25
Diagnostic of Rotating Machines Insulation
Figure 6-4:
Power Factor "Tip-Up"
according to IEEE 286
Figure 6-5:
"Δ tan
δ" as a function of
the test voltage
according to IEC 60894
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.
% Power Factor
2.5
2.0
1.5
1.0
0.5
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Applied Test Voltage, x U
n
kV
PowerFactorTip-up
tan δ
0 0.20.40.60.81.0
U/U
N
Δ tan δ
tan δ 0.2 tan δ 0.4 tan δ 0.6 tan δ 0.8 tan δ 1.0
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