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IDAX User's Manual 10
© 2009 Megger AB
the current is measured by an ammeter or electrometer which acts as a current-to-voltage
converter. The analogue signals (voltages) are then converted to digital samples of the signals
that are used in subsequent calculations.
1.4.2 Sample Modelling
The impedance, Z, can be presented directly or by using different impedance models. Two ways
of presenting Z directly are the polar and the rectangular, as follows:
Polar:
Rectangular:
Two simple models which are usually used in circuit analysis, although more seldom in
insulation analysis, are capacitance, C, and resistance, R. The equivalent RC circuit models
available are series and parallel models (see Fig.1) calculated as follows:
a) b)
Fig.1. Equivalent circuit models: a) series RC circuit, b) parallel RC circuit.
Parallel:
Series:
where
w
= 2
p
f and f is frequency.
Another model, more often used in insulation diagnostics, is the complex capacitance model
describing the insulation impedance as a complex capacitance, where the imaginary part of the
capacitance represents the losses. The complex capacitance model is defined as follows:
where