Rated Current
The continuous current rating should be specified to be equal to or larger than the maximum required by the synchronous machine field under any allowed continuous operating conditions. Note that some machines have a continuous overload rating.

In addition, ANSI C50.12-1982 [2], ANSI C50.13-1989 [3], and ANSI C50.14-1977 [4] allow all machines to operate at rated MVA and within ± 5% of rated terminal voltage. Some machines may require an even wider operating range.

The need for off-frequency operation must also be considered in establishing the rating. Some machines, such as combustion-turbine-drive units, have a variable rating depending on ambient air temperature.

The excitation system for these machines may require a variable rating based on ambient air temperature. In the past, some excitation systems have had a small continuous negative current rating. This was utilized when the machine was operated as a synchronous condenser and the negative field current allowed a slightly greater transmission line charging capability.

The negative current was relatively easily supplied from commutator-type exciters, with little additional complexity. Modern exciters employing solid-state rectifiers do not normally have inherent capability for negative currents.

It could be obtained with great complexity, which cannot justify the associated minimal performance improvement (except for synchronous condensers). Some modern exciters will generate a transient negative voltage to force the decay of field current toward zero.

This should not be confused with the concept of a continuous negative current rating, which is mentioned above. For exciters that are specified with redundant current paths or cooling elements, the continuous rating should apply with the redundant parts out of service. The exciter efficiency and losses should be measured at the rated current and voltage point with all redundant parts in service, since this is the normal operating mode.

Rated Voltage
The continuous voltage rating of a system should be such that the voltage is sufficient to supply the necessary continuous current to the synchronous machine field, with the field at its maximum temperature under rated load conditions.

In addition, the continuous voltage capability should allow operation of the synchronous machine at rated MVA and within ±5% of rated terminal voltage unless otherwise specified.

In determining the required voltage for the continuous as well as the transient ratings mentioned later, all voltage drops, including interconnecting bus or cable run voltage drop, up to the field winding terminals should be considered. Any brush drop voltage should be considered part of the synchronous machine field circuit.

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