The word feeder here means the connecting link between two circuits. The feeder cod be in the form of a transmission line, short, medium or long, or this could be a distribution circuit. The various methods of protecting the feeders are:
1. Over current protection.
2. Distance protection.
3. Pilot relaying protection.
of these, overcurrent protection is the simples and cheapest form of protection. It is most difficult to apply and needs readjustment, should a change in the circuit occur. This may even have to be replaced depending upon the circuit conditions.
Overcurrent relaying for distribution circuits besides being simple and cheap provides the following advantages:
- Very often the relays need not be directional and hence no ac voltage source is required.
- Two-phase and one earth fault relays are required for the complete protection of three-phase circuits as shown in Fig.
Where distance protection is costly, overcurrent protection is used for phase and ground faults on station service, electric utilities (distribution circuits) and on some sub transmission lines. Over current protection is normally used as back up protection where the primary protection is provided with distance schemes.
The discrimination using overcurrent protection is achieved in the following ways:
(i) Time graded system.
(ii) Current graded system.
(iii) Time-current graded system.
Time Graded System
The selectivity is achieved based on the time of operation of the relays. Consider a radial feeder in Fig. The feeder is being fed from one source and has three substations indicated by the vertical lines. The crosses represent the location of the relays. The relays used are simple overcurrent relays. The time of operation of the relays at various locations is so adjusted that the relay farthest from the source will have minimum time of operation and as it is approached towards the source the operating time will be increases. This is the main drawback of grading the relays in this way because it is required that the more severe a fault is, lesser should be the operating time of the relays whereas in this scheme the operating time will be increases. The main application of such a grading is done on systems where the fault current does not vary much with the location of the fault and hence the inverse characteristic is not used.
Current Graded System
This type of grading is done on a system where the fault current varies appreciably with the fault. This means as we go towards the source the fault current will be increases. With this if the relays are set to pick at a progressively higher current towards the source, then the dis-advantage of the long-time delay that occurs in case of time graded systems can be partially overcome. This is known as current grading.
Since it is difficult to determine the magnitude of the current accurately and also the accuracy of the relays under transient conditions is likely to suffer, current grading alone cannot be used. Usually a combination of the two gradings, i.e. current time grading is used.
This type of grading is achieved with the help of inverse time over current relays and the most widely used is the IDMT relay. The other inverse characteristics, e.g. very inverse or extremely inverse are also employed depending upon the system requirements are slow at low values of overloads, extremely inverse relays are used and if the fault current reduces substantially as the fault location moves away from the source, very inverse type of relays are used.
Selection of Current Setting
For proper coordination between various relays on a radial Seeder, the pickup of a relay should be such that it will operate for all short circuits in its own line and should provide backup protection for short circuits in immediately adjoining line For back equal to the value of the current when the fault is at the far end of the adjoining section with minimum generation connected to the system A 3-phase fault under maximum generation gives the maximum fault current and line-to-line fault under minimum generation gives the minimum fault current. The relay must respond between these two extreme limits. On a radial system the current setting of the relay farthest from the source should be minimum and it goes on increasing as we go towards the source. According to Indian Standard specifications the operating value should be exceed 1.3 times the setting, i.e.
Min short circuit current ≥ 1.3Isetting
Selection of Time Setting
For proper coordination between various relays on a radial feeder the operating time of the relay farthest from the source should be minimum and it should increase as we go towards the source. Referring to Fig. if the time of operation of relay 1 is say T1, that of the relay 2 say T2 then T2=T1+t where t is the time step between successive relays and consists of time of operation of C.B. at 1, over-travel of relay at 2 and the factor of safety time. Here over-travel of relay at 2 means, the travel of the relay at 2 due to inertia of the moving system of the relay even after the fault at location1I is removed. A suitable value of over-travel is 01l sec. Similarly factor of safety time is taken as 0.1 sec. The time grading should be done at the maximum fault currents because at lower values it will automatically have a higher selectivity as the curves are more inverse in that range.
The characteristics of the various IDMT relays used on system in Fig. on a simple graph should look like the ones in thick lines in Fig.2. If suppose characteristic 2 intersects 1 at a current I, this means that if the current exceeds I amperes relay at 2 will operate faster than 1 which is not desirable if the fault is in the zone of relay 1.
Therefore, for proper coordination the characteristics should not intersect.
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