Micom P63X Differential Relay Setting Calculation Derivation

Unreadable....You might try dividing the paper up into 3 or 4 sections to enlarge, and darkening the lines by using paint program to rewrite...

Dear SolarEagle,

Here is the readable picture,

Surely a telephone call to the equipment manufacturer's Technical Helpline would be far quicker?

The relay manufacturer designates all applicable equations pertaining to their specific relay.

Applicable equations are based on design of the relay circuitry which is proprietary information of the relay manufacturer.

I suggest you contact the relay manufacturer technical support group otherwise you most likely will not get accurate information.

I think you have got the arithmetic right. But you did not define Is1, Ibias. K1 and K2 can be assumed as line slopes.

How the equations are arrived at?

Does that mean how the relay achieves that operating curve? I do not know this relay - it might have a set of rectifiers, filters and amplifiers to do the job or thousands of lines of software.

Does that mean "why does a differential relay have an operating current like this?

Well, even at normal steady load, differences between CTs of nominally same ratio: also plant like transformers with on-load tap changing are going to cause a differential at the relay, without any fault in the protected zone. This is the likely reason for Is1.

At many times normal load (current feeding a fault in plant outside the zone) the absolute magnitude of differential CT errors increases - to counteract this the differential trip point is biased up proportional to the magnitude of the through current. This would be the likely reason for Ibias, K1 dependent slope.

Finally, at very high through currents, serious saturation of CTs will occur causing more differential current error. Also, if one CT is seriously in error, the value of the bias current (say this is the mean of the current in the two CTs) adds more error. This may be reason for a second bias slope line using K2.

A last point, the relay is not perfect, when fed with currents from two CTs, one more distorted than the other, should the relay compare mean, rms or peak and over what time period?

67model