commercial applications.
Engine-generator sets in stand alone service
(isolated bus) may utilize either droop or
isochronous governors. The designer should analyze
the application and loads to determine if the more
expensive isochronous unit is actually required.
Droop units provide added stability (less engine
cycling) in single unit applications where constant
speeds are not critical and are less expensive than
isochronous governors.
defines the allowable steady state variation in
frequency and is typically quite small for
commercially available governors (typically less
than + 0.4% with + 0.25% readily available). The
predominant type of device loads which are
susceptible to steady state frequency deviations
less that + 0.4% are those which employ switching
power supplies (computers and variable frequency
drives). The designer should select the least
restrictive value for bandwidth for the application.
Voltage Regulators. Solid state regulators are
easily available which maintain the voltage level
(regulation or voltage droop) to + 0.5%. Voltage
regulator bandwidth is important relative primarily
to transient response. EGSA Standard 100R-1992
defines three performance classes for voltage
regulators: standard (2% bandwidth); high (1%
bandwidth); and precision (0.5% bandwidth). Select
the least restrictive bandwidth necessary to satisfy
the application requirement.
Generator frequency, and voltage should be shown on
the engine-generator set schedule. (For example:
60 Hz, 208Y/120 volts, 3-phase, 4-wire).
Subtransient Reactance. The subtransient reactance
determines current during the first cycle after a
system short circuit condition is presented to the
generator. Therefore, it is used to determine the
necessary interrupting capacity of the genset
circuit interrupting device. It also is utilized to
predict generator response to non-linear loads.
Typical values for generator subtransient reactance
are found in IEEE Std 141. Subtransient reactance
is specified in per unit of the generator rated kVA.
Also, see the following discussion on non-linear
loads.
Non-linear Loads: Non-linear loads are addressed in
IEEE Std 519. They are loads that draw a
non-sinusoidal current wave form when supplied by a
sinusoidal voltage source. Typical non-linear loads
include solid state switching power supplies,
computer power supplies (including those found in
SECTION 16264A
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