Photovoltaic system and ground fault detection method

Abstract

The present application discloses a photovoltaic system, comprising: a first end of a grounding circuit is connected to a negative output end of a DCDC circuit, a second end of the grounding circuit is connected to a positive output end of the DCDC circuit, and a third end of the grounding circuit is grounded; a DC source is connected between the second end and the third end of the grounding circuit; a controller controls the second end and the third end of the grounding circuit to be disconnected and controls the first end and the third end of the grounding circuit to be connected when at least one condition is met; at least one condition includes: a change in the voltage of the N line to the ground is greater than a first preset threshold, a change in the voltage of the positive pole of a photovoltaic string to the ground is greater than a second preset threshold, a change in the voltage of the BUS+ to the ground is greater than a third preset threshold, or a change in the voltage of the BUS- to the ground is greater than a fourth preset threshold; when it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground or between the BUS+ and the ground, the photovoltaic string is controlled to be disconnected, the fault is located, and the fault is removed in time to avoid shutdown of the subarray and excessive power generation loss.

Description

Technical Field

[0001] The present application relates to the field of photovoltaic power generation technology, and in particular to a photovoltaic system and a ground fault detection method. Background Art

[0002] At present, in order to reduce the voltage stress of each switching device in the power converter in the photovoltaic system, a bipolar photovoltaic system has emerged. For example, for a ±1500V photovoltaic system, the bus voltage is 3000V, and the system voltage is designed as half of the bus voltage. Since the system voltage is designed as half of the bus voltage, the safety regulations require that the system voltage does not exceed half of the bus voltage (if ±1500V is used as an example, the system voltage does not exceed 1500V).

[0003] At present, when photovoltaic power generation is carried out, there are multiple photovoltaic systems in one subarray, and the N lines of multiple photovoltaic systems are connected together, that is, they have a common potential. When a photovoltaic string in one photovoltaic system or multiple photovoltaic systems is short-circuited to the ground, the potential of the N line in a subarray will be problematic. Therefore, it is impossible to locate the change in the potential of the N line to the ground caused by the PV string with a short-circuit fault.

[0004] In order to ensure the safe operation of the sub-array, in the prior art, all photovoltaic systems of the entire sub-array are shut down, which results in a huge loss in power generation. Summary of the invention

[0005] The present application provides a photovoltaic system and ground fault detection method, which can accurately locate the photovoltaic system where a short circuit fault occurs, remove the fault in time, avoid shutdown of the entire sub-array, and thus avoid excessive power generation loss.

[0006] The present application provides a photovoltaic system, comprising: a DCDC circuit grounding circuit and a controller;

[0007] The positive input terminal and the negative input terminal of the DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the DCDC circuit is connected to the positive bus BUS+;

[0008] The first end of the grounding circuit is connected to the negative output end of the DCDC circuit, the second end of the grounding circuit is connected to the positive output end of the DCDC circuit, and the third end of the grounding circuit is grounded; a DC source is connected between the second and third ends of the grounding circuit;

[0009] The controller is used to control the second end and the third end of the grounding circuit to be disconnected and the first end and the third end of the grounding circuit to be connected when at least one voltage condition is met; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the voltage of the positive pole of the photovoltaic string to the ground is greater than the second preset threshold, the change of the voltage of the BUS+ to the ground is greater than the third preset threshold, or the change of the voltage of the BUS- to the ground is greater than the fourth preset threshold; and is also used to control the photovoltaic string to be disconnected when it is determined by the voltage of the photovoltaic string that the positive pole of the photovoltaic string is faulty to the ground or the BUS+ is faulty to the ground.

[0010] Preferably, the grounding circuit comprises: a first switch and a second switch;

[0011] The impedance and the first switch are connected in series between the first end and the third end of the grounding circuit;

[0012] The DC source and the second switch are connected in series between the second end and the third end of the grounding circuit;

[0013] The controller is specifically used to control the first switch to be turned on so that the first end of the grounding circuit is connected to the third end; and control the second switch to be turned off so that the second end of the grounding circuit is disconnected from the third end.

[0014] Preferably, the controller is used to determine whether the positive pole of the photovoltaic string is faulty to the ground according to the voltage of the photovoltaic string, specifically:

[0015] When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string;

[0016] or,

[0017] When the voltage between the positive pole of the photovoltaic string and the ground is less than a first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground;

[0018] or,

[0019] When the voltage between the negative pole of the photovoltaic string and the ground is greater than a second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground;

[0020] or,

[0021] When the voltage of the photovoltaic string is less than a third voltage threshold, it is determined that a positive pole of the photovoltaic string is faulty relative to ground, and the voltage of the photovoltaic string is a difference between a positive pole voltage relative to ground and a negative pole voltage relative to ground.

[0022] Preferably, the grounding circuit further comprises: an impedance;

[0023] The impedance is connected between the first end and the third end of the grounding circuit.

[0024] Preferably, the grounding circuit further includes: a first capacitor;

[0025] The first capacitor is connected in parallel at both ends of the DC source.

[0026] Preferably, the grounding circuit further comprises: a current limiting resistor;

[0027] The current limiting resistor, the DC power supply and the second switch are connected in series.

[0028] Preferably, it further comprises: a first diode and a second diode;

[0029] An anode of the first diode is connected to the second end of the grounding unit, and a cathode of the first diode is connected to the positive input end of the first DCDC circuit;

[0030] An anode of the second diode is connected to the second end of the grounding unit, and a cathode of the second diode is connected to the negative output end of the first DCDC circuit.

[0031] Preferably, the DCDC circuit comprises: a first DCDC circuit and a second DCDC circuit;

[0032] The positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+;

[0033] A first end of the grounding circuit is connected to a negative output end of the first DCDC circuit, and a second end of the grounding circuit is connected to a positive output end of the first DCDC circuit;

[0034] The positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-.

[0035] Preferably, it further comprises: a first DCAC circuit and a second DCAC circuit;

[0036] The positive input terminal of the first DCAC circuit is connected to the BUS+, and the negative input terminal of the first DCAC circuit is connected to the N line;

[0037] A positive input terminal of the second DCAC circuit is connected to the N line, and a negative input terminal of the second DCAC circuit is connected to the BUS-.

[0038] The present application provides a ground fault detection method for a photovoltaic system, and the photovoltaic system includes: a first DCDC circuit, a second DCDC circuit, a grounding circuit and a controller; the positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-; the first end of the grounding circuit is connected to the negative output terminal of the first DCDC circuit, the second end of the grounding circuit is connected to the positive output terminal of the first DCDC circuit, and the third end of the grounding circuit is grounded; a DC source is connected between the second and third ends of the grounding circuit;

[0039] The method includes:

[0040] When at least one voltage condition is met, the second end and the third end of the grounding circuit are controlled to be disconnected, and the first end and the third end of the grounding circuit are controlled to be connected; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the positive pole voltage to the ground of the photovoltaic string is greater than the second preset threshold, the change of the BUS+ voltage to the ground is greater than the third preset threshold, or the change of the BUS- voltage to the ground is greater than the fourth preset threshold;

[0041] When it is determined through the voltage of the photovoltaic string that a positive pole of the photovoltaic string fails to ground or the BUS+ fails to ground, the photovoltaic string is controlled to be disconnected.

[0042] Preferably, judging that a positive pole of the photovoltaic string is faulty to ground by the voltage of the photovoltaic string specifically includes:

[0043] When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string;

[0044] or,

[0045] When the voltage between the positive pole of the photovoltaic string and the ground is less than a first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground;

[0046] or,

[0047] When the voltage between the negative pole of the photovoltaic string and the ground is greater than a second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground;

[0048] or,

[0049] When the voltage of the photovoltaic string is less than a third voltage threshold, it is determined that a positive pole of the photovoltaic string is faulty relative to ground, and the voltage of the photovoltaic string is a difference between a positive pole voltage relative to ground and a negative pole voltage relative to ground.

[0050] It can be seen from the above technical solutions that the present application has at least the following advantages:

[0051] The photovoltaic system provided in the embodiment of the present application adds a grounding circuit, which is connected between the PV+ of the photovoltaic string and the ground, and between the PV- of the photovoltaic string and the ground. Under normal circumstances, PV+ is grounded through the DC source in the grounding circuit, and PV- is disconnected from the ground through the grounding circuit. First, it is determined whether a grounding fault has occurred in PV+ by whether the voltage suddenly changes. If a grounding fault has occurred in PV+, PV+ is disconnected from the DC source in the grounding circuit, and PV- is grounded through the impedance in the grounding circuit, which is equivalent to an impedance connected between PV+ and PV-, thereby detecting the voltage of the photovoltaic string to locate the photovoltaic string with a grounding fault, and disconnecting the faulty photovoltaic string from the photovoltaic system, so that the normal photovoltaic string continues to work, so that all photovoltaic systems of the sub-array do not need to be shut down, reducing the loss of power generation, and also reducing the large fluctuations in the output of the photovoltaic power station. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0053] Figure 1 A schematic diagram of a photovoltaic system provided in an embodiment of the present application;

[0054] Figure 2 A schematic diagram of another photovoltaic system provided in an embodiment of the present application;

[0055] Figure 3 A schematic diagram of a grounding circuit provided in an embodiment of the present application;

[0056] Figure 4 The photovoltaic system provided in the embodiment of the present application includes Figure 3 A schematic diagram of a grounding circuit is shown;

[0057] Figure 5 This is an equivalent diagram when a ground fault occurs in PV+ provided in an embodiment of the present application;

[0058] Figure 6 A schematic diagram of another grounding circuit provided in an embodiment of the present application;

[0059] Figure 7 The photovoltaic system provided in the embodiment of the present application includes Figure 6 A schematic diagram of a grounding circuit is shown;

[0060] Figure 8 A schematic diagram of another photovoltaic system provided in an embodiment of the present application;

[0061] Fig. 9 A flow chart of a ground fault detection method for a photovoltaic system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0062] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0063] It will be understood that the words “first”, “second”, “third” and “fourth” in the embodiments of the present application are only for the convenience of description and do not constitute a limitation of the present application.

[0064] The photovoltaic system provided in the embodiment of the present application can be a bipolar photovoltaic system or a unipolar photovoltaic system, which is not specifically limited in the present application, and can be a string type or a centralized type. The bipolar photovoltaic system will be taken as an example for introduction below.

[0065] See also Figure 1 , which is a schematic diagram of a photovoltaic system provided in this application.

[0066] The photovoltaic system provided in the embodiment of the present application includes a positive busbar BUS+, an N line and a BUS-, wherein the voltage between the BUS+ and the N line is equal to the voltage between the BUS- and the N line under normal circumstances. For example, the voltage of BUS+ is 1500V, the voltage of BUS- is -1500V, and the voltage of the N line is 0V.

[0067] The photovoltaic system provided in the present application includes: a first DCDC circuit 10, a second DCDC circuit 20, a first DCAC circuit 30 and a second DCAC circuit 40;

[0068] The positive input terminal and the negative input terminal of the first DCDC circuit 10 are respectively used to connect the positive electrode PV+ and the negative electrode PV- of the photovoltaic string; the positive output terminal of the first DCDC circuit 10 is connected to the positive bus BUS+;

[0069] The positive input terminal and the negative input terminal of the second DCDC circuit 20 are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit 10; the positive output terminal of the second DCDC circuit 20 is connected to the N line, and the positive output terminal of the second DCDC circuit 20 is connected to the negative bus BUS-;

[0070] The positive input terminal of the first DCAC circuit 30 is connected to BUS+, and the negative input terminal of the first DCAC circuit 30 is connected to the N line;

[0071] A positive input terminal of the second DCAC circuit 40 is connected to the N line, and a negative input terminal of the second DCAC circuit 40 is connected to BUS-.

[0072] In a photovoltaic power station, a photovoltaic array includes many Figure 1 The photovoltaic system shown, for example, a subarray may include more than a dozen photovoltaic systems, but the N lines of the dozen photovoltaic systems are connected together, that is, they have a common potential, for example, the N line has the same potential as the ground. Therefore, if a photovoltaic system fails, the potential of the N line of the photovoltaic system will change. Since the N lines of all photovoltaic systems in the same subarray are connected together, the potential of the N lines of all photovoltaic systems in the subarray will change, and the failed photovoltaic system cannot be located. Therefore, all photovoltaic systems in the entire subarray need to be shut down, which will cause a large waste of electricity.

[0073] The photovoltaic system provided in the embodiment of the present application can solve the above technical problems. When a photovoltaic system fails, the failed photovoltaic string can be located, so that the failed photovoltaic string can be removed, and the remaining normal photovoltaic systems in the subarray can continue to operate normally, thereby avoiding excessive power loss and making full use of electrical energy.

[0074] See also Figure 2 , which is a schematic diagram of another photovoltaic system provided in an embodiment of the present application.

[0075] The photovoltaic system provided in this embodiment includes: a first DCDC circuit 10, a second DCDC circuit 20, a grounding circuit 50 and a controller (not shown in the figure);

[0076] The positive input terminal and the negative input terminal of the first DCDC circuit 10 are respectively used to connect the positive electrode PV+ and the negative electrode PV- of the photovoltaic string; the positive output terminal of the first DCDC circuit 10 is connected to the positive bus BUS+;

[0077] The positive input terminal and the negative input terminal of the second DCDC circuit 20 are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit 10; the positive output terminal of the second DCDC circuit 20 is connected to the N line, and the positive output terminal of the second DCDC circuit 20 is connected to the negative bus BUS-;

[0078] A first end of the grounding circuit 50 is connected to the negative output end of the first DCDC circuit 10, a second end of the grounding circuit 50 is connected to the positive output end of the first DCDC circuit 10, and a third end of the grounding circuit 50 is grounded; a DC source is connected between the second and third ends of the grounding circuit 50;

[0079] The controller is used to control the second end and the third end of the grounding circuit 50 to be disconnected and the first end and the third end of the grounding circuit 50 to be connected when at least one voltage condition is met; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the voltage of the positive pole of the photovoltaic string to the ground is greater than the second preset threshold, the change of the voltage of BUS+ to the ground is greater than the third preset threshold, or the change of the voltage of BUS- to the ground is greater than the fourth preset threshold; and is also used to control the photovoltaic string to be disconnected when it is determined by the voltage of the photovoltaic string that the positive pole of the photovoltaic string is faulty to the ground or the BUS+ is faulty to the ground.

[0080] It should be understood that when BUS+ fails to ground, it will cause PV+ to fail to ground.

[0081] Since the technical solution provided in the embodiment of the present application is applied to the case of a ground fault in PV+, that is, the potential of PV+ is 0, but since PV+ and PV- are the two output terminals of the photovoltaic string, the voltage difference between PV+ and PV- will not change for the photovoltaic string. When PV+ is 0, the voltage of PV- will be lower. For example, under normal circumstances, the voltage difference between PV+ and PV- is 1000V, then when PV+ is 0V, PV- is negative 1000V. Due to the relationship between many voltages in the photovoltaic system, when PV+ is grounded, the voltage between BUS+ and BUS- will change. Therefore, at least one voltage condition mentioned above includes multiple voltage judgment methods, all of which are for judging whether PV+ has a ground fault. When PV+ fails to ground, the voltage between the N line and the ground, the voltage between PV+ and the ground, the voltage between PV- and the ground, the voltage between BUS+ and the ground, and the voltage between BUS- and the ground will all mutate.

[0082] The technical solution provided in the embodiment of the present application controls the second end and the third end of the grounding circuit 50 to be disconnected and the first end and the third end of the grounding circuit 50 to be connected when it is determined that PV+ has a ground fault. That is, the impedance is connected between PV+ and PV-, and the connection between the DC source and PV+ is disconnected. The purpose of controlling the grounding circuit 50 in this way is to locate the specific fault and determine which photovoltaic string has a ground fault.

[0083] Combine the following Figure 3 and Figure 4 A specific implementation method of a grounding circuit is introduced.

[0084] See also Figure 3, which is a schematic diagram of a grounding circuit provided in an embodiment of the present application.

[0085] See also Figure 4 , which is a photovoltaic system provided by an embodiment of the present application, including Figure 3 Schematic diagram of the grounding circuit shown.

[0086] For details, please refer to Figure 3 In a possible implementation, an impedance R1 may be connected between the first end and the third end of the grounding unit 50. This embodiment is introduced by taking the impedance including the resistor R1 as an example. It should be understood that the impedance may also include a capacitor or an inductor, that is, the impedance may be at least one of a resistor, a capacitor and an inductor, that is, it may include a single impedance or a combination of multiple impedances, and the series-parallel combination form is not specifically limited.

[0087] The grounding circuit 50 includes: a first switch S1 and a second switch S2;

[0088] The impedance and the first switch S1 are connected in series between the first end and the third end of the grounding circuit;

[0089] The DC source DC and the second switch S2 are connected in series between the second end and the third end of the grounding circuit;

[0090] The controller is specifically used to control the first switch S1 to be turned on so that the first end of the grounding circuit 50 is connected to the third end; and control the second switch S2 to be turned off so that the second end of the grounding circuit 50 is disconnected from the third end.

[0091] It should be understood that the function of the direct current source DC is to achieve potential induced degradation (PID, Potential Induced Degradation) regulation of the photovoltaic string, and the controller can control the output voltage of the direct current source DC as needed.

[0092] The photovoltaic system provided in the embodiment of the present application adds a grounding circuit, which is connected between the PV+ of the photovoltaic string and the ground, and between the PV- of the photovoltaic string and the ground. Under normal circumstances, PV+ is grounded through the DC source in the grounding circuit, and PV- is disconnected from the ground through the grounding circuit. First, it is determined whether a grounding fault has occurred in PV+ by whether the voltage suddenly changes. If a grounding fault has occurred in PV+, PV+ is disconnected from the DC source in the grounding circuit, and PV- is grounded through the impedance in the grounding circuit, which is equivalent to an impedance connected between PV+ and PV-, thereby detecting the voltage of the photovoltaic string to locate the photovoltaic string with a grounding fault, and disconnecting the faulty photovoltaic string from the photovoltaic system, so that the normal photovoltaic string continues to work, so that all photovoltaic systems of the sub-array do not need to be shut down, reducing the loss of power generation, and also reducing the large fluctuations in the output of the photovoltaic power station.

[0093] Combine the following Figure 5 The working principle is analyzed by using the equivalent diagram when a ground fault occurs in PV+ as shown.

[0094] See also Figure 5 , which is an equivalent diagram when a ground fault occurs in PV+ provided in an embodiment of the present application.

[0095] When PV+ has a ground fault, it is equivalent to PV+ being directly grounded. Since there is a resistor R1 connected between PV+ and PV-, the voltage between PV+ and PV- depends on the voltage on R1. Therefore, by judging the voltage of the PV string, it is possible to judge whether the PV string has a ground fault. It should be understood that this scheme is to judge the voltage of the PV string of each PV system in the subarray, so as to locate the faulty PV string, remove the PV string, and other normal PV strings can continue to operate.

[0096] It should be noted that if there is no controllable switch at the input end of the faulty PV string, Figure 1 The PV system shown is shut down to avoid affecting other PV systems in the subarray.

[0097] In a possible implementation, the photovoltaic system provided in the embodiment of the present application further includes: a first diode D1 and a second diode D2;

[0098] An anode of the first diode D1 is connected to the second end of the grounding unit, and a cathode of the first diode D1 is connected to the positive input end of the first DCDC circuit 10;

[0099] An anode of the second diode D2 is connected to the second end of the grounding unit, and a cathode of the second diode D2 is connected to the negative output end of the first DCDC circuit 10 .

[0100] like Figure 3 As shown, if D1 and D2 are ignored, PV- is grounded through the first switch S1 and the impedance R1 connected in series, and PV+ is grounded through the second switch S2 and the DC source DC connected in series.

[0101] The first diode D1 and the second diode D2 are used to prevent the current from flowing in the reverse direction, that is, to play the role of reverse cutoff.

[0102] The above describes various methods for determining whether a PV+ ground fault has occurred. The following describes various methods for locating a PV string ground fault.

[0103] The first one is that the controller is used to determine whether the positive pole of the photovoltaic string is faulty to the ground through the voltage of the photovoltaic string. Specifically:

[0104] When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string.

[0105] When the PV strings are normal, the PV+ voltage to ground and the PV- voltage to ground should be equal, or the difference is not big, that is, the ratio of their absolute values ​​is close to 1. However, when the PV+ voltage to ground is faulty, the PV+ voltage to ground is 0 or very small, and the PV- voltage to ground is larger. Therefore, the ratio of the two becomes smaller. It should be understood that the ratio here refers to the ratio of the absolute values ​​of the voltages, and is the absolute value of the PV+ voltage to ground divided by the absolute value of the PV- voltage to ground.

[0106] The second type is a controller, which is used to determine whether the positive pole of the photovoltaic string is faulty to the ground through the voltage of the photovoltaic string. Specifically:

[0107] When the voltage between the positive pole of the photovoltaic string and the ground is less than the first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground.

[0108] When PV+ fails to ground, the voltage between PV+ and ground is 0 or very small.

[0109] The third method is to determine whether the positive pole of the photovoltaic string is faulty to the ground by the voltage of the photovoltaic string, which includes:

[0110] When the voltage between the negative pole of the photovoltaic string and the ground is greater than the second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground.

[0111] Here, the PV-to-ground voltage refers to the absolute value of the voltage.

[0112] There is another way to determine whether the positive pole of the photovoltaic string is faulty to the ground, that is, when the voltage of the photovoltaic string is less than the third voltage threshold, it is determined that the positive pole of the photovoltaic string is faulty to the ground, and the voltage of the photovoltaic string is the difference between the positive pole voltage to the ground and the negative pole voltage to the ground. According to the IV characteristics of the photovoltaic panel, when PV+ is short-circuited to the ground, the voltage presented to the outside by the photovoltaic string will drop. For example, the voltage of the photovoltaic string was 1000V before, and when PV+ is short-circuited to the ground, the voltage of the photovoltaic string may drop to 500V.

[0113] The photovoltaic system provided in the embodiment of the present application further includes: a first DCAC circuit 30 and a second DCAC circuit 40;

[0114] The positive input terminal of the first DCAC circuit 30 is connected to BUS+, and the negative input terminal of the first DCAC circuit 30 is connected to the N line;

[0115] The positive input terminal of the second DCAC circuit 40 is connected to the N line, and the negative input terminal of the second DCAC circuit 40 is connected to the BUS-.

[0116] See also Figure 6 , which is a schematic diagram of another grounding circuit provided in an embodiment of the present application.

[0117] See also Figure 7 , which is a photovoltaic system provided by an embodiment of the present application, including Figure 6 Schematic diagram of the grounding circuit shown.

[0118] The grounding circuit further includes: a first capacitor C1;

[0119] The first capacitor C1 is connected in parallel to both ends of the direct current source DC.

[0120] The first capacitor C1 functions to stabilize voltage, that is, to stabilize the voltage across the DC source DC.

[0121] It should be understood that the first capacitor C1 may be integrated inside the direct current source DC, or may be provided independently of the direct current source DC.

[0122] The grounding circuit also includes: a current limiting resistor R2;

[0123] The current limiting resistor R2, the DC power supply DC and the second switch S2 are connected in series.

[0124] The photovoltaic system described in the above embodiments is introduced by taking the photovoltaic system including two DCDC circuits to form a bipolar photovoltaic system as an example. The inverter in the photovoltaic system is introduced as a string inverter as an example below. It should be understood that the grounding circuit provided in the above embodiments is also applicable to the string system.

[0125] See also Figure 8 , which is a schematic diagram of another photovoltaic system provided in an embodiment of the present application.

[0126] The photovoltaic system provided in this embodiment includes: a DCDC circuit 10 , a grounding circuit 50 and a controller (not shown in the figure); and also includes a DCAC circuit 30 .

[0127] The positive input terminal and the negative input terminal of the DCDC circuit 10 are respectively used to connect the positive electrode PV+ and the negative electrode PV- of the photovoltaic string; the positive output terminal of the DCDC circuit 10 is connected to the positive bus BUS+;

[0128] A first end of the grounding circuit 50 is connected to the negative output end of the DCDC circuit 10, a second end of the grounding circuit 50 is connected to the positive output end of the DCDC circuit 10, and a third end of the grounding circuit 50 is grounded; an impedance is connected between the first end and the third end of the grounding circuit 50, and a DC source is connected between the second end and the third end of the grounding circuit 50;

[0129] The controller is used to control the second end and the third end of the grounding circuit 50 to be disconnected and the first end and the third end of the grounding circuit 50 to be connected when at least one voltage condition is met; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the voltage of the positive pole PV+ of the photovoltaic string to the ground is greater than the second preset threshold, the change of the voltage of BUS+ to the ground is greater than the third preset threshold, or the change of the voltage of BUS- to the ground is greater than the fourth preset threshold; and is also used to control the photovoltaic string to be disconnected when it is determined by the voltage of the photovoltaic string that the positive pole PV+ of the photovoltaic string is faulty to the ground.

[0130] Since PV- and N lines are connected together, the voltage of N line to ground is equal to the voltage of PV- to ground.

[0131] The working principles and advantages described in the above embodiments are applicable to Figure 8 The embodiments shown will not be described in detail here.

[0132] It should be understood that the first DCDC circuit diagram in the above embodiment illustrates a DCDC circuit. In the actual product, it may include multiple first DCDC circuits connected in parallel, and the input end of each DCDC circuit is connected to multiple photovoltaic strings, i.e., PV. The figure only illustrates that the input end of a DCDC circuit is connected to a photovoltaic string. In addition, in the actual product, a switch may be provided between the photovoltaic string and the first DCDC circuit, that is, when it is necessary to disconnect the connection relationship between the photovoltaic string and the input end of the first DCDC circuit, the switch can be disconnected. In the actual product, multiple photovoltaic strings may share a switch, for example, two photovoltaic strings correspond to a first DCDC circuit, and multiple first DCDC circuits share a switch at an input end. When it is necessary to disconnect the failed photovoltaic string, the switch can be directly disconnected, but since the switch controls multiple photovoltaic strings, all photovoltaic strings corresponding to the switch will be disconnected at the same time. In addition, there is another possible implementation method, which disconnects the connection relationship between the DCDC circuit and the DCAC circuit, thereby cutting off the connection between the failed photovoltaic string and the DCAC circuit.

[0133] Method Embodiment

[0134] Based on a photovoltaic system provided in the above embodiment, an embodiment of the present application further provides a ground fault detection method for a photovoltaic system, and its working principle is described in detail below with reference to the accompanying drawings.

[0135] See also Fig. 9 , which is a flow chart of a ground fault detection method for a photovoltaic system provided in an embodiment of the present application.

[0136] The photovoltaic system grounding fault detection method provided in this embodiment comprises: a first DCDC circuit, a second DCDC circuit, a grounding circuit and a controller; the positive input terminal and the negative input terminal of the first DCDC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DCDC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DCDC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DCDC circuit; the positive output terminal of the second DCDC circuit is connected to the N line, and the positive output terminal of the second DCDC circuit is connected to the negative bus BUS-; the first terminal of the grounding circuit is connected to the negative output terminal of the first DCDC circuit, the second terminal of the grounding circuit is connected to the positive output terminal of the first DCDC circuit, and the third terminal of the grounding circuit is grounded; a DC source is connected between the second and third terminals of the grounding circuit;

[0137] The method includes:

[0138] S901: When at least one voltage condition is met, the second end and the third end of the grounding circuit are controlled to be disconnected, and the first end and the third end of the grounding circuit are controlled to be connected; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the voltage of the positive pole of the photovoltaic string to the ground is greater than the second preset threshold, the change of the voltage of the BUS+ to the ground is greater than the third preset threshold, or the change of the voltage of the BUS- to the ground is greater than the fourth preset threshold; S902: When it is judged by the voltage of the photovoltaic string that the positive pole of the photovoltaic string is faulty to the ground or the BUS+ is faulty to the ground, the photovoltaic string is controlled to be disconnected.

[0139] It should be noted that the above judgment logic for the positive pole of the photovoltaic string to ground fault is the same as the judgment logic for the BUS+ to ground fault, which will not be repeated here.

[0140] The method provided in the embodiment of the present application is applied to a photovoltaic system, and a grounding circuit is added to the photovoltaic system, and the grounding circuit is connected between the PV+ of the photovoltaic string and the ground, and between the PV- of the photovoltaic string and the ground. Under normal circumstances, PV+ is grounded through a DC source in the grounding circuit, and PV- is disconnected from the ground through the grounding circuit. First, it is determined whether a grounding fault has occurred in PV+ by whether the voltage suddenly changes. If a grounding fault has occurred in PV+, PV+ is disconnected from the DC source in the grounding circuit, and PV- is grounded through an impedance in the grounding circuit, which is equivalent to an impedance connected between PV+ and PV-, thereby detecting the voltage of the photovoltaic string to locate the photovoltaic string with a grounding fault, and disconnecting the faulty photovoltaic string from the photovoltaic system, so that the normal photovoltaic string continues to work, so that all photovoltaic systems of the sub-array do not need to be shut down, reducing the loss of power generation, and also reducing the large fluctuations in the output of the photovoltaic power station.

[0141] The above describes various methods for determining whether a PV+ ground fault has occurred. The following describes various methods for locating a PV string ground fault.

[0142] The first one is that the controller is used to determine whether the positive pole of the photovoltaic string is faulty to the ground through the voltage of the photovoltaic string. Specifically:

[0143] When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string.

[0144] When the PV strings are normal, the PV+ voltage to ground and the PV- voltage to ground should be equal, or the difference is not big, that is, the ratio of their absolute values ​​is close to 1. However, when the PV+ voltage to ground is faulty, the PV+ voltage to ground is 0 or very small, and the PV- voltage to ground is larger. Therefore, the ratio of the two becomes smaller. It should be understood that the ratio here refers to the ratio of the absolute values ​​of the voltages, and is the absolute value of the PV+ voltage to ground divided by the absolute value of the PV- voltage to ground.

[0145] The second type is a controller, which is used to determine whether the positive pole of the photovoltaic string is faulty to the ground through the voltage of the photovoltaic string. Specifically:

[0146] When the voltage between the positive pole of the photovoltaic string and the ground is less than the first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground.

[0147] When PV+ fails to ground, the voltage between PV+ and ground is 0 or very small.

[0148] The third method is to determine whether the positive pole of the photovoltaic string is faulty to the ground by the voltage of the photovoltaic string, which includes:

[0149] When the voltage between the negative pole of the photovoltaic string and the ground is greater than the second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground.

[0150] Here, the PV-to-ground voltage refers to the absolute value of the voltage.

[0151] Fourth, when the voltage of the photovoltaic string is less than the third voltage threshold, it is determined that the positive pole of the photovoltaic string is faulty to the ground. It should be understood that the voltage of the photovoltaic string meets the IV characteristics of the photovoltaic panel. When PV+ is short-circuited to the ground, the voltage presented to the outside by the photovoltaic string will drop.

[0152] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A photovoltaic system, characterized in that: include: DC / DC circuit, grounding circuit and controller; The positive input terminal and the negative input terminal of the DC / DC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the DC / DC circuit is connected to the positive bus BUS+; The first end of the grounding circuit is connected to the negative output end of the DC / DC circuit, the second end of the grounding circuit is connected to the positive output end of the DC / DC circuit, and the third end of the grounding circuit is grounded; a DC source is connected between the second and third ends of the grounding circuit; The controller is used to control the second end of the grounding circuit to be disconnected from the third end, and control the first end of the grounding circuit to be connected to the third end when at least one voltage condition is met; The at least one voltage condition includes: a change in the voltage of the N line to the ground is greater than a first preset threshold, a change in the voltage of the positive pole of the photovoltaic string to the ground is greater than a second preset threshold, a change in the voltage of the BUS+ to the ground is greater than a third preset threshold, or a change in the voltage of the negative bus BUS- to the ground is greater than a fourth preset threshold; and is also used to control the photovoltaic string to be disconnected when it is determined by the voltage of the photovoltaic string that a fault occurs to the positive pole of the photovoltaic string to the ground or a fault occurs to the BUS+ to the ground.

2. The system according to claim 1, characterized in that The grounding circuit includes: a first switch and a second switch; An impedance connected in series and the first switch are connected between the first end and the third end of the grounding circuit; The DC source and the second switch are connected in series between the second end and the third end of the grounding circuit; The controller is specifically used to control the first switch to be turned on so that the first end of the grounding circuit is connected to the third end; and control the second switch to be turned off so that the second end of the grounding circuit is disconnected from the third end.

3. The system according to claim 1 or 2, characterized in that: The controller is used to determine whether the positive pole of the photovoltaic string is faulty to the ground according to the voltage of the photovoltaic string, specifically: When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string; or, When the voltage between the positive pole of the photovoltaic string and the ground is less than a first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground; or, When the voltage between the negative pole of the photovoltaic string and the ground is greater than a second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground; or, When the voltage of the photovoltaic string is less than a third voltage threshold, it is determined that a positive pole of the photovoltaic string is faulty relative to ground, and the voltage of the photovoltaic string is a difference between a positive pole voltage relative to ground and a negative pole voltage relative to ground.

4. The system according to claim 1, characterized in that The grounding circuit further includes: an impedance; The impedance is connected between the first end and the third end of the grounding circuit.

5. The system according to claim 1, characterized in that The grounding circuit further includes: a first capacitor; The first capacitor is connected in parallel at both ends of the DC source.

6. The system according to claim 2, characterized in that The grounding circuit also includes: a current limiting resistor; The current limiting resistor, the DC source and the second switch are connected in series.

7. The system according to claim 1, characterized in that The DC / DC circuit comprises: a first DC / DC circuit and a second DC / DC circuit; The positive input terminal and the negative input terminal of the first DC / DC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DC / DC circuit is connected to the positive bus BUS+; A first end of the grounding circuit is connected to a negative output end of the first DC / DC circuit, and a second end of the grounding circuit is connected to a positive output end of the first DC / DC circuit; The positive input terminal and the negative input terminal of the second DC / DC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DC / DC circuit; the positive output terminal of the second DC / DC circuit is connected to the N line, and the positive output terminal of the second DC / DC circuit is connected to the negative bus BUS-.

8. The system according to claim 7, characterized in that Also includes: a first diode and a second diode; An anode of the first diode is connected to the second end of the ground circuit, and a cathode of the first diode is connected to the positive input end of the first DC / DC circuit; An anode of the second diode is connected to the second end of the ground circuit, and a cathode of the second diode is connected to the negative output end of the first DC / DC circuit.

9. The system according to claim 8, characterized in that Also includes: a first DC / AC circuit and a second DC / AC circuit; The positive input terminal of the first DC / AC circuit is connected to the BUS+, and the negative input terminal of the first DC / AC circuit is connected to the N line; A positive input terminal of the second DC / AC circuit is connected to the N line, and a negative input terminal of the second DC / AC circuit is connected to the BUS-.

10. A method for detecting ground faults in a photovoltaic system, characterized in that: The photovoltaic system comprises: a first DC / DC circuit, a second DC / DC circuit, a grounding circuit and a controller; the positive input terminal and the negative input terminal of the first DC / DC circuit are respectively used to connect the positive electrode and the negative electrode of the photovoltaic string; the positive output terminal of the first DC / DC circuit is connected to the positive bus BUS+; the positive input terminal and the negative input terminal of the second DC / DC circuit are respectively connected to the negative output terminal and the positive output terminal of the first DC / DC circuit; the positive output terminal of the second DC / DC circuit is connected to the N line, and the positive output terminal of the second DC / DC circuit is connected to the negative bus BUS-; the first end of the grounding circuit is connected to the negative output terminal of the first DC / DC circuit, the second end of the grounding circuit is connected to the positive output terminal of the first DC / DC circuit, and the third end of the grounding circuit is grounded; a DC source is connected between the second and third ends of the grounding circuit; The method includes: When at least one voltage condition is met, the second end and the third end of the grounding circuit are controlled to be disconnected, and the first end and the third end of the grounding circuit are controlled to be connected; the at least one voltage condition includes: the change of the voltage of the N line to the ground is greater than the first preset threshold, the change of the positive pole voltage to the ground of the photovoltaic string is greater than the second preset threshold, the change of the BUS+ voltage to the ground is greater than the third preset threshold, or the change of the BUS- voltage to the ground is greater than the fourth preset threshold; When it is determined through the voltage of the photovoltaic string that a positive pole of the photovoltaic string fails to ground or the BUS+ fails to ground, the photovoltaic string is controlled to be disconnected.

11. The method according to claim 10, characterized in that The determining, based on the voltage of the photovoltaic string, whether the positive pole of the photovoltaic string is faulty relative to the ground specifically includes: When the ratio of the positive pole-to-ground voltage to the negative pole-to-ground voltage of the photovoltaic string is less than a preset ratio, it is determined that a positive pole-to-ground fault occurs in the photovoltaic string; or, When the voltage between the positive pole of the photovoltaic string and the ground is less than a first voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground; or, When the voltage between the negative pole of the photovoltaic string and the ground is greater than a second voltage threshold, it is determined that a fault occurs between the positive pole of the photovoltaic string and the ground; or, When the voltage of the photovoltaic string is less than a third voltage threshold, it is determined that a positive pole of the photovoltaic string is faulty relative to ground, and the voltage of the photovoltaic string is a difference between a positive pole voltage relative to ground and a negative pole voltage relative to ground.

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