Distance protection (ANSI 21) — impedance protection with zone 1/2/3 on MV and HV lines

The [guide on directional overcurrent protection (ANSI 67)](/guides/nen-3140/richtingsafhankelijke-overstroombeveiliging-67-richtingsrelais) covers a protection that responds to the magnitude and direction of the current. The guide on generator field-failure protection (ANSI 40) covers a protection that uses impedance to recognize a specific generator fault. This article covers a third application of the impedance principle, this time aimed at line protection: distance protection, designated by ANSI code 21.

The principle: impedance as a measure of distance

A distance relay measures the voltage and current at its own location and calculates from these the apparent impedance to the fault. Because the impedance of a line (approximately) increases proportionally with length, this calculated impedance gives a direct indication of how far the fault is from the relay: a nearby fault produces a small measured impedance, a fault further away a larger one. This makes distance protection fundamentally different from an ordinary overcurrent protection, which looks solely at the magnitude of the current and is therefore sensitive to changes in available fault current (for example due to a varying amount of active generation in the network). Because distance protection responds to impedance — that is, to a ratio between voltage and current — rather than to an absolute current value, the configured reach remains relatively insensitive to this kind of variation, which makes the function attractive for meshed or strongly interconnected networks.

Staggered zones: from fast and limited to slow and far

A distance relay almost never applies a single impedance threshold, but a staggered scheme of several zones, each with its own reach and time delay:

Zone 1 typically covers roughly 80 to 85% of the protected line and trips instantaneously. The reach is deliberately not set to 100% — this is called underreaching — to prevent measurement inaccuracies in the current and voltage transformers or in the relay itself from causing zone 1 to mistakenly assess a fault just beyond the end of the line as its own fault and trip without delay.
Zone 2 deliberately overreaches beyond the end of the own line — a common setting is on the order of roughly 120% of the own line impedance, so that the remaining part of the line (the 15 to 20% that zone 1 deliberately does not cover) is still protected, plus a portion of the first adjacent line as backup. Zone 2 trips with a short but deliberate delay — a common practical value is on the order of 0.3 seconds — so that a fault on the adjacent line first gets the chance to be cleared by that line's own, closer protection.
Zone 3 reaches even further, typically well into the next line section, and serves as further backup protection with a longer time delay — on the order of roughly 1 second — in case the closer-lying protections fail to operate for whatever reason.

This staggered scheme combines fast, primary protection for the majority of the own line with a stepwise slower, but further-reaching backup protection for the rest of the line and the adjacent network sections.

Impedance characteristic: mho or quadrilateral

As with the offset-mho characteristic of field-failure protection (see the related guide), the reach of a distance relay is typically represented as an area on the R-X impedance diagram. For line protection, a mho characteristic (a circle through the origin) or a quadrilateral characteristic (a four-sided area, which allows independent setting of the reach in the resistive and reactive direction) is often applied — the choice depends on the type of fault (phase-to-phase versus phase-to-earth) and on the desired sensitivity to arc-related resistance at the fault location.

Note: the exact zone reaches, time delays, and characteristic shape follow from the line impedance, the network configuration, and the coordination study of the specific connection; this article covers the principle, not a ready-made setting table for every line.

Practical relevance

Distance protection is mainly applied on medium-voltage and high-voltage lines — particularly in meshed or interconnected networks where the available fault current can vary considerably with the network configuration — and less on simple, radial low-voltage distribution networks, where overcurrent protection is generally sufficient. During commissioning of a distance relay it is important to verify that the configured zone reaches are aligned with the actual, current line impedance — a setting based on outdated or incorrect line data can result in a zone 1 that reaches too far or too little.

Common mistakes

Setting zone 1 to (nearly) 100% of the line length — this increases the risk of an instantaneous, incorrect trip for a fault just beyond the own substation, as a result of normal measurement inaccuracies.
Not coordinating the time delay of zone 2 and zone 3 with the protections of adjacent lines — this can lead to an unnecessary trip of the own line for a fault that should actually have been cleared by the protection of the adjacent line.
Not reviewing impedance settings after a change to the line configuration (for example a longer or shorter line section, or a new tap) — the zone reaches are directly derived from the current line impedance.
Applying a mho characteristic without accounting for arc-related fault resistance for phase-to-earth faults — a characteristic that is too tight can miss a fault with considerable arc resistance, particularly for faults near the edge of the protected zone.