50/51 Differential

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mbrooke

mbrooke

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What would be the cutoff for differential relays without operate and restrain ie feeding a differential zone into an IAC or classical over current relay instead of an SEL487 or B90?
 
mbrooke said:
What would be the cutoff for differential relays without operate and restrain ie feeding a differential zone into an IAC or classical over current relay instead of an SEL487 or B90?
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You just won’t have the same error bounds. But no reason to use those SELs if you settle for say a Basler.The SELs are for far more complicated cases.
 
paulengr said:
You just won’t have the same error bounds. But no reason to use those SELs if you settle for say a Basler.The SELs are for far more complicated cases.
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Explain. My understanding is that for 50/51 diffy you need to know at what point your CTs will saturate.
 
I’m not quite understanding the question..
IAC is 50/51. There isn’t 87 protection there.
BDD is old 87 relay, and operate and restraint is the way it works..
 
Conceptually 87 relaying means the sum of the currents within a zone stays at zero or nearly so. If you wire your CTs in series by superposition they sum to zero. So tripping on any current over a threshold is an 87 relay. But it has no restraint, etc., so the limit is absolute and not relative to the absolute currents. It’s not a scheme I recommend. I’ve run into it in old 1960s vintage partial bus differential relaying on main-tie-main protection using IAC relays.
 
paulengr said:
Conceptually 87 relaying means the sum of the currents within a zone stays at zero or nearly so. If you wire your CTs in series by superposition they sum to zero. So tripping on any current over a threshold is an 87 relay. But it has no restraint, etc., so the limit is absolute and not relative to the absolute currents. It’s not a scheme I recommend. I’ve run into it in old 1960s vintage partial bus differential relaying on main-tie-main protection using IAC relays.
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Bingo! Now your thinking!

Can you tell me more about the 1960s scheme you encountered?
 
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