Abstract
The rise of the renewable energies in the electricity mix has given potential to emerging offshore wind sourced technologies. The difficulty of offshore operations and the large capital investment that this projects suppose has led to finding topologies that can export large amounts of energy over long distances. In this sense, High Voltage DC transmission has an advantage over High Voltage AC systems, as they become more affordable for higher power transmission over longer distances. Moreover, Modular HVDC generators allow HVDC power production due to the segmented stator and Voltage Source Converters connected in series, which avoids a transforming step to achieve HVDC.
This thesis studies electrical faults that can happen in the generator modules to analyse the operating behaviour of the system and ultimately determine if operation can be continued through healthy modules.
First, the origin of faults occurring in the modules is identified and the faults are split into two categories: the internal module faults and the inter-module faults. Furthermore, detection and protection techniques for the faults are researched to further simulate and analyse the faults.
The simulations of a single module, and later on eight modules are done in Matlab Simulink. The model contains a module rated at 625 kW of power and 26.7 Hz fundamental frequency and a DC output voltage of 9.44 kV per module, taken from Casper L. Klop's machine design in the thesis "Forces and vibrations in a Modular HVDC Generator". The faults are simulated without fault detection and protection at first, then a detection and protection scheme is added to the generator control. The fault protection is based on the power converter's action and the strategies used are Six Switch Open or diode flywheel mode and Active Short Circuit protection.
Based on the simulation results, it is concluded that operation might be possible in some fault cases if n-1 operation control was added, so that the system can operate with the healthy modules and the faulty module can be disconnected. Moreover, to implement this n-1 operation a bypass switch for the DC side should be added in further work to disconnect the faulty module, as well as a switch in the module neutral to open fault paths between modules.
