DC/DC converter for offshore DC collection network

Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes.Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes

DC collection systems for offshore wind farms

Power generation through natural resources has found to be one of the best options to minimise climate change and global warming concerns. Among the naturally replenish sources, power generation from offshore wind accounts for a larger share. This has been showcased by the rapid development of offshore wind farms (OWF)s especial in the North sea. At the OWF collection system level, only alternating current (ac) technology is being used at present. Conversely, the use of direct current (dc) technology could provide additional benefits in terms of control flexibility, minimising system losses, and increasing power density of components. However, there are still a number of technical challenges that require addressing. One of the major aspects is the reliability of this concept as a whole. The research work presented in this thesis is aimed to address the existing challenges, in particular, from the component level to the system level from the perspective of reliability. The main contributions of this research work comprise of four parts, namely, (1) reliability analysis of semiconductors of dc-wind turbine machine side converter, (2) propose a new selection guideline based on reliability and costs to identify the most suitable multi-level converter topology for offshore wind power dc collection systems at different voltage levels and power levels, (3) identification of the most suitable dc collection system topology in terms of reliability and other economic factors, and (4) development of an analytical methodology to asses the availability of offshore wind farms considering the cable network dependency. One of the key building blocks of a dc collection system is the dc wind turbine (dcWT). The lifespan of a wind power system is highly influenced by the reliable operation of its power converter. A mission-profile based reliability assessment technique considering long-term and short-term thermal cycles are used to evaluate the lifetime of power electronic components of a dual active bridge based dcWT. Further, to ensure an effective lifetime evaluation of the entire converter system, a Monte Carlo method is used to generate the lifetime distributions and entire unreliability functions for power semiconductors. To utilise the full capacity of the dc technology in the context of the OWF collection system, the selection of a suitable power electronic converter topology is a key aspect. iii iv A selection criterion based on the optimal redundancy level with the consideration of the converter reliability, preventive maintenance interval, operational efficiency, the total cost of ownership and return on investment is proposed. The primary motivation of this work is to investigate the feasibility of utilising suitable multi-level voltage source converter topologies at different medium voltage dc levels and power levels. To select a suitable dc collection system topology, a comprehensive analytical reliability evaluation method based on Universal Generating Function (UGF) is proposed with associated economic factors. This strategy combines the stochasticity of wind with multiple power output states of a single wind turbine (WT). Subsequently, the relationship between the output states and corresponding state probabilities of WTs are combined using the UGF technique considering the network structure. To identify the best topology, the investment- and operating- costs (which includes network losses) are incorporated. The OWF collection system is made up of a considerable number of inter-array cables. The effectiveness of the OWF to export energy to the grid depends on the availability of that network. Therefore, it is imperative to include the reliability of the collection system in the overall availability assessment. However, this increases the number of components significantly, introducing the dimension curse. This combined with wind turbine output dependence makes the inclusion of the collection system in OWF availability assessment computationally intractable. An analytical reliability model based on the UGF technique is proposed accounting for the cable network dependency. Further, the impact of modelling wind farm components using a binary Markov model rather than a multi-state one is also investigate

Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

Design and Optimization Considerations of Medium-Frequency Power Transformers in High-Power DC-DC Applications

Recently, power electronic converters are considered as one of the enabling technologies that can address many technical challenges in future power grids from the generation phase to the transmission and consequently distribution at different voltage levels. In contrast to the medium-power converters (5 to 100 kW) which have been essentially investigated by the automotive and traction applications, megawatt and medium-voltage range isolated converters with a several kilohertz isolation stage, also called solid-state transformers (SST), are still in an expansive research phase. Medium-frequency power transformers (MFPT) are considered as the key element of SSTs which can potentially replace the conventional low-frequency transformers. The main requirements of SSTs, i.e., high power density, lower specific losses, voltage adaptation and isolation requirements are to a great extent fulfilled through a careful design of MFPTs. This work proposes a design and optimization methodology of an MFPT accounting for a tuned leakage inductance of the transformer, core and winding losses mitigation, thermal management by means of a thermally conductive polymeric material as well as high isolation requirements. To achieve this goal, several frequency-dependent expressions were proposed and developed in order to accurately characterize such a transformer. These expressions are derived analytically, as in frequency-dependent leakage inductance expression, or based on finite element method (FEM) simulations, as in the proposed expression for high-frequency winding loss calculation. Both derived expressions are experimentally validated and compared with the conventional methods utilizing detailed FEM simulations. Utilizing the proposed design method, two down-scaled prototype transformers, 50 kW/5 kHz, have been designed, manufactured and measured. The nanocrystalline-based prototype reached an efficiency of 99.66%, whereas the ferrite-based transformer showed a measured efficiency of 99.58%, which are almost the same values as the theoretically predicted ones. Moreover, the targeted value of prototype’s leakage inductances were achieved through the proposed design method and were validated by measurements. Finally, using SiC MOSFETs and based on the contribution above, the efficiency and power density of a 1 / 30 kV, 10 MW turbine-based DC-DC converter with MFPT are quantified. It was found that, with respect to the isolation requirements, there is a critical operating frequency above which the transformer does not benefit from further volume reduction, due to an increased frequency

TIDAL STREAM DEVICES: RELIABILITY PREDICTION MODELS DURING THEIR CONCEPTUAL & DEVELOPMENT PHASES

Tidal Stream Devices (TSDs) are relatively new renewable energy converters. To date only a few prototypes, primarily horizontal-axis turbine designs, are operational; therefore, little reliability data has accumulated. Pressure to develop reliable sources of renewable electric power is encouraging investors to consider the technology for development. There are a variety of engineering solutions under consideration, including floating tethered, submerged tethered, ducted sea-bed bottom-mounted and sea-bed pile-mounted turbines, but in the absence of in-service reliability data it is difficult to critically evaluate comparative technologies. Developing reliability models for TSDs could reduce long-term risks and costs for investors and developers, encouraging more feasible and economically viable options. This research develops robust reliability models for comparison, defining TSD reliability block diagrams (RBD) in a rigorous way, using surrogate reliability data from similarly-rated wind turbines (WTs) and other relevant marine and electrical industries. The purpose of the research is not to derive individual TSD failure rates but to provide a means of comparison of the relative reliabilities of various devices. Analysis of TSD sub-assemblies from the major types of TSDs used today is performed to identify criticality, to improve controllability and maintainability. The models show that TSDs can be expected to have lower reliability than WTs of comparable size and that failure rates increase with complexity. The models also demonstrate that controls and drive train sub-assemblies, such as the gearbox, generator and converter, are critical to device reliability. The proposed developed models provide clear identification of required changes to the proposed TSD system designs, to raise availability, including duplication of critical systems, use of components developed for harsh environments and migration of equipment onshore, wherever practicable