This paper presents a method to detect the presence of capacitor in the fault loop under consideration by calculation of average harmonic distortion for 3,5,7 Harmonics from post fault one cycle data using Fast Fourier Transform (FFT) (Sim power System Blockset, MATLAB).The value of the same is considered as a decision-making criterion for the operation/prevention of the mal-operation of the relay. The paper also discusses how to select the system configurations in order to get the fruitful results towards finding solution for problems associated with protection of series compensated transmission line.

Increased transmittable power, improved system stability, reduced transmission losses, improved voltage profiles and more flexible power flow control are techno-economical reasons behind installing Series Capacitors (SCs) on long transmission lines (Logland et al.,1984; and Saadat, 1999). SCs and their over voltage protection device Figure 1 (typically Metal Oxide Varistors, MOVs), in spite of their beneficial effects on the power system performance, introduce additional problems and make operating conditions unfavorable for the protective relays that uses conventional techniques (Elkateb and Cheetham, 1980). During a power system fault the non-linear behavior of series capacitor arrangement, the rapidly changing characteristic of circuit impedance, and the high frequency noise generated from the non-linear protective devices of the compensation capacitors affects the voltage and current signals and thus create the problems with relay functionality (Saha et al., 2001 and 2003). Following paragraph explains the problem in detail.

The relay will attempt to look at the ratio of voltage to current to determine the distance to the fault in order to decide if the fault is in or out of its zone of protection. It is, of course, possible to correct the settings of the relay when it is known that the capacitor is always going to be a part of the fault circuit. However, that is not always known. By cancelling some of the line's series inductance, the series capacitor can make remote forward faults look as if they were in zone one of the relay when the capacitor is switched into the transmission line circuit and the relay setting rules are based on no capacitor in the fault loop (i.e., they can cause the relay to `overreach' Figure 2). Under these conditions, close-in faults can appear to be reverse faults due to voltage reversal (voltage inversion).

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