Grid integration of large offshore wind farms using VSC-HVDC transmission system in parallel with AC submarine cable

The wind electricity generated has increased significantly in the last decade, the causes for this increase are many, such as, it has been used to meet CO2 and greenhouse gas emission reduction goals set by the World governments; it can be used as a safe guard against fuel price volatility; and its competitiveness with conventional electricity generation in term of cost of electricity and construction. With increased wind power penetration into the grid, its influence on the power system operation is becoming significantly and requirements for its integration are becoming more restricted. The main two requirements are: 1. Capable of providing voltage/reactive power support to the grid. 2. Capable of riding through the faults (fault ride-through capability). Synchronous connection of offshore wind farms using ac submarine cable has several shortcomings, such as flow of charging current in the cables which increase the power losses and reduces its current carrying capability; fault in the grid may significantly affect wind farm operation due to its poor fault ride through capability and increases the risk of voltage collapse due to lack of reactive power support; and it has distance limitations. It has been recognized that the grid integration of a large offshore wind farms using dc transmission systems (HVDC) based on voltage source converters (VSC) has several features, such as: 1. Ability to transmit large amount of power over long distances with minimum losses. 2. Converter stations can be used as (STATCOM) to regulate voltage of both AC sides. 3. It improves fault ride-through capability of the wind farms; in other words, fault in the grid side will not affect the wind farm operation due to decoupling feature of asynchronous connection. In this paper, grid integration of a large offshore wind farm using VSC-HVDC transmission system in parallel with ac subsea cable is presented. The converter stations of dc transmission are used as static synchronous compensators to provide dynamic reactive power support in both sides of the ac submarine cable. Whilst the ac submarine cable provides a reference frequency to the wind farm and simplifies its converter control. The advantages of the proposed configuration are: it improves system stability and reliability, It eliminates the need for STATCOM in the wind farm side in order to increase the effective short circuit ratio and potential lose of supply due to fault in dc or ac transmission system is minimal. Computer simulation in MATLAB-SIMULINK environment will be used to validate the effectiveness of the proposed configuration