Generator Rotor Protection

Generator Rotor Protection

The rotor is one of the important part of Generator and it should be well protected against Earth faults or Inter turn faults these are caused by thermal and mechanical stresses.

The field system is normally ungrounded so when a single fault between field winding and rotor body exist does not give rise to any fault current. But when second earth fault exist it short circuit the rotor winding and therefore produce an unsymmetrical field system and unbalance force on the rotor. This causes vibration of the rotor and damage to the bearings. So rotor earth fault protection is provided to restrict the fault spreading.

Because of this fault, unbalance in three phase stator currents exists. As unbalance three phase currents have a Negative Sequence Component, it rotates in a opposite direction at synchronous speed giving rise to double frequency currents. This results in overheating of the rotor and other damage to the rotor. So rotor temperature indicators are used for detecting the rotor overheating.

Rotor open circuit faults are very rare to exist causes arcing problems and reduced excitation. Loss of excitation or field failure occurs due to short circuit or open circuit in field winding. In this case generator start running as induction generator, supplying power at leading power factor. Due to loss of excitation loss of synchronism and system stability occur. In this case rotor protection can be done by using tripping scheme which opens field circuit breaker which will trip the generator unit breaker.  

Rotor Protection

Different Protection schemes are used for protection of faults occurring in rotor. These schemes are:- 

Rotor earth fault protection:-

As the field circuit are operating unearthed a single earth fault does not affect the operation of the generator. But this fault increases the stress to the ground because stator transients induce an extra voltage in the field winding. So if there is only single earth fault but relay should be provided to give the knowledge that fault has occurred so the generator may take out of service until second fault occurs and cause serious damage. Two methods are used for rotor earth fault protection.

 Method I- 

In this method a high resistance is connected across the rotor circuit and its mid point is grounded through a sensitive relay. This relay detects the earth fault for whole circuit except the rotor center point.

 

Method II- 

In this method dc injection or ac injection method is used. In it either DC or AC voltage is connected between the field circuit and ground through a sensitive over voltage relay and current limiting Resistor or Capacitor. A single earth fault in the rotor circuit will complete the circuit including voltage source, sensitive over voltage relay and earth fault. DC injection method is simple and has no problems of leakage currents. If we use dc the over voltage relay will be more sensitive than if we use ac because in case when we use ac the relay not picking up the current that flows normally through capacitance to ground and also care should be taken to avoid resonance between capacitance and inductance.

Rotor overheating Protection:-

Negative sequence component of the unbalanced currents of the stator winding causes double frequency current to be induced in the rotor winding due to this component overheating of the rotor take place. In case of over current due to over excitation in the rotor circuit, a dc relay is used. This relay senses and initiates alarm. Application of such relay is limited because relaying of DC quantities is relatively uncommon.

Rotor Temperature Alarm:- 

This kind of protection is only provided in case of large generators. It gives the level of temperature. In it resistance is measured by comparing voltage and current by a double actuating quantity moving coil relay. The operating coil being used as voltage coil and restraining coil used as current coil. The relay measure the ratio of voltage and current because resistance gives the measure of rotor temperature.
 

Automatic Field Suppression and Use of Neutral circuit Breaker:-

When a fault on the generator winding exist even through the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist. For the quick removal of the fault, it is necessary to disconnect the field simultaneously with disconnection of the generator. So it is very necessary to discharge its magnetic field as soon as possible in short duration. Hence it should be ensured that all protection system not only the trip the generator circuit breaker but also trip the automatic field discharge switch.