Floating DC system and DC earth fault relay

Introduction: DC supply is widely used as auxilliary supply for control and protection system. For larger systems 110V or 220V DC supply is used. For reliability both positive and negative terminals of DC supply are isolated from ground. This is system is called floating DC system.  

Floating DC system has advantage that single earth fault will not cause any outage. Second earth fault may blow the fuse or maloperation of protection. 

DC earth fault relay: For identification of earth faults in early stages, sensative DC earth fault relay is used. 

DC earth fault relay is connected to midpoint of two high value resistors (~ 20kΩ). In normal condition as there will be no path for current to earth, there will be no current in relay element. During a DC earth fault in +ve or -ve terminal, current will flow in to earth by completing return path through DC earth fault relay element. Typical setting for DC earth fault relay is ~ 5mA.

This will cause operation of DC earth fault relay and give alarm to operator. Earth fault can be identified and rectified by maintenance staff before second earth fault. As high value resistance is used in earth fault circuit, fault current will be very low and identification of exact feeder and loaction of fault is difficult. Following methods are generally used for identification of location:

1. Switching off feeders one by one.
2. Using DC earth fault locator instrument, which inject low voltage low frequency signal of ~5Hz in DC system. Faulty feeder is identified by measuring leakage current using tuned clamp on meters.

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