Dielectric strength and protection of modern dry-type air-cooled transformers

TODAY there are in service over 1,000,000 kva of large dry-type air-cooled transformers, some in ratings as high as 4,000 kva and including the 15-kv voltage class. This wide acceptance in industry of the modern dry-type transformer and its continual growth, naturally has increased interest in its application and standardization. In a prior AIEE paper¹ the design characteristics, particularly with regard to thermal performance, have been discussed. The purpose of this paper is to review the development of the modern dry-type transformer, specifically with reference to its dielectric strength, and to present recommendations on impulse levels and methods of protection for applications which may be affected by lightning surges.