Maximum demand and demand factors in underground coal mining

Maximum demand is an important parameter in electric-utility rate structures and in establishing demand factors. If meaningful demand factors for a specific-type electrical system are available, they can be applied in the estimation of demand charges and in the design of new systems. However, the basis for the demand factor is not necessarily the same for both uses. It is the differences in the intended application that are problematic because the measurement and subsequent calculation of utility demand and demand factor is based upon billing practices, not design considerations. Maximum demand, and hence demand factor, which is intended for use in design, should relate in some way to the thermal characteristics of primary electrical components since equipment capacity is determined by allowable temperature-rise limits. A brief review of maximum demand is presented. Then appropriate applications of demand factors are described, as well as recommended methods for their calculation, based upon usage. Included in this is an examination of the thermal characteristics of transformers, motors, and cables, so that a partial relationship may be found between demand and electrical component temperature rise. Finally the conclusion contains a description of the data collection activities and necessary calculations to arrive at meaningful demand factors for an American retreating longwall system.