Protection Riddle No.45 - Relay setting of new feeders
A ring of transformers supplied from main substation using 2 feeders. The main substation feeding from 15MVA transformer. We decided to chang the supply from main substatin to another one witch feeding from 5MVA transformer. How can we changing the setting of the incoming feeders?

#1
Sun, August 9th, 2009 - 19:18
Below figure shows a simple ring main, with a single infeed at Bus A and three load busbars which I think is very close to your system conceptually. I transfer this example from ALSTOM PROTECTIVE RELAYS APPLICATION GUIDE in here; you can try to new setting calculation with following this example.
Settings for the directional relays R2- R7 and non-directional relays R1/R8 are required. Maximum load current in the ring is 785A (maximum continuous current with one transformer out of service), so 1000/1A CT’s are chosen.
The first step is to establish the maximum fault current at each relay location. Assuming a fault at Bus B (the actual location is not important), two possible configurations of the ring have to be considered, firstly a closed ring and secondly an open ring. For convenience, the ring will be considered to be open at CB1 (CB8 is the other possibility to be considered, but the conclusion will be the same).

Three-phase fault currents I1 and I’1 can be calculated as 2.13kA and 3.67kA respectively, so that the worst case is with the ring open (this can also be seen from consideration of the impedance relationships, without the necessity of performing the calculation).

Above Table shows the fault currents at each bus for open points at CB1 and CB8.
For grading of the relays, consider relays looking in a clockwise direction round the ring, i.e. relays R1/R3/R5/R7.

For R7 relay:

Load current cannot flow from Bus D to Bus A since Bus A is the only source. Hence low relay current and TMS settings can be chosen to ensure a rapid fault clearance time. These can be chosen arbitrarily, so long as they are above the cable charging current and within the relay setting characteristics. Select a relay current setting of 0.8 (i.e. 800A CT primary current) and TMS of 0.05. This ensures that the other relays will not pick up under conditions of normal load current. At a fault current of 3376A, relay operating time on the SI characteristic is:

For R5 relay:

This relay must grade with relay R7 at 3376A and have a minimum operation time of 0.54s. Relay R5 current setting must be at least 110% of relay R7 to prevent unwanted pickup, so select relay R5 current setting of 0.88 (i.e. 880A CT primary current).

Relay R5 operating time at TMS = 1.0