Overcurrent protective schemes are widely used for the protection of distribution lines. A radial feeder may be sectionalized and two or more over-current relays may be used, one relay for the protection of each section of the feeder, as shown in figure 2.9. If a fault occurs beyond C, the circuit breaker at substation C should trip. The circuit breakers at A & B should not trip as far as the normal operation is concerned. If the relay at C fails to operate, the circuit breaker at B should trip as a back-up protection.
Similarly, if a fault occurs between B & C, the circuit breaker at B should trip; the circuit breaker at A should not trip. But in the case of failure of a relay and / or the circuit breaker at B, the circuit breaker at A should trip. Thus, it is seen that the relays must be selective with each other. For proper selectivity of the relays, one of the following schemes can be employed, depending on the system conditions.
Time graded system
Current graded system
A combination of time and current grading
Time Graded System:
In this scheme, definite time overcurrent relays are used. When it operates for a fault current, it starts a timing unit which trips the circuit breaker after a preset time, which is independent of the fault current. The operating time of the relays is adjusted in increasing order from the far end of the feeder, as shown in figure 2.9. The difference in time setting of two adjacent relays is usually kept at 0.5 second (0.4 or 0.3 second with fast circuit breakers and modern accurate relays). This difference is to cover the operating time of the circuit breaker and errors in the relay and CT.
Fig. 2.9 Time graded overcurrent protection of a feeder
When a fault occurs beyond C, all relays come into action as the fault current flows through all of them. The least time setting is for the relay placed at C. so it operates after 0.5 second and the fault is cleared. Now the relays at A and B are reset. If the relay or circuit breaker at C fails, the fault remains uncleared. In this situation, after 1 second the relay at B will operate and the circuit breaker at B will trip. If the circuit breaker at B also fails to operate after 1.5 second the circuit breaker at A will trip.
The drawback of this scheme is that for faults near the power source, the operating time is more. If a fault occurs near the power source, it involves a large current and hence it should be cleared quickly. But this scheme takes the longest time in clearing the heaviest fault, which is undesirable because the heaviest fault is the most destructive.
This scheme is suitable for a system where the impedance (distance) between substations is low & the fault current is practically the same if a fault occurs on any section of the feeder. This is true for a system in which the source impedance ZS is more than the impedance of the protected section Z1. If the neutral of the system is grounded through a resistance or impedance ZS is high and ZS / (ZS + Z1) is not sufficiently lower than unity. In this situation, the advantage of inverse time characteristics can not be obtained. So definite relays can be employed, which are cheaper than IDMT relays.
Current Graded System:
In this scheme, the relays are set to pick-up at progressively higher values of current towards the source. The relays employed in this scheme are high set (high speed) instantaneous overcurrent relays. The operating time is kept the same for all relays used to protect different sections of the feeder, as shown in figure 2.10. The current setting for a relay corresponds to the fault current level for the feeder section to be protected.
Ideally, the relay at B should trip for faults anywhere between B and C. But it should not operate for faults beyond C. similarly, the relay at A should trip for faults between A and B. the relay at C should trip for faults beyond C. this ideal operation is not achieved due to the following reasons:
2.10 Instantaneous over-current protection of a feeder
The relay at A is not able to differentiate between faults very close to B which may be on either side of B. If a fault in section BC is very close to station B, the relay at A ‘understands’ that it is in section AB. This is due to the fact that there is very little difference in fault currents if a fault occurs at the end of the section AB or in the beginning of the section BC.
The magnitude of the fault current can not be accurately determined as all the circuit parameters may not be known.
During a fault, there is a transient condition and the performance of the relays is not accurate.
Consequently, to obtain proper discrimination, relays are set to protect only a part of the feeder, usually about 80 %. Since this scheme can not protect the entire feeder, this system is not used alone, but in conjunction with IDMT relays, as shown in figure 2.11
This scheme is used where the impedance (distance) between substations is sufficient to create a margin of difference in fault currents. For such a system ZS is smaller compared to Z1.
The advantage of this system as compared to the time graded scheme is that the operating time is less near the power source.
Combination of Current and Time Grading:
This scheme is widely used for the protection of distribution lines. IDMT relays are employed in this scheme, which have the combined features of current and time setting arrangements. The current setting of the relay is made according to the fault current level of the particular section to be protected. The relays are set to pick-up progressively at higher current levels, towards the source. The difference in operating times of two adjacent relays is kept 0.5 second.
An inverse time-current characteristic is desirable where ZS is small compared with Z1. If a fault occurs near the substation, the fault current is
I = E / ZS
If a fault occurs at the far end of the protected section, the fault current is
I = E / (ZS + Z1)
If Z1 is high compared to ZS, there is an appreciable difference in the fault current for a fault at the near end and for fault at the far end of the protected section of the feeder. For such a situation, a relay with inverse time characteristic would trip faster for a fault near a substation, which is a very desirable feature.
Definite time characteristic is desirable where ZS is large compared to Z1.
An IDMT characteristic is a compromise. At lower values of fault current, its characteristic is an inverse time characteristic and at higher values of fault current, it gives a definite time characteristic.