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Phase Transformers -> 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.
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