January/February 2021 | Vol. 26 No. 1
by Viacheslav Levashov, Swedish Neutral AB; Jesse Rorabaugh, Southern California Edison; Franz Stadtmueller, Pacific Gas & Electric; and Niklas Winter, Swedish Neutral AB
On the power grid, the grounding and protection system must properly function when lines come in contact with vegetation, wildlife, or other lines, that could trigger faults. Without adequate protection on the lines, the results could be catastrophic.
“What keeps us up at night,” explains Matthew Pender, Director of the PG&E Community Wildfire Safety Program, “is the exposure—how many miles, how many things could go wrong. It only takes one tree.”
Some commonly employed grounding/protection technologies might not always detect certain kinds of faults in the network or might not sufficiently reduce the fault’s energy, leading to ignition. A relatively new technology to the North American market—the Ground Fault Neutralizer (GFN)—seeks to remedy this. GFN technology detects faults that traditional protection cannot by being roughly 25 times more sensitive. Then it neutralizes the fault ultra-fast, preventing a fire before one can even begin. This is achieved by combining hardware (a passive Petersen coil and an active residual current compensating inverter) to neutralize the fault current with advanced software (differential zero sequence admittance) to detect the faults.
Though new to North America, GFN technology was developed in the 1980s in Sweden and first demonstrated in field applications in the early '90s. At the time, Swedish regulators had begun requiring the state-owned utility to cut power supply immediately following an earth fault, eliminating any residual current on the line. Regulators also required increasing fault detection sensitivity to be able to detect all earth faults of up to 5kOhm. Recognizing that earth fault current on ungrounded systems consists of two components—capacitive (charging) current and residual current—engineers developed GFN to eliminate both.
Today’s GFN systems are installed and controlled from the substation, protecting the entire network from a single point. The technology connects to the neutral of a power transformer and is comprised of three major components: 1) an arc suppression coil (aka Peterson coil) that compensates for the capacitive part of the fault current; 2) a residual current compensating Inverter that compensates for the residual part of the fault current; and 3) a computer controller that detects ultra-high impedance faults and controls the two other components to neutralize the fault current ultra-fast. Mathematically, this could be expressed as follows:
Total Fault Current = (capacitive fault current – arc suppression coil current) + (residual fault current – inverter current) = 0
Configuration of a GFN system (referred to here as Rapid Earth Fault Current Limiter, or REFCL)
Until somewhat recently, use of the technology was largely limited to Sweden and nearby European countries. That all began to change in 2009 when the Victorian Government (Australia) implemented the Rapid Earth Fault Current Limiter Program across the state in response to the Black Saturday bush fires which led to 173 deaths. An extensive test program of available technologies showed that the Ground Fault Neutralizer was the most effective at preventing ignition. The Victorian utilities were mandated to install them to protect a total of 31,000 km of circuitry by 2023. Conversions have already been completed on 15,000 km of circuitry. These conversions included testing to demonstrate that every single circuit can detect a 25.4kOhm earth fault.
The Ground Fault Neutralizer appears to have been extremely effective in its first years of use. In the 2019- 20 bushfire season—the worst ever in Australia—the technology performed well, protecting the lines against 57 faults including 33 permanent faults. None of these faults resulted in an ignition.
Well aware of the success in Victoria, two major investor-owned utilities in California have both begun Ground Fault Neutralizer pilot projects: Southern California Edison and Pacific Gas & Electric.
Southern California Edison
The small city of Neenach, California, lies about 100 miles north of Los Angeles and is home to one of Southern California Edison’s distribution substations. The substation covers about 180 miles of 12kV circuity, 70 of which run through geography designated as “high fire risk areas” by the state of California.
“We chose this area for the pilot,” explains Jesse Rorabaugh, Senior Engineer and Project Lead “because it closely resembled the system design, ratings and application parameters of what was done in Australia. We hope to replicate their success as we familiarize ourselves with the technology, then scale it to other parts of our system, where it makes sense to do so.”
In February, SCE will begin installation of a GFN into the Neenach substation, just as was done through Victoria, to protect the entire length of circuitry originating from that substation. The project timeline allows for field testing in time for the 2021 fire season.
Pacific Gas & Electric
Many of the electric distribution circuits in the PG&E service territory are like those in Victoria where GFN technology has been successfully deployed, namely three-wire, un-grounded configurations. Through the Electric Program Investment Charge (EPIC) program, Pacific Gas & Electric Co. is demonstrating GFN technology at one of its substations in Napa County. The technology adds an additional resilience and protection layer, rapidly reducing the fault current if a ground fault occurs somewhere within the 160 circuit miles connected to the substation.
PG&E is operationalizing and testing the technology through controls, simulations, and real-world
tests to measure the effectiveness of the technology. Specifically, fault sensitivity, fault location, and fault current are being assessed.
“Deploying GFN technology is not plug and play,” confides Franz Stadtmueller of PG&E. “A detailed engineering design of the supporting substation and distribution equipment is required. All of the primary connected equipment needs to be fully rated for phase- phase voltage to ground for example.”
To maximize the greatest level of sensitivity, the PG&E demonstration involves balancing the capacitive (charging) currents from each of the phases at the substation. Long single-phase tap lines or single- phase underground cables cause unbalance, so PG&E is installing capacitive balancing units to maintain balance and high sensitivity to ground faults. The demonstration project is scheduled for completion in June 2021. ei
Mr. Levashov is a software department manager at Swedish Neutral; Mr. Rorabaugh is a senior engineer at Southern California Edison; Mr. Stadtmueller is an electrical engineer at Pacific Gas & Electric; and Mr. Winter is executive vice president at Swedish Neutral.