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23.11.4 Load sharing
When two or more engines are operating in parallel, some kind of load
sharing must be provided. Load sharing means that each engine will
contribute equally to the total power demand, and it ensures that load
changes are absorbed evenly by the engines.
When the engine is operated in speed control mode, load sharing with
other engines is normally provided with the use of speed droop. Drop
control is a load sharing method, by which parallel running engines
share the load by decreasing their speed reference proportionally to an
increase in load. The droop value is normally set to 4 %, but the setting
is adjustable. Too low droop value means that the load can start
oscillating between the engines. Too high droop value means that the
plant’s frequency decreases steeply with the load level. Load sharing
based on droop, means that the power management system must
compensate the effect derived from the droop slope. Therefore, this
system must operate the speed increase/decrease inputs of WECS (in
so called cascade control) to ensure that the net frequency is kept
constant regardless of the load level. Speed control with droop is used
on plants operating in island mode, but it can also be used when
operating against a grid utility. In the latter case, load control is
however recommended.
23.11.5 Load control
The load control function is enabled if kW-mode is selected. This mode
can only be activated when the generator breaker is closed. Also the
grid breaker must be closed, if the plant configuration is such, that
kW-mode is only applicable when this breaker closes. The load control
mode has most benefits in base load applications where the grid
frequency stability is low. The engine load will not fluctuate according
to the frequency in the same way as if it would do in speed control mode
with droop. In load control mode, the load reference is compared with
the actual load of the engine. The difference between these signals
constitutes the input to a PID-controller. The regulation output of this
controller will accordingly vary, to sustain the reference level. This
output will control the opening duration of the gas valves. The PID-
controller’s dynamic settings have load dependent mapping.
If the frequency deviates outside a fixed window, the control mode will
automatically trip back to speed control. The speed reference is updated
continuously by the speed control loop also in load control, which means
that if a trip occurs, the transfer will basically be bumpless (no load
swing). By giving a reset, the load control mode will be restored,
providing that all enabling conditions are met.
WECS 3000
34SG-200408-01
Содержание 41508
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