Electrical Structure of Future Off-shore Wind Power Plant with a High Voltage Direct Current Power Transmission

Abstract

The increasing demand of electric power and the growing consciousness towards the changing climate has led to a rapid development of renewable energy in the recent years. Among all, wind energy has been the fastest growing energy source in the last decade. But the growing size of wind power plants, better wind conditions at off-shore and the general demand to put them out of sight have all contributed to the installation of large wind power plants in off-shore condition. However, moving wind power plants far out in the off-shore comes with many associated problems. One of the main challenges is the transmission of power over long distance. Historically, the power transmission from off-shore wind power plants has been done via HVAC submarine cables. This provides a simple solution, but AC cables cannot be arbitrarily long. It is shown in the report that major issues with HVAC cable transmission system are related to surplus reactive power and added losses. On the other hand, HVDC transmission system can be arbitrarily long and for long distance power transmission requirement it provides much better efficiency compared to a corresponding HVAC system. HVDC may provide a viable solution for high power transmission over long distances, but some issues related to fulfilling different grid code requirements still need further clarification. A transmission system should foremost provide a stable power transmission and participate in network stabilizing by providing efficient support for AC voltage control and frequency response requirements. These objectives are discussed and verification with simulation results is included in the report. A concept of negative sequence voltage compensation during small voltage unbalances and asymmetrical faults at the grid are also discussed. Secondly, a large WPP is not allowed to trip off during temporary grid side faults, commonly described as low voltage fault-ride-through requirement. There are four different fault-ride-through options discussed in the report. The first option includes controlling of collector network frequency. This provides a very good opportunity to use simple fixed speed wind turbines in the wind power plant. Induction generators attached to a large rotating mass show good response to frequency rise by allowing the rotor to speed up while reducing the active power output. However, it is observed that the post fault recovery process is very difficult to control and as such a high current capacity of the WPP side VSC might be required. Detailed simulation results are included in the report. The other option is to use a DC chopper, the results of which are also presented in detail in the report. It is observed that a DC chopper can provide a simple solution but the efforts required to remove the total heat during power dissipation is enormous. Alternatively, a telecommunication signal may be used, but the reliability and speed of such a system is in doubt. Finally, a controlled AC voltage drop at the collector network is derived and discussed in detail. It is illustrated in the report that such an option is advantageous in the sense that a fault at the grid side and at the wind power plant side can be dealt in the same way. More importantly, a similar wind turbine type can be used regardless of HVAC or HVDC connection strategy. A good co-ordination between the full-scale wind turbine and wind power plant side voltage sourced converter is also verified in the laboratory model based on real time digital simulation of wind turbine connected to an external voltage source converter via a power amplifier . The overall results show that the power transmission from long distance off-shore wind power plant is viable via HVDC system and at the same time the strict gird code requirements can also be fulfilled by selecting proper control methods.