Independent speed control of tidal turbine rotors in an array

• Main Author: Reynolds, Simon Mark
• Format: Dissertation
• Language: English
• Published: University of Edinburgh 2020
• Subjects: control; IGBT; Insulated Gate Bipolar Transistor; renewable; renewable energy; tidal energy
• DOI: 10.7488/era/341

A novel control strategy is proposed for the control of two fixed pitch tidal turbines subjected to discrete flow speeds whilst interconnected in an array. It is suggested that reliability will be increased by locating all power conversion and control equipment onshore, thereby reducing Operating Expenditure (OPEX) incurred through offshore equipment failure. An added benefit is utilisation of a single shared export cable for both turbines, thereby reducing Capital Expenditure (CAPEX). By lowering the cost of installing and maintaining an array, it is anticipated that the commercial viability of the project will be enhanced. Speed regulation of each Induction Generator (IG) is carried out individually through Field Oriented Control (FOC) with the torque reference calculated using Optimal Torque Control (OTC). This provides Maximum Power Point Tracking (MPPT) whilst allowing individual generators to run at different speeds by varying the Tip Speed Ratio (TSR) at which each turbine operates. Estimated current waveforms calculated by FOC are averaged between both turbines and fed to a Pulse Width Modulation (PWM) generator for comparison against actual averaged currents from both generators. A shared Insulated Gate Bipolar Transistor (IGBT) rectifier is controlled by the PWM pulses to maintain both current and voltage in phase across the two generators. A Simulink model was constructed to test this control strategy and assess if two turbines could be operated in parallel as suggested, whilst still maintaining an acceptable level of efficiency. The results obtained from this proved the function of the control strategy and allowed for an economic analysis to be carried out. A cost comparison estimate was made for parallel and individual turbine arrays and energy yields were calculated for both from the results of the modelling. The final outcome of this analysis was that the efficiency loss for the parallel connected array was low enough to make the cost of energy produced lower than from an array with individual cables.