Tettex MIDAS 2881, Operating Instructions Manual

Page: 19 / 164

Theory 9
4 Theory
4.1 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 order to define acceptable loss factor values, a „data record service“ was developed based upon
statistical data that related to specific equipment types and models. Standard measurements of
capacitance and loss factor of the electrical insulating medium were carried out to ensure that the data
was comparable. The loss factor was calculated and the results were corrected by energy
comparison to a value for a test voltage of 10kV. Some test results were further multiplied by a
temperature correction factor to produce 20°C compatible values. Any results that are now acquired
at different Test Levels and temperatures are recalculated for 10kV and / or 20°C and then recorded
and compared. 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.
4.2 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.