Page 41 - Energize April 2021

TECHNICAL

by Megger’s AIACompact solution can help to keep them in service for longer. The signal in switchgear installations, and although
emitted by a PD is trapped inside the GIS, but the company’s AIACompact can detect and alternative gases are available, most new
amplify those signals, converting them into actionable readings. installations over the past decade still use
PD failures can be detected with both acoustic and UHF sensors, with the combination SF 6 gas.
making the portable, rugged, and easy to use AIACompact suitable for a range of installations. A 2018 study from the University of
Inside a GIS, faults that can give rise to PD include free metal particles moving inside Cardiff that collated data from the UK’s six
the chamber, loose contacts, or surface protrusions. To detect these, UHF sensors can DNOs found that the installed inventory of
be attached to unshielded flanges, or the surface of an inspection window. On large GIS GIS switchgear rose every year from 2010-
compartments, more than one measurement would be necessary. 11 to 2015-16, by 30-40 tonnes per year.
Alternatively, most modern GIS systems are manufactured with embedded ultra-high While the DNOs’ returns indicate that SF 6
frequency (UHF) sensors to enable survey measurements to be taken; as the signals from emissions did not increase, the increased
embedded sensors can be strong, these are routed through an Input Protection Unit before volume of gas in operational use creates a
reaching the monitor unit. higher potential risk of leaks.
Free metal particles sometimes even “hop” due to the impulse given when a PD occurs; Improved insight into PD and the
here, acoustic sensors – used with headphones – are particularly useful at detecting the fault. insulating health of the gas means that
Acoustic sensors are often fixed underneath the tubes as “hopping particles” tend to be found unnecessary maintenance work on GIS
at the bottom. can be avoided, while also improving asset
As well as PRPD patterns generated from UHF signals, the AIACompact will display owner’s ability to predict and pre-empt
results of acoustic tests in “time of flight” mode, indicating the distance travelled by particles, failures. Health and safety are also important
analysis that helps to quantify certain defects and their severity. drivers. Under the Electricity, Safety, Quality
and Continuity Regulations (ESQCR), DNOs
Wider and emerging benefits of PD testing report any unplanned outages which result
Typically, network operators implement PD testing as part of their condition-based in damage to equipment to the Health and
maintenance (CBM) strategies: in order to avoid unplanned outages and improve network Safety Executive.
reliability; to deliver better safety outcomes; to extend the lifespan of existing assets and delay Overall incidence rates are not
capital replacement. declining: from 2014/15 to 2018/19 there
In some cases, it may be possible to manage relatively low risk conditions by reducing were 14 000 reported incidents a year on
the operational demands on a particular transformer or piece of switchgear until removing average, but this rose to 14 992 in 2018/19.
it from service is more convenient; in other cases, an immediate and urgent repair may be While PD is only one factor of many, it
necessary. PD measurements can provide information about the nature of the repairs before is acknowledged as a leading cause of
the repair process is started, eliminating the need for exploratory investigations to locate electrical failures.
the dielectric damage and making it easier to prepare for what is likely to be found when
transformers or other equipment is opened. This, in turn, helps shorten outage times and Conclusion
improve security of supply. Network operators usually expect a precise read-out of the PD Partial discharge has been present in HV
status of new assets at purchase. electrical equipment for as long as there
“When a customer buys a HV transformer, they will set a PD specification so the has been HV electrical equipment, and PD
manufacturer will have to prove the PD value at the factory gate to test the insulation is of testing methodologies were first developed
good quality,” says Wills. more than half a century ago. But with the
In addition, if switchgear has been taken offline for testing or maintenance, there must number and type of network applications
typically be a PD test measurement before the equipment can be put back into service. constantly growing, and the price of failure
Testing during an asset’s operational life often starts by comparing PD levels with site or rising, network operators are constantly
factory acceptance tests, and previous diagnostic tests and measurements. searching for new ways to improve their
Insurance underwriters and brokers may also set out criteria on PD testing as a pre- insight into the underlying health of assets.
condition of insuring operational assets, such as transformers and switchgear, where values The PD testing range from Megger, with its
are calculated in millions of rand. modular, interoperable components that
“Testing means that they can be confident in their underwriting,” notes Wills. flex to suit the needs of every circuit, can
There are also environmental and sustainability arguments to managing PD. In the upgrade operators from guidance-level
first place, networks are currently under pressure to accommodate an increasing number PD test results to a full diagnostic picture.
of import and export connections in order to assist with the transition to renewable green Today, better data is improving outcomes
electricity and electric vehicle charging. With many local networks offering little headroom to in multiple areas of our lives; in the arena
safely accommodate new connections, better management of PD – including more accurate of PD testing, Megger’s comprehensive
scheduling or when reinforcement work or capital investment is necessary – helps in the suite of tools offer the data needed
overall network management agenda. to underpin robust maintenance and
But GIS poses a particular environmental challenge. The switchgear is typically housed in replacement strategies. n
chambers filled with SF 6 gas, which can extinguish arc flashes and stop short circuits. While
SF 6 is a highly effective insulation, it also has an extremely high global warming potential. Contact Silvia Paulino, Megger,
New renewable energy connections added in recent years have resulted in an overall increase Silvia.Paulino@megger.com

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