Continuous and reliable electric energy supply is the objective of any power system operation. Series compensated transmission lines utilize series capacitors to cancel a portion of the inductive reactance of the line, thereby improving power transmission capability of line and securing better system stability. However, installation of series capacitors (SCs) equipped with nonlinear metal oxide varistors (MOVs) (for overvoltage protection), on transmission lines causes certain problems for protective relaying and fault location. Accurate fault location requires offsetting the series compensation effect and the reactance effect resulting from the remote end in-feed under resistive faults. For a parallel line system, countermeasures are needed for mutual impedance effect between lines also.

This project work deals with digital protection and fault location algorithms for series compensated transmission lines. The series capacitor (SC) protected by metal-oxide varistor (MOV) is modeled to offset series compensation effect. The SC-MOV is modeled by two approaches, i.e. (i) estimation of instantaneous voltage across SC-MOV using line current samples and (ii) estimation of equivalent resistance and reactance offered by SC-MOV as a function of line current. The later approach is presented in two alternative ways, i.e. (a) estimation using analytical model and (b) estimation using empirical formula developed by Goldsworthy. The performance of different computer relaying algorithms such as DFT full cycle algorithm, DFT half cycle algorithm, Differential equation algorithms (with least square error, neglecting line capacitance etc.) have been critically examined considering fault current and voltage signals obtained from ATP-EMTP simulation study considering 400kV 3-phase transmission lines.

Two alternative fault locating algorithms have been investigated considering (i) a single circuit line compensated in the middle, (ii) a single circuit line compensated at the both ends and (iii) double circuit lines with mutual coupling. The effectiveness in terms of accuracy and speed has been analyzed for these algorithms considering different fault locations and fault types.

Investigations reveal that (i) fault location algorithms give better results for SC-MOV modeling by empirical formula. (ii) Differential equation algorithm gives excellent results on uncompensated lines but very poor results on compensated lines. DFT full cycle algorithm is found to give very good results for both compensated as well as uncompensated lines. (iii) the fault locating algorithms taking countermeasures for series compensated effect, remote end in-feed effect and mutual coupling effect (for double circuit lines) are found to give accurate results.

The system modeling has been done using ATP-EMTP software package and voltage and current signals are taken from there at the rate of 20 samples per cycle. All impedance measurements are done using digital relaying algorithms. MATLAB6.5 software package has been used for all programming work.