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TP-6200 10/12204 Appendix
Reactive Droop
Reactive droop refers to another compensation
technique used in excitation control systems. Reactive
droop means that the generator set voltage droops with
increasing reactive current. Although this sounds like an
undesirable effect, it is quite beneficial in paralleling
applications with multiple generator sets. Because the
terminals of the generator set are connected to another
generator set(s), the voltage at the terminals is not solely
determined by either generator set’s excitation. Rather,
it is determined by the combination of the excitation
level, the generated voltage, and the voltage drop
across the armature impedance or armature reactance
for each generator set.
Normally the generated voltage is higher than the
voltage at the terminals because the generator set
current causes a drop across the armature impedance.
In a parallel application, the generated voltage of one
generator set may be slightly higher than the generated
voltage of another generator set. Differences in
potential between the generator sets will cause current
to flow into the lower voltage generator set and will also
cause the generator sets to share the load current
disproportionately. Both results are undesirable.
By introducing reactive droop, the reactive current can
be better predicted and controlled. If the current is
measured, the regulator/controller can adjust the
excitation up or down accordingly, reducing excitation as
more current is supplied or increasing excitation as the
reactive current decreases. If all the parallel generator
sets incorporate this type of compensation, the reactive
current can be shared equally based on the proportional
size of the generator sets. For an example, see below.
The stability and accuracy of this technique depends on
several factors. Most important, the regulation point for
each generator set must be equal. That is, each voltage
adjust setting must be the equal to the other(s). This is a
basic requirement prior to the actual paralleling
connection. Also, the effects of the reactive current in
each generator set must be compensated for
individually, which requires an adjustable droop for each
generator set. This adjustment happens to be the
reactive droop adjust. The reactive droop adjust is
quantified as the droop in operating voltage from the
adjusted setting when full rated load with 0.8 power
factor (PF) is applied. A droop setting of 4% voltage at
full rated load is a recommended starting point. If the
reactive current is not shared proportionately in each
generator set, the respective droops may need
adjustment. Adjust those generator sets that have
proportionately higher current for more droop and those
generator sets with lower reactive current for less droop.
If the reactive current is not stable in the system, adjust
the droop lower in all generator sets.
As implied above, the reactive droop is not usually
necessary in stand-alone applications. Therefore,
some means of disabling the feature is provided. If the
generator set will not be paralleled with other generator
sets, the reactive droop feature should be disabled. A
reactive droop setting of 0 will also effectively disable the
reactive droop feature. It should be noted that reactive
droop applies strictly to the reactive current or
volt-ampere-reactive (VAR) loading. Primarily, the
fueling or speed governing system controls the real
current which contributes to watts loading.
The gain of the reactive droop function is determined by
the voltage droop setting. For most applications, a
droop of 3%--5% of rated voltage at rated load at 0.8 PF
is adequate. Prior to actually connecting the generator
sets in parallel, test the droop by applying full rated load
at 0.8 PF. The system is operating correctly if this test
shows a reduction in voltage equal to the voltage droop
setting. If the available load is less than full load, the
correct voltage droop should be proportional to the
applied VAR load as a fraction of the rated VAR output for
the generator set. For instance, a 480-volt generator set
with a voltage droop setting of 4% should drop 19.2 volts
with full rated (0.8 PF) load applied (480 x 0.04) or
9.6 volts with half the rated load applied (480 x 0.04 / 2).
When a generator set will be connected in parallel with
the utility, VAR or PF control should be ENABLED. If
there are multiple generator sets in parallel as well, then
reactive droop should be ENABLED also.
Example
Two 100 kilowatt (kW) generator sets are paralleled to
provide 150 kW of power at 0.8 PF and wired for a
277/480-volt wye system.
Total kVA load:
kVA = kW / PF
187.5 = 150 / 0.8
KVAR load:
kVAR = kVA * sin (acos [ PF ] )
112.5 = 187.5 * 0.6
Line current:
I =(VA/3)/V
L-N
226 amps = (187500 / 3) / 277
Reactive current:
I =(VAR/3)/V
L-N