RU2304334C1 - Method for protection of three-phase network with insulated center-tap from single-phase ground short-circuits - Google Patents

Abstract

FIELD: electric networks, applicable for protection of the three-phase network with an insulated center-tap from single-phase ground short-circuits.

SUBSTANCE: the state of network insulation and the zero-sequence currents in the outgoing lines are monitored, at a reduction of the network insulation level a circuit is formed for a short-time flow of current of a double short-circuit to ground, the variation of the zero-sequence currents in the outgoing lines is monitored, if at the instant of flow of current of a double short-circuit to ground is varied in one of the outgoing lines, a signal is formed for disconnection of this line because of its ground short-circuit.

EFFECT: enhanced protection selectivity.

1 dwg

Description

The invention relates to electrical networks and is intended to protect a three-phase network with an isolated neutral from single-phase earth faults (OZZ).

There is a method of protecting a three-phase network from OZZ, when along with a general non-selective signaling device that monitors the state of insulation in a given voltage system, selective protection is provided at each connection. This protection is carried out with the relay switched on to the zero sequence current filter and comes into effect due to the passage through the damaged section of the zero sequence current, due to the capacity of the entire electrically connected network, without taking into account the capacity of the damaged line. The current in the relay winding appears only during the SCR, it is proportional to the zero sequence current, in normal mode and during interphase short circuits, only the unbalance current flows through the relay winding. To carry out protection, three-transformer and one-transformer zero-sequence current filters are used as a zero-sequence current filter [V.A. Andreev, E.V. Bondarenko. Relay protection, automation and telemechanics in power supply systems. - M .: Higher. school, 1975. Pages 70-71, 140-141 - prototype].

The disadvantage of this method is its non-selective operation, because it is very difficult to carry out protection using a zero-sequence current filter, detuned from unbalance currents, but sensitive enough to OZZ.

The objective of the invention is to increase the selectivity of protection against OZZ due to the receipt and use of additional information features.

This problem is solved due to the fact that in the known method of protecting a three-phase network with isolated neutral from single-phase earth faults, namely, that the state of the insulation of the network and the zero-sequence currents in the outgoing lines are controlled, according to the invention form a circuit for short-term the current flow of the double circuit to ground, while controlling the change in the currents of the zero sequence in the outgoing lines, and if at the time of the current of the double circuit n and the ground there is a change in the zero sequence current in one of the outgoing lines, they form a signal to turn off this line after it is shorted to ground.

The essence of the invention is illustrated in the drawing, which shows a structural diagram of the protection of a three-phase network with isolated neutral from single-phase earth faults.

The circuit (see drawing) contains: a power transformer 1, an input circuit breaker 2, low voltage busbars 3, an insulation monitoring device 4, a control and control unit 5, a line number indicator with OZZ 6, outgoing lines 17, 18 and 19, switched by three-phase switches 7, 8 and 9, single-phase switches 10 and 11, current sensors (zero sequence current filters) 12, 13, 14, current sensor 15, current limiting resistance 16.

The proposed method works as follows.

When a fault occurs in any of the lines of a three-phase network, for example, in line 17, in phase A, an insulation monitoring device 4 connected to the low-voltage busbars 3 of the power transformer 1 detects the appearance of a zero-sequence voltage and generates a corresponding signal. The signal arrives at the control and control unit 5, which issues a command to one of the switches 10 or 11, for short-term switching on of the undamaged phase, in this case phase C, of the low-voltage busbars 3 to ground via current-limiting resistance 16, i.e. a circuit is formed for the current of a double circuit to ground.

When two signals appear simultaneously, a signal from a current sensor (zero sequence current filter) 12 of line 17, about a change in current, and a signal from a current sensor 15, about the flow of a double-fault current to the ground, coming to the control and control unit 5, the latter generates a signal switch 7 to disconnect line 17 upon the fact of its OZZ. The line number with OZZ is displayed by the line number indicator 6.

If, in the absence of a signal about a change in current from current sensors (zero-sequence current filters) 12, 13 and 14 of the outgoing lines 17, 18 and 19, only one signal from the current sensor 15 is received from the current sensor 15 about the flow of a double circuit current to ground, then it generates a signal to turn off the input switch 2 upon the fact of OZZ on the low voltage busbars 3 of the power transformer 1.

The proposed method provides selective protection of a three-phase network with an isolated neutral from single-phase earth faults, which improves the reliability of power supply and the safety of consumers.

Claims (1)

A method of protecting a three-phase network with an isolated neutral from single-phase earth faults, namely, that they monitor the state of the insulation of the network and zero sequence currents in the outgoing lines, characterized in that when the insulation level of the network is reduced, a circuit is formed for the short-term flow of the double-fault current to earth in this case, the change in the zero sequence currents in the outgoing lines is controlled, and if at the moment of the flow of the double circuit fault to the earth, the zero current changes in series In one of the outgoing lines, they form a signal to turn off this line upon the fact of its ground fault.