Fundamentals of Main Tie Main schemes and state machines

Often large facilities have processes that can justify the added expense of having a redundant source and a means to transfer load without interrupting the process. This paper reviews the fundamentals of main tie main schemes, methods of transferring load (fast, in-phase, and residual/fixed time) and how state machines can be used to make the relay logic more readable and easier to troubleshoot.

I. Introduction

There are many industrial facilities that have high cost associated with plant interruptions. At refineries and chemical facilities, an interruption in the process can require flaring and purging the process after an interruption and involve a long restart process. The cost associated with this is not just in wasted chemical feedstock but also lost production. Facilities like these can justify having a redundant connection to the grid. Often these facilities are fed by two independent electrical sources. A common arrangement to make use of the two independent connections to the grid is a main tie main arrangement. This setup can be used to transfer load from one source to the other for planned and emergency transfers.