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When two diesel engines
are synchronised either electrically or mechanically (geared) then obviously
the speeds must be constant. It is the governor characteristics which
determines how the load is shared between the two machines.
The figure shows the
characteristic for a governor with droop compared to one which is isochronous.
It can be seen that there is a fall in speed as the load is increased.
This droop is necessary
for parallel operation of the machines if stable operation is to be achieved.
If we consider the case
of two diesel alternators operating in parallel then the required characteristics
and the load sharing operation is shown in the following figure.
The top left diagram
shows the situation after synchronising with all the load taken by machine
number 1. As the speed control of number 2 is increased it takes part of the load
and at the same time the frequency is increased. The bottom centre diagram
shows the situation after the speed control on number one machine is reduced
giving more load to number 2 machine and returning frequency back to normal.
From this it should be
obvious that load sharing is achieved by alteration of the fuel setting of the
machines.
If the governor
characteristics are isochronous then there will be no crossing points along the
characteristics and the load sharing will be unstable. If droop is incorporated
on at least one machine then there will always be a crossing point at any given
load and stable operation will be achieved.
This is shown on the
following characteristics for the case of isochronous governors and for both
having droop.
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