David Tomás Díaz

Control of a small wind turbine in the high wind speed region

This paper proposes a new soft-stalling control strategy for grid-connected small wind turbines operating in the high and very high wind speed conditions. The proposed method is driven by the rated current/torque limits of the electrical machine and/or the power converter, instead of the rated power of the connected load, which is the limiting factor in other methods. The developed strategy additionally deals with the problem of system startup preventing the generator from accelerating to an uncontrollable operating point under a high wind speed situation. This is accomplished using only voltage and current sensors, not being required direct measurements of the wind speed nor the generator speed. The proposed method is applied to a small wind turbine system consisting of a permanent magnet synchronous generator (PMSG) and a simple power converter topology. Simulation and experimental results are included to demonstrate the performance of the proposed method. The paper also shows the limitations of using the stator back electromotive force to estimate the rotor speed in PMSG connected to a rectifier, due to significant d-axis current at high load.

An inrush current limiter as a solution of injection transformer oversizing in dynamic voltage restores

In this paper, the problem of injection transformer saturation in dynamic voltage restoration (DVR) applications is analyzed. The inrush currents caused by the injection transformer saturation are one of the most serious problems that affects to this kind of systems. The high values of these inrush currents can cause faults in the power electronic system or can generate trips of the converter overcurrent protection. Furthermore, the inrush currents provoke high voltage drops that distort the injected voltage. As a result, a bad dynamic restoration of the voltage during sags is obtained. The most adopted solution is the oversizing of the injection transformer, however this solution drastically increments the cost of the system. What the authors propose is a solution based in a DC reactor type inrush current limiter (ICL). This solution is based on a diode H-bridge and a DC reactor connected in series with the primary winding of the injection transformer. The use of this solution will reduce the oversizing requirements in the injection transformer, reducing the cost of the system.