Research On Improved Apparent Impedance Algorithm Considering Influence Of STATCOM On Distance Protection

Distance protection is widely applied for the primary and backupprotection of transmission lines, due to its simple principle and insensitiveto the system conditions. As the representative of the second generation ofthe Flexible AC transmission systems (FACTS) devices, the StaticSynchronous Compensator (STATCOM) has the superior performance onvoltage support, var compensation, transient stability of system, and so on.Both the structure of the transmission lines and the apparent impedance ofthe distance protection change after installation with the STATCOM, whichaffect the operation condition of the distance protection.In this dissertation, based on dynamic control property of theSTATCOM, the apparent impedance and operation condition of paralleltransmission lines with the STATCOM are studied. Take the single line shuntcompensated, consider zero-sequence coupling reactance between theparallel lines, and assure the distance protection before composition is notvulnerable to the fault resistance. With balanced and unbalanced controlstrategies respectively, the effects of the STATCOM on the operationcondition of distance protection on parallel transmission lines are analyzedand validated. The improved apparent impedance algorithm with theSTATCOM is newly proposed. The main contents and contributions areoutlined as follows:Firstly, effect of the STATCOM under balanced control on the operationcondition of the distance protection on parallel lines is analyzed. It isconfirmed that the STATCOM under balanced control strategy may causeunder-reach of the fault phase, but no effect on non-fault phase.Secondly, effect of the STATCOM under unbalanced control strategy onoperation condition of the distance protection on parallel lines is analyzed.STATCOM under balanced control will suppress symmetric voltage drop.However with asymmetric voltage drop, control strategy should be based ondual dq reference frames. It is validated that STATCOM under unbalancedcontrol strategy may cause under-reach of fault phase and over-reach of non-fault phase.Finally, the fault location method with the existence of STATCOM isimproved to reduce the location error under different control strategies tothe STATCOM. The apparent impedance of distance protection is modifiedby combining fault distance with the positive sequence impedance of the line.The simulation results show that the modified algorithm guarantees correctoperation of impedance relays, avoiding re-setting distance protection afterinstalled with STATCOM.