SU650151A1 - Method of protection of synchronous machine excitation circuit from grounding in a single point - Google Patents

Description

protection of the generator excitation winding with the superposition of the operating current on both outputs of the excitation winding and measuring the sum of the operational currents of these conclusions. ,, ,,, 4

The overlay of the operating current on the terminals of the excitation winding 4 DRAIN is detected with the help of the source 5 of the operating alternating current I connected by one of its conclusions to the terminals of the excitation winding 4 through the resistances Z1 and Z2, and to the other leads to ground through the resistance Z3. Resistances Z1 and Z2 are chosen such that the minimum voltage applied to the operating current source 5 on the side of the excitation winding is minimal. Resistance Z3 is chosen such that the current at the earth fault point does not exceed the maximum allowable value.

As a source of alternating operational current, a frequency generator, such as a 25 Hz generator, used in KZR-3 type earth fault protection, can be used, but it can also be a different frequency generator. Since in this method, the sum of the operational currents of the outputs of the protected circuit is taken as the measured value, the effect of the voltages of different frequencies arising on the excitation winding is significantly reduced and therefore the source frequency as well as its voltage can be chosen mainly from conditions provide a given sensitivity of protection.

Measurement of the sum of operational currents of the field winding leads is made using current transformers 7 and a measuring element — relay 8. Relay 8 also compares this sum with a reference value (relay setting) and generates a tripping signal if the sum of currents exceeds the relay setting .

It is advisable to choose a relay that reacts to the current or voltage of a given frequency, in this case the frequency of the operating current source. But it can also be a relay that reacts to the active component of the operational source current and is intended for protection of the type KZR-3. The sum of the currents in the outputs of the protected circuit can be measured using one transformer, passing both outputs of the protected circuit into its window in one direction.

Achieving selectivity and eliminating the negative effect of the voltage of the field winding when the protection is performed by this method is explained as follows.

When an excitation winding S from the source 5 to the closure point in parallel through the positive and negative leads of the winding 4 closes to ground, two approximately identical currents flow, which in sum are equal to the current of source 5 and have

source frequency. Since these currents flow through the transformers 7 in the same voltage, in relay 8 they add up and the last is triggered. When a ground fault is not in the winding 4 of the windings, but in another generator excitation circuit, for example in the exciter 6, the currents of the source 5 flow to the location of the fault mainly along two parallel

chains, the first of which contains resistance Zi and a positive output of the exciter 6, and the second one - resistance Z2 and a negative output of the drivers b. At the same time, in the value of the coil 4, current will flow through the current transformers 7, which is determined only by the resistance difference between the two circuits. In case of equality of these resistances, the operational current in the circuit of transformers 7 will be zero and relay 8 will not

will work. The relay will not operate even in the case of a significant difference of this current from zero, since the current flows through transformers 7 in different directions and due to which the sum of the currents in the relay will be zero.

The appearance on the excitation winding of voltages of different frequencies for the method in question is only important if it is significantly reflected in the generator excitation current. Obviously, due to the significant inductance of the winding, the frequency of change of the excitation current is not large and the offset from it by choosing the source frequency does not

presents significant difficulties. Moreover, the effect of excitation current oscillations is eliminated by the fact that the excitation current flows through transformers 7 in different directions, thanks to

The secondary currents of the transformers flowing in the relay 8 are counter-current and only the unbalance current flows in the relay, due to the nonidentity of the characteristics of the current transformers.

Of course, due to the fact that relatively large alternating currents can flow through current transformers, the requirements for the identity of their characteristics can be high.

In order to jointly protect the excitation winding 4 and the exciter 6, current transformers must be installed in the resistance circuit Z1 and Z2, and in order to protect the exciter 6,

circuit of the exciters.

The proposed method is simple and fairly reliable, since due to the use of superposition of the operational current and the sum of the operational currents in the terminals

the protected circuit allows protection not only of all the excitation circuits together, but also of their individual sections, which is especially important when carrying out the protection of the excitation winding of the generator with the action

to disconnect.

The proposed method can reliably protect the excitation circuits operating on both direct and rectified currents with sensitivity independent of the operating mode of the generator and the place of the occurrence of a circuit in the circuit to be protected.

Claims (2)

1. Chernikov G. B. Protection against the earth fault of the rotor of the generator with valve-type exciter, the magazine “Electricity, L. 5, 1965. 2. USSR author's certificate number 258431, cl. H 02 H 7/06, 1968.