Research On Application Of UPFC With Fault Current Limiting In 10kV Power Grid

The modern power system has developed into a large scale AC/DC interconnected power system,the distribution of power generation and load in the region is not balanced,and the problem of uneven distribution of power flow in power transmission and transformation equipment is becoming more and more prominent.In addition,due to the restriction of urban planning,the difficulty of circuit transformation and power grid expansion is increasing daily,and because of restriction by many conditions of complicated system structure,heavy operation task,high power quality requirement,market and environment protection,the requirement of reliable and economical operation of transmission network is higher and higher.However,the lack of traditional control methods and the low level of automation is an important factor to limit the power transmission.UPFC can adjust the transmission parameters-voltage,impedance and phase angle to realize the flexible control of power flow.However,UPFC can easily be damaged by the impact of system high voltage and large short-circuit current when short-circuit fault occurs in power system,so it is of great significance to study UPFC with current limiting function to improve the stability of power system.This paper is based on UPFC-FCL topology of the dynamic model laboratory at Zhejiang University,it improves the current limiter two side topology structure,designs RLC series resonant circuit,analyzes its working principle and limiting dynamic process,establishes mathematical model when the fault occurs.Considering the defect of the DC capacitor voltage of UPFC-FCL in the limiting process is rising,easily to exceed the converter IGBT voltage resistance,this paper analyzes the mechanism of DC side capacitor voltage rise when the fault occurs,designs the bridge type capacitor voltage limiter to change short-circuit current flow path.The conventional UPFC topology is analyzed in this paper,the shunt converter and the series converter are respectively replaced by controllable current source and controllable voltage source,and the equivalent circuit model of UPFC is established.Based on the model and the relevant circuit theory,the mathematical relation of line power flow in the system with UPFC-FCL is derived,and the working principle of UPFC regulating power flow is analyzed qualitatively.Three dimensional power surface diagram of each part of the system with UPFC-FCL is drawn,and the power operating range of each part is determined.The UPFC-FCL input voltage balance,system active power balance and reactive power balance are analyzed,which establishes control objectives for the converter.This paper aims at the main components of the 10kV/1MVA UPFC-FCL to design parameters,the technical specifications of UPFC-FCL are determined.The range of filter inductance is calculated according to the compensation capacity of the converter,the operating range and the loss of inductance voltage drop.The DC side capacitor voltage is calculated by the maximum compensation degree of UPFC-FCL injected voltage and the maximum phase voltage utilization ratio of the space voltage vector.The DC capacitor is calculated from the point of view of the operating state mutation of shunt converter and transient active power injected into system by series converter.The current limiting inductance,current limiting resistor and current limiting capacitor are calculated according to the principle of optimal energy and minimum volume of current limiting inductance.According to the current limiting process of UPFC,the stress analysis of the switch S is calculated.The mathematical model of the converter in the dq coordinate system is established and the double closed-loop control of shunt converter is determined.The shunt converter maintains the DC side capacitor voltage and UPFC-FCL input voltage stability by controlling the active and reactive components of the AC side current respectively.The cross coupling control scheme of current inner loop,voltage intermediate loop and power outer loop is determined.UPFC-FCL regulates the reactive power and active power of the line by controlling the active and reactive components of the injected voltage respectively to track changes in power flow commands in real time.The UPFC-FCL simulation models of startup,normal operation and fault current limiting are established by using Matlab/Simulink software,the simulation results show that the topology,parameters design methods and cross coupled power flow control scheme of UPFC-FCL have the rationality and effectiveness in this paper.Although the line power decoupling is not complete,it can track the control commands in real time to meet the requirements of fast regulation power flow.The input voltage and DC side capacitor voltage of UPFC-FCL only oscillate slightly in power flow regulation.The short-circuit current peak is limited to 2kA and decays to zero within 7ms,which effectively protects the safety of the UPFC device.