Operation and control design of an input-series-input-parallel-output-series conversion scheme for offshore DC wind systems

High-power converters for high voltage direct current (HVDC) transmission systems and collecting networks are attracting increasing interest for application in large offshore wind farms. Offshore wind farms are capable of generating more electric energy at lower cost when compared with onshore wind systems. In this context, DC/DC voltage conversion should be achieved with a power converter that uses readily available semiconductor devices. A modular DC/DC converter can achieve the required system currents and voltages without exceeding semiconductor ratings. In this paper, the operation and control strategy for an Input-Series-Input-Parallel-Output-Series (ISIPOS) energy conversion system for wind systems are presented. The ISIPOS system allows the direct connection of wind turbines to the DC grid. In this research, the design process to control the input and output currents and voltages is explained. In addition, a new method to ensure voltage and current sharing between the different modules is presented and explained. The basic structure, control design, and system performance are tested using MATLAB/SIMULINK. Practical results validate the control design flexibility of the ISIPOS topology when controlled by a TMSF280335 DSP.