| dc.description.abstract | DC-grids have no need to have frequency and phase control, and there is no problems regarding loses through reactive effect. Even so, there have been issues related to the security of DC-grids. One of the problems which have led to safety issues is the challenge to cut the current, for instance during a short circuit fault. As the voltage in a DC-grid does not have any zero crossing, large plasma arc arise when trying to mechanically cut the current.
In later years, both Siemens and ABB have come up with solutions to cut a dc current in a quick and safe fashion. This has led to an increase in the popularity of DC-grids.
With DC-grids it is also possible to reduce the amount of power electronic needed. This can reduce the total electric losses, weight and volume of the installation. This is beneficial for marine applications where space is limited or needed for other applications such as cargo or passenger. Weight is also important as a heavier vessel is more submerged than a lighter one, and therefore will need more fuel and power for thrusters and maneuvering.
Such grids on boats also go under the name of Microgrids. DC-Microgrid also has an advantage over AC-Microgrids when it comes to integration of renewable energy sources, and energy storing systems. Several renewable energy sources either delivers DC directly, or AC that is normally converted to DC before being transformed back to AC. This makes DC-Microgrid more and more important as building blocks for Smartgrids, where you have an increased introduction for decentralized renewable energy sources.
As mentioned before there are not any need for synchronization of the distributed generators, there are no problem in connection with reactive-power flow, harmonic currents, and three phase unbalances. In a DC-grid the voltage stability is on the other hand of utmost importance in order to provide the needed power to the different loads in the system at any given instance.
This thesis, studies the stability of a DC-system through simulations in Matlab. A diesel engine connected to a rectifier, an induction motor in series with an inverter, a battery and a supercapacitor connected through each own DC-converter. All these components are then connected in parallel with each other to the DC-bus. The focus in this report lies on the control of the DC-DC converters, the energy storing system and its management. | en |
