Purpose of 87N Circuit

[Mbrooke]

What is the purpose of the circuit encircled in orange? What is the 35 ohm resistor for? What doesn't the 51N IAC relay on the neutral suffice, transformer differential and primary 50/51? What is this circuit looking to catch?

 

[cranky108]

Sensitive earth fault protection. The resistor is to force current through a saturated CT, and not the low impedance over current relay.
This circuit is to detect a ground fault in the transformer winding.

You can achieve the same thing with a high impedance differential relay, which is what I would use.

 

[stevenal]

Also known as restricted earth fault, or REF.
I recommend the IEEE guide to transformer protection, C37.91. For this question and some other recent ones.

 

[Mbrooke]

Got it, but why can't a ground fault be detected via the normal differential circuit?

 

[charz]

Because fault currents are very less as the faults get closer to neutral, this can be detected by REF whereas Differential may not detect this. As most of the times, the faults of those lesser magnitude comes under the slope setting.

 

[Mbrooke]

Can the primary differential be made to catch such faults? Or must you use REF, even in microprocessor relays?

 

[dpc]

The short answer is that the standard differential relay can never be as sensitive as the ground differential (and still be secure). There's nothing that says that ground diff is required, and there a many, many installations without it. But the ground diff will always be able to detect low level ground faults that the standard phase differential will not.

 

[stevenal]

The arguments against it once were the expense and complexity. I count nine relays and 14 CTs above, and still see no overcurrent protection on the wye side. Now it requires one relay and 7 CTs. The function is likely to be in the relay already, so all one needs to do is enable and test it to get the added protection.

 

[Kiribanda]

It is the stabilizing resister of REF relay/ GROUND DIFF PROTECTION relay for the high side.
But normally REF is basically applied for a transformer with an NGR so that ground faults
close to the neutral point are covered. For solid grounding transformers there is no
such uncovered portion.

 

[stevenal]

It's more important for impedance grounded transformers, but faults near the wye point of solidly grounded transformers are still difficult to detect via 87T. Your minimum pickup and slope settings are likely to put you in the no trip zone. See thread238-477392

 

[wattyeng]

87N is restricted earth fault protection REF
It is restricted to the zone protected - normally transformer LV winding and LV cable feeding the LV switchboard - the NE link and CT will be sited in the LV switchboard incomer, some distance from the transformer

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