Parallel Operation
3. After the engine has run long enough to warm up,
bring it up to synchronous speed (the same frequency as
the unit on the line). The synchronizing lights will begin
to blink.
4. Using the governor control adjust the speed until the
lights blink very slowly.
5. The lights are off when the voltages of the two units
are in phase. At this point very quickly close the breaker
while the lights are out.
6. Use governor controls to share KW load between
engines
7. After generator temperature has stabilized (1 hr.),
adjust the droop control of each generator so as to share
the reactive load and to limit the circulating currents.
Less droop (moving control CCW) increases the reactive
current carried by the generator.
The frequency of the incoming unit should be slightly
greater than the line frequency. This will allow the
incoming unit to assume some of the load rather than
add to the system load.
Load Division--(Speed Droop)
Once two units have been paralleled their share of the
load is determined by the governor control setting. If two
units of the same capacity and the same governor
characteristics have the same governor control settings
they will share the load equally.
To transfer the load from one engine to the other follow
this procedure:
The total load must not exceed the capacity of the
engine.
1. Increase the governor speed control of one unit to
increase the load.
2. Reduce the governor speed control of the other unit
to decrease the load on that unit.
3. Raise or lower the governor speed control of both
units to change system frequency.
Circulating Currents
When two units are paralleled there will be circulating
currents. These currents are not doing useful work, but
are flowing between the generators. By determining the
total generator amperage and subtracting the amperage
going to the load, the amount of circulating current can
be determined.
Circulating currents are caused by voltage differences
between the two units.
With cold generator sets, circulating current may be as
high as 25% of rated amperes, without being considered
harmful. Circulating current is part of the total generator
current and this total must not exceed the amperage
rating.
As the generators warm, the circulating currents will
decrease. The ammeter readings should decrease
slightly, but the voltage meter readings should remain
constant.
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