Below question involves power protection relay For the syste

Below question involves power protection relay

For the system shown in Figure 2, the fault at F produces these differing \"espouses at various times. They are listed below: R_3 B_3 and R_4 B_4 operate; R2 82, R3, R4 84, and R5 85 operate; R3 83, R4 84 and Ri Bi operate; R6 86. R4 84, and Ri Bi operate. B_1 to B_6 are circuit breakers. R_1 to R_6 are protective relays which provide primary protection and all necessary backup protect ions. Operation of a particular relay could be due to its primary or backup protection. Assuming that there is only one failure at the maximum for each of the cases listed, analyze each of these responses for fault at F, and discuss the possible sequence of events that may have led to these operations. Classify each of the abovementioned responses as \"correct\" or \"incorrect\" operation of the relay \"correct\" or \"incorrect\" operation (or non-operation) of the breaker

Solution

i) Mentioned that the fault F is at maximum which can be considered as PERMANENT FAULT. Any relay first operates to check whether it is Transient , Semi-Permanent or Permanent Faults.
   Case (a): R3 B3 & R4 B4 operate
This case the fault occurance is sensed by relays R3 & R4 first depending upon the type of relay used and they send signal to the corresponding breakers resulting in the isolation of section onsisting of fault.
  Case (b) : Occurance of this case results in introduction of subtransient currents into the bus bar resulting in decreasing the reliability of system. Here R3 sensed the fault but B3 didnt operate indicating that either the fault current is less than the operating value of B3. R5 B5 makes sure that these subtransient currents doesn\'t enter into the subsection.
Case (c): Fault occurance is near to R3 hence it operates so does R4 and B4 . But in this case even R1 and B1 operate results in black out of the section preceeding to them. Point to be noted is that R2 didnt operate means the fault might be a temporary fault and well in control. Hence before R1 sensing the fault itself there are chances that it might be cleared by auto reclosers present at the bus bar.
   Case (d): In this case the fault is first sensed by R6 which indicates that the fault components or transient currents might have already entered into the system i.e., Grid. This case of operation might result in blackout as the relays R3 which is near to fault is not sensing it.

ii) 1. Correct Operation of relays Case (a) , (b) and (c)
Incorrect operation Case (d)
   2. Correct operation of breaker Case (a) and (c).
   Incorrect operation Case (b) and (d).

iii) Case (a): In this case only the fault section is isolated hence there will not be any loss of dependability or loss of security.
   Case (b) : In this case the entire subsection between the bus bars is lost resulting in loss of dependability and security as R6 section still get the power transfer from the other end.
   Case(c): In this case there was loss of dependability but there will be no kloss of security as the breaker R2B2 is entirely dependent on R1B1 operation
   Case (d) : This case results in induction of sub transient currents into the entire system hence dependability will be entirely lost.