Page 8 - Energize September 2022

COVER STORY

Stage 1: PF analysis
An increase in power factor, measured at power frequency, was
observed on two of the three bushings (X 1 and X 3), as shown in
Table 2.
According to IEEE C57.152, the line frequency PF values for
X 1 and X 3 at 20°C exceed acceptable limits, while according to
CIGRE TB 445, the PF values of bushings X 1 and X 2 are within limits
with only bushing X 3 outside acceptable limits. Based on these
results alone, we may say that bushings X 1 and X 3 are not in good
condition, and that bushing X 3 should be further investigated as
soon as possible.

Figure 2: Example of a normalised DFR curve alongside the measured curve Bushing Nameplate % PF Measured % PF

Using the ITC method to accurately normalise results X1 0.23 0.47
eliminates the need to wait until the temperature is close to
20˚C to test a bushing, or to rely on generic correction factors X2 0.23 0.27
that may or may not be valid, depending on the insulation
condition. X3 0.24 1.4
Table 2: Increase in power factor at power frequency
Dielectric frequency response (DFR) measurements
The insulation system of an OIP bushing electrically behaves as Stage 2: NB DFR analysis
multiple series capacitors that are formed of conductive foil and The NB DFR test was carried out on the three bushings at
oil-impregnated Kraft paper, as previously mentioned. The oil and frequencies from 1 Hz to 500 Hz. Figure 3 shows an increase in
paper form a composite dielectric system, and DFR results are a DF% for bushings X 1 and X 3 at lower frequencies. The thermal
combination of the responses from the oil and paper components. response curves are shown in Figure 4 and are different for each
The responses from both of these components vary with bushing because of the variations in the dielectric frequency
frequency, but the way they vary is different and this difference is response between the bushings. Table 3 shows the temperature
exploited in DFR testing. corrected PF% values.
When NB DFR testing does not provide a definitive indication For a bushing in good condition, its C 1 PF test result is only
of the condition of a bushing, DFR testing, which is carried slightly temperature dependent. As its insulation ages and
out over a wider range of frequencies, can be used for a more
in-depth analysis. Analysis of DFR curves can yield important
information, such as the moisture content of the paper and the Bushing % PF at 60 Hz % PF at 10Hz % PF at 1 Hz
conductivity of the oil. In addition, the presence of contamination
X1 0.454 0.514 1.32
or other physical issues can result in atypical responses with the
most prominent deviations seen at the lower frequencies. X2 0.271 0.278 0.66
DFR measurements are often carried out at 140 V. Because
the test covers a wide range of frequencies and uses low X3 1.14 2.76 11.9
currents, electrical noise can adversely affect the accuracy of the
measurements, particularly when testing already relatively low Table 3: Temperature corrected PF values
capacitance specimens, like bushings. The solution is to test at a
higher voltage. The use of a voltage amplifier in conjunction with
a DFR test set greatly improves the signal-to-noise ratio, allowing
accurate and reliable measurements to be made even in noisy
environments.

In the field
To illustrate and confirm the value of the tests described in this
article, the same series of tests was performed in the field on
three OIP HV bushings. The testing was carried out in three
stages: PF analysis, NB DFR analysis and DFR analysis. Figure 3: Increase in percentage of DFR at lower frequencies

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