Fault Energy Limiting – Rapid Earth Fault Current Limiter (REFCL)

Risk Reduction Category

Protection & Detection for Reduced Fault/Ignition Energy

Technology Description

First, some background. An arc suppression coil (ASC) is an inductance which is connected to the neutral of a transformer to cancel out the current caused by the capacitance of the system. This device can be a fixed inductance, a variable inductance which is automatically adjusted with a plunger or a fixed inductance which is varied by the switching of capacitors. The terms Petersen Coil, Arc Suppression Reactor are synonyms [1]. Figure 1 shows an ASC on a distribution system, schematically.

Figure 1: Arc Suppression Coil (ASC) [1]

Figure 1: Arc Suppression Coil (ASC) [1]

As new feeders are added to distribution grids or taken out of service, the arc-suppression coil may no longer be tuned to match the zero-sequence capacitance of the downstream grid. In such cases, the level of ground fault current would increase. Specifically for delta distribution systems without grounded neutrals and line-to-ground fault scenarios, Rapid Earth Fault Current Limiter Technology (REFCL) has been applied to provide a dynamically variable neutral impedance, which will respond to single-line-to ground faults to choke off any remaining ground fault current. With this technology, a power electronic switch is connected across a neutral grounding reactor on the supply transformer. When a downstream ground fault occurs, the system detects the ground fault current and injects a current of equal magnitude but opposite polarity. Field deployments have shown that this impedance tuning effect completes within approximately 5 cycles of fault initiation and can reduce ground fault current from the order of amps to tens of amps at the time of fault initiation to below 0.5 A once tuning completes. Because the fault current is very low, fault location can be difficult to calculate, but the fault arc should self-extinguish. Therefore, in the majority of cases, circuit breakers are not required to trip unless the fault persists for an extended period (such as more than 30 seconds). Source: Distribution Protection Options to Reduce Damage and Improve Public Safety, Tom Short et al. EPRI 3002018773

A ground fault neutralizer is one type of REFCL technology. The term is loosely defined as a device consisting of an arc suppression coil and an inverter used to restrict fault current. Figure 2 shows an example of a residual current compensator system with ASC.[1].

Figure 2: Arc Suppression Coil and Residual Current Compensator [1]

Figure 2: Arc Suppression Coil and Residual Current Compensator [1]

Southern California Edison (SCE) have piloted multiple variations of REFCL technology to better understand how the available alternatives apply to the wide variety of circuit designs on the SCE system. The diversity of SCE circuit types means that no single variation is cost-effectively applicable across the entire SCE system. For example, for large substations feeding tens to hundreds of miles of high fire risk circuitry, Ground Fault Neutralizer projects modeled on the Australian REFCL program may be a good choice. However, the Ground Fault Neutralizer is neither economically viable nor necessary for fire risk reduction for smaller distribution systems. For these smaller facilities grounding conversion projects to unground or resonant ground them can achieve a similar reduction in risk at a much lower cost [1].

An interesting finding by SCE is that it is necessary that the charging current on the three phases be balanced. Voltage regulators can cause voltage imbalance if the three phases are being set to different voltages. This can occur either from open delta connections where only two regulators are used or from settings which allow independent operation of the three phases. Open delta voltage regulators therefore require upgrade to closed delta with all three phases controlled by a single controller [1].

The distribution network requires capacitive balancing, replacement of equipment intolerant to overvoltage and the installation of equipment to support REFCL operation. The cost of this aspect of work is material. HV customers also require equipment to be replaced or isolated where existing equipment is not rated to the expected higher voltages from REFCL operation[2].

Technical Readiness (Commercial Availability)

REFCL technologies have been trialed and rolled out by AusNet (report, 2018) and Powercor (report, 2018) 22-kV grids since 2017.

“Rapid Earth Fault Current Limiter (REFCL) Program: HV Customer Policy for REFCL Protected Networks (Load & Generator),” AusNet Services, 2018. https://www.aer.gov.au/system/files/Attachment%2018%20-%20REF%2030-10%20REFCL%20Program%20HV%20Customer%20Policy%20Issue%202%20-%20Public.pdf

“Rapid Earth Fault Current Limiter Explained,” PowerCor, 2018. https://media.powercor.com.au/wp-content/uploads/2018/11/23143431/2018-09-04-a4-hvc-collateral-final.pdf

Vendors with Hardware and Software

Swedish Neutral – Ground Fault Neutralizer http://www.swedishneutral.se/main.php?name=start

Implementations / Deployments

Approximately 100 installations of Swedish Neutral Ground Fault Neutralizer mostly in Europe. Primary customers are electric utilities and large manufacturing plants who have substation equipment on site. This product is being used to improve reliability and reduce shock hazards in Europe and is also being used in Australia to prevent wildfires.

Pilot test of Ground Fault Neutralizer technology in southern California involving at least one substation is ongoing as of 2023.

Innovations as of Mid 2023

Potential Enrichment Work Opportunity

References

[1] Rapid Earth Fault Current Limiter (REFCL) Projects at Southern California Edison, Jesse Rorabaugh Nicole Rexwinkel Austin William Fresquez, 12/29/2022.

[2] REFCL Tranche 3 Program, PowerCor Austalia, July 2020