generators in parallel [RE-wrenches]

matthew tritt solarone at charter.net
Tue Jul 1 17:14:03 PDT 2003


Daryl,

When you say "your smaller generators", what do you consider small? If  you
are paralleling two high quality sets, such as Lister/Petter, Deutz, etc.,
you will find very precise speed control. Less expensive equipment will have
less expensive, less precise control gear. I am assuming that you are mating
identical generators?

Matt T----- Original Message ----- 
From: "Daryl Thayer" <daryl_solar at yahoo.com>
To: "re Wrenches" <RE-wrenches at topica.com>
Sent: Monday, June 30, 2003 9:37 PM
Subject: generators in parallel [RE-wrenches]


I have worked with several generators that are
paralleled.  They are larger generators, with high
quality speed regulation.   I have no experience with
small generators.   That said, the way larger
generators are synchronized is to use a synchronize
lamp.  The lamps are for example 230, 240 volts for a
120-240 volt system.  The neutrals are tied together
and the hot wire from each generator is connected to
each side of the synchronize lamps.  Often two lamps
are used for each leg of a single phase system or
three lamps for a three phase system. When the
generators are run the lamps flicker at the
synchronize difference.  One generator is speed varied
until the synchronize difference is a very slow
flicker, in the order of seconds.  Then the contactor
is closed and generators are synchronized.  If a load
is applied the generators tend to change speed and the
governors respond with increased throttle, or
injection rate to carry the load.  Now here is the
problem, if the generators have a different
sensitivity to frequency the more sensitive generator
carries the greater load.  As the more sensitive
generator picks up load it loses sensitivity to load
and the second generator picks up more load.  The load
capacity of paralleled generators is less than the sum
of the load capacities.

I have not experienced the voltage problem, however I
assume the voltages of the larger generators is fixed
precisely, whereas  the voltage of your smaller
generators is different.  My first thought is this
does not matter under load except the effect the load
balance.  The generator with the higher output voltage
will "hog" the load.

Daryl


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