By Steve Willard, Avistar Inc., and Rodney Fickler, Siemens Power T&D
Ernest Sanchez, PNM’s manager of electric station services, needed a more efficient way to dispatch crews to maintain high-voltage circuit breakers throughout New Mexico. Too often, he dispatched crews on a non-routine basis to maintain the levels of sulfur hexafluoride (SF6), a di-electric gas that quenches arcs when breakers operate. Sanchez wanted the ability to monitor the SF6 levels and then use that information to schedule the breaker maintenance.
Sanchez is not alone. In the interest of both continuous maintenance improvement and environmental stewardship, many utilities are investigating better ways to monitor equipment that uses SF6. According to the U.S. Environmental Protection Agency, the electric power industry uses roughly 80 percent of all SF6 produced worldwide. Even a relatively small amount of SF6 can have an effect on the environment due to its long life span and high global warming potential (GWP). Classified as a greenhouse gas, SF6 has a GWP 23,900 times greater than CO2, along with an atmospheric life of 3,200 years. Put in different terms, one pound of SF6 has the same GWP as 11 tons of CO2, according to the EPA. SF6 emissions from electric power systems depend on the type and age of the breaker, and the handling and maintenance procedures practiced by electric utilities.
Available SF6 detection/monitoring technologies range from very high-cost, advanced leak-imaging technology to lower-cost, on-line density monitors that survey and report the measured gas content within a utility breaker. Even the lower-cost solutions, however, do not yield an adequate cost-benefit ratio to justify widespread monitoring. Their true installed costs include expensive installation and wiring. Their accuracy also suffers from the ambient-driven diurnal and annual pressure variations, which impact the baseline pressure measurements and therefore limit detection and accurate forecasting of leak rates.
Because breakers are critical to operations, diligent management ofSF6 gas content has become essential. Utilities expend significant operations and maintenance resources to monitor and maintain proper SF6 levels. In most instances, unplanned maintenance occurs when breakers alarm low levels of gas. This translates to expensive overtime or unscheduled personnel dispatch.
So Sanchez and PNM turned to Avistar Inc., PNM Resources’ research and development subsidiary, to develop a new monitor that would not only provide a cost-effective SF6 leak notification system, but also provide an easily expandable wireless communications system and network gateway.
The resulting SF6 monitor, which is now installed on PNM breakers, detects and forecasts leak rates much more accurately by accounting for the annual and diurnal pressure variations. The sensor system couples improved SF6 content-measurement algorithms with a complementary low-cost sensor, battery and wireless communication package. This combination allows utility personnel to manage and maintain the SF6 breakers more efficiently.
The SF6 monitor uses a mesh network topology. With a mesh network each wireless radio transmits, receives and retransmits signals from neighboring radios.
The algorithms compensate for diurnal pressure variations by using two different temperature measurements to derive gas temperature. The ideal gas equation gives the initial molar content of the gas, which is then compared to ensuing molar content calculations. In short, the technique involves deriving molar ratios and examining the trends of these ratios. By analyzing the trends, the system can accurately identify leaks and forecast service requirements for the breaker. Accuracy tests place the system’s leak-detection capability in the range of 0.2 kg per year.
If one radio drops out, others pick up and support the signals.
The SF6 monitor takes advantage of current technologies by incorporating a wireless radio that uses a mesh network topology. With a mesh network, each radio not only transmits but also receives and retransmits signals from its neighboring radios. Each radio supports the network dynamically: If one radio drops out, others pick up and support the signals it was conveying in a self-healing fashion. The radios typically install on any breaker type within five minutes, significantly reducing installation costs due to labor costs and complicated wiring.
The radios adapt to a range of operating parameters and power usages. Engineers tested the equipment at a variety of frequencies in a 345-kV substation environment and determined the optimal configuration for communications within a 120-foot range, which allowed them to disperse the sensors throughout the substation yard.
The engineers also geared the mesh network radio’s data transmission requirements to minimize power usage. Sensor specifications also favored highly accurate, silicon-based and micro-machined sensors and low power-consumption circuitry. The sensor’s overall low power consumption meant that a commercially available lithium-ion battery would handle the system, and design tests indicate that the battery should last three to five years. Engineers also positioned the battery in the overall package to allow easy replacement.
Concurrent with design and development of the sensor product, analysts reviewed utility O&M records and industry data to determine how much savings could be generated by better information about SF6 gas content in the utility breakers. Findings indicated that most SF6 maintenance occurred in response to low-pressure alarms. Most of these events occurred outside regular business hours, requiring overtime maintenance labor. The potential savings of eliminating unscheduled breaker maintenance, with its associated travel and overtime labor costs, combines with the sensor’s low cost to produce attractive paybacks. Especially for remote substations, with their higher travel time, the solution would yield a one-year payback-and in some cases faster.
Reliability impact analysis showed that in critical applications, avoiding the typical four-hour unscheduled breaker maintenance job could substantially impact associated wholesale market transactions. One conservative analysis estimated that a four-hour unplanned outage due to an SF6 leak on one breaker could easily cost tens of thousands in lost revenue. Additionally, in cases where an unscheduled outage on one breaker leaves a utility dependent on a single breaker at a large generation asset, the reliability impact becomes enormous if the remaining breaker fails and forces the generation asset to shut down. The bottom line showed that for a small incremental cost, the ability to accurately detect SF6 levels and forecast the molar ratio trend allowed for prevention of overtime call outs and, more importantly, would allow operators to align O&M activities on the breaker without impacting system reliability.
Though developed initially for SF6 detection, this system has much broader implications. The implementation’s wireless infrastructure entailed not only placement of sensor-based radios but also a low-cost gateway device to collect the data and place it securely on the utilities’ LAN or SCADA. The gateway communicates using DNP3 over a TCP/IP link, allowing for integration into higher-level networks. The gateway can support as many as 250 sensors of various types. This opens the possibility of additional sensors and other devices at very low incremental communication costs.
Avistar has entered into a working relationship with Siemens to develop this system. Siemens views the Avistar approach to collecting and delivering sensor data as a potential route to automating older substations without requiring expensive and invasive retrofits. Siemens is currently evaluating the monitoring system for other utilities.
PNM’s Sanchez has tested other approaches to SF6 monitoring and has looked at many proposals to advance substation systems. He’s found, as have others, that traditional approaches to substation upgrades focus on broad implementation of enhanced monitoring and control systems. These efforts have, for the most part, failed to overcome market barriers because they did not address specific issues.
The new SF6 detection technology Sanchez is using at PNM demonstrates that an incremental upgrade approach yields clearer definition of the costs and benefits for specific operational issues, which in turn increases the probability of implementation. Further, by incorporating recent technologies into these initial projects, utilities can construct easily scalable communications platforms that will accommodate future upgrades at a much lower cost.
Steve Willard joined Avistar Inc. as new product development manager in 2005. His career spans more than 22 years of experience in the energy industry in regulated and unregulated markets. Steve holds BSME and MBA degrees, both from the University of New Mexico, and is a professional engineer in the state of New Mexico.
Rodney Fickler is manager of business development for the service solution division of Siemens Transmission and Distribution. Before joining Siemens, Fickler spent 27 years with General Electric Canada and USA servicing the utility and industrial markets. Mr. Fickler holds a BSEE from Concordia University-Montreal and is a professional engineer from the province of Ontario.