Many of the electric transmission grids in service around the world were designed and built more than a half century ago. While changes and upgrades have obviously been made over the years, the systems were generally developed with very different resources in mind than what is regularly coming online today. However, most of the problems created by renewables and distributed energy resources have workable solutions that can lead to a reliably functioning modern power grid.
When the first electric power generation, transmission, and distribution systems were created in the 1870s and 1880s, electricity was generated locally in small power plants and distributed via direct current (DC) circuits. Toward the end of the 19th century, the industry transitioned. The construction of larger alternating current (AC) power stations became the preferred solution for many national electric grids. That central power station design remains the norm in most parts of the world today.
However, as renewable energy continues to grow, needs are changing. Many nuclear and coal-fired power stations have been retired, causing a few challenges for grid operators.
Integrating Renewable Energy
Peter Lundberg, global product manager HVDC with Hitachi ABB Power Grids, said many grids around the world need refurbishment, not only due to age, but also to improve reliability and increase functionality so they can accommodate new elements that have added complexity to the transmission grid. “The big thing, and the big challenge for us, is the integration of renewable power—solar and wind—and the shift to less conventional generation,” Lundberg told POWER.
One grid problem created by renewables is that solar panels and wind turbines are often located far from where retired coal and nuclear plants were sited. Conventional power plants were generally fairly close to load centers; whereas, renewable generation could be spread out or built in wide-open spaces far from urban areas.
“You might get new bottlenecks in the grid that cause strains. You have new power flows, and renewable generation is a lot more fluctuating than the conventional,” said Lundberg. “So, you need a much smarter and faster controller for your transmission grid,” he said, adding that Hitachi ABB Power Grids has good solutions to support a smarter and faster grid.
However, not all grid-related problems are technical; there are also political and regulatory challenges that must be addressed. “To renovate the aging grid and make it more agile, we still lack some of the political and regulatory frames that come with changing market pictures,” Lundberg said. “We are continuously working with the regulators and the political side to get clarity on how our customers should get paid with the new schemes,” he said.
Lundberg pointed to Germany’s Energiewende (energy transition) initiative as one example of success. The government has said Energiewende is “a fundamental restructuring and realignment of German energy policy.” The country’s goal is to phase out nuclear and fossil power, and switch entirely to renewable energy. While it hasn’t been a trouble-free process, Lundberg said Germany’s progressive policies have resulted in positive changes.
Jonathan Marmillo, co-founder and vice president of Product Management with LineVision, an overhead-line monitoring company, agreed that aging power lines are a concern. He noted the average transmission line in the U.S. is more than 40 years old and has been exposed to a lot of stress over time. LineVision provides a tool for utilities to better understand the stress that their grid is under, while helping them optimize the performance of assets.
The tool has two sensors: an electromagnetic field sensor and a light detection and ranging (LiDAR) sensor. It can be mounted on transmission or distribution towers (Figure 1) and is voltage agnostic. The system measures critical line properties, such as motion, current, MWs, Vars, power factor, and more. With the information, advanced analytics determine the conductor temperature, tension, and full range of motion—including sag and blowout—all without touching the lines.
1. LineVision can help utilities better understand overhead line health by continuously monitoring the motion and clearance of conductors, as well as temperatures and other parameters that affect dynamic load ratings. Courtesy: LineVision
“This is a fundamental game-changer in the industry, because taking an outage or putting equipment onto a conductor while it’s live is sometimes prohibited by utilities for operational, safety, or risk reasons, and it can be very challenging to schedule an outage, or they have to use ‘hot sticks’ and bucket trucks or helicopters to mount equipment, which is extremely expensive and time-consuming. So, we take a number of the barriers to the adoption of this type of technology out of the equation by mounting sensors directly to the towers, structures, or poles themselves,” Marmillo said. “And, we’re gathering advanced analytics around this data, and sending it to the utilities, along with helping them understand the data, so they can have this actionable information to make decisions.”
LineVision systems have been deployed in North America, Europe, and Oceania. The company was also recently selected to demonstrate its technology as part of the Electric Power Research Institute’s (EPRI’s) Incubatenergy Labs Challenge. That project is being hosted by the Tennessee Valley Authority (TVA). “It’s meant to be a quick-impact, short-term project to demonstrate not just to TVA, but really the broader EPRI community and the industry, about the impact that these novel datasets can have for operational decisions for utilities,” said Marmillo. EPRI said in a press release that project facilitators will present findings during the Incubatenergy Labs Challenge Demonstration Day, which is scheduled to be hosted by Ameren Corp. in St. Louis, Missouri, on Oct. 14, 2020.
Dynamic Line Ratings
Another benefit of LineVision’s monitoring system is that it allows line ratings to be adjusted dynamically. For example, all power lines have static and/or seasonal current/amperage limits to prevent overloading and damaging lines. However, the actual load that a line can safely carry depends greatly on ambient conditions at any given time.
“It’s been shown that an increase of wind speed perpendicular to the line of three feet per second would increase the capacity of that line by 44%,” Marmillo said, referencing a Department of Energy study. “So, this is going to be able to help reduce congestion on those areas that have thermal limitations, and it can allow us to help incorporate additional renewable energy onto the grid.”
Marmillo noted that wind turbines are often located in sparsely populated high-wind areas away from load centers. That means power must be transmitted long distances sometimes through congested grid areas. However, if there’s wind spinning the turbines, that same wind is likely to be cooling conductors too, which boosts their effective capacity. So, by monitoring the line temperatures closely, capacities can be increased, preventing curtailments.
Dynamic line ratings can also benefit fossil and nuclear generators by freeing up congestion on the grid. There are at least a few examples of summertime constraints in the Northeastern U.S. that forced less-advantageous resources to be dispatched, but if dynamic load ratings had been in effect, more-favorable generation could have been deployed.
Marmillo pointed to a study LineVision did with the Southwest Power Pool (SPP), a regional transmission operator in the central U.S., to stress the point. “We showed that during times when [SPP was] sending the market signal of congestion on a particular transmission line, our sensor actually showed that there was extra capacity above the static limit, and that effectively, the congestion was fictitious. It didn’t need to occur, if they had been using dynamic line ratings provided by our system,” he said.
Reactive Power, Short-Circuit Current, and Frequency Stability
Bernd Niemann, business development manager for FACTS (flexible AC transmission systems) at Siemens Energy, explained a few problems he has witnessed due to the shift from fossil and nuclear generation to renewables. Niemann said there are three major effects: Reactive power, short-circuit current, and frequency stability are all reduced when large conventional rotating generators are removed from the grid.
“Lack of reactive power means we have voltage fluctuations,” Niemann told POWER. “We have to insert additional reactive power to the grid, which was usually done by conventional power plants and some static compensating devices across the grid. Now it’s changing. We have a new topology and we need flexible and powerful reactive power compensation solutions.”
Additionally, the disconnection of conventional power plants has reduced short-circuit current levels in the system. “We have in the past had these big rotating generators, and in case of short circuits somewhere in the net, which can happen—it’s not wanted, but it happens—the task is to disconnect lines or parts from the grid, which are affected by the short circuit,” Niemann explained. “Therefore, we have devices that recognize and localize short circuits. And of course, you need to have a certain current flowing into the short circuit, otherwise you cannot distinguish between a short-circuit current or regular load current.”
Niemann said wind farms and inverters cannot really cover or ensure enough short-circuit current because the distance between the power source and the load may be quite long. Furthermore, inverters are often restricted to a maximum current, which means they limit current, block, or shut down when exposed to an overcurrent condition; therefore, they do not contribute much to short circuits. If short-circuit currents get too low because rotating masses or rotating generators have been retired, then other measures must be taken to compensate. One solution is adding synchronous condensers (Figure 2) to the grid to provide a certain amount of short-circuit current.
2. The displacement of existing power plants by renewables results in reduced system strength. A solution to cover the missing short-circuit current and inertia is a synchronous condenser, shown here. Courtesy: Siemens Energy
Another consequence resulting from the loss of rotating mass in the system is frequency instability. Electric grids are generally 50-Hz or 60-Hz systems, depending on a country’s design. In either case, grid operators must take measures to balance the system and maintain nominal frequency, and it must happen very quickly to avoid blackouts. When a large load rejection or addition occurs on the grid, and there is not enough rotating mass in the system to provide frequency stability, there are problems.
“There are also solutions covered by flexible AC transmission systems in the portfolio of Siemens that provide inertia to the system,” Niemann said. “When there is a frequency event, it emulates inertia. For example, it could be a synchronous condenser with a flywheel connected to have more mass rotating behind the shaft, or it could be solved with specially designed power electronic devices with super capacitors, a STATCOM [static synchronous compensator] with additional grid supporting performance.” ■
—Aaron Larson is POWER’s executive editor.