Denis Ruest · IPQDF.com · March 2026
From 42 A to 337 A: Zig-Zag Reactors and Ground Fault Detection — A Practical Case Study
Single-phase-to-ground faults account for 70–80% of all fault events on medium-voltage power systems worldwide. On a conventional utility network, these faults are readily detected — the short-circuit capacity of the grid produces a fault current large enough to operate overcurrent and earth fault relays with comfortable margin. On an islanded or weak power system, however, the situation is fundamentally different. Inverter-based renewable energy sources — solar photovoltaic, wind, battery storage — limit their output current to approximately 1.0–1.2 times rated during a fault. Diesel generators, operating in remote area power supply (RAPS) configurations with high-impedance neutral earthing, contribute even less. The result is a single-phase ground fault that may produce a fault current of 20% of rated — indistinguishable from a normal load unbalance, and invisible to any conventional protection relay. The fault persists, the arc burns, and the probability of escalation to a catastrophic multi-phase event grows with every second.
The zig-zag reactor solves this problem directly. By providing a stable, well-characterised zero-sequence impedance path at an appropriate point on the network, it creates a controlled fault current source that operates independently of the generation technology. Where a 13.8 kV / 5 MVA islanded renewable energy site would otherwise produce only 42 amperes of ground fault current — undetectable by any standard relay — a correctly specified zig-zag reactor raises that figure to 337 amperes, a sensitivity margin of 3.4 times a 100 A relay pickup. The fault is detected, the circuit breaker operates, and the arc is extinguished within the relay operating time.
This article provides the complete engineering treatment of the zig-zag reactor for this application. The operating principle is developed from first principles — the air-gapped three-leg core, magnetomotive force cancellation under positive-sequence load, and free conduction of zero-sequence fault current. The comparison with the star-delta transformer is made explicit: both are equivalent from the network perspective, but the zig-zag delivers the same zero-sequence shunt at 40–60% lower cost, without the ferroresonance susceptibility of an ungapped transformer core on a capacitive cable network.
The specification methodology is fully parametric and worked through a detailed numerical example: system impedances, arc resistance using the Warrington formula, sequence network calculation, fault current with and without the reactor, and phase voltages at four measurement points — source bus, reactor bus, load feeder, and the fault itself. A key finding that emerges from the phasor analysis is that voltage alone cannot discriminate between an upstream and a downstream fault — the voltage difference at the reactor bus is only 1.4% between the two cases. Current direction is the reliable discriminator, and the neutral current transformer alone cannot provide it. The article explains what instrumentation is needed and why.
Arc resistance sensitivity is addressed quantitatively: the design basis of 20 Ω is demonstrated to be conservative relative to the Warrington formula for a 0.3 m arc, and a sensitivity table from bolted fault to the relay detection limit gives the engineer the full operating envelope.
A downloadable calculation sheet covering all equations, the complete voltage table, the arc sensitivity analysis, and a blank adaptation template for other voltage levels accompanies the article.
Read the full article on IPQDF.com → https://ipqdf.com/renewables-energies/zig-zag-reactors-zero-sequence-current-supply-weak-power-systems/