Research On Receiving-End Frequency Control Of DC Power Suppling Isolated Grid With High Proportion Renewable Energy

Our country has a vast territory,population and resources are inversely distributed.In some coastal and remote mountainous areas,electricity for garrisons and residents are not connected to the main network,power supply reliability is poor.The Line Commutated Converter-Voltage Source Converter(LCC-VSC)hybrid DC power supply method is superior to AC power supply in the context of high voltage and long-distance power supply,and has the advantages of traditional DC and flexible DC transmission,which is a better choice for power supply than isolated microgrids.However,due to the characteristics of"high proportion of power electronic equipment" and "high proportion of new energy",frequency regulation is a very important issue for this system.The main work of the paper is as follows:Firstly,the structure and modeling of hybrid DC power supplying microgrid system are introduced,the reasons for the LCC-VSC hybrid DC power supply architecture of this system are analyzed,the reasons for cascaded three-level converters are introduced for the receiving converter station,and the control methods of the receiving and sending end converter stations are analyzed.The model of LCC-VSC and microgrid consisting of photovoltaic equipment,wind turbine,diesel generator,energy storage system and comprehensive load model are established on the PSCAD/EMTDC platform.Secondly,it is hoped that after the disturbance,the fast-moving DC is preferentially used for frequency regulation.So a DC frequency regulation method which can adapt to disturbance changes is proposed in the LCC-VSC supplying microgrid system.Firstly,the critical droop coefficient required to pull the system frequency into the specified range after disturbance is theoretically deduced.Then a primary frequency regulation method with variable droop coefficient adaptive to the disturbance is proposed,the frequency is controlled quickly due to fast reaction of DC.Because the primary frequency regulation is still a differential regulation,the secondary frequency regulation strategy of variable reference power is proposed.A simulation model is established on the PSCAD/EMTDC platform.The results show that under the control of primary frequency regulation,the frequency deviation can be restored to within 0.2Hz when the load changes within 12.5%,or the illumination changes within 10%.When the load changes within 1 5%,the secondary frequency regulation can achieve zero-error frequency regulation.Finally,if the disturbance is too large,or the system power approaches its boundary after the DC power increases or decreases,a part of the additional power needs to be allocated to the wind power、photovoltaic and energy storage.Therefore,based on the above DC adaptive frequency regulation method,a multi-source coordinated frequency regulation method with DC power、photovoltaic、wind power and energy storage is further proposed.The initial frequency regulation area which determined by the disturbance magnitude and LCC-VSC increase/decrease margin is qualitatively analyzed.Then,the action boundary expression between each frequency regulation area are obtained.Thus the multi-source coordinated frequency regulation method which can adaptively increase or decrease power according to disturbances for LCC-VSC,energy storage,wind power and photovoltaic system is proposed.A simulation model is established on the PSCAD/EMTDC platform,the action boundary value under specific examples are calculated.The simulation results verify the rationality of the proposed initial frequency regulation area.