Investigation of multi-phase tubular permanent magnet linear generator for wave energy converters

In this article, an investigation into different magnetization topologies for a long stator tubular permanent magnet linear generator is performed through a comparison based on the cogging force disturbance, the power output, and the cost of the raw materials of the machines. The results obtained from finite element analysis simulation are compared with an existing linear generator described in [1]. To ensure accurate results, the generator developed in [1] is built with 3D CAD and simulated using the finite-element method, and the obtained results are verified with the source.The PRIMaRE project

Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

A study of rotor eccentricities effects on brushless doubly fed machines performance

This paper studies the unbalanced magnetic pull (UMP) in the Brushless Doubly-Fed Machine (BDFM). Analytical study is performed to derive the UMP, then, Finite element (FE) analysis, which has been verified experimentally, is used to verify the analytical method. The BDFM with different types of rotor eccentricities including static and dynamic eccentricities, are also modeled in FE method and their resultant UMPs are obtained. The results are compared with the case at which a perfectly constructed rotor is considered. The study has been carried out on a prototype D400 250 kW BDFM

Vernier hybrid wind turbine generators

A great deal of research is currently being undertaken in the field of renewable energy, much of it dedicated to wind power conversion and focussing on a machine topology that increases efficiency. The vernier hybrid machine, VHM, is one topology considered and has promising features for use in direct drive and variable speed operation. Using the VHM topology, two prototype generators were built and tested. The machines use magnets that are buried within the stator and orientated in a flux concentration arrangement. The flux paths in the machines are inherently 3D in nature and thus require complicated modelling methods to achieve sufficient design accuracy. Various tests were conducted in an effort to find results that will describe the machines characteristics and operating mechanisms. It was found that increased torque handling capabilities can be observed at the detriment of power factor and construction complexity. With the use of power factor correction and suitable construction techniques the VHM is a viable wind turbine generation technique. To determine whether it is suitable for renewable energy applications would require an in depth economic feasibility study over the operating lifetime of the machine

Wind Turbine Generator Reliability: An Exploration of the Root Causes of Generator Bearing Failures

Increasing the availability of multi-megawatt wind turbines (WT) is necessary if the cost of energy generated by wind is to be reduced. Reliability surveys have shown that WT generator bearings have a relatively high failure rate, with failures happening too early to be due to classical rolling contact fatigue. It has been the purpose of the present work to demonstrate the value of models which may help to explain some of the failure modes of wind turbine generators (and their root causes). The work has considered two potential root causes of wind turbine generator failure. Firstly, gearbox-generator misalignment caused by deflection of the compliant drivetrain under loading. Secondly, electrical discharge machining (EDM) of the generator bearings due to the common-mode voltage caused by pulse width modulation (PWM) of the power electronics. Numerical simulations have been used to investigate these potential failure modes and to show how they bring about premature failure. It has been shown that there exists a mechanism by which gearbox-generator misalignment can exacerbate EDM. This demonstrates the importance of holistic analyses of WTs, which are complex electromechanical systems with non-trivial interactions between sub-assemblies. The need for further research has been shown

Investigation into the use of variable speed drives to damp mechanical oscillations

Research report to School of Electrical and Information EngineeringAn investigation was conducted into how a variable speed drive can provide a damping torque when mechanical oscillations are present. The modeling of mechanical oscillations via an analogous electrical circuit was performed. Simulation was used to demonstrate how a variable speed drive is able to damp speed oscillations using Direct Torque Control (DTC). Damping of mechanical oscillations is done by means of the variable speed drive providing a damping torque component that is in-phase with the speed deviation. The simulation showed that by applying a small torque component with the speed variation results in torque oscillations being damped by 60% after the initial disturbance. Damping is further improved by applying a torque component equal to the speed variation resulting in the oscillations being damped by 80% when compared to the initial disturbance.MT201