- Rotor Earth Fault Protection Definition: Rotor earth fault protection involves methods to detect and fix faults in the rotor’s field winding to prevent damage.
- Potentiometer Method: Uses a centrally tapped resistor connected to a relay, but can miss faults at the center of the winding.
- AC Injection Method: Connects a relay via a capacitor and transformer but can cause leakage currents and may become inactive if the AC supply fails.
- DC Injection Method: Uses a DC relay and external DC source, eliminating leakage current issues and providing reliable fault detection.
- Importance of Detection: Detecting and rectifying rotor earth faults is crucial for preventing unbalanced magnetic fields and mechanical damage in alternators.
Rotor earth-fault protection watches the field winding of an alternator for a connection to earth. One earth fault on an otherwise isolated field does not circulate current. A second earth fault shorts part of the winding, unbalances the air-gap magnetic field and can damage the bearings.
Find the first field earth fault and take the machine off load, or alarm, before a second fault can form. The relay circuit closes through the earth path. IEEE practice is usually to alarm on the first fault and trip only when the maker requires it.
Three older rotor earth fault protection schemes are used for this purpose.
- Potentiometer method
- AC injection method
- DC injection method
The three methods are described below.
Potentiometer Method of Rotor Earth Fault Protection in Alternator
A resistor is connected across the field winding and the exciter. The resistor is centre-tapped and that tap goes to earth through a voltage-sensitive relay.
An earth fault on the field or exciter then closes the relay circuit through earth. A voltage appears across the relay by potentiometer action of the resistor.
The scheme cannot see an earth fault at the electrical centre of the field winding.
A fault at the electrical centre of the field puts no voltage across the relay. That rotor earth fault protection is therefore blind at mid-winding. A second tap, offset from centre and selected by a push-button, shifts the divider so a mid-winding fault then produces voltage on the relay.
AC Injection Method of Rotor Earth Fault Protection in Alternator
A voltage-sensitive relay is connected to a point on the field and exciter circuit. The other relay terminal goes to earth through a capacitor and the secondary of an auxiliary transformer, as shown.
An earth fault on the field or exciter closes that path. Auxiliary-transformer voltage then appears across the relay and the relay operates.
A standing leakage current can flow through the coupling capacitors into the field even when the winding is healthy. That current is a detection limit of AC injection. It does not by itself unbalance the air-gap magnetic field or load the bearings. Bearing damage still belongs to a second field earth fault.
The scheme also needs a separate AC source. If that supply fails, this rotor-earth-fault function is dead.
DC Injection Method of Rotor Earth Fault Protection in Alternator
DC injection avoids the AC capacitive coupling of the previous scheme. One terminal of a DC voltage-sensitive relay goes to the exciter positive. The other terminal goes to the negative of an external DC source. That source is an auxiliary transformer and bridge rectifier, with its positive terminal earthed.
A field or exciter earth fault then puts the external DC positive onto the relay terminal that sits on the exciter positive. Rectifier voltage appears across the relay and the relay operates. The scheme still depends on that auxiliary DC source. Low-frequency square-wave injection is a later alternative on numerical relays.
