Onboard bus voltage regulation for all-electric ships using the DC microgrid concept

Abstract

Shipboard power system of marine vessels has been based on alternating current (AC) system for a very long time. Nowadays, marine engineers are starting to opt for an electric power distribution system. Manufactures in the marine industry are adopting the direct current (DC) system in All-Electric Ships (AES). This technology provides cleaner, more efficient, flexibility in the power system design and is more eco-friendly. In marine vessels the integrated power system (IPSs) integrated with renewable energy sources (RES) and energy storage systems (ESSs) can be classified as shipboard microgrids (SMGs) in which energy sources and electrical loads are connected in parallel. When the ship is at sea, the SMG is considered as an islanded microgrid. The main focus of this dissertation is to design, model and simulate two main architectures for an all-electric ship based on the DC concept. The electric ship being considered in this dissertation is currently being developed by DAMEN group. These architectures mainly differ in the way that the DC bus voltage is achieved and regulated. The project was divided into four main stages. Stage 1 is concerned with the design and modelling of the marine synchronous generator. This includes: 1) the electric machine used as a generator, 2) the speed governor which regulates the output frequency of the generator and 3) the automatic voltage regulator (AVR) which regulates the output voltage amplitude. Stage 2 consists of design and modelling of the propulsion motor and the voltage/frequency drive (VFD). The model parameters from for both the generator and the propulsion motor used in the simulations were extracted from the datasheets provided by DAMEN group. In Stage 3, a 6-pulse rectifier and an active rectifier were introduced into the power system. Vector control was also implemented in the active front end to get optimal functionality. Cascaded loops, modelled in the synchronous (DQ) frame, regulate the AC input current and the DC voltage of the active front end rectifier. Finally, Stage 4 consisted of integrating the previous components into two main architectures, one with the 6-pulse rectifier and the other with the active front end. Various simulations of these two architectures were carried out in order to verify the designs and test their behaviour under various operating conditions. The results obtained show an integrated shipboard power system operation using either an uncontrolled 6-pulse rectifier and an active front end. Analysis of the overall system power quality such as harmonics, controllability and maximum output power are also conducted.