METERING FOR LINEMEN

 

Electrical Metering for Linemen and Techicians

 

Voltage Transformers

Purpose

The voltage transformer (VT) was formerly referred to as a potential transformer (PT). In recent years, voltage transformer or VT is considered the more modern term. Voltage transformers are always connected in parallel with the circuit being measured. Voltage transformers have only one purpose, to reduce the voltage to the meter to a lower level. When connecting voltage transformers for three phase metering, it is not necessary to connect them with the same configuration as the power transformer secondary (i.e. Wye or Delta). They should simply be thought of as a voltage reduction device.

Ratio

The ratio of a voltage transformer refers to the turns ratio of the windings. For example, a 7200/120 transformation is equivalent to a 60 to 1 ratio. (7200 divided by 120 is 60). Knowledge of this ratio is required when calculating the dial multiplier. Nominal secondary voltages are 115 volts when transforming voltages above 34.5kV and 120 volts for below 34.5kV.

Polarity

A meter stator contains a potential coil and one or more current coils. These coils provide both voltage and current signals to the meter. The stator must be able to compare these two signals at any moment in time. Therefore, to establish forward rotation of the meter disk, polarity marks on instrument transformers must be observed. Incorrect polarity connections will result in reverse rotation of the meter disk. All instrument transformers are wound subtractive. This simply means that H1 and X1 polarity marks are physically on the same side of the instrument transformer.

Accuracy

Simply stated, voltage transformers should be rated for plus or minus .3% (3 tenths of one percent) accuracy when used for metering.

Burden Rating

The burden on a voltage transformer is the connected load. As more devices are connected to the voltage transformer the burden increases. This burden is usually calculated in volt amps. Two burden ratings are usually stated. The burden rating at the stated accuracy class is important for metering. Exceeding this rating will cause the voltage transformer to operate outside the required .3% accuracy class. The thermal burden rating is usually several times the burden rating stated for metering accuracy. Exceeding the thermal burden rating will shorten the life of the voltage transformer.