50/51 Differential

[mbrooke]

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?

 

[paulengr]

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?

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.

 

[mbrooke]

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.

Explain. My understanding is that for 50/51 diffy you need to know at what point your CTs will saturate.

 

[Hv&Lv]

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..

 

[paulengr]

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.

 

[mbrooke]

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..

Right. 50/51 is 87 operate without the restraint.

 

[mbrooke]

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.

Bingo! Now your thinking!

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