Protection Riddle No.99 - Why REF is set to time delayed tripping?
I was wondering why REF is set to time delayed tripping of around 200 ms as REF will only cause tripping when the part of winding has grounded and obviously REF wont trip for imbalance and other factors which causes other type of differential relays to trip?In that case it should be set to instantaneous.isnt it?
Secondly, i want to know that if high impedance differential is employed for over-coming C.T saturation problem,Miss-match b/w CT's and different magnetizing curves of C.Ts, Same problems can be faced in simple or low impedance differential as well.Then why main and other differential relays are not set as high impedance too?

#1
Sat, February 23rd, 2013 - 10:07
I don’t know about 200 ms time delay, as I have heard, in case of internal fault the differential current is going up to the knee point of the main current transformer being involved in REF protection. The ratio of knee point voltage to stability voltage determines the tripping time of that protection. This ratio should be chosen higher than 2 in order to ensure tripping times of less than 2 cycles at 50 HZ. Typical tripping time as a function of mentioned ratio can be taken from the following diagram.

The most important difference between classical high impedance REF protection and new low-impedance REF protection is the input impedance. As with all numerical relays, the input impedance of the low-impedance REF is very low compared to high-impedance relays. For example, a low impedance relay typically has an input impedance of 0.1 VA.
At 1 A nominal rating, this computes to 0.1 W. On the other hand, for a high-impedance REF relay with a voltage setting of 100 V and a 20 mA operating current, the input impedance is 5 kW. This is a significant difference.
Low-impedance REF protection does not have the same inherent stability against CT saturation for external faults as does high-impedance REF protection.
A second significant difference is that the operating current of the low-impedance REF protection is not realized by CT connection. With low-impedance REF, the relay measures all four CTs necessary to realize the element.
A very important advantage of low-impedance REF protection
is the fact that the CT ratios for the phase CTs and neutral
CTs do not have to be the same.
Most low-impedance REF relays use an operating and a restraint current. The difference between different relays from different manufacturers lies in the way these relays determine the restraint quantities and in the CT saturation detection algorithm of each relay. Note that, in the case of low-impedance REF protection, there is no inherent immunity to CT saturation, as is the case with high-impedance REF protection. The following different methods are used to determine the restraint and operating current:
1. Use of the residual current Ir = Ia + Ib + Ic as the restraint current and the differential current Id = Ia + Ib + Ic – In as the operating current.
2. Use of the residual current Ir = Ia + Ib + Ic as the operating current and the neutral current In as the restraint current.

References:
1- A Comparison Between High-Impedance and
Low-Impedance Restricted Earth-Fault
Transformer Protection
Casper Labuschagne, Schweitzer Engineering Laboratories, Inc.
Izak van der Merwe, Eskom Enterprises

2- ALSTOM MICOM 30 series (Restricted earth fault protection application guide)