Types of Unsymmetrical faults in Power System:

There are two types of unsymmetrical faults in power system namely,

Shunt Type Fault:

  • Single Line to Ground Fault (LG)
  • Line to Line Fault (LL)
  • Double Line to Ground Fault (LLG)

Series Type Fault:

(i) Open conductor (one or two conductors open) fault.

It was stated, that a three-phase (3L) fault being the most severe must be used to calculate the rupturing capacity of circuit breakers, even though this type of fault has a low frequency of occurrence, when compared to the Unsymmetrical Fault Types listed above.

There are, however, situations when an single line to ground fault can cause greater fault current than a three-phase fault (this may be so when the fault location is close to large generating units).

Apart from this, unsymmetrical fault analysis is important for relay setting, single-phase switching and system stability studies.

The probability of two or more simultaneous faults (cross-country faults) on a power system is remote and is therefore ignored in system design for abnormal conditions.