Research On The Key Techniques Of A Novel Dynamic Voltage Restorer With Fault Current Limiting Ability

Electricity is the central link of energy transformation in China,and grid revolution is the key to energy transformation.In recent years,people have constantly deeper understanding of power production,transmission,consumption and other issues,which inevitably requires the safe,reliable,efficient and high-quality operation of the power grid.Power electronic devices have penetrated into all aspects of power system and become the core of smart grid construction,with its advantages of energy saving,environmental protection,high efficiency,flexibility and intelligence.Of these,series power electronic devices are connected with the power grid at both ends or even at multiple ends,with a high degree of system coupling,which directly affects the voltage,current and power flow of the power grid.At present,series power electronic devices have been applied in power flow optimization control,fault current limiting,voltage quality control and other aspects,which can effectively solve the new problems such as uneven distribution of power flow,continuous improvement of short circuit capacity,complex voltage fluctuations and increasing frequencies.Therefore,series power electronic devices have broad application prospects,and the research of related key technologies is very valuable,which is of great significance for the safe,reliable,high quality and efficient development of the new generation of power grid.Aiming at the efficiency and the reliability of series power electronic devices,under the support of the National Natural Science Foundation of China "The research of transient characteristics and fault ride-through mechanism of the series-connected voltage source converter(51707014)","Key technology of novel hybrid controller for power quality and fault current(51377051)" and the key Scientific and Technological Projects of China Southern Power Grid Corporation “Research on Key Technologies of New Solid State Current Limiter for Distribution Network with Power Quality Control Function",this paper mainly studies the modeling and output characteristics of DVR,phase angle jump suppression method for DVR under grid side voltage disturbance,fault current limitation strategy for DVR under load side short circuit fault,series capacitor coupled DVR and its control considering multi-objective optimization,and engineering application of novel DVR.The main work and innovations are as follows:(1)The output characteristics and stable operation interval of DVR system are emphatically analyzed,which provides theoretical guidance for parameter design and operation optimization.On the basis of comparing the working characteristics of two typical structures-energy storage DVR and back-to-back DVR,considering that the output capacity of back-to-back DVR will be affected by voltage sag,load fluctuation and the coupling of parallel and series side converters,the output voltage range under stable operation is analyzed in depth.Based on the instability mechanism of the parallel part of back-to-back DVR under heavy load,its carrying capacity is theoretically deduced,and the mathematical model of the maximum carrying capacity of the back-to-back DVR parallel part is obtained.Combining equivalent models of parallel and series converters,and from the perspective of maintaining the load capacity of parallel converters with stable voltage on the DC side,the relationship between the modulation ratio of series converters and the system voltage,load capacity,DC side voltage and circuit parameters of the converter is obtained.(2)The phase angle jump suppression method for DVR under grid side voltage disturbance is proposed.On the basis of analyzing the causes and influencing factors of phase angle jump during DVR compensation,a flexible control strategy based on smooth phase angle transition of DVR output voltage is proposed,which ensures that DVR can smoothly move from standby mode to minimum active power compensation mode,and can also smoothly withdraw from operation.Based on the characteristics of phase angle control and energy flow relationship of energy storage DVR,an energy self-recovery strategy is proposed,which can compensate for the energy consumed during the compensation period of DVR and effectively ensure that there is no large phase angle jump during the energy self-recovery period,and no additional charging link is needed.In addition,the influence of DVR stable operation boundary on its operation mode is analyze,the corresponding adjustment strategy of DVR compensation reference value is proposed to avoid the occurrence of over-modulation.(3)A fault current limiting method for DVR under load side short circuit fault is proposed.Considering the response characteristics and topology evolution of DVR when short-circuit fault occurs on the load side of power grid,a typical FCL-DVR topology based on filter inductance multiplexing and uncontrolled rectifier bridge multiplexing is proposed respectively.The working principle of the two schemes is analyzed in detail,and the electrical model under fault current limiting mode is established.The transient switching process from voltage compensation mode to fault current limiting mode,and from fault current limiting mode to dynamic voltage compensation mode is deeply analyzed.(4)A series capacitor-coupled DVR and its control method considering multi-objective joint optimization are proposed.Based on the characteristics of in-phase compensation and minimum energy compensation in the operating voltage of series converter and active power required for compensation,the series capacitor-coupled DVR is preliminarily studied,and the operation principle of LCC-DVR is analyzed in detail and compared with the characteristics of traditional strategies.The influence of load fluctuation on the operation boundary of LCC-DVR is studied,and the adjustment strategy of compensation reference value is given.In addition,the fault ride through capability of LCC-DVR under short circuit fault at load side is discussed,and the resonance suppression method during current limiting period is given to ensure the operation reliability.Further,the design principles of parameters and control strategies are given.(5)A low-voltage and low-power experimental platform and a hardware-in-the-loop experimental platform based on RT-LAB are establish to verify the operation mechanism and function of the new DVR.Based on the background of the integrated control project of power quality control and fault current limitation in a 110 kV substation of China Southern Power Grid Corporation,a complete set of design ideas of 10kV/1.5MVA new DVR engineering machine,power module,series transformer,current limiting branch and control system is put forward.The detailed equipment manufactured,field installation and test scheme have worked out.This paper takes DVR,a typical representative of series power electronic devices in distribution network,as the research object,takes voltage quality regulation technology and fault current limiting technology as the research core,and focuses on the key issues arising from the interaction between DVR and its derivative systems,and the power grid.The systematic research has carried out in the aspects of topology structure,control strategy and engineering application.Further,in-depth research has carried out in improving the reliability of the device,reducing the cost of the device,increasing power density and improving the quality of power supply.The research results can provide some reference for other series power electronic devices and composite systems,and provide useful reference for promoting the practicality of power electronic devices.