Why not have 2 bakups for Protective Relays?
Why not have 2 bakups for Protective Relays?
(OP)
I was wondering why utilities don't have 2 backups to their primary relay. If one relay had to be pulled out for whatever reason they would not have to replace it because they would still have 2 relays. Wouldn't this save them money? How often does a relay get pulled out? What would the cost be to install the 3rd relay? The commissioning time would be a lot less. Thanks for your input.






RE: Why not have 2 bakups for Protective Relays?
I'm not following your logic. If a relay has to be pulled out, they still have functioning protection. Assuming a failure of the remaining relay while one is out of service is a double contingency and normally no one can afford to design for that.
The commissioning time would be greater because there would be 50% more things to test and 50% more problems to troubleshoot.
Also, you are assuming that the only failure mode is the relay. This is not the case. For true redundancy, each relay would need its own CT and PT inputs, its own breaker trip coil and its own battery. Outside of nuclear power plants, this is not practical.
Breaker failure schemes provide backup protection, although it is slower than the primary protection.
To be honest, a lot of protective systems don't even have one backup and just depend on the next device upstream to eventually trip.
David Castor
www.cvoes.com
RE: Why not have 2 bakups for Protective Relays?
RE: Why not have 2 bakups for Protective Relays?
Bill
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"Why not the best?"
Jimmy Carter
RE: Why not have 2 bakups for Protective Relays?
Its depend on the voltage level and countries rules.
Its depend on the protected object.
For example:
lines 220/330/500kV one main protection and two back-ups protection.
big transformers, one 87T , second 50/51 and next 87N.
But for the MV, waht David and Bill saied...depend
Good Luck.
Slava
RE: Why not have 2 bakups for Protective Relays?
Back-up protections are still common to see, but we do not rely on them to provide redundancey.
Mark
RE: Why not have 2 bakups for Protective Relays?
The problem with duplicating things is the maintenance and cost. Having two circuit breakers, is expencive. Having two batteries is expencive and ups the maintenance. Having two PT's also increases your chances of failure.
Having two communications schemes becomes problomatic, expencive, and also ups the maintenance.
There are limits, and we prefer to make the limit the impact on the system, and cost.
RE: Why not have 2 bakups for Protective Relays?
RE: Why not have 2 bakups for Protective Relays?
There are two philosophies on wheather or not to use the same or different manufactures for duplicated systems. I think I do prefer using different manufactures when it is available, but arguments can be made for both sides.
Mark
RE: Why not have 2 bakups for Protective Relays?
The issue with common mode failure is where do you draw the line? Few people will install totally redundit systems as they cost to much.
RE: Why not have 2 bakups for Protective Relays?
RE: Why not have 2 bakups for Protective Relays?
That being said, different situations call for different approaches. When dealing with critical system elements redundancey is a bigger issue than when dealing with the LV side of a station.