Alternator Stator E/F Protection

5%

The fault within the 5% winding close to neutral is difficult to measure by the relay as the voltage at that point is close to zero and very less current will flow through the neutral in-case if earth fault. So it is very difficult to sense this fault by current measurement.

This fault close to neutral can be detected either by injection the frequency signal through the neutral transformer (25Hz square wave) or by measuring the 3^rd harmonic voltage at the neutral. Any method can be applied.

Regards

Manish

thank you very much manish sir...

how we can inject 25hz square wave...can you tell me what is first harmonic over voltage and third harmonic under voltage

I suggest that you attend to our 4-days duration training programme on Protective Realying starting on Oct. 12th at our training centre at Coonoor (near Ooty in Tamilnadu, India). Mail me your ID to send you further details.

Yaar the Injection of this low frequency signals is generally done through the grounding transformer of the machine into the stator of the machine which we want to protect. The voltage level of the injection i need to confirm once again.

The method 3^rd harmonic is also used to detect the fault close to neutral of the machine. The measurement of voltage for this scheme is done at both neutral and phase side (all 3 phase) by forming an open delta connections across the PT secondary of phase side, and across the grounding resistor at neutral side.

The 3^rd harmonic voltage varies with the load, so the setting of this really is really very tough. Incase of fault the voltage level goes down.

The 3^rd harmonic voltage varies with the load, so the setting of this really is really very tough. Generally the 3^rd harmonic voltage of healthy machine of rating 155MW on base load remains 2V approximately and tries to go below incase of earth fault.