ADVANTAGES AND DISADVANTAGES OF DIFFERENT SUBSTATION SCHEMES COMPARISON OF CONFIGURATIONS


Below is a summary of comparison of switching schemes for substations.

A. SINGLE BUS SCHEME
Advantages
1. Lowest cost.

Disadvantages
1. Failure of bus or any circuit breaker results in shutdown of entire substation.
2. Difficult to do any maintenance.
3. Bus cannot be extended without completely deenergizing substation.
4. Can be used only where loads can be interrupted or have other supply arrangements.

B. DOUBLE BUS DOUBLE BREAKER SCHEME
Advantages
1. Each circuit has two dedicated breakers.
2. Has flexibility in permitting feeder circuits to be connected to either bus.
3. Any breaker can be taken out of service for maintenance.
4. High reliability.

Disadvantages
1. Most expensive.
2. Would lose half of the circuits for breaker failure if circuits are not connected to both buses.

C. MAIN AND TRANSFER BUS SCHEME
Advantages
1. Low initial and ultimate cost.
2. Any breaker can be taken out of service for maintenance.
3. Potential devices may be used on the main bus for relaying.

Disadvantages
1. Requires one extra breaker for the bus tie.
2. Switching is somewhat complicated when maintaining a breaker.
3. Failure of bus or any circuit breaker results in shutdown of entire substation.

D. DOUBLE BUS, SINGLE BREAKER SCHEME
Advantages
1. Permits some flexibility with two operating buses.
2. Either main bus may be isolated for maintenance.
3. Circuit can be transferred readily from one bus to the other by use of bus-tie breaker and bus selector disconnect switches.

Disadvantages
1. One extra breaker is required for the bus tie.
2. Four switches are required per circuit.
3. Bus protection scheme may cause loss of substation when it operates if all circuits are connected to that bus.
4. High exposure to bus faults.
5. Line breaker failure takes all circuits connected to that bus out of service.
6. Bus-tie breaker failure takes entire substation out of service.

E. RING BUS SCHEME
Advantages
1. Low initial and ultimate cost.
2. Flexible operation for breaker maintenance.
3. Any breaker can be removed for maintenance without interrupting load.
4. Requires only one breaker per circuit.
5. Does not use main bus.
6. Each circuit is fed by two breakers.
7. All switching is done with breakers.

Disadvantages
1. If a fault occurs during a breaker maintenance period, the ring can be separated into two sections.
2. Automatic reclosing and protective relaying circuitry rather complex.
3. If a single set of relays is used, the circuit must be taken out of service to maintain the relays. (Common on all schemes.)
4. Requires potential devices on all circuits since there is no definite potential reference point. These devices may be required in all cases for synchronizing, live line, or voltage indication.
5. Breaker failure during a fault on one of the circuits causes loss of one additional circuit owing to operation of breaker-failure relaying.

F. BREAKER AND A HALF SCHEME
Advantages
1. Most flexible operation.
2. High reliability.
3. Breaker failure of bus side breakers removes only one circuit from service.
4. All switching is done with breakers.
5. Simple operation; no disconnect switching required for normal operation.
6. Either main bus can be taken out of service at any time for maintenance.
7. Bus failure does not remove any feeder circuits from service.

Disadvantages
1. 1 1/2 breakers per circuit.
2. Relaying and automatic reclosing are somewhat involved since the middle breaker must be responsive to either of its associated circuits.

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