It's not clear what your question is. Where is the fault?
Typically a fault on a transmission line would be cleared without the need to trip any generators.
But in some cases if a fault is not cleared fast enough, the generator would accelerate so much that it is danger of losing syncronism and it will be removed by out-of-step protective relaying to prevent that.
As electricpete mentioned there are many faults that affect a generator, the following may not be all but perhaps you can use them as a starting point:
1. Synchronization faults:
Synch out of phase (i.e. beyond the allowable synch window) this causes a huge stress on the windings and may destroy the generator.
2. Differential Fault
When detected this usually causes the trip of the whole train (i.e. generator and prime mover)
3. Load Rejection
This usually causes an overspeed condition on the prime mover
4. Ground fault
When detected should cause a trip
5. Overvoltage
6. Over/under frequency
7. Over/under excitation
Under excitation may result in a loss of synchronization (out of step).
8. High vibration
9. High Temperature
10. Fire detected
11. Loss of Hydrogen purity (when used)
The control/protective system should have the relaying/sequencing to detect all the faults and is usually defined by the Owner in the Technical Spec.
In general the faults cause the following results:
1. Alarm only
2. generator Breaker trip (prime mover unloads to the full speed no load or synch idle condition)
3. generator lockout (Breaker trip + Prime Mover trip)
Hope this helps.
a.
Suggestion: I just would like to add to the previous posting the following:
1. Protection against reverse power flow, 32
2. Negative sequence protection, 46
3. Overcurrent protection, 50
4. Bus differential, 87B
5. Field relay, 40
6. Etc.
Actually, the generators protection is somewhat related to a generator size. Therefore, the fault will affect the generator of a small size differently from the generator of a big size.
What are the effects of a fault on a generator? As stated above, that depends on what type of fault.
- A phase-phase external short circuit will cause the generator current to increase and the terminal voltage to decrease; the degree will depend on the impedance between the generator and the fault point. The short circuit current will tend to cause shifting of the generator coils and these must be adequately braced to withstand the worst-case situation. If the external short circuit remains on long enough, the generator will indeed lose synchronism.
- Phase-phase faults will cause the generator excitation to increase to the maximum forcing value. The type of excitation system will determine the steady-state value of current available from the generator after the initial subtransient and transient currents have decayed. For example, a static exciter that is fed from the generator terminals and with no boost CT, will not sustain a steady state current output from the generator.
- The effect of single phase to ground faults will depend on the generator and system grounding conditions. Some form of high impedance grounding is normally used, as this minimizes any damage to the generator iron for an internal fault - but this depends on the external system requirements and the network topology.
All good comments. It would seem to me the original poster was looking at the effect of fault external to generator from a stability standpoint, since loss of syncronism was metnioned. Are there any other comments along those lines?
Can the original poster clarify whether the question has been answered?
as electripete has mentioned in his latest post, i was indeed looking at the effect of fault external to generator from a stability standpoint.
AFter peterb's post, i can say that my question has been answered. Thanx peterb for providing such wonderful insight to my question. Thanx also to the rest for providing xtra info on possible causes of fault to a generator
