System Requirements And Guidelines For Diesel-Electric Propulsion - Wartsila 31 Product Manual

Wärtsilä 31 Product Guide
13.5 System requirements and guidelines for diesel-electric
propulsion
Typical features to be incorporated in the propulsion control and power management systems
in a diesel-electric ship:
1. The load increase program must limit the load increase rate during ship acceleration and
load transfer between generators according to the curves in chapter 2.2 Loading Capacity.
● Continuously active limit: "normal max. loading in operating condition".
● During the first 6 minutes after starting an engine: "preheated engine"
If the control system has only one load increase ramp, then the ramp for a preheated engine
is to be used.
The load increase rate of a recently connected generator is the sum of the load transfer
performed by the power management system and the load increase performed by the
propulsion control, if the load sharing is based on speed droop. In a system with isochronous
load sharing the loading rate of a recently connected generator is not affected by changes in
the total system load (as long as the generators already sharing load equally are not loaded
over 100%).
2. Rapid loading according to the "emergency" curve in chapter 2.2 Loading Capacity may
only be possible by activating an emergency function, which generates visual and audible
alarms in the control room and on the bridge.
3. The propulsion control should be able to control the propulsion power according to the
load increase rate at the diesel generators. Controlled load increase with different number of
generators connected and in different operating conditions is difficult to achieve with only
time ramps for the propeller speed.
4. The load reduction rate should also be limited in normal operation. Crash stop can be
recognised by for example a large lever movement from ahead to astern.
5. Some propulsion systems can generate power back into the network. The diesel generator
can absorb max. 5% reverse power.
6. The power management system performs loading and unloading of generators in a speed
droop system, and it usually also corrects the system frequency to compensate for the droop
offset, by adjusting the speed setting of the individual speed control units. The speed reference
is adjusted by sending an increase/decrease pulse of a certain length to the speed control
unit. The power management should determine the length of the increase/decrease pulse
based on the size of the desired correction and then wait for 30 seconds or more before
performing a new correction, in particular when performing small corrections.
The relation between duration of increase/decrease signal and change in speed reference is
usually 0.1 Hz per second. The actual speed and/or load will change at a slower rate.
7. The full output of the generator is in principle available as soon as the generator is connected
to the network, but only if there is no power limitation controlling the power demand. In practice
the control system should monitor the generator load and reduce the system load, if the
generator load exceeds 100%.
In speed droop mode all generators take an equal share of increased system load, regardless
of any difference in initial load. If the generators already sharing load equally are loaded beyond
their max. capacity, the recently connected generator will continue to pick up load according
to the speed droop curve. Also in isochronous load sharing mode a generator still on the
loading ramp will start to pick up load, if the generators in even load sharing have reached
their max. capacity.
8. The system should monitor the network frequency and reduce the load, if the network
frequency tends to drop excessively. To safely handle tripping of a breaker more direct action
can be required, depending on the operating condition and the load step on the engine(s).
Wärtsilä 31 Product Guide - a1 - 18 October 2016
13. Automation System
13-9