Under Voltage Protection of 6.6 kV Bus

so what is your problem now??

What is the use of 3rd relay 27N , having different type and range?

The 27A and 27B relays are for undervoltage protection of the bus. You have not stated what their actual pickup setting or time delay is, but I expect it is at the upper end (80% of nominal bus voltage) and 30-120 cycles, maybe somewhat more.

The 27N is usually used in ground fault detection schemes, often in conjunction with a 59N (neutral overvoltage) relay. The "N" gives it away - for neutral. See the following link, page 13 for an explanation of typical use:

http://www.beckwithelectric.com/main/doccenter/techpapers/Impact-of-Green-Power-Generation-on-Distribution-Systems.pdf

You should check the schematics of your system to see how the relays are connected to their respective VTs. That will tell you a lot.

Peter, thanks for ur interest in the topic. Actually, i think, i need to specify my problem more. In the bus pt of 6.6 kv board, VT of ratio 6.6kv/(sq.rt 3) to 110v/(sq.rt 3) is used. 27A is connected between R & y phase of sec. of PT, 27B is between B & R, and 27N between Y & B. That is, I m not able distinguish 27N from 27A & B with the help of schematic drawing. So, why 27N is used, though 27A & B is sufficient and why 27N has different range? I hope I m clear now. Waiting for the reply.

You have given the range of settings available in each relay, but what is the actual pickup value? The time delay? Also, from your schematics, where do the output contacts of the relays go? What do they operate? That will tell you much about their function. Do they all trip the same circuit breaker(s)?