Speed Control of Multiple Tidal Turbines Connected to a Shared Onshore Converter

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) due to 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 Induction Generators (IG) is carried out through Field Oriented Control (FOC) with reference torque calculated by 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) of each turbine. Estimated current waveforms calculated via 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 PWM pulses to maintain the currents of both generators in phase. A Simulink model was constructed to test this control strategy; the results generated demonstrate that two turbines can be operated in parallel, whilst still maintaining an acceptable level of efficiency.