When electrical systems are installed in residences, safety is a primary consideration. Among other technologies designed to keep people safe, ground fault interrupters (GFIs) have been commonplace in homes since the 1970s, and are used to prevent electrocution.
Unfortunately, transmission and distribution lines have not been installed with the same level of safety precaution when it comes to the risk of a faulted power line sparking a wildfire. This is not to say that safety considerations are not a primary concern of the electric utility industry. However, safety measures to minimize wildfire risk should be implemented proactively, rather than after the fact, as they often are today.
To decrease the risk of power lines sparking wildfires, new devices called ground fault detectors (GFDs) should be implemented. GFDs mimic the functionality of a GFI; where GFIs prevent electrocution, GFDs prevent a fault from sparking a wildfire. If electrical systems are constructed so that single phase loads are served via phase-to-phase connections to power lines, neutral current can be minimized and GFDs can be used to rapidly isolate faulty electric lines, thereby minimizing the risk of wildfire.
What are GFIs and GFDs?
Ground fault interrupters (GFIs) are devices that rapidly detect and isolate a fault. GFIs have eliminated the possibility of electrocution when people accidentally contact energized conductors. They are often found in kitchen or bathroom electrical outlets. The first GFI was created in 1961; they became commonplace in households in the 1970s. GFIs are vital components of a safe electrical system in homes, offices, and more.
Ground fault detectors (GFDs) are devices that detect low-level ground fault current. Traditional ground fault relays are necessarily set above circuit unbalance. Like GFIs, GFDs require the installation of insulated neutrals. This allows GFDs to actuate for very low levels of ground current. GFDs should be located in substations where circuit breakers are installed, and as part of recloser installations in distribution lines.
It's important to note that GFDs are still in the design phase at Prescient. If you'd like to learn more about them, contact us.
Update Best Practices for Rapid Fault Isolation
Currently, it is generally accepted as a best practice to install circuit breakers with protective relays in substations. This practice protects customer service transformers with fuses, and coordinates fuses and protective relays so that fuses open before protective relays operate.
This best practice is not necessarily the best throughout the entire year. An updated grid must have different practices for two different seasons: wildfire/dry season and non-wildfire/rainy season. These two seasons would vary in length and months of the year depending on the location; for example, in California, wildfire season may extend from April through November, while non-wildfire season is only December through March.
When updating the electric power grid to incorporate wildfire risk reduction strategies, traditional best practices must be modified to accommodate the two seasons. During the non-wildfire/rainy season, general best practices would be the norm. However, during wildfire season, a new best practice needs to be implemented: rapid fault detection and isolation.
Two sets of critical factors must be considered. The first set of critical factors, 90NFR, align with traditional settings, that detect and isolate short-circuited transformers, capacitors, insulators, etc. These risk factors are expressed in number of failures repaired in 90 days (90NFR).
The second set of critical factors includes wildfire risk factors (WRF) such as relative humidity, fuel moisture, ambient temperature, wind speed, and wildfire ignition/spread time.
During the non-wildfire/rainy season, the first set of critical factors can be used to determine protective relay settings, including GFD settings. During the wildfire/dry season, the second set of critical factors should be prioritized over the first set, so that the threat of wildfire can be eliminated.
Distribution Line Fault Detection
During extreme wildfire risk conditions, interrupting fault current quickly is more important than opening the protective device closest to a fault. Consider a 12.47 KV distribution line that is equipped with one hundred 25 KVA transformers with 3 amp fuses; forty 50 KVA transformers with 6 amp fuses; and twenty 100 KVA transformers with 12 amp fuses. When wildfire risk is extreme, circuit breakers in the utility substation should be allowed to trip if any of the 160 transformers fail, regardless of their proximity to the fault. Settings in the GFD would implement this updated practice.
This would be in the place of a preemptive outage (rolling blackout), which is often the current wildfire prevention practice. During a preemptive outage, thousands of customers lose power to prevent a fault from sparking a wildfire. Instead, with new fault detection practices in place, only a few hundred customers will lose power only if a fault occurs.
Since this method sacrifices protective relays and fuse coordination, recovery time may take up to 24 hours. This can have an impact on utility performance indicators, such as Customer Average Interruption Duration Index (CAIDI). However, despite the potential for a negative impact on CAIDI, this new fault detection and isolation method will allow service to continue to customers unless a fault occurs. To compare risk factors, the likelihood of a transformer failing in any ninety day window is 0.001%, but the likelihood that a faulted power line will ignite a wildfire is 10% during extreme wildfire conditions.
GFDs: A Long-Term Solution for Wildfire Prevention
To decrease the risk of faulted power lines sparking wildfires, circuit breakers should be equipped with GFDs with dynamic settings, which are automatically lowered as the risk of wildfires increases. During the non-wildfire/rainy season, GFDs could actuate when ground current exceeds 120 amps. During wildfire/dry season, GFDs could actuate when ground current exceeds 12 amps. Algorithms using the WRF factors could change relay settings to eliminate the possibility of wildfire initiation.
In the same way that GFIs eliminate accidental electrocutions, GFDs can eliminate accidental wildfire ignition by operating quickly at low fault current levels. The tools to create new GFDs are available today. With dedication to innovation, GFDs can be tested and implemented within the existing grid in two-three years.
Immediate Steps for Wildfire Risk Reduction
Before this new technology can become a reality, steps must still be taken to reduce the risk of wildfires sparked by faulted power lines in the short term. At Prescient, the goal of our wildfire risk assessment and analysis service is exactly that. We assess clients’ power lines and right of ways using a proprietary risk matrix to determine the risk of specific power lines sparking a wildfire. Our algorithm reliably predicts if vegetation is likely to ignite when a fault occurs, and provides a specific risk score, which allows clients to understand their risk. We also help clients determine prudent improvements to existing power lines based on cost and time.
To learn more about our wildfire risk assessment methodology, join us for our free Power Lines: Wildfire Risk Assessment & Prevention webinar on May 27 at 11:00 a.m. PDT. Sign up today to guarantee your spot! Check out our wildfire blog collection to learn more about Prescient’s innovative concepts for wildfire risk reduction, and contact us with questions.
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