Enhanced load transfer schemes for very reliable service

Modern production processes require very reliable service. A loss of electric power, even for a few seconds, causes a large loss of income to electricity users with time-critical and uninterruptible processes. To raise power system reliability to a higher level, multiple sources are used in combination with an automatic load transfer scheme. This paper examines several different load transfer schemes in use at utility sites today and their operation in preserving load continuity during system interruptions. Examples of successful and unsuccessful load transfer schemes are given. This paper addresses how settings are incorporated to maximize the advantages and minimize the drawbacks of different schemes. Analysis of correct and incorrect operations, with applicable event reports, is included. A root cause investigation of problems encountered in performing the expected load transfer, with solutions implemented to correct those problems, is included for each incorrect operation. Transfer requirements to meet reliability needs for different end-user facilities are presented. Different communications channels and methods for interconnection and interlocking of the incoming feeders are discussed along with the coordination requirements between the feeds. Economics of both the controls and primary equipment required to implement the system are evaluated and compared with the end-user costs of lost service. Conclusions and recommendations are presented to assist power providers in determining the preferred throw-over methodology for given conditions, available communications, and end-user needs.