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Glossary · Energy industry & energy transition

Earth-fault compensation, Petersen coil

earth fault compensation Petersen coil earth fault medium-voltage grid grid protection electricity grid Operational safety

Earth-fault compensation is a method of neutral-point treatment in three-phase AC networks. It deliberately reduces the current that flows during a single-phase earth fault. For this purpose, an adjustable reactor coil is connected between the neutral point of a transformer or of a neutral-point former and the earthing system. This coil is called an arc-suppression coil or, after its inventor Waldemar Petersen, a Petersen coil. Other names for this mode of operation are resonant neutral earthing, resonant-earthed network or compensated network. In the event of a fault, the coil generates an inductive current that largely cancels the capacitive earth-fault current of the network.

The idea originates from Waldemar Petersen, an electrical engineer at TH Darmstadt. AEG filed a patent application for the device on 24 January 1917 (DE 304 823). From 1921, AEG manufactured the coil in series production. To this day, earth-fault compensation remains a widely used form of neutral-point treatment in many European distribution networks.

It is most commonly used in medium-voltage networks. In Germany, however, many 110 kV networks are also operated in compensated mode, especially those with many overhead lines. Single-phase earth faults are a frequent type of fault there. They arise, for example, from damaged cables, insulation faults, branches, overvoltages, animals or moisture in equipment. Earth-fault compensation is therefore not an individual protective device but a mode of operation of the network: a single-phase fault to earth is not supposed to lead immediately to a high fault current and thus to disconnection.

Physical principle

Earth-fault compensation is based on the electrical capacitance of the conductors to earth. Every overhead line and every cable, together with the earth, forms a kind of distributed capacitor. In an earth fault, a phase conductor is conductively connected to earth or to earthed parts. As a result, the entire voltage system shifts relative to earth. The so-called neutral displacement voltage then appears between the neutral point and earth. In a solid fault, it reaches approximately the phase-to-earth voltage of the network. The two healthy conductors then sit at roughly the line-to-line voltage relative to earth, that is, up to √3 times their normal value. A capacitive current flows to the fault location via their earth capacitances.

At the same time, the displacement voltage drives an inductive current through the Petersen coil, which also flows via the fault location. The capacitive and inductive currents are phase-shifted by 180 degrees relative to each other. If the coil is correctly tuned, these two reactive currents largely cancel each other at the fault location. The coil and the earth capacitances form a parallel resonant circuit tuned to the network frequency. What remains is a residual earth-fault current. It consists of the active residual current, which arises from coil losses, leakage currents and corona losses, of the uncompensated reactive component in the case of detuning, and of components from harmonics and asymmetries. The Petersen coil compensates only the fundamental component. If the residual current is small enough, an arc at the fault location often extinguishes on its own. This is the origin of the name arc-suppression coil.

An example illustrates the order of magnitude: an extensive 20 kV cable network can have a capacitive earth-fault current of several hundred amperes. If it is well compensated, only a few percent of this flows through the fault location, predominantly as active current. Cable networks have low damping. There, the active component comes predominantly from the coil itself, whose damping is often around 3 percent. In overhead-line networks, by contrast, corona and leakage losses predominate. The total damping there is significantly higher, at about 6 to 10 percent.

Extinction limit and tuning

Self-extinction works only up to a certain residual current, the extinction limit. The former standard VDE 0228 Part 2 (edition December 1987) contained experimentally determined guide values for this, depending on the nominal voltage. For 20 kV networks with compensation, the limit was about 60 A, for isolated networks about 30 A. Technical literature still gives as a guideline about 60 A for medium voltage (10, 20 and 30 kV) and about 120 to 132 A for 110 kV networks. The extinction limit restricts how far a resonant-earthed network may extend. The active residual current grows with line length and cannot be compensated with a classical coil.

How high the capacitive earth-fault current is depends on the length, design and switching state of the lines and cables. Cables have significantly higher earth capacitances than an overhead line. If a distribution network is increasingly undergrounded, the capacitive earth-fault current therefore rises, and with it the active residual current. Many Petersen coils can be adjusted under load, for example with a movable plunger core or with taps. A controller adjusts them automatically. During commissioning and during checks, the coil is stepped through its settings. In doing so, the resonance curve of the displacement voltage is recorded. From it, the capacitive earth-fault current and the active residual current can be estimated.

In practice, the coil is not set exactly to resonance. In Germany, compensated networks are usually operated slightly overcompensated. The coil current is then above the capacitive earth-fault current by a detuning of the order of 5 percent. This keeps the network from going into resonance when line sections are switched off and the network capacitance decreases. In Austria, undercompensation is more common. If the network is detuned too much, the residual current rises and self-extinction deteriorates.

Distinction from other neutral-point treatments and from short circuits

Earth-fault compensation is not the same as simple earthing of the network. The question is not only whether a neutral point is earthed, but how it is earthed. A solidly or low-impedance earthed neutral point leads to a high fault current in a single-phase fault, which the protection disconnects quickly. An isolated neutral point has no direct connection to earth. Nevertheless, capacitive earth-fault currents flow via the earth capacitances, and their extinction limit is lower. The compensated network lies in between: its neutral point is earthed via a coil tuned to the network capacitance. An overview of all variants is given in the entry neutral-point treatment.

A distinction must also be made from the short circuit. A short circuit is a low-impedance connection between conductors of different potential, in which very high currents can flow. In low-impedance earthed networks, a fault between conductor and earth is also a short circuit, the earth short circuit. In the compensated network, by contrast, the single-phase earth fault is characterised precisely by its small fault current. It therefore does not have to be disconnected immediately. However, if a second earth fault occurs on another conductor and at another location, a double earth fault arises. This is a short circuit via earth with high current, which the protection must disconnect selectively.

Compensation is not suitable for very high voltages. From about 220 kV, in some cases already at 110 kV, the residual current from the active component and detuning is too large. In addition, it would be uneconomical there to design the insulation for the increased phase-to-earth voltage of the healthy conductors. Extra-high-voltage networks are therefore low-impedance earthed.

Significance for network protection and network operation

Earth-fault compensation does not replace network protection, but it changes its task. With high short-circuit currents, current magnitude and time grading are often sufficient. A compensated earth fault, by contrast, is harder to detect and locate: the fault current is small, its active component weak, and its direction must nevertheless be determined reliably. Compensated networks therefore need measurement of the displacement voltage, earth-fault directional relays, evaluation of the active residual current or of transients, and often further location methods. One aid is, for example, a resistor briefly connected in parallel with the coil, which increases the active current. All of this must fit together in the protection concept.

The most important advantage: the network can continue to be operated during a single-phase earth fault while the fault is searched for and specifically disconnected or eliminated. Many temporary arc faults, for example from a flashover, extinguish on their own without customers noticing anything. This avoids numerous supply interruptions. A compensated earth fault is nevertheless not a harmless condition. Insulation, surge arresters, cable accessories and other equipment must withstand the increased voltage to earth. An existing earth fault also increases the risk of a double earth fault.

How long an earth fault may persist is not determined by a uniform figure. It depends on the technical rules, the network concept, the design of the equipment, the requirements for the earthing systems and the operating instructions of the respective network operator. The Petersen coil is therefore not a component that creates operational safety by itself. It works only in interaction with network planning, protection technology and disciplined operational management.

Changing distribution networks

Photovoltaic and wind energy plants do not provide earth-fault compensation themselves. The energy transition changes the framework conditions through the expansion and conversion of distribution networks: there are more cables, more switching states, more power electronics, more automated stations and more infeeders in medium voltage. With undergrounding, the capacitive earth-fault current and the active residual current rise. At the same time, self-extinction loses importance because cable faults are mostly permanent faults. Some large cable networks are therefore divided into smaller network areas with their own compensation or switched to low-impedance neutral earthing. In addition, there are systems that use power electronics to also compensate the active and harmonic components of the residual current (residual current compensation).

Common misconceptions

  • "A compensated network simply keeps running during an earth fault." Only time-limited continued operation under defined conditions is permissible. The fault must be detected, located and eliminated. If an earth fault is ignored permanently, the risk of consequential damage and multiple faults increases.

  • "The coil makes the fault location harmless." The Petersen coil reduces the fault current and can extinguish arcs. However, touch and step voltages as well as the local earthing conditions remain safety-relevant. Earthing systems, protective disconnection and fault location must therefore be considered together.

  • "The arc-suppression coil extinguishes every earth fault." An arc extinguishes only if the residual current is below the extinction limit and the fault conditions are suitable. With solid earth contact, contaminated insulators, damp cable terminations or damaged cables, the fault persists. In that case, compensation mainly limits the current and buys time for an orderly switchover.

  • "Exact resonance is ideal." In practice, operation is deliberately slightly detuned. This keeps the displacement voltage limited in normal operation, and the network does not go into resonance when the switching state changes.

Earth-fault compensation shows that security of supply does not depend on strong lines or sufficient generation alone. Equally important is how fault currents are limited, faults are detected, network sections are selectively separated and equipment is designed against voltage increases. The Petersen coil is a good example of this: a passive piece of equipment with great effect. Its benefit, however, arises only through correct tuning, suitable protection technology and orderly operational management.

Sources

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Edited by eCMS on 11.11.2026 19:47

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