Parallel Operation of Governors
This section is a general description of the function of
the engine governor in relation to load division between
parallel electric sets. For detailed information on
governor controls and adjustments, see the Operation
Guide and Service Manual for the engine.
It is very important that two basic facts be understood
concerning
load
division
between
generator
sets
operating in parallel. First, the power supplied to the
generator and thus to the load is a function of the
engine. The engine governor settings and the positions
of the governor controls determine the amount of power
delivered by the engine and the KW load carried by the
generator. If the governor control setting is advanced,
the engine and generator will assume more KW load.
Likewise, decreasing the governor control setting will
result in a reduction of load on the unit. Any other units
on the line will, conversely, either reduce or gain load at
the same time, assuming no change in total load or no
change in the governor settings of the other units has
taken place. Second, the division of power is not
determined by generator excitation or terminal voltage.
The Power Factor at which a generator will operate
when paralleled with other generators is determined by
its excitation. For more discussion on this subject, refer
to the section on Parallel Operation of generators.
Governors furnished with Caterpillar Powered Electric
Sets can be either of two types, governors with fixed
speed droop or governors with adjustable speed droop.
The values of speed droop used are commonly 3% and
0%. Governors with adjustable speed droop can be
adjusted so their characteristics match quite closely the
characteristics of governors with fixed speed droop. The
operating characteristics of the following combinations of
governors on paralleled electric sets will be described.
1. Two 3% governors.
2. One 3% governor and one 0% (isochronous)
governor.
Example 1 - Two 3% Governors
When paralleling A.C. Generators, the engine governors
must have the same speed droop characteristics if the
sets are to divide the load in proportion to their ratings
throughout the entire operating range.
The governor speed droop characteristics of these two
units are similar. This is shown by lines "A" and "B"
coinciding in Figure 1. If both units are started, set for
high idle speed and paralleled at no load, the system
frequency will be 61.8 cycles or 103% of rated
frequency. As load is applied to the system, the
frequency
will
decrease
along
the
speed
droop
characteristic until the frequency at full load is 60 cycles.
Figure 1 - Paralleling two units with similar speed
droop characteristics.
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