Switchgear dangerous incident

[CGGP]

A few days ago while carrying out some basic maintenance work in a 20kV ring main unit fitted with bushing mounted current transformers the following sequence of events happened:
-While the circuit breaker was in the "open" position and the earth switch was in the "on" position. When we opened the earth switch,at the moment of doing this the bushings became momentarily live with the subsequent risk for the operator, etc.
The three cable bushings had been shorted prior to the opening of the earth switch as a further precaution.
This resulted in the operation of the protection relay which picked up a fault without reaching the trip state thus indicating to us that the bushings only became live for a very short period of time. Approximately 25ms.
Has anybody encountered this kind of problem with ring main units?
I can not name the manufacturer of the switchgear for obvious reasons.
Any information of similar type of occurrences would be of interest.
Thank you all

 

[alehman]

Do you know for a fact that the braeker operated or that current flowed? Was there any damage? What type of relay is it? Does it use voltage sensing in any way?

It seems possible that operating the earthing switch somehow caused a false operation of the protective relay.

 

[buzzp]

How long was the ground switch in the ON position? (I assume this is the shorting ground for the CT leads)
May not have been long enough, depending on the size of the CTs and the wire length size to the ground switch. Could also be induced currents from some neighboring energized cable running somewhere next to the CT leads.

 

[swgrmfg]

CGGP:

Assuming that your main is a vacuum circuit breaker,
and the opposite to cable side was live....

if you visualize a capacitor in parallel with each vacuum interrupter you'll see that the grounding switch coupled your system to a virtual neutral point through these capacitors. Opening the switch floated the neutral point to a different potential, and subsequently current flows through the CT's (C.dV/dt).

 

[Mazoon]

Be careful quite RMU like a bomb!!

I tried to imaging the situation, if somebody could correct me that fine, I think one of the following two possible problems is exist.

1. The RMU oil insulation is not in good condition or RMU tank was not topped properly, for long use of RMU, you have to maintain the RMU and change the oil.

2. Check the earth switch; it might be the knife / contacts is not in proper position, which could cause such a problem.

 

[panou]

Hi all
Can you tell me what are the main considerations of IEC in order to have an MCC panel with protection class IP 20 ?
Thanks in advance for your time.

 

[matwong]

Mazoon is right. It is no joke for RMU (especially at 20kV) to have this abnormal symptom. It can be either low insulation (oil or SF6-low gas) or mis-alignment of the contact blades. Have you tried on other similar unit ? Check with the manufacturer for explanation.