Interaction between power electronic components and distribution transformers — risk evaluation and mitigation techniques

The Geneva city utility SIG operates a large medium / low voltage distribution network with a high number of distribution transformers in place. Within the last approx. 4 years an unusual high number of transformer failed due to a premature aging. These units were all delivered between beginning of 1996 until mid 1999. Only sizes of 630 kVA and 1000 kVA were affected. Within other MV networks as well 400 kVA and 1250 kVA units failed, however a smaller number.