175
Appendix
The DF value for 60Hz is about 0.30%. The character of the DF curve over the
frequency range is interesting and should be retained as a fingerprint result for
future diagnosis of the insulation and its degradation.
Figure 12-32 Frequency scan HV-LV (154 kV-20 kV)
12.9.15 Capacitance Measurements on Transformer Windings
The capacitance test measures the capacitance between the high- and low-
voltage windings, between the high-voltage winding and the tank & core
(ground), and between the low-voltage winding and the tank & core (ground).
Nearby lightning strikes or through faults can cause a change of the measured
capacitance values. This indicates winding deformation and structural problems
such as displaced wedging and winding support.
12.9.16 Capacitance and DF Measurements on Transformer
Bushings
The high-voltage bushings are critical components of the power transformer and
particularly, capacitive high-voltage bushings need care and regular tests to
avoid sudden failures. These bushings have a measurement tap-point at their
base and both the capacitance between the top of the bushing and the bottom
tap (normally called C1) and the capacitance between the tap and ground
(normally called C2) are measured. An increase of C1 indicates partial
breakdowns of the internal layers.
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