Tang Delta
Tang Delta
Tang Delta
6.2.1
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Figure 6-4:
Power Factor "Tip-Up"
according to IEEE 286
2.5
% Power Factor
2.0
1.5
1.0
0.5
0
0
Figure 6-5:
" tan " as a function of
the test voltage
according to IEC 60894
0.1
0.2
0.3
1.0
1.1 1.2
tan
tan
tan 0.2
0.2
0.4
tan 0.4
0.6
U/UN
tan 0.6
0.8
tan 0.8
tan 1.0
1.0
Historically, the DF test was first applied to high voltage stator bars and coils, to
ensure that the insulation was completely impregnated. However, since the late
1950s, some motor and generator operators have applied the test to complete
windings to detect various aging mechanisms that produce PD.
Measurement of the tip-up is complicated by the presence of silicon carbide
stress control coatings on coils rated 6 kV or above. At low voltage, the silicon
carbide is essentially a very high resistance coating, and no current flows
through it. Thus, there is no power loss in the coating. However, when tested at
rated voltage, by design the silicon carbide coating will have a relativity low
resistance.
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CP CR500
Capacitive charging currents flow through the insulation and then through the
coating. The charging currents flowing through the resistance of the coating
produce an IR loss in the coating.
The DF or PF measuring device measures this loss. Since the loss is nearly zero
at low voltage, and non-zero at operating voltage, the coating yields its own
contribution to tip -up. It is not uncommon for the tip-up due to the stress relief
coating to be 2 or 3 %. This coating tip-up creates a minimum tip-up level. Very
significant PD must be occurring in most windings for the PD loss to be seen
above the silicon carbide tip-up. When manufacturers test individual coils and
bars in the factory as a quality assurance test, the tip-up contribution - due to the
stress relief coating - can be negated by guarding.
Unfortunately, it is not practical to guard out the coating tip-up in complete
windings. It makes sense to do Tan-Delta and -Tan-Delta tests upwards and
downwards. The area between upwards and downwards curves is a measure
for the PD activity, because ignition and extinguishing of PD will occur at
different voltage levels.
6.2.2
Interpretation
As maintenance tools for complete windings, -Tan-Delta and tip-up tests are
used for trending. If the tip-up is measured every few years and the tip-up starts
increasing from the normal level, then it is likely that the winding has significant
PD activity. To increase the tip-up above the normal level requires widespread
PD. The most likely causes of this PD are:
thermal deterioration
load cycling
An acceptable power factor offers assurance that the coil or the bar was properly
fabricated with inherently low-loss materials and was properly processed. A low
power factor tip-up reflects the quality of the construction and compactness (lack
of gaseous inclusions or voids) of a coil or bar, the composition of the
impregnating material and quality of the impregnating process, and the quality
and condition of the semiconductive surface treatment in the slot area.
Differences in the tip-up measured for coils or bars of similar composition and
fabrication are generally attributed to a variation in the incidental void content.
The power factor measured at a low voltage, e.g., 2 kV rms, is, for the most part,
unaffected by partial discharge and is an indication of:
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the inherent dielectric losses of the insulation and its general condition
the quality of the contact of the semi-conductive surface with the core
0.85 %
Tan Delta
0.75 %
0.65 %
0.55 %
0.45 %
0.35 %
0.25 %
0.0 Hz
100.0 Hz
200.0 Hz
300.0 Hz
400.0 Hz
500.0 Hz
The test is made at low test voltages of about 100 V to 200 V from 15 Hz to
400 Hz. In this case also high capacitance can be tested without compensating
reactors. The value at rated frequency (50 / 60 Hz) can be compared to the value
at test frequency of e.g. 55 Hz, which is used to get resonance with the
compensating reactor coil.
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