Page 54 - Offshore Electrical Engineering Manual

Load–Voltage Characteristics (Regulation) 41

LOAD SHARING OF GENERATORS

Unlike an ‘infinite busbar’ system as experienced in grid systems, where the fre-
quency is maintained by a large number of generators and the output of each genera-
tor is controlled mainly by controlling the prime mover power output (governor), in
the offshore or ship system the frequency is dependent on not only individual gen-
erator governors but also the type of load-sharing system operating. With a simple
‘droop characteristic’ control where each machine’s speed decreases for the increas-
ing kilowatt loading, if there is a tendency for the load on one machine to increase, it
will tend to drop its speed thereby shedding some of its load. This arrangement works
well with machines of equal size and characteristics (and with compounding when
AVRs are utilised), but an electronic active load-sharing scheme will be required if
machines of different size and characteristics are to share load successfully in most
operating conditions. Such systems are available commercially as a modular part of
a full multigenerator power management system (see PART 4 Chapter 8).
Once the generators have been paralleled, the power outputs of the two machines
should be equalised by operating in governor droop mode, or by a load-sharing sys-
tem. Reactive power is equalised by adjusting excitation. It is possible to parallel
dissimilar machines, but there is a tendency for the differences in governor and AVR
characteristics to cause one machine to take the load until the other is tripped on
reverse power.

NOTE ON COMPOUNDING
To share kilovolt-ampere reactive between paralleled machines, a drooping volt-
age characteristic is required. This is provided inherently with hand regulation but
machines under AVR control maintain virtually constant voltage (i.e., no droop),
and instability will result unless the characteristic is modified. This can be achieved
by using a compounding or quadrature droop current transformer to develop a volt-
age across a resistor. This voltage is added vectorially to the system voltage and the
resultant applied to the AVR. The effect is to reduce the excitation of any machine
supplying excessive kilovolt-ampere reactive.
Hand regulators should always be provided for testing/commissioning purposes.
If part of the automation fails, it may also be necessary to operate one of the machines
in hand regulator control, with other machines on AVR control providing for any load
variations.

LOAD–VOLTAGE CHARACTERISTICS (REGULATION)
In machines of normal design, the drop between no load and full load voltage with a
power factor of 0.8 will be of the order of 65%–75%, assuming constant excitation.
At unity power factor the voltage drop is caused by stator resistance and by distortion