Transformer Case Ground protection for 5MVA Transformer 1

[Uquresh1]

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.

 

[PHovnanian]

Is this just like having a 50/51N OC relay?

Not really. The IEEE excerpt describes one important difference: With the TCG, it is possible to 'bypass' the single point ground (and sensing CT) with another metallic connection to ground. This will produce a secondary path for subsequent fault current that bypasses the CT, reducing the protection sensitivity. With the 50/51N, the CTs measure the three phases and sum the currents (which should sum to zero for an unfaulted delta). The sensing locations and failure modes of these protections are different.

only thing I can trip is low side recloser - will that help any?

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.

 

[odlanor]

I did not understand. The CT is connected to the 4,2kV neutral winding transformer or is CT tank protection?

 

[rcw retired EE]

The transformer tank is insulated from earth if a Case Ground is used. Our older rectifier transfomers were mounted on rails to allow easy removal for maintenance and the railroad style wheel mounting was insulated. A large bar type CT connected the tank to earth. All control raceways were isolated to maintain the single point earth connection. Internal faults on the HV or LV winding would eventually fault to the tank and cause current flow in the tank ground CT.

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.

 

[odlanor]

I have seen some transformer tank protection for ( = 5MVA or less ) transformers with natural ventilation(OA). Trip with security.

 

[prc]

This is a protection practice found in French influenced areas.When such protection is provided all motor mountings (cooling fans, oil pumps and tap changer drive) are insulated from transformewr tank and separately earthed to avoid the mal-operations described by rcwilson.

 

[Uquresh1]

odlanor - there are no CTs on this unit as of yet. If we go with this TCG scheme, then I would have to tell the customer to buy the TCG CT that connects the tank to earth (as rcwilson mentioned).

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 ).

 

[PHovnanian]

The skid has this transformer, Station service transformer, Regulators, and a recloser on it

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.

 

[jghrist]

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.

But wouldn't a fault current on something mounted on the insulated skid return to the tranformer secondary neutral through the skid without going through the ground and not go through the TCG CT?