HVDC Solution for Offshore Wind Park Comprising Turbines Equipped with Full-Range Converters

This paper describes a power transmission technique from an offshore wind farm to an on-land AC (alternating current) grid through a HVDC (high voltage direct current) transmission system. The limitations of an AC transmission system for long distances are presented together with the advantages of a HVDC transmission system. The power system under study includes an offshore wind farm comprising turbines equipped with full range converters. The collection network is a local AC grid. Power transmission is done through HVDC system. The grid side VSC (voltage source converter) controls the DC voltage of the DC transmission together with the reactive power to the grid. However, the control of the wind farm side VSC needs to be adaptive. The paper presents control methods for wind farm side VSC during the normal and the fault conditions of the connecting AC grid. During normal operation, the wind farm VSC is controlled to operate as an infinite voltage source. The voltage and frequency of the collection grid is kept constant. When the connecting AC grid side fault occurs, the fault-ride-through is achieved by adjusting the voltage of the wind farm side AC collection grid through the wind farm side VSC. When a voltage drop is created at the collection grid, the wind turbines go into fault-ride-through mode. The power output from each of the wind turbines is thus reduced to balance the system power. The detailed explanation of the strategy is presented in the paper. Matlab simulation model was prepared and some of the results are presented in the paper.