Research On Power Flow Regulation Characteristics And Control Strategy Of UPFC

The unified power flow controller(UPFC)is recognized as the most advanced and universal flexible AC transmission systems(FACTS),which combines the advantages of parallel and series type FACTS,and can provide multiple aspects of fast and accurate adjustment functions,such as voltage control,reactive or active power compensation,line impedance compensation,power flow adjustment,damping of low-frequency oscillation,also with the enhancement of the phase angle stability and voltage stability of power systems,and so on.Especially with the gradual development and maturity of MMC(Modular Multilevel Converter)-type voltage source converters in recent years,the MMC-type UPFC has been designed,manufactured and used in practical trial operation.Up to now,there have been a lot of studies on the operation principles of UPFC,but some deficiencies or defects are still remained to be solved urgently.In terms of the mathematical modeling for UPFC embedded system,most of the existing research uses the equivalent modeling methods such as current injection or power injection models,which are not based on the real circuits of the system,resulting in that the obtained results have certain differences with those in the real conditions.In regards to the existing research on the steady-state power flow principles of the UPFC embedded system,on the one hand,the power flow distribution and operation principles of the UPFC embedded system are lacked to be researched and analyzed from the global and local regional perspectives of the coordinate,and the steady-state power flow principles at different critical points in the system are also ignored to be analyzed.On the other hand,the detailed regulation methods by the output series inserted voltage of UPFC,the characteristics of power flow gradients and regulation efficiencies of the power flow in the system have also never been specifically analyzed.In regard to the research on the control strategies of UPFC,the existing control strategies are difficult to meet the regulating requirements in multiple aspects of speed,accuracy,and practicality at the same time.In terms of the simulation modeling and design,the existing research has not taken different types of steady-state and transient mutation incidents and different system conditions into consideration,and also has not analyzed and compared the simulation results at different critical points in the system.This thesis aims to make up for the deficiencies of the above existing research.Based on the real circuit model of the UPFC embedded double-end power system,specific theoretical modeling,design,tests and analysis at different critical points in different typical operating conditions of the system in multiples aspects of the steady-state power flow mathematical modeling,the power flows distribution and operation principles,the regulation principles of power flow gradients,the practical control strategy,and the electromagnetic transient simulation are carried out comprehensively,so as to conclude the characteristics of UPFC in aspects of steady-state power flow operation rules,power flow regulation principles and the performance of the control strategy comprehensively.The contributions of this thesis mainly include the following aspects.(1)The original detailed mathematical model for the steady-state power flows in the UPFC embedded double-end power system are achieved,and the partitioned power flow mathematical model including the global and local power flow variation ranges are also established,theoretical analysis,tests and summarizations for the steady-state power flow principles are carried out,respectively.Based on the real circuit of the UPFC embedded power system,five representative critical points from the sending end to the receiving end in the system are selected,where the power flows have significantly changed after the insertion of UPFC.The mathematical expressions of the active and reactive power flow at each SCP are derived,respectively,so that the original detailed mathematical model of the power flows in a typical UPFC embedded double-ends power system is established.Next,the built original detailed mathematical model is further decomposed and recombined into two parts containing the local power flow model(LPFM)and the global power flow model(GPFM),which are collectively referred to as partitioned power flow model.Multiple typical operating conditions of the system are designed,theoretical analysis,case studies at different SCP are carried out from the GPFM and LPFM perspectives,and the test results are also analyzed and compared in the two-dimensional planes and three-dimensional spaces,respectively.Finally,the steady-state power flow distribution principles inside the local power flow operation regions of UPFC at each SCP in different operation conditions dependent on the magnitudes and phase angles of the output series inserted voltage are analyzed and compared.Test and analysis demonstrate that during the operation process of UPFC,the power flows at each critical point of the system all have two changing rules including the rotation of the power flow curve in the local power flow region and the movement of the local power flow region in the global coordinates.The active and reactive power flow distribution points(P-Q point)in different system operation conditions at different critical points can be adjusted to the same coordinate area or points by UPFC,so as to adapt different operation conditions.In the general condition,the regulation for the phase angle of the output series inserted voltage of UPFC always concentrate in the limited specific regions,which is beneficial to improve the efficiency and accuracy of the power flow regulation by UPFC.(2)Modeling and analysis for the regulation principles of power flow gradients(PFG)to multiple characteristic variables of UPFC are carried out.Based on the established original detailed mathematical model above,three characteristic independent variables(CIV)for the power flow regulations containing the magnitude and the phase angle of output series inserted voltage(OSIV),and the phase difference of the system are selected.The mathematical model of the active and reactive power flow gradients to each CIV at each SCP is derived out.Next,two typical regulation modes for PFG are designed through combining different incremental types of the magnitude of OSIV and the phase angle of OSIV,and several typical regulation scenarios for PFG are also designed based on the phase difference variation types of the third CIV.Finally,taking the degrees of freedom for the variable regulation and regulation efficiencies in the regulation process of PFG as analytical basis,case studies for the PFG regulation principles by different regulation modes and regulation scenarios at different SCP are performed and compared.Test and analysis show that by the regulations of different CIV in the coordinates domain,the PFG at different SCP can be regulated to different levels according to different system operating conditions and the demands of the application scenarios,so as to realize the comprehensive coordination of the relationship between the power flow regulation efficiency and the requirements of the system protection and stability,which are beneficial to make the efficient and stable operation adjustment options for the power grids.(3)The design and performance analysis for a novel feedforward control strategy of MMC type UPFC(MMC-UPFC)is achieved.Based on the real circuit of the UPFC embedded double-end power system,the inner-current loops and outer-voltage loops the shunt and series converter of UPFC in the basic cross-coupling control scheme is designed,respectively.On this basis,the feedforward control modules and corresponding control loops regarding the d-axis and q-axis components of the output current from the shunt converter and the output voltage from the series converter,respectively.At the same time,necessary feedforward compensation signals are injected into the input sides of the above control loops through theoretical deduction,respectively,which improve the precision of the control strategy efficiently,thus the whole feedforward control strategy is performed.On the other hand,the open-loop and closed-loop transfer functions of the inner-current loops,the outer-voltage loops and the feedforward control loops of the feedforward control strategy are deduced and designed.Finally,the complex frequency domain and time domain indices of all the above control loops are computed and analyzed.Theoretical analysis shows that all the relevant control loops of UPFC including the inner loops,outer loops and the feedforward loops can operate in the respective frequency regions without overlap and interference,and show favorable performance in aspects of the frequency domain characteristics containing damping ratio and phase margin,and the time domain characteristics containing settling time and maximum overshoot.Thus validating the rapidity,accuracy and stability of the designed control strategy theoretically.(4)The electromagnetic transient modeling and simulation for MMC-UPFC in regard to multiple types of steady-state and transient mutant incidents are achieved.Taking a MMC-UPFC embedded 220 kV transmission system project as an example.First of all,the composition structure,the transformer connection types,and the input parameters of the MMC-UPFC embedded 220 kV power system are designed,respectively.Then,different mutant incidents in three types of incident conditions including step regulations of power flow references,the variations of system operation conditions and multiple types of horizontal and load shedding transient faults at different critical locations of the system are designed by constant 500 ms simulation time interval in the same time domain coordination.This simulation scheme design can realize the comparison of the simulation results in the same time domain coordination for different mutation incidents in the same incident condition,which is helpful to analyze the UPFC performance in different incident condition,as well as to compare and analyze the simulation results at different critical points in the same time domain coordination.The electromagnetic models for the MMC-UPFC in the PSCAD platform are built and the time domain simulations are carried out,and several key indicators containing the power flows,the voltage and current waves,and the total harmonic distortions(THD)at different critical points of the system are analyzed and compared specifically.The simulation results demonstrate that the step regulations of power flow and damping for the power flow oscillations caused by transient grounding faults are mainly achieved by the phase angle of the series inserted voltage of UPFC.Also,the adaptivity for the variations of system operation conditions is mainly achieved by the regulation of the output power of the series inserted voltage of UPFC.Thus validating the designed feedforward coordination control strategy of the MMC-UPFC has advantages of high control efficiency,nice quality waves and favorable regulation stability in the 220 kV power system.