MMC-STATCOM Applied To DC Research On Reactive Power Compensation Characteristics

China's vast territory and vast resources,resources and population distribution do not match;under the development of the 'One Belt and One Road' and global energy interconnection,UHV transmission technology has the characteristics of large transmission capacity,long distance and high efficiency,and can meet China's current energy transmission requirements.The traditional UHVDC transmission project usually configures a reactive capacitor bank with corresponding capacity on the AC side of the converter station for reactive power compensation.However,when the DC transmission capacity is relatively large and the strength of the AC system at the receiving end is weak,the static reactive power compensation device cannot improve the dynamic reactive power compensation capability of the system.The transient voltage stability of the DC system in the event of large disturbance will be faced with severe tests.Therefore,in order to improve the stability of the system and power quality,in many practical projects that have been completed,the State Grid plans to configure dynamic reactive power compensation devices in the converter stations on the weak AC system side of the HVDC system.In order to ensure the voltage stability and power quality of the power system,the Static Synchronous Compensator(STATCOM)has been proposed as a dynamic reactive power compensated mid-flow mast,and it has been widely used recently.However,due to its inherent topology,the inverter has limited the inverter.The pressure level of the device.Modular multilevel converter(MMC)is a new type of topology converter.The series connection of submodules can theoretically meet the requirements of high power and high withstand voltage levels.Applying this topology to STATCOM can satisfy High-voltage and high-power demand improves power quality.In view of the above problems,the ±800kV Qilianshan-Laoshan UHVDC power transmission system is mainly used to establish a simulation model based on actual engineering parameters in PSCAD/EMTDC.Simulation experiments are performed on the DC-side grounding fault and bus-to-source bus grounding fault conditions.The results show that when there is only static reactive power compensation,the stability of the system's commutation bus voltage is poor and commutation failures are likely to occur.After analyzing the advantages of MMC-STATCOM large-capacity dynamic reactive power compensation method,its topological structure and control model are established and applied to the converter station of the UHVDC power transmission system;the result of reactive power compensation is then performed.Compared with the static reactive power compensation and traditional STATCOM with small capacity,this method improves the stability of the system and effectively improves the commutation failure.However,there are a series of problems such as difficult parameter setting,poor adaptability,slow outer-loop voltage control,and circulation in the interior of the MMC converter due to.traditional double closed-loop control.In view of the above problems,this paper innovatively designs an improved control strategy for the controller of MMC-STATCOM:replace all the double closed loop control with the sliding mode controller to control the outer loop voltage,and the inner model controller decouple control the inner loop current.And the circulation suppression scheme has been added.Finally,it is applied to the Lushan converter station,and its modeling and simulation are carried out.The compensation effects of the improved control strategy and the traditional control strategy under different working conditions are compared and analyzed,which shows that the improved control strategy is reasonable and effective.