All synchronous generators must have stator protection, and most units have several
types of protection, each of which guards against different types of problems. There is little
disagreement-regarding the need for basic stator short-circuit protection, and most engineers
agree that this is best done using differential protection. Beyond this basic protection the
opinions vary as to the need for special relaying. Some argue that too many protective systems
increase the preventive maintenance and testing time, resulting in a reduction of protective
system reliability, and therefore tend to favor simpler systems and frequent testing. Others
prefer more elaborate protective systems on the theory that generator outages are very costly
and even moderate limitations on the amount of damage will more than pay for the additional
relaying and testing. The final decision rests on the benefit derived from the added complexity
in terms of improved generator availability. This is not easy to measure accurately since major
outages are rare events.
This chapter does not discuss the general physical structure of synchronous machines.
The reader is referred to one of the many excellent books on electric machinery for this type
of information.