JPH0271168A - Method for earth detection - Google Patents

Abstract

PURPOSE:To detect earth irrespective of a grounded point of a d.c. circuit by a method wherein an a.c. current is detected by injecting a.c. signals between a supply line and the ground while a.c. voltage of said supply line with respect to the ground is detected. CONSTITUTION:At a signal injection port 15, a.c. signals I1 from an a.c. power supply 18 are injected into + and - supply lines 12, 13, respectively, which connect a d.c. power supply 11 and load 14 via capacitors 16, 17. Current flowing from the power supply 18 is combination of earth resistance Rx and current flowing in an electrostatic capacity to ground 25. During normal operation, most of the signal current I1 becomes current flowing in the capacity 25, and shows a value advancing approximately. 90 deg. in phase with respect to a.c. power supply voltage E. When grounding occurs in the load 14, current I2 flowing in the resistance Rx overlaps the current I1 flowing in the capacity 25. Since the current I2 and the voltage E are equal in phase, signal current I3=I1+I2 at the time of grounding occurrence is reduced in phase difference from the voltage E. An identification device 23 calculates phase difference between the signal voltage E and current I3 and identifies grounding when the preset value is reaches.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a ground fault detection method for detecting a ground fault in a DC circuit that includes a DC power source, a load, and a supply line connecting the DC power source and the load. It is about the method.

(Conventional technology) Power plants and substations that have traditionally used DC circuits,
In plants, ground faults in installed loads such as DC motors are detected to prevent leakage current. As a method of detecting this ground fault, a method using a bridge circuit shown in Fig. 5 is known. In Fig. 0, 1 is a DC power supply;
2 is a ground fault detection circuit, 3 is a positive wire of the DC power source 1, 4 is a negative wire of the DC power source 1, and 5 is a load. The ground fault detection circuit 2 consists of a bridge circuit including resistors R1, Rt+R@, and a relay Ry, and the resistors R1 and R2 form a series circuit between the positive supply line 3 and the negative supply line 4, One side of the resistor R3 is connected to the connection point of the resistors Rt and R2, and the other side of the relay RJ is grounded. Rx is a ground fault resistance.

When there is no ground fault in the load 5, Rx corresponds to leakage resistance and has a large value, so the current flowing through the relay Ry is almost zero.

If a ground fault occurs outside the vicinity of the neutral point of the load 5, the balance of the bridge will be disrupted and current will flow through the relay RY.

This activates relay Ry and detects a ground fault.

This can be similarly detected even if a ground fault occurs in the supply line.

(Problem to be solved by the invention) However, in the above ground fault detection method, the load 5
When a ground fault occurs near the neutral point, the imbalance of the bridge is small, so the current flowing through relay Ry is weak, and as shown in Figure 6, relay R7 near the neutral point of load 5 There was a problem in that a dead area occurred, making it impossible to detect ground faults near the neutral point. Further, a similar problem occurs when a ground fault occurs inside the DC power supply.

(Object of the Invention) Therefore, this invention was made in view of the above circumstances, and its purpose is to detect a ground fault that can detect a ground fault regardless of the position of the ground fault in a DC circuit. The object of the present invention is to provide a detection method.

(Means for Solving the Problem) In order to achieve the above object, the present invention uses a DC power supply and
A ground fault detection method for detecting a ground fault in a DC circuit comprising a load and a supply line connecting the DC power source and the load, the method comprising: injecting an AC signal between the supply line and the ground; In addition to detecting the alternating current flowing through the ground, the alternating current voltage of the supply line to the ground is also detected, and a ground fault in the direct current circuit is detected from the detected alternating current and alternating voltage.

(Function) According to the ground fault detection method according to the present invention, the signal current and signal voltage of the AC signal flowing in the supply line can be detected regardless of the position of the ground fault point in the DC circuit, and the signal current and signal voltage can be detected. A ground fault is detected from the voltage.

(Example) Hereinafter, an example in which the ground fault detection method according to the present invention is applied to a DC motor will be described with reference to the drawings.

In Figure 1, 11 is a DC power supply, P is its anode, and N
is its cathode. 12 is a positive supply line connected to the anode P of the DC power supply 11, 13 is a negative supply line connected to the cathode N of DC type all, 14 is a DC motor (load) connected to the supply line 12.13, It is shown here as an equivalent circuit.

Reference numeral 15 denotes a signal injection section, which consists of capacitors 16 and 17 and an AC power source 518 that outputs an AC signal, and the capacitors 16 and 17 form a series circuit between the positive supply line 12 and the negative supply line 13. AC power supply 18
One output terminal of is connected to the connection point of capacitors 16 and 17, and the other output terminal of AC power supply 18 is grounded,
An AC signal from an AC power supply 18 is injected into the positive supply line 12 and the negative supply line 13.

20 is a signal detection section, which consists of a current transformer 21 and a capacitor 22, and the current transformer 21 is connected to a positive supply line 1.
2 and the negative supply line 13 are installed so that the DC component flowing to the load 14 is canceled out so that only the signal current of the AC signal is detected. The capacitor 22 is connected to the negative supply line 13 and is used to cut off the DC voltage and detect the signal voltage of the AC signal flowing through the negative supply line 13. Note that this capacitor 22 may be connected to the positive supply line 12 to detect the signal voltage of the positive supply line 12.

23 is a signal voltage detected via the capacitor 22;
This determination device determines the phase difference with the signal current detected by the current transformer 21 and determines a ground fault based on this phase difference and the magnitude of the signal current. 24 is a leakage resistance, and 25 is a ground capacitance.

Now, when a DC current is supplied from the DC power supply 11 to the load 14 and an AC signal is injected into the positive supply line 12 and the negative supply line 13 by the AC power supply 18, the current transformer 21 connects the positive supply line 12 and the negative supply line. The capacitor 22 detects the signal current of the alternating current signal flowing through the line 13, and the capacitor 22 detects the signal voltage of the alternating current signal flowing through the negative supply line 13.

By the way, the current flowing from the AC power supply 18 is the sum of the ground fault resistance Rx and the current flowing through the ground capacitance fi25, and when there is no ground fault in the load 14, the ground fault resistance Rx is due to ground leakage. Only the resistor 24 is used, which has a large value.

Therefore, most of the signal current flowing from the AC power supply 18 becomes the current flowing to the ground capacitance fi25, and as shown in FIG.

On the other hand, when a ground fault occurs, the current flow 1 flowing through the ground capacitance 25 is superimposed on the current flow 2 flowing through the ground fault resistance Rx. Since this current generator 2 is in phase with the AC power supply voltage E,
As shown in FIG. 2, the phase difference between the signal current l5 (=Ix+I2) and the AC power supply voltage E when a ground fault occurs becomes small.

The determination device 23 determines the phase difference between the signal voltage E and the signal current 3, determines whether a preset value (phase difference, magnitude of the signal current) has been reached, and if so, a ground fault is detected. It is determined that

FIG. 3 shows a simplified equivalent circuit of FIG. Since the impedance of the load 14 is generally sufficiently small compared to Rx, as shown in FIG. 3, as long as the ground fault resistance Rx is small, the AC power source 18 The impedance seen from the AC power supply 18 is small, and the supply line 12.13
A large alternating current signal can be passed through. Therefore, even if there is a ground fault near the neutral point of the load 14, each supply line 12.1
Since a large alternating current signal flows through 3, the ground fault can be detected reliably.

FIG. 4 shows the relationship between the ground fault point 14p of the load 14 and the detection area. As is clear from this figure, if the ground fault resistance Rx is below a predetermined value, the ground fault point 14p of the load 14 is detected. This shows that ground faults can be detected regardless of the situation. Note that when the ground fault resistance Rx is greater than a predetermined value, a ground fault cannot be detected, but there is no problem because the leakage current in this case is weak.

Although the above embodiment describes the case of detecting a ground fault in the load, if the current transformer 21 for detecting the signal current is installed on the DC power supply side from the injection point where the signal voltage is injected, it can be used in general. It is also possible to detect ground faults in DC power generation equipment such as DC power supplies and solar cells.

Further, in the above embodiment, one current transformer 21 is used, but if a plurality of current transformers are installed at several locations, the location of the ground fault point can be determined.

(Effects of the Invention) According to the present invention, a ground fault can be reliably detected regardless of the position of the ground fault point of the load, and leakage current of the load can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment in which the ground fault detection method according to the present invention is applied to a DC circuit that drives a DC motor, Fig. 2 is an explanatory diagram showing the phase relationship between signal current and signal voltage, and Fig. 3 The figure is a simplified equivalent circuit diagram of the circuit diagram shown in Figure 1. Figure 4 is an explanatory diagram showing the relationship between ground fault points and detection areas. Figure 5 is an explanatory diagram showing the conventional ground fault detection method. , FIG. 6 is an explanatory diagram showing the problems of the conventional ground fault detection method. 18...AC power supply 21...Current transformer 22...Capacitor 23...Determination device 11...DC power supply 12...Positive supply line 13...Minus supply line

Claims (1)

[Scope of Claims] A ground fault detection method for detecting a ground fault in a DC circuit comprising a DC power supply, a load, and a supply line connecting the DC power supply and the load, the method comprising: a ground fault between the supply line and the ground; It is characterized by injecting an alternating current signal, detecting the alternating current flowing through the supply line, detecting the alternating current voltage of the supply line with respect to the ground, and detecting a ground fault in the direct current circuit from the detected alternating current and alternating voltage. Ground fault detection method.