Leakage current suppression method in relay self-inspection

Abstract

Disclosed in the present invention is a leakage current suppression method in relay self-inspection, which is applied to a grid connection process of an inverter system. An output end of a DC / AC unit of the inverter system is connected to a power grid by means of a relay set; the relay set comprises a positive relay set and a negative relay set, the positive relay set comprises at least two relays connected in series to a positive bus, and the negative relay set comprises at least two relays connected in series to a negative bus; a bidirectional switch unit is further connected between positive and negative ends of the output end of the DC / AC unit; and when the positive relay set is closed, the inverter system forms a suppression loop by controlling the DC / AC unit and the bidirectional switch unit, and the suppression loop is configured to generate the voltage for suppressing leakage current. The problem of tripping in a residual current device caused by excessively large leakage current generated during relay self-inspection is effectively solved, and the overall implementation mode is simple and easy to implement.

Description

A method for suppressing leakage current in relay self-test Technical Field

[0001] The present invention relates to the technical field of power generation, and in particular to a leakage current suppression method in relay self-test. Background Art

[0002] As shown in FIG1 , a typical topology of an existing photovoltaic system is shown. The front stage is a DC / DC unit 110, which may adopt a Boost topology, but is not limited to this; the back stage is a DC / AC unit 120, which may adopt a Heric topology, but is not limited to this; the output end of the DC / AC unit 120 is connected to the power grid through a relay group 130, and the photovoltaic system is connected to the grid by closing the relay group 130.

[0003] However, in the actual grid connection process, it is found that during the relay self-test stage and the relay fully closed stage, when the relays T1 and T3 in FIG1 are closed but before the DC / AC unit 120 generates a wave, the leakage current i g0 Distortion will occur if the output leakage current i g0 If the distortion is too large, the non-50Hz component will increase, and the leakage protector is more sensitive to non-50Hz components, so the leakage current i g0 This can easily cause the leakage protector to trip, thus affecting the normal grid connection process of the photovoltaic system. Therefore, it is necessary to suppress the leakage current generated during the grid connection process of the photovoltaic system. Summary of the Invention

[0004] An object of the present invention is to provide a method for suppressing leakage current in relay self-test that can solve at least one of the drawbacks of the above-mentioned background technology.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is: a leakage current suppression method in relay self-test, applied to the grid-connected process of an inverter system, wherein the output end of a DC / AC unit of the inverter system is connected to the power grid via a relay group; the relay group includes a positive relay group and a negative relay group, the positive relay group includes at least two relays connected in series to the positive bus, and the negative relay group includes at least two relays connected in series to the negative bus; a bidirectional switch unit is also connected between the positive and negative output ends of the DC / AC unit; when the positive relay group is closed, the inverter system forms a suppression loop by controlling the DC / AC unit and the bidirectional switch unit, and the suppression loop is used to generate a voltage to suppress leakage current.

[0006] As a preferred embodiment, the DC / AC unit includes a switch tube S connected in a bridge type. a1 、S a4 、S b1 and Sb4 , wherein the switch tube S a1 and S a4 are connected to form a first branch, the switch tube S b1 and S b4 The first branch and the second branch are connected to the positive and negative busbars in parallel, and the switch tube S a1 and S b4 Close to the positive bus; when the positive relay group is closed, by controlling the switch tube S a1 、S a4 、S b1 、S b4 and the bidirectional switch unit to form the suppression loop.

[0007] As a preferred embodiment, when the positive relay group is closed, the bidirectional switch unit is in an open state, and the DC / AC unit is in a closed state through the conducting switch tube S a4 and S b4 The DC / AC unit is connected to the power grid so as to output a differential-mode DC voltage to form a suppression loop.

[0008] As a preferred embodiment, when the positive relay group is closed, the bidirectional switch unit is in an open state, and the DC / AC unit is in a closed state through the conducting switch tube S a1 and S b1 The DC / AC unit is connected to the power grid so as to output a differential-mode DC voltage to form a suppression loop.

[0009] As a preferred embodiment, when the positive relay group is closed, the bidirectional switch unit is in an open state, and the DC / AC unit is in a closed state through the conducting switch tube S a1 It is connected to the power grid so that the live wire clamp of the power grid is located at the positive busbar to form a suppression loop.

[0010] As a preferred embodiment, when the positive relay group is closed, the bidirectional switch unit is in an open state, and the DC / AC unit is in a closed state through the conducting switch tube S a4 It is connected to the power grid so that the live wire clamp of the power grid is located at the negative busbar to form a suppression loop.

[0011] As a preferred embodiment, the bidirectional switch unit includes switch tubes S2 and S3 connected in anti-series. When the positive relay group is closed, the switch tubes S2 and S3 are both in the on state and are connected to the switch tube S2 in the DC / AC unit. a1 Or the switch tube S is turned on a4 Form an inhibitory loop.

[0012] As a preferred embodiment, the withstand voltage of the ground capacitance of the inverter system is greater than (vg +V dc ), the withstand voltage of the negative ground capacitance is greater than v g , where V dc is the bus voltage, v g is the grid voltage.

[0013] As a preferred embodiment, the withstand voltage of the ground capacitance of the inverter system is greater than v g , the withstand voltage of the negative capacitance to ground is greater than (v g -V dc ), where V dc is the bus voltage, v g is the grid voltage.

[0014] As a preferred embodiment, the positive relay group includes relays T1 and T3, the negative relay group includes relays T2 and T4, and the positive relay group includes relays T1 and T3, and the positive ... IO To determine the working condition of the relay group.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] During the relay self-test process, when the positive relay group is closed, the inverter system controls the suppression loop formed by the DC / AC unit and the bidirectional switch unit. The suppression loop is able to generate a voltage that suppresses the leakage current, effectively solving the problem of the leakage protector tripping caused by excessive leakage current generated during the relay self-test process. The overall implementation method is simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG1 is a schematic diagram of the ground capacitance distribution structure of an existing inverter system;

[0018] FIG2 is a timing diagram of a conventional inverter system;

[0019] FIG3 is a simulation waveform diagram of an existing inverter system;

[0020] FIG4 is a waveform diagram of a conventional inverter system;

[0021] FIG5 is a schematic diagram of the ground capacitance distribution structure of the existing inverter system in process 1 shown in FIG4 ;

[0022] FIG6 is a schematic diagram of the ground capacitance distribution structure of the existing inverter system in the second process shown in FIG4 ;

[0023] FIG7 is a schematic diagram of the ground capacitance distribution structure of the existing inverter system in process three shown in FIG4 ;

[0024] FIG8 is a schematic diagram of the ground capacitance distribution structure of the existing inverter system in process 4 shown in FIG4;

[0025] FIG9 is a timing diagram of the inverter system of the present invention;

[0026] FIG10 is a timing diagram of the inverter system according to the first embodiment of the present invention;

[0027] FIG11 is a schematic diagram of the distribution structure of the ground capacitance of the inverter system forming a suppression loop according to the first embodiment of the present invention;

[0028] FIG12 is a schematic diagram of an equivalent circuit of the structural schematic diagram shown in FIG11 of the present invention;

[0029] FIG13 is a simplified circuit diagram of the equivalent circuit diagram shown in FIG12 of the present invention;

[0030] FIG14 is a timing diagram of the inverter system according to the second embodiment of the present invention;

[0031] FIG15 is a schematic diagram of the distribution structure of the ground capacitance of the inverter system forming a suppression loop according to the second embodiment of the present invention;

[0032] FIG16 is a schematic diagram of an equivalent circuit of the structural schematic diagram shown in FIG15 of the present invention;

[0033] FIG17 is a simplified circuit diagram of the equivalent circuit diagram shown in FIG16 of the present invention;

[0034] FIG18 is a timing diagram of the inverter system according to the third embodiment of the present invention;

[0035] FIG19 is a schematic diagram of the distribution structure of the ground capacitance of the inverter system forming a suppression loop according to the third embodiment of the present invention;

[0036] FIG20 is a schematic diagram of an equivalent circuit of the structural schematic diagram shown in FIG19 of the present invention;

[0037] FIG21 is a simplified circuit diagram of the equivalent circuit diagram shown in FIG20 of the present invention;

[0038] FIG22 is a timing diagram of the inverter system according to the fourth embodiment of the present invention;

[0039] FIG23 is a schematic diagram of the ground capacitance distribution structure of the inverter system forming a suppression loop according to the fourth embodiment of the present invention;

[0040] FIG24 is a schematic diagram of an equivalent circuit of the structural schematic diagram shown in FIG23 of the present invention;

[0041] FIG25 is a simplified circuit diagram of the equivalent circuit diagram shown in FIG24 of the present invention;

[0042] FIG26 is a timing diagram of the inverter system according to the fifth embodiment of the present invention;

[0043] FIG27 is a schematic diagram of the ground capacitance distribution structure of the inverter system forming a suppression loop according to the fifth embodiment of the present invention;

[0044] FIG28 is a schematic diagram of an equivalent circuit of the structural schematic diagram shown in FIG27 of the present invention;

[0045] FIG29 is a simplified circuit diagram of the equivalent circuit diagram shown in FIG28 of the present invention.

[0046] In the figure: 110, DC / DC unit; 120, DC / AC unit; 130, relay group; 131, positive relay group; 132, negative relay group; 140, bidirectional switch unit. DETAILED DESCRIPTION

[0047] The present invention will be further described below 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.

[0048] In the description of the present invention, it should be noted that, for directional words, such as the terms "center", "horizontal", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like, 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 describing the present invention 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 invention.

[0049] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0050] The terms "comprises" 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 that includes a series of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, product or apparatus.

[0051] As shown in Figure 1, a conventional inverter system includes a DC / DC unit 110 and a DC / AC unit 120 connected in sequence. A bidirectional switch unit 140 is connected in parallel between the DC / AC unit 120 and a relay group 130. The output of the relay group 130 is connected to the power grid. When the relay group 130 is closed, the inverter system is connected to the power grid. Furthermore, the input of the inverter system can be connected to a photovoltaic module (PV) or other power generation device. Relay group 130 includes relays T1 to T4, with relays T1 and T3 connected in series to the positive busbar, and relays T2 and T4 connected in series to the negative busbar.

[0052] Generally speaking, the operation of the inverter system mainly includes the following four stages. The first stage: the bus voltage V dc The DC / DC unit 110 starts to boost the voltage. Phase 2: Relays T1-T4 are activated individually to perform a self-test. Phase 3: Relays T1-T4 are fully closed. Phase 4: DC / AC unit 120 performs PWM modulation (Pulse Width Modulation) to generate power for grid connection.

[0053] Specifically, as shown in FIG2 , in the second stage, relay T3 is disconnected and the other relays are closed; relay T1 is disconnected and the other relays are closed; relay T2 is disconnected and the other relays are closed; relay T4 is disconnected and the other relays are closed; and by detecting the port voltage v IO Whether it is the grid voltage, this can be used to determine whether relays T1 to T4 are normal.

[0054] As shown in FIG3 , when relay T2 or T4 is self-testing, relays T1 and T3 are in a closed state. At this time, before the DC / AC unit 120 generates a wave, the leakage current i g0 Will produce distortion, if the output leakage current i g0 If the distortion is too large, the non-50Hz component will increase, and the leakage protector is more sensitive to non-50Hz components, so the leakage current i g0 It is easy to cause the leakage protector to trip.

[0055] In order to facilitate the understanding of the solution of this application, the leakage current i caused by the self-test of the relay group 130 can be firstly g0 When relays T1 and T3 are in the closed state and the DC / AC unit 120 is not emitting waves, the grid voltage V g The waveform is shown in Figure 4, and one cycle is divided into four processes for easy analysis.

[0056] In process 1, that is, the period [t0,π / 2ω] shown in Figure 4, combined with Figure 5, it can be understood that from time t0, the voltage of the grid is greater than the voltage v of the capacitor to ground. dc+ At this time, the switch tube S of the DC / AC unit 120 a1 The power grid passes through the switch tube S a1 The negative ground capacitance is charged until the grid voltage reaches its positive peak at π / 2ω.

[0057] In the second process, i.e., the period [π / 2ω, π / ω+t0] shown in FIG4 , it can be understood from FIG6 that, starting from the moment π / 2ω, the grid voltage begins to decrease from the positive peak. At this time, the DC / AC unit 120 is disconnected, and the voltage of the negative ground capacitor remains unchanged until the grid voltage reaches 0 at the moment π / 2ω.

[0058] In process three, that is, the period [π / ω+t0, 3π / 2ω] shown in Figure 4, combined with Figure 7, it can be understood that starting from the moment π / ω+t0, the voltage v of the negative ground capacitor dc- is greater than the voltage of the grid, then the switch S of the DC / AC unit 120 a2 The negative capacitance to ground passes through the switch tube S a1 Discharge to the grid until the moment 3π / 2ω when the grid reaches a negative peak.

[0059] In process four, i.e., the period [3π / 2ω, 2π / ω+t0] shown in FIG4 , it can be understood from FIG8 that, starting from the moment 3π / 2ω, the grid voltage starts to rise from the negative peak. At this time, the DC / AC unit 120 is disconnected, and the voltage of the negative ground capacitor remains unchanged until the moment π / 2ω, when the grid voltage reaches 0.

[0060] It can be understood from the above process 1 to process 4 that when the DC / AC unit 120 is not generating the wave, the switch tube S a1 and S a2 The switch is switched alternately at intervals, so that the leakage current i g0 It changes nonlinearly, causing distortion. If the output leakage current i g0 If the distortion is too large, it will affect the normal grid connection process of the inverter system. Therefore, as shown in FIG9 , it is necessary to make the DC / AC unit 120 generate a wave to output a DC voltage or an AC voltage during the relay self-test phase, thereby making the leakage current i g0 The state is close to sinusoidal.

[0061] Example 1:

[0062] As shown in Figures 10 to 13, a leakage current suppression method in relay self-test is applied to the grid-connected process of an inverter system. The output end of the DC / AC unit 120 of the inverter system is connected to the grid through a relay group 130; the relay group 130 includes a positive relay group 131 and a negative relay group 132. The positive relay group 131 includes at least two relays connected in series to the positive bus, and the negative relay group 132 includes at least two relays connected in series to the negative bus; a bidirectional switch unit 140 is also connected between the positive and negative output ends of the DC / AC unit 120; when the positive relay group 131 is closed, the inverter system controls the DC / AC unit 120 and the bidirectional switch unit 140 to form a suppression loop, which is used to generate a voltage to suppress leakage current.

[0063] As shown in FIG11 , the DC / AC unit 120 includes a bridge-connected switch tube S a1 、S a4 、S b1 and S b4 , where the switch tube S a1 and S a4 Connected to form the first branch, the switch tube S b1 and S b4 The first branch and the second branch are connected in parallel to the positive and negative busbars, and the switch tube S a1 and S b4 Close to the positive bus, the input end of the bidirectional switch unit 140 is connected to the midpoint of the first branch and the second branch respectively; the bidirectional switch unit 140 includes switch tubes S2 and S3 connected in anti-series; when the relays T1 and T3 are closed, the switch tubes S2 and S3 are controlled to a1 、S a4 、S b1 、S b4 , S2 and S3 to form a suppression loop. a1 、S a4 、S b1 、S b4 There are many specific structures of S2 and S3, such as field effect tubes and thyristors, etc. In this embodiment, field effect tubes are preferably used.

[0064] Specifically, as shown in FIG10 and FIG11, when the positive relay group 131 is closed, the bidirectional switch unit 140 is in the open state, and the DC / AC unit 120 is turned on by the switch tube S a4 and S b4 Connected to the grid, the DC / AC unit 120 outputs a differential mode DC voltage to form a suppression loop. At this time, the voltage generated by the suppression loop is -V dc / 2, where V dc is the bus voltage.

[0065] It can be understood that the positive relay group 131 in this embodiment includes relays T1 and T3, and the negative relay group 132 includes relays T2 and T4, but in other embodiments, the positive relay group 131 and the negative relay group 132 can also respectively include three or even more relays, and this application does not impose specific limitations.

[0066] According to Figure 11, the impedance circuit of the inverter system can be equivalent to the equivalent circuit diagram shown in Figure 12. The main impedance to ground considered when the inverter system is connected to the grid is the positive and negative ground capacitance C of the output side of the photovoltaic module PV. PV+ and C PV- , the capacitance of the positive and negative busbars to ground C dc+ and C dc- Generally speaking, C PV+ =C PV- =C PV , C dc+ =C dc- =Cdc, V PV =V dc , then by simplifying the equivalent circuit shown in FIG12 , we can obtain the simplified circuit schematic diagram shown in FIG13 .

[0067] According to Figure 13, the leakage current i g0 The AC component is: i g0 =-jω(2C PV +2C dc )v g , then we know that the leakage current i g0 The leakage current i is determined by the capacitance to ground on the grid side and the capacitance to the positive and negative busbars. The frequency of the photovoltaic module PV and the grid is usually 50Hz, so the leakage current i g0 The AC component is also 50Hz, which can avoid leakage current i g0 The waveform is distorted to prevent the leakage protector from tripping.

[0068] Furthermore, the voltage from point O in Figure 13 to ground is v g +V dc / 2, combined with Figure 11, the voltages of the positive and negative busbar capacitors to ground are: v dc+ =v g +V dc ;v dc- =v g Therefore, the withstand voltage of the positive busbar's capacitance to ground should be greater than (v g +V dc ), the withstand voltage of the negative busbar's capacitance to ground should be greater than v g .

[0069] Furthermore, by detecting the port voltage v IOIt is possible to determine whether the relay group 130 is normal. Specifically, when the port voltage v IO DC-V dc When the terminal voltage v IO For AC v g When the relay is stuck.

[0070] Example 2:

[0071] Compared with the first embodiment, the difference of this embodiment is that: as shown in Figures 14 and 15, when the positive relay group 131 is closed, the bidirectional switch unit 140 is in the open state, and the DC / AC unit 120 is connected to the DC / AC unit 120 through the conductive switch tube S a1 and S b1 Connected to the grid, the DC / AC unit 120 outputs a differential mode DC voltage to form a suppression loop. At this time, the voltage generated by the suppression loop is -V dc / 2.

[0072] It can be understood that the positive relay group 131 in this embodiment includes relays T1 and T3, and the negative relay group 132 includes relays T2 and T4, but in other embodiments, the positive relay group 131 and the negative relay group 132 can also respectively include three or even more relays, and this application does not impose specific limitations.

[0073] According to Figure 15, the impedance circuit of the inverter system can be equivalent to the equivalent circuit shown in Figure 16. The main impedance to ground considered when the inverter system is connected to the grid is the positive and negative ground capacitance C of the output side of the photovoltaic module PV. PV+ and C PV- , the capacitance of the positive and negative busbars to ground C dc+ and C dc- Generally speaking, C PV+ =C PV- =C PV , C dc+ =C dc- =C dc , V PV =V dc , then by simplifying the equivalent circuit shown in FIG16 , we can obtain the simplified circuit schematic diagram shown in FIG17 .

[0074] According to Figure 17, the leakage current i g0 The AC component is: i g0 =-jω(2C PV +2C dc )v g , then we know that the leakage current i g0The leakage current i is determined by the capacitance to ground on the grid side and the capacitance to the positive and negative busbars. The frequency of the photovoltaic module PV and the grid is usually 50Hz, so the leakage current i g0 The AC component is also 50Hz, which can avoid leakage current i g0 The waveform is distorted to prevent the leakage protector from tripping.

[0075] Furthermore, the voltage from point O in Figure 17 to ground is v g +V dc / 2, combined with Figure 15, we can get that the voltages of the positive and negative busbar capacitors to ground are: v dc+ =v g ;v dc- =v g -V dc Therefore, the withstand voltage of the positive busbar's capacitance to ground should be greater than v g , the withstand voltage of the negative busbar's capacitance to ground should be greater than (v g -V dc ).

[0076] Furthermore, by detecting the port voltage v IO It is possible to determine whether the relay group 130 is normal. Specifically, when the port voltage v IO is DC V dc When the terminal voltage v IO For AC v g When the relay is stuck.

[0077] Example 3:

[0078] Compared with the first embodiment, the difference of this embodiment is that: as shown in Figures 18 and 19, when the positive relay group 131 is closed, the bidirectional switch unit 140 is in the open state, and the DC / AC unit 120 is connected to the DC / AC unit 120 through the conductive switch tube S a1 Connected to the grid, the live wire clamp of the grid is located at the positive busbar to form a suppression loop, which can change the flow path of the common mode current and prevent the common mode current from flowing through the switch tube S a1 The diode prevents leakage current i g0 The waveform is distorted.

[0079] It can be understood that the positive relay group 131 in this embodiment includes relays T1 and T3, and the negative relay group 132 includes relays T2 and T4, but in other embodiments, the positive relay group 131 and the negative relay group 132 can also respectively include three or even more relays, and this application does not impose specific limitations.

[0080] According to FIG19 , the impedance circuit of the inverter system connected to the grid can be equivalent to obtain the equivalent circuit diagram shown in FIG20 . Among them, the differential mode voltage generated by the DC / AC unit 120 itself is V dc / 2. When the inverter system is connected to the grid, the main impedance to ground is the positive and negative ground capacitance C of the output side of the photovoltaic module PV. PV+ and C PV- , the capacitance of the positive and negative busbars to ground C dc+ and C dc- Generally speaking, C PV+ =C PV- =CPV, C dc+ =C dc- =C dc , V PV =V dc , then by simplifying the equivalent circuit shown in FIG20 , we can obtain the simplified circuit schematic diagram shown in FIG21 .

[0081] According to Figure 21, the leakage current i g0 The AC component is: i g0 =-jω(2C PV +2C dc )v g , then we know that the leakage current i g0 The leakage current i is determined by the capacitance to ground on the grid side and the capacitance to the positive and negative busbars. The frequency of the photovoltaic module PV and the grid is usually 50Hz, so the leakage current i g0 The AC component is also 50Hz, which can avoid leakage current i g0 The waveform is distorted to prevent the leakage protector from tripping.

[0082] Furthermore, the voltage from point O in Figure 21 to ground is v g +V dc / 2, combined with Figure 19, the voltages of the positive and negative busbar capacitors to ground are: v dc+ =v g ;v dc- =v g -V dc Therefore, the withstand voltage of the positive busbar's capacitance to ground should be greater than v g , the withstand voltage of the negative busbar's capacitance to ground should be greater than (v g -V dc ).

[0083] Furthermore, by detecting the port voltage v IO It is possible to determine whether the relay group 130 is normal. Specifically, when the port voltage v IO For AC v gSince the grid's live wire clamp is located on the positive busbar of the inverter system, it can avoid affecting the system's differential mode circuit. Even if the relay sticks, the system will not experience overcurrent.

[0084] Example 4:

[0085] Compared with the third embodiment, the difference of this embodiment is that: as shown in Figures 22 and 23, when the positive relay group 131 is closed, the bidirectional switch unit 140 is in the open state, and the DC / AC unit 120 is connected to the DC / AC unit 120 through the conductive switch tube S a4 Connected to the grid, the live wire clamp of the grid is located at the negative busbar to form a suppression loop, which can change the flow path of the common mode current and prevent the common mode current from flowing through the switch tube S a4 The diode prevents leakage current i g0 The waveform is distorted.

[0086] It can be understood that the positive relay group 131 in this embodiment includes relays T1 and T3, and the negative relay group 132 includes relays T2 and T4, but in other embodiments, the positive relay group 131 and the negative relay group 132 can also respectively include three or even more relays, and this application does not impose specific limitations.

[0087] According to FIG23, the impedance circuit of the inverter system connected to the grid can be equivalent to obtain the equivalent circuit diagram shown in FIG24. Among them, the differential mode voltage generated by the DC / AC unit 120 itself is -V dc / 2. When the inverter system is connected to the grid, the main impedance to ground is the positive and negative ground capacitance C of the output side of the photovoltaic module PV. PV+ and C PV- , the capacitance of the positive and negative busbars to ground C dc+ and C dc- Generally speaking, C PV+ =C PV- =C PV , C dc+ =C dc- =C dc , V PV =V dc , then by simplifying the equivalent circuit shown in FIG20 , we can obtain the simplified circuit schematic diagram shown in FIG25 .

[0088] According to Figure 25, the leakage current i g0 The AC component is: i g0 =-jω(2C PV +2C dc )v g , then we know that the leakage current i g0The leakage current i is determined by the capacitance to ground on the grid side and the capacitance to the positive and negative busbars. The frequency of the photovoltaic module PV and the grid is usually 50Hz, so the leakage current i g0 The AC component is also 50Hz, which can avoid leakage current i g0 The waveform is distorted to prevent the leakage protector from tripping.

[0089] Furthermore, the voltage from point O in Figure 25 to ground is v g +V dc / 2, combined with Figure 23, we can get that the voltages of the positive and negative busbar capacitors to ground are: v dc+ =v g +V dc ;v dc- =v g Therefore, the withstand voltage of the positive busbar's capacitance to ground should be greater than (v g +V dc ), the withstand voltage of the negative busbar's capacitance to ground should be greater than v g .

[0090] Furthermore, by detecting the port voltage v IO It is possible to determine whether the relay group 130 is normal. Specifically, when the port voltage v IO For AC v g Since the grid's live wire clamp is located on the negative busbar of the inverter system, it can avoid affecting the system's differential mode circuit. Even if the relay sticks, the system will not experience overcurrent.

[0091] Embodiment 5:

[0092] Compared with the fourth embodiment, the difference of this embodiment is that: as shown in Figures 26 and 27, when the positive relay group 131 is closed, the switch tubes S2 and S3 are both in the on state and are connected to the switch tube S3 in the DC / AC unit 120. a1 Or the conducting switch S a4 Form an inhibitory loop.

[0093] It can be understood that the positive relay group 131 in this embodiment includes relays T1 and T3, and the negative relay group 132 includes relays T2 and T4, but in other embodiments, the positive relay group 131 and the negative relay group 132 can also respectively include three or even more relays, and this application does not impose specific limitations.

[0094] According to FIG27, the impedance circuit of the inverter system connected to the grid can be equivalent to obtain the equivalent circuit diagram shown in FIG28. Among them, the differential mode voltage generated by the DC / AC unit 120 itself is -V dc / 2. When the inverter system is connected to the grid, the main impedance to ground is the positive and negative ground capacitance C of the output side of the photovoltaic module PV.PV+ and C PV- , the capacitance of the positive and negative busbars to ground C dc+ and C dc- Generally speaking, C PV+ =C PV- =C PV , C dc+ =C dc- =C dc , V PV =V dc , then by simplifying the equivalent circuit shown in Figure 28, we can obtain the simplified circuit schematic shown in Figure 29.

[0095] According to Figure 29, the leakage current i g0 The AC component is: i g0 =-jω(2C PV +2C dc )v g , then we know that the leakage current i g0 The leakage current i is determined by the capacitance to ground on the grid side and the capacitance to the positive and negative busbars. The frequency of the photovoltaic module PV and the grid is usually 50Hz, so the leakage current i g0 The AC component is also 50Hz, which can avoid leakage current i g0 The waveform is distorted to prevent the leakage protector from tripping.

[0096] Furthermore, the voltage from point O in Figure 29 to ground is v g +V dc / 2, combined with Figure 27, we can get that the voltages of the positive and negative busbar capacitors to ground are: v dc+ =v g +V dc ;v dc- =v g Therefore, the withstand voltage of the positive busbar's capacitance to ground should be greater than (v g +V dc ), the withstand voltage of the negative busbar's capacitance to ground should be greater than v g .

[0097] Furthermore, by detecting the port voltage v IO It is possible to determine whether the relay group 130 is normal. Specifically, when the port voltage v IO is DC V dc When the terminal voltage v IO For AC v g When the relay is stuck.

[0098] The above describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and description merely illustrate the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for suppressing leakage current in relay self-test, applied to the grid-connected process of an inverter system, wherein the output terminal of a DC / AC unit of the inverter system is connected to the grid via a relay group; the relay group includes a positive relay group and a negative relay group, the positive relay group includes at least two relays connected in series to a positive bus, and the negative relay group includes at least two relays connected in series to a negative bus; a bidirectional switch unit is further connected between the positive and negative output terminals of the DC / AC unit; characterized in that: When the positive relay group is closed, the inverter system forms a suppression loop by controlling the DC / AC unit and the bidirectional switch unit, and the suppression loop is used to generate a voltage for suppressing leakage current.

2. The method for suppressing leakage current in relay self-test according to claim 1, characterized in that: The DC / AC unit includes a bridge-connected switch tube S a1 、S a4 、S b1 and S b4 , wherein the switch tube S a1 and S a4 are connected to form a first branch, the switch tube S b1 and S b4 The first branch and the second branch are connected to the positive and negative busbars in parallel, and the switch tube S a1 and S b4 Close to the positive bus; when the positive relay group is closed, by controlling the switch tube S a1 、S a4 、S b1 、S b4 and the bidirectional switch unit to form the suppression loop.

3. The method for suppressing leakage current in relay self-test according to claim 2, characterized in that: When the positive relay group is closed, the bidirectional switch unit is in the open state, and the DC / AC unit is switched on by the conducting switch tube S a4 and S b4 The DC / AC unit is connected to the power grid so as to output a differential-mode DC voltage to form a suppression loop.

4. The method for suppressing leakage current in relay self-test according to claim 2, characterized in that: When the positive relay group is closed, the bidirectional switch unit is in the open state, and the DC / AC unit is switched on by the conducting switch tube S a1 and S b1 The DC / AC unit is connected to the power grid so as to output a differential-mode DC voltage to form a suppression loop.

5. The method for suppressing leakage current in relay self-test according to claim 2, characterized in that: When the positive relay group is closed, the bidirectional switch unit is in the open state, and the DC / AC unit is switched on by the conducting switch tube S a1 It is connected to the power grid so that the live wire clamp of the power grid is located at the positive busbar to form a suppression loop.

6. The method for suppressing leakage current in relay self-test according to claim 2, characterized in that: When the positive relay group is closed, the bidirectional switch unit is in the open state, and the DC / AC unit is switched on by the conducting switch tube S a4 It is connected to the power grid so that the live wire clamp of the power grid is located at the negative busbar to form a suppression loop.

7. The method for suppressing leakage current in relay self-test according to claim 2, characterized in that: The bidirectional switch unit includes switch tubes S2 and S3 connected in anti-series. When the positive relay group is closed, the switch tubes S2 and S3 are both in the on state and are connected to the switch tube S3 in the DC / AC unit. a1 Or the switch tube S is turned on a4 Form an inhibitory loop.

8. The method for suppressing leakage current in relay self-test according to claim 3, 6 or 7, characterized in that: The withstand voltage of the ground capacitance of the inverter system is greater than (v g +V dc ), the withstand voltage of the negative ground capacitance is greater than v g , where V dc is the bus voltage, v g is the grid voltage.

9. The method for suppressing leakage current in relay self-test according to claim 4 or 5, characterized in that: The withstand voltage of the ground capacitance of the inverter system is greater than v g , the withstand voltage of the negative capacitance to ground is greater than (v g -V dc ), where V dc is the bus voltage, v g is the grid voltage.

10. The method for suppressing leakage current in relay self-test according to any one of claims 1 to 7, characterized in that: The positive relay group includes relays T1 and T3, the negative relay group includes relays T2 and T4, and by detecting the port voltage v IO To determine the working condition of the relay group.

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