Unsymmetrical Fault Types:
Various Unsymmetrical Fault Types that occur in power systems are:
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 LG 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.