PACADmodelreport-faultarc916.pdf

1“Fault arc”model 1.Introduction Although three-phase bus fault is the most serious fault on transmission lines,experiences and statistical data indicate that the Single Phase to Ground(SLG)is the most common fault,especially in EHV transmission systems.An arc occurs during SLG fault.It can be classified into the primary arc duration(before breakers open)and the secondary arc duration(after the breakers trip).In the primary arc duration,the heavy short circuit current flows through the primary arc,and there is no significant elongation of the arc.In the secondary arc duration,the faulted phase is isolated from both ends by the related protective relays and single pole breakers.The faulted conductor(line)is capacitively and inductively coupled to the two sound conductors and other conductors of parallel circuits(ex.Double circuit lines),which are still energized at approximately normal circuit voltage and carrying load current.This coupling has two effects 1:Before the extinction of the fault arc,it feeds current to the fault and maintains the arc.After the arc current becomes zero,the coupling causes a recovery voltage across the arc path.If the rate of rise of recovery voltage is too great,it will reignite the arc.The secondary is the dominant fact of the arc extinction.A detailed fault arc model is extremely important for the fault transient studies such as auto-reclosure and protection design.This report introduces a fault arc component developed in PSCAD/EMTDC based on the model proposed by A.T.Johns 2.The mathematic arc model is introduces first,then a component based on the model in PSCAD/EMTDC is introduced,finally validation and applications of this component are conducted.2 2.The Mathematic Model of Fault Arc From a modelling point of view,the fault arc can be classified as the high current primary arc during the fault and the low current secondary arc(after the faulted phase is isolated)and which is sustained by mutual coupling between the healthy and faulted phases.The details of primary and secondary arc models refer 2.This report only gives a brief introduction on them.2.1.Model of primary arc The arc conductance is a time variable.=Where:time varying primary arc conductance:stationary primary arc conductance:primary arc time constant:peak value of primary arc current :a coefficient 2.85e-5 Subscripts:p:primary 2.2.Model of secondary arc 2.2.1.Secondary arc conductance The secondary arc is a highly complex phenomenon.The V-I characteristic is hysteretic(see figure 1),which can be modeled by a time vary arc conductance.=3=()=750.4 V/cm=1.4 Where:time varying secondary arc conductance:stationary secondary arc conductance,time varying :secondary arc time constant:peak value of secondary arc current:constant voltage parameter per unit length of secondary arc:the secondary arc length:the time from the initiation of secondary arc:a coefficient 2.51e-3 Subscripts:s:secondary Figure 1 V-I cyclogram of secondary arc 2.2.2.Reignition voltage of secondary voltage 4 During the secondary arc period,each current zero point may result arc extinction if the arc voltage remains below a threshold voltage required for reignition.This threshold voltage is obtained from following experimental equation:()=5+16202.15+()()103 V/cm Where:reignition voltage of secondary voltage:the time from the initiation of secondary arc:the time from the initiation of secondary arc to a current zero 5 3.“Fault Arc”component in PSCAD/EMTDC This section introduces the“Fault arc”component in PSCAD/EMTDC.Then the validation is conducted by comparing with simulation results in A.T.Jones paper 2.Finally an application of the component in Single Phase Auto-Reclosure(SPAR)study is given.3.1.The“Fault arc”component in PSCAD/EMTDC From design point of view,the most important objective is to determine the extinction time of the secondary arc.The extinction depends on many factors such as the arc current,recovery voltage,length of the arc path,wind velocity.In this component,the arc length,the primary and secondary arc currents are chosen as the typical parameters.The component graph the is shown in figure 2 Figure 2 the Fault arc component The component settings are shown in figure 3.ARCBrkIFltJ 6 Figure 3 the setting of“fault arc”Where:Initial arc length(m)Real The initial arc length Peak steady state primary arc(kA)Real The peak value of primary arc current Peak steady state secondary arc(kA)Real The peak value of secondary arc current Arc conductance Text the name of arc conductance for output Arc name Text 7 Reference:1.Improved techniques for modelling fault arcs on faulted EHV transmission systems,A.T.Johns et.el.IEE Proceedings,Generation,Transmission and,Distribution,Vol.141,No.2,March 1994.2.E.W.Kimbark,Suppression of Ground-Fault Arcs on Single-Pole Switched EHV Lines by Shunt Reactors,IEEE Transactions on Power Apparatus and Systems,vol PAS-83,pp.285-290,March/April 1964.