Photovoltaic system DC side ground impedance suppression protection system

Abstract

The invention discloses a photovoltaic system direct current side ground impedance suppression protection system, which comprises a blocking device arranged on a direct current side of a photovoltaic system and having unidirectivity; the blocking device is suitable for forming a suppression circuit with an alternating current side when the negative end of a PV unit of a photovoltaic system is short-circuited so as to block or shunt short-circuit current. The blocking device and the alternating current side form a suppression circuit to block a short circuit loop of a PV unit to ground short circuit fault, so that a power switch device and the like are prevented from being burnt due to overlarge short circuit current, the service life of electronic elements in the circuit is effectively prolonged, meanwhile, an arc discharge phenomenon is avoided when a grid-connected relay is turned off, and the safety of a photovoltaic system is higher.

Description

Technical Field

[0001] The present application relates to the technical field of photovoltaic power generation, and in particular to a suppression protection system for the impedance of the DC side of a photovoltaic system to the ground. Background Art

[0002] At present, when a photovoltaic system has a PV (photovoltaic system) short-circuit fault to the ground due to reasons such as line aging, even if all the switching devices in the photovoltaic system are turned off, a short-circuit loop may be formed through the reverse-parallel diode of the power switching device of the inverter circuit. The large short-circuit current may damage the power switch and cause secondary damage. Therefore, in the prior art, the short-circuit loop is cut off by turning off the grid-connected relay or a diode is connected in parallel at both ends of the power switching device of the inverter circuit to increase the current carrying capacity when a short-circuit fault to the ground occurs, thereby protecting the system when a PV short-circuit fault to the ground occurs. However, this solution has a disadvantage that when the grid-connected relay is turned off, a large short-circuit current may cause arcing, which may cause the system to catch fire in severe cases.

[0003] The arcing phenomenon refers to the phenomenon that when the voltage between two conductors in a circuit is high enough, the current will pass through the air or other insulating media to produce a visible arc. This arc is usually accompanied by strong light, heat and electrical noise.

[0004] On the basis of directly shutting off the grid-connected relay, there is a method of not shutting off the grid-connected relay in the half grid cycle that will form a short-circuit loop, but shutting off the grid-connected relay in the next half grid cycle when the body diode of the inverter circuit power switch tube is cut off and the short-circuit loop will not be formed. Although this method avoids the occurrence of arcing, at this time, the body diode of the inverter circuit power switch device needs to withstand at least half a grid cycle of short-circuit current, and a large short-circuit current may damage the power switch device.

[0005] Therefore, it is very necessary to provide a protection system for suppressing the impedance of the DC side of a photovoltaic system to the ground, so as to promptly block or shunt the short-circuit current when a short-circuit fault occurs to the ground at the negative end of the PV to avoid damage to the power switching device. Summary of the invention

[0006] An object of the present application is to provide a photovoltaic system DC side impedance to ground suppression protection system that can solve at least one of the defects in the above-mentioned background technology.

[0007] In order to achieve the above objectives, the technical solution adopted in this application is: a suppression protection system for the DC side of a photovoltaic system to ground impedance, comprising: a blocking device, which is arranged on the DC side of the photovoltaic system and is unidirectional; the blocking device is suitable for forming a suppression circuit with the AC side when the negative end of the PV unit of the photovoltaic system is short-circuited, thereby blocking or shunting the short-circuit current.

[0008] As a preferred embodiment, the DC / AC unit of the photovoltaic system includes three switch bridge arms corresponding to the three-phase power grid; there are three groups of blocking devices corresponding to the three switch bridge arms respectively, one end of each group of blocking devices is connected to the midpoint of the switch bridge arm, and the other end is connected to the negative bus on the DC side of the photovoltaic system, so that each group of blocking devices forms a suppression circuit for shunting the short-circuit current passing through the corresponding switch bridge arm.

[0009] As a preference, the blocking device is installed on the negative busbar on the DC side of the photovoltaic system, and the blocking device is suitable for forming a suppression circuit for blocking the short-circuit current passing through the negative busbar on the DC side.

[0010] As another preferred embodiment, there are multiple blocking devices and they are respectively installed at the negative end of each PV unit of the photovoltaic system; the blocking device is suitable for forming a suppression circuit for blocking the short-circuit current passing through the negative end of the PV unit.

[0011] Further preferably, the blocking device is a diode or a transistor.

[0012] Further preferably, the suppression protection system also includes a detection circuit, a multiplexer and a controller; the number of the detection circuits corresponds to the number of PV units in the photovoltaic system; the detection circuit is used to collect power and perform fault judgment on the power devices corresponding to the PV units, and the multiplexer is suitable for summarizing the judgment results of all the detection circuits and sending them to the controller, and the controller locates the position of the PV unit that is short-circuited to the ground according to the judgment results of the detection circuit.

[0013] Further preferably, the blocking device installed at the negative end of the PV unit is used as a power device for the detection circuit to collect power; the detection circuit includes a protection circuit and a comparison unit; the protection circuit is connected in parallel to the blocking device, and the input end of the comparison unit is connected to the protection circuit to obtain the terminal voltage or reverse current of the blocking device, and the comparison unit is suitable for judging the short circuit fault of the PV unit to ground based on the data obtained at the input end.

[0014] Further preferably, the protection circuit also includes a collection unit, the collection unit includes a pair of sampling resistors connected in series, and the two input terminals of the comparison unit are connected in parallel to one of the sampling resistors; if the terminal voltage of the sampling resistor on one side close to the PV unit is higher than the terminal voltage on the other side, the comparison unit will output a short-circuit fault signal of the PV unit.

[0015] Further preferably, the acquisition unit includes a pair of sampling resistors connected in series, one of the input terminals of the comparison unit is connected to the midpoint of the series branch formed by the two sampling resistors, and the other input terminal of the comparison unit is given a voltage threshold V ref ; If the midpoint voltage of the blocking device terminal voltage divided by the sampling resistor is greater than the voltage threshold V ref , the comparison unit will output the ground short circuit fault signal of the PV unit.

[0016] Further preferably, the protection circuit comprises at least one current limiting resistor connected in parallel to the blocking device, one of the input terminals of the comparison unit is connected to the protection circuit, and the other input terminal of the comparison unit is a given current threshold I ref ; If the reverse current value on the protection circuit is greater than the current threshold I ref , the comparison unit will output the ground short circuit fault signal of the PV unit.

[0017] Compared with the prior art, the beneficial effects of this application are: The blocking device is arranged on the DC side of the photovoltaic system, and the blocking device is unidirectional. When the negative end of the PV unit of the photovoltaic system is short-circuited, the blocking device and the AC side form an inhibition circuit to block the short-circuit loop of the PV unit to the ground short-circuit fault, and prevent the short-circuit current from being too large to cause the power switching devices to burn out, effectively improving the service life of the electronic components in the circuit. At the same time, no arcing will occur when the grid-connected relay is turned off, making the photovoltaic system safer. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 PV in this application n Schematic diagram of the circuit structure when a short circuit to ground occurs at the negative terminal.

[0019] Figure 2 This is a schematic diagram of the circuit structure of one example of the blocking device in the present application when it plays a shunting role.

[0020] Figure 3 This is a schematic diagram of the circuit structure of another example of the blocking device in the present application when it plays a shunting role.

[0021] Figure 4 This is a schematic diagram of a circuit structure of one example when the blocking device in this application plays a blocking role.

[0022] Figure 5 This is another example circuit structure diagram of the blocking device in the present application when it plays a blocking role.

[0023] Figure 6 Schematic diagram of the circuit structure in which the blocking device in the present application is applied to the flying capacitor Boost circuit.

[0024] Figure 7 Schematic diagram of the circuit structure in which the blocking device in the present application is applied to the TL-Boost circuit.

[0025] Figure 8 Schematic diagram of the circuit structure of the suppression protection system in this application.

[0026] Fig. 9This is a schematic diagram of the circuit structure of one example of the detection circuit in this application.

[0027] Fig.10 This is a schematic diagram of the circuit structure of another example of the detection circuit in this application.

[0028] Fig.11 This is a schematic diagram of a circuit structure of another example of a detection circuit in the present application.

[0029] In the figure: 110, blocking device; 120, DC side negative bus; 130, PV unit negative terminal; 140, detection circuit; 141, protection circuit; 142, comparison unit; 143, acquisition unit; 143a, first sampling resistor; 143b, second sampling resistor; 144, current limiting resistor; 145, isolation unit; 146, current sensor; 150, multiplexer; 160, controller. DETAILED DESCRIPTION

[0030] Below, the present application is further described in conjunction with specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0031] In the description of the present application, it should be noted that directional words, such as the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating directions and positional relationships are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of narrating the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and cannot be understood as limiting the specific scope of protection of the present application.

[0032] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0033] The terms "including" and "having" and any variations thereof in the specification and claims of this application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatuses.

[0034] In order to facilitate the understanding of the technical solution of the application, the following can be specifically analyzed when the PV unit of the photovoltaic system is short-circuited to the ground; Figure 1 As shown, there are multiple PV units in the photovoltaic system, which can be marked as PV1, PV2, ..., PV n Each PV unit is connected to the DC side of the DC / AC unit through the corresponding DC / DC unit, and the AC side of the DC / AC unit is connected to the power grid. A diode is set between the positive end of each PV unit and the DC / DC unit. The diodes corresponding to each PV unit can be marked as D1, D2, ..., D n .

[0035] Taking PV1 as an example, the method is also applicable to other PV units. Here, only PV1 is used as an example. Figure 1 As shown in the figure, the positive end of PV1 is connected to a diode D1. When the positive end of PV1 is short-circuited to the ground, the diode D1 can block the current, and the loop between the grid and the positive end of PV1 is blocked, and no short-circuit current will be generated. n As an example, the method is also applicable to other PV units. n Let's give an example. Figure 1 As shown, PV n The negative terminal is short-circuited to the ground; when one phase of the power grid is in the negative half cycle, Figure 1 V ga For example, the DC / AC unit of the photovoltaic system includes six power switches S1 to S6; wherein the power switches S1 and S4 are connected in series to form a switch bridge arm, the power switches S2 and S5 are connected in series to form a switch bridge arm, and the power switches S3 and S6 are connected in series to form a switch bridge arm; the midpoints of the three switch bridge arms are respectively connected to the three-phase power grid. When a short circuit fault occurs at the negative terminal 130 of the PV unit, even if the power switches S1-S6 of the inverter circuit are turned off, a Figure 1 The “PV n Negative terminal → power switch S4 → coil L a →Grid-connected switch S a →grid→ground" short-circuit loop, at this time the short-circuit current I=V ga / Z eq , where V ga is the A-phase grid voltage; Z eq is the equivalent impedance in the short-circuit loop. A larger short-circuit current may damage the switch tube, which shows that even if the switch tube in the inverter circuit is disconnected, the power switch device cannot be effectively protected.

[0036] From the above analysis process, it can be seen that when the positive end of the PV unit is short-circuited to the ground, the diode installed at the positive end can block the short-circuit current of the PV unit, and when the negative end of the PV unit is short-circuited to the ground, a short-circuit current will be generated through the power switching device of the DC / AC unit. Therefore, this embodiment will suppress the short-circuit current when the negative end of the PV unit is short-circuited to the ground.

[0037] Therefore, the present application provides a photovoltaic system DC side to ground impedance suppression protection system, such as Figures 2 to 5 As shown, one preferred embodiment includes: a blocking device 110, which is installed on the DC side of the photovoltaic system and has unidirectionality. When the negative terminal 130 of the PV unit of the photovoltaic system is short-circuited, the blocking device 110 can form a suppression circuit with the AC side of the photovoltaic system, and then block the short-circuit current through the unidirectional cutoff of the blocking device 110, or shunt the short-circuit current through the unidirectional conduction of the blocking device 110; thereby effectively preventing the short-circuit current from being too large and avoiding damage to the power switch and other devices of the photovoltaic system.

[0038] It should be known that, from the above analysis, the main reason why the power switch device is damaged when the PV unit is short-circuited to the ground is that the short-circuit current is too large, causing the power switch device to be burned. Therefore, in this embodiment, the short-circuit current can be directly blocked, that is, the short-circuit loop formed when the PV unit is short-circuited to the ground is disconnected, or the short-circuit current in the short-circuit loop is shunted so that the current value flowing through the power switch device is reduced to a value lower than the normal working current value of the power switch device. At the same time, in order to ensure the normal operation of the photovoltaic system, the blocking device 110 needs to be set to be unidirectionally conductive; that is, when the PV unit is working normally, the blocking device 110 used to block the short-circuit loop is in a unidirectional conductive state. At this time, the blocking device 110 will not affect the circuit. Only when the short-circuit loop is formed will the blocking device 110 form a unidirectional cutoff. Similarly, when the PV unit is operating normally, the blocking device 110 for shunting the short-circuit current in the short-circuit loop is in a unidirectional cut-off state. At this time, the blocking device 110 is equivalent to an open circuit and will not affect the circuit. Only when the short-circuit loop is formed will the blocking device 110 be unidirectionally conducted to shunt the short-circuit current.

[0039] It is understandable that, since the blocking device 110 has different ways of suppressing short-circuit current, the specific installation method of the blocking device 110 will also be different; for ease of understanding, three specific embodiments will be used to illustrate each of them.

[0040] Embodiment 1: A scenario in which the blocking device 110 shunts the short-circuit current.

[0041] In this embodiment, Figure 2 and Figure 3 As shown, three groups of blocking devices 110 are provided, and they correspond to the three switch bridge arms of the DC / AC unit respectively, one end of each group of blocking devices 110 is connected to the midpoint of the corresponding switch bridge arm, and the other end is connected to the negative bus 120 of the DC side of the photovoltaic system. Therefore, when the negative end of the PV unit is short-circuited to the ground, each group of blocking devices 110 can form a parallel suppression circuit with the corresponding switch bridge arm, and then the short-circuit current flowing through the switch bridge arm can be shunted.

[0042] It should be known that there are various specific structures of the blocking device 110 that can achieve the above functions. Common unidirectional blocking devices 110 include diodes and transistors, etc. The specific type of the blocking device 110 can be selected according to the actual needs of technical personnel in this field.

[0043] It is understandable that the specific number of blocking devices 110 in each group can be one or more, and can be selected according to the actual needs of technical personnel in this field. If the short-circuit current is too large, the number of blocking devices 110 is preferably multiple, and multiple blocking devices 110 are connected in parallel. Then, the short-circuit current can be shunted into multiple strands, so that the shunting amount of the short-circuit current flowing through the switch bridge arm is greatly reduced; of course, the resistance value of the blocking device 110 can also be increased so that the blocking device 110 can shun more short-circuit current to greatly reduce the shunting amount of the short-circuit current flowing through the switch bridge arm. For ease of understanding, the short-circuit current shunting process will be described in detail below by taking one phase of a three-phase power grid as an example; wherein, the number of blocking devices 110 in each group is taken as one. Specifically, Figure 2 As shown, the blocking device 110 is exemplified by a shunt diode connected in parallel with the switching device body diode in the same direction, and is marked as D a , D b , D c PV n When the negative terminal is shorted to the ground, take the grid phase A as an example in the negative half cycle. In addition to the “PV n Negative terminal → power switch S4 → coil L a →Grid-connected switch S a →Grid→Ground”, and also added “PV n Negative terminal → Shunt diode D a →Coil L a →Grid-connected switch S a →grid→ground” circuit, the two circuits are as follows Figure 2 The bold solid line in indicates that Figure 2 The two circuits shown in the figure form a parallel circuit, so that the short-circuit current originally flowing through the power switch S4 is shunt diode D aThe current is shunted, reducing the short-circuit current flowing through the power switch S4, thereby preventing the short-circuit current from being too large and burning the power switch S4. Therefore, the blocking device 110 has the ability to protect the body diode of the switching device when a short-circuit fault occurs to the ground at the negative terminal 130 of the PV unit, preventing the current from being too large during the short circuit and damaging the body diode of the switching device.

[0044] Additional explanation: Figure 2 The shunt diode D is connected in parallel with the switching device body diode in the same direction. a , D b , D c For example, but not limited to, Figure 3 A blocking device 110 composed of a triode is provided, and its working principle is similar to that of a diode, which will not be described in detail here.

[0045] Embodiment 2: Scenario 1 in which the blocking device 110 blocks the short-circuit current.

[0046] In this embodiment, Figure 4 As shown, the blocking device 110 is arranged on the negative bus 120 of the DC side of the photovoltaic system. When the negative terminal 130 of the PV unit of the photovoltaic system is short-circuited to the ground, the blocking device 110 blocks the short-circuit current of the negative terminal 130 of the PV unit. When the system works normally, the current flows from the anode of the blocking device 110 to the cathode; when the negative terminal 130 of the PV unit is short-circuited to the ground and a phase of the power grid is in the negative half cycle, the cathode voltage of the blocking device 110 is higher than the anode voltage, the blocking device 110 is cut off, and the short-circuit loop is blocked to protect the power switch and other devices and avoid damage due to excessive short-circuit current.

[0047] To supplement, when the blocking device 110 uses a diode, if the diode is made of silicon, then when the cathode voltage of the diode is 0.7V higher than the anode voltage, the silicon diode is cut off and the short circuit is blocked; if the diode is made of germanium, then when the cathode voltage of the diode is 0.3V higher than the anode voltage, the germanium diode is cut off and the short circuit is blocked.

[0048] It should be known that when the negative terminals of multiple PV units in a photovoltaic system are short-circuited to the ground, the short-circuit loop formed between the negative terminal of each PV unit and the ground will pass through the blocking device 110, that is, a single blocking device 110 needs to have the ability to block the short circuit to the ground of all PV units. Especially for high-power photovoltaic systems, the blocking device 110 is also required to have a strong current-carrying capacity, which will increase the difficulty of the busbar heat dissipation design in the actual design process. Therefore, the above-mentioned embodiment 2 is more suitable for low-power photovoltaic systems. For the short-circuit current suppression of high-power photovoltaic systems, the following will be described in detail through embodiment 3.

[0049] Embodiment 3: Scenario 2 of the blocking device 110 blocking the short-circuit current.

[0050] In this embodiment, Figure 5 As shown, there are multiple blocking devices 110, and each blocking device 110 is installed at each PV unit negative terminal 130 of the photovoltaic system. When the PV unit negative terminal 130 is short-circuited to the ground and a phase of the power grid is in the negative half cycle, the blocking device 110 blocks the short-circuit current passing through the PV unit negative terminal 130. Specifically, each PV unit negative terminal 130 is added with a blocking device 110, which can reduce the heat generation of a single blocking device 110 compared with the above-mentioned embodiment 2. When the system works normally, the current flows from the anode to the cathode of the blocking device 110. When a PV unit negative terminal 130 is short-circuited to the ground and a phase of the power grid is in the negative half cycle, taking the PV1 negative terminal short-circuited to the ground as an example, the cathode voltage of the blocking device 110 is higher than the anode voltage, the blocking device 110 is cut off, the short circuit loop is blocked, and the blocking device 110 is arranged at the PV unit negative terminal 130, which is conducive to the modular design of the system.

[0051] In addition, the blocking device 110 is a diode or a triode, so that the blocking device 110 can select a variety of electronic components to achieve the purpose of blocking or shunting. Diodes and triodes play a role in controlling current in the circuit. When the diode is forward biased, it allows current to flow in one direction, and when it is reverse biased, it cuts off the current. The triode also requires the correct polarity to work properly. For example, in an NPN triode, the base must maintain a positive potential relative to the emitter to make it conductive.

[0052] It should be known that the solution of this embodiment can be applied not only to the traditional two-level Boost circuit, but also to the three-level Boost circuit. Figure 6 As shown, the blocking device 110 is applied in a flying capacitor Boost circuit; Figure 7 FIG. 1 shows the application of the blocking device 110 in a TL-Boost circuit. Therefore, the blocking device 110 of this embodiment can be applied to a variety of circuits and has greater practicality.

[0053] As can be seen from the above content, the blocking device 110 can suppress the short-circuit current generated when the PV unit is short-circuited, but the blocking device 110 cannot eliminate the short-circuit fault of the PV unit. Therefore, when the PV unit is short-circuited to the ground, maintenance personnel are required to repair or replace it. However, the number of PV units included in the photovoltaic system is large, and it is difficult or impossible for maintenance personnel to determine which PV unit has a short-circuit fault to the ground, so maintenance personnel often need to detect all PV units in turn, which will greatly reduce the maintenance efficiency of maintenance personnel. At the same time, the long-term continuation of the short-circuit fault of the PV unit will also increase the safety risk of the photovoltaic system. Therefore, this embodiment can not only suppress the short-circuit current through the blocking device 110, but also locate the specific PV unit that is short-circuited to the ground, thereby facilitating the subsequent maintenance of maintenance personnel.

[0054] In another preferred embodiment of the present application, Figure 8 As shown, the suppression protection system also includes a detection circuit 140, a multiplexer 150 and a controller 160. The number of the detection circuits 140 corresponds to the number of PV units in the photovoltaic system. The detection circuit 140 collects power and makes fault judgments on the power devices corresponding to the PV units. The multiplexer 150 summarizes the judgment results of all the detection circuits 140 and sends them to the controller 160. The controller 160 locates the position of the PV unit that is short-circuited to the ground according to the judgment results of the detection circuit 140, so that when a fault occurs, the operating personnel can know the fault location in time, shorten the maintenance time, and improve the maintenance efficiency.

[0055] It is understandable that when a PV unit is short-circuited to the ground, the power parameters of the power device connected to the PV unit will change, so the short-circuit fault of the PV unit to the ground can be determined by detecting the power parameters of the power device. There are many types of power devices connected to the PV unit, but the PV unit is generally a packaged structure when in use, so when the power of the power device is detected, the packaging structure of the PV unit may be damaged. In the aforementioned third embodiment, the blocking device 110 is installed at the negative end of the PV unit, and the power parameters of the blocking device 110 will also change when the PV unit is short-circuited to the ground. Therefore, in this embodiment, the power collection object of the detection circuit 140 preferably uses a blocking device 110 installed at the negative end of the PV unit.

[0056] In addition, the multiplexer 150 summarizes the judgment results of all the detection circuits 140 and sends them to the controller 160, saving the interface resources of the controller 160. The specific structure and working principle of the multiplexer 150 are well-known technologies to those skilled in the art, so they are not elaborated in detail here; the common multiplexer 150 can adopt a logic gate circuit structure.

[0057] In this embodiment, there are multiple specific structures of the detection circuit 140. For ease of understanding, one of the structures will be described in detail below. Figures 9 to 11 As shown, the detection circuit 140 includes a protection circuit 141 and a comparison unit 142. The protection circuit 141 is connected in parallel with the blocking device 110, and the protection circuit 141 can be protected by the comparison unit 142; that is, the protection circuit 141 can be equivalent to the power information of the blocking device 110 by being connected in parallel with the blocking device 110, and then the input end of the comparison unit 142 is connected to the protection circuit 141 to obtain the equivalent terminal voltage or reverse current of the blocking device 110, and the comparison unit 142 determines whether the PV unit has a short circuit to ground fault according to the terminal voltage or reverse current.

[0058] It is understandable that the terminal voltage and reverse current can be used to determine whether a PV unit has a short circuit to ground fault; however, the judgment parameters and judgment structures used in different fault judgment methods are different. For ease of understanding, three examples will be used for detailed explanation below.

[0059] Example 1: Fig. 9 As shown, the protection circuit 141 also includes a collection unit 143, which is provided with a pair of sampling resistors connected in series, namely a first sampling resistor 143a and a second sampling resistor 143b; the two input terminals of the comparison unit 142 can be connected in parallel with one of the sampling resistors, specifically taking the comparison unit 142 and the second sampling resistor 143b in parallel as an example. The voltages of the two input terminals of the comparison unit 142 are equal to the two terminal voltages of the second sampling resistor 143b; if the terminal voltage of the second sampling resistor 143b close to the PV unit is higher than the terminal voltage of the other side, the comparison unit 142 outputs a short-circuit fault signal of the PV unit.

[0060] It should be noted that the total resistance of the acquisition unit 143 should be set large enough so that the acquisition unit 143 can be equivalent to an open circuit when the PV unit is performing normal operation, thereby ensuring that the blocking device 110 can perform normal unidirectional conduction operation. At the same time, the resistance value R1 of the first sampling resistor 143a is much greater than the resistance value R2 of the second sampling resistor 143b. In the case of a short circuit to the ground of the PV unit, the current in the protection circuit 141 is effectively reduced to prevent the protection circuit 141 from being damaged by excessive current. Alternatively, in the case of a circuit breaker failure in the blocking device 110, the first sampling resistor 143a can shunt more current to reduce the current flowing through the second sampling resistor 143b to protect the comparison unit 142.

[0061] It should be known that the specific resistance value of the sampling unit 143 can be selected according to the actual needs of those skilled in the art; for example, the resistance value of the first sampling resistor 143a is R1=99MΩ, and the resistance value of the second sampling resistor 143b is R2=1MΩ.

[0062] Example 2: If Fig.10 As shown, the first sampling resistor 143a and the second sampling resistor 143b are connected in series to form a series branch, and the series branch has a midpoint. One of the input terminals of the comparison unit 142 is connected to the midpoint of the series branch, and the other input terminal of the comparison unit 142 is given a voltage threshold V ref ; Obtain the midpoint voltage of the blocking device 110 terminal voltage divided by the acquisition unit 143, and divide the midpoint voltage and the voltage threshold V ref By comparison, the midpoint voltage is greater than the voltage threshold V ref , the comparison unit 142 will output the short-circuit fault signal of the PV unit to ground, the voltage threshold V ref The value of is set according to the actual situation and is not limited here.

[0063] Supplementary explanation: The calculation method of midpoint voltage is: V 中 =(V PV- -V BUS- )*R2 / (R1+R2), V 中 is the midpoint voltage, V PV- is the voltage at the negative terminal 130 of the PV cell, V BUS- is the voltage of the DC side negative bus 120, R1 is the resistance value of the first sampling resistor 143a, R2 is the resistance value of the second sampling resistor 143b, if V 中 >V ref In this case, the comparison unit 142 will output a short-circuit fault signal of the PV unit.

[0064] Example 3: Fig.11 As shown, the protection circuit 141 includes a plurality of current limiting resistors 144, which are connected in parallel with the blocking device 110. One input end of the comparison unit 142 is connected to the protection circuit 141 through a current sensor 146, and another input end of the comparison unit 142 is given a current threshold I ref If the reverse current value on the protection circuit 141 is greater than the current threshold I ref , the comparison unit 142 will output a short-circuit fault signal of the PV unit to ground, and the current threshold I ref The value of is set according to the actual situation and is not limited here.

[0065] Supplementary explanation: the reverse current is calculated as: I-=V g / R,V gis the grid voltage, R is the equivalent resistance of the current limiting resistor 144; taking the A phase grid voltage as an example, I-=V ga / R,V ga is the grid voltage of phase A. The specific structure and working principle of the current sensor 146 are well known to those skilled in the art, so they will not be elaborated in detail here; the common current sensor 146 adopts a Hall sensor.

[0066] During actual operation, the current sensor 146 is selected according to the actual working conditions, and then the resistance value and series-parallel relationship of the current limiting resistor 144 are reasonably set according to the specifications of the current sensor 146. After the current limiting resistor 144 and the current sensor 146 are determined, the comparison unit 142 is designed to make the selection of electronic components of the protection circuit 141 more reasonable.

[0067] The above three judgment methods can be selected according to the actual working conditions and hardware conditions. When a short circuit to ground occurs, the PV unit where the fault occurs can be located in time, and there is no need for maintenance personnel to check one by one, which greatly reduces the workload of maintenance personnel and enables maintenance personnel to quickly determine which PV unit has the short circuit to ground fault, and can quickly carry out maintenance, shortening the maintenance time.

[0068] The comparison unit 142 can directly send the fault signal to the multiplexer 150, but considering the safety of signal transmission, that is, whether the isolation unit 145 needs to be installed according to the grounding condition of the controller 160, if it needs to be installed, the comparison unit 142 will send the judgment result to the isolation unit 145, and the isolation unit 145 will generate an MPPT_error signal to the multiplexer 150.

[0069] When the controller 160 and the DC side negative bus 120 are not grounded together, the isolation unit 145 needs to be set; if the controller 160 and the DC side negative bus 120 are grounded together, the isolation unit 145 may not be set. If the comparison unit 142 and the controller 160 are not grounded together, the output of the comparison unit 142 is connected to the isolation unit 145, and the isolation unit 145 outputs the MPPT_error signal when a short-circuit fault occurs to the ground at the negative terminal 130 of the PV unit. The isolation unit 145 and the controller 160 are grounded together, and a high level indicates that a short-circuit fault occurs to the ground at the PV negative terminal of the corresponding PV unit.

[0070] When it is necessary to install the isolation unit 145, the comparison unit 142 transmits the comparison result to the isolation unit 145, and the isolation unit 145 outputs the MPPT_error signal to the multiplexer 150. The multiplexer 150 integrates the collected signals and transmits them to the controller 160. The controller 160 determines the location of the fault based on the integrated signals to remind the maintenance personnel and perform maintenance in time.

[0071] In addition, the output logic of the MPPT_error signal is determined by the connection between the current sensor 146 and the comparison unit 142. Fig.10 As shown, the output of the current sensor 146 is connected to the negative input terminal of the comparison unit 142. When the MPPT_error signal is at a high level, a short-circuit fault of the PV negative terminal to the ground occurs in the corresponding PV unit.

[0072] The above describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above embodiments, and the above embodiments and the specification only describe the principles of the present application. The present application may have various changes and improvements without departing from the spirit and scope of the present application, and these changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the attached claims and their equivalents.

Claims

1. A photovoltaic system DC side impedance to ground suppression protection system, characterized in that: include: A blocking device, which is arranged on the DC side of the photovoltaic system and has unidirectionality; the blocking device is suitable for forming a suppression circuit with the AC side when the negative end of the PV unit of the photovoltaic system is short-circuited, thereby blocking or shunting the short-circuit current; The DC / AC unit of the photovoltaic system includes three switch bridge arms corresponding to the three-phase power grid; the blocking devices have three groups and correspond to the three switch bridge arms respectively, one end of each group of the blocking devices is connected to the midpoint of the switch bridge arm, and the other end is connected to the negative busbar on the DC side of the photovoltaic system, so that each group of the blocking devices respectively forms the suppression circuit for shunting the short-circuit current passing through the corresponding switch bridge arm; The blocking device is installed on the negative busbar on the DC side of the photovoltaic system, and the blocking device is suitable for forming the suppression circuit for blocking the short-circuit current passing through the negative busbar on the DC side; There are a plurality of blocking devices, which are respectively installed at the negative end of each PV unit of the photovoltaic system; the blocking devices are suitable for forming the suppression circuit for blocking the short-circuit current passing through the negative end of the PV unit.

2. The suppression protection system as claimed in claim 1, characterized in that The blocking device is a diode or a triode.

3. The suppression protection system according to claim 1, characterized in that: The suppression protection system also includes a detection circuit, a multiplexer and a controller; the number of the detection circuits corresponds to the number of PV units of the photovoltaic system; the detection circuit is used to collect power and perform fault judgment on the power devices corresponding to the PV units, and the multiplexer is suitable for summarizing the judgment results of all the detection circuits and sending them to the controller, and the controller locates the position of the PV unit that is short-circuited to the ground according to the judgment results of the detection circuits.

4. The suppression protection system according to claim 3, characterized in that: The blocking device installed at the negative end of the PV unit serves as a power device for the detection circuit to collect power; the detection circuit includes a protection circuit and a comparison unit; the protection circuit is connected in parallel to the blocking device, and the input end of the comparison unit is connected to the protection circuit to obtain the terminal voltage or reverse current of the blocking device, and the comparison unit is suitable for judging the short-circuit fault of the PV unit to ground based on the data obtained from the input end.

5. The suppression protection system according to claim 4, characterized in that: The protection circuit also includes a collection unit, which includes a pair of sampling resistors connected in series, and the two input ends of the comparison unit are connected in parallel to one of the sampling resistors; if the terminal voltage of the sampling resistor on one side close to the PV unit is higher than the terminal voltage on the other side, the comparison unit will output a short-circuit fault signal of the PV unit.

6. The suppression protection system according to claim 4, characterized in that: The acquisition unit includes a pair of sampling resistors connected in series. One of the input terminals of the comparison unit is connected to the midpoint of the series branch formed by the two sampling resistors. The other input terminal of the comparison unit is given a voltage threshold V ref If the midpoint voltage of the blocking device terminal voltage divided by the sampling resistor is greater than the voltage threshold V ref , the comparison unit will output a short-circuit fault signal of the PV unit to ground.

7. The suppression protection system according to claim 4, characterized in that: The protection circuit includes at least one current limiting resistor connected in parallel to the blocking device. One of the input terminals of the comparison unit is connected to the protection circuit. The other input terminal of the comparison unit is given a current threshold I ref If the reverse current on the protection circuit is greater than the current threshold I ref , the comparison unit will output a short-circuit fault signal of the PV unit to ground.

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