Question

For the system shown in Figure 10.44, directional overcurrent relays are used at breakers B12, B21, B23, B32, B34, and B43. Overcurrent relays alone are used at B1 and B4. (a) For a fault at P1, which breakers do not operate? Which breakers should be coordinated? Repeat (a) for a fault at (b) P2, (c) P3. (d) Explain how the system is protected against bus faults.

Short answer

Answer

(a) For fault at P1, the breakers B12, B21, B23 and B32 do not operates.

(b) For fault at P2, the breakers B12, B21, B34 and B43 do not operates.

(c) For fault at P3, the breakers B23, B32, B34 and B43 do not operates.

(d) The system protection, with respect to the fault on the various buses.

Fault bus	Breaker operates
1	B1 and B21
2	B12 and B32
3	B23 and B43
4	B4 and B34

Step-by-step solution

Write the given data from the question:

The directional overcurrent relay B12, B21, B23, B32, B34 and B43 are used at breakers.

The Overcurrent relay B1 and B4 are used alone in the given system.

Which overcurrent relay do not operate when fault occurred at P1.

(a)

The breakers under the protection zone P1 are B34 and B43.

For the fault at P1, the breaker under the protection zone P1, B34 and B43 operate and other do not operate. The breaker B34 coordinates with B23, B12 and B1. If the breaker B34 fails to operate under the fault condition only that situation other breaker operate in the same sequence.

Similarly, breaker B43 coordinates with B4. Breaker B4 operate only after the failure of breaker B43.

Therefore, for the fault at P1, the breakers B34 and B43 will operate.

Hence, for fault at P1, the breakers B12, B21, B23 and B32 do not operates.

Which overcurrent relay do not operate when fault occurred at P2.

(b)

The breakers under the protection zone P2 are B23 and B32.

For the fault at P2, the breaker under the protection zone P2, B23 and B32 operate and other do not operate. The breaker B23 coordinates with B12 and B1. If the breaker B23 fails to operate under the fault condition only that situation other breaker operate in the same sequence.

Similarly, breaker B32 coordinates with B43 and B4. Breaker B43 and B4 operate only after the failure of breaker B32.

Therefore, for the fault at P2, the breakers B23 and B32 will operate.

Hence, for fault at P2, the breakers B12, B21, B34 and B43 do not operates.

Which overcurrent relay do not operate when fault occurred at P3.

(c)

The breakers under the protection zone P3 are B12 and B21.

For the fault at P3, the breaker under the protection zone P3, B12 and B21 operate and other do not operate. The breaker B21 coordinates with B32, B43 and B4. If the breaker B21 fails to operate under the fault condition only that situation other breaker operate in the same sequence.

Similarly, breaker B12 coordinates with B1. Breaker B1 operate only after the failure of breaker B12.

Therefore, for the fault at P3, the breakers B12 and B21 will operate.

Hence, for fault at P3, the breakers B23, B32, B34 and B43 do not operates.

Determine the system protection with respect to fault on various buses.

(d)

The following table shows how the system protection works with respect to the fault on the various buses.

Fault bus	Breaker operates
1	B1 and B21
2	B12 and B32
3	B23 and B43
4	B4 and B34