To Lock Out or to Just Trip, That is the Question

[Thriggen]

Im relatively new to protective relaying and I am trying to understand existing schemes that our utility uses.

I have a bus that is feeding a transformer with a NC tie breaker on one side and what I'll call an upstream breaker on the other side of the bus. There is differential protection for the bus that gets the Tie, the upstream, and the circuit switcher leading to the transformer.

My question is that if the differential trips, we lockout the bus. Locking out the bus will ask the tie, and the upstream and the circuit switcher to trip to isolate the fault. Should those breakers also be locked out? I'm thinking they only need to be locked out if they fail, in which it would then lock out and ask their upstream breakers to trip.

To me thats the best way to do it, but I have not been in the protection business for that long. Will each utility do it different, or is there a protection bible out there that shows you the standard way of doing things. Thanks for all comments.

 

[DTR2011]

Generally Speaking..

If a device is tripped for a permanent, serious fault (transformer differential), you want to isolate (trip) all sources and block closing / reclosing until further investigation is performed. For this reason, the lock out relay is also in series with the close circuit of the devices tripped. The LOR must be reset to enable closing.

You will normally see this with any type of differential protection as
the protection scheme provides zone protection and has determined that the fault is within that defined zone. For transformers, you may also
see Sudden Pressure / Gas Accumulator relays tripping the LOR as well.

Your application sounds like a bus differential scheme. Again all sources are tripped by the LOR and block closed. If it's a bus fault, anything within the 87B Zone of protection could have caused a fault (bus, breakers, VT/CT, ...) There must be an inspection of the area to determine the cause of the fault.

When digital relays are used, sometimes the LOR is omitted and a latched trip is used as well as some logical supervision of the close circuit through the relay.

There is an excellent FAQ......check in the FAQ Section
What are good references for a Power Engineer? faq238-1287

 

[cranky108]

A lockout can be used in several ways. 1. is to cause someone to travel to the site, and look at the conditions. 2. is to make more contacts available. 3. to prevent a closure of the circuit breakers. 3. prevent reclosing.

 

[oldfieldguy]

I concur with cranky's ideas. The lockout function is, in applications I specify, used to differentiate between capacity type faults like overcurrent and undervoltage. /The lockout function is initiated by severe faults that indicate component failure.

With microprocessor relays and their ability to bring many functions together, I send instantaneous overcurrents to the lockout, as well as differential and ground faults.

In industrial applications, I further train affected personnel that in the case of an outage, the circuit cannot be reset from lockout unless an investigation is completed by knowledgeable people.

old field guy

 

[racookpe1978]

Speaking from the standpoint of somebody is being protected BY the lockout/tagout (LOTO) - rather than a person who is designing the lockout! - I REALLY REALLY don't want anybody to be able to close that breaker or re-enegerize the circuit I am working on remotely. Unless they have manually gone to the panel or LOTO point and had to physically remove the locking device.

tripping "off" is"safe" -> re-energizing remotely without removing a LOTO lock or tag is going to kill me or my workers.

 

[jghrist]

racookpe1978

You are confusing LOTO with a simple lockout relay for bus differential. A lockout relay for bus differential is not done for purposes of maintaining the equipment, but is part of the protection scheme.

 

[racookpe1978]

OK.