Around a quarter of the insulators, known as 'bushings', currently sold by Alstom are of the resin type but 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," said Cardano. "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.”
In the case of such a bushing - a glass tube with a silicon shell, for example, the only damage from an explosion would be a crack to the casing, he said.
The safety of transformer bushings was bought into focus with the introduction of a safety test in Italy two years ago, which has been applied to current transformers for more than 20 years but was extended to the bushings.
It 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
“When a bushing fails, there could be huge damages to surrounding machines and people, easily resulting in loss of life,” said Cardano.
Other bushing types are:
Gas - typically used where space is limited' but costly.
Oil-impregnated-paper - Oldest, cheapest and most widespread bushing technology. Damaged, they can leak into the environmental and catch 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," said Cardano.
Dry resin-based - eliminate the risk of leakage but can still explode.
“All of the bushing types we offer at Alstom Passoni Villa have passed the test but even so, some are still safer than others,” said Cardano.
Cardano will present a paper ‘EHV and UHV bushing design optimisation from a safety point of view’ at the CWIEME coil winding show in Berlin on Wednesday 25th June.