Research On Distance Protection Of Series Compensated Lines

With the development of large-capacity long-distance transmission, series capacitor compensation has been widely applied on long-distance EHV transmission. Series capacitor compensation can reduce the line inductance, shorten the distance of electric power transmission, reduce the transmission losses, improve voltage quality, improve transmission power, improve transmission power distribution, and improve the dynamic stability and static stability of the power system.The existence of series compensation capacitors destroys the uniformity of the transmission line impedance. So when fault occurs in the series compensated line, the phase of the voltage and the phase of the current may be reversed, and there may be low frequency transient current. All of these brings adverse effect to the line protection.This paper introduces the component and working principle of the series compensation device firstly, then analyzes the volt-ampere characteristics of the MOV, and then introduces the equivalent model and the different operating modes of series compensation devices. The characteristics of the fault voltage and fault current in series compensated line have been analyzed in detail based on the lumped parameter model, and the effect to distance protection has also been analyzed. Impedance relay may refuse operation when the fault occurs at the positive direction by the series compensation capacitor; and impedance relay may operate incorrectly when the fault occurs at the reverse direction by the series compensation capacitor; impedance relay at the side without the series compensation capacitor may overreach when the fault occurs at the positive direction by the series compensation capacitor and the protection range shortens.To work out the problem of the distance protection in series compensated lines, a new distance protection theory is proposed in this paper. In this essay, compensated point impedance is defined as the margin of the line impedance between the distance protection relay with the series capacitor and the measuring impedance. The impedance angle of the compensated point impedance is between 70°and 90°when the fault happened before the series capacitor and it is between 90°and 270°when the fault after the series capacitor. According to the character, a new method of fault point recognition is derived, integrating it with the traditional distance protection, a new series compensated line distance protection principle is proposed. The new theory obviously increases the distance range of distance protection and effectively solves the steady state overreach problem.