Faults in Generator

Generators are the most expensive equipment in the whole plant and the generator faults are considered to be the serious since they may cause severe and costly damage to the insulation, windings and the core. They may cause severe mechanical damage to the shafts and couplings, thereby increasing the amount of fault damage.

In the Power system, around 80% of the faults are due to the Short-circuit and during this short-circuit a very high magnitude of short-circuit current flows in the system. So now-a-days high-speed protection schemes are employed to minimize the damage.

 

Classification of Generator Protection

CLASS-A

All the Heavy electrical faults in the Generator, Generator -Transformer and Unit-auxiliary Transformer come under this class and for this class of faults the tripping action can’t be delayed.

Sequence of Operation:

1. Trip the GCV(Generator Circuit Breaker)
2. Trip the Field Breaker
3. Shut Down Turbine or Close the MSV(Main Steam Valves)
4. Trip the UAT(Unit Auxiliary Transformer ) Breaker

 

CLASS-B

All the Mechanical faults in the Turbine or the Boiler side, which results in tripping of the turbine and subsequent closing of MSV (Main Steam Valve), come under class B.

Sequence of Operation:

1. Trip the GCV(Generator Circuit Breaker)
2. Trip the Field Breaker
3. Trip UAT Breakers

 

CLASS-C

The faults referring to the external disturbances in the Grid come under this category. For example if any fault is there in the Grid then we only need to isolate our plant system from the grid for a certain amount of time.

In this case the  control system initiates the HP-LP bypass after the Generator  getting disconnected from the Grid and then it continues to feed the unit auxiliary load through unit auxiliary transformers.

Sequence of Operation:

1. Trip generator Breaker
2. Initiate HP-LP Bypass

 

CLASS-D

All the incipient electrical faults come under this class for which the tripping of the Generator can be delayed until the fault is cleared.For example if there is an increase in the Temperature of the Oil in Transformer then we can wait for a certain span of time and if the fault is not cleared then we can go for Tripping.

 

SEVERE FAULTS IN THE GENERATOR (Class A)

 

• In Stator

  • Phase to Phase fault.
  • Inter – turn fault
  • Earth fault.

• In Rotor

  • Rotor Earth Fault
  • Over voltage in the rotor.

-: Protection Schemes :-

DIFFERENTIAL PROTECTION for Phase to Phase fault:

Differential protection is the most reliable method for providing protection to stator winding phase to phase faults. In this scheme, the stator currents on both sides of the generator are compared or you can say the inlet and outlet currents in the windings are compared.

Under normal condition, the current i1s is equal to current i2s. Therefore, the current values in the CT secondaries are also equal, which is i1s=i2s then no current will flow through the relays. If a fault is generated in the protected zone, current i1s and i2s are no longer equal, i1s will be very much higher than i2s, therefore i1s and i2s are not equal and therefore current flows in the current relay and that difference in two currents drives the relay to open the circuit breaker. For three phases in the generator, as you can see below in the diagram, we have 3 separate CT assemblies and relays provided.

( Differential arrangement[Oa,Ob & Oc are the Relay Operating Coils] )

INTER – TURN FAULT:

In the generators, instead of single path for each phase, Double Layer windings are used. That means for each phase there will be two parallel paths. So if there is an inter turn fault between the two parallel paths of each phase then there will be heavy fault current. This mainly happens due to the insulation damage between two conductors of parallel paths of each phase. This is almost same as that of Differential protection scheme, in this kind of protection scheme, the primaries of the CT(Current Transformer)s are inserted in the parallel paths of each phase and Secondary are inter connected.

Under normal condition, the current flowing through the two parallel paths of each phase of the stator winding will be same and no current will be flowing through the relay. Under the inter turn fault, current flowing through the two parallel Path will be different and this difference in current flowing through the operating coil And thus causes the circuit breaker to trip. This kind of Protection is very sensitive.

Stator Earth Fault:

(0-95% stator earth fault protection)

Any fault involving earth results in shift of the Neutral Point from its actual position. This shift in neutral point can be detected by measuring the Voltage across the Grounding Resistor. The Over Voltage Relay monitors the voltage developed across the secondary side of the neutral grounding transformer in case of ground faults.

The protection is provided for the 0 to 95% of the generator stator conductor. Because for the rest 5% region nearer to the Neutral Point, if there is a fault then the voltage induced in the secondary side of the NGT (Neutral Grounding Transformer) will be very very less or you can say, it can’t be detected by the relay. So for that reason, to detect the fault in that 5% region nearer to the Neutral, 3rd Harmonics Voltages induced are taken into consideration.

(95-100% Stator Earth Fault Protection)

We use 3rd harmonic voltage relay for the rest 5% region of the stator conductors. It protects 95 to 100% of stator windings. For a Stator Phase-to-ground fault at or near the Generator Neutral, there will be an increase in 3^rd Harmonic Voltages at The Generator Output Terminals, which will Cause the Relay to operate. The Increase in the 3^rd Harmonics Voltage can be seen from the below mentioned Graph.

 

ROTOR EARTH FAULT:

Rotor Single earth fault is not dangerous, 
but the Rotor Double earth fault is Severe and after a single earth fault,
the chances of Double earth fault are more.

This is caused mainly by the insulation failure due to moisture and ageing of the insulation or vibration of the rotor etc. But existence of a single ground fault increases the chance of a second ground fault. The occurrence of the second earth fault can cause bypassing of the field current through the ground, which ultimately isolates a portion of the rotor windings from the flow of current. This results in unsymmetrical flux distribution. The air gap flux distribution is affected and the rotor is displaced enough to rub the stator and may lead to severe vibrations and can damage the bearing.

Remember, a machine can continuously run on a single earth fault but if the second rotor earth fault is allowed to occur then it should be detected immediately and the generator should be tripped. So there is always an Alarm on 1^st Rotor earth fault and tripping on 2^nd Rotor earth fault. The various schemes employed for Rotor Protections are;

Potentiometer Method

DC Injection Method

AC Injection Method

Potentiometer method is now-a-days not used due to certain disadvantages, in this scheme a resistor is connected across the field winding and according to the potentiometer scheme a tapping is taken from the center of the resistor and connected to the ground through a Voltage relay. So whenever there is a fault in generator rotor apart from the center of the field then the potentiometer circuit gets complete and it measures the voltage difference across the relay for further action.

Disadvantages:

1. It can’t sense any fault occurred in the center of the field winding as the potentiometer tapping is given at the center of the field, but we can provide a push-button to cheek the system in the occurrence of the fault at the center of the field.
2. It also senses if any fault is there in the field circuit apart from the Field windings.

In DC injection method, a tapping is taken from any point in the field circuit and it is connected to a Voltage sensitive relay through a DC supply to the ground. So during the occurrence of fault, the path through the ground gets connected and the current flows through the relay and depending on the current and the voltage value the relay decides whether it’s a single earth fault or a double earth fault and it is actuated. The reason we are providing a DC supply is to assist the relay to sense the current and voltages during the fault in voltage build-up condition in the generator.

(AC injection method)

(DC injection method)

Both AC and DC injection methods are same, but the disadvantage in case of the AC injection is that there is always a chance of leakage current to the rotor field through the capacitor, which will affect the field in the rotor and will create a distortion.