Investigation on Fault-ride Through Methods for VSC-HVDC Connected Offshore Wind Farms - Overview and New Proposal for Fault Ride Through
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- Institutt for elkraftteknikk 
Recently, there has been a fast development and deployment of wind energy to meet the increasing electrical power demand and to limit the use of fossil fuels. More and more wind farms are planned far from shore because of good wind condition and less visual impact. This is so called offshore wind farm (OWF). In such a situation, high voltage direct current (HVDC) transmission is a favorable option for integrating these OWFs to the onshore grid, because HVDC, compared with high voltage alternating current (HVAC), has lower losses and higher transmission efficiency. For HVDC transmission, voltage source converter (VSC) has some advantages over current source converter (CSC), e.g. independent control of active power and reactive power, bidirectional power transfer for fixed voltage polarity. When a fault occurs at the onshore ac grid which connects OWFs via VSC-HVDC, the active power cannot be fully transmitted to onshore grid, while OWFs still produce active power. The imbalanced power will increase the HVDC-link voltage. This increased dc voltage will lead to high electrical stress for the insulated gate bipolar transistor (IGBT) modules, capacitors as well as cables, and even damage them. There have been different proposed methods to deal with this problem, e.g. chopper controlled resistor, wind turbine generator power setpoint adjustment, wind turbine grid side converter active current reduction, offshore voltage reduction. Chopper resistor method limits dc-link voltage by dissipating the imbalanced power . The second and third method reduce the power output from each wind turbine to limit the dc-link voltage increase. These two methods need communication between HVDC converter and each wind turbine. Offshore voltage reduction method initiates a controlled voltage drop by offshore converter to achieve a fast power reduction. All these four fault ride through (FRT) methods will be implemented in a test system and the effectiveness of these methods are evaluated with simulations made in PSCAD environment. Finally, based on the proposed methods, an enhanced FRT method is developed and its effectiveness is tested with the system. The advantages and disadvantages of different FRT methods are compared and summarized.