if the Power supply was off & some maintenance was done in the control wiring, which resulted in opening of CT secondary, then when you switch On the Power supply , the CT can explode.
Was it an FMJL oil-filled type manufactured by GEC by any chance? There have been recorded failures due to moisture ingress into the insulation of this particular type of CT.
----------------------------------
If we learn from our mistakes I'm getting a great education!
When a CT is left with open secondary you end up with:
- high level of voltage in the secondary circuit of the CT and the equipments connected to that secondary winding. This can affect people working there, equipments and magnetic core of the CT.
- noise
- heating
- you may get an explosion too, that depends of the primary winding current and the CT ratio.
What happens when a secondary winding of a CT is left open?
Primary current ---> Primary flux
Primary flux ---> E in CT's secondary
E in CT's secondary ---> secondary current
secondary current ---> secondary flux (oposite direction to Primary flux)
Resultant flux will be: Primary flux minus secondary flux
When you open the secondary winding circuit there's no secondary current and therefore no secondary flux ---> the resultant flux will be = Primary flux only and you end up with dangerous voltages.
But that's not the only reason a CT can explode. Unfortunately the oil is very hygroscopie and 20g of water can make 1t of oil useless.
May you grow up to be righteous, may you grow up to be true...
Any catastrophic failure of an oil-filled IT is going to be a result of HV insulation failure. The question is what cause the failure and there are many possibilities.
You would need to provide a lot more information to get a reasonable response.
I'll add, it's unlikely it's an open-circuit on a secondary winding.
I agree with scottf that open circuiting of a CT secondary can not cause a CT explosion.The secondary voltage developed (7-12 kV) cannot puncture the HV insulation.Explosion is consequent to HV insulation failure and power flow to ground.Many times failure is due to moisture ingress through poor oil seals, insulation slippage/damage during transit,ageing of insulation etc,etc.
I didn't say it "can't" cause failure...I said it is unlikely it is the cause.
Open circuited CTs, not only develop high voltages across the secondary (can often be <30 kVp), but it also drives the core into saturation, which can generate a lot of heat. Normally, if there are enough cores and/or enough heat, the oil expands past normal levels and a pressure relief device operates. Normally the oil leak is seen prior to failure, but I've seen some explosive failures result from this...normally due to the oil-leak not being detected in time.
I'm wondering if that first transformer energization spike may cause an internal breakdown and arc in an open circuited transformer? Such an event may quickly lead to oil degradation and a high voltage insulation failure.
That said, the probability is extremely high that there was pre-existing damage or degradation in the CT.
But we haven't heard back from the original poster. The transformer may have failed at energization, or it may have failed in normal service.
I strongly suspect that the CT had a pre-existing condition.
Failure may have been a result of progressive deteriorization or failure may have been triggered by some transient event such as a lightning strike, a switching surge or an energization surge.
An open secondary circuit may have been a supporting factor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
