Question

For the three-bus power system shown in the figure, the trip signals to the circuit breakers $B_{1$ to $B_{9}$ are provided by overcurrent relays $R_{1}$ to $R_{9}$, respectively, some of which have directional properties also. The necessary condition for the system to be protected for short circuit fault at any part of the system between bus 1 and the R--L loads with isolation of minimum portion of the network using minimum number of directional relays is:} \begin{center} \includegraphics[width=0.5\textwidth]{06.jpeg} \end{center}

• A. $R_{3}$ and $R_{4}$ are directional overcurrent relays blocking faults towards bus 2
• B. $R_{3}$ and $R_{4}$ are directional overcurrent relays blocking faults towards bus 2 and $R_{7}$ is directional overcurrent relay blocking faults towards bus 3
• C. $R_{3}$ and $R_{4}$ are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively, $R_{7}$ is directional overcurrent relay blocking faults towards Line 3 and $R_{5}$ is directional overcurrent relay blocking faults towards bus 2
• D. $R_{3}$ and $R_{4}$ are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively

Hint:

In meshed or parallel networks, always check for the possibility of current reversal. Directional relays are only necessary at points where fault current may flow from either direction. Adding extra directional relays unnecessarily increases cost and complexity.

Answer 1

The Correct Option is D

Step 1: System description.
The network has three buses: Bus 1 (generator source), Bus 2 (junction), and Bus 3 (feeding an $R$--$L$ load). - Line 1 connects Bus 1 to Bus 2 through breakers $B_{1}$ and $B_{3}$ with relays $R_{1}$ and $R_{3}$. - Line 2 connects Bus 1 to Bus 2 through breakers $B_{2}$ and $B_{4}$ with relays $R_{2}$ and $R_{4}$. - Line 3 connects Bus 2 to Bus 3 through breakers $B_{5}$ and $B_{7}$ with relays $R_{5}$ and $R_{7}$. - Loads are connected at Bus 2 (via $B_{6}$ and $R_{6}$) and at Bus 3 (via $B_{8}$ and $R_{8}$). The source breaker is $B_{9}$ with relay $R_{9}$.

Step 2: Condition for selective protection.
The objective is to isolate only the faulty section with minimum disturbance to the healthy system. Overcurrent (OC) relays can detect excessive current, but when there are multiple sources or parallel paths, OC relays alone cannot distinguish the fault direction. In such cases, directional overcurrent relays (DOCRs) are required.

Step 3: Where are directional relays needed?
- For faults on Line 1: Current can flow from Bus 1 to Bus 2 or from Bus 2 to Bus 1 (depending on fault location). To avoid tripping from the wrong side, $R_{3}$ must be directional (blocking reverse current into Bus 2). - For faults on Line 2: Similar reasoning applies, so $R_{4}$ must also be directional. - For faults on Line 3: Only one source (Bus 1 via Bus 2) supplies Bus 3, so current direction is unidirectional. Thus, simple overcurrent relays $R_{5}$ and $R_{7}$ are sufficient; no directional relay is required. - For loads at Bus 2 and Bus 3: The fault current always comes from Bus 1; hence simple overcurrent relays ($R_{6}$, $R_{8}$) are adequate.

Step 4: Minimum number of directional relays.
To achieve selectivity with minimum directional relays: - Only $R_{3}$ and $R_{4}$ need to be directional (to discriminate between through-faults and actual line faults). - Adding more directional relays (such as $R_{5}$ or $R_{7}$) is unnecessary and would not reduce the isolated portion.

Step 5: Verify the options.
- Option (A): Incomplete, because it specifies blocking towards Bus 2 but not line-specific direction. - Option (B): Requires an extra directional relay $R_{7}$, which is unnecessary. - Option (C): Uses four directional relays, which violates the minimum requirement condition. - Option (D): Correct, since only $R_{3}$ and $R_{4}$ (on Bus 2 ends of Line 1 and Line 2) must be directional. % Final Answer \[ \boxed{\text{The correct option is (D): $R_{3}$ and $R_{4}$ as directional relays on Line 1 and Line 2.}} \]