University of South Wales Master of Sciences Thesis
Standalone hybrid generation system for the remote area of Thar, Pakistan
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Standalone hybrid generation system for the remote area of Thar, Pakistan Fig. 2.11 demonstrates the steady-state relationship between extracted aerodynamic power and wind speed. The dotted line represents the power in the unimpeded wind passing through the rotor swept area, while the solid curve represents the power extracted by a typical variable speed wind turbine. Classic control techniques such as proportional, integral and derivative (PID) control of blade pitch are typically used to limit power and speed on both the low and high-speed shafts for turbines operating in region 3, while generator torque control is usually used in region 2. For a variable speed wind turbine operating in region 2, the control objective is to ensure maximum energy capture by operating the wind turbine at the peak of the Cp – TSR as shown in Fig. 2.11. The power coefficient Cp (λ, β) is a function of the tip speed ration (TSR) λ and the blade pitch β. The TSR is defined as [24,25]: 2.21 From (2.16), the rotor aerodynamic power P increases with Cp. As a result, the wind turbine should be operated at the maximum power coefficient Cpmax. The relationship between TSR λ and blade pitch can be expressed as follows: 2.22 The steady-state power curve of the wind turbine for different wind speeds is given in Fig. 2.12. The steady-state power curve of the wind turbine for different wind speeds is given in Fig. 2.13. Standalone hybrid generation system for the remote area of Thar, Pakistan 2.4. Permanent Magnet Synchronous Generator (PMSG) Model PMSGs are synchronous ac machines. The PMSG consist of 3-phase stator winding similar to the SCIG, while the rotor winding is replaced by the permanent magnets. The advantages of eliminating the rotor field winding are reduced copper losses, higher power density, lower rotor inertia and more robust rotor construction. The demerits are loss of flexibility in field flux control, possible demagnetization/saturation of magnetic material and parameter variation over time. Depending on the magnet placement on the rotor, PMSGs are divided into two categories: • Surface permanent magnet machines (SPM) • Interior permanent magnet machines (IPM) In SPM synchronous machines, the permanent magnets are mounted on the rotor surface as shown in Fig. 2.14(a). The rotor has an iron core that may be solid or made of punched laminations with skewed poles to minimize cogging torque, and the simple design makes it easy to build. This configuration is used for low speed operation, since the magnet may fly during high-speed operation. The permeability of magnetic material approximates air, producing an effectively large air gap. Moreover, the smooth rotor surface design minimizes saliency in the rotor, contributing to a low armature reaction effect due to low magnetization inductance. In IPM synchronous machines, magnets are installed inside the rotor as shown in Fig. 2.14(b). The IPM rotor is difficult to fabricate, although the robust design makes it more suitable for high speed applications. The unequal effective air gap distribution renders it a salient pole machine, where the direct axis inductance is less than quadrature axis inductance (Ld |