18.1 Introduction

There are several types of equipment that interface with an isolated phase bus connecting the generator to the generator transformer with tap-off to a unit auxiliary transformer. Depending upon the project, unit size, and system requirements they comprise: 

Since the generator connections are expected to be extremely reliable, any equipment or accessory interfacing with or connected to it must also be equally reliable. The electrically connected product must be engineered to ensure that the line to line fault is eliminated and the probability of a line to ground fault is minimal. Depending upon the zone of protection in which the connected equipment is located, a fault can trigger a unit shut down resulting in significant loss of revenue. When two phases are brought close for connection inside a cubicle or equipment, a ground fault or arcing could develop into a line to line fault with disastrous consequences due to very high fault levels. 

18.2 Generator Circuit Breakers

Generator circuit breakers (GCB) for use in conjunction with isolated phase bus have been in operation for the past few decades. 

As generator sizes increased, the rated continuous current, rated breaking, and short-time withstand current under fault conditions required of the associated generator circuit breaker, also increased. 

Medium voltage circuit breakers, used in the distribution network, could not keep pace with this increase. Most have a maximum current rating of 3 kA, a breaking capacity of 40 kA and a short time fault current withstand capability for 1 second. The absence of a higher-rated capacity of these breakers resulted in generators being directly connected to the generator transformer with tap-off to auxiliary loads, excitation system, and surge protection voltage transformer cubicles. The synchronizing with the grid was carried out at the HV end of the generator transformer.