Earth Fault Detection

[pace]

With a 3-phase Delta(HV)-Star(LV) transformer with the Star point connected to earth, what fault current would flow in the HV side for a line-to-earth fault on the LV side?
Would overcurrent or earth fault relays on the HV side detect the fault?

 

[DiscoP]

An earth fault on the star side would look like a phase to phase fault on the HV side.

An earth fault relay on the HV side would not see the fault, however an overcurrent relay might see it, depending on the amount of current.

 

[dpc]

For a line-to-ground fault on the secondary of 1.0 per unit, there will be current in two of the three phases of the primary of only 0.58 per unit. This is why it is difficult to provide adequate through-fault protection for delta-wye transformer from only the primary side.

There will be no ground current on the primary, only phase current, so any ground fault protection on the primary will not see the fault.