Interlocking schemes

Introduction: In power system following switchgears are used for operation purpose:

1. Circuit Breaker (CB): It has capability to close or open electric circuits under load or fault conditions. 
2. Disconnector (or Isolator): It can close or open electric circuits under no load condition.
3. Earth switch: It is used to earth primary conductors for maintenance purposes.

Above description leads to following requirements:

1. CB can be opened or closed on no load or full load or fault condition. Therefore, no check is required for disconnector or earth switch status. However, for safety and security of grid, synchronization check is carried out in grids.
2. Disconnector should be operated only when load current is not flowing, i.e. CB should be open. Further, if earth switch is closed, closing of disconnector may cause fault. Therefore, before closing disconnector, associated earth switch status needs to be checked.
3. Earth switch should be operated only when primary conductor is already de-energized. Therefore, status of associated disconnectors needs to be checked. In case of long transmission lines, that may be charged from remote end, status of remote end switchgear may not be available. In this case presence of voltage can be checked for interlocking.

For example, let's take a simple single bus bar scheme:

• 189A can be operated when: 152 is open, Bus earth switch is open
• 189B can be operated when: 152 is open
• 289A can be operated when: 252 is open, Bus earth switch is open
• 289B can be operated when: 252 is open
• Bus earth switch can be operated when: 189A is open, 289A is open

For one and half breaker scheme:

Disconnector 389A can be operated when:

• CB 352 is open
• Earth switch 389AE and 389BE is open
• Bus-2 Earth switch is open

Disconnector 389B can be operated when:

• CB 352 is open
• Earth switch 389AE and 389BE is open

Disconnector 389L can be operated when:

• CB 352 & CB 252 is open
• Earth switch 389LE is open

ES 389AE can be operated when:

• Disconnector 389A & 389B is open

ES 389BE can be operated when:

• Disconnector 389A & 389B is open

ES 389LE can be operated when:

• Disconnector 389L should be open
• There should be no voltage in line