JPS6158424A - Ground-fault detector - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a ground fault detection device for a neutral point grounded distribution line.

An example of a device in which the neutral point on the load side is grounded in conventional thyristor power supply equipment for heaters is shown in Figure 3 (al). In this figure, each phase output end of the three-phase AC power supply 1 is Thyristor converters 2a and 2bl connected in antiparallel
2ci to a load 3 whose neutral point is grounded. In a device configured in this way, the conventional method for detecting a ground fault is, for example, by installing a power source (not shown) that connects the neutral point on the side of the AC power supply 1 and the earth, and using these transformers to reduce the zero-sequence current or zero-sequence voltage. A method was used to detect ground faults by monitoring

Problems to be Solved by the Invention In the device of FIG. 3(a), 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. 3(b), the υ phase and Y'@ thyristors are in the on state, and the W phase and Z thyristors are in the on state.
When the phase thyristor is in the off state, the neutral point of the load 3 becomes the neutral point of the U phase and the Y phase, and is no longer a neutral point where the three phases are balanced. For this purpose, thyristor converters 2a, 2b are used.

The gate-on cycle when controlling with 2ct-control angle α = 70° is as shown in Fig. 4(a), and at this time, the load 3
A potential as shown in FIG. 4(b) is generated at the neutral point on the side.

Moreover, the control angle α=
The gate-on cycle when controlled at 45° is shown in Figure 5 (
At this time, a potential as shown in FIG. 5(b) is generated at the neutral point on the load 3 side.

That is, the neutral point potential on the load 3 side changes according to the phase control angle α. A ground return current constantly flows due to the potential difference between the neutral point on the load 3 side and the neutral point on the power source 1 side. 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 ground faults regardless of the magnitude of the phase control angle α of the thyristor converter, and is capable of detecting ground faults without causing the ground fault relay to malfunction due to constant ground return current. The purpose of this invention is to provide a fault detection device.

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 contact point of the converter and the ground, and the phase control angle α of the thyristor converter is
is detected by the thyristor control circuit, and the value of the resistor is changed depending on the magnitude of the detected output, thereby suppressing the constantly flowing earth return current within a range that does not cause the earth fault relay to malfunction. An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, the same parts as in FIG. 3(a) are shown with the same reference numerals, and the explanation thereof will be omitted. A common connection point 11a between the AC power source 1 and the thyristor converter 2a is a resistor 12d'.
It is connected to one end of the resistor 12a via t-. 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. A common connection point 11C between the AC power supply 1 and the thyristor converter 2C is connected to one end of a resistor 12C via a resistor 12fi. These resistors 123 to 12f constitute a current suppressing means. The other ends of the resistors 12a, 12b, and 12c are all grounded through a grounding resistor 13f, and a current detecting means, such as a zero-phase current transformer 14, is inserted in the electrical path connecting the grounding resistor 13. The output side of this current transformer 14 is connected to a ground fault relay 15. The resistors 12a, 12b,
At both ends of 12c are contacts 16a of an electromagnetic contactor, which will be described later.
, 16b, and 16c are each connected in parallel. 17 is a transformer Ω 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 rear cylinder 1 matching circuit 19. 20a and 201) are current transformers that detect the current flowing through each phase. This current transformer 20a
, 20 b output i! The current is passed through a converter 21 consisting of a rectifier etc.
Rear cylinder 2 matching circuit 2 converted into DC current by
2 is input. 23 is a setting device for setting the reference voltage. The output voltage of this setting device 23 is determined by the first matching circuit 1.
9, the voltage is matched with the output voltage of the converter 18, 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. The output of the current amplifier 25 is input to the α detection comparator 26 and also to the phase control circuit 28 via the integrator 27.

A phase control circuit 28 controls the phase of each thyristor of the thyristor converters 2a, 2b, and 2c based on the output of the integrator 27. The α detection comparator 26 detects a preset phase control angle (α0) and the second matching circuit 22.
The deviation output is amplified and compared with the obtained phase control angle α, and an electromagnetic contactor (not shown) is excited by the comparison output. For example, when the phase control angle α is smaller than the set phase control angle α0, the magnetic contactor is excited,
The contacts 16a, 16b, and 16c are configured to close. In this way, the resistor 12a
, 12b12c are short-circuited to switch the resistance value of the current suppressing means.

In the M8 device as described above, the earth return current flowing due to the potential difference between the neutral point of the load 3 and the neutral point of the power source 1 is limited by current suppressing means, that is, resistors 12a to 12f. For example, thyristor converters 2a, 2b
, 2c is smaller than the set phase control angle α0, and therefore the earth 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, 113b,
16c is closed. As a result, the resistor 12a,
12b.

Since both ends of 12c are short-circuited, the resistance value of the current suppressing means becomes small. Further, it is assumed that the phase control angle α becomes larger than the set phase control angle α0, and therefore the earth return current becomes larger. In this case, α detection comparator 2
The output of 6 puts the electromagnetic contactor in a de-energized state, and its contacts 16a, 16b.

16c is opened. 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. Next, when a ground fault occurs, the current passing through the zero-phase current transformer 14 increases significantly. For this reason, ground fault relay 1
5 is certain: C works.

Here, in FIG. 2 (aL (bL (C)), the phase control angle α is 90°.

The waveforms of the current flowing through the zero-phase current transformer 14 at 60° and 300° are shown, respectively. As is clear from these waveform diagrams, the current during normal operation and the current during a ground fault accident can be easily distinguished by the amount of current increase, and as a result, the ground fault relay 15 does not malfunction.

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 a detection comparator 26, but the present invention is not limited to this, and the suppression amount of the current suppression means may be controlled in accordance with the size of the phase control angle α. It may be composed of other things, if any.

Effects of the Invention As described above, according to the present invention, the earth return current during normal operation is suppressed in accordance with the phase control angle α.
No matter what the value is, the earth fault %'5 appliance will not malfunction during normal operation. 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]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2(a)
, (1)), and (C) are current waveform diagrams for explaining the operation of the circuit in Figure 1, and Figures 3 (a) and (b) are both current waveform diagrams for explaining the conventional ground fault detection method. Circuit diagram, 4th
Figure (a) is a waveform diagram showing the gate-on cycle when the phase control angle α of the thyristor is 70°, and Figure 4 (b) is the voltage waveform at the load side neutral point when the phase control angle α is 70'. Figure 5(a) is a waveform diagram showing the gate-on cycle when the phase control angle α is 45°, and Figure 5(b) is a waveform diagram of the load side neutral point when the phase control angle α is 45°. It is a voltage waveform diagram. DESCRIPTION OF SYMBOLS 1... Three-phase AC power supply, 2a, 2b, 2c... Thyristor converter, 3... Load, 12a-12f... Resistor, 14... Zero-phase current transformer, 15... Ground Fault relay, 16
a. 16b, 16c...Contact points of electromagnetic contactor, 19.22.
-・Matching circuit, 23... Setting device, 24, 2.5
...Amplifier, 26...Comparator for α detection, 28
...Phase control circuit.

Claims (1)

[Claims] In a device configured by inserting a thyristor converter into a distribution line connecting an AC power source and a load whose neutral point is grounded,
a current suppressing means that is inserted in an electric path connecting a common connection point of the AC power supply and the thyristor converter to the ground and suppresses the current flowing in the electric path; a current detecting means that detects the current flowing in the electric path; and the thyristor converter. and α detection means for detecting the phase control angle α of the converter, and controls the suppression amount of the current suppression means according to the output signal of the α detection means, and detects a ground fault by the output of the current detection means. A ground fault detection device characterized by operating a relay.