Page 40 - Energize April 2021

TECHNICAL

modular equipment ranges can provide transformer has a life of 40 to 50 years, you need to be able to monitor it for that time, so the
both types of testing, in both offline and equipment needs to be resilient and robust. But our modular system allows customers to
operational scenarios, for the full range of swap out the components or sensors, for instance, while retaining the rest of the assembly. It
network applications. has a high specification, it’s easy to use and easy to upgrade,” says Wills.
“It’s an extensive range, and it’s PD activity also produces sound waves at audible frequencies. Compared with electrical
modular: customers can choose to grow signals, acoustic signals travel relatively slowly, at speeds determined by the medium they
their systems from basic to advanced,” are travelling in, whether oil, metal, or gas. By using several acoustic sensors, it is possible to
says Wills. locate PD faults by analysing differences in the delay of acoustic signals. However, acoustic
To perform testing based on the ultra- measurements alone are unlikely to provide actionable information, Wills adds. “Usually it’s
high frequency electrical signals emitted by just a reference measurement – a first step for locating the PD.”
PD activity, pulses of known charge values In the case of oil-filled transformers, a decision to test for PD will often be triggered by the
are applied to the test circuit for calibration, results of dissolved gas analysis (DGA) tests, as PD activity will liberate gases inside the unit.
then deviations from the expected The rate at which the gas is being generated can also indicate the severity of the PD activity.
impedance values can be measured to However, DGA provides little or no useful information about the location of the PD activity.
determine if PD is present. If the PD is “It’s an extensive range, and it’s modular: customers can choose to grow their systems
lower than accepted and no increasing from basic to advanced”, Wills says.
tendency in present, equipment has passed
the PD test. The values are set out in the How PD testing can protect essential gas-insulated switch gear
international standard, IEC 60270. Gas-insulated switchgear (GIS) is of high importance for every grid: with high electric fields
Measurements rely on coupling in comparatively small spaces, failure can lead to extensive damage and long outages. In
devices that detect the PD pulses; the addition, the SF 6 typically found inside insulating chambers must be handled with special care,
high frequency signal is converted and as it has the highest global warming potential of any known substance.
sent to the data acquisition unit, often As GIS units reach the end of their projected life spans, systems such as Power Diagnostix
passing through a preamplifier that tunes
out other background signals. To detect PD
currents in cables, or in the earthing cable
of transformers, radio frequency current
transformers (RFCT) are often the preferred
coupling devices, as these can easily be
clamped on to cable terminations.
Power Diagnostix by Megger’s
digital monitoring equipment plots the
electrical PD signal as “Phase resolved
partial discharge patterns” [PRPD],
where characteristic graph patterns are
associated with different types of molecular
breakdown. The evaluation of the PRPD
pattern can therefore help to identify the
type of defect.
Some PRPD outputs show typical
and recognisable problems, for example,
delamination within paper layers in a
transformer has a very clear PRPD pattern;
in other cases, more advanced analysis
may be needed to pinpoint the exact fault
to within a few centimetres.
“You often need expertise to interpret
the data; colleagues in Germany offer this
analysis,” says Wills.
Every application requires robust kit
that can withstand life in the field, but
sturdiness becomes a key consideration
when continuous monitoring of equipment
is required.
“With continuous monitoring, the kit
you’re using to do the monitoring becomes
critical. It needs to be reliable: when a

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