Distance protection · Glossary

Distance protection is a method of power system protection. It detects faults on electrical lines from the impedance measured by the protection device. This impedance describes the apparent AC resistance between the relay location and the fault location. The device receives current and voltage via current and voltage transformers, calculates an impedance from them and compares it with the set line impedance. The smaller the measured impedance, the closer the fault is from the device's point of view. In the English-speaking world, the function is often referred to as "21" after its ANSI device number.

The "distance" in distance protection is therefore not a geographical distance. Two lines of equal length can appear to the protection to be of different lengths if conductor cross-section, material, voltage level or design differ. An overhead line, a cable and a series-compensated line have different electrical characteristics. Distance protection works with these network parameters and not with a site plan.

Distance protection is used mainly in the high-voltage and extra-high-voltage grid, that is, in the transmission grid and in 110 kV grids. There, lines transmit large amounts of power, the grids are meshed and faults must be limited very quickly. Distance protection functions are also found in devices for medium-voltage cables. In a short circuit, high currents flow, voltages collapse, and generators, converters and loads react to the disturbance. The protection must then disconnect the affected section within fractions of a second while taking as little intact equipment as possible out of the grid.

Impedance instead of kilometres

The central measured quantity of distance protection is impedance. It is evaluated as a complex quantity, that is, with a real part (resistance R) and an imaginary part (reactance X). For overhead lines and cables in the high-voltage grid, the reactive part is predominantly inductive. A simple resistance value is therefore not sufficient. Protection devices display the measured values in the R-X impedance plane and check whether a measuring point lies within a set tripping characteristic. Circular characteristics (mho) and polygonal characteristics are common. With polygons, the reach can be set separately in the R direction and in the X direction. For reach, digital devices rely mainly on the reactance, because arc resistance mainly increases the resistive part.

An illustrative example is a bolted (metallic) short circuit on a line. The voltage at the relay location drops while the current rises sharply. This results in a small measured impedance. If the fault is farther away, the measurement includes a larger part of the line impedance. A simplified numerical example: if a line has a total reactance of 10 ohms and the device measures 4 ohms, the fault lies, under ideal conditions, at about 40 percent of the line length. Before measuring, the device must detect a fault. This so-called starting (pickup) takes place via overcurrent or impedance criteria. It must not respond falsely even at high operating load and during switching operations.

This estimate is never just a calculation exercise with ideal values. Faults can occur as single-phase, two-phase or three-phase faults. They arise, for example, through arcs, trees, insulation damage or faults in the switchgear. In the case of earth faults, the return path through the earth plays a role. The device takes it into account via a set earth impedance compensation factor. Fault resistances, infeeds from several sides (intermediate infeed), parallel lines with magnetic coupling and series compensation can distort the measured impedance. The protection then measures too short (underreach) or too far (overreach). Distance protection is therefore always also a question of parameterisation.

Zones, grading plan and selectivity

Distance protection devices work with graded zones that are combined in a grading plan. The first zone trips without intentional delay, that is, in instantaneous time. However, it does not cover the whole of its own line. The reason is inaccuracies in line data, transformers and measurement. If the zone extended to the end of the line, the device could reach into the adjacent line and trip a fault just beyond the next station in instantaneous time (overreach). A typical reach of the first zone is about 80 to 90 percent of the line impedance. A common setting rule uses a grading factor of 0.85. The Swiss industry guideline gives 0.85 without communication and 0.8 with communication for transmission grid lines. In the direction of a transformer, 0.7 is given.

The second zone extends beyond the end of the line. It thus reliably covers the remaining part of the line and acts with a delay. Further zones serve as backup for adjacent grid sections in case a protection device or a circuit breaker fails there. There is often also a backward-looking zone. The delay between two steps is called the grading time. For digital devices, about 0.3 seconds is often assumed, for example 0 s, 0.3 s and 0.6 s. For electromechanical relays, about 0.4 seconds is used. Swiss practice gives a range of 0.25 to 0.5 seconds. These values are guide values from technical literature and guidelines. The respective requirements of the grid operator are binding.

This makes a basic principle of power system protection visible: selectivity. As far as possible, only the faulty item of equipment should be disconnected, while the rest of grid operation is maintained. A setting that is too tight can mean that a fault is not cleared or is cleared too late. A setting that is too wide or too fast can disconnect healthy lines and enlarge a disturbance.

Without communication, faults in the last section of the line are cleared only in the delayed second zone. For this reason, in the transmission grid the distance protection devices at both ends of the line are often coupled via signal comparison. One example is the POTT scheme (permissive overreach transfer trip). In it, both sides signal to each other that they see a fault in the forward direction. This allows the entire line to be protected in instantaneous time.

In this respect, distance protection differs from simple overcurrent protection. The latter responds when a current value is exceeded, if applicable with time grading. This can be sufficient in radial distribution grids, but in meshed grids it is often too coarse. There, the short-circuit current depends not only on the fault location but also on the grid configuration, infeeds and the number of connected generators. Distance protection additionally evaluates the voltage and thereby obtains location information that is largely independent of the short-circuit current.

Distinction from other protection methods

Distance protection is not the same as line protection in the general sense. Line protection is the umbrella term for all methods that protect lines against faults. Besides distance protection, these include differential protection, overcurrent protection, earth fault protection and other functions. Distance protection is one particular method within this protection technology.

It differs fundamentally from differential protection. Line differential protection compares the currents at both ends of the line. If more current flows into the line than out of it, the fault lies within the protected section. This method is very selective but needs a reliable communication link between the line ends. In principle, distance protection can work with measured values from only one end. With signal comparison it becomes faster, but does not require a continuous exchange of measured values. It is therefore often used as main protection or as backup protection, for example when the communication for differential protection fails. In modern transmission grids, both methods are often combined.

Distance protection must also not be equated with security of supply. It is a technical means of limiting faults, not security of supply itself. Security of supply arises from many elements: sufficient generation and flexibility options, resilient grids, system operation, protection technology, maintenance, market rules and crisis preparedness. Distance protection acts within this at a very concrete level. In the event of a fault, it decides which line is disconnected and how quickly.

Significance for grid operation

In a meshed grid, a fault on one line can redirect currents onto other lines. If the faulty section is disconnected correctly, the power flow is redistributed. If too much is disconnected, the disturbance can spread. If too little is disconnected, or too late, short-circuit currents persist. Equipment is then subjected to thermal and mechanical stress, and the voltage situation deteriorates. In the event of faults, protection devices, circuit breakers, automatic reclosing, control technology and operational requirements must fit together. A device that measures correctly but has unsuitably set zones can be just as problematic as a defective device.

The far-reaching backup zones are particularly delicate. At very high line loading or during power swings after disturbances, the measured operating impedance can become so small that it enters a tripping zone although there is no short circuit. This is called load encroachment. Such unwanted trips are discussed in the technical literature as a contributing cause of large-scale cascading outages. Countermeasures are adapted characteristic shapes, load blinders and power swing blocking. Power swing blocking blocks tripping during stable power swings without overlooking real faults.

The settings are not produced in isolation. Grid operators take into account line data, short-circuit capacities, switching states, the protection of neighbouring sections, transformers, busbars and backup concepts. Changes in the grid can alter protection calculations, for example new lines, substations, changed switching states or new generation plants. Protection technology is thus not an accessory installed once, but part of ongoing grid planning and system operation. Simulation tools are used today for verification.

Common misinterpretations

A widespread misunderstanding is to regard distance protection as an electrical rangefinder that determines the fault location unambiguously. In fact, it delivers a protection decision on the basis of measured electrical quantities. Fault location can be derived from the same or supplementary data, but it is a separate function and is subject to uncertainties due to fault resistances and infeeds. For fault clearance, it helps to know whether a fault lies at 17 or at 43 kilometres. For the protection function, what counts first is whether it lies within the zone that is to trip.

A second misunderstanding concerns speed. Fast disconnection is necessary in the event of a short circuit, but speed alone is not a quality criterion. A protection that trips falsely very quickly endangers grid operation. A protection that waits although the fault clearly lies in its own area endangers equipment and stability. The task is to coordinate sensitivity, selectivity, time grading and backup protection with one another.

The notion that protection devices are unaffected by the energy transition is also misleading. With more converter-connected plants, short-circuit contributions and dynamic responses change. These include wind and photovoltaic parks, battery storage and high-voltage direct current links. Synchronous generators supply high, physically determined currents in the event of a fault. Converters usually limit their current to values close to their rated current and control it quickly according to the specifications of the grid connection rules. In some cases they supply no zero-sequence current. Technical reports show that starting, directional decision and impedance measurement of distance protection devices can be impaired as a result. This does not affect every distance protection equally, but it increases the need for careful protection studies and adapted settings.

Institutional and economic significance

Distance protection initially appears to be a purely electrical engineering function. In practice, it touches on responsibilities, investments and rules. Grid operators are responsible for the safe operation of their grids. They define protection concepts, parameterise devices, test protection chains and coordinate with neighbouring grid operators. Manufacturers supply devices and functions, but the operator is responsible for the integration into the specific grid.

Economically, distance protection acts through avoided damage and limited consequences of disturbances. A grid area disconnected in error can lead to redispatch, supply interruptions, industrial outages or additional grid loading. A fault that is not disconnected can damage lines, switchgear and transformers. Good protection technology therefore costs not only devices, transformers and switches, but also engineering, testing, data maintenance and coordination. This ongoing work is less visible than the construction of a line, but indispensable for its usability.

Grids do not consist only of copper, aluminium, steel and transformers, but also of measurement, protection, control, communication and operational rules. A line can be physically present and yet be operated safely only if its protection concept fits the rest of the grid. With higher utilisation, changed load flows and more weather-dependent generation, this level gains weight. Distance protection thus denotes a protection function that translates electrical measured quantities into a rapid disconnection decision: measured locally, evaluated according to grid parameters, graded in time and aimed at preserving the operation of the rest of the grid.

Sources

Reichweiten und Staffelzeiten – Grundsätze — Netzschutz-Magazin (Walter Schossig); 2017-09-18. Einleitung Leitungsschutz, Abschnitte Anregung und Staffelzeit
Leitfaden zum Einsatz von Schutzsystemen in elektrischen Netzen — Verband Schweizerischer Elektrizitätsunternehmen (VSE); 2011-11. Abschnitte 8.2.2 Staffelzeiten und 8.2.3 Zonenreichweiten
Application of Overreaching Distance Relays — IEEE PES Power System Relaying and Control Committee; unbekannt. Abschnitte zu Zone 3, Load Encroachment und Power Swing Blocking
Impact of Inverter Based Resources on Utility Transmission System Protection — IEEE PES Power System Relaying and Control Committee; unbekannt. Abschnitt 3.1 Distance Relay
Considerations and Benefits of Using Five Zones for Distance Protection — Schweitzer Engineering Laboratories; unbekannt. Einleitung zum Stufendistanzschutz

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Edited by eCMS on 11.11.2026 19:26