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

VT supervision

Introduction: For protection relays, analogue inputs are connected from Current Transformers (CT) and Voltage Transformers (VT). VT is a voltage source, any short circuit in wiring will cause heavy current to flow in VT winding. This may result in failure of VT. Therefore, Connection from VT is always with fuse or MCB to take care of short circuit / overload.

Now it is evident that in case of fuse failure, relay will not get any voltage from VT. Some of the protections may see this as abnormal condition in system and may cause unwanted trip or may not trip in actual fault. For example:

1. Distance relays measure Impedance from Voltage and current (Z = V/I). If there is no voltage (V=0), Z will also be Zero. This will cause trip of distance protection.
2. Directional Overcurrent relay derives direction by comparing angle of voltage with current. If there is no voltage, it can not measure its angle. Therefore, it will not operate in case of actual fault.

Absence of voltage or lower voltage to relay may be due to one of the following reason:

1. Actual fault on primary side of power system, which has caused voltage to dip. One disturbance record for this condition with B-N fault is given below. We can see presence of VN (3V0) with significant IN (3I0).
   
   
   
2. No actual fault on primary side, only secondary fuse fail. One disturbance record for this condition with C-phase fuse failure is given below. We can see presence of VN (3V0) without any significant IN (3I0).
   

It is important for protection relays to differentiate between above two conditions and declare VT fuse fail only when there is no fault in primary side. VT fuse fail condition leads to blocking of certain protection functions and generation of alarm to operator.

Method of detection: VT fuse fail may be classified in two types:

1. Single phase VT fuse fail, when fuse is used in VT secondary circuit and it has blown due to short circuit.
2. Three phase VT fuse fail, when MCB is used in VT secondary circuit and it has tripped. There may be another case when VT selection is used and VT selection relay has failed to operate.  

Single phase VT fuse fail: Its detection is easy and reliable. In case of actual single phase fault, voltage will decrease and current will increase for that phase, ie. there will be zero sequence current and zero sequence voltage present in system. 

However, in case of VT fuse fail, currents will remain same and only voltage will be decreased. Most of the relays detect it by presence of Zero sequence voltage without presence of Zero sequence current in system. In some case Negative sequence voltage and Negative sequence current is used for detection.

Three phase VT fuse fail: It is a little unreliable and works to some extent. Due to absence of all three phases there will not be any zero sequence or negative sequence voltage in the system. 

It can be detected by change in voltages without any change in currents by comparing with previous cycles values (ΔV and ΔI). But in case three phase fuse fail condition is persisting, and current goes below a certail level, due to load variation. Relay may detect it as dead line condition (no voltage and no current). This will reset three phase VT fuse fail condition. Now sudden rise of current can cause trip of relay. Therefore, three phase fuse fail is a little unreliable. 

Or, relay can detect MCB trip through auxilliary contact of MCB, if MCB is used in secondary circuit.