Tettex MIDAS 2881, Руководство по эксплуатации

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Functional Description 19
Figure 6- Function Schematics
I
X
Current trough Device Under Test C
X
I
N
Current trough known Standard
Capacitor C
N
I
RX
Losses of the Device Under Test C
X
C
X
Test Object (ideal capacitance)
C
N
Standard capacitor (with tan

< 10
-5
)
R
X
Measuring shunt for I
X
, C
X
R
N
Measuring shunt for I
N
, C
N
V Low voltage point of the HV supply and
reference point of the measurement
ADC Analogue to Digital Converter
t
1
, t
2
Time stamps of the measured values
5.2 V-potential point and Guarding
This measuring system is able to measure capacitances with highest accuracy to determine trending
analysis of insulating materials. In the range of normal insulation capacitances the always existent
stray capacitances - measured together wit the DUT - are influencing the measuring values
significant. So these unwanted stray capacitance effects have to be eliminated.
This is realized by the so called “guarding” of the relevant elements. That means that the complete
high voltage source, the supply and measuring cables have to be shielded with the so called “V-
potential” which is the low voltage point (reference) of the high voltage supply. All capacitances
connected to this reference point are bypassed and are therefore not influencing the measuring value.
Several parts have to be double shielded (Guard and Ground) to compensate other side effects and
to ensure the specified measuring accuracy. Due to this guarding concept the supplied shielded coax
measuring cables (for High Voltage Supply, Input A and Input B) have to be used always. If the
system is connected with normal unshielded cables the measuring values will be incorrect.
To keep in mind for the user of the system is that capacitances related to the V-point are bypassed.
Make sure that all unwanted capacitances are related to the V-potential point and their current is
flowing directly into the V-point and not through the measuring shunt R
X.
This has to be evaluated for every measuring setup. The most common ones are described in this
manual – for the other ones the user has to make sure that only the desired capacitances are
measured with the chosen test setup. Most cases can be solved by setting the internal Test Mode
Switch matrix correctly which sets unused measuring cables and connected parts to the V-potential
automatically.