TESTING AND ADJUSTING
Currents - Circulating in Paralleled Generators
Building Services, Marine, Material Handling
When two or more generator sets are operated in
parallel, a current may circulate between the generators.
This current will exist when the internal voltage
generated by each generator is slightly different, but the
terminal or bus voltage is the same. In the most
elementary form, current will flow out the line leads of
one generator, through the paralleling bus and into the
second generator. It does not flow into the load. This
current, called "circulating current," is in addition to the
normal line current supplied to the connected load.
When more than two generators are in parallel, current
could flow out of any generator and into one or more of
the other generators. Circulating currents can take
many paths into and out of the several generators.
We are concerned with these "wattless amperes" only
when they interfere with normal generator set operation
or when the normal on-line kVA capacity of the
generators must be reduced because of excessive line
currents. With no-load (zero kilowatts) on a generator in
a parallel system, Caterpillar generators can readily
tolerate a circulating current equal to 20%-25% of the
line ampere rating shown on the generator name plate.
generators will tolerate a circulating current of up to 10%
of the rated line amperes.
Since circulating currents pass through the generator
coils, these currents heat the coils the same as does the
load current. Further, since circulating currents are
superimposed on the load current passing through the
circuit breaker, circulating currents can cause a breaker
to trip as the breaker could "see" an actual ampere
overload. More complex control systems include
"reverse current" relays which sense counter flow
currents. Currents in excess of the relay setting will
actuate the circuit breaker trip mechanism.
Observed line current (as indicated by panel ammeters)
in a parallel generator set system is a summation of two
or three currents:
Load current -- that current which is supplied to
the load. It may be in phase with the voltage
(unity power factor) or somewhat out of phase
with the voltage (power factor less than unity).
Harmonic current -- usually third harmonic
current which flows through the entire system
when "Y" connected paralleled generators have
their neutral leads connected, either directly or
through an earth or ground connection.
Circulating current -- that current which flows
between generators for reasons explained
Each of the above currents contribute heat to the
generator coils, the amount being equal to the square of
the sum total current times the resistance of the coils.
Thus, if the current doubles, the heat loss increases by a
factor of four. Coil heating reflects in possible
overheating and lowered efficiency. In very large
Significance of efficiency decreases with smaller
generators. However, coil heat is always a factor, as it
must be removed by ventilation or radiation to keep coil
temperatures to an acceptable maximum.
The load alone determines the load current. Reactors or
switches can be placed in neutral leads to reduce or
eliminate third harmonic currents. Proper generator
voltage adjustment can bring operating circulating
currents to a minimum.
Circulating currents perform an important function: they
account for misadjustment of the generator voltage
control system as well as slight variations in the control