Page 6 - Energize September 2022

COVER STORY

Diagnostic testing of bushings

For almost a century, capacitance and power factor tests have been routinely performed on
transformer bushings. In more recent times however, dielectric frequency response (DFR) testing
has been recognised as an expanded form of power factor testing that offers significant benefits,

as Sanket Bolar and Ankit Porwal from Megger explain.

By Sanket Bolar and Ankit Porwal, Megger

n the early 1900s it was shown that adopting a capacitance-
graded design for transformer bushings provided better radial
Ivoltage distribution, thereby allowing smaller bushings to be used
for a given working voltage. Today, condenser bushings are almost
universal for applications at 25 kV or higher. These bushings can be
classified according to the materials used in the insulation system:
oil-impregnated paper (OIP), resin-impregnated paper (RIP), resin-
bonded paper (RBP) and resin-impregnated synthetics (RIS). OIP
bushings are the most widely used.
In substations, bushings provide the means of making
connections to transformers. They are designed to withstand
high voltage stresses during operation, to carry large currents
and to operate at high temperatures. They are typically used
outdoors, where they are exposed to potentially harsh atmospheric
conditions. Furthermore, before installation, they can be easily
damaged during transportation and, after installation, they are a
target for vandalism.
For these reasons, bushing problems are among the most
common causes of transformer failure, and transformer failure Figure 1: Capacitance components of a condensor bushing
can be very expensive. It is important, therefore, to monitor the
condition of bushings throughout their service life to ensure that In an OIP bushing, capacitance grading is achieved by wrapping
defects and deterioration can be dealt with before they lead to Kraft paper multiple times around the conductor core and placing
major failures. Traditionally, condition monitoring has been carried conductive foil inserts at specific intervals during wrapping;
out using capacitance and power factor tests, but now dielectric afterwards, this main insulation system is impregnated with oil. In
frequency response (DFR) testing – an advanced form of power bushings rated 69 kV (or 350 kV BIL), the outermost paper layer
factor testing – is growing in popularity, either to supplement or in is connected to a test tap. In service, the test tap is grounded via
place of traditional tests. To find out why, let us take a closer look at the tap cover. In bushings rated >69 kV (i.e., >350 kV BIL), one
each of the techniques. of the outermost layers is connected to a potential tap while the
outermost layer is grounded internally. The potential tap ‘floats’ in
Line frequency power factor testing (LFPF) on OIP service with the tap cover in place.
bushings The capacitance of the multiple layers between the core
The term ‘power factor’ is typically used in the USA, but in and the test tap, or between the core and the potential tap, is
other parts of the world, references to tan delta and dissipation represented by C 1. The test tap is isolated from the grounded
factor are more usual. Note that power factor and dissipation flange when the tap cover is removed, and the capacitance of
factor are calculated in different ways but, for our purposes, this insulation is represented by C 2. The C 2 insulation system
they are numerically equal, and the terms can therefore be used of a bushing with a potential tap includes the outermost, oil-
interchangeably when values are below 10%. impregnated paper layers of the main core as well as insulation
The insulation system of a condenser bushing has two between the potential tap and the grounded flange.
capacitive components: C 1 and C 2 (Figure 1). Capacitance and power factor tests measure, usually at 10 kV,

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