Research On UPFC Fault Ride-Through And Its Access Line Protection And Reclosure Technology

With the rapid development of social economy and the gradual expansion of the power grid construction scale,especially the construction of large-scale AC/DC hybrid transmission networks,the existing distribution of power generation and consumption areas has become increasingly imbalanced,and the uneven distribution of power flows between regions has become more prominent.The Unified Power Flow Controller(UPFC),as a new type of Flexible AC Transmission System(FACTS)device,can coordinate the balance between power generation,transmission,and transformation in different regions,and fully utilize the power supply capacity of the grid.As a new power electronic device that applies Modular Multilevel Converter(MMC),UPFC has unique structural characteristics and complex fault characteristics.Based on a comprehensive study of the basic structure,working principle,and equivalent methods of UPFC,this paper relies on the actual engineering of the 500kV UPFC in the southern Jiangsu to conduct a series of research on the fault characteristics,protection configuration,fault ride-through,and the impact on line protection and reclosing.The main contents and achievements of this paper are as follows:(1)Studied the fault characteristics of the UPFC main body and the adaptability of protection.Analyzed in detail the basic structure,working principle,and equivalent methods of the MMC converter in UPFC.By building a high-voltage multi-level UPFC fast simulation model,the protection configuration and fault characteristics of each region of the UPFC main body were analyzed.In response to the problem of insufficient reliability of the bridge arm differential protection in single-phase grounding faults,a protection method based on fault component current differential for the bridge arm of the converter was proposed.(2)Proposed a fault ride-through strategy for UPFC based on virtual impedance and current limiting reactor.After a thorough analysis of the fault characteristics of the UPFC series and shunt sides when the connected line experiences a fault,the fault ride-through of the UPFC was defined.The key factors causing the overcurrent blocking of the MMC on the series and shunt sides under external faults were studied.Based on these,a targeted fault ridethrough strategy using a virtual impedance control loop and a DC bus connected current-limiting reactor,as well as the values determination method,was proposed.Simulation experiments verified the effectiveness of the fault ridethrough strategy.(3)Proposed countermeasures for line protection when UPFC is connected.Conducted a thorough analysis of the impact of UPFC series and shunt sides on the measured impedance of distance protection,and derived the expression for additional impedance.In response to the rejection of distance protection due to the influence of UPFC,a fast blocking and bypassing method with series transformers was proposed.On the other hand,based on the analysis of the action criteria for positive sequence fault component direction elements,studied the impact of UPFC connection on the reliability of direction elements.Utilizing two sets of complementary action regions,a new type of direction element based on fuzzy logic was proposed.(4)Proposed a single-phase adaptive reclosing scheme based on UPFC characteristic voltage injection.Utilizing the superposition theorem,analyzed the composition of the recovery voltage after single-phase transient faults with parallel reactors in ultra-high voltage transmission lines,and revealed the shortcomings of the single-phase adaptive reclosing scheme based on beatfrequency voltage.Furthermore,taking advantage of the flexible control of UPFC,achieved fault type identification and adaptive reclosing even when the beatfrequency voltage characteristics at the faulted phase end are not obvious by injecting characteristic voltage into the faulted phase through series resonance.