5 Theory
5.1 Why is Insulation Tested?
All transformers, high voltage switchgear, motors and electrical equipment accessories have a high voltage
lifespan. From the first day of use the equipment is subject to thermal and mechanical stresses, foreign particle
ingress and variations in temperature and humidity. All of these influences raise the working temperature of the
equipment when switched on. This heating accelerates chemical reactions in the electrical insulation, which result
in a degradation of the dielectric characteristics. This process has an avalanche character i.e. the changing
electrical characteristics of the insulation increase the loss factor and produce heating which further degrades the
insulation. If the loss factor of the insulation is periodically monitored and recorded, it is possible to predict and / or
avoid catastrophic failure of the electrical equipment.
At the beginning of the public electricity supply industry, methods and processes were sought to avoid unexpected
losses caused by equipment defects. One method that provided repeatable data and offered simple on-site
measurement was the measurement of capacitance and loss factor (power factor) of the equipment insulation.
In cases where loss factor tests were regularly carried out, and the relevant test results compared with earlier
results, the deterioration of the insulation was noted and necessary preventative measures were carried out.
Based on this groundwork, a series of test procedures were developed and described in various IEEE, ANSI and
IEC documents and standards to specify the insulation quality for various types of electrical equipment.
In this way the degradation of the insulation characteristics over a specified period of time can be determined.
With the test result history an experienced engineer is able to take the necessary maintenance actions based upon
changes in the value of loss factor.
5.2 What is Loss Factor?
Loss factor is the total energy that will be used by the equipment during normal service. In particular, the insulation
loss factor is any energy that is taken by the flow of current through the resistive component of the insulation. The
earth path varies according to the type of electrical equipment. For example, switchgear will probably develop
tracking to earth at right angles to the floor connections. In transformers paths can develop in the insulation
resistance between the windings or between the windings and housing (tank). In all cases the result is a loss
factor in the form of heating.
Note: In this text loss factor (losses, watts) is referred to, in contrast with total loss factor. Total loss factor is
normally used to describe the total losses of the transformer under load and should not be confused with the
energy that is lost due to degradation of the insulation.
5.3 What is Dissipation Factor tan δ?
To specify the insulation loss factor, the test object must be considered in the test arrangement as a capacitor.
Consider all test objects e.g. transformers, bushings, bus bars, generators, motors, high voltage switchgear etc.
are constructed from metal and insulation, and therefore possess associated capacitive properties. Every test
object consists of various capacitances together with the insulation and the internal capacitance to earth. The
figure shows the components that comprise a capacitance and the diagram for a simple disc capacitor.
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