Enhancing the performance of distance protection relays under practical operating conditions.

Abstract

This thesis examines the use of real time digital simulation as a tool for enhancing the performance of modern numerical distance protection relays under complex, practical fault scenarios and operating conditions, in particular those associated with distance protection of closely-coupled parallel lines. Double-circuit and parallel transmission lines, in various topologies, are widely used in South African electrical networks due to servitude and cost constraints. The particular topology and separation of adjacent transmission lines affects the individual lines’ electrical characteristics and dynamics, and hence, in-turn, affects the performance of the protection schemes designed to protect the lines against short circuit faults. The design of protection schemes to accommodate the effects of this environment presents a number of challenges. The particular challenges that are considered in this thesis are: the effect of mutual coupling between double-circuit and parallel transmission lines on distance protection relays; the effect of untransposed transmission lines on distance protection relays; the effects of cross country faults on distance protection relays.

The effects of each of these practical operating conditions on the performance of distance protection relays can be complex and interacting, and therefore detailed analysis is required in order to avoid the occurrence of a protection mal-operation. However, with the emergence of powerful simulation tools, like the Real Time Digital Simulator (RTDS), it is possible to model the characteristics of untransposed and closely-coupled transmission lines in detail for the purposes of such analysis. A Real Time Digital Simulator also allows one to connect actual protection relay hardware in closed-loop with the real time model of the protected transmission lines in order to be able to analyse the performance of actual distance protection relays in response to complex fault scenarios and operating environments that may occur on the power system.

The thesis begins by presenting a review of the fundamental theory and prior research into distance protection in complex line topologies, including mutual coupling and non-transposition. The thesis then presents a detailed, first-principles analysis of the mathematical models of transmission lines and the theoretical impedance seen by a distance protection relay for faults, firstly on mutually-coupled lines, and then on untransposed lines, in order to establish the validity of the mathematical models and hardware-in-loop test procedures used. The findings of the hardware-in-loop studies confirm the true extent of the impact of mutual coupling and non-transposition of lines and show that these issues need to be taken into account when designing a protection scheme.

Finally, the thesis considers the practical issue of cross country faults, in which simultaneous, or near-simultaneous, occurrences of ground faults on nearby lines may result in incorrect performance of distance protection relays. Techniques used to minimise the impact of cross-country faults in modern protection relays are reviewed, and a detailed real time simulation study is presented to assess the performance of such schemes under practical conditions in a utility environment in which the nearby lines are both mutually coupled and untransposed.