Trafo electromechanical trips. 1

[slavag]

Dear Folks.
We work now on commissioning of new substation with two 80MVA trafo's Y/D with zig zag trafos in main trafo zones.
Today we tested signal from external devices:
Bucholz, oil level, oil temp., etc..
This signals are connected to Binary ( digital) inputs of transformer protection relay. It's common practic in last years.
But in this project designer and customer used new concept.
This external trips was connected via N.C. contacts to protection relay for extremly safety.
This concept is new for me.
What is your opinion? Maybe someone use same concepts?
Thanks in advance.
Best Regards.
Slava.
additional technical data.
220V dc supply.
digital input: low level voltages about 80V dc.

 

[dpc]

So the contacts OPEN to trip? This is a "fail-safe" technique that is pretty common for critical interlocks in the process control world - things like boiler safety interlocks etc. It protects against sensor failure, broken wires, etc, but at the expense of nuisance trips.

I have (almost) never seen this done for protective relaying systems. Energize-to-trip is the standard, at least in the US. Often we use fail-safe techniques for transformer alarms, but not for trips.

The overriding principle in the US is to use energize-to-trip but to take great care to make this system as reliable as possible, using station batteries, dual battery chargers, etc. In high voltage breakers dual trip coils and dual lockout relays are used to improve reliability.

 

[slavag]

DPC.
Thanks a lot for your back-up.
After discussion with designer, I start think maybe something not O.K. with me.
We use exactly same standards ( I think all EE also).
Today, w/o many discussion I change all signals to N.O. contacts.
Best Regards.
Slava

 

[dpc]

Back in the olden days of electro-mechanical control, for the boiler master fuel trip, we often used a 2 out of 3 de-energize-to-trip logic into a 1 out of 2 energize-to-trip lockout relay system.

This was done for the most critical interlocks such as boiler drum level, etc.

But it was a big pain in the neck, as I recall.

 

[slavag]

Hi.
Not standard solutions
1.We commissioned 3 breakers ( same water company ) 170kV, that have two trip coil and one NV ( control voltage 220V dc) according to customer requst.
2. 2006-2007, UAT's (25MW)of power plant: AND gate for two signal from two pressure relief relys.
3. I think same logic, 1 out 2 operated in nuclear power station.
Regards.
Slava