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.