Petersen coil (arc suppression coil) — resonant earthed networks and why an earth fault is not tripped immediately

The guide on the zigzag earthing transformer briefly mentions that an earthing resistor or Petersen coil can be connected to the artificial star point of such a transformer. This article looks deeper at what a Petersen coil actually does and why a resonant earthed network behaves, in certain respects, like the IT system covered in the guide on the IT system and insulation monitoring.

The problem: capacitive earth-fault current in a cable-rich network

Every phase conductor of a network has, relative to earth, a certain capacitance — limited for overhead lines, but significant for an extensive underground cable network. At a fault from one phase to earth, this distributed capacitance of the remaining, healthy phases charges and discharges through the fault location: a capacitive earth-fault current arises that is entirely determined by the network's total capacitance to earth, independent of how the neutral point is earthed. At a sufficiently large capacitive current, this current can sustain an arc at the fault location rather than let it extinguish, with a risk of the arc propagating into a double or three-phase fault.

How a Petersen coil compensates the capacitive current

A Petersen coil (also called an arc suppression coil or "ground-fault neutralizer", after the German engineer Waldemar Petersen who introduced the principle in 1916) is an adjustable reactor connected between the network's neutral point (directly at a suitable power transformer, or via a zigzag or earthing transformer as covered in the related guide) and earth. At an earth fault, this coil supplies an inductive current that is in antiphase with the network's capacitive fault current. If the coil is correctly tuned to the network's total capacitance, the inductive and capacitive currents at the fault location cancel each other out almost entirely: the remaining residual current at the fault location is then much smaller than the original capacitive fault current and is determined solely by the active (resistive) losses in the network. At a sufficiently small residual current, the arc at the fault location self-extinguishes, without the fault needing to be tripped out.

Tuning to the network capacitance

The inductance of the Petersen coil is typically adjusted, via a tap changer or a continuously variable (plunger) core, to the network's actual capacitance to earth. Because this network capacitance changes as cable circuits are switched in or out, the tuning must be checked periodically and adjusted if necessary — a network that is no longer accurately tuned leaves a larger residual current at the fault location and thereby (partly) loses the self-extinguishing effect.

Why this resembles, but is not the same as, an IT system

Like an IT system, a resonant earthed (Petersen-compensated) network can keep operating through a first earth fault without immediate tripping, because the fault current stays small enough not to pose a hazard. The underlying mechanism differs, however: an IT system has no (or a very-high-impedance) neutral-point earthing, which keeps the fault current low on a first fault due to the high impedance of the path; a resonant earthed network instead has a low-impedance, tuned inductive connection to earth that actively compensates the capacitive fault current. Both, like an IT system, require some form of insulation monitoring or earth-fault detection to signal the presence of a first fault, so it can be located and cleared before a second, independent fault occurs.

Note: resonant earthed (Petersen) networks are applied in the Netherlands by regional distribution network operators in parts of the cable-rich medium-voltage network; the exact design parameters (tuning accuracy, residual-current threshold, detection method) are system-specific and follow from the network operator's system study, not from a single fixed standard value. IEC 60076-6 covers the basic requirements for the arc suppression coil as a reactor.

Practical relevance

When assessing a medium-voltage connection supplied via a Petersen-compensated network, it is important to recognise that a reported "first earth fault" does not by definition lead to an immediate interruption — as with an IT system, though, rapid location and clearance of the fault is essential to prevent a second fault from leading to a double earth fault with a much higher fault current.

Common mistakes

Confusing a resonant earthed network with a solidly or low-resistance earthed network — the presence of a Petersen coil specifically means the fault current on a first fault is kept small rather than tripped out quickly.
Assuming the Petersen coil never needs re-tuning after installation — a changing network capacitance (as cable circuits are switched in or out) requires periodic re-tuning.
Thinking a resonant earthed network can do without insulation monitoring or earth-fault detection — as with an IT system, a first fault must still be signalled and cleared, even though the network does not trip immediately.
Confusing the Petersen coil with the zigzag earthing transformer itself — the zigzag transformer creates an artificial star point where needed; the Petersen coil is the separate, adjustable reactor connected to that star point to compensate the capacitive earth-fault current.