Megger IDAX 300, Руководство пользователя

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AG033586EE ZP-AG03E IDAX 300/322/350 41
7 APPENDIX BMEASuREMENT TECHNIquE
Insulation diagnostics is based on material
characterization and therefore material models are
often used. To be able to define material parameters
from measured impedance Z the geometry of the
sample, described in terms of the geometrical
capacitance C
0
, has to be defined. In the illustration
below, a vacuum (or air-filled) capacitor of defined
geometry is shown. Since no “material” is between
the electrodes, the capacitance of a) is the geometrical
capacitance.
I
a)
U
C
0
I
c)
U
C
ε' & σρ
I
b)
U
C
ε' & ε’’
Material parameter models based on a geometrical capaci-
tance C
0
and material parameters.
In the above illustration b) and c) a material is inserted
between the electrodes and it will influence the
current, I, flowing in the circuit. The influence of the
material can be described by different parameters
using either a dielectric model or a conductive model.
In the dielectric model the “material capacitance”, the
permittivity, is a complex function describing both the
capacitance and the loss. Whereas in the conductive
model the capacitance is described by a permittivity
and the loss by a conductivity (or resistivity). The
dielectric and resistive models are derived as follows:
Z =
1
jωC
C = C
0
(ε’ – j
ε
’’)
Dielectric:
ε’
= Re� �
1
jωC
0
Z
Δ
ε’
=
ε’
+ k
ε’’
= –Im� �
1
jωC
0
Z
Resistive:
ε’
= Re� �
1
jωC
0
Z
ρ
=
1
ε
0
Re
� �
Z
C
0
σ
=
1
ρ
If geometrical capacitance, C
0
, is unknown it can be
set by entering a permittivity (dielectric constant),
ε
‘,
for the material. Entering a permittivity,
ε
‘, will let
the system calculate an approximate C
0
making use
of the material models available. Yet, one must be
aware that the accuracy of the absolute values are
limited by the accuracy of the entered permittivity.
If the geometrical capacitance is unknown and an
approximate permittivity is given, the geometrical
capacitance is calculated as follows:
C
0
= =
.
Re� �
1
jωZ
ε
r
C’
ε
r
1
The impedance value, Z, used in the calculation is the
first measurement point in the actual measurement.