The Service Current Rating of Power Conversion Equipment

The conventional method which has been used for specifying the load current/time capability of power conversion equipment is based on the provision of continuous and short-time current ratings under specified conditions of service. This method is not suitable for establishing the capability of the equipment for supplying arbitrary load current/time demands on a repetitive duty cycle basis. This paper develops the basis for a method of rating which can be used for this purpose. It is a method that is application rather than equipment oriented. It involves the construction of a service current profile which is actually a function rather than a value. The method has been incorporated in standards which have been proposed for thyristor power conversion equipment for dc motor drives by the appropriate committees within NEMA, IEEE, and the IEC. The concept, in fact, was developed through the deliberations of those committees. It should be remarked that the method can also be applied to other types of power equipment.