figure 2 This is a photovoltaic system diagram of a grid-connected inverter system using the second embodiment of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with the embodiments shown in the accompanying drawings.

[0022] Embodiment 1: As shown in the attached Figure 1 As shown, the photovoltaic system is composed of a photovoltaic module, a photovoltaic inverter, a circuit breaker, and an isolation transformer. The photovoltaic inverter is connected to the photovoltaic module and mainly includes a DC / AC conversion module. The output side of the photovoltaic inverter is connected to the low-voltage side of the isolation transformer via a circuit breaker, and the high-voltage side of the isolation transformer is connected to the power grid. In the above photovoltaic system, the photovoltaic inverter and the isolation transformer constitute a grid-connected inverter system as the main parts, and the grid-connected inverter system may also include a circuit breaker.

[0023] In order to achieve the function of preventing the potential induced attenuation of photovoltaic components, the scheme of the grid-connected inverter system is as follows: the center point of the low-voltage side of the isolation transformer is connected to the photovoltaic inverter, and the center point of the low-voltage side of the isolation transformer is not grounded. At this time, the center point of the low-voltage side of the isolation transformer is floating when the system is working. The center point of the low-voltage side of the traditional isolation transformer is grounded. When the photovoltaic system is working, due to the three-phase inverter topology NPC, ANPC, TNPC and other topologies in PWM modulation output, the BUSN and the center point of the isolation transformer are at the same potential. Since the center point of the low-voltage side of the isolation transformer is grounded, the BUSN is equivalent to the earth at the same potential. The PV-end and BUSN are connected through capacitor C2, and the voltage between PV- and BUSN is -0.5Vbus. Therefore, the voltage between PV- and PE is -0.5Vbus, which will cause the PID effect. In the wiring method where the center point of the low-voltage side of the isolation transformer is not grounded, when the grid-connected inverter is working, since the center point and the earth PE are floating, for P-type components, PV- can be connected to the earth, so that the voltage of PV- to PE is 0, preventing the PID effect from occurring. Similarly, for N-type components, PV+ is connected to the earth, so that the voltage of PV+ to PE is 0, preventing the PID effect from occurring. The photovoltaic inverter includes a grounding circuit for grounding the output positive pole or output negative pole of the photovoltaic component.

[0024] The grounding circuit includes a positive grounding circuit connected to the positive output of the photovoltaic module and a negative grounding circuit connected to the negative output of the photovoltaic module. The positive grounding circuit includes a positive switch S1 connected between the positive output of the photovoltaic module PV+ and the ground, and a positive resistor R1 connected in series with the positive switch S1; the negative grounding circuit includes a negative switch S2 connected between the negative output of the photovoltaic module PV- and the ground, and a negative resistor R2 connected in series with the negative switch S2. Therefore, according to the different types of photovoltaic modules at the power station site, P-type modules or N-type modules, the corresponding mode is selected. When the photovoltaic module at the site is a P-type module, it is connected to the ground through the negative switch S2 (not limited to relays, electronic switches, etc.) and the series negative resistor R2. After the photovoltaic inverter completes the safety self-test, the negative switch S2 is closed and the grid is connected to work. During the operation, the PID effect is prevented. When the on-site component is an N-type component, it is connected to the ground through the positive switch S1 (not limited to relays, electronic switches, etc.) and the series positive resistor R1. After the photovoltaic inverter completes the safety self-check, the positive switch S1 is closed, and the photovoltaic inverter starts to connect to the grid. During the operation, the PID effect is prevented. Therefore, the positive switch S1, the negative switch S2, the positive resistor R1, and the negative resistor R2 together constitute a PID prevention circuit. The photovoltaic inverter also includes a first type of controller for controlling the on and off of the positive switch or the negative switch.

[0025] Therefore, the idea of ​​PID prevention is: through the connection between the positive output PV+ of the photovoltaic module / the negative output PV- of the photovoltaic module and the earth, the voltage between the positive output PV+ of the photovoltaic module or the negative output PV- of the photovoltaic module and the earth is 0V or close to 0V. This electrical connection mode needs to consider that the inverter can work safely under any circumstances, so there are the following protection logic and measures:

[0026] (1) When a human body touches the positive output PV+ of a photovoltaic module or the negative output PV- of a photovoltaic module, the PID prevention circuit works, the positive switch S1 or the negative switch S2 is turned on, and the positive output PV+ of the photovoltaic module and the negative output PV- of the photovoltaic module form a circuit through the earth and the human body, causing electric shock to the human body. At this time, the sampling circuit can detect the voltage on the positive resistor R1 or the negative resistor R2 to determine whether the human body is electrocuted, and immediately disconnect the positive switch S1 or the negative switch S2. Therefore, the photovoltaic inverter also includes a first type of sampling circuit for detecting the voltage across the positive resistor R1 or the negative resistor R2, and the first type of sampling circuit is connected to the first type of controller. The first type of controller also controls the on and off of the positive switch S1 or the negative switch S2 according to the voltage across the positive resistor or the negative resistor and the type of photovoltaic module. When the photovoltaic module is an N-type module, the first type of controller controls the positive switch S1 to be turned on and the output positive pole of the photovoltaic module is grounded, and controls the negative switch S2 to be turned off; when the photovoltaic module is a P-type module, the first type of controller controls the negative switch S2 to be turned on and the output negative pole of the photovoltaic module is grounded, and controls the positive switch S1 to be turned off;

[0027] (2) When the photovoltaic inverter is connected to the grid for power generation, if the PID prevention circuit does not work, the positive switch S1 or the negative switch S2 is disconnected. Since the center point of the low-voltage side of the isolation transformer is not grounded and the live wire is short-circuited to the ground, the inverter can still be connected to the grid for power generation. When the PID prevention circuit works, the positive switch S1 or the negative switch S2 is turned on, the center point of the low-voltage side of the isolation transformer is not grounded, and the live wire is short-circuited to the ground, the AC and DC form a loop through the ground, which may cause damage to the inverter circuit transistors in the photovoltaic inverter. Therefore, for P-type components, a fuse F2 is connected in series between the output negative pole PV- of the photovoltaic component in the PID prevention circuit and the ground to prevent the power IGBT from being damaged by the short circuit of the live wire on the low-voltage side of the isolation transformer to the ground when the PID is working. For N-type components, a fuse F1 is connected in series between the output positive pole PV+ of the photovoltaic component and the ground to prevent the power IGBT from being damaged by the short circuit of the live wire on the low-voltage side of the isolation transformer to the ground when the PID is working. Therefore, the positive grounding circuit also includes a positive fuse F1 connected in series with the positive switch S1, and the negative grounding circuit also includes a negative fuse F2 connected in series with the negative switch S2;

[0028] (3) In order to prevent power semiconductor failure caused by live wire short circuit to ground, it is necessary to detect whether the live wire output by the inverter is short circuited to ground. There are many ways, such as using high-voltage differential resistors or voltage sensors to sample the AC live wire to ground voltage or the bus negative pole BUS- to ground voltage. The voltage detection method here is not limited to voltage sensor detection and high-voltage differential resistor differential detection. If the live wire is detected to be short circuited to the ground, the positive switch S1 and the negative switch S2 of the PID prevention circuit will not be switched. Therefore, the specific solution is: the photovoltaic inverter also includes a second type of sampling circuit for detecting the voltage of the low-voltage side of the isolation transformer to the ground, and a second type of controller for judging whether the live wire is short circuited to the ground (i.e., whether the live wire output by the inverter is short circuited) according to the voltage of the low-voltage side of the isolation transformer to the ground and controlling the on and off of the positive switch S1 and the negative switch S2 accordingly. Among them, the second type of sampling circuit includes a high-voltage differential resistor or a voltage sensor, etc. When it is determined that the live wire is short-circuited to the ground based on the monitored voltage between the low-voltage side of the isolation transformer and the ground, the second type of controller controls both the positive switch S1 and the negative switch S2 to be turned off.

[0029] The first type of controller and the second type of controller mentioned above can be integrated into a controller of a photovoltaic inverter.

[0030] The working mode logic of the above PID prevention circuit is: A) the illumination power of the PV module meets the grid-connected power; B) the PV inverter is turned on; C) the PV inverter performs safety self-test (PV insulation impedance detection, judging whether the module is short-circuited to the ground, fan self-test, leakage current detection, etc.); D) the PV inverter starts to be connected to the grid, and uses the second type of sampling circuit to detect the voltage between the live wire and the ground on the low-voltage side of the isolation transformer to judge whether the live wire is short-circuited to the ground; E) when the live wire is not short-circuited to the ground, the first type of controller determines whether it is a P-type module. If it is a P-type module, the negative switch S2 is switched on. If it is an N-type module, the positive switch S1 is switched on.

[0031] Embodiment 2: As shown in the attached Figure 2As shown, the difference between the second embodiment and the first embodiment is that the photovoltaic inverter also includes a DC / DC conversion module located at the front end of the DC / AC conversion module, that is, a boost circuit, such as a traditional two-level boost circuit, a symmetrical three-level boost circuit, a flying capacitor three-level boost circuit, etc. The output end of the boost circuit constitutes the positive bus BUS+ or the negative bus BUS- in the photovoltaic inverter. Then the photovoltaic inverter includes a grounding circuit for grounding the positive bus BUS+ or the negative bus BUS-. The structure of the grounding circuit is the same as that of the first embodiment, and the only difference is the connection position, that is, at this time, the positive switch S1 and the negative switch S2 of the PID prevention circuit are respectively connected to the positive DC bus BUS+ and the negative DC bus BUS-. In the second embodiment, the voltage of the negative DC bus BUS- to the earth is detected by the second type of sampling circuit in the non-state, and then it is judged whether the live wire on the low-voltage side of the isolation transformer is short-circuited to the earth in the grid-connected state, and the positive switch S1 and the negative switch S2 are controlled to be turned on and off accordingly.

[0032] The solution of the present application provides a new grid-connected inverter system for preventing the PID effect, which is relatively low in cost and easy to implement as an industrial product. It effectively prevents the PID effect, helps to increase the power generation of the photovoltaic system, increase customer benefits, and extend the service life of photovoltaic modules.

[0033] This solution is a new type of PID prevention solution and inverter system solution, which focuses on providing measures to solve the PID effect and has great application value in extending the service life of photovoltaic modules. At the same time, the traditional PID prevention solution requires the addition of additional external equipment. This new PID prevention solution does not require additional external PID prevention equipment or manual laying and debugging of external equipment lines, which reduces the investment in manual construction costs, enhances the competitiveness of system products, and enhances the convenience of the entire photovoltaic system.

[0034] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A grid-connected inverter system for preventing potential-induced attenuation of photovoltaic modules, comprising a photovoltaic inverter connected to the photovoltaic module, and an isolation transformer connected to the photovoltaic inverter on the low-voltage side. Features: The center point of the low-voltage side of the isolation transformer is not grounded, and the photovoltaic inverter includes a grounding circuit for grounding the output positive pole or output negative pole / bus positive pole or bus negative pole of the photovoltaic module; The grounding circuit includes a positive grounding circuit connected to the output positive electrode of the photovoltaic module / the positive electrode of the busbar, and a negative grounding circuit connected to the output negative electrode of the photovoltaic module / the negative electrode of the busbar; The positive grounding circuit includes a positive switch connected between the output positive pole of the photovoltaic module / the positive pole of the busbar and the ground, and a positive resistor connected in series with the positive switch; the negative grounding circuit includes a negative switch connected between the output negative pole of the photovoltaic module / the negative pole of the busbar and the ground, and a negative resistor connected in series with the negative switch; The photovoltaic inverter also includes a second type of sampling circuit for detecting the voltage between the low voltage side of the isolation transformer and the ground, and a second type of controller for determining whether a live wire is short-circuited to the ground based on the voltage between the low voltage side of the isolation transformer and the ground, and controlling the on and off of the positive switch and the negative switch accordingly.

2. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 1, Features: The positive electrode grounding circuit further includes a positive electrode fuse connected in series with the positive electrode switch, and the negative electrode grounding circuit further includes a negative electrode fuse connected in series with the negative electrode switch.

3. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 1, Features: The photovoltaic inverter further includes a first type controller for controlling the on and off of the positive switch or the negative switch.

4. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 3, Features: The photovoltaic inverter also includes a first type of sampling circuit for detecting the voltage across the positive resistor or the negative resistor. The first type of sampling circuit is connected to the first type of controller. The first type of controller controls the on and off of the positive switch or the negative switch according to the voltage across the positive resistor or the negative resistor and the type of the photovoltaic component.

5. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 4, Features: When the photovoltaic component is an N-type component, the first type of controller controls the positive switch to be turned on and the output positive pole / bus positive pole of the photovoltaic component is grounded, and controls the negative pole switch to be turned off; when the photovoltaic component is a P-type component, the first type of controller controls the negative pole switch to be turned on and the output negative pole / bus negative pole of the photovoltaic component is grounded, and controls the positive pole switch to be turned off.

6. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 1, Features: The positive switch and the negative switch are relays or electronic switches.

7. The grid-connected inverter system for preventing potential induced degradation of photovoltaic modules according to claim 1, Features: The second type of sampling circuit includes a high voltage differential resistor or a voltage sensor.

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