Over-current protection has a wide range of applications. It can be applied where there is an abrupt difference between fault current within the protected section and that outside the protected section and these magnitudes are almost constant. The over-current protection is provided for the following:
Motor protection: Over-current protection is the basic type of protection used against overloads and short-circuits in stator windings of motors. Inverse time and instantaneous phase and ground overcurrent relays can be employed for motors above 1200 HP. For small and medium size motors where cost of CT’s and protective relays is not economically justified, thermal relays and HRC fuses are employed, thermal relays are used for overload protection and HRC fuses for short-circuit protection.
Transformer protection: Transformers are provided with over-current protection against faults, only, when the cost of different relaying cannot be justified. However, over-current relays are provided in addition to differential relays to take care of through faults. Temperature indicators and alarms are always provided for large transformers.
Small transformers below 500 KVA installed in the distribution system are generally protected by drop-out fuses, as the cost of relays plus circuit breakers is not generally justified.
Protection of utility equipment: The furnaces, industrial installations, commercial, industrial and domestic equipment are all provided with over-current protection.
Line protection: The lines (feeders) can be protected by
Instantaneous overcurrent relays
Inverse time overcurrent relays
Directional overcurrent relays
Lines can be protected by impedance, or carrier current protection also. In radial feeders the discrimination is obtained by means of the relay time and current setting adjustments only. To obtain discrimination in other feeders, it is usually necessary to incorporate a directional feature in the protection, as will be clear from the following three cases: a) parallel feeders, b) tee’d feeders and c) ring feeders
Figure 2.19 shows an overcurrent protective scheme for parallel feeders. At the sending end of the feeders at A and B, non-directional relays are required. The symbolindicates a non-directional relay. At the other end of feeders at C and D, directional over-current relays are required to ensure the discrimination and these are direction sensitive such that they operate for faults occurring (away from the bus). Furthermore directional relays should operate before the non-directional relays A & B, hence given with lower time and current settings than A & B. directional and non-directional relays are graded in the same way. The arrow mark for directional relays placed at C and D indicate that the relay will operate if current flows in the direction shown by the arrow. If a fault occurs at F, the directional relay at D trips, as the direction of the current is reversed. The relay at C does not trip, as the current flows in the normal direction. The relay at B also trips for a fault at F. Thus, the faulty feeder is isolated and the supply of the healthy feeder is maintained.
2.19 Protective scheme for parallel feeder
If non-directional relays are used at C and D, both relays placed at C and D will trip for a fault at F. this is not desired as the healthy feeder is also tripped. Due to this very reason, relays at C and D are directional over-current relays. For faults at feeders, the direction of current at A and B does not change and hence relays used at A and B are non-directional.
Figure 2.20 (i) shows an over-current scheme for the protection of a ring feeder. Figure 2.20 (ii) is another way of drawing the same scheme.
2.20 (i) Protective scheme for ring feeder
Compared with radial feeders, the protection of ring feeders is costly and complex. In order to achieve discrimination for faults the relays associated with a faulty section of the ring should operate. Each feeder requires two relays. A non-directional relay is required at one end and a directional relay at the other end. It will be again noted that the relays at the source bus A & A’ need not be directional relays. Otherwise for faults close to the source bus there may be non-operation. The operating times for relays are determined by considering the grading, first in one direction and then in another direction, working backward to the power source as shown in figure 2.20. If a fault occurs at F1 as shown in figure 2.20 (i), the relays at C’ and D’ will trip to isolate the faulty feeder. The relay at C will not trip as the fault current is not flowing in its tripping direction though its operating time is the same as that of C’. Similarly, the relays at B and D will not trip as the fault currents are not in their tripping direction, though their operating time is less than the operating time of B’ and D’ respectively.
Figure 2.20 (ii) is an alternative way of drawing the same scheme. In this figure, loads, though present, are not shown on buses A, B and D so as to make the figure simple to understand. If a fault occurs at F2, the relays at A’ and D will trip.
Figure 2.20 (iii) shows a scheme involving an even greater number of feeders.
2.20 (ii) Alternative way of protection of ring feeders & (iii) protection with 4 no. of feeders
Figure 2.21 shows the case of a tee’d parallel feeder, where again for discriminative reasons the relays on the source end must be non-directional while the relays on the load end buses must be directional relays with their direction for operation corresponding to fault current flowing into the feeder. Furthermore, the directional relays are set for lower time and current settings than the non-directional ones, so that the former operate before the later.
2.21 Protective scheme for tee’d parallel feeder