Megger DELTA 4000, Справочное руководство

Страница: 7 / 48

ZM-AH02E DELTA 4000 7
1 INTRODUCTION
Figure 2: UST connection for measuring CHL in a two-wind-
ing transformer
Figure 3: GST connection for measuring CH in a two-wind-
ing transformer
Current, capacitance and
dissipation factor relationship
In an ideal insulation system connected to an alternating
voltage source, the capacitance current I
c
and the voltage
are in perfect quadrature with the current leading. In ad-
dition to the capacitance current, there appears in practice
a loss current I
r
in phase with the voltage as shown in
Figure 5.
The current taken by an ideal insulation (no losses, I
r
= 0)
is a pure capacitive current leading the voltage by 90 ° ( q =
90 ° ). In practice, no insulation is perfect but has a certain
amount of loss and the total current I leads the voltage by
a phase angle q ( q < 90 ° ). It is more convenient to use the
dielectric-loss angle d , where d = (90 ° - q ). For low power
factor insulation I
c
and I are substantially of the same mag-
nitude since the loss component I
r
is very small.
The power factor is defined as:
Power factor= cos Θ = sin δ = Ir
I
and the dissipation factor is defined as:
Dissipation factor = cot Θ = tan δ = Ir
Ic
PF =
DF
•
1+DF
2
DF =
PF
•
1 – PF
2
The DELTA 4000 is able to display either dissipation factor
or power factor based on user’s choice.
Figure 5: Vector diagram insulation system
In cases where angle d is very small, sin d practically equals
tan d . For example, at power factor values less than 10
percent the difference will be less than 0.5 percent of read-
ing while for power factor values less than 20 percent the
difference will be less than 2 percent of reading.
The value of I
c
will be within 99.5 percent of the value I
for power factor (sin d ) values up to 10 percent and within
98 percent for power factor values up to 20 percent.