Haefely 2830, User Manual, Page: 18 / 124

10 Theory
5 Theory
5.1 Dissipation Factor tanδ
To specify the insulation loss factor, the test object must be considered in the test arrangement as
a capacitor. Consider the liquid test cell and solid test cell. Are constructed from metal and
insulation, and therefore possess associated capacitive properties. Every test cell consists of two
electrodes: a high voltage and a guarded measuring electrode. The capacitance of the insulating
material between the two electrodes will be measured. The figure shows the components that
comprise a capacitance and the diagram for a simple disc capacitor.
Disc Capacitor
d
A
C

=

where:
A electrode face
d distance between the electrodes
C capacitance

0
dielectric constant of air (

0
=8,8542 x 10
-12
F/m)

r
relative dielectric constant dependent upon
material
 
=

0
•

r
, dielectric constant
In an ideal capacitor the resistance of the insulation material (dielectric) is infinitely large. That
means that, when an AC voltage is applied, the current leads the voltage by exactly 90° as it flows
as pure current.
After further consideration it must be realized that every insulation material contains single free
electrons that show little loss under DC conditions with P= U
2 /R. Under AC a behaviour called
dielectric hysteresis loss occurs which is analogous to hysteresis loss in iron.
As losses therefore occur in every insulation material, an equivalent diagram of a real capacitance
can be constructed as follows: