JP2010187513A - Dc ground fault detector and system linkage inverter system including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC ground fault detector that reliably detects a ground fault accident even when a ground fault accident occurs between batteries connected in series inside a DC power supply. <P>SOLUTION: In a detection unit 51 of the DC ground fault detector 5, a positive electrode and a negative electrode of a DC power supply 1 are connected by a connecting wire via resistors R1-R4 and connection between a grounding wire and ground points c1, c2 in the middle of the connecting wire is changed by a switch SW in a prescribed period on the basis of signals from a switching signal generating part 51a. By this, the ground point is switched between c1 and c2 such that a resistance ratio between a resistance value from the ground point to the positive electrode and a resistance value from the ground point to the negative electrode can be changed in the prescribed period. Whenever the resistance ratio is changed, a ground fault detection means 52 detects a current flowing from the ground point to the ground so as to detect the occurrence of a ground fault by using the detection value, thereby reliably detecting a ground fault accident. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a DC ground fault detection device and a grid interconnection inverter system provided with the DC ground fault detection device.

  2. Description of the Related Art Conventionally, an inverter system has been developed that converts DC power output from a DC power source including a solar cell or a fuel cell into AC power by an inverter and outputs the AC power. A DC ground fault detection device for detecting a ground fault in the DC power source of the inverter system or in a connection line connecting the DC power source and the inverter has been developed.

  FIG. 11 is a block diagram for explaining a grid interconnection inverter system provided with a conventional DC ground fault detection device. In this grid-connected inverter system, DC power output from a DC power supply 100 including a solar cell is converted into AC power by an inverter 200 and supplied to a commercial power system 300.

  In this system, a circuit breaker 400, a DC ground fault detection device 500, and a control device 600 are provided in order to prevent an electric shock due to a ground fault current due to a ground fault or a breakdown of the inverter 200. The DC ground fault detection device 500 detects a DC ground fault and outputs a detection signal to the control device 600 when a DC ground fault is detected. When the detection signal is input from the DC ground fault detection device 500, the control device 600 outputs a cutoff signal to the circuit breaker 400 and outputs a stop signal to the inverter 200. The breaker 400 breaks the connection between the DC power supply 100 and the inverter 200 when a break signal is input from the control device 600. Inverter 200 stops when a stop signal is input from control device 600. Thereby, it can prevent that a ground fault electric current flows into the inverter 200. FIG. Thus, the DC ground fault detection device 500 and the control device 600 constitute a protective relay that protects the inverter 200 from the ground fault current when a DC ground fault occurs.

  The DC ground fault detection device 500 includes a detection unit 510 and a determination unit 520. The detection unit 510 includes a positive terminal a and a negative terminal b connected to two connection lines connecting the DC power source 100 and the inverter 200 of the grid-connected inverter system, and a positive terminal a and a negative terminal b. A connection line connected via two resistors Ra and Rb, a ground line connecting the connection line at a connection point c between the two resistors Ra and Rb, and a current transformer CT for detecting an electric current flowing through the ground line And. The detection unit 510 outputs a current signal detected by the instrument current transformer CT to the determination unit 520. The determination unit 520 receives the current signal from the detection unit 510 and determines whether a ground fault has occurred. That is, when the current value indicated by the input current signal is within a predetermined range provided to eliminate measurement errors, it is determined that a ground fault has not occurred, and the current value is out of the predetermined range. It is determined that a ground fault has occurred. When determining unit 520 determines that a ground fault has occurred, it outputs a detection signal to control device 600.

  FIG. 12 is a diagram for explaining a state in which a ground fault has occurred in the grid interconnection inverter system of FIG. 11. In FIG. 12, only the DC power supply 100 and the detection unit 510 of the DC ground fault detection device 500 are shown, and the other parts are omitted for simplification. In the figure, a ground fault has occurred at a point d on the connection line between the positive electrode of the DC power supply 100 and the inverter 200 (not shown). The resistance Rg shown in the figure represents the resistance value at the time of ground fault. At this time, a ground fault current Ig indicated by a broken-line arrow flows so as to pass from the ground fault occurrence point d through the connection point c (hereinafter referred to as “grounding point c”). The determination unit 520 determines that a ground fault has occurred when the current value indicated by the current signal input from the instrument current transformer CT exceeds a predetermined value due to the ground fault current Ig.

JP 2002-233045 A

  The DC power supply 100 is configured by connecting a plurality of solar cells in series when a high output voltage is required. When a ground fault occurs between the solar cells connected in series, the DC ground fault detection device 500 may not be able to detect the ground fault.

  FIG. 13 is a diagram for explaining a state in which a ground fault has occurred between solar cells connected in series inside the DC power supply 100 in the grid-connected inverter system of FIG. In FIG. 13, only the DC power supply 100 and the detection unit 510 of the DC ground fault detection device 500 are illustrated as in FIG. 12. In the figure, a ground fault has occurred at an intermediate point d ′ of the solar cells connected in series inside the DC power source 100.

  In this case, the ground fault current Ipg shown by the broken line arrow flowing from the positive electrode of the DC power supply 100 to the intermediate point d ′ via the positive electrode terminal a and the ground point c, and from the intermediate point d ′ to the ground point c and the negative terminal b. As a result, a ground fault current Ing indicated by a broken arrow flowing through the negative electrode of the DC power supply 100 is generated. If the output voltage of the DC power supply 100 is E, and the resistance values of the resistors Ra and Rb are equally R, Ipg = Ing = (1/2) E / (R + Rg), and the current flowing through the ground line is the ground fault current Ipg and Ing Is canceled out, so that no ground fault current appears in the current signal detected by the instrument current transformer CT. Therefore, determination unit 520 cannot determine that a ground fault has occurred. Even when the ground fault currents Ipg and Ing are not the same, if the current value indicated by the current signal detected by the current transformer CT is less than or equal to a predetermined value, the determination unit 520 determines that a ground fault has occurred. Can not do it.

  The present invention has been conceived under the above-described circumstances, and even when a ground fault occurs between batteries connected in series inside a DC power source, it is possible to reliably detect a ground fault. An object of the present invention is to provide a DC ground fault detection device that can be used.

  In order to solve the above problems, the present invention takes the following technical means.

  In the DC ground fault detection device provided by the first aspect of the present invention, a resistance circuit is connected between a positive electrode and a negative electrode of a DC power supply, and a grounding point to be grounded is provided in the resistance circuit. A ground fault detection device for detecting the occurrence of a ground fault based on a voltage with respect to the positive and negative electrodes of the ground point or a current flowing between the ground point and the ground, wherein the resistance circuit A resistance ratio between a resistance value from a point to the positive electrode and a resistance value from the ground point to the negative electrode can be changed, and a resistance ratio changing unit that changes the resistance ratio at a predetermined cycle; and the resistance circuit Each time the resistance ratio is changed, the voltage of the ground point with respect to the positive and negative electrodes or the current flowing from the ground point to the ground is detected, and the occurrence of the ground fault is detected using those detected values. And the ground fault detection means that, characterized in that it is provided with a.

  According to this configuration, since the resistance ratio is changed at a predetermined cycle, even if the ground fault is not detected when the ratio is a certain value, the ground fault is caused when the ratio is changed to another value. Can be detected.

  In a preferred embodiment of the present invention, the resistance ratio changing unit alternately changes the resistance ratio between two values.

  According to this configuration, as compared with the case where the resistance ratio is switched between three or more values, the necessary parts are reduced and the design is simplified, so that the apparatus can be downsized and the manufacturing cost can be reduced. Can do.

  In a preferred embodiment of the present invention, the resistance circuit is configured such that the position of the grounding point can be changed, and the resistance ratio changing means changes the resistance ratio by changing the position of the grounding point. To do.

  In a preferred embodiment of the present invention, the resistor circuit is configured such that both ends of some of the resistors constituting the resistor circuit can be short-circuited, and the resistance ratio changing unit changes the number of resistors that short-circuit both ends. Thus, the resistance ratio is changed.

  In a preferred embodiment of the present invention, in the resistor circuit, a part of the resistors constituting the resistor circuit is a variable resistor, and the resistance ratio changing means changes the resistance value of the variable resistor by changing the resistance value. Change the resistance ratio.

  In a preferred embodiment of the present invention, the ground fault detection means includes current detection means for detecting a current flowing in a pair of lines between the positive electrode, the negative electrode, and the resistance circuit, and the current detection means To detect a current flowing between the grounding point and the ground, and detect the occurrence of the ground fault using the detected current.

  A grid-connected inverter system provided by the second aspect of the present invention is generated by the DC ground fault detection device according to any one of claims 1 to 6 and a DC power source to which the DC ground fault detection device is connected. An inverter that converts the DC power to be converted into AC power and supplies the power to the power system, the system-connected inverter system comprising: a disconnecting unit that disconnects the connection between the DC power source and the inverter; And a control device that stops the inverter when the ground fault is detected by the fault detection device, or shuts off the connection between the DC power source and the inverter by the shut-off means.

  Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

It is a block diagram for demonstrating the grid connection inverter system provided with 1st Embodiment of the direct-current ground fault detection apparatus which concerns on this invention. FIG. 2 is a diagram for explaining a state where a ground fault has occurred between batteries connected in series inside a DC power supply in the grid-connected inverter system of FIG. 1. It is a figure for demonstrating the state by which the switch was switched from the state of FIG. It is a figure for demonstrating the state where the ground fault accident generate | occur | produced in the place different from FIG. It is a figure for demonstrating the state by which the switch was switched from the state of FIG. It is a figure for demonstrating another Example of 1st Embodiment. It is a figure for demonstrating another Example of 1st Embodiment. It is a figure for demonstrating 2nd Embodiment of the direct-current ground fault detection apparatus which concerns on this invention. It is a figure for demonstrating the state by which the switch was switched from the state of FIG. It is a figure for demonstrating 3rd Embodiment of the direct-current ground fault detection apparatus which concerns on this invention. It is a block diagram for demonstrating the grid connection inverter system provided with the conventional direct-current ground fault detection apparatus. FIG. 12 is a diagram for explaining a state where a ground fault has occurred in the grid-connected inverter system of FIG. 11. FIG. 12 is a diagram for explaining a state in which a ground fault has occurred between batteries connected in series inside the DC power supply in the grid-connected inverter system of FIG. 11.

  Embodiments of the present invention will be specifically described below with reference to the drawings.

  FIG. 1 is a block diagram for explaining a grid-connected inverter system provided with a first embodiment of a DC ground fault detection apparatus according to the present invention.

  The grid interconnection inverter system A includes a DC power source 1, an inverter 2, a commercial power system 3, a circuit breaker 4, a DC ground fault detection device 5, and a control device 6. The DC power source 1 is connected to the inverter 2 through the circuit breaker 4. The inverter 2 is connected to the commercial power system 3. The DC ground fault detection device 5 is connected to two connection lines that connect the DC power source 1 and the inverter 2. The grid interconnection inverter system A converts the DC power output from the DC power source 1 into AC power by the inverter 2 and supplies the AC power to the commercial power system 3.

  The DC power source 1 outputs DC power, and in this embodiment, includes a plurality of solar cells that are connected in series to convert solar energy into electrical energy. Note that the DC power source 1 is not limited to one that generates DC power using a solar cell, but may be one that generates DC power using, for example, a fuel cell or a storage battery, or AC power generated by wind power generation or the like. It may be converted into DC power and output.

  The inverter 2 converts DC power input from the DC power source 1 into AC power. The inverter 2 converts DC power into AC power by turning on and off a plurality of switching elements (not shown) based on a PWM signal generated by a control circuit (not shown). Further, when a stop signal is input from the control device 6, the inverter 2 stops its operation by stopping the generation of the PWM signal by a control circuit (not shown).

  The circuit breaker 4 cuts off the connection between the DC power source 1 and the inverter 2. The circuit breaker 4 normally connects the DC power source 1 and the inverter 2, but when the cutoff signal is input from the control device 6, the circuit breaker 4 disconnects the connection between the DC power source 1 and the inverter 2.

  The DC ground fault detection device 5 detects a DC ground fault and outputs a detection signal to the control device 6 when a DC ground fault is detected. The DC ground fault detection device 5 includes a detection unit 51 and a determination unit 52.

The detection unit 51 detects a current signal for detecting a ground fault current, and outputs the detected current signal to the determination unit 52. The detection unit 51 includes a positive terminal a and a negative terminal b connected to two connection lines that connect the DC power source 1 and the inverter 2 of the grid-connected inverter system A, and the positive terminal a and the negative terminal The output voltage of the DC power source 1 is applied from b. The positive electrode terminal a and the negative electrode terminal b are connected by a connection line through four resistors R1, R2, R3, and R4. The resistance values of the resistors R1, R2, R3, and R4 are determined according to the maximum output voltage of the DC power supply 1 and the accuracy of an instrument current transformer CT described later. If the resistance value is too small, useless current flows through the resistors R1, R2, R3, and R4. However, if the resistance value is too large, the ground fault current that flows when a ground fault occurs is reduced. Depending on the accuracy, the detection becomes difficult. In this embodiment, since the maximum output voltage of the DC power supply 1 is 500V, the resistance R
The resistance values of 1, R2, R3, and R4 are each 10 kΩ.

  A connection point c1 between the resistors R1 and R2 and a connection point c2 between the resistors R3 and R4 are grounded through a switch SW. The switch SW switches the connection with the ground line between the connection point c1 (hereinafter referred to as “first ground point c1”) and the connection point c2 (hereinafter referred to as “second ground point c2”). The switch SW1 that connects the ground line and the first ground point c1 and the switch SW2 that connects the ground line and the second ground point c2 are provided. The switches SW1 and SW2 are connected based on a switching signal output from the switching signal generator 51a. The switching signal generator 51a outputs a switching signal to the switch SW. In the present embodiment, the switching signal is a pulse signal that switches between a high level and a low level at a predetermined period. When the switching signal is high level, the switch SW1 is connected and the switch SW2 is released, and when the switching signal is low level, the switch SW2 is connected and the switch SW1 is released. That is, the switch SW1 and the switch SW2 are alternately switched on and off.

  The switching cycle of the switch SW (that is, a cycle that is half the cycle of the switching signal) may be the same cycle as the ground fault detection timing cycle, or may be a natural number multiple. When the switching cycle of the switch SW and the ground fault detection timing cycle are the same cycle, the state where the first ground point c1 is grounded and the state where the second ground point c2 is grounded are switched at each ground fault detection timing. Be When the switch SW switching cycle is twice the ground fault detection timing cycle, the first ground point c1 is grounded and the second ground point c2 is grounded once every two times of the ground fault detection timing. The state is switched. In this embodiment, for example, 1 second is set.

  Note that the configuration of the switching signal generator 51a and the switch SW is not limited to this, and it is only necessary that the first ground point c1 and the second ground point c2 can be alternately connected to the ground line. For example, the switching signal generator 51a may output a pulse signal for connecting the switch SW1 and a pulse signal obtained by inverting the pulse signal and connecting the switch SW2, respectively. . The switches SW1 and SW2 may be switching elements such as transistors. Further, instead of the switches SW1 and SW2, a single switch that switches the connection with the ground line between the first ground point c1 and the second ground point c2.

  An instrumental current transformer CT is arranged on the ground line. The instrument current transformer CT detects a current flowing through the ground line, and outputs the detected current signal to the determination unit 52.

  The determination unit 52 receives a current signal from the detection unit 51 and determines whether a ground fault has occurred. When the determination unit 52 determines that a ground fault has occurred, the determination unit 52 outputs a detection signal to the control device 6. The determination unit 52 includes a filter unit 52a and a ground fault determination unit 52b.

  The filter unit 52a removes an AC component and a switching frequency component due to the characteristics of the inverter 2 from the current signal input from the detection unit 51. The filter unit 52a is constituted by, for example, a low-pass filter that passes only frequency components of several Hz or less.

  The ground fault accident determination unit 52b determines whether a ground fault has occurred depending on whether or not the current value indicated by the current signal input from the filter unit 52a is within a predetermined range set to eliminate measurement errors. Determine whether or not. That is, when the current value is within the predetermined range, it is determined that the measurement error is within the range and no ground fault has occurred, and when the current value is outside the predetermined range, it is determined that the ground fault has occurred. In the present embodiment, the predetermined range is ± 0.05 A, but an appropriate range may be determined as appropriate through experiments. When it is determined that a ground fault has occurred, the ground fault determination unit 52b outputs a detection signal to the control device 6.

  When the detection signal is input from the DC ground fault detection device 5, the control device 6 outputs a cutoff signal to the circuit breaker 4 and outputs a stop signal to the inverter 2. The circuit breaker 4 to which the interruption signal is input disconnects the connection between the DC power source 1 and the inverter 2, and the inverter 2 to which the stop signal is input stops its operation. Thereby, it can prevent that a ground fault electric current flows into the inverter 2. FIG. Thus, the DC ground fault detection device 5 and the control device 6 constitute a protective relay that protects the inverter 2 from the ground fault current when a DC ground fault occurs.

  Next, the operation of the DC ground fault detection device 5 will be described.

  When no ground fault has occurred, the current value indicated by the current signal detected by the instrument current transformer CT and noise removed by the filter unit 52a is within a predetermined range regardless of the state of the switch SW. The DC ground fault detector 5 does not output a detection signal.

  On the other hand, when a ground fault occurs, a ground fault current flows through the grounding wire. Therefore, the current value indicated by the current signal detected by the instrument current transformer CT and noise removed by the filter unit 52a is out of the predetermined range. Therefore, the DC ground fault detection device 5 outputs a detection signal. Further, based on the switching signal input from the switching signal generator 51a, the switch SW switches the connection with the ground line between the first ground point c1 and the second ground point c2. Therefore, even if it is not possible to detect a ground fault when switching to one, it is possible to detect a ground fault when switching to the other.

  Hereinafter, detection of the ground fault current by switching the switch SW will be described with reference to FIGS.

  FIG. 2 is a diagram for explaining a state in which a ground fault has occurred between the batteries connected in series inside the DC power supply 1 in the grid-connected inverter system A of FIG. In FIG. 2, only the DC power supply 1 and the detection unit 51 of the DC ground fault detection device 5 are illustrated, and the other components and the switching signal generation unit 51 a are omitted for simplification. The same applies to the following FIGS. In FIG. 2, a ground fault occurs at an intermediate point d ′ (hereinafter referred to as “ground fault point d ′”) of batteries connected in series inside the DC power source 1. The resistance Rg shown in the figure represents the resistance value at the time of ground fault.

  In the figure, the switch SW is switched so that the ground line and the first ground point c1 are connected. In this case, the ground fault current Ipg shown by the broken arrow flowing from the positive electrode of the DC power source 1 to the ground fault point d ′ via the positive terminal a and the first ground point c1 and the first ground point c1 from the ground fault point d ′. Then, a ground fault current Ing indicated by a broken line arrow flowing through the negative electrode terminal b through the negative electrode of the DC power source 1 is generated. Assuming that the output voltage of the DC power supply 1 is E and the resistance values of the resistors R1 to R4 are equal R, Ipg = (1/2) · E / (R + Rg), Ing = (1/2) · E / (3R + Rg) Since the ground fault currents Ipg and Ing do not match, the ground fault current of the difference is superimposed on the current signal detected by the current transformer CT. Therefore, the resistance value R is set so that the ground fault current (Ipg−Ing = ER / ((R + Rg) · (3R + Rg))) superimposed on the current signal is outside the predetermined range set in the ground fault determination unit 52b. Is determined, the DC ground fault detector 5 can detect a ground fault.

  FIG. 3 is a diagram for explaining a state in which the switch SW is switched from the state of FIG. 2 and the ground line and the second ground point c2 are connected. In this case, Ipg = (1/2) · E / (3R + Rg), Ing = (1/2) · E / (R + Rg), and the ground fault current Ipg and Ing do not match, and the difference in ground fault current Since (Ipg-Ing = −ER / ((R + Rg) · (3R + Rg))) is outside the predetermined range, the DC ground fault detector 5 can detect a ground fault.

  4 and 5 are diagrams for explaining a state in which a ground fault has occurred at a point d ″ (hereinafter referred to as “ground fault point d ″”) that divides the voltage of the DC power supply 1 into 1: 3. . In FIG. 4, the switch SW is switched so that the ground line and the first ground point c1 are connected. In this case, Ipg = (1/4) · E / (R + Rg), Ing = (3/4) · E / (3R + Rg), and the ground fault current (Ipg−Ing = − (1/2) · ERg / ((R + Rg) · (3R + Rg))) may be within a predetermined range depending on the magnitude of Rg. However, as shown in FIG. 5, when the switch SW is switched and the ground line and the second ground point c2 are connected, Ipg = (1/4) · E / (3R + Rg), Ing = (3/4) ) · E / (R + Rg), and the difference in ground fault current (Ipg-Ing =-(1/2) · E · (4R + Rg) / ((R + Rg) · (3R + Rg))) is outside the specified range. The DC ground fault detector 5 can detect a ground fault.

  Next, the operation of the DC ground fault detection device 5 will be described.

  In the present embodiment, when a ground fault occurs between batteries connected in series inside the DC power supply, the point to be grounded is switched between the first ground point c1 and the second ground point c2, The ground fault currents Ipg and Ing change. Therefore, even when the ground fault current Ipg and Ing cancel each other when switching to one and the ground fault cannot be detected, the ground fault can be detected when switched to the other.

  In the first embodiment, the current signal flowing through the ground line is detected by the current transformer CT and input to the determination unit 52, but the present invention is not limited to this. For example, the zero-phase current may be detected by a zero-phase current detection current transformer, or the voltage between the positive electrode and the negative electrode may be detected.

  FIG. 6 is a diagram for explaining another example of the first embodiment. In this embodiment, a zero-phase current detection current transformer CT ′ is arranged on two connection lines connecting the DC power source 1 and an inverter (not shown) instead of the instrument current transformer CT arranged on the ground line. Then, the zero-phase current signal detected by the zero-phase current detection current transformer CT ′ is input to the determination unit 52 (see FIG. 1). Also in the present embodiment, since the point to be grounded is switched between the first ground point c1 and the second ground point c2, the ground fault currents Ipg and Ing change. Therefore, even when the ground fault current Ipg and Ing cancel each other when switching to one and the ground fault cannot be detected, the ground fault can be detected when switched to the other.

FIG. 7 is a diagram for explaining still another example of the first embodiment. In this embodiment, in place of the instrument current transformer CT arranged on the ground line, a voltmeter Vp between the ground line and the positive terminal a is used.
A voltmeter Vn is provided between the ground line and the negative electrode terminal b, respectively, and a positive-to-ground voltage signal detected by the voltmeter Vp and a negative-to-ground voltage signal detected by the voltmeter Vn are determined. 52 (see FIG. 1).

  In this embodiment, since the ratio of the voltage to be detected by the voltmeter Vp and the voltmeter Vn is switched by switching the switch SW, the determination unit 52 switches the voltage ratio for reference according to the switching of the switch SW. Thus, it is necessary to determine whether or not the ratio of the input voltages matches the referenced voltage ratio. For example, when the resistance values of the resistors R1, R2, R3, and R4 are the same, the ratio of the voltages detected by the voltmeter Vp and the voltmeter Vn is 1: 3 when the grounded point is the first grounding point c1. If the point to be grounded is the second ground point c2, it should be 3: 1.

  Also in the present embodiment, since the point to be grounded is switched between the first ground point c1 and the second ground point c2, the positive ground voltage and the negative ground voltage change. Therefore, even if a ground fault cannot be detected when the ratio of the detected voltage matches the referenced voltage ratio when switched to one, the ratio of the detected voltage when switched to the other Will not match the referenced voltage ratio, so a ground fault can be detected.

  Note that the resistance provided in the detection unit 51 is not limited to the above. The ratio between the resistance value between the ground line and the positive terminal a and the resistance value between the ground line and the negative terminal b may be changed by switching the switch SW, and the resistors R1, R2, and R3. , R4 may have different resistance values, or the number of resistors may be different. The resistors R1 and R2 may be one resistor, and the first ground point c1 may be a point provided in the middle of the resistor. Further, the resistance value ratio is not limited to switching between two values, and the resistance value ratio may be switched between three or more values.

  In the first embodiment, the case where both the resistance value between the ground line and the positive terminal a and the resistance value between the ground line and the negative terminal b change has been described, but only one of them changes. You may comprise as follows.

  FIG. 8 is a diagram for explaining a second embodiment of the DC ground fault detection apparatus according to the present invention. 2, only the DC power supply 1 and the detection unit 51 ′ of the DC ground fault detection device 5 are described, as in FIG. 2, and the other components and the switching signal generation unit 51a are omitted for the sake of brevity. ing. The detection unit 51 'of the present embodiment differs from the detection unit 51 of the first embodiment in that a switch SW3 for short-circuiting both ends of the resistor R1 is provided instead of the switch SW. In the present embodiment, only the resistance value between the ground line and the positive terminal a is switched by turning on and off the switch SW3.

  In FIG. 8, the switch SW3 is in an off state. The resistance values of the resistors R1 to R4 are equal to R, the output voltage of the DC power supply 1 is E, and the ground fault is a ground fault point d ′ that is an intermediate point of the series-connected batteries in the DC power source 1. Suppose that it has occurred. In this case, the ground fault current Ipg shown by the broken arrow flowing from the positive electrode of the DC power source 1 to the ground fault point d ′ via the positive terminal a and the ground point c, and the ground point c and the negative terminal b from the ground fault point d ′. A ground fault current Ing indicated by a broken line arrow flowing through the negative electrode of the DC power source 1 is generated. Since Ipg = Ing = (1/2) · E / (2R + Rg) and the ground current Ipg and Ing cancel out the current flowing in the grounding wire, the ground fault is detected in the current signal detected by the instrumental current transformer CT. Current does not appear. Therefore, a ground fault cannot be detected.

  FIG. 9 is a diagram for explaining a state where the switch SW3 is turned on from the state of FIG. In this case, since both ends of the resistor R1 are short-circuited, Ipg = (1/2) · E / (R + Rg). Therefore, if the ground fault current (Ipg-Ing = (1/2) · ER / ((R + Rg) · (2R + Rg))) is set outside the specified range, a ground fault is detected. can do.

  In the first and second embodiments, the case where a fixed resistor is used has been described, but a variable resistor may be used.

  FIG. 10 is a diagram for explaining a third embodiment of the DC ground fault detection apparatus according to the present invention. This figure shows only the detection unit 51 ". The detection unit 51" of this embodiment is provided with a variable resistor R5 between the positive terminal a and the ground line, and between the negative terminal b and the ground line. A fixed resistor R6 is provided between them. The resistance value of the variable resistor R5 is switched between two values according to the switching signal input from the switching signal generator 51a (see FIG. 1). Also in the present embodiment, since the ratio between the resistance value between the ground line and the positive terminal a and the resistance value between the ground line and the negative terminal b is changed, the same as in the first and second embodiments. The effect of can be produced. The resistance value of the variable resistor R5 is not limited to being switched between two values, but may be configured to be switched between three or more resistance values.

  Although the case where the DC ground fault detection device is used in the grid interconnection inverter system has been described in the above embodiment, the present invention is not limited to this. The DC ground fault detection device of the present invention can appropriately detect a ground fault when used in a system in which a DC current flows. Therefore, it can be used in a system that directly uses DC power, or can be used directly in solar cells, fuel cells, storage batteries, and the like that generate DC power.

  Moreover, the DC ground fault detection device of the present invention is used not only when the system is stopped when a ground fault is detected, but also when detecting a ground fault to notify the detection of the ground fault. be able to.

  The DC ground fault detection apparatus according to the present invention is not limited to the above-described embodiment. The specific configuration of each part of the DC ground fault detection device according to the present invention can be varied in design in various ways.

DESCRIPTION OF SYMBOLS 1 DC power supply 2 Inverter 3 Commercial power system 4 Circuit breaker 5 DC ground fault detection apparatus 51,51 ', 51 "Detection part 51a Switching signal generation part (resistance ratio change means)
52 determination unit (ground fault detection means)
52a Filter unit 52b Ground fault determination unit 6 Control device

Claims (7)

A resistance circuit is connected between the positive electrode and the negative electrode of the DC power supply, and a ground point is provided in the resistor circuit to be grounded. The voltage of the ground point with respect to the positive electrode and the negative electrode or the ground point and the ground A DC ground fault detection device that detects the occurrence of a ground fault based on the current flowing between
The resistance circuit can change a resistance ratio between a resistance value from the ground point to the positive electrode and a resistance value from the ground point to the negative electrode,
Resistance ratio changing means for changing the resistance ratio at a predetermined period;
Each time the resistance ratio of the resistance circuit is changed, the voltage of the ground point with respect to the positive and negative electrodes or the current flowing from the ground point to the ground is detected, and the detected value is used to generate the ground fault. A ground fault detecting means for detecting
A DC ground fault detection device comprising:   The DC ground fault detection device according to claim 1, wherein the resistance ratio changing unit alternately changes the resistance ratio between two values. The resistance circuit is configured such that the position of the ground point can be changed,
The resistance ratio changing means changes the resistance ratio by changing the position of the ground point.
The DC ground fault detection apparatus according to claim 1 or 2. The resistor circuit is configured such that both ends of some of the resistors constituting the resistor circuit can be short-circuited,
The resistance ratio changing means changes the resistance ratio by changing the number of resistors that short-circuit both ends.
The DC ground fault detection apparatus according to claim 1 or 2. In the resistor circuit, some of the resistors constituting the resistor circuit are variable resistors,
The resistance ratio changing means changes the resistance ratio by changing a resistance value of the variable resistor.
The DC ground fault detection apparatus according to claim 1 or 2.   The ground fault detection means includes current detection means for detecting a current flowing in a pair of lines between the positive electrode, the negative electrode, and the resistance circuit, and the current detection means causes a current between the ground point and the ground. 6. The DC ground fault detection device according to claim 1, wherein a current flowing in the ground is detected, and the occurrence of the ground fault is detected using the detected current. A DC ground fault detection device according to any one of claims 1 to 6,
An inverter that converts DC power generated by a DC power source to which the DC ground fault detection device is connected to AC power and supplies the AC power;
A grid interconnection inverter system comprising:
Blocking means for cutting off the connection between the DC power source and the inverter;
A control device that stops the inverter when a ground fault is detected by the DC ground fault detection device, or shuts off the connection between the DC power source and the inverter by the shut-off means;
A grid-connected inverter system.

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