Protection designs in medium voltage distribution networks, time to reconsider

The quality of the electricity supply has always been a point of major interest. Worldwide there is an increasing concern about Power Quality [1,2]. On the other hand the electricity-market is quite conservative. This implies that long used solutions are often not questioned, so in general no reconsideration takes place anymore, although more economical or technical optimised solutions may exist. In addition to this the technical know-how in the utilities nowadays seems subsidiary to the (short term) economical concerns. This article explains what the real backgrounds are for the widely used fuse protection of transformers. The fuses dealt with are considered to be of the current limiting type, as mainly used in Europe; so expulsion fuses are not taken into account. With the help of a few examples it is demonstrated that a lot of misinterpretations exist about application aspects. After a historical overview of MV-transformer protection in the Netherlands, as applicable for many other countries, the nowadays considerations for this protection are dealt with. This results in listing the pros and cons of protection with fuses compared to circuit-breakers (CB´s). The conclusions of these considerations were used for the design and main specifications of a new generation of indoor switchgear for 12 and 24kV networks. (Fig.1) [3].