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Title: Maximum power control of permanent magnet synchronous generator based wind power generation systems
Author: Li, L.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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Wind power has been one of the fastest growing and most competitive renewable sources in the past decade. After the massive installation of new wind power generation systems (WPGSs), efficient and reliable operation of them has become one main concern of the wind power industry. This thesis focuses on improving the generation efficiency of WPGSs. The study is carried out based on permanent magnet synchronous generator (PMSG)-WPGS, which is one main stream WPGS due to the merits such as high reliability, high efficiency, low noise, fast dynamic response and self-excitation on the rotor. A PMSG based wind turbine connects to the power grid via back to back full-rate power electronic converters and can be controlled to rotate at a varying speed in variable wind power to achieve high efficiency. Chapter 1 starts an overview of typical WPGSs, including different types of wind turbines, generators and power electronic devices. Then the three typical operation regions of a variable speed variable pitch wind turbine are presented, i.e. the first one variable speed operation region, the second one fixed rated rotation speed operation region, and the third one rated power operation region. In the first operation region, the control objective is achieving maximum power extraction from the wind, so called maximum power point tracking (MPPT). Methods for WPGSs to achieve MPPT are reviewed. A reliable and efficient operation of a WPGS requires smooth switching between three operation regions when the wind speed changes between the cut-in and the cut-out speed. As different region has different control strategy, thus the switching between different operation regions is required. Non-smooth and excessive switching between different control strategies will introduce fatigue loads during the transitions period thus damage the mechanical part of a WPGS. Thus a smooth region switching logic is proposed in Chapter 2 to minimise sudden changes during switching. The proposed method is verified via simulation on a 5th-order nonlinear variable-speed wind turbine model and by hardware-in-the-loop test based on Speedflo and dSPACE platform. Among all the MPPT methods, perturbation&observation (P&O), also called hill-climb-search (HCS) method, is commonly used in industry for small-scale wind turbine as it does not require wind speed measurement and prior knowledge of the wind turbine characteristics. However, it may fail to track the optimal power points, or even lose its trackability in variable wind speed environment. To tackle this challenge, a wind speed variation detection method is proposed to improve performance of the HCS method. With the wind speed variation detection, the wind turbine can change its rotational speed according to the changes in wind speed. The proposed method is verified in simulation and hardware-in-the-loop tests with a PMSG based WPGS. The simulation results show the proposed method can detect the wind speed variation during the operation such that the misleading of HCS during variable wind speed can be avoided. Results of hardware-in-the-loop tests show that compared with the conventional HCS method, the developed method has higher MPPT efficiency. It can generate 4% more power than the conventional HCS method with the same wind speed input. Besides HCS method, power signal feedback (PSF) is another feasible MPPT method. PSF method needs the wind turbine's optimal power-speed curve as the reference to control the rotational speed to track maximum power points. In industry, the wind turbine manufacturers need additional aerodynamic experiments to obtain the accurate optimal power-speed curve of wind turbines. However, the optimal power-speed curve of each type of wind turbines is different. Even for the same wind turbine, the optimal curve may change after installation, due to the change of operation environment such as temperature variation and dust pollution. To avoid those special tests, a method is proposed in Chapter 4 for wind turbines to on-site detect the optimal power-speed curve. With an accurate optimal power-speed curve, the WPGS can efficiently achieve MPPT by PSF control. Both simulation and hardware-in-the-loop tests show that the proposed method can detect the wind turbine optimal power-speed curve in variable wind speed. It can be used to calibrate PSF reference and improve the MPPT efficiency. To verify the proposed MPPT algorithm, in Chapter 5, a designed power electronics hardware is tested with a PMSG based WPGS and finally implemented in a small scale prototype WPGS. The small scale prototype implementation and field tests are given in Chapter 6. In Chapter 5, a diode rectifier with DC-DC boost converter is designed and implemented as the generator-side converter in a PMSG based WPGS. The operation and model of DC-DC boost converter are given. Due to the low cost and high reliability of diode rectifier, it is widely used in PMSG based WPGSs. A boost converter controls the dc-side voltage and current for MPPT and steps up the voltage for grid connection.
Supervisor: Jiang, L. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral