Minnesota Power’s solution to optimize grid stability

On a windswept hill overlooking one of the world's largest freshwater lakes, a whitewashed power substation serves as the operational core of a sophisticated electric grid connecting Minnesota's North Shore to a sprawling mining district to the west. Owned and operated by Minnesota Power, the state's second-largest investor-owned utility, the North Shore Switching Station on a hill near Lake Superior helps stabilize the regional electric grid to allow for less dependence on coal-fired generation, making way for more renewable energy. Built a few years ago, the substation northeast of Duluth hosts several technologies, including the SVC PLUS (STATCOM), developed by Siemens Energy.

The STATCOM technology irons out the dips and surges that come when utilities face rapid, variable power from wind, solar, and other intermittent generation. The static synchronous compensator (STATCOM) in modular multilevel converter (MMC) topology uses fast-acting semiconductor elements called "IGBTs" to control reactive power through creating a sinusoidal voltage out of many small DC voltages. Utilities use the total SVC PLUS approach to regulate voltage quickly, manage reactive power and utilize assets more efficiently.

Siemens Energy's solutions seem to have fit well with Minnesota Power's needs as it enters a new era of greater dependence on renewable energy. A subsidiary of ALLETE, Inc., Minnesota Power has embarked upon a dramatic transformation by retiring coal plants and switching to renewable energy to decarbonize its grid. Called "Energy Forward," the utility wants to reach 100% carbon-free energy by 2050 and has already reached 50% renewable energy this year, the first power company in Minnesota to hit that milestone.

Minnesota Power's progress comes even as it faces a unique electricity load unlike almost any other utility in the country. More than 60% of Minnesota Power's electricity serves just a handful of taconite mines and paper mills in northern and central Minnesota, with the rest consumed by 145,000 residential, commercial, and light industrial customers. In contrast, Xcel Energy, Minnesota's largest investor-owned utility, serves several million ratepayers in Minnesota and Wisconsin.

Minnesota Power now has just two coal units as it relies more and more on electricity from 870 megawatts (MW) of wind energy, more than 370 MW of hydro, and 11 MW of solar energy. Plans call for adding more clean energy to the mix. Currently, half the utility's power comes from clean energy. With power coming in from multiple sources, some of which are located far away from the main load centers, the utility required more sophisticated equipment and continuing operational digitization.

Minnesota Power's supervising engineer of transmission and distribution planning, Christian Winter, described the North Shore Loop's history. The loop runs up the North Shore for around 70 miles and another 70 miles to the east end of the Mesabi Iron Range, creating a transmission loop. From the 1950s until 2015, the loop connected local coal-fired plants with customers throughout that swath of territory. In 2005 coal still generated about 95% of the utility's power.

As the utility retired or idled those plants while adding new generation sources outside the North Shore Loop, a technology that could stabilize electricity flow in the loop became necessary. From 2015 to 2019, all baseload coal-fired generators in the North Shore Loop were idled or retired, leaving the area with no local generation running. "The analogy that I like to use is that the generators are like the load-bearing walls of the system," Winter said. "If you renovate your homes, and you want that open concept living space, maybe you remove a wall, and if it is a load-bearing wall, you have to do something to replace the structural support it was providing to your house. You have to probably put up a beam to span that space that you're creating."

Baseload electric plants once provided stability to the transmission system, but that support is becoming less available as new, cheaper variable clean energy sources take over as utilities strive to decarbonize the grid. The SVC PLUS system does not generate power but instead modulates and manages electricity that flows from distant sources hundreds of miles away. Instead of coal plants generating electricity just down the road from customers, the utility now receives wind power from North Dakota, hydro from Manitoba Hydro, and various smaller hydro operations in the area. A small amount of solar also serves the utility's power supply needs. The SVC PLUS system ensures stable, predictable operation of the grid in the North Shore Loop regardless of where the power is coming from.

"The real power, the power that people use in the area now, is being delivered from outside of that system because it's not being generated (in the North Shore Loop) anymore," Winter said. The SVC PLUS system helps the utility manage transmission support. With generation located much farther away than in the past, the new technology stabilizes loads, "holding the network up" rather than generating electricity for the use of customers in the loop. Other stations located between generation, transmission and distribution manage that aspect of the utility's electric grid. The SVC PLUS technology controls for optimal robust and predictable voltage, he said, along with providing redundancy "so we can withstand the loss of one or two (transmission or generation) elements unexpectedly," Winter said.

Siemens Energy's business development manager Bernd Niemann said the SVC PLUS's ability to stabilize electrical grids becomes even more critical as Minnesota Power adds more clean energy and storage. Depending on the situation SVC PLUS adds capacitive or inductive power to stabilize voltage where wind and other clean energy sources might cause volatile power infeed and voltage fluctuations. Offering an example, Niemann said the Minnesota Power system generally maintains a 115-kilovolt (kV) level in the transmission grid. If the voltage drops down, Siemens Energy SVC PLUS increases the voltage up to the target level, so the system can function smoothly and any potential blackouts or other issues can be avoided, he said.

Only a short list of companies exist that offer the kind of technology needed by Minnesota Power. Winter and his colleagues began looking at different vendors in 2016; the same year the company announced an upcoming slate of plant closures. Winter and others, however, had been considering the potential transmission impacts of massive changes to the company’s aging coal fleet as early as 2012. "We went through a process of doing a lot of study work on our own, vetting a lot of different potential solutions to this issue, and defining what our needs were so that we could put that into a specification and then issue that out," he said.

The company selected a consulting engineer, Winnipeg-based RBJ Engineering, to assist with evaluating the vendors and the eventual design of the system. The request for proposals went out in 2017, and about six months later, the utility chose Siemens Energy to provide the solution. As Minnesota Power began installing the technology, Winter and his colleagues studied scenarios with the help of RBJ Engineering and worked with Siemens Energy on a long list of technical issues they wanted to resolve before going live with the SVC PLUS solution.

Their work on navigating the challenges resulted in a technical paper for CIGRE, an international power systems organization. "This technology has turned out to be ideal for the application and ideally suited for helping us to navigate the challenges along the way," Winter said. "But that doesn't mean that we didn't encounter unexpected things along the way that we had to deal with that were very deeply technical and difficult".

The main challenge derived from potential system events that would push the SVC PLUS to the limits of its technical capabilities and could lead to a loss of control over the system. Using simulations and performance studies, Siemens Energy modified the software to handle different events involving transmission outages and other factors. "We haven't experienced those types of things yet, thankfully, but we know the system is designed to withstand them if they come," Winter said.

The Siemens Energy SVC PLUS helps utilities capturing electricity from distant renewable sources because of its ability to tune voltage while maintaining grid integrity, according to Winter. The data gathered allows for the granular study of grid activity such as "bumps" that may cause tiny, temporary outages due to lightning or other weather events or human-made interruptions. "We really have a lot more insight into what's happening on our system, and that's just going to help with our ability to model it and simulate it and design it and ultimately operate it reliably," he said. "A lot of things happen on the system so very quickly, in such fractions of a second that unless you have a recorder out there, that's able to record that type of (disruption) you don't get to see it or analyze it afterward".

Siemens Energy sees Minnesota Power's project as a harbinger. Utilities have begun to transition to clean energy sources to combat the global climate crisis. Driving Minnesota Power's ability to decarbonize will be technology capable of managing variability inherent in clean energy. For Siemens Energy and Niemann, the North Shore Switching Station presents a learning opportunity. "It's really a laboratory environment for us," Niemann said.

August 6, 2021

Combined picture and video credits: Ackermann & Gruber