Transformer Case Ground protection for 5MVA Transformer
Transformer Case Ground protection for 5MVA Transformer
(OP)
I have to implement a Transformer Case Ground protection on a 5MVA Delta (23)/Wye (grounded)(4.2) transformer. The transformer has no high side breaker, only Fuses; Low side has a recloser. I have never used this scheme before.I have included an IEEE excerpt about the TCG protection below. What should I be worried about when implementing this? Is this just like having a 50/51N OC relay? Whats special about this. If this TCG relay picks up, only thing I can trip is low side recloser - will that help any?
6.4.3 Case ground
On a grounded neutral system, it is possible to isolate the transformer case from ground except for a single
point. A CT and overcurrent relay at this grounding point would detect any internal ground fault or bushing
flashover. Although effective, several problems are encountered. The system should be tested periodically to
determine that no accidental grounds have been added. Incorrect operations can result from accidental
grounds from power tools and transformer auxiliary equipment. Careful coordination between auxiliary
equipment circuit breaker or fuse curves, arrester characteristics, and a time overcurrent trip relay can minimize this danger.
6.4.3 Case ground
On a grounded neutral system, it is possible to isolate the transformer case from ground except for a single
point. A CT and overcurrent relay at this grounding point would detect any internal ground fault or bushing
flashover. Although effective, several problems are encountered. The system should be tested periodically to
determine that no accidental grounds have been added. Incorrect operations can result from accidental
grounds from power tools and transformer auxiliary equipment. Careful coordination between auxiliary
equipment circuit breaker or fuse curves, arrester characteristics, and a time overcurrent trip relay can minimize this danger.






RE: Transformer Case Ground protection for 5MVA Transformer
Not really, unless there is a source on the low side that needs tripping (bus tie, motor load, etc.). If there is no remote feeder breaker to trip upstream, an alarm might be all that is available to you.
RE: Transformer Case Ground protection for 5MVA Transformer
RE: Transformer Case Ground protection for 5MVA Transformer
The only way to clear a fault like that is to trip the upstream breaker. So this system does not work for yoour case.
One problem we had was the CT also picked up fault current from the 208 V cooling fans mounted on the radiators and tripped the plant when a fan motor failed. The CT can't tell the difference between 23 kV, 4.2 kV or 120V amps. An Amp is an Amp.
RE: Transformer Case Ground protection for 5MVA Transformer
RE: Transformer Case Ground protection for 5MVA Transformer
RE: Transformer Case Ground protection for 5MVA Transformer
Another thing I didn't mention is that this transformer is on a skid. The skid has this transformer, Station service transformer, Regulators, and a recloser on it. They call it MITS (Modular Integrated Transportable Substation ).
RE: Transformer Case Ground protection for 5MVA Transformer
If you insulate this skid from ground and ground it via a TCG CT, the protection will only trip for a ground fault in something mounted on that skid. There is no way to discriminate between a transformer, regulator or station service transformer ground fault. This might not be a problem since the desired action for each fault may be the same: disconnect the source from the entire substation. But its something to think about.
RE: Transformer Case Ground protection for 5MVA Transformer