That type of transformer has been involved in at least three incidents (where I was also involved) over the years. All due to partial discharge. One construction seems to be more reliable than normal cast resin transformers: the Geafol types. They are dry type transformers but have a wound insulation that is superior to the cast insulation where voids are formed and where P. D. takes place and causes Ozon (O[sub]3[/sub]) build up.
I would be very cautious if the application is the least critical (most are).
Cast resin transformer technology has improved a lot during the past few years and today commercial units are available up to 40 MVA 72.5 kV. These are definitely much superior, maintenance free and less prone to moisture ingress than standard dry type or VPI(vacuum pressure impregnated) dry type units. These fire proof units are widely used in residential buildings, traction substations, wind turbine transformers and inside power stations.
Cast coil are standard transformers used indoors for traction power. We have had them for at least 15 years with no problems.
Mostly German SGB or WTW. ABB makes cast coils in Virginia. Virginia Transformers makes UNICLAD pretty close to cast coil.
They are sealed and generally don't give problems.
Schneider seem to have missed partial discharge as a problem, or at least seem to think their transformers don't experience it. I agree with Gunnar that the 'Geafol' type are among the best of the breed. I'm not convinced that Schneider have PD completely fixed. Equally I'm not sure how many users actually look for PD to know that they have it. Being PD-free on commissioning day doesn't always equate to being PD-free after years of service, especially with organic insulation materials.
