| Recently, the voltage source converter(VSC), as a high-performance energy interface, has been widely used in electric power systems, such as renewable energy integration(e.g. wind turbine, photovoltaic, etc.), flexible AC transmission system(FACTS), high voltage DC(HVDC) transmission and adjustable speed drives with power regeneration. In the existing literatures,two important types can be identified from the published control strategies for VSCs: voltage oriented control(VOC) and direct power control(DPC). The line current and instantaneous power are utilized as the control variables in the former and latter, respectively. This thesis conducts the control strategy research of three-phase VSCs based on the instantaneous power theory and virtual flux technique.This thesis develops a AC voltage sensorless predictive direct power control(PDPC) strategy for three-phase VSCs based on accurate virtual flux(VF) estimation. The proposed strategy achieves a constant switching frequency operation by adopting a space vector modulation(SVM)modulator. AC voltage sensors are removed by using the following two techniques. One is that an accurate VF observer is employed to compensate the magnitude and phase errors introduced by the first-order low pass filter(LPF). The other is that grid voltages are estimated using the VF based estimation of instantaneous active and reactive power as intermediate variables. In addition, the time delay existing in a real system is taken into account in the proposed control model.Comparative results demonstrate the excellent performance of the proposed PDPC compared with the conventional look-up-table DPC(LUTDPC) and the conventional PDPC.So far, the control strategies for three-phase VSCs have been deeply investigated under ideal grid voltage conditions. Nevertheless, due to the existence of various grid disturbances, the actual grid voltage is different from the ideal case, e.g. symmetrical sinusoid and rated frequency. Then, the actual grid condition would degenerate the controller performance which is established based on the ideal assumption, even trip the converters. In order to solve this problem, this thesis proposes two AC voltage sensorless schemes for DPC, i.e. VF based positive-sequence power control and VF based fluctuating power compensation. In their hardware configurations,AC voltage sensors are both replaced by a second order generalized integrator(SOGI) based VF observer, which achieves accurate VF estimation independent from the grid frequency deviation. Meanwhile,the gird voltage vector is replaced by the grid VF vector. Regarding the control strategy design, the former employs the active and reactive power derived from the fundamental positive-sequence component as control variables, to perform the unbalance regulation and harmonic suppression of line currents. The waveform control of line currents is also contributed by using the fundamental negative-sequence VF feedforward and SOGI-based sequence extraction. In the latter, through specific fluctuating power compensations carried out based on the estimated VF, balanced sinusoidal line current, constant active power and constant reactive power operations are ensured, respectively. Moreover,this thesis presents two improved versions of the proposed VF based PDPC based on the methods mentioned above. Simulation and experimental results verify the feasibility and validity of the proposed improvement schemes.Based on programmable AC power source and dSPACE system, the author develops an experimental platform for control strategy verifications of three-phase VSCs operating under different gird conditions. The control strategies proposed in this thesis and conventional control strategies are implemented on this platform, and the experimental results together with the simulation results obtained by using MATLAB/Simulink demonstrate the feasibility and validity of the proposed strategies, and excellent performances of the proposed strategies compared with the conventional ones. |
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