Voltech AT3600 User Manual page 297

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OLTECH SER
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Capacitance occurs in transformers due to the physical proximity of, and electrostatic
coupling between, different turns of wire. In general, the capacitance is distributed
between the different layers within a winding, and between the outside layer of one
winding and the inside layer of the next.
Although the capacitance is distributed, it is usually represented by a simple
equivalent circuit showing a single capacitance from one winding to the next.
Inter-winding capacitance may be of interest in transformers used in audio, medical
and instrumentation applications, where isolation between primary and secondary
windings is important. It can also play an important part in the circuit operation of
switch-mode transformers where, for example, too large a capacitance may give rise
to a large amount of noise at the switching frequency being coupled into sensitive
circuits connected to the secondary windings.
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To measure capacitance, the tester applies an ac voltage between the windings to be
tested, usually with all taps on each winding shorted together. It then measures the
voltage between the windings, and the resulting current using harmonic analysis.
Dividing the voltage by the current gives the inter-winding impedance, from which
the capacitance may be calculated.
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