Tettex MIDAS 2881, Operating Instructions Manual

Page: 143 / 164

Applications Guide 133
Rotating Machine Stator Test connections to measure C
1G
, C
12
and C
13
.
Test Connections
DUT INPUT A
to INPUT B
to INPUT
HV GND
to Test Mode High Voltage
to
C
1G
2 3 Casing GST gA+B 1
C
12
2 3 Casing UST A 1
C
13
2 3 Casing UST B 1
C
23
1 3 Casing UST B 2
C
2G
1 3 Casing GST gA+B 2
C
3G
2 1 Casing GST gA+B 3
C
1G
+ C
2G
+ C
3G
- - Casing GST gA+B 1 + 2 + 3
14.3.2 Measuring Data Interpretation
An increase in dissipation factor above a certain voltage is a guide to the rate at which ionization is
occurring and gives guidance how the ionization action may be expected to accelerate. If voids are
short-circuited when ionization occurs, some increase of capacitance with voltage may also result.
In general, the coils nearest to the line terminals operate at the highest voltage to ground and are
therefore most affected by ionization. The remaining life in a winding can often be extended by
obtaining dissipation factor versus voltage curves on all coils, replacement only the worst, and
regrouping them so that the coils with the least increase of dissipation factor, and preferably lower
value of dissipation factor, are nearest the line terminals. Considerable extension of winding life can
also be realized in many cases by measuring dissipation factor versus voltage on groups of coils
without removal and rearranging the line and neutral connections accordingly. This can be done
several times in a lifetime so that the coils are evenly deteriorated.
A reduction in the phase-to-ground capacitance (charging current) of a new winding after an initial
period of operation may be an indication of incomplete curing of the winding. This can lead to corona
in the slot sections or to a loss of compactness.