Symmetrical Fault :
- In symmetrical faults, all phases are shorted to each other or to earth (L-L-L)or(L-L-L-G).
- The nature of this type of fault is balanced.
- In this type of fault,fault currents in all phases are symmetrical i.e. their magnitudes are equal and they are equally displaced by angle 120o as shown in figure
- It is more severe type of fault but it occurs rarely.
- For this reason balanced short circuit calculation is performed to determine these large currents.
- Because of balanced nature of fault, only one phase need to be considered in calculations since in the other two phases will also be similar.
These faults involve only one or two phases.
In this type of fault ,three phase lines become unbalanced.
Those faults on the power system which gives rise to unsymmetrical fault currents(i.e, unequal line current with unequal phase displacement)are called Unsymmetrical fault.
Such fault occur between line to ground and two lines.
This type of fault mainly have two types
(a) shunt fault
(b) series fault.
Shunt faults are Single Line to Ground fault (L-G) Fault, Line to Line fault (L-L Fault),Line to Line To Ground fault(L-L-G) fault as shown in figure below.
- Example of Series fault is open conductor circuit.
- These faults mostly occurs on power system.
- The calculations of these currents are made by Symmetrical Component method.
- Unbalanced fault analysis is important for Relay setting, single phase switch and system stability studies.