Study On Influence Of UPFC On Distance Protection Of Transmission Line

Voltage,phase angle and impedance can be regulated by Unified Power Flow Controller(UPFC)and then the power flow can be controlled smoothly and quickly,resulting in less transmission losses,high conveying capacity and stability.However,voltage,current,impendence and other parameters for relay protection are also changed when system performance is improved,distance protection is influenced the most among all the protections.Firstly,this paper introduced the basic structure,principle and function of UPFC and analyzed the influence on distance protection of UPFC theoretically;secondly,factors have influence on distance protection are analyzed and especially UPFC and transition resistance for most fault in power system is not metallic.And then,a new single phase grounding distance protection scheme immune to transition resistance was presented and its feasibility and accuracy was analyzed and tested theoretically and by simulations.Firstly,based on geometrical property of vectors of voltage and current of protective relay and fault point,similar triangles were established.On this basis,equation of fault distance was formulated to eliminate effect of transition resistance.Then using phase relationship between zero sequence current measured by distance relay and voltage at fault point,above equation of fault distance was simplified,beneficial to its quick solution.As for line with UPFC,there is a principal criterion—ratio of voltage of the faulted line and the setting voltage and an auxiliary criterion-fluctuation of the main criterion.With the cooperation of principal and auxiliary criterion,the new scheme can distinguish between internal and external fault using one-terminal electric measurements and is not affected by operational mode and control parameters of UPFC and transition resistance,whose characteristics are testified by simulations on PSCAD/EMTDC.