Page 7 - Energize September 2022

COVER STORY

the dielectric losses in the insulation represented by C 1. The These guidelines are in line with those provided in IEEE C57.19.100
values obtained are commonly expressed as percentages, and – IEEE Guide for Application of Power Apparatus Bushings.
typical values for new bushings are in the range 0,2% to 0,4%. A point that is often overlooked when analysing power factor
Tests at the factory provide reference values that are included test results is the effect of temperature. Measuring at different
on the bushing nameplate, and field test values are compared temperatures yields different values, so it is important to normalise
with these references. Any significant deviation suggests that the measured values by correcting them to a reference temperature
insulation of the bushing may have deteriorated. (20°C). Tables of correction factors are available from various
Changes in the capacitance value of C 1 are also important. sources, but these are generic values and cannot always be relied
An increase in capacitance may be the result of short-circuited upon. We will look at this again later.
layers, while a decrease in capacitance most often results from
problems with the tap connection. Capacitance and power Narrowband dielectric frequency response (NB DFR)
factor measurements can also be made on the insulation measurements
represented by C 2, at 500 V (test tap) or 2 kV (potential tap). Some 10 kV power factor test sets can be used to make power
Particularly, when a gasket fails on a bushing that, in turn, factor measurements at multiple frequencies from 1 Hz to 500
allows for moisture ingress, water typically accumulates in the Hz. The curve obtained by plotting these measurements is the
tap compartment and attacks the main insulation core from “narrowband dielectric frequency response” (NB DFR) and it
the outermost layers first. A C 2 test primarily includes this provides additional information about the insulation condition
most susceptible insulation and, in such instances, provides of the bushing. This relatively narrow frequency band does not
notification that moisture ingress, or other contamination, is a provide quantitative information about the moisture content of the
problem before a C 1 test does. insulation, but it does provide an indication that moisture and/or
Several standards provide guidelines on interpretation impurities may be present.
and validation of power factor test results. Examples for NB DFR testing is an advancement from measuring power factor
factory acceptance testing are IEEE C57.19.01 – IEEE Standard at line frequency only and, in about three minutes, it provides
Performance Characteristics and Dimensions for Outdoor valuable additional information. It can, for example, deliver an
Apparatus Bushings, and IEC 60137 – Insulated bushings for early warning of insulation ageing and degradation, suggesting that
alternating voltages above 1000 V. maintenance should be prioritised or that the condition should be
The limits for C 1 power factor prescribed by these standards investigated further using more advanced test techniques.
are shown in Table 1. All values are either measured at 20˚C or NB DFR is typically carried out at 250 V and, because it is a low
are normalised to 20˚C. energy test, it is safer to perform than the routine 10 kV LFPF test. It
Standards applicable to field testing include: IEEE C57.152 allows visualisation of the unique dielectric signature of the object
– IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power under test, thereby facilitating graphical comparison of results from
Transformers, Regulators and Reactors. multiple bushings on the same transformer. It also provides a way
of measuring the temperature sensitivity of a particular insulation
This standard states that: system so that an accurate individual temperature correction factor
• A change from the initial reading by 1,5 to 2 times warrants (ITC) can be determined for normalising results to 20˚C.
more frequent testing of bushings Power factor is temperature dependent and Note 3(b) of Section
• A change from the initial reading by more than three times 10.10.4 of IEEE C57.12.90 – IEEE Standard Test Code for Liquid-
warrants removal of the bushing from service Immersed Distribution, Power and Regulating Transformers – states
• A change in capacitance by more than 5% is a cause to “Experience has shown that the variation in dissipation factor with
investigate the suitability of bushing for continued service temperature is substantial and erratic, so that no single correction
curve will fit all cases”. Fortunately, the frequency response and
the thermal response of a dielectric system are related, and this
Bushing type C1 acceptance C1 acceptance relationship reveals a more accurate, alternative means to ‘correct’
power factor power factor or normalise a power factor test result to its 20°C equivalent.
limits as per IEEE limits as per IEC
C57.19.01 [2] 60137 [3] For an OIP bushing, the dielectric frequency response plot
shifts horizontally with a change in temperature, but the shape of
OIP 0.5% 0.7% the curve remains unchanged. It is possible to determine the exact
amount by which the curve is shifted horizontally for a given change
RIP 0.85% 0.7& in temperature, which means that power factor measurements
made at the most commonly encountered bushing temperatures
RBP 2% 1.5% can be accurately and reliably normalised to 20˚C. Figure 2 shows an
Table 1: Limits for C1 power factor example of a normalised DFR curve alongside the measured curve.

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