There is a one circuit (one voltage source), two phases and

[lavetzzi]

Hello Electricians,

This is my first post and I'm glad to be one of this forum member.
However, to clarify my question. I've a "Skin Effect Heating System", fed by 1240 V. The circuit is quit simple, Phase U, Neutral in the middle and Phase V. Phase U and Neutral feeding the heating cable#1 (Length 2100 m), and Phase V and Neutral feeding the heating cable#2 (Length 2100 m). I'm facing a small problem, the amp meter for Phase U shown 234 A and Phase V shown 236 A. Nothing affected everything is normal the 87T relay (Unbalance current relay) didn't operate and the system working well. Now my question is, why do I've Amps difference knowing that the both phases fed by one voltage source? Is it because the heating cable maybe aren't the same length accurate?


Thank U

 

[telectrix]

YOU COULD TRY MEASURING THE RESISTANCE OF THE HEATING CABLES. oops caps. lock. a small difference could be the reason for the 2A difference.

 

[marconi]

Also measure the U to N and V to N voltages at the supply end and the load end of the cable.

 

[davesparks]

There may be a tiny variation in the manufacturing tolerances of the heating cable which results in the very slight difference you have.

They are pretty long cables, with a relatively small difference between them.

 

[Julie.]

87t is specifically a transformer differential relay, so would be connected to cts on the primary and secondary of the transformer, usually with settings for the phase shift of the windings (older ones connect the current transformers in phase shift), this would only detect faults within the transformer and connections itself, not any external load.

If it's actually a 87/60 relay - single high impedance differential, then this could be connected in a multitude of ways.

If it's connected as a single ct around the outgoing cable u, v & n then it would only see an unbalance between all three - which you don't have (87gf or 87rgf).

Again if it's connected with cts in parallel u, v & n then it would not operate as all three conductors add to zero - this is a sensitive arrangement, so there needs to be a stabilising resistor fitted, calculated based on the setting, this is likely to be more than 2A anyhow (87 or more usually 60gf)

If the differential relay is just over phase conductors only then a two phase - neutral load would not sum to zero and any 60 or 87 relay would operate/trip.

In addition, 2 in 235A is a small error, less than 0.9% the voltages could be this much out, or indeed the current transformers themselves could have errors greater than this.