Voltage Transformer selection (VT selection)

Voltage Transformer selection (VT selection) is used in different protection  schemes where multiple VTs are installed and the requirement of VT secondary voltage to be used can change with the operation of dis-connectors.

In protection schemes, VTs are installed for three puposes:

1. Protection
2. Metering
3. Synchronization

Normally VTs are installed on busbars, and lines for EHV system. For voltages < 66kV, VTs are not installed in lines to reduce cost. In this case, Bus VTs is used for metering and protection of lines. As there will be no VT in line, synchronization is not possibe in this case. But in most of the case <66kV lines are radial type and synchronization is not required.

Double Bus Scheme: In the image below, having double bus scheme, only Bus VTs are used. Now, let us suppose we want to install Distance protection on feeder-1. As there is no VT in feeder, we have to use bus VT for protection purpose. But which bus VT? As the feeder-1 can be connected to Bus-1 or Bus-2 at any time and there may be cases when one of the bus is under shutdown (Bus voltage zero). So we require VT switching.

Now let us take one case of meshed system, where we have used VT in feeder, so that Protection and metering is done through Line VT. In this case, we have to check synchronization before closing of circuit breaker. We have to compare voltage of incoming feeder with existing bus voltage. But existing bus voltage of which bus? We need VT selection for synchronisation check, so that we may compare line VT voltage with selected bus voltage.

Bus selection is decided by position of bus side dis-connectors
. 
Case-1: If Bus-1 disconnector is closed, Bus-1 VT is selected for synchonization. 
Case-2: If Bus-2 disconnector is closed, Bus-2 VT is selected for synchronization. 
Case-3: If both disconnectors for Bus-1 & Bus-2 are closed, there is always priority for one of the bus, generally bus-1. Therefore, Bus-1 VT will be selected. Bus-2 VT can not be selected untill Bus-1 VT is deselected.

One and a half Breaker Scheme: Typical location of VTs is shown below. No VT is installed for transformer in this case. 

For protection and metering of Transformer, any one VT out of three VTs: Bus-1 VT, Bus-2 VT or Feeder VT may be selected depending on following:

Case-1: CB-4 Close, Bus-1 VT selected
Case-2: CB-4 Open, CB-5 & CB-6 close, Bus-2 VT selected
Case-3: CB-4 & CB-6 Open, CB-5 and Feeder disconnector closed, Feeder VT selected.

In this case, while closing CB, voltage on both side of CB is also required for synchronization checks. The voltages are also selected based on positon of CB / Dis-connectors.

Therefore, VT switching is required in feeders with position of Circuit breakers / dis-connectors for protection, metering and synchronization purposes.

Current Transformer switching (CT swithing)

CT switching is used in different protection schemes where protection boundaries can change with the operation of isolators (dis-connectors). 

In protection schemes, having differential protections, protection boundaries are defined by the locations of CTs. Let us look into the Double Bus Scheme, which is used widely upto 400kV levels. 

Every Feeder is having three isolators, one for Bus-1, second for Bus-2 and third for Feeder. In this case any feeder can be connected to either Bus-1 or Bus-2 by changing the position of isolators.

In the diagram above, Feeder-1 and Feeder-3 are connected to Bus-1. Feeder-2 is connected to Bus-2. Now, let us come to bus bar protection. Bus bar protection is differential protection, which calculates the vector sum of all the feeders terminating on a bus. In above figure differential current for Bus-1 shall be vector sum of current measured by CT1 + CT3 + CT4. Differential current for Bus-2 shall be vector sum of CT2 + CT4.

In the image above all the feeders are connected to Bus-1, bus coupler is kept out. Now in above figure differential current for Bus-1 shall be vector sum of current measured by CT1 + CT2 + CT3. Differential current for Bus-2 shall be zero as no feeder is connected to it.

In this way we can see that with the change in isolator positions of primary circuits, secondary circuits of CTs are required to be changed, so that the bus bar relay could calculate the differential current correctly. For this we need CT switching relay, which would change (switch) the secondary CT circuits depending on primary CT circuits.