Grid stability is a growing issue today. This is because peak power is increasing and the variation in consumption is getting larger and larger. And these grid stability issues experienced on land are the same as those onboard ships that crisscross the seas.
The typical peak time starts early in the morning when people are waking up and starting to become active. From 5 to 6 am until around 11 am to noon, the requirements for power increases steadily as people start off on their day. Then the need for power reduces slightly until about 2 to 3 pm. The peak starts again from about 3 pm onwards as people rush home, make dinner and prepare for bed. Only after about 8 or 9 pm in the evening does the requirement for power start to go down again.
The problem with this uneven power requirement is the need for ramp-up power.
Another power variation is caused by short-term variations. In this case, there is a sudden unbalance of power production, and consumption can create wide swings in the network frequency or voltage.
To support the grid during such variations, some countries require power plants to have a certain reserve for use. This reserve is normally about 10-20% of the nominal power.
Frequency variation affects a larger area. It can be nationwide or as wide as the power pool. Therefore, every nation aims to keep the frequency within set limits. If the variation is more than the limit, the national grid company must have an alternative solution that can kick in.
If the frequency is higher than the limit, power production must be reduced. And in case a fast reaction is needed, energy storage can act as a buffer. If the frequency hits a lower limit, the grid must be supported by increased power production. Again, energy storage is the solution for a fast reaction.
Locally, voltage levels can also experience variations. When production is too high compared with consumption, the voltage will increase. Likewise, when production is too low, this will cause the voltage to drop below the set variation.
Energy storage solutions based on semiconductors are the best from a response point of view. The energy behind a power converter can be used within tens of milliseconds. Energy can be stored in compressed air, a flywheel or battery.
With a vessel power grid, the scale can be similar to that of a single city. A ship’s grid has all kinds of loads and more than one power source. For fast variations, the combusting engine control may be slow to maintain the balance. Also the optimum point of operation is not so easy to reach.
If variable speed generation is in place, control is easy. A shaft generator based on permanent magnet (PM) technology and an electronic-based power converter makes the production control easy and the reaction time fast to handle the demand variations. PM technology also enables high efficiency when compared with other types of generators, especially with partial loads, where ships normally operate most of their time.
That’s why green shipping requires exceptional efficiency from the power system. And shaft generators are one good solution to achieve this efficiency. A shaft generator can optimize the operation point of the main propulsion engine and even allow additional gensets to be turned off during sailing.
From an economical point of view, fuel consumption per kWh can be reduced significantly and savings increase. Also additional savings can be gained because the fuel cost for the main propulsion is typically 1.5 times lower than that of gensets.
Jari Riikonen
Marketing Manager, Marine Systems