Earth Fault on Auxiliary Transformers

Electrical Systems and Equipment: Incorporating Modern ...

First of all I am not a protection specialist so what I am saying it's my general opinion.

Usually, between a generator and the step-up transformer no breaker is provided.

Also, usually, the system at generator voltage level is ungrounded so Earth Fault trip directly is not - logically- possible [has to be at first a warning of first-to-ground fault ].

However, a vector surge relay could detect the islanding situation of a generator and to stop the generation. I this case -without an actually ground fault-a ground fault on auxiliary transformer could be signalized with no reason.

I don't think the Grid Circuit Breaker could trip on a Earth fault on the auxiliary transformer,

but since we don't know what it was the actual reason this could be the source of the entire

scenario.

Dear All. thanx for valuable comments. We have installed relay 7sj6025 on auxiliary trafo and I have downloaded the fault logging of relay by digsi software of Siemens. It is not showing any trip. We are not reaching the root cause. Any valuable comment is needed. Because DCS history is showing that auxiliary oil pump and Motor operated valves stopped which is feeding by same auxiliary trafo and resulted in turbine generator tripping

Most all of what I am about to say is IMHO (in my humble opinion). Every generating unit has control switchgear that either makes intial phase in with grid possible (if on a grid that is interconnected and is actually not a stand-alone power island). There are also other switches (86 relay, dV/dt switch, etc.) that can drop the unit "offline" in a couple of milliseconds. Most generating units also have a start-up auxilliary transformer, and a main transformer.

IF the unit is still on aux. transformer while the field breaker opens, then oil auxilliaries continue, etc. With some luck, the field breaker may be phased back in and re-closed, if conditions are suitable.

IF the unit has been online, and the actual field transformer trips, resulting in power turbine trip, there should be a fast transition (by instrument relays) back to aux. transformer. If the aux. transformer cannot supply power to the unit auxilliaries (it is receiving no power) then the unit will continue to hold in a tripped state, and can be damaged if there is no backup source of oil pressure (unless there is contained oiled bearings for short duration spin-down on a magnetic bearing system).

Ground faults are a typical result in that the auxilliary is now not actually connected to anything since its source of power is dead and isolated (the field transformer). In these situations, common is no longer "common". The only ground available to the sensing and instrumentation circuitry is hard earth ground, which may or may not exist at all points, and even if it does, there can be shifts of potential due to surges in shorting current on the "common" line.

I hope in attempting to clarify the situation I have done the reverse and simply muddied the water.