High-Voltage Multilevel Converters: Application of Multilevel Converters in Direct Grid Connection of Wind Turbines
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- Institutt for elkraftteknikk 
In this thesis a modular PMSG and converter topology, suitable for utility interface of large wind turbine has been has been explored. A comprehensive analysis on different sinusoidal pulse width modulation has been performed an Matlab/Simulink® environment. Through analytical calculation, it is proved that multilevel converters can have higher energy efficiency compared to traditional two-level inverters. The possible solutions to draw sinusoidal current from the generator windings, where they are interfaced with single phase diode rectifiers are investigated and the proposed solution is simulated and the results are confirmed by laboratory tests. The laboratory results show that the combination of a single phase diode rectifier with a boost chopper which operates at continuous mode of operation presents a cost effective solution for reducing the torque ripple and controlling power factor of the generator. Moreover, a mathematical modeling of the boost chopper interconnected with a single phase diode rectifier performed and an algorithm for maximum power point tracking without measuring wind speed has been proposed. The algorithm is simulated and the results show satisfactory performance. The algorithm is implemented in dSPACE, but due the limited available RAM the dSPACE could not operate satisfactory. A more powerful DSP card is needed to implement the algorithm. The Robicon Perfect Harmony drive has been chosen to investigate the possibility of utilizing available VFD for direct grid connection of wind energy systems. The simulation results demonstrate that in order to utilize this drive some modifications must be done in the topology of the drive. It is shown that the capacitances of the dc-links are a decisive factor for direct grid connection of PMSG. A value of less than 4.5 mF for a 23-level multilevel converter causes an increase in the third harmonic of the output voltage of the multilevel converter which makes it unacceptable for direct grid connection. A laboratory test is performed on a 50 kW PMSG to investigate the voltage stress in the generator coils when the output voltage of the PMSG is in maximum amplitude. It was observed that the amplitude of the voltage stress was less than induced voltage in each coil, but high dv/dt appears across each coil which may finally destroy the insulation. It is supposed that the switching functions of the IGBTs in the inverter side cause the problem and by using a multilevel converter instead of two-level inverter a smoother transition for the voltage stress from one level to another can be achieved. More practical tests are needed to accurately determine the level of voltage stress. Also an alternative topology for the grid side inverter has been proposed to reduce the losses and improve the THD profile of the converter output voltage. It is suggested to do analytical and practical tests on the proposed topology. All the simulations have been performed in Matlab/Simulink environment.