DC power supply leakage detection device

Abstract

To provide an electric leakage detection device that can cause a detection current to flow which is sufficiently large and easily detected without power consumption of a DC power by a ground fault current.SOLUTION: An electric leakage detection device has: resistance elements R1 and R2 that are series-connected between power transmission lines L1 and L2; a constant voltage power source PS that outputs a low output voltage; a switch element Q1 that has one end of a current path in which ON / OFF is controlled connected to a middle point n, and has other end of the current path connected to a positive pole output end 13 of the constant voltage power source PS; a switch element Q2 that has one end of the current path in which ON / OFF is controlled connected to the middle point n, and has the other end of the current path connected to a negative pole output end 14 of the constant voltage power source PS; a rectifying element D1 that is connected between the power transmission line L1 and the positive pole output end 13 of the constant voltage power source PS in an opposite direction with respect to the DC power source; a rectifying element D2 that is connected between the power transmission line L2 and the negative pole output end 14 of the constant voltage power source PS in the opposite direction with respect to the DC power source; and a detection circuit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ground fault detection device of a neutral grounding type for detecting a ground fault in a power transmission line of a DC power supply. [Background technology]

[0002] A known conventional leakage current detection device for a DC power supply is a neutral grounding system shown in Fig. 7. The neutral grounding system typically has a configuration in which two resistor elements R1 and R2 are connected in series and their connection point is grounded to set a neutral point n between a power transmission line L1 (potential φ+) connected to a positive output terminal 1 of the DC power supply and a power transmission line L2 (potential φ-) connected to a negative output terminal 2 (see, for example, Patent Documents 1 to 3). In a steady state where no ground fault occurs, the potential φn of the neutral point n is the ground potential. When a ground fault occurs on the power transmission line L1 or L2, the ground fault current flowing through the resistor element R2 or R1 is passed as a detection current through a detection resistor element Rd, for example, and the voltage across the resistor element Rd is measured to detect the ground fault. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-296316 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-261039 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-130536 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional earth leakage detectors with a neutral grounding system, it is preferable to reduce the steady-state current by using high-resistance resistors R1 and R2 from the viewpoint of reducing the power consumption of the DC power supply during steady-state operation. However, since the detection current during a ground fault is measured in a highly sensitive range of, for example, a few mA, the device is sensitive to noise and prone to malfunctions.

[0005] Therefore, in the neutral grounding method, if the resistance values ​​of the resistor elements R1 and R2 are reduced to increase the detection current and reduce sensitivity, the system will be more resistant to noise, but the steady-state current will also increase, resulting in the problem of increased power consumption from the DC power supply.In addition, since the magnitude of the earth fault current depends on the output voltage of the DC power supply, there is also the problem that if the output voltage fluctuates, the detection current value will also change.

[0006] In view of the above problems, an object of the present invention is to provide a leakage detection device for a DC power supply that does not consume power from the DC power supply due to ground fault current, can pass a detection current of a sufficient magnitude that is easy to detect, and is not affected by fluctuations in the output voltage of the DC power supply. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides the following configuration: Note that the reference numerals in parentheses are reference numerals in the drawings to be described later, and are provided for reference.

[0008] An aspect of the present invention is a leakage current detection device connected between a first power transmission line (L1) connected to a positive output terminal (1) of a DC power supply and a second power transmission line (L2) connected to a negative output terminal (2) of the DC power supply, a first resistor element (R1) and a second resistor element (R2) connected between the first power transmission line (L1) and the second power transmission line (L2) and connected in series to each other at a connection point (a); a constant voltage power supply (PS) that outputs a constant DC output voltage lower than the output voltage of the DC power supply between a positive output terminal (13) and a negative output terminal (14); a first switch element (Q1) including a first control terminal to which the potential of the connection point (a) is applied, and a first current path whose on / off is controlled by the potential of the first control terminal, one end of the first current path being connected to a midpoint (n) and the other end of the first current path being connected to a positive output terminal (13) of the constant voltage power supply (PS); a second switch element (Q2) including a second control terminal to which the potential of the connection point (a) is applied, and a second current path whose on / off is controlled by the potential of the second control terminal, one end of the second current path being connected to the midpoint (n) and the other end of the second current path being connected to the negative output terminal (14) of the constant-voltage power supply (PS); a first rectifier element (D1) connected between the first power transmission line (L1) and a positive output terminal (13) of the constant voltage power supply (PS) so as to be in a reverse direction with respect to the output voltage of the DC power supply; a second rectifier element (D2) connected between the second power transmission line (L2) and the negative output terminal (14) of the constant voltage power supply (PS) so as to be in a reverse direction with respect to the output voltage of the DC power supply; and a detection circuit connected between the midpoint (n) and a ground point. In the above aspect, it is preferable that the output voltage of the DC power supply is 100 to 1000V and the output voltage of the constant voltage power supply (PS) is 15 to 50V. In the above aspect, it is preferable that two Zener diodes (ZD1, ZD2) are connected in series in opposite directions between the connection point (a) and the midpoint (n). In the above aspect, it is preferable that the detection circuit has a plurality of resistance elements each having a different resistance value and connected in a switchable manner. Another aspect of the present invention is a leakage current detection device connected between a first power transmission line (L1) connected to a positive output terminal (1) of a DC power supply and a second power transmission line (L2) connected to a negative output terminal (2) of the DC power supply, a first constant voltage power supply (PS1) that outputs a constant DC output voltage lower than the output voltage of the DC power supply between a positive output terminal (13) and a negative output terminal (14), the negative output terminal (14) being connected to a midpoint (n); a second constant voltage power supply (PS2) that outputs the same constant DC voltage as that of the first constant voltage power supply (PS1) between a positive output terminal (23) and a negative output terminal (24), the positive output terminal (23) of which is connected to the midpoint (n); a first rectifier element (D1) connected between the first power transmission line (L1) and a positive output terminal (13) of the first constant voltage power supply (PS1) so as to be in a reverse direction with respect to the output voltage of the DC power supply; a second rectifier element (D2) connected between the second power transmission line (L2) and the negative output terminal (24) of the second constant-voltage power supply (PS2) so as to be in a reverse direction with respect to the output voltage of the DC power supply; A detection circuit connected between the midpoint (n) and a ground point. death, The output voltage of the DC power supply is 100 to 1000 V, and the output voltage of the first and second constant voltage power supplies (PS1, PS2) is 15 to 50 V. . [Effects of the Invention]

[0009] According to the present invention, a leakage detection device for a DC power supply is realized that does not consume power from the DC power supply due to ground fault current, can pass a detection current of a sufficient magnitude that is easy to detect, and is not affected by fluctuations in the output voltage of the DC power supply. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of a circuit configuration of a first embodiment of an electric leakage detection device according to the present invention. [Figure 2] FIG. 2 shows the flow of a ground fault current when a ground fault occurs on the negative side line in the leakage detection device of FIG. [Figure 3] 3 is a diagram showing the flow of a ground fault current when a ground fault occurs on the positive electrode side line in the earth leakage detection device of FIG. [Figure 4] FIG. 4 is a diagram showing an example of a circuit configuration of a second embodiment of an electric leakage detection device according to the present invention. [Figure 5] FIG. 5 is a diagram showing the flow of a ground fault current when a ground fault occurs on the negative electrode line in the leakage detection device of FIG. [Figure 6] FIG. 6 is a diagram showing the flow of a ground fault current when a ground fault occurs on the positive electrode side line in the leakage detection device of FIG. [Figure 7]FIG. 7 is a diagram showing a schematic diagram of a conventional earth leakage detection device of a neutral grounding type. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of a leakage current detection device for a DC power supply according to the present invention will be described with reference to the drawings. The leakage current detection device of the present invention is a device used to detect leakage current from a transmission line of a DC power supply to the ground, i.e., a ground fault. In this specification, leakage current due to a ground fault will be referred to as a "ground fault current."

[0012] The "neutral point grounding method" in a leakage current detection device is a method in which the midpoint of the voltage between the power transmission lines is configured to be at ground potential during steady-state operation when no ground fault has occurred, and a ground fault is detected by utilizing the fact that the potential at the midpoint deviates from the ground potential when a ground fault occurs, or that current flows through the midpoint to the ground point when a ground fault occurs.

[0013] (1) First embodiment (1-1) Circuit configuration Fig. 1 is a diagram schematically illustrating an example of the circuit configuration of a first embodiment of a leakage detection device for a DC power supply. The DC power supply is not illustrated, and only a positive output terminal 1 and a negative output terminal 2 of the DC power supply are shown. A first power transmission line L1 is connected to the positive output terminal 1 of the DC power supply, and a second power transmission line L2 is connected to the negative output terminal 2. The leakage detection device is connected between the power transmission line L1 and the power transmission line L2. The leakage detection device is connected in parallel with the DC power supply and its load (not shown).

[0014] The DC power source may be, for example, a solar power generation device or a storage battery. For example, the output voltage between the positive output terminal 1 and the negative output terminal 2 is several hundred volts, e.g., 300 V. The symbol φ+ represents the potential of the power transmission line L1 relative to the ground potential, e.g., +150 V. The symbol φ- represents the potential of the power transmission line L2 relative to the ground potential, e.g., -150 V.

[0015] The leakage current detection device shown in Figure 1 has a first resistor element R1 and a second resistor element R2 connected in series at connection point a. Resistor element R1 is connected between power transmission line L1 and connection point a, and resistor element R2 is connected between power transmission line L2 and connection point a. Here, "resistor element" means a linear resistor element. Resistor elements R1 and R2 have the same resistance value. Symbol φa represents the potential of connection point a relative to the ground potential. Under steady-state conditions when no ground fault occurs, φa is the ground potential, i.e., 0 V.

[0016] The earth leakage detection device further includes a constant voltage power supply PS. The constant voltage power supply PS is a DC power supply separate from the DC power supply having output terminals 1 and 2. The constant voltage power supply PS outputs a constant output voltage between the positive output terminal 13 and the negative output terminal 14, and this output voltage is lower than that of the DC power supply. For example, when the output voltage of the DC power supply is 100 to 1000 V, the output voltage of the constant voltage power supply PS is preferably about 15 to 50 V. As an example, when the output voltage of the DC power supply is 300 V, the output voltage of the constant voltage power supply PS is set to 24 V.

[0017] In the illustrated example, input terminals 11 and 12 of the constant-voltage power supply PS are connected to and supplied with power from power transmission lines L1 and L2, respectively. Because output terminals 1 and 2 of the DC power supply are in normal mode (differential mode) with respect to the ground, even if the input terminals 11 and 12 of the constant-voltage power supply PS are connected to the power transmission lines L1 and L2, this does not affect the detection of ground fault currents. In this case, the input terminals 11 and 12 and the output terminals 13 and 14 of the constant-voltage power supply PS must be isolated from each other. For example, the constant-voltage power supply PS can be an isolated switching power supply (step-down converter) using a transformer. Alternatively, the constant-voltage power supply PS does not need to be supplied with power from the power transmission lines L1 and L2. For example, the constant-voltage power supply PS can be a storage battery.

[0018] The leakage current detection device further includes a first switch element Q1 and a second switch element Q2. The first switch element Q1 is configured as an N-channel field effect transistor (FET). The second switch element Q2 is configured as a P-channel FET. Alternatively, the switch element Q1 may be configured as an NPN transistor, and the switch element Q2 may be configured as a PNP transistor.

[0019] The switch element Q1 has a control end (gate) and a current path (drain-source current path) whose conduction and interruption, i.e., on / off, is controlled by the potential of the control end. The control end of the switch element Q1 is connected to the connection point a between the resistor elements R1 and R2, and the potential φa of the connection point a is applied to the control end. One end (source) of the current path of the switch element Q1 is connected to the midpoint n, and the other end (drain) of the current path is connected to the positive output terminal 13 of the constant-voltage power supply PS.

[0020] The switch element Q2 also has a control end (gate) and a current path (source-drain current path) whose conduction and interruption, i.e., on / off, is controlled by the potential of the control end. The control end of the switch element Q2 is also connected to the connection point a between the resistor elements R1 and R2, and the potential φa of the connection point a is applied to the control end. One end (source) of the current path of the switch element Q2 is also connected to the midpoint n, and the other end (drain) of the current path is connected to the negative output terminal 14 of the constant-voltage power supply PS. Therefore, one end (source) of the switch element Q1 and one end (source) of the switch element Q2 are connected to each other at the midpoint n. The symbol φn represents the potential of the midpoint n with respect to the ground potential.

[0021] In a preferred example, two Zener diodes ZD1 and ZD2 are connected in series in the opposite directions as protective elements to prevent excessive voltage from being applied between one end (source) and the control end (gate) of the switch elements Q1 and Q2, thereby suppressing the gate-source voltage to the Zener voltage.

[0022] The leakage detection device further includes a first rectifying element D1 and a second rectifying element D2, each connected in a reverse direction to the output voltage of the DC power supply.

[0023] The first rectifier element D1, which is a rectifier diode, is connected between the power transmission line L1 and a connection point b1, with its anode connected to the connection point b1 and its cathode connected to the power transmission line L1. The connection point b1 is the positive output terminal 13 of the constant-voltage power supply PS and also the other terminal (drain) of the switch element Q1. The symbol φb1 represents the potential of the connection point b1 relative to the ground potential.

[0024] The second rectifier element D2, a rectifier diode, is connected between the power transmission line L2 and a connection point b2, with its anode connected to the power transmission line L2 and its cathode connected to the connection point b2. The connection point b2 is the negative output terminal 14 of the constant-voltage power supply PS and also the other end (drain) of the switch element Q2. The symbol φb2 represents the potential of the connection point b2 relative to the ground potential.

[0025] The earth leakage detection device further includes a detection circuit for detecting the occurrence of a ground fault. The detection circuit is connected between output terminals 3 and 4. The output terminal 3 of the detection circuit is common to the midpoint n, and the output terminal 4 is grounded. The detection circuit includes four detection resistor elements R6, R7, R8, and R9, each having a different resistance value. Each resistor element has a resistance value of, for example, several hundred to several kilo-ohms. Each resistor element can be selectively connected between the midpoint n and the ground point by a switch SW. The midpoint n is connected to the ground point via a selected resistor element (R8 in the illustrated example). One of the resistor elements R6 to R9 is selected and connected in advance depending on the expected ground fault resistance and magnitude of the ground fault current.

[0026] This single detection circuit is used to detect ground faults on either side of the power transmission lines L1 and L2. A positive or negative voltage is output between output terminal 3 and output terminal 4 (ground point) depending on the direction of the ground fault current. Capacitor C is a smoothing capacitor for noise suppression. Although not shown, various circuits that utilize the output voltage of the detection circuit, such as a rectifier circuit or an alarm output circuit, can be provided downstream of output terminals 3 and 4.

[0027] As another example of a detection circuit, instead of multiple resistor elements, multiple constant current diodes with different constant current values ​​can be used. In this case, each resistor element is replaced with two constant current diodes connected in series in the opposite direction. If the ground fault current flowing through the constant current diode attempts to exceed the constant current value, the voltage across the constant current diode increases, thereby detecting the fault.

[0028] (1-2) Steady state In Figure 1, the dotted line indicates the steady-state current Ia that flows through the resistor elements R1 and R2 during steady-state operation when no ground fault occurs. The resistor elements R1 and R2 have a resistance value that is sufficiently large to suppress power consumption of the DC power supply during steady-state operation, for example, a resistance value of several hundred kilohms to several megaohms. For example, if the output voltage of the DC power supply is 300 V and the resistor elements R1 and R2 are each 1 MΩ, the steady-state current Ia that flows during steady-state operation is small, at approximately 0.15 mA. In the earth leakage detection device of the present invention, the current that flows through the resistor elements R1 and R2 during a ground fault is not used for ground fault detection, so the resistor elements R1 and R2 can have a sufficiently high resistance value.

[0029] In steady state, the potential φn of midpoint n and the potential φa of connection point a are at ground potential (0 V). Therefore, the potential at output terminal 3 of the detection circuit is also at ground potential (0 V). Switching elements Q1 and Q2 are both in the off state. Rectifying elements D1 and D2 are in the opposite direction to the voltage between transmission lines L1 and L2, so no current flows through rectifying element D1, switching element Q1, switching element Q2, or rectifying element D2.

[0030] A voltage of, for example, 24 V is output between output terminals 13 and 14 of constant-voltage power supply PS. In this case, the potential φb1 at node b1 and the potential φb2 at node b2 are indefinite because both switch elements Q1 and Q2 are in the off state, and no output current flows. Therefore, in a steady state, the power consumption of the leakage detection circuit is almost zero.

[0031] (1-3) Operation in the event of a ground fault Next, the operation of the earth leakage detection device of FIG. 1 when a ground fault occurs will be described with reference to FIGS.

[0032] <Negative ground fault> Figure 2 shows a schematic diagram of a ground fault occurring at point c on the negative transmission line L2. A ground-fault resistance Re is shown between point c and the ground point. A ground-fault current Is1 flows from the ground point through the ground-fault resistance Re to point c on the transmission line L2.

[0033] A ground fault causes the potential φ- of the transmission line L2 to rise from, for example, -150V to almost ground potential. This causes the potential φa at the junction point a between the resistor elements R1 and R2 to rise to half the output voltage of the DC power supply (for example, 300V), but the protective Zener diode ZD1 prevents the gate-source voltage of the switch element Q1 from rising to a maximum of the Zener voltage (for example, 15V). However, this causes the switch element Q1 to be fully on. At the same time, the rise in the potential φ- of the transmission line L2 causes the rectifier element D2 to switch forward and become conductive. As a result, the ground-fault current Is1 flows along the following path, using the output voltage of the constant-voltage power supply PS as a power source: Positive output terminal 13 → switch element Q1 → midpoint n → resistor element R8 → ground point → ground fault resistor Re → transmission line L2 (point c → point d) → rectifier element D2 → negative output terminal 14

[0034] At this time, the voltage across rectifier element D2 is nearly zero. When the output voltage of constant-voltage power supply PS is 24 V, the potential φb1 at point b1 of output terminal 13 is +24 V, and the potential φn at midpoint n is also +24 V, resulting in a positive detection voltage being output from output terminal 3. If resistor element R8 is, for example, 400 Ω, a ground-fault current of 60 mA will flow. The leakage current detector of the present invention can also handle ground-fault currents of several tens of mA to several A. Therefore, ground faults can be easily detected.

[0035] Meanwhile, switch element Q2 remains in the off state. Rectifier element D1 remains reversed, and a voltage close to the output voltage of the DC power supply is applied across it, so it needs to have appropriate voltage resistance characteristics. Conversely, since only the voltage resistance of the rectifier element needs to be considered, it can easily accommodate higher voltages.

[0036] The ground fault current Is1 flows from a constant voltage power supply PS with a relatively low output voltage (e.g., 24 V) as a current source, so the switch element Q1 does not need to be a high-voltage type. Furthermore, because the switch element Q1 is fully on and its resistance is nearly zero, the ground fault current Is1 does not generate much heat. Furthermore, the detection resistor elements R6 to R9 generate heat only equal to the output voltage (e.g., 24 V) of the constant voltage power supply PS multiplied by the ground fault current Is1. Therefore, the power consumption of the leakage detector due to the flow of the ground fault current Is1 is small.

[0037] Furthermore, the ground fault current Is1 does not pass through the output terminals 1 and 2 of the DC power supply, and therefore does not consume power from the DC power supply.

[0038] Furthermore, the ground fault current Is1 passes between points c and d on the power transmission line L2. Therefore, if current detection were performed at three points - to the right of point d, between points c and d, and to the left of point c - a large increase in current would be observed only between points c and d, but not at the other two points, making it easier to find the ground fault.

[0039] Furthermore, since the ground fault current Is1 flows from the constant voltage power supply PS with a relatively low output voltage as a current source, there are no high voltage parts in the part through which the ground fault current Is1 flows, and there is no risk of electric shock, making it safe.

[0040] Furthermore, since the ground fault current Is1 flows using the constant voltage power supply PS as a current source, even if the output voltage of the DC power supply fluctuates, the magnitude of the ground fault current Is1 does not fluctuate.

[0041] <Positive side ground fault> Figure 3 shows a schematic diagram of a situation where a ground fault occurs on the positive-side power transmission line L1. The operation when a ground fault occurs on the positive side is fundamentally the same as the operation when a ground fault occurs on the negative side described above, except that the polarity is reversed, so only a brief explanation will be given.

[0042] If the potential φ+ of the transmission line L1 drops from, for example, +150 V to nearly ground potential, the potential φa at node a drops, and switch element Q2 turns fully on. At the same time, rectifier element D1 turns forward. As a result, ground-fault current Is2 flows via the following path, using the output voltage of constant-voltage power supply PS as a power source: Positive output terminal 13 → rectifier element D1 → transmission line L1 → earth fault resistor Re → ground point → resistor element R8 → midpoint n → switch element Q2 → negative output terminal 14

[0043] At this time, the voltage across the rectifier element D1 is almost zero, and when the output voltage of the constant voltage power supply PS is 24V, the potential φb2 at point b2 of the output terminal 14 is, for example, −24V, and the potential φn at the midpoint n is also −24V, and a negative detection voltage is output to the output terminal 3.

[0044] Meanwhile, the switching element Q1 remains in the off state. Also, the rectifying element D2 remains in the reverse direction, and a voltage close to the output voltage of the DC power supply is applied, so a withstand voltage is required. Other effects are the same as those in the case of a negative-side ground fault described above.

[0045] (2) Second embodiment (2-1) Circuit configuration 4 is a diagram schematically illustrating an example of a circuit configuration of a second embodiment of a leakage detection device for a DC power supply. The DC power supply is not illustrated, and only the positive output terminal 1 and negative output terminal 2 of the DC power supply are shown. A first power transmission line L1 is connected to the positive output terminal 1 of the DC power supply, and a second power transmission line L2 is connected to the negative output terminal 2. The leakage detection device is connected between the power transmission line L1 and the power transmission line L2. In other words, the leakage detection device is connected in parallel with the DC power supply and its load (not shown).

[0046] The earth leakage detection device shown in FIG. 4 has a first constant-voltage power supply PS1 and a second constant-voltage power supply PS2. The constant-voltage power supplies PS1 and PS2 are DC power supplies separate from the DC power supply having output terminals 1 and 2. The constant-voltage power supply PS1 outputs a constant output voltage of the same value between positive output terminal 13 and negative output terminal 14, and the constant-voltage power supply PS2 outputs a constant output voltage of the same value between positive output terminal 23 and negative output terminal 24. These output voltages are lower than the output voltage of the DC power supply. For example, if the output voltage of the DC power supply is 100 to 1000 V, the output voltages of the constant-voltage power supplies PS1 and PS2 are preferably approximately 15 to 50 V. As an example, if the output voltage of the DC power supply is 300 V, the output voltages of the constant-voltage power supplies PS1 and PS2 are each set to 24 V.

[0047] In the illustrated example, input terminals 11 and 12 of constant-voltage power supply PS1 and input terminals 21 and 22 of constant-voltage power supply PS2 are connected to power transmission lines L1 and L2, respectively, and receive power. Because output terminals 1 and 2 of the DC power supplies are in normal mode (differential mode) with respect to ground, even if input terminals 11, 12, 21, and 22 of constant-voltage power supplies PS1 and PS2 are connected to power transmission lines L1 and L2, this does not affect ground-fault current detection. Constant-voltage power supplies PS1 and PS2 are connected in parallel to the DC power supplies on the input side. In this case, the input terminals 11 and 12 of constant-voltage power supply PS1 must be insulated from output terminals 13 and 14, and the input terminals 21 and 22 of constant-voltage power supply PS2 must be insulated from output terminals 23 and 24. For example, constant-voltage power supplies PS1 and PS2 can each be an isolated switching power supply (step-down converter) using a transformer. As another example, the constant voltage power supplies PS1 and PS2 may not be supplied with power from the power transmission lines L1 and L2, and may be, for example, storage batteries.

[0048] The constant-voltage power supplies PS1 and PS2 are connected in series on the output side. That is, the negative output terminal 14 of the constant-voltage power supply PS1, which is located on the high-potential side, and the positive output terminal 23 of the constant-voltage power supply PS2, which is located on the low-potential side, are both connected to a midpoint n. The symbol φn represents the potential of the midpoint n relative to the ground potential.

[0049] The leakage detection device further includes a first rectifying element D1 and a second rectifying element D2, each connected in a reverse direction to the output voltage of the DC power supply.

[0050] The first rectifier element D1, which is a rectifier diode, is connected between the power transmission line L1 and the positive output terminal 13 of the constant-voltage power supply PS1, with its anode connected to the positive output terminal 13 and its cathode connected to the power transmission line L1. The symbol φ13 represents the potential of the positive output terminal 13 with respect to the ground potential.

[0051] The second rectifier element D2, which is a rectifier diode, is connected between the power transmission line L2 and the negative output terminal 24 of the constant-voltage power supply PS2, with its anode connected to the power transmission line L2 and its cathode connected to the negative output terminal 24. The symbol φ24 represents the potential of the negative output terminal 24 relative to the ground potential.

[0052] The earth leakage detection device further includes a detection circuit for detecting the occurrence of a ground fault. The detection circuit is connected between output terminal 3 and output terminal 4. Output terminal 3 of the detection circuit is common to midpoint n, and output terminal 4 is grounded. The detection circuit includes four detection resistor elements R6, R7, R8, and R9, each having a different resistance value. Each resistor element has a resistance value of, for example, several hundred to several kilo-ohms. Each resistor element can be switched by a switch SW so that it is selectively connected between midpoint n and the ground. Midpoint n is connected to the ground via the selected resistor element (R8 in the illustrated example). This detection circuit has the same configuration as in the first embodiment, so further detailed description will be omitted.

[0053] (2-2) Steady state In steady state where no ground fault occurs, the potential φn at midpoint n is ground potential (0V). Therefore, the potential at output terminal 3 of the detection circuit is also ground potential (0V). Because rectifier elements D1 and D2 are reversely oriented relative to the voltage between power transmission lines L1 and L2, no current flows through rectifier elements D1 and D2. Therefore, even if a voltage of, for example, 24V is output between output terminals 13 and 14 of constant-voltage power supply PS1 and between output terminals 23 and 24 of constant-voltage power supply PS2, no output current flows. Therefore, in steady state, the power consumption of the earth leakage detection circuit is zero.

[0054] (2-3) Operation in the event of a ground fault Next, the operation of the leakage detection device of FIG. 4 when a ground fault occurs will be described with reference to FIGS.

[0055] <Negative ground fault> Figure 5 shows a schematic diagram of a ground fault occurring at point c on the negative transmission line L2. A ground-fault resistance Re is shown between point c and the ground point. A ground-fault current Is1 flows from the ground point to point c on the transmission line L2 through the ground-fault resistance Re.

[0056] A ground fault causes the potential φ- of the transmission line L2 to rise from, for example, -150 V to almost ground potential. As a result, the potential φn at the midpoint n attempts to rise to half the output voltage of the DC power supply (for example, 300 V). However, the rise in the potential φ- of the transmission line L2 causes the rectifier element D2 to switch to forward conduction and become conductive, so the rise in the potential φn at the midpoint n is limited to the potential of the positive output terminal 23 of the constant-voltage power supply PS2 (for example, +24 V). As a result, a ground-fault current Is1 flows via the following path, using the output voltage of the constant-voltage power supply PS2 as a power source: Positive output terminal 23 → midpoint n → resistor element R8 → ground point → earth fault resistor Re → transmission line L2 (point c → point d) → rectifier element D2 → negative output terminal 24

[0057] At this time, the voltage across rectifier element D2 is nearly zero, and when the output voltage of constant-voltage power supply PS2 is 24 V, the potential φn at positive output terminal 23 (midpoint n) is +24 V, and a positive detection voltage is output to output terminal 3. If resistor element R8 has a resistance of, for example, 400 Ω, a ground-fault current of 60 mA will flow. The leakage detector of the present invention can also handle ground-fault currents of several tens of mA to several A. Therefore, ground faults can be easily detected.

[0058] On the other hand, rectifier D1 remains reversed, and a voltage close to the output voltage of the DC power supply is applied across it, so it needs to have appropriate voltage resistance characteristics. Conversely, since only the voltage resistance of the rectifier needs to be considered, it can easily accommodate higher voltages.

[0059] The ground fault current Is1 flows from the constant voltage power supply PS2, which has a relatively low output voltage (for example, 24 V), as a current source, so that the detection resistor elements R6 to R9 do not generate heat to any significant extent, and therefore the power consumption of the leakage detector due to the flow of the ground fault current Is1 is small.

[0060] Furthermore, the ground fault current Is1 does not pass through the output terminals 1 and 2 of the DC power supply, and therefore does not consume power from the DC power supply.

[0061] Furthermore, the ground fault current Is1 passes between points c and d on the power transmission line L2. Therefore, if current detection were performed at three points - to the right of point d, between points c and d, and to the left of point c - a large increase in current would be observed only between points c and d, but not at the other two points, making it easier to find the ground fault.

[0062] Furthermore, since the ground fault current Is1 flows from the constant voltage power supply PS2 with a relatively low output voltage as a current source, there are no high voltage parts in the part through which the ground fault current Is1 flows, and there is no risk of electric shock, making it safe.

[0063] Furthermore, since the ground fault current Is1 flows using the constant voltage power supply PS2 as a current source, even if the output voltage of the DC power supply fluctuates, the magnitude of the ground fault current Is1 does not fluctuate.

[0064] <Positive side ground fault> Figure 6 shows a schematic diagram of a situation in which a ground fault occurs on the positive-side power transmission line L1. The operation in the event of a positive-side ground fault is fundamentally the same as the operation in the event of a negative-side ground fault described above, except that the polarity is reversed, so only a brief explanation will be given.

[0065] If the potential φ+ of the transmission line L1 drops from, for example, +150 V to nearly ground potential, the potential φn of the midpoint n will attempt to drop to half the output voltage of the DC power supply (for example, 300 V). However, the drop in the potential φ+ of the transmission line L1 causes the rectifier element D1 to switch to forward conduction and become conductive, so the drop in the potential φn of the midpoint n is limited to the potential of the negative output terminal 14 of the constant-voltage power supply PS1 (for example, -24 V). As a result, a ground-fault current Is2 flows through the following path using the output voltage of the constant-voltage power supply PS1 as a power source. Positive output terminal 13 → rectifier element D1 → transmission line L1 → earth fault resistor Re → ground point → resistor element R8 → midpoint n → negative output terminal 14

[0066] At this time, the voltage across the rectifier element D1 is approximately zero, and when the output voltage of the constant voltage power supply PS1 is 24V, the potential φn at the midpoint n is −24V, and a negative detection voltage is output to the output terminal 3.

[0067] On the other hand, rectifier D2 remains in the reverse direction, and a voltage close to the output voltage of the DC power supply is applied across it, so it needs to have an appropriate withstand voltage. Other effects are the same as those in the case of a negative ground fault described above.

[0068] Although the embodiments of the present invention have been described above with reference to exemplary configurations, the specific circuit configurations are not limited to these. Various modifications are also included within the scope of the present invention as long as they comply with the principles of the present invention. [Explanation of symbols]

[0069] 1 Positive output terminal of DC power supply 2 Negative output terminal of DC power supply 3 Detection circuit output terminal 4 Detection circuit output terminal (ground terminal) 11, 21 Positive input terminal of constant voltage power supply 12, 22 Negative input terminal of constant voltage power supply 13, 23 Positive output terminal of constant voltage power supply 14, 24 Negative output terminal of constant voltage power supply L1 and L2 transmission lines PS, PS1, PS2 constant voltage power supplies ZD1, ZD2 Zener diodes D1, D2 Rectifying diodes R1, R2 Resistor elements R6, R7, R8, R9 Detecting resistor elements Q1, Q2 switch elements Re Earth fault resistance SW switch

Claims

1. A leakage current detection device connected between a first power transmission line (L1) connected to a positive output terminal (1) of a DC power supply and a second power transmission line (L2) connected to a negative output terminal (2) of the DC power supply, a first resistor element (R1) and a second resistor element (R2) connected between the first transmission line (L1) and the second transmission line (L2) and connected in series to each other at a connection point (a); a constant voltage power supply (PS) that outputs a constant DC output voltage lower than the output voltage of the DC power supply between a positive output terminal (13) and a negative output terminal (14); a first switch element (Q1) including a first control end to which the potential of the connection point (a) is applied, and a first current path whose on / off is controlled by the potential of the first control end, one end of the first current path being connected to a midpoint (n) and the other end of the first current path being connected to a positive output terminal (13) of the constant-voltage power supply (PS); a second switch element (Q2) including a second control end to which the potential of the connection point (a) is applied, and a second current path whose on / off is controlled by the potential of the second control end, one end of the second current path being connected to the midpoint (n) and the other end of the second current path being connected to the negative output terminal (14) of the constant-voltage power supply (PS); a first rectifier element (D1) connected between the first power transmission line (L1) and a positive output terminal (13) of the constant-voltage power supply (PS) so as to be in a reverse direction with respect to the output voltage of the DC power supply; a second rectifier element (D2) connected between the second power transmission line (L2) and the negative output terminal (14) of the constant-voltage power supply (PS) so as to be in a reverse direction with respect to the output voltage of the DC power supply; and a detection circuit connected between the midpoint (n) and a ground point.

2. 2. The leakage detection device according to claim 1, wherein the output voltage of the DC power supply is 100 to 1000V, and the output voltage of the constant voltage power supply (PS) is 15 to 50V.

3. 3. The leakage detection device according to claim 1, further comprising two Zener diodes (ZD1, ZD2) connected in series in opposite directions between the connection point (a) and the midpoint (n).

4. 4. The earth leakage detection device according to claim 1, wherein the detection circuit has a plurality of resistance elements each having a different resistance value and connected in a switchable manner.

5. A leakage current detection device connected between a first power transmission line (L1) connected to a positive output terminal (1) of a DC power supply and a second power transmission line (L2) connected to a negative output terminal (2) of the DC power supply, a first constant voltage power supply (PS1) that outputs a constant DC output voltage lower than the output voltage of the DC power supply between a positive output terminal (13) and a negative output terminal (14), the negative output terminal (14) being connected to a midpoint (n); a second constant voltage power supply (PS2) that outputs the same constant DC voltage as that of the first constant voltage power supply (PS1) between a positive output terminal (23) and a negative output terminal (24), the positive output terminal (23) of which is connected to the midpoint (n); a first rectifier element (D1) connected between the first power transmission line (L1) and a positive output terminal (13) of the first constant-voltage power supply (PS1) so as to be oriented in a reverse direction with respect to the output voltage of the DC power supply; a second rectifier element (D2) connected between the second power transmission line (L2) and the negative output terminal (24) of the second constant-voltage power supply (PS2) so as to be in a reverse direction with respect to the output voltage of the DC power supply; a detection circuit connected between the midpoint (n) and a ground point; The leakage detection device is characterized in that the output voltage of the DC power supply is 100 to 1000V, and the output voltages of the first and second constant voltage power supplies (PS1, PS2) are 15 to 50V.

6. 6. The earth leakage detection device according to claim 5, wherein the detection circuit has a plurality of resistance elements each having a different resistance value and connected in a switchable manner.

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