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Experimental and Simulation Results for Sliding Mode Dynamic Wind Turbine Control using a DC Chopper
G.Riahy,P.Freere,D.G.Holmes 전력전자학회 2001 ICPE(ISPE)논문집 Vol.2001 No.10
Wind speeds can vary rapidly and wind turbines cannot easily follow these variations because of their inertia and aerodynamic characteristics. For maximum energy extraction, the turbine blades should operate at their optimum tip speed ratio, but with rapid changes in wind speed, this is usually not possible. To improve the energy extraction from turbulent wind, it is necessary to establish an effective measure of the high frequency component of the wind, and then to use this measure to optimise the operation of the turbine controller for maximum energy extraction. This paper presents an approach for combining readings from three anemometers into a composite wind speed measurement, and using this signal to control the operation of a permanent magnet generator to achieve maximum energy extraction. The method combines simulation and experimental investigations into a heuristic algorithm, and demonstrates its effectiveness with field trials.<br/>
An Improved Three Phase Variable Band Hysteresis Current Regulator
D. G. Holmes,R. Davoodnezhad,B. P. McGrath 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5
This paper presents an improved variable band hysteresis current controller for a two level three phase voltage source inverter. The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMFs, and uses this result to vary the hysteresis bands to maintain constant phase leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronise the zero crossings of the phase leg current errors with a fixed reference clock, to ensure nearest space vector switching and hence achieve the best possible harmonic switching performance. Finally, a technique is proposed to replace the third phase leg current regulator with a fixed frequency open loop PWM modulator, where its commanded reference is generated from the average switched output voltage of the other two phases. This avoids the hazard of three independent current regulators in a conventional system adversely interacting as an over constrained control problem, and allows the linear modulation range to be increased by adding a common mode third harmonic component to the third phase leg reference command.
J. Riedel,D. G. Holmes,B. P. McGrath 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6
Dual Active Bridge (DAB) converters offer an unmatched capability to transfer energy in either direction between two DC sources, while also providing galvanic isolation and high conversion efficiency. However, to operate at high efficiencies, the bridges must operate with Zero-Voltage-Switching (ZVS) over as wide an operating range as possible. The conventional approach to determine ZVS operation uses time domain analysis with ideal AC coupling inductances, which only approximately identifies the ZVS boundaries. This paper proposes a new approach using frequency domain analysis of the bridge switching patterns, which accurately predicts the ZVS boundaries over a full range of operating conditions while also accommodating more complex AC coupling structures and practical impedance non-idealities. An exact theoretical analysis is presented for two level modulation of the two bridges coupled through a general impedance structure. An analytical solution is then presented for a single coupling impedance, while boundaries for more complex coupling structures and practical impedances are solved by numerical integration. ZVS boundaries for selected systems are validated by matching simulation and experimental results.
A. A Nazib,D.G Holmes,B.P McGrath 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5
Current regulation of inductive-capacitiveinductive (LCL) grid-connected inverters typically require multiple current sensors to actively damp the filter resonance. Unfortunately, while inverter side current regulation avoids this issue because of its inherent LCL filter damping properties, it does not work well with distorted grids. Furthermore, both strategies require voltage sensors for grid synchronization. This paper proposes a novel current control strategy for LCL grid-connected inverters that requires only inverter side current sensors, with the grid voltage phase and capacitor current estimated from the Proportional Resonant current regulator to achieve self-synchronized indirect regulation of the grid-side current. The effectiveness of the approach has been verified using simulation and matching experimental investigations.
Evolution of strain throughout gallium nitride deposited on silicon carbide
김지현,M.A. Mastro,N.D. Bassim,J.A. Freitas Jr.,M.E. Twigg,C.R. Eddy Jr.,D.K. Gaskill,R.L. Henry,R.T. Holm,P.G. Neudeck,A.J. Trunek,J.A. Powell 한양대학교 세라믹연구소 2007 Journal of Ceramic Processing Research Vol.8 No.5
During GaN growth on an on-axis SiC substrate, a large density of dislocations (~109 cm−2) is unavoidably generated by the GaN/SiC misfit, uncontrolled misorientation in the substrate and defects present at the surface of the substrate. Recently a unique SiC substrate was developed with step-free and stepped mesas as well as continuous surface areas similar to a standard wafer all in close proximity. It is now possible to isolate the influence of defects and steps on the deposition of GaN on SiC. This paper provides a link between the dislocation environment and the strain state in the GaN film as well as the change of this strain with increasing thickness.