An earth fault on a 22 kV overhead powerline is normally something we think about in terms of protection. Detect the fault, trip the circuit breaker and isolate the affected feeder. In Victoria, however, earth fault protection has another critical objective, preventing bushfires.
Following the 2009 Black Saturday bushfires, Victoria embarked on a major program to reduce the risk of powerlines igniting vegetation. One of the key technologies adopted was the Rapid Earth Fault Current Limiter (REFCL). REFCLs are now installed at zone substations protecting significant portions of Victoria's 22 kV distribution network in high bushfire consequence areas.
Why simply tripping isn't always enough
Consider a 22 kV conductor falling onto dry ground or vegetation. A conventional protection system needs to detect the fault and then open the appropriate circuit breaker. For a high-impedance earth fault, the current may be relatively low, but potentially still sufficient to sustain arcing and ignite vegetation. Even where conventional protection successfully clears the fault, the energy released before the circuit breaker opens can be important when conditions are conducive to bushfire ignition.
A REFCL approaches the problem differently. Rather than relying solely on disconnecting the feeder, it acts to dramatically reduce the current and energy released at the earth fault itself. If the fault persists, conventional protection can subsequently isolate the affected feeder.
First, a little about neutral displacement
To understand a REFCL, it helps to first consider what happens to the neutral of a three-phase network during an earth fault.
On a normal 22 kV system, the nominal phase-to-earth voltage is:
22 kV / √3 = 12.7 kV
If A-phase develops an earth fault on a system where the neutral is not solidly connected to earth, the system neutral can shift relative to earth. This phenomenon isn't unique to REFCLs. Neutral displacement also occurs on conventionally impedance-earthed systems, including systems using a Neutral Earthing Resistor (NER). The extent of the displacement depends on the network and the impedance between neutral and earth.
With resonant earthing, a phase-to-earth fault can result in almost complete neutral displacement. For an A-phase fault, the network can move approximately towards:
A-E ≈ 0 kV
B-E ≈ 22 kV
C-E ≈ 22 kV
The phase-to-phase voltages remain approximately 22 kV. What has changed is the position of the three-phase voltage system relative to earth. This neutral displacement is an important part of how a REFCL operates, but the interesting part is what the system does with it.
Enter the Petersen coil
At the heart of most REFCL systems is an arc-suppression coil, commonly called a Petersen coil. The Petersen coil is an adjustable reactor connected between the system neutral and earth.
Every distribution network has capacitance between its phase conductors and earth, and capacitive charging current is therefore present during normal operation. When a single-phase earth fault occurs, the healthy phase-to-earth voltages increase and their capacitive currents can flow through earth and return via the faulted conductor. This capacitive earth-fault current contributes to the current flowing through the fault and can help sustain an arc. The Petersen coil is tuned so that its inductive current approximately opposes this network capacitive current:
Iₗ ≈ -I꜀
The two currents substantially cancel, an arrangement known as resonant earthing. Rather than an earth fault producing hundreds or potentially thousands of amps, the Petersen coil can reduce the remaining earth-fault current to a much smaller value. Earth-fault arcs are therefore more likely to self-extinguish, while the amount of energy released at the fault is also significantly reduced.
So what makes a modern REFCL different?
The Petersen coil isn't new technology. In fact, the principle dates back more than 100 years. German engineer Waldemar Petersen developed the Petersen coil around 1916 to 1917, and resonant earthing has subsequently been used extensively around the world, particularly in European distribution networks.
A traditional Petersen coil, however, isn't perfect. It primarily compensates the reactive or capacitive component of earth-fault current. Network losses, imperfect tuning and the resistive component of the fault leave some residual current flowing. For conventional network reliability purposes, that residual current may be acceptable. For preventing a conductor lying in dry grass from starting a fire, it may not be.
Modern REFCL systems therefore add power electronics and active residual-current compensation. The Petersen coil does most of the heavy lifting by cancelling the network capacitive current, while the active compensation system injects a controlled current into the neutral system to compensate for the remaining residual components.
The combination allows the voltage and current at the earth fault to be driven extremely low. In simple terms, the earth fault causes the system neutral to displace and the faulted phase voltage to fall. The Petersen coil compensates the network's capacitive earth-fault current, while active compensation deals with the remaining residual current. The result is a significant reduction in the energy released at the fault.
Why does this reduce bushfire risk?
Imagine a conductor falling into dry vegetation. Even a relatively small sustained current concentrated through a small contact area can produce enough heat and arcing to ignite surrounding material.
The REFCL seeks to rapidly collapse the voltage and current at that fault. Victorian testing found that conventional resonant earthing alone was not sufficient to achieve the extremely low fault-energy levels required for bushfire mitigation, so modern REFCLs use active compensation to further reduce the residual current.
This is an important distinction. The objective is not simply to detect the earth fault faster. It is to change the electrical conditions of the network so that the fault itself becomes substantially less energetic.
The engineering trade-off
There is, however, a consequence. Under normal operation, equipment on a 22 kV network sees approximately 12.7 kV phase-to-earth. During substantial neutral displacement associated with REFCL operation, the two healthy phases can rise towards the system phase-to-phase voltage with respect to earth.
Victorian REFCL requirements can result in phase-to-earth voltages of approximately 22 kV and potentially up to 24.2 kV rms under specified conditions. This has important consequences for HV customer installations.
A piece of equipment that is perfectly suitable for a conventional 22 kV network may not necessarily have adequate phase-to-earth insulation performance for a REFCL-protected network. Transformers, switchgear, HV cables and terminations, voltage transformers, surge arresters and other equipment connected between phase and earth all need to be considered.
It is therefore not enough to simply ask whether equipment is "22 kV rated". The insulation coordination and temporary power-frequency overvoltage withstand capability of the equipment need to suit the actual network earthing and operating conditions.
Was REFCL technology invented in Victoria?
The underlying principle of resonant earthing is more than a century old. Petersen coils have been used internationally for decades, particularly across European distribution networks. More sophisticated Ground Fault Neutraliser systems combining resonant earthing with active compensation were also developed overseas, primarily to improve network safety, reliability and continuity of supply.
What is unusual about Victoria is the application and performance requirement. Following the Black Saturday bushfires, Victoria investigated whether this established power-system principle could be pushed far enough to substantially reduce the likelihood of a fallen conductor igniting vegetation.
Extensive testing showed that conventional resonant earthing alone was not enough. Even relatively small residual currents could still cause ignition under adverse conditions. Combining the Petersen coil with sophisticated active compensation allowed the remaining fault energy to be driven considerably lower, and Victoria subsequently undertook a major rollout of REFCL technology across high-bushfire-consequence areas of its 22 kV network.
The Victorian program has since attracted international interest, including from utilities in California facing similar powerline-initiated wildfire risks.
A century-old idea solving a modern problem
There is something particularly interesting about the engineering history of the REFCL. The underlying physics isn't new. Petersen developed resonant earthing more than 100 years ago to suppress earth-fault arcs, and the principle went on to become widely used internationally for network reliability and safety. Modern power electronics then provided a means of actively compensating the residual current that the Petersen coil could not eliminate.
Following the 2009 bushfires, Victoria took those technologies and applied them specifically to reducing powerline bushfire ignition risk. It is a good example of innovation not necessarily meaning inventing entirely new physics. Sometimes it means taking an established engineering principle, combining it with modern technology and applying it to a different problem.
There is also a broader lesson for engineers designing HV installations in Victoria. Reducing bushfire risk changes the way the network behaves during an earth fault. That change in network behaviour results in higher phase-to-earth voltages on the healthy phases, which in turn affects the insulation requirements of connected equipment.
Understanding the network you're connecting to can therefore be just as important as understanding the equipment you're installing.
