Omicron CP CR500, User Manual

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CP CR500
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Capacitive charging currents flow through the insulation and then through the
coating. The charging currents flowing through the resistance of the coating
produce an I²R 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
Δ
-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,
Δ
-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
• improper impregnation during manufacture.
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:
• the inherent dielectric losses of the insulation and its general condition
• the quality of the contact of the semi-conductive surface with the core
• the moisture content and degree of cleanliness
• the degree of curing of materials.