Distance protection: Why have we started with a circle, does it matter, and what else is out there?

We look back at the history of distance protection, explain the first principles, and discuss why our industry settled on designs we know and appreciate today. We look at why, after a century of refinements, a typical distance element still uses heavily filtered voltages and currents and operates on the order of one power cycle. In the second part of the paper, we explain the principles of time-domain distance protection based on incremental quantities, and operating by processing samples of voltages and currents without band-pass filtering to retrieve phasors. We discuss various choices for a time-domain distance element and present test results and field cases of an implementation with operating times of just a few milliseconds. In the third part of the paper, we discuss the feasibility of a distance element based on traveling waves and operating even faster.

I. Introduction

Controlled reach is the key attribute of a distance element applied for short-circuit protection of power lines. Using only local voltages and currents, a distance element responds to faults located within a predetermined reach explicitly set by the user. Moreover, the reach is specified in units proportional to the physical distance to the fault, hence the element's name. Ideally, the element's actual reach is independent of the fault current level, pre-fault load, fault type, or fault resistance. In practice, these factors affect fault coverage, but only slightly compared with that of an overcurrent element.