MVDC Distribution Fed High Power Multi-Motor Drives

High power variable speed drives play a major role in many industrial applications. At the moment, these high power loads are supplied by the source (grid/generator) through the state-of-the-art MVAC power distribution network. The voltage levels, in these systems, are dependent on the power levels of the loads, other technical or historical requirements. These MVAC PDNs are used for energy intensive land based industrial and offshore applications that include, e.g., hot and cold rolling mills for metal industry, ore mining, processing plants, liquefied natural gas (LNG) tankers, drill ships and exploration platforms, etc. and, in such systems, the installed power capacity can reach up to 250 MW. With the recent improvements in the power electronic technologies, MVD) PDNs are being promoted for such installations, especially marine applications. There are a number of motives behind such a push, one of them being the opportunity to optimize the use of prime movers by operating them at the most optimal speed/efficiency with respect to the loading conditions, thus improving fuel efficiency. As today these prime movers are driving generators that are all synchronized to the same MVAC PDN, hence, to achieve decoupling of the speeds of the different prime movers there is a need to remove synchronization requirement first. This is easily achievable if generator's output ac voltages are rectified and connected together to MVDC PDN. Another motivation is the removal of the bulky transformers, which would lead to space and installation cost savings. Additionally, MVDC PDN provides flexibility in design of these systems and possibilities to include new technologies, e.g., gas turbines, permanent synchronous generators, energy storage, high power dc-dc converters, etc. In this thesis, a number of technical issues pertaining to a possible MVDC PDN are discussed and multi-converter dynamic interactions are investigated. In the first part of the thesis, a discussion and evaluation of the possible technologies is provided, commercially available or proposed in the literature, which can be adopted by the MVDC PDNs, e.g., prime movers, generators, rectifiers, cables, filters, inverters and motors. This is followed by a critical discussion on the adoption of MVDC PDN by analyzing the opportunities and the challenges of replacing the state-of-the-art MVAC PDN in the existing LNG tanker and drill ship. These two systems work in entirely different conditions, thus they provide distinct technological challenges for the adoption of the MVDC PDN. Taking into account the earlier evaluation, possible high quality multi-phase multi-pulse MVDC supplies, for different power levels, are proposed, designed, and simulated. In the second part, small signal models are developed for different commercially available rectifiers and inverters, mapping the impact of their respective control schemes. These small signal models are then used to investigate the dynamic interactions between the sources and loads that are connected through distribution cables. The effects of the variations of the different passive elements are studied, which show that lower filtering effort and higher inductance in the system lead to instability. Furthermore, state space modeling techniques are investigated for developing models to analyze multi-converter interactions in multi-port MVDC PDNs. This analysis is further used to identify unstable system configurations.