JPH0520975B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a ground fault detection device in a neutral point grounding type power distribution line.

2. Description of the Related Art An example of a thyristor power source equipment for a heater in which the neutral point on the load side is grounded is shown in FIG. 3a. In this figure, each phase output end of a three-phase AC power supply 1 is connected to a load 3 whose neutral point is grounded via thyristor converters 2a, 2b, and 2c formed by connecting thyristors in antiparallel. In a device configured in this way, a conventional method for detecting a ground fault is to install a current transformer (CT) or a grounding transformer (GPT) in the electrical path connecting the neutral point on the AC power supply 1 side and the earth, for example. (not shown), a method was used in which ground faults were detected by monitoring zero-sequence current or zero-sequence voltage using these transformers.

Problems to be Solved by the Invention In the device of FIG. 3a, the thyristor converter 2
a, 2b, and 2c are phase-controlled by a control circuit (not shown), but in this case, a potential difference occurs between the neutral point on the AC power supply 1 side and the neutral point on the load 3 side. For example, in Fig. 3b, when the U-phase and Y-phase thyristors are in the on state and the W-phase and Z-phase thyristors are in the off state, the neutral point of the load 3 is the neutral point of the U and Y phases. The three phases are no longer a balanced neutral point. For this reason, thyristor converter 2
When a, 2b, and 2c are controlled at a control angle α=70°, the gate-on cycle is as shown in Figure 4a, and at this time, the neutral point on the load 3 side has a potential as shown in Figure 4b. arise.

Furthermore, when the thyristor converters 2a, 2b, and 2c are controlled at a control angle α=45°, the gate-on cycle is as shown in Fig. 5a, and at this time, the neutral point on the load 3 side is as shown in Fig. 5b. A potential as shown is generated. That is, the neutral point potential on the load 3 side changes according to the phase control angle α. And the neutral point on the load 3 side and the power supply 1
Due to the potential difference between the neutral points on both sides, a ground return current always flows. If the earth return current flows all the time in this way, it will become indistinguishable from the earth fault current and the earth fault relay will malfunction.

The present invention has been made in view of the above points, and is capable of detecting a ground fault regardless of the magnitude of the phase control angle α of the thyristor converter, and is capable of detecting a ground fault relay by a constant ground return current. The purpose is to provide a ground fault detection device that does not malfunction.

Means and Effects for Solving the Problems The present invention provides a device configured by inserting a thyristor converter in a distribution line connecting an AC power source and a load whose neutral point is grounded. A resistor and a current detector are provided in the electrical path connecting the common connection point of the converter and the ground, and the phase control angle α of the thyristor converter is detected from the thyristor control circuit, and the resistance of the resistor is adjusted according to the magnitude of the detected output. By changing the value, the ground return current that constantly flows is suppressed within a range that does not cause the ground fault relay to malfunction, and the ground fault current in the event of a ground fault accident is reliably detected by the current detector. It is characterized by what it did.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, the same parts as those in FIG. 3a are designated by the same reference numerals, and their explanation will be omitted. A common connection point 11a between the AC power source 1 and the thyristor converter 2a is connected to one end of a resistor 12a via a resistor 12d. A common connection point 11b between the AC power supply 1 and the thyristor converter 2b is connected to one end of a resistor 12b via a resistor 12e. AC power supply 1
A common connection point 11c of the thyristor converter 2c and the thyristor converter 2c is connected to one end of the resistor 12c via a resistor 12f. These resistors 12a to 12f constitute current suppressing means. The resistors 12a, 12b, 12
The other end of c is collectively grounded via a grounding resistor 13. Current detecting means, for example, a zero-phase current transformer 14 is inserted in the electrical path connecting the other ends of the resistors 12a, 12b, 12c and the grounding resistor 13. The output side of this current transformer 14 is connected to a ground fault relay 15. At both ends of the resistors 12a, 12b, 12c are contacts 16a, 16b, 16 of a magnetic contactor, which will be described later.
c are connected in parallel. 17 is a transformer that detects the voltage between each phase. The output voltage of the transformer 17 is converted into a DC voltage by a converter 18 comprising a rectifier or the like, and then input to a first matching circuit 19. Both 20a and 20b are current transformers that detect the current flowing through each phase. This current transformer 20
The output currents of a and 20b are input into a second matching circuit 22 after being converted into direct current by a converter 21 comprising a rectifier or the like. 23 is a setting device for setting the reference voltage. The output voltage of this setting device 23 is applied to the converter 1 in the first matching circuit 19.
8 and its deviation output is amplified by a voltage amplifier 24. The output of the voltage amplifier 24 is matched with the output current of the converter 21 in a second matching circuit 22, and the deviation output thereof is amplified in a current amplifier 25. current amplifier 2
The output of 5 is input to the α detection comparator 26, and is also sent to the phase control circuit 2 via the integrator 27.
8 is input. The phase control circuit 28 is an integrator 27
Based on the output of the thyristor converters 2a, 2
The phase of each thyristor b and 2c is controlled. The α detection comparator 26 compares a preset phase control angle (α ₀ ) with a phase control angle α obtained by amplifying the deviation output of the second matching circuit 22,
An electromagnetic contactor (not shown) is excited by the comparison output. In this case, for example, the electromagnetic contactor is energized and the contacts 16a, 16b, 16c are closed when the phase control angle α is smaller than the set phase control angle α ₀ . In this way, both ends of the resistors 12a, 12b, and 12c are short-circuited, and the resistance value of the current suppressing means is switched.

In the device configured as described above, load 3
The earth return current flowing due to the potential difference between the neutral point and the neutral point of the power source 1 is limited by current suppressing means, that is, resistors 12a to 12f. For example, the phase control angle α of the thyristor converters 2a, 2b, 2c
It is assumed that the set phase control angle α is smaller than ₀ , and therefore the ground return current is relatively small. In this case, the electromagnetic contactor is excited by the output of the α detection comparator 26, and its contacts 16a, 16b, and 16c are closed. As a result, the resistors 12a, 12b,
Since both ends of 12c are short-circuited, the resistance value of the current suppressing means becomes small. Further, the phase control angle α
Suppose that the phase control angle α becomes larger than the set phase control angle α ₀ , and therefore the earth return current becomes large. In this case, the output of the α detection comparator 26 brings the electromagnetic contactor into a de-energized state, and the contacts 16a, 16
b, 16c are open. This increases the resistance value of the current suppressing means. Since the resistance value of the current suppressing means is changed in accordance with the magnitude of the phase control angle α in this manner, the earth return current during normal operation is suppressed within a range in which the earth fault relay 15 does not malfunction. If a ground fault occurs next, the current passing through the zero-phase current transformer 14 will increase significantly. Therefore, the ground fault relay 15 operates reliably.

Here, in Fig. 2 a, b, and c, the phase control angle α is 90°,
The waveforms of the current flowing through the zero-phase current transformer 14 at 60° and 30° are shown, respectively. As is clear from these waveform diagrams, the current during normal operation and the current during a ground fault fault can be easily distinguished based on the amount of current increase, thereby preventing the ground fault relay 15 from malfunctioning.

Note that the current suppressing means is not limited to the series resistor as in the embodiment, but may be constructed of other means. Further, in the embodiment, the electromagnetic contactor is excited by the output of the α detection comparator 26, but the present invention is not limited to this, and the suppression amount of the current suppression means may be controlled according to the size of the phase control angle α. It may be composed of other materials as long as they are suitable.

Effects of the Invention As described above, according to the present invention, the earth return current during normal operation is suppressed according to the phase control angle α, so that no matter what value the phase control angle α is, the earth fault relay will not operate during normal operation. There is no malfunction. Further, when a ground fault occurs, the ground fault can be reliably detected, and the reliability of ground fault detection can be improved.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
Figures a, b, and c are current waveform diagrams for explaining the operation of the circuit in Figure 1, Figures 3 a and b are circuit diagrams for explaining the conventional ground fault detection method, and Figure 4 a. is a waveform diagram showing the gate-on cycle when the phase control angle α of the thyristor is 70°, Fig. 4b is a voltage waveform diagram at the load side neutral point when the phase control angle α is 70°, and Fig. 5a is a waveform diagram showing a gate-on cycle when the phase control angle α is 45°, and FIG. 5b is a voltage waveform diagram at the load side neutral point when the phase control angle α is 45°. 1... Three-phase AC power supply, 2a, 2b, 2c... Thyristor converter, 3... Load, 12a to 12f...
...Resistance, 14...Zero-phase current transformer, 15...Ground fault relay, 16a, 16b, 16c...Contact of electromagnetic contactor, 19, 22...Matching circuit, 23...Setter, 24, 25 ...Amplifier, 26...Comparator for α detection, 28...Phase control circuit.

Claims (1)

[Claims] 1. In a device configured by inserting a thyristor converter in a distribution line connecting an AC power supply with a load whose neutral point is grounded, a a current suppressing means for suppressing the current flowing in the electric circuit, a current detecting means for detecting the current flowing in the electric circuit, and an α detecting means for detecting the phase control angle α of the thyristor converter; A ground fault detection device, characterized in that a suppression amount of the current suppression means is controlled according to an output signal of the detection means, and a ground fault relay is operated by the output of the current detection means.