Analysis of current control of a unity power factor rectifier in a standalone wind energy conversion system feeding a DC bus
Date of Issue2016-04-13
School of Electrical and Electronic Engineering
A stand-alone isolated wind energy conversion system (WECS) is comprised of a wind turbine, a mechanically coupled generator, a power electronic converter, a battery system and the electric load. Such a system does not have a connection to the utility grid. Traditionally, a diode rectifier is used to convert the generator AC output voltage to a DC voltage in order to connect to a DC-bus, which is interfaced with DC loads and the battery system. However, owing to their nonlinear switching nature, diode-bridge rectifiers inject harmonic components into the system, at the interface point, leading to an increased total harmonic distortion (THD) of generator current, which leads to increased AC side losses leading to heating, equipment malfunction and reduced utilization efficiency of the WECS system. To mitigate these power quality issues, a unity power factor (UPF) rectifier scheme is explored as a suitable alternative in place of the fixed diode rectifier structure, as the front end AC-DC converter for the wind energy conversion system. In this thesis, the focus is on developing a novel current control strategy for the modified Unity Power Factor Rectifier in a standalone wind energy conversion system, that ensures high-power factor operation and low current harmonics at the rectifier AC side, as well as the extraction of maximum mechanical power from the wind turbine (maximum power point tracking). The parameters under study are the input Power Factor (PF), Total Harmonic Distortion (THD) of the AC input currents to the AC-DC converter and the coefficient of performance (Cp) for the wind turbine. A wind-turbine interfaced permanent magnet synchronous generator (PMSG) and a unity power factor current-controlled rectifier are modelled in the PSIM® software environment, to validate the performance of the proposed new WECS topology using the UPF AC-DC rectifier converter. Firstly, two current control methods were tested, the Average Current Control and Hysteresis Current Control, are employed using the UPF rectifier as the front end converter, to test their effectiveness in meeting the system objectives. A traditional diode bridge rectifier without the current control is used as the base case front-end converter to compare the performance with the UPF rectifier AC-DC converter. The performance of the UPF rectifier AC-DC converter is tested for constant wind speed conditions as well as for varying wind speed profile under rated load. In addition to high power factor operation, the power electronic converter must also maximize the power extracted from the wind turbine. A maximum power point tracking (MPPT) algorithm, based on Perturb and Observe and a modified Hill Climbing Search technique, is included in the current control to facilitate maximum power tracking. Rotor-speed control of the wind-turbine is performed using a unique combination of maximum power tracking using Particle Swarm Optimization (PSO) technique and power factor correction. Dynamic and steady state operations are investigated in PSIM® software environment which validates the performance of the current-controlled Unity Power Factor Rectifier Converter. Finally, the performance of the MPPT is further improved by utilizing the PSO algorithm approach using a static PSO algorithm for constant wind speeds and dynamic PSO algorithm for varying wind speeds operations. The combined current control algorithm with Hysteresis Current Control and PSO is validated for excellent dynamic performance in meeting the operational system objectives, viz. maximum power point tracking, Unity Power Factor Control and reduced total harmonic distortion of input line AC side currents. The UPF rectifier AC-DC converter is therefore capable of maximum power extraction from the wind turbine system for a wide range of wind speed variations while maintaining higher power factor operation at AC side, good voltage reference tracking and reduced switching voltage stress. It is proposed as an efficient AC-DC converter for operation in a stand-alone WECS.