Tettex MIDAS 2881, Руководство по эксплуатации

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10 Theory
10
4.3 What is Dissipation Factor tan

?
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, busbars, 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.
Figure 1- Disc Capacitor
d
A



C
where:
A electrode face
d distance between the electrodes
C capacitance

0
dielectric constant of air (

0
=8,8542•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:
Figure 2 - Parallel equivalent diagram of a lossy
capacitance with vector diagram
Loss factor (Dissipation Factor)
R C R
X
I
I
Q
P
C
C
R
C
R
 
   


1
tan
Power Factor
PF
I
I
P
S
R R
C
   

cos
tan
tan



1 2
U
Test
applied test voltage
I
C
current through capacitance
I
R
current through resistance (insulating material)
C ideal capacitance
R ideal resistance