Electrical Supply and Distribution Systems

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Please refer to the attachment.

Solution:
1 (a) The entire transmission and distribution system is equipped with circuit breakers (CBs) and protective relays (to operate the CBs) at strategic locations to protect the system from damage or collapse in case of a fault at any point in the system. Hence, as soon as there is a fault at any point in the system, several over current relays in the system start sensing abrupt increase in the current levels and are initiated towards the process of operating the corresponding circuit breaker. However, ideally only that minimum number of CBs should operate which is required to isolate the faulty section(s) of the system. Operation of any other CB is counterproductive and superfluous leading to unnecessary disruption of power supply in the healthy sections of the system. This ideal situation is sought to be ensured by means of what is called "time discrimination". The overcurrent relays do not operate the corresponding circuit breakers instantly; they operate the CBs with a time delay, the magnitude of time delay being such that nearer the relay to the fault location, faster it operates. Hence, a relay nearest to the fault location will operate first isolating the fault. Other relays farther away, with the currents returning to normal levels after isolation of the fault will get reset. However, if due to any reason (say relay being faulty), the nearest relay to the fault location fails to operate, the second nearest relay operates isolating the fault (and some section of the healthy system) but sparing the rest of the healthy system from collapse.
(b) The fault current flows through the relays A, B and C. As the relay C is nearest to the fault, it will operate first isolating the fault.
(c) Following is the single line diagram of a ring feeder (for detailed description of a ring feeder please see the next answer).
A B
Fault
A' B'
Relays A,B and A',B' protect two parallel segments of the ring (there could be many more relays depending upon the size of the system). Let us consider a fault at the point shown in the diagram. Directions of fault current through different relays are indicated by arrows. As can be seen, the directions of fault current through relays B and B' are opposite to each other. If these relays are non-directional type (relays which operate for fault current flow in either direction), both the relays will operate and isolate both the parallel segments. However, the main advantage of a ring arrangement is that isolation of any segment of the ring does not disrupt flow of power to any part of the ring. Hence, the relays B and B' must be directional type such that for the fault condition shown, relay B should operate and B' should not operate (as can be seen, for a fault on the other segment, the directions of current through B and B' will reverse and B' will operate and B will not operate).

2(a) Electric power can be distributed from a distribution point (say a substation or the home distribution ...