Over Voltage on The Other Two Phases...

[Elecme]

In case of phase to ground short circuit, will we experience a case of over voltage on the other two phase in a specific earthing system? Please elaborate. Elecme

 

[electricpete]

An ungrounded three-phase system will normally float (as a result of capacitances and possible instruement transformers) in a balanced manner with respect to ground. That means voltage with respect to ground is VLL/sqrt(3).
If you get a ground on one phase which is not immeidately removed by protective relaying, the other phases will rise to VLL above ground.

Draw an equilateral triangle representing the three phases. In the unfaulted case, ground lies in the center of the triangle.

In the case of ground fault, ground moves toward one corner of the triangle... distance from ground to the other corners is longer.

 

[busbar]

Add that in an oscillatory or resonant condition, the "neutral" may be Displaced Outside the ø-ø voltage triangle.

 

[jbartos]

Suggestion: The previous posting suggests that there is a dc offset with a decrement involved. These are of transient nature. If the fault is remote from the source, then the dc ofset is small. If the fault is close to the source then the dc offset is high.
See W.D. Stevenson Jr "Elements of Power System Analysis," 3rd Ed., McGraw-Hill Book Company, 1975, Chapter 11 Symmetrical Three-Phase Faults on Synchronous Machines.
Equation 11.1
Vmax sin(wt + alfa)=Ri + Ldi/dt
The di/dt will cause the higher voltage.

 

[busbar]

See:

thread238-3370 High Resistance Grounding

thread238-6870 Alarm system for 480V ungrounded systems