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3. The voltage regulation characteristics of the
individual generators should be similar.
4. The generators must have the same phase
rotation.
5.The driving engines should have the same
speed regulation characteristics and the
governors should be adjusted to give the same
speed regulation.
Before operating generator sets in parallel,
each set should be checked by starting,
operating, and adjusting the sets as individual
units before attempting paralleling.
REACTIVE LOAD CONTROL
When two identical generators are operating
together in parallel and an unbalance occurs in
field excitation, circulating currents begin to
flow between the generators. This current will
appear as a lagging power factor or inductive
load to the highly excited generator, and as a
leading power factor or capacitive load to the
generator with the lower field current. This is
known as the reactive circulating current and
there are two methods of controlling it in
parallel operation:
1. Reactive droop compensation. (Formerly
known as parallel droop compensation.)
The bus voltage droops, or decreases, as the
reactive lagging power factor load is
increased.
2. Reactive differential compensation.
(Formerly known as cross current
compensation.) The reactive differential
compensation circuit allows parallel
generators to share reactive loads with no
decrease or droop in generator voltage.
The circuit must meet the following criteria:
A. All paralleling current transformers for all the
generators being paralleled must be included in
the secondary interconnection loop.
B. When different size generators are paralleled
all paralleling current transformers must have the
same proportional ratios that give approximately
the same secondary current.
C. Voltage regulator paralleling circuitry must be
the same.
D. Current transformer secondary and the
generator lines must be isolated electrically.
E. It is also desirable to have an auxiliary contact
on the main generator breaker to short the parallel
CT secondary when that breaker is open (not
connected to the load bus).
Because of the above criteria, reactive differential
compensation cannot be used when paralleling
with the utility power grid. There is no limit,
however, in the number of generators that can be
included in this type of circuit.
PARALLELING CIRCUITRY
Because of the number of variables involved in
paralleling generator sets, every installation will
have its own circuitry and methods or procedure
of bringing paralleled units on line. There are
numerous ways of connecting paralleled units and
an almost unlimited variety of applications and
associated equipment.
When parallel operation is desired, it is important
that the control manufacturer, the generator manu-
facturer, and the systems engineer work together
to insure the proper selection of all components.
Please refer to Marathon Electric for application
assistance.
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