Resin to the rescue in transformer safety

Recent developments in transformer equipment design have led not only to increased efficiency but also increased safety – a recurring theme at CWIEME Berlin, the world’s largest electrical manufacturing exhibition. Indeed, how to increase the safety of transformers – from the insulating oils to the bushings – will be the subject of several seminars at this year's show.

One the key speakers, Paolo Cardano, a senior expert in OIP bushings at Alstom Passoni Villa says: “After the introduction of a new safety test in Italy two years ago, the safety of transformer bushings has been brought more sharply into focus than ever before.”

The test in question has been applied to current transformers for more than 20 years but has only now been extended to the bushing – a safety-critical component in any electrical transmission system.

“When a bushing fails, there could be huge damages to surrounding machines and people – easily resulting in loss of life,” he says.

All transformer bushings sold in Italy must pass the test, which essentially involves creating an explosion inside the bushing to measure its integrity. If the bushing remains intact in its air side part – even if there is internal damage – it passes. If the bushing cracks and projects parts externally, or is expelled completely, it is no longer able to guarantee the safety of the transformer.

“All of the bushing types we offer at Alstom Passoni Villa have passed the test but even so, some are still safer than others,” he says.

Mr. Cardano will use his seminar at CWIEME Berlin to discuss the relative safety of each bushing type – gas, oil-impregnated paper and resin-impregnated paper for AC and DC applications.

Resin rules

Gas bushings are typically used where space is limited but are costly to produce. Oil-impregnated-paper is the oldest, cheapest and most widespread bushing technology but likewise has its drawbacks. A damaged oil-based bushing could leak, leading to potential environmental damage or fire.

“If there is a fire due to a power arc on a bushing, the transformer could catch light and be extremely difficult to extinguish – not to mention the replacement costs and disruption to the supply,” he says.

Dry resin-based bushings eliminate the risk of leakage but could still explode, causing damage to surrounding equipment and people in the event of failure.

To avoid explosions or leakage, Mr. Cardano recommends the resin type together with a composite casing, such as a glass tube with a silicon shell, removing this possibility. In this case, the only damage would be a crack to the casing.

Around a quarter of the bushings currently sold by Alstom are of the resin type but Mr. Cardano anticipates an increase in their popularity in coming years.

“Even though the resin type was introduced 20 years ago, that’s still relatively recent in the power transmission and distribution industry. Given that the typical service life of a transformer bushing is 30 years, the industry is still evaluating its performance. From our perspective, however, a resin bushing with a composite casing is the safest option and I believe it will soon become the norm in Europe, if not the world.”

Member of the IEC

Mr. Cardano received his doctoral degree in Electrical Engineering from the Milano Polytechnic University and has worked in the field of transformer bushings for almost 20 years. Today he forms part of the R&D department at Alstom Passoni Villa as a senior expert on OIP bushings. He is president of the IEC subcommittee for bushings in Italy, convener of a working group for the revision of seismic bushing standards, as well as a member of several other IEC/CEI working groups.

Mr. Cardano’s presentation, entitled ‘EHV and UHV bushing design optimization from a safety point of view’, will take place at CWIEME Central on Wednesday 25th June, 14:25-15:05.

CWIEME Central seminars are held in English and free for all CWIEME Berlin visitors.