In this scheme of protection, discrimination of time is incorporated. In other words, the time setting of relays is so graded that in the event of a fault, the smallest possible part of the system is isolated by operating the relay nearer to the fault location.
Radial feeder:
Fig. shows the radial system in which power flows only in one direction, i.e. from a generator or supply-side to load end.
Fig. also indicates the overcurrent protection provided for radial feeder by definite relays.
In a time graded protection system the time of operation of each relay is kept fixed and is independent of the operating current (i.e. magnitude of fault current).
Farther the relay location from generating station smallest will be its operating time thus the relay D has an operating time of 0.5 seconds. While another relay is successively increased by 0.5 seconds.
On the occurrence of a fault on a section beyond point 'D', the relay and circuit breaker at 'D' will clear the fault in 0.5 seconds. Meanwhile, all other relays will be inoperative because all other relays have higher operating times. In this way only section beyond D so the system will be isolated.
But due to certain reasons if the relay at D' fails to operate, then the relay at 'C' will operate after a time delay of 0.5 seconds i.e after 1 second from the occurrence of a fault.
Similarly, if the relay at C fails to operate, then the relay at 'B' will operate after a time delay of 0.5 seconds i.e after 1.5 seconds from the occurrence of a fault.
In this way, the previous relays will provide backup protection to the next one.
The radial system has a major drawback that continuity of supply cannot be maintained at receiving end in the event of a fault. Thus the radial system is used where the discontinuity of supply does not matter much,
Disadvantages of graded time Lag overcurrent relaying:
The operating time of each relay is fixed and independent of the magnitude of fault current, therefore for large fault current relay provides time lag, which may disturb the system considerably.
The method is suitable for radial lines with supply at the one end only and not suitable for ring mains or interconnected lines.
Any modification in the system does not coordinate with the previous system.
This system is not suitable for important, long transmission lines where rapid fault clearing is necessary to ensure system stability.
By using inverse time relays these disadvantages can be overcome to a reasonable extent.
Fig. shows the aver current protection of radial feeder using inverse time relays.
Inverse time relays have their operating time inversely proportional to its operating current i.e. larger the fault current smaller will be its operating time and vice versa.
By this arrangement, the farther the circuit breaker from the generating station, the shorter is the relay operating time.
Suppose a fault occurs in section QR, then relay at Q will allow breaker at Q to trip out before the breaker at 'P'.
Parallel feeders:
As mentioned above radial system has a major drawback that continuity of supply cannot be maintained at receiving end in the event of a fault.
Therefore to overcome the above drawback two feeders may be installed in parallel to maintain continuity of supply.
Fig. shows the system where two feeders are connected in the parallel between the generating station and substation.
Referring to the above figure if a fault occurs on any one feeder (say on feeder 1 at point F), then this feeder is disconnected from the system, and continuity of supply is maintained from the other feeder (feeder 2).
In this type of protection scheme, each feeder has a non-directional overcurrent relay at the generator end and have inverse time characteristic.
Along with this, each feeder has an inverse power or directional relay at the substation end.
Directional relays should be of an instantaneous type and operate only when power flows in the reverse direction i.e. in the direction i.e. of the arrow at P and Q.
On the occurrence of a fault on feeder 1, the fault current is fed by two routs
(a) Directly from feeder 1 via the relay A.
(b) From feeder 2 via B, Q substation, and P
Therefore only circuit breakers at A and P should open to clear the fault and circuit breakers at B and Q of feeder 2 should remain intact to maintain the continuity of supply.
Since the power flow in the relay, Q is in a normal direction but is reversed in the relay P. This causes the opening of the circuit breaker at P.
Also, relay A will operate while relay B remains inoperative. It is because these relays have inverse time characteristics and the current flowing in relay A is in excess of that flowing in relay B.
Thus opening the circuit breakers at points A and P only the faulty feeder (feeder 1) is isolated.
Ring main system:
Fig. shows the single line diagram of a typical ring main system.
The above figure consists of one generator G supplying to the four substations namely S1, S2, S3, and S4.
The major disadvantage of the ring system is that under faulty conditions power flows in both directions.
Therefore protection scheme should possess directional relays to detect the power flow under faulty conditions.
Referring to Fig. suppose a fault occurs at the point as shown then, it is desired that only circuit breakers at E and F should open to clear the fault whereas all other sections of the ring should remain stable to maintain continuity of supply. The above arrangement accomplishes this job.
The power will be fed to the fault via two routes viz.
1. Form generator G around s, and S2
2. From generator G around Ss and S.
It is clear that relays at A, B, C, and D as well as J, I, H, and G will operate before any other relay operates because of their lower time lag.
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