Completely Self-Protected Transformer Evaluation


Distribution transformers are one of the most important purchases a distribution utilty makes. Investment in transformers constitutes a large percentage of the distribution plant investment each year. The types of transformers used and protection systems they employ also have a significant impact on continuity of service — a factor which is becoming much more important in contemporary systems. Distribution transformer safety is of prime importance since these transformers are placed in public locations and operating personnel are exposed to them.

Types of distribution transformers range from conventional transformers to Completely Self-Protected (CSP) units that incorporate overvoltage protection, internal primary fusing and secondary breakers. Selection of appropriate transformer types can have significant long term economic and reliability benefits for distribution systems.

The purpose of this paper is to present the purchase options for overhead transformers, and to provide information on the relative merits of each option.


Basic features of overhead distribution transformers include:

  • Internal primary fusing, either weak-link or current limiting
  • Secondary voltage circuit breaker
  • Tank-mounted lightning arrester

If overvoltage and overcurrent protection are not provided by accessories integral to the transformer, similar protection must be provided by external equipment. As with features integral to the transformer, external equipment items must be chosen for the particular conditions prevalent on the distribution system.

In present-day electric distribution utilities there are concerns about several features of CSP transformers. These concerns involve economic, power quality, and safety factors. This paper identifies these concerns and aims to help quantify evaluation of them by transformer users.

The 21-page report was commissioned by the NRECA Cooperative Research Network (a member of DSTAR) and is provided to DSTAR members under a reciprocactive arrangement.

This study was conducted in: P10-7