Research On Key Problems Of Operation Control In Unified Power Quality Conditioner Based On MMC

Under the background of global economic competition and scientific and technological competition,the structural upgrading of the economic and Technological Development Zone is facing a series of challenges,of which the further improvement of the power supply environment is one of the problems to be solved in the development zone.The enterprises of high and new technology industry,modern manufacturing industry and modern financial service industry in the economic and Technological Development Zone have a large number of complex,precise and sensitive electrical equipment for power quality,which put forward higher requirements for the quality of electric energy and the reliability of power supply.At the same time,with the development of the grid itself,especially the development of the micro grid,the application of the intermittent distributed powersupply,the wave load and the high proportion of the power electronic transform equipment,resulting in the coexistence of many kinds of power quality problems,and more and more outstanding.Therefore,there must be new theories to enhance the understanding of power quality and new technologies to improve power quality.The quality of modern power often has the complex characteristics of multi type,multi time scale and mutual coupling.It is necessary to carry out comprehensive and multi angle management in accordance with the concept of "near compensation,localsuppression and regional coordination".Moreover,power quality also involves many stakeholders.It involves the responsibility of power quality and the allocation of responsibilities.It needs innovative governance mode.On the other hand,with the rapid development of the electricity market,the provision of high quality power supply service is of great practical significance to the power supply company or the new sales company.When supplying high quality power supply service to the power supply Park,the power supply company or the sale company needs to carry out the green and centralized management of the power quality under the condition of medium and high voltage and large capacity,and the unified power quality conditioner(UPQC),which is widely concerned,fits the target.However,due to the limitation of the withstand voltage capability of power electronic devices,most of the existing UPQC can only be applied to low voltage distribution systems.With the emergence and progress of modular multilevel converter(MMC)technology,it is possible for UPQC technology to be applied to high-voltage and large capacity occasions.Therefore,it has very important technical significance and market value to study the key problems of the operation control of medium high voltage and large capacity UPQC based on MMC structure and the application of industrial prototype.In this paper,focus on the key problems in the operation and control of high-voltage and large-capacity industrial prototype based on MMC-UPQC are studied,study work that carried out as below:(1)The topology and basic working principle of MMC type UPQC are studied,and the key parameters of MMC type UPQC are optimized with corresponding functions and performance indexes.The relationship between the voltage sags compensation capacity of the MMC converter and the capacitance of the MMC sub module is deduced,and the design method of the sub module capacitance for the MMC converter temporary drop compensation is put forward,and the method for the design of the submodule capacitance for the MMC converter is presented.The main circuit parameters of 10kV/4MVA's MMC UPQC industrial prototype.(2)In view of the problem that the capacitance voltage fluctuation of the sub module is caused by the working mode of the MMC,on the basis of the analysis of the mechanism of the unbalanced capacitance and voltage,a double balance control strategy is proposed for the phase voltage balance control and the voltage balance control of the sub module.The operation control of the UPQC shunt side MMC converter is studied and the parallel side MMC exchange is considered.The flow device mainly compensates the continuous current distortion,and has high requirement for the steady state error.A mathematical model of the two phase rotating coordinate system is set up.The double closed loop control strategy of the external voltage loop and current inner loop is put forward.The operation control of the UPQC series side MMC converter is studied,and the main suppression of the series side MMC converter is taken into account.In order to make the event type voltage drop,the response speed is high,the detection method of voltage sags and the control strategy of the single voltage loop are proposed.The response speed is improved effectively.The operation control of the UPQC parallel side and the series side MMC converter is studied,and the series parallel converter is based on the series parallel converter.The coordinated control strategy of DC bus voltage sags is proposed,which ensures the safe operation of UPQC.The proposed control strategy is verified by PSCAD/EMTDC.(3)On the basis of analyzing the MMC sub module charging and discharging mechanism,a multi stage starting control scheme is given.Especially for the series side MMC converter,the soft start control strategy of the sub module linear modulation boost is put forward,which can realize the smooth start of the UPQC without shock and the smooth switching with the normal operation formula.The proposed method passes through the PSCAD/E.The MTDC simulation is verified.For the topology of the MMC type UPQC and the multiple connection points of the series parallel converter and the system,the characteristics of multiple circuit breaker access systems are required.The MMC type UPQC protection configuration system and the logical control strategy considering the timing coordination of multi switch are proposed,and verified by digital physical closed loop simulation experiment.(4)The simulation of real time digital physical closed loop control is carried out.The establishment of the platform is very important for the recognition of the characteristics of the MMC UPQC system,the control of the control law,the tuning and debugging of the parameters,and the prediction of the control defects.On the basis of analyzing the modeling conditions of RTDS simulation scale and interface resources,the 5 level MMC type UPQC main circuit model based on small step length is set up,and the logic coding pulse input scheme based on small step length is proposed.The scheme is simple,the hardware platform resource cost is less,and the real time and precision are full.The experimental platform of digital physical closed loop simulation of MMC type UPQC is used to verify the correctness and effectiveness of the proposed control strategy under normal operation and start-stop state.(5)Aiming at the application demand of medium high voltage and large capacity UPQC in China Southern Power Grid,a 10kV/4MVA MMC type UPQC industrial prototype has been developed.On the basis of field power quality measurement and analysis,the access scheme of UPQC industrial prototype is put forward.The main circuit adopts the compact design of the container type,the MMC converter uses 33 level structure.Based on the structure of the industrial control machine +DSP+FPGA,the control of the sub module power unit layer,the converter layer and the system layer three level MMC UPQC is established.The function and performance of the parallel side and the series side converter are studied by using the MMC UPQC industrial prototype,and the experience is accumulated for the further extension and application of the MMC type UPQC.