Research On Control Strategy Of HVDC Transmission System Based On Second Order Generalized Integrator

With the continuous development of economy and society,China's electrification level continued to improve and the country's overall electricity demand showed rapid growth.From China's energy endowment,energy resources are mainly distributed in the northwest region,while the energy demand is located in the eastern region.Therefore,in China,such a large countries with rapid economic development,High voltage AC and DC transmission will be rapidly development.In view of the global warming situation and the economic pressure facing China's economic development,the development of clean energy is urgent.High voltage DC transmission(HVDC)is widely used in new energy access and remote transmission with its unique technological advantages.On the one hand,compared with the AC transmission system,HVDC transmission system has a large transmission capacity,narrow line corridor,more flexibility and other characteristics,which are suitable for high-power,long-distance transmission occasions.On the other hand,high-voltage DC transmission technology can achieve large power grid asynchronously interconnected,enhance the wind energy,solar energy and other clean energy grid capacity,and improve the stability of the power system.Modular multi-level converter(MMC)HVDC transmission system has been applied in many engineering practice as a promising transmission mode.The modular structure of MMC can be easily extended to a very high voltage level and power level with low demand for power electronics.Due to the large number of levels,the distortion of output waveform is small,and there is no need to install the AC filter device.Redundancy control can be achieved by setting up redundant sub-module and system loss can be smaller by using proper modulation mode to control switching frequency.So,MMC has better steady and transient performance than traditional VSC.With cascaded structure including large numbers of sub-modules,MMC also brings the corresponding control problems,for example,a smooth pre-charge and start-up links,sub-module capacitance voltage balancing control,greater requirement on fault protection than the traditional VSC.Therefore,the optimal control of MMCHVDC is a hot topic in the current research,and experts and scholars have put forward a lot of control algorithms with excellent characteristics.In this paper,the optimal control of MMC-HVDC is studied from the following two aspects: on the one hand,in order to improve the accuracy of phase-locked link and voltage calculation,the dual second order generalised integrator based quadrature signals generator(DSOGIQSG)is used to estimate the AC voltage of the network side.The improved model predictive control(MPC)is used to reduce the computational complexity of the prediction link.So the MPC method can be used in the large level MMC-HVDC system.In addition,due to the frequency adaptive characteristics of DSOGI-QSG,phase angle can be accurately locked when the AC side is asymmetric.Finally,the proposed method is verified on the PSCAD / EMTDC simulation experiment platform.The simulation results show that the proposed method has a good dynamic response and can achieve the expected control effect.