The performance of transmission line protection relays when a fault occurs in the system is important for improvements in the stability of the system and reduction of their effect on sensitive loads. Reducing the fault clearing time for more possible fault conditions is one of the main goals in the development, application and setting of such relays.

The operating time of a transmission line protection relay is a function of many different factors. Some of them are related to the operating principle and the design of the relay itself. The paper analyzes the impact of errors in the line impedance parameters on the accuracy of the short circuit currents and voltages calculation, the settings of the distance and overcurrent relays and the fault clearing times for different line lengths and fault locations. The accuracy of the fault location calculation is also affected. This paper explains the difficulty of k-Factor settings and points out cost effective solutions for preventing incorrect behaviour of distance protection schemes.

The inaccurate values of the mutual coupling of parallel transmission lines are another important factor that may affect the operation of the relays for faults involving ground. This is also discussed in the paper.

The transmission line impedances used for short circuit currents calculation and the setting of distance relays are normally derived from the results of a line constants program calculation or systems studies. Due to the large number of influencing factors (e.g. wire types, spiraling and average sag of the wires, shield handling on cables, specific soil resistivity) these calculations can be prone to error.

Actual measurement of the fault-loop impedance is the best way to ensure that the distance and overcurrent relay settings are correct. The second part of the paper describes an advanced method for these measurements and calculations that provide the impedance data for the different applications that use it. Comparisons of estimated and measured line impedances are presented at the end of the paper.

Measuring mutual coupling between power lines can be done using a similar method.