Operation and protection of DC shipboard power system

Marine vessels integrated with electrical propulsion have conventionally been based on fixed-voltage, fixed-frequency (50/60 Hz) ac generation and distribution system. In recent years, dc power system in the marine vessels has been proposed primarily to take advantage of the fuel-efficient operation, which is enabled through the integration of the variable frequency diesel generators. Such dc shipboard power system (SPS) also enables interconnection of the alternative power generation and energy storage technologies, which helps in peak shaving of the generators in the event of wide load variation. In spite of the advantages, one of the impediments to the widespread adoption of dc SPS is the lack of comprehensive short-circuit fault management strategies. These vessels are dominated by a significant number of active loads and a finite number of dc generation sources. As a result, the network configuration is expected to be dynamically altered to fulfil the required generation and load demands to cater for the desired marine mission. Such varying network configurations make the transient responses significantly different from the conventional ac grids and the prospective dc grids and hence making the fault management strategies more challenging. Thus, the modeling and control of dc generation sources, loading scenarios, and system operation become important aspects to effectively understand and analyze transient responses. The aim of this thesis is to address the modeling and control aspects of dc shipboard power systems and devising protection algorithms. This thesis considers the platform supply vessel (PSV) as the target dc marine vessel and covers detailed investigation on the challenges in the modeling and control and the solutions of the dc generation and load systems. PSV is taken as an example of the marine vessel due to its complex operating scenarios and wider applicability in the marine industry. Voltage source converter (VSC) based dc generation system is chosen owing to its improved operational benefits. The real-time transient framework and operation of the dc PSV are discussed along with the possible contingency scenarios, such as the outage of the generation systems, abrupt load changes, effect of the energy storage systems and so on. The disadvantage of the dc system is the lack of zero current crossing which worsens the problem of arc quenching. In dc power system, the converters will be used as interfaces between the generators and the marine loads. During the fault, the dc-link capacitor will discharge rapidly, releasing high current. This capacitive discharge current represents a serious challenge in fault detection and protection as the current profile depends on the circuit parameters. Moreover, the time required to detect the dc fault current is very low. It is thus required to devise suitable protection algorithm for fault detection and isolation. Proper operation of the dc SPS calls for high fidelity control and modeling of the system components. After the modeling and operation, this thesis also covers systematic transient studies to devise the short-circuit fault detection technique for the dc PSV. The transient response of the VSC-based dc generation system in the dc PSV is generally characterized by rapidly rising capacitive discharge current which is different from the ac counterpart. The limitations of the traditional time-domain based fault detection techniques for the varying network conditions of the dc PSV are also discussed. The rapidly rising fault current is expected to have high-frequency components which could be an effective indicator of the transient condition. With this regard, this thesis also covers a short-time Fourier transform (STFT) based quantitative analysis of the high-frequency components in the dc fault currents. Detailed operating principles, factors affecting the STFT operation and the sensitivity analysis are also discussed. For the enhanced selectivity in the dc PSV, a novel directional protection is also proposed which uses directional zonal interlocking as a directional element and STFT as fault detector. The efficacy of this proposed directional protection is also presented which is verified against a range of fault impedances initiated at the generator terminals, load terminals, lines and buses of the dc PSV. The thesis is concluded by discussing the future work and recommendations on the fault-tolerant architectures to external short-circuit faults.Doctor of Philosoph