Wartsila 31 Product Manual page 13

Wärtsilä 31 Product Guide
2.2.2 Diesel electric propulsion and auxiliary engines
Fig 2-3
In diesel electric installations loading ramps are implemented both in the propulsion control
and in the power management system, or in the engine speed control in case isochronous
load sharing is applied. If a ramp without knee-point is used, it should not achieve 100% load
in shorter time than the ramp in the figure. When the load sharing is based on speed droop,
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.
The "emergency" curve is close to the maximum capability of the engine and it shall not be
used as the normal limit. In dynamic positioning applications loading ramps corresponding to
20-30 seconds from zero to full load are however normal. If the vessel has also other operating
modes, a slower loading ramp is recommended for these operating modes.
In typical auxiliary engine applications there is usually no single consumer being decisive for
the loading rate. It is recommended to group electrical equipment so that the load is increased
in small increments, and the resulting loading rate roughly corresponds to the "normal" curve.
In normal operation the load should not be reduced from 100% to 0% in less than 15 seconds.
If the application requires frequent unloading at a significantly faster rate, special arrangements
can be necessary on the engine. In an emergency situation the full load can be thrown off
instantly.
2.2.2.1 Maximum instant load steps
The electrical system must be designed so that tripping of breakers can be safely handled.
This requires that the engines are protected from load steps exceeding their maximum load
acceptance capability. The load steps are in three equal steps. The resulting speed drop is
less than 10% and the recovery time to within 1% of the steady state speed at the new load
level is max. 5 seconds.
When electrical power is restored after a black-out, consumers are reconnected in groups,
which may cause significant load steps. The engine must be allowed to recover for at least
10 seconds before applying the following load step, if the load is applied in maximum steps.
2.2.2.2 Start-up time
A diesel generator typically reaches nominal speed in about 20 seconds after the start signal.
The acceleration is limited by the speed control to minimise smoke during start-up. If requested
faster starting times can be arranged.
Wärtsilä 31 Product Guide - a1 - 18 October 2016
Maximum recommended load increase rates for engines operating at
nominal speed
2. Operating Ranges
2-3