27-11-2012, 01:02 PM
Distribution systems
Distribution systems.ppt (Size: 2.56 MB / Downloads: 60)
Choice of a particular arrangement of busbar:
System voltage
Position of the substation in the system
Flexibility
Reliability of supply (immunity from total shut down)
Cost
Technical considerations:
Simplicity
Easy maintenance
Possibility of alternate arrangements during faults
Future expansion
economical
Classification of bus schemes
The first category is passive failure, such as wire open circuit and improper operation of breaker, and such failure mode does not lead to junction tripping of protective devices like circuit breaker.
The second category is active failure, generally
referring to short-circuit failure occurring in components.
Such failure mode may result in junction tripping of protective devices like circuit breaker, and isolation of some failure-free devices from the system, thus causing power supply interruption.
In addition, when tripping of primary protector fails, backup protector needs tripping
Modes of failure
• Passive failure events
• Active failure events
• Stuck-condition of breakers
• Overlapping failure events
Passive failure events are component failures that do not activate the protection system such as unknown open circuit conditions or unintentional operation of a circuit breaker.
An active failure is an event that causes the protection system to operate and isolate a failed component. A simple example of an active failure event would be a fault on a bus and the subsequent operation of breakers to “seal off” the area from the rest of the station.
If during the above fault one of the primary breakers failed to operate and a backup or secondary breaker had to operate to isolate the faulted area, this would be termed a ‘stuck-condition of breakers’ failure mode. The station may still remain in operation, but a larger portion has become inoperable than in the active failure mode.
An overlapping failure is when a failure has occurred and before the failure has been fixed, another failure occurs.
Evaluation of failure effects
For each failure mode, the effects of the failure and what action must be taken to correct the failure need to be determined. The effect of each failure can then be listed according to the likelihood of the event.
The following steps can provide a framework for gathering the needed information from each failure mode.
1. Protection system status and resulting breaker action.
2. Have breaker actions caused load interruption?
3. Have any performance criteria violations occurred?
a. If yes, then determine actions to mitigate violations
i. Transfer possible? ii. Repair required?
4. Record all effects by terminal affected, along with the probability of the event and its duration.
Summary of evaluation
The final step is to list all system failures by the probability of occurrence. To find the system reliability (or substation reliability), combine the system failure probabilities and frequencies.
Each failure state is an exclusive state, so the probability of occurrence of system failure is the sum of all the failure event probabilities. The product of occurrence of failure event and the duration = the probability of the failure state.