SU71434A1 - Generator ground fault protection device - Google Patents

Description

Device for protection against ground fault generators

December 29, 1945 for the USSR Commissariat of Power Plants

for N ° 575 (342212)

Published May 31, 1948

The object of the present invention is a device for protection against the earth fault of generators connected to buses by a bundle of parallel cables. The circuits of each cable include one Ferranti-type three-winding current transformer; The secondary winding of each transformer flows around the zero-sequence current, and a relay is connected to the third winding.

The proposed device is characterized in that in order to reduce unbalance currents, the secondary windings of Ferranti transformers are connected to the generator zero-sequence reverse current circuit in such a way that they compensate for the currents caused by the uneven distribution of currents in individual cables and thereby reduce the current unbalance in the relay parallel to to all third windings of the Ferranti current transformer.

The invention is illustrated in the drawing, which is a schematic diagram of the device.

In the drawing, the following notation is accepted: 7 - current relay; 2 - Ferranti transformer cores; 3 - Ferranti transformer main windings; 4 - additional windings of Ferranti transformers; 5 - generator cables; 6 - zero transformer with a transformation ratio K p iV.j, where

p - the number of cables (in the drawing of p 2)

W is the number of turns of the additional winding 4.

In known devices for the protection of generators connected to buses by several cables, parallel connection of the same type of primary windings of Ferranti transformers is provided. In this case, there is an uneven distribution of currents in the windings and, as a result of this, inadmissibly large unbalance currents in the relay circuit.

In the proposed device, one group of primary windings of Ferranti transformers is interconnected in parallel (these windings are the cables of the protected generator themselves), and the other group of windings

167

(windings 4) are interconnected in series. This eliminates the possibility of an uneven distribution of currents between the windings 4 and prevents large unbalance currents in the relay circuit.

Each cable is equipped with Ferranti transformers with additional windings 4 connected in series with each other and powered from the zero current transformer 6.

The connection scheme of the windings 4 and the windings of the transformer 6 is such that when grounding outside the protection zone, the turns of the windings 4 are equal and opposite to the resulting cable turns. Therefore, the current in the relay is theoretically zero. If the currents in the generator cables are all the same, then in the windings 3 currents do not flow. If, however, due to the unequal resistance of the cables, the currents in them are somewhat different, then the unbalance of the amperages of the windings 4 and the resulting cables of the amperes of the cables is compensated by the currents that then appear in the windings 3. However, these currents circulate only in the windings 3 and connecting them wires and relay 7 do not fall.

If a ground fault occurs in the protection zone, then the amps of the cables and windings 4 are folded, as a result of which currents appear in the windings 3, which are added to the relay 7 and cause it to act.

Subject invention

A device for protection against grounding of generators connected to buses by a bundle of parallel cables, the circuits of each of which include a three-winding Ferranti-type current transformer, the secondary winding of which flows around the zero-sequence current, and a relay connected to the third winding, characterized in that unbalance currents, the secondary windings of Ferranti transformers are included in the generator’s zero-sequence reverse current circuit in series in such a way that they compensate the flows caused by the uneven distribution of the currents in the individual cables and thereby reduces the current unbalance in the relay, adventured parallel to all the third windings of the Ferranti current transformers.