Earthing (grounding) transformer trip
Earthing (grounding) transformer trip
(OP)
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.
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.






RE: Earthing (grounding) transformer trip
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
RE: Earthing (grounding) transformer trip
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
RE: Earthing (grounding) transformer trip
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.
RE: Earthing (grounding) transformer trip
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.
RE: Earthing (grounding) transformer trip
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.
RE: Earthing (grounding) transformer trip
RE: Earthing (grounding) transformer trip
RE: Earthing (grounding) transformer trip
RE: Earthing (grounding) transformer trip
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.