Time-Graded Protection of Transmission Lines (Applications) | Electricity

The application of time-graded protection to radial feeders, parallel feeders, and ring main system will be discussed here:

Application # 1. Protection of Radial Feeders:

The main characteristic of a radial system is that power can flow in one direction only, from generator or supply end to the load end. It has the drawback that continuity of supply cannot be maintained at the load end in the event of fault.

In radial system when number of feeders are connected in series, is desired that the smallest possible part of the system should be off. This is conveniently achieved by employing time-graded protection. In this system the time setting of overcurrent relays is adjusted in such a way that farther the relay from the generating stations the lesser the time of operation. When the fault occurs on the line far side SS₄ the relay OC₅ should operate first and not any other, i.e., the time required to operate the relay OC₅ should be less than that required for relay OC₄.

Similarly the time required to operate the’ relay OC₄ must be less than that required for OC₃ and so on. This shows that the time setting required for these relays must be properly graded. The minimum interval of time which can be allowed for the two adjacent circuit breakers depends on its own clearance time, plus a small time for safety margin. With normal circuit breakers in use minimum discriminating time between adjacent breakers should be about 0.4 second. The time settings for relay OC₁, OC₂, OC₃, OC₄ and OC₅ will be 2.0 seconds, 1.5 seconds, 1.0 second, 0.5 second and instantaneous respectively.

In addition to this grading, it is also essential to have the time of operation dependent on the severity of fault. For severe fault the time of operation should automatically be less. This is achieved by using time-limit fuses in parallel with the trip coils. Its additional advantage is that the relay will not operate under normal overload conditions of very short duration. The graded time lag relays when connected in series require that their time current characteristics are similar in shape. In no case they should cross each other at any point. From this point of view induction type inverse definite minimum time (IDMT) relays are most suitable. Their ratings provide complete discrimination under all fault conditions and they are most widely used.

Time-graded overcurrent protection for phase faults is supplemented by time-graded earth fault protection. The earth-fault relay is residually connected. In general, two relays are used for phase faults and one for earth fault.

The drawbacks of graded time lag overcurrent protection are given below:

1. Time lag is to be provided which is not desirable on short circuits.

2. This scheme is suitable for radial feeders with supply at one end only; not for ring mains or interconnected lines.

3. It is difficult to coordinate and requires changes with the addition of loads.

4. It is not suitable for important long distance transmission lines where rapid fault clearance is necessary to ensure stability of the systems.

Application # 2. Protection of Parallel Feeders:

For important installations continuity of supply is a matter of vital importance and at least two lines are used and are connected normally in parallel so as to share the load. These lines may or may not run on the same towers or the same right of way. Special means are available for protecting such parallel feeders. In the event of a fault occurring, protective device will select and isolate the defective feeder while the other instantly assumes the increased load.

The simplest method of obtaining such protection is by providing time-graded overload relays with inverse time characteristic at the sending end and instantaneous reverse power or directional relays at the receiving end.

When heavy fault or short circuit occurs on any one feeder, say at F on feeder No. II, the power is fed into the fault from the sending end as well as from the receiving end bus-bars. The direction of power flow will be reversed through the relay on D, which will open. The excess current is then confined to B until its overload relay operates and trips the circuit breaker, thus completely isolating the faulty feeder and supplying power through healthy feeder.

This method is not satisfactory unless the fault is heavy and reverses the power at D. Hence differential protection in addition to overload protection must be provided to both ends of the parallel feeders. A differential overcurrent relay is a centre zero induction type of instrument in which a pivoted spindle carries two discs. The torques on the two discs are caused by the currents flowing in the two feeders, and are in opposition. Under normal condition the currents in the two feeders will be equal and, therefore, torque acting on the relay will be zero. But in the event of fault, current in one of the feeders will increase and thus cause the relay to operate.

Application # 3. Protection of Ring Main System:

The ring main is a system of interconnection between a series of power stations by an alternative route. The peculiarity of the ring main is that direction of power flow can be changed at will, particularly when an interconnector is used, consequently, the reverse power relay will not serve the purpose in this system. In case the reversal of power does not take place, reverse power relays can be employed.

An elementary diagram of such a system is shown in Fig. 5.6 where G is the generating station and A, B, C and D is substations. At the generating station, the power flows only in one direction i.e., away from the bus-bars so non-directional time-lag overload relays are used.

The time-graded directional relays are used at both ends of the substation and they are set so that they will only trip when an overload flows away from the substation which they protect. Going round the ring in the direction GABCD; the relays on the farther side of each station are set with decreasing time lags; for instance at generating station 2 seconds at stations A, B, C and D 1.5 seconds, 1.0 second, 0.5 second and instantaneous respectively.

Similarly going round the ring in the opposite direction, the relays on the outgoing sides would be set as follows:

Generating station G = 2 seconds; substations D = 1.5 seconds; C = 1.0 second; B = 0.5 second and A instantaneous. Now remembering that, the reverse-power relays are set so as to operate only when power flows away from the substation at which they are installed, a study of the diagram will make it clear that a short circuit occurring at any point on the system will cause only the two immediately adjacent circuit breakers to operate.

If a fault occurs at point F, the power is fed into the fault through two paths ABF and DCF. The relays to operate are that between substation B and fault point F and substation C and fault point F. Thus fault on any section will cause the relays on that section only to operate and the healthy sections will be operating uninterruptedly.

Limitations:

In this system necessary stability of operation is achieved at the cost of slightly longer time intervals between adjacent relays. The time grading cannot be closer than 0.33 or 0.5 second and as the longest time that can be fed is 2 to 2.5 seconds, the maximum number of sections that can be protected in this way are six.

In order to overcome the above limitations, an interlock system that can be employed for any number of sections with a very short time delay has been designed. It may be employed for radial or ring main systems. Figure 5.7 illustrates the application of this method to a radial system.

If a section is healthy the same current flows at both ends and the overload relays operate. The operation of the relay completes the circuit of the trip coil (contact B), but the operation of the relay at the end of the section closes contact A, and causes the locking relay to operate and thus break the trip circuit. When the fault occurs within the section, the current entering is high, and causes the contact B to close, but the current leaving is small, and the locking relay does not operate. It is necessary that locking relay shall be capable of operating before contact B is closed, and as such the relay B is set to operate in 0.3 to 0.5 second.

For application of this method to a ring main or any interconnected system, it is necessary merely to have directional relays to close the circuit for the locking relays.