Overcurrent Protection: Discrimination by Current or Time of Operation

Overcurrent current protection is obviously implements as the first protection in electrical system. At its basic level it was just a fuse wire protection. With the advancement in technology and electrical system requirements; over current relays and breakers has evolved as protection system. We will discuss the philosophy behind adopting over current protection with the help of an example.

  Consider a power system consisting of three Sub-Stations as shown in the figure. For this electrical system we will discuss the setting philosophy for relays R1, R2 and R3. In this figure fault current shown are for 3-phase faults and calculated correctly for the shown power system.

Logic-1: Discrimination by Current

Case-A: No appreciable change in impedance.

Theoretically relay R1 expected to protect Line-A-B against fault at Bus-B. Relay R2 expected to protect Line-B-C against fault at Bus-C and relay R3 expected to protect the transformer against fault at Bus-D. Now let we have set the relay at R1 to operate whenever current exceeds 3715 Amp and likewise other relays too. However, there are two important practical points that affect this method of co-ordination

a. it is not practical to distinguish between a fault at X and a fault at Y, since the distance between these points may be only a few metres, corresponding to a change in fault current of approximately 0.1%

b. in practice, there would be variations in the source fault level, typically from 5250 Amp to  2550 Amp.  At this lower fault level the fault current would not exceed 3715 A, even for a line fault close to B (say at point X).  A relay set at 3715 A would not protect any part of the line section concerned.

Discrimination by current is therefore not a practical proposition for correct grading of R1 and R2.

Logic-2: Discrimination by Time of Operation

As explained in previous section it is not possible to discriminate between fault at X and Y by current measurement. The alternative to this is discrimination by time of operation. Hence to protect the Line-A-B by relay R1 and Line-B-C by relay R2 we will adopt its operational current setting equal but operational time setting different (t1 and t2 respectively). Let operational current for relay R1 and R2 is kept same and considering safety margin 1.5 times for fault at Bus-C it would be 3385/1.5 = 2256 A with t1 > t2 it will discriminate the operation of relay R1 for fault on Line-A-B and operation of relay R2 for fault on Line-B-C.

The practical problem with such type of discrimination are

a)    Irrespective of fault current; time of relay operation is same. Actually high current faults are sever and needs to be cleared early.

b)    Relay near the source has higher time of operation. Actually whenever the relay is near the source, the element protected by the relay exposed to more fault current hence needs to be cleared early.