Tettex MIDAS 2881, Operating Instructions Manual

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20 Functional Description
The V-potential point is accessible over a 4mm plug on the instruments side panel where the user can
connect external parts of a test setup.
Example: Bypass the leakage current on bushing surface with guarding.
Figure 7 - Measurement without Guard (V-Potential)
Normal connection in GST gA+B mode to measure high voltage winding to tank C
HG
. But with this
connection the stray capacitance C
stray
(surface leakage current on bushing surface) is measured in
parallel and therefore causes a minor error on the measurement. The measured value is C
HG
. + C
stray
Figure 8 - Measurement with connected V-Potential point to the powered bushings (guarding)
Normal connection in GST gA+B mode to measure high voltage winding to tank C
HG
. With guard collars
mounted on the bushings surface close to the tank (not touching). These electrodes, connected to the
V-potential point bypass now the leakage current and therefore also the stray capacitance C
stray
The
measured value is now only C
HG
. and the best accuracy is reached.
Note: As guard collar you can use any conducting material as aluminium foil, copper band, etc