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.

MEASUREMENT CONDITIONS

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.

The test voltage can be in the range of 1mV to 5V at a frequency of 20Hz to 1MHz. The table below gives the recommended test conditions for different values of capacitance:

Capacitance range	Preferred test signal
Capacitance range	Frequency	Voltage
1pF → 10pF 10pF → 100pF 100pF → 1nF 1nF → 10nF 10nF → 100nF	100KHz 100KHz 10KHz 1KHz 100Hz	5V 5V 5V 5V 5V

The Test Conditions for Capacitance Measurement

When choosing the test conditions, the following potential problems should be considered:

CURRENT LEVELS
For larger capacitance, particularly at higher frequencies, the current flowing during the measurement can be very high, and similarly the measured current could also be very small for small capacitance at lower frequencies and voltages.

Where possible, you should use the recommended test signal levels in the table above to ensure that the currents which flow can be measured accurately.
NON-LINEAR CAPACITANCE
Normally non-linearities in the stray capacitance of transformers are not a problem, and therefore capacitance is measured with as large a voltage as possible.
EQUIVALENT CIRCUIT
As with inductance, capacitance is actually measured as a complex impedance, and therefore the result can be expressed in terms of either a series or a parallel equivalent circuit.

It was explained in section 1.5 of this chapter that parallel and series equivalent inductance do not necessarily have the same values. The same is true for capacitance; parallel and series equivalents can also be different.

The tester always uses a parallel equivalent circuit for capacitance measurements.

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