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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