الفهرس 
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Abstract
Over the last several decades, the production of electricity has been progressively influenced by renewable energy sources like solar, wind, and hydropower that produce less carbon emissions and more environmentally friendly electrical systems. Offshore wind farms are one of these renewable energy resources which have been expanded and developed widely during recent years due to their benefits such as strong wind with less turbulence. The rapid growth of offshore wind farms increases the researchers’ interest of their collection and conversion systems. Collection power networks of wind farms have different configurations based on whether the network voltage is medium voltage direct current (MVDC) or medium voltage AC (MVAC). Since offshore wind farms need less installation area and cost, MVDC collection grids are the preferred topology for these farms. Medium voltage DC-DC and AC-DC power converters with high voltage gain are primary components for the windfarms’ collection schemes. As the power converters installed in offshore wind farms should have limited size and weight, the non-isolated converters are the most suitable choice for these farms. In this thesis, three power converters are proposed for DC collection power grids of offshore wind farms.
The proposed converters I and II are MV DC-DC power converters. The first one is a modular input parallel output series non-isolated DC-DC converter. It is a hybrid interleaved structure which combines two buck-boost topologies into a single converter. The proposed converter has the benefit of high voltage gain with reduced voltage and current stress on the converter’s components. Moreover, the interleaved PWM offers limited output voltage ripple and supply current ripple. The proposed converter II is also a modular DC-DC converter with high voltage gain based on the non-inverting buck-boost topology for high-power applications. The proposed converter consists of three non-inverting buck-boost modules connected as input parallel output series (IPOS). This proposed DC-DC converter offers also reduced voltage/current stress on power devices, the increased voltage gain, and enhanced power conversion efficiency. The third proposed converter is a novel power factor corrected (PFC) single-phase bridgeless AC-DC converter. This converter’s structure integrates the inverting and non-inverting buck-boost dc-dc topologies that are connected to the ac supply through an LC filter. To ensure sinusoidal supply current, the proposed converter is operated in discontinuous current mode (DCM). The AC-DC proposed converter has many merits, such as a reduced number of components, enhanced efficiency, and soft start-up capability.
The operation modes and the steady-state performance of the proposed converters are analysed. In addition, small signal models based on the state space averaged method are developed for the proposed converters. The accuracy of the developed models is verified using the converters’ switching model. Furthermore, the output voltage control scheme utilizing a PI controller for each proposed converter is presented. These controller systems achieve a fast response with negligible oscillations and steady-state error. MATLAB/Simulink software package is used to simulate the proposed converters, investigate their operation, and examine the converters’ dynamic behaviour. Moreover, an 850W prototype is developed for converter III to validate the features of the proposed converter experimentally. Tight voltage regulation, sinusoidal supply current, and near-to-unity power factor operation are observed from the results, regardless of supply or load disturbances.
This Thesis includes six chapters as follows,
1- Chapter 1 includes a literature review of the offshore wind farms’ benefits and challenges, the different DC collection power schemes, the DC-DC converters, and the PFC AC/DC converters.
2- Chapter 2 presents the structure of a proposed hybrid interleaved DC-DC converter, including the converter’s steady-state characteristics, dynamic performance, and converter modeling. Moreover, the converter’s output voltage controller is designed.
3- Chapter 3 introduces a modular, non-inverting buck-boost DC-DC Converter along with details on its dynamic performance, steady-state characteristics, efficiency evaluation, and converter modeling. Additionally, the output voltage controller of the converter is designed.
4- Chapter 4 includes the steady state and dynamic performance evaluation for a proposed hybrid bridgeless PFC AC-DC converter. The converter’s performance analysis includes efficiency evaluation. Moreover, the converter’s output voltage controller is designed based on a developed state space average model.
5- Chapter 5 includes two wind energy conversion systems using two of the proposed converters to investigate the capability of using the proposed converters in the medium voltage power collection networks and evaluate their performance.
6- Chapter 6 includes the thesis contributions and the suggested future work.