Research On Control And Optimization Of Power Systems With UPFC

As one of the most advanced flexible ac transmission system(FACTS)devices,the unified power fow controller(UPFC)can simultaneously realize the functions of power flow control,ac bus voltage control,and damping low frequency oscillations,which provides a powerful way to tackle the challenges faced in the development of China's power grid.The commissioning of the Nanjing UPFC demonstration project and the Suzhou 500 kV UPFC project reflects the broad prospect of UPFC applications in China.Study on the control and optimization of power systems with UPFC,and expiration of methods and principles of applying UPFC in actual power grid,are the premises to achieve the effect of the UPFC.Related research on these issues can be used to solve the problems encountered in the operation of the current power grid of China,and is of great significance for accelerating the transformation of the traditional power grid into a strong,flexible,and controllable smart grid.Based on the previous research,and taking into consideration the actual needs of the Nanjing UPFC demonstration project and the Suzhou 500 kV UPFC project,related issues on the control and optimization of power systems with UPFC are systematically studied.The thesis is organized as follows:(1)A novel UPFC-based line overload control scheme is proposed for power system security enhancement.Firstly,sensitivities of line loads to the variations of UPFC system-level control settings(e.g.active/reactive power settings)are deduced based on electrical network theories.On this basis,the feasibility of line overload control using UPFC system-level control functions is analyzed.Secondly,the control laws of the line overload control are developed,which adjusts the active power setting of the UPFC intelligently in case of line overloads.The implementations of the above control laws in actual power grids are also provided.Finally,the control sensitivity analysis and the proposed control scheme are verified in the Nanjing western power grid and the Suzhou 500 kV power grid.(2)A novel voltage regulation scheme of the UPFC series and shunt converters is proposed.The method of alleviating voltage violations by enhancing the reactive power support ability through the UPFC installed line is investigated.Firstly,sensitivities of bus voltages to the variations of UPFC system-level control settings are deduced.On this basis,the voltage control characteristics of the series and shunt converters are discussed.Secondly,the control laws of the voltage regulation are developed,and the principles of the control parameter selection are discussed.Finally,the sensitivity analysis and the proposed control scheme are verified in the Suzhou 500 kV power grid.The proposed line overload control and voltage regulation schemes constitute the whole system-level control strategy of a UPFC in transmission grids,which enhances the security of the power grid while reducing the operation complexity.(3)The influence of a UPFC on the low frequency oscillation characteristics of the system is studied,and a novel design method of the UPFC damping controller is developed.Firstly,the low frequency oscillation analysis model of power systems with UPFC is established.On this basis,in a simplified study system where the infinity system is powered by a generator through a UPFC installed transmission section,the influence of the UPFC on the low frequency oscillation characteristics and its key influencing factors are analyzed.Then,a novel design method of the UPFC damping controller are proposed,including the input signal selection method based on the proposed residual index and the controller parameter tuning method based on the test signal method.Case study in the Suzhou 500 kV power grid verifies the proposed design method of the UPFC damping controller.Also,the results show that main conclusions obtained by the analysis in the simplified system are also of instructive and guiding significance in actual power grids.(4)Critical factor analysis and optimization principles of the placement and sizing of UPFC are studied.Firstly,formulas to determine the capacity requirement of the UPFC device relating the amount of transferred power flow,the location of the UPFC and the grid structure parameters are deduced.On this basis,critical factors concerning the capacity requirement are studied.In two typical power grid configurations(radial and loop configurations),the impacts of the location of the UPFC and the grid structure parameters on the required capacity per transferred power flow are analyzed.Then,the power transfer characteristics of UPFC are analyzed in several common transmission scenarios.Furthermore,combined with the voltage control characteristics and the influence on low frequency oscillation characteristics,the optimization method and principles of the placement and sizing of the UPFC device are discussed.The validity of the deduced formulas,the critical factor analysis and the proposed optimization method of the placement and sizing of the UPFC is verified in IEEE 39 bus system and the Suzhou 500 kV power grid.(5)A novel control scheme for the UPFC series converter is proposed in this paper to achieve line loss reduction and security enhancement in distribution systems with a high penetration of renewable energy.Firstly,the line loss minimum conditions of a general distribution system with loop configurations are deduced.Secondly,security constraints including the permissible voltage range,the line loading limits and the UPFC ratings are considered.System security enhancement with the least increase in line loss is tackled by solving a much reduced optimal power flow(OPF)problem.The computational task of the OPF problem is remarkably reduced by deducing the security-constrained line loss minimum conditions and removing the equality constraints with real-time measurements.Thirdly,a hybrid control scheme is proposed.Line loss minimization is achieved through a dynamic feedback controller,while an OPF calculator is integrated to generate corrective action for the dynamic controller when the security constraints are violated.The validity of the proposed control scheme is verified in a modified IEEE 33 bus test system.