Earthing (grounding) transformer trip 1

[132kVshunt]

We have a 90MVA 132/33kV YNd1 transformer with a ZNyn1 earthing transformer on the 33kV side. Recently, during major road works, the contractor damaged a 33kV cable and the earthing transformer tripped on Buchholtz. This happened twice. After the first trip, gas was found in the Buchholtz relay and the earthing transformer was replaced by an identical transformer. A few days later, the same contractor damaged another 33kV cable and the earthing transformer tripped on Buchholtz, but no gas was collected int the relay. This transformer was returned to service without any further intervention.

Has anyone had such experience? Is it normal for the Buchholtz relay to operate on oil surges following external faults? Any comments would be appreciated.

 

[RalphChristie]

Is the NEC (zig-zag transformer) solidly grounded? How long did it took before the Buchholtz tripped?
I have seen similar effents on our system. (although I can't recall a buchholtz trip - but I have to check it up) It seems as if the fault-current have a "shock-effect" on the NEC if it is solidly grounded.

See also thread238-100206

 

[RalphChristie]

Just to add:

It should not trip for a downstream earth-fault. It can be either due to a fault in the transformer or due to the mechanical shock during a fault

 

[132kVshunt]

The cable protection operated correctly and cleared the fault in 0.117 seconds
The Buchholz protection on the earthing initiated the tripping of the 33kV and 132kV circuit breakers to isolate the earthing transformer 0.418 seconds after the start of the cable fault and the earthing transformer was switched off 0.797 seconds after the start of the cable fault.
Note times above include delays by the recording instruments. I hope this could be of some help.

 

[prc]

Buchholtz relay can trip for external line faults.When a fault current passes through transformer,the windings violently moves up and down causing a sudden movement of oil from main tank to conservator through Buccholtz relay.This is especially so in trfs with small oil content.
Normal solution in such cases is to slightly desensitize the relay

But in the first case gas collected -a matter of concern .Is it due to internal faults (resulting from the overcurrent flow,consequent to external fault )or just release of blocked up air.You should have checked the collected gas for inflammability or analysis.Now you may do a DGA,in addition to low voltage excitation current test,for ensuring transformer integrity.

 

[mc5w]

In addition what what prc stated a ground fault can heat up the windings by quite a bit and then the heat flows into the oil which creates a fast local volume expansion. Your 6 cycle or so clear time would liberate quite a bit of heat in the windings. You have to remember that ordinary building wire can reach an internal peak temperature of 250 degrees Celsius without causing loss of insulation life or integrity. The heat spreads out very quickly.

I have also encountered an instance back in 1971 where the wiring in a house that we rented started blowing out because the licensed electrician who built the place NEVER installed antishort bushings in the ends of the armored cables. The heat liberated during the fault clearing time melted the insulation everywhere upstream of the fault.

Between the winding vibration and heating you will have some shock waves in the oil.

By the way, how come this road contractor is not using compressed air and a vacuum cleaner to excavate around your cables? I know that backhoe operators never ever see the warning ribbons above underground installations and we charge them a fortune when they dig our stuff up. Even happens to brand new wiring when we happen to be on the job and they could just ask where we buried stuff.

 

[prc]

Sorry mc5w,the oil and winding heating from a current flow for 6cycles is negligible.Even with a fault current flow of 100 cycles ,usually winding temperature reaches only 150-200C (from the normal value of 100 C).This heat in copper will not spread quickly.Heating time constant of transformer winding is 6-8 minutes and of oil is 2-3 hours.Windings oscillataes with respect to their end supports and this causes oil surge in to conservator.I have noticed this phenomena in large trfs when subjected to terminal faults.Maloperation of relay can also occur due to tank vibrations (normal when heavy fault currents flow in windings ).If the relay pipe is long and unsupported ,sometimes the slender pipe may go for sympathetic violent swings which can operate the mercury switches of the relay.

 

[RalphChristie]

Good explanation, prc

 

[132kVshunt]

Thanks all. We shall do further tests and I will keep you informed of our findings.

 

[mc5w]

I stand corrected. I should have stated that in the case of the BX cable that was blowing out, everything between the circuit breaker and the short turned into toast.

On the other hand, I am still a leery about the heat liberated during a 3 cycle or 6 cycle clear time. I have seen some stuff get pretty doggone hot during shorts. I would also be a little more worried about the integrity of that 33 KV cable if the excavation contractor is that bad at digging. You could have damage that this excavating contractor did what he could to hide.

As I stated above this excavating contractor must not be using the latest digging techniques for avoiding cable damage.