Ground Fault on Delta Side of Transformer 2

[HeinzM]

We have a 13.2 kV Delta to 208/120 Wye/gnd transformer, supplying facility power.
The utility had a single phase ground fault on the 13.2kV primary side and the Neutral conductor on the grounded wye got destroyed/burnt.
Any ideas how that neutral conductor picked up extensive current?

 

[magoo2]

Assuming the ground fault either blew a line fuse or locked out a single phase recloser, you would wind up with one phase of the delta being energized by the remaining good phases.
The faulted phase with an open fuse or recloser, would result in the two other phase-to-phase voltages in series and adding up to 13.2 kV, so each is seeing only half voltage.

On the secondary, this gives you 120 V on the good phase and about 60 V on each of the other two phases (phase to neutral). What types of loads were being served from the 208 V supply?

 

[RRaghunath]

I agree with magoo2.
Single phasing on utility side can cause huge circulating currents in the neutral if the single phase load are substantial in the LV system.
The 'neutral protection' if set in the 208V incomer 'protection release' should help prevent recurrence.

Rompicherla Raghunath

 

[HeinzM]

Hi,
The loads 208/120 are regular power distribution in a school, single phase 120V and 3ph 208 for AC and other equipment.
However, there is also a Solar Power system (PV) on the roof with a Wye gnd/ Wye (floating) transformer 208/480V 225kVA. The neutral to this transformer is the one having taken the hit!

 

[wroggent]

Is the wye on the PV side really floating or just not grounded?

 

[HeinzM]

BTW, the inverters were off when the neutral got taken out, so no generation was adding load into the system.

 

[HeinzM]

Floating. The inverters do not need the neutral