Alstom Rotor E/Second F CAEM33AF601D

Alstom Rotor E/Second F CAEM33AF601D Technical Details:

CAEM 33 MK II Second Rotor Earth Fault Relay

Challenge

The field circuit of the generator, comprising the field winding of the generator and the armature of the exciter together with any associated field circuit breaker if it exists, is an isolated D.C. circuit, which is not normally earthed. If an Earth fault occurs, there will be no steady state fault current and the need for action will not be evident.

Danger arises if a second earth fault occurs at a separate point in the field system, to cause the high field current to be diverted, in part at least from the intervening turns.

Solution

The only way to find out the first and the actual second rotor earth fault is, with the help of VAEM and CAEM schemes, along with a null detector. Null detector is adjusted to zero position with the help of potentiometers provided in the circuit.

In this scheme the first rotor earth fault is detected by VAEM and then the VAEM becomes out from the circuit. Null detector is again adjusted to the zero position. CAEM makes a bridge circuit between the faulted portion. Occurrance of the second rotor earth fault disturbs the balancing of the bridge and gives trip command to the machine.

Application

A single Earth Fault (E/F) detected in the D.C. field circuit of a machine requires the machine to be taken out of service at the first opportunity. This is because, if allowed to run with an Earth fault on the ?rotor?, a subsequent second earth fault can cause severe damage to the machine.

However, a relay which can detect such a second E/F and trip out the machine can make it possible to run even with a single E/F, without any such risks, thus helping to preserve the generation risks, thus helping to preserve the generation capacity. CAEM 33 has been designed for the above application.

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