SU411570A1 - - Google Patents

Description

one

A known method of protecting synchronous electric motors with transformer-free thyristor excitation from short circuits of the excitation winding to ground, based on measuring the current of the excitation winding.

In order to increase the sensitivity of the proposed method, the variable component of the current is separated from the field current of the field winding from the thyristor group of the field valves and, if it is present, the damage is recorded.

FIG. 1 given an electrical circuit of the excitation and supply network; in fig. 2 - graphs explaining the described method.

During normal operation, the exciter is under the action of the linear voltage Aw of the network, which creates an operating mode current Id in the excitation winding 1. This current is state-of-the-art, i.e. it has practically no pulsation, since the inductance of the excitation windings of synchronous electric motors is large enough to smooth the shape of the current.

In the case of a ground fault winding, for example, at point a, the exciter, in addition to the voltage UAB, is affected by two phase voltages UA and UB, which create additional currents superimposed on the operating current. The voltage UA is phase shifted relative to the voltage of the UAB by 30 ° (see

FIG. 2). In the interval -t, the voltage UA is positive, i.e., is directed from point b to point c. This voltage tends to create a current in the circuit; point b - thyristor 2 - section g-a

field windings - ground - grounded neutral 3 of the transformer 4 of the network - point c. However, the current in this circuit can flow only from the moment fe, when the thyristor 2 of the driver is opened. The moment tz is determined by the angle a of the control with which the pathogen operates.

At time 4, the current is stopped due to the thyristor 2 being locked due to the transfer of the UAB through zero. In the range of 4–4, the voltage UA is negative, i.e. it is directed from point c to point b. This voltage cannot create a reverse direction in the specified current circuit, since the exciter is applied to the thyristors in a nonconductive direction.

The voltage UB is phase shifted relative to the voltage UA by 120 °. In the interval /. for example: UB is positive, directed from point d to point e, and tends to create

current in the circuit: point d - thyristor 5 - section ga –– grounded neutral 3 –point e.

through zero value). In the next two periods of the period, the voltage Ov through a zero value). In the subsequent half of the period, the voltage Ujg becomes negative, but it cannot create a current in the opposite direction, since the exciter is applied to the thyristors in the non-conductor direction.

Thus, during one period, two current pulses appear superimposed on the constant excitation current. The duration of the first impulse Ki p - a,

and the second KZ - - - a. In the current flowing 6

through the section d - and of the excitation winding, a variable component appears, which is a sign of its closure to ground. Turning on the thyristor group of the CASS exciter in series with this site

Any device that reacts to the appearance of a variable component, such as a current transformer, can be used to protect the motor from closing the exciter winding to ground.

The subject invention The method of protection of a synchronous motor with a transformer-free thyristor

excitation from grounding of the excitation winding to earth, based on measuring the current of the excitation winding, characterized in that, in order to increase the sensitivity of protection, the alternating component of the current is isolated from the excitation winding current from the side of the thyristor group of excitation valves and, if present, they detect damage.

ten

-W

-W