Performance Analysis of a Four-Switch Three-Phase Grid-Side Converter with Modulation Simplification in a Doubly-Fed Induction Generator-Based Wind Turbine (DFIG-WT) with Different External Disturbances

This paper investigates the performance of a fault-tolerant four-switch three-phase (FSTP) grid-side converter (GSC) in a doubly-fed induction generator-based wind turbine (DFIG-WT). The space vector pulse width modulation (SVPWM) technique is simplified and unified duty ratios are used for controlling the FSTP GSC. Steady DC-bus voltage, sinusoidal three-phase grid currents and unity power factor are obtained. In addition, the balance of capacitor voltages is accomplished based on the analysis of current flows at the midpoint of DC bus in different operational modes. Besides, external disturbances such as fluctuating wind speed and grid voltage sag are considered to test its fault-tolerant ability. Furthermore, the effects of fluctuating wind speed on the performance of DFIG-WT system are explained according to an approximate expression of the turbine torque. The performance of the proposed FSTP GSC is simulated in Matlab/Simulink 2016a based on a detailed 1.5 MW DFIG-WT Simulink model. Experiments are carried out on a 2 kW platform by using a discrete signal processor (DSP) TMS320F28335 controller to validate the reliability of DFIG-WT for the cases with step change of the stator active power and grid voltage sag, respectively

Split DC bus converters for power electronic and AC-DC Microgrid applications

Power electronic converters are used extensively for electrical power conversion in applications such as renewable energy systems, utility applications, and electric vehicles. Such converters are needed as it is rare for a source voltage to fit the needs of a load or a set of loads for any particular application. They consist of active semiconductor switches and passive components that are combined in circuit structures (topologies) that are operated with a control strategy. The focus of this thesis is on AC-DC and DC-DC converters and their applications in AC-DC microgrids. AC-DC converters are typically two-stage converters that consist of a front-end AC-DC converter followed by a DC-DC back-end converter. The AC-DC front-end converter converts AC voltage from an AC source such as the grid to a DC bus voltage that has been filtered by an intermediate DC bus capacitor; the DC-DC converter then converts this DC voltage into the desired output voltage. A less expensive alternative to this two-stage approach is to have just one converter perform AC-DC and DC-DC conversion. This thesis examines isolated single-stage AC-DC converters and back-end DC-DC converters for two-stage converters that have a split DC bus, with either two capacitors in series across the bus to split the voltage or with two parallel current paths to split the bus current. These converters have fewer components or fewer light-load losses than converters with conventional topologies. Four new power converters with a split DC bus are proposed in this thesis: a reduced-switch three-phase AC-DC converter, two lower power DC-DC converters, and an AC-DC converter that can be used to simplify the architecture and control of AC-DC hybrid microgrids. The proposed converters increase efficiency and reduce the control complexity of hybrid microgrids. The operation of each converter is explained, the steady-state characteristics and the dynamic model of each converter are determined by mathematical analysis, and a procedure that can be used for their power and control stages design is developed. Experimental and simulation results are used to confirm the feasibility of the converters and simplified AC-DC hybrid microgrid, and conclusions that resulted from the thesis work are stated