Wind farms must contribute to the stability and reliability of the transmission grid, if they are to form a robust component of the electrical network. This includes providing grid support during grid faults, or voltage dips. Transmission system grid codes require wind farms to remain connected during specified voltage dips, and to supply active and reactive power into the network. Doubly-fed induction generator (DFIG) technology is presently dominant in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially-rated power converter. However, the DFIG is sensitive to dips in supply voltage. Without specific protection to 'ride through' grid faults a DFIG risks damage to its power converter due to over-current and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed-circuit leads to poor power output and sustained suppression of the stator voltages. This thesis presents a detailed understanding of wind turbine DFIG grid fault response, including flux linkage behaviour and magnetic drag effects. A flexible 7.5kW test facility is used to validate the description of fault response and evaluate techniques for improving fault ride-through performance. A minimum threshold rotor crowbar method is presented, successfully diverting transient over-currents and restoring good power control within 45ms of both fault initiation and clearance. Crowbar application periods were reduced to 11-16ms. A study of the maximum crowbar resistance suggests that this method can be used with high-power DFIG turbines. Alternatively, a DC-link brake method is shown to protect the power converter and quench the transient rotor currents, allowing control to be resumed; albeit requiring 100ms to restore good control. A VAr-support control scheme reveals a 14% stator voltage increase in fault tests: reducing the step-voltage impact at fault clearance and potentially assisting the fault response of other local equipment.

The increasing interest among architects and planners in designing environmentally friendly buildings has led to a desire to explore and integrate renewable sources of energy within the built environment. Roof mounted wind turbines is a technology that presents a high potential for integration within the built environment. However, there is a state of uncertainty regarding the viability of these wind turbines. This thesis argues that part of this uncertainty is attributed to uninformed decisions about positioning and locating urban wind turbines. This is underpinned by lack of consideration to the wind accelerating effect of different roof shapes, buildings’ heights and surrounding urban configurations. This thesis aims to investigate the effect of different roof shapes on wind acceleration and positioning of roof mounted wind turbines covering different buildings’ heights within different urban configurations under different wind directions. To achieve the aim of the thesis, the commercial Computational Fluid Dynamics (CFD) code Fluent 12.1, implementing the Realizable k-ε turbulence model, is used to simulate wind flow around different roof shapes, different buildings’ heights and different urban settings. Predictions are comparatively analysed to identify the optimum roof shape for mounting wind turbines. Simulation results indicate that the barrel vaulted roof has the highest wind accelerating effect. The barrel vaulted roof shape case was carried further to investigate the effect of building height and surrounding urban configurations on the energy yield and positioning of roof mounted wind turbines. The optimum mounting location for each of the investigated roof shapes namely: flat, domed, gabled, pyramidal, barrel vaulted and wedged roofs is identified. Results from the investigation predict a possible increase up to 56.1% in energy yield in the case of a barrel vaulted roof if an informed wind assessment above buildings’ roofs is carried out. However, changing the building height and surrounding urban configuration had an effect on choosing the optimum mounting location and the energy yield at that location.

Wind power energy conversion is growing rapidly in the world. There are two main wind farm types, namely ac grid-based and dc grid-based wind farms. The dc grid-based approach reduces the size and weight of the magnetic components and cables. In the dc system, the step-up dc/dc converter is the key component when interfacing the wind turbine to the ac grid, via its low/medium voltage generator. This thesis focuses on the control and design of a wind energy conversion system based on dc/dc current source converters. An optimized One-Power-Point method for maximum power tracking is proposed. It incorporates One-Power-Point control and Maximum Power Differential Voltage control to allow the wind turbine to extract more energy during rapid wind speed changes. A current output hard-switched full bridge converter and serial-parallel resonant converter with an intermediate high frequency transformer are investigated for interfacing wind turbines to a local dc grid. These converters are assessed and compared in terms of semiconductor stresses and losses. A new modified One-Power-Point control method is proposed for the dc/dc converter, which tracks the maximum power during wind speed changes. A design procedure for the serial-parallel resonant converter is presented, based on its characteristics specific to a wind energy conversion system (WECS). A current source dc/dc converter based multi-terminal dc WECS is presented, investigated, and simulated. A practical multi-terminal dc WECS verifies its feasibility and stability, using two dc current output wind turbine units. Furthermore, a coordinated de-loading control scheme for the current sourcing based WECS is proposed, to cater for ac grid demand changes. It combines pitch control, dc dumping chopper control, and dc/dc converter control, to safely and quickly establish de-loading control. Both simulation and experimental results verify the de-loading scheme.

The residual scepticism surrounding the installation of wind turbines, is mainly due to the perceived lack of reliability and expense of these energy converters. Because of the commercial significance of reliability, few sources of such data are publicly available, with the result that knowledge of the true reliabihty of wind turbines is fragmentary and sometimes anecdotal, even among operators and professionals in the wind energy industry.

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Wind energy has been widely used around the world, but there are still many problems associated with the operation of the wind turbines. The increase penetration of wind power in power system further brings new challenges for the transmission system operators. The grid codes now require the wind turbines have a high quality output power and can be operated to provide network support in similar ways as for conventional generators. To improve the performance of the wind turbines and provide network support, extra energy is required. In order not to disturb the operation of wind turbines, small energy storage systems are connected to fully rated converter based wind turbines to improve their performance. Coordinate DC voltage control methods of the line side converter and energy storage system are investigated to provide power smoothing of the wind turbine output power. Wind turbines are also expected to provide damping to system. As system oscillation is often induced by AC faults, it is desirable for the wind turbines to ride through the fault first and then provide a damping effect. During the fault period, the energy storage system is controlled to assist the fault ride through process, and the line side converter is controlled to provide AC voltage support in accordance with grid code. Methods based 'on regulating the active power output of energy storage system and reactive power of line side converter is proposed to enhance power system damping. Assessment of variable speed wind turbine's inertial response for power system frequency support is made, and an improved control method to carry out inertial response is proposed. The embedded energy storage system helps smooth the wind turbine 's rotor speed transition from acceleration to deceleration while carrying out inertial response. The frequency support effect with AC and DC connected wind farms is studied.

The modern power system has undergone a significant transformation in the way it generates electrical energy. Increased levels of renewable generation has resulted in a reduction of conventional thermal plant and as a consequence introduced new challenges for network managers. Monitoring of recent power transfer activity between the regional power systems of Ireland has identified periods of sustained electromechanical mode resonance. As a matter of technical coincidence power pulsations at a frequency that coincides with existing system electrical modes have been detected at fixed-speed wind farms connected to the Northern Ireland power system. Prompted by the prevalence of fixed-speed wind turbine technology in Ireland, analysis carried out by research presented in this thesis sought to establish if low-frequency oscillations created by blade-passing (3-p) phenomenon of grid-connected fixed-speed wind farms are capable of influencing power system small-signal stability either by damping or reinforcing existing electromechanical modes. With the aid of electrical data measurements of grid-connected fixed-speed wind farms, visual-recognition software was developed to extract blade angle information from video frame sequences and correlate it with time-synchronised data. Visual analysis of turbine blade movement identified periods of synchronised blade-passing between groups of turbines that resulted in oscillations of greater magnitude at the wind farm level. A unique wavelet-Prony method developed as part of this investigation was used to characterise blade-passing properties of two fixed-speed wind farms over a full range of outputs. Blade-passing characteristics of both wind farms displayed greater mode amplitudes at rated output but with faster decay times. Eigen-analysis performed on a small inter-connected system with high levels of wind penetration exhibiting 3-p oscillatory modes revealed a significant improvement in intra and inter area mode damping. Studies on a larger system operating under high levels of dynamic system demand however, demonstrated electromechanical mode instability under the influence of blade-passing power pulsations. Based on simulation studies it can be concluded that there is a necessity for blade-passing rotational dynamics to be incorporated in future small-signal analysis especially in studies where there are large concentrations of fixed-speed wind farms.

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