Bespoke Instrument Transformers

These are transformers designed for monitoring voltages on power systems where they are providing isolation between the voltage being measured and the measuring instrument. Often they are also required to provide voltage scaling to convert the system voltage down to a level within the range of the meter input capability.

Design

• Due to the high impedance nature of voltmeters very little current is required so instrument transformers are usually low VA rated typically less than 10VA for single phase or 30VA for 3 phase units although they can be specified higher
• Small variation of the output voltage with the loading drawn by the voltmeter
  This is expressed as the ‘Accuracy Class’ typically 0.5% or 1.0% and is directly related to the regulation of the transformer. To achieve this transformers are physically much larger than a standard transformer of the required VA rating this is to reduce the winding resistances to acceptable levels
• Low waveform distortion in order that the output gives an accurate reflection of the system voltage. For this reason instrument transformers are operated at a lower flux level than standard panel transformers

Accuracy Class to BS EN 60044

This is defined by the standard as the maximum allowable variation in the output voltage for a defined stable input voltage where the loading on the output can be varied from 25% of the VA rating to 100% of the VA rating

e.g. A 10VA transformer with an output of 110V and an accuracy class of 0.5% must give a maximum output of 110.55V at 2.5VA load and 109.45V at 10VA load for the defined nominal input voltage.