Strong Grid Architecture is Vital to Expand Renewable Resources
By Carl Laverghetta, Senior Energy Advisor
According to the Rocky Mountain Institute data collected in 2019, 72% of global net additions of generating capacity came from renewable energy. The ‘Institute’ has produced an informative report entitled, Reimagining Grid Resilience. The overall thrust of the report is that variable renewable energy resources and the growing popularity of “distributed energy resources,” are impacting the current architecture of the transmission grid. https://rmi.org/wp-content/uploads/2020/07/reimagining_grid_resilience.pdf
The transformation occurring across the world’s electrical systems represents one of the greatest technological challenges industrialized societies have undertaken. Reconfiguring a grid designed to carry power one way from reliable generation sources managed by few agents to a system increasingly laden with less reliable wind and solar energy while involving millions more participants using advanced technologies, will introduce a high degree of uncertainty and variability into the future grid.The ongoing transition from an electrical system substantially dependent on fossil fuels to one substantially powered by intermittent renewable energy represents not only technological transformation, but also a change from traditional concepts of grid reliability and stability. RTOs, ISOs, and electrical engineers focused on the existing transmission grid and grid transformation, are applying the term “operational risk management” in this new paradigm, which will require a redefinition of reliability and the methods to assess system conditions before they occur. Specifically, the increased uncertainty and complexity in the grid.
The transmission grid is not an electric circuit; it is a network of structures, highly coupled and replete with constraints. Changes to grids must account for impact across the set of structures, not just in limited siloes. Grids are not just complex, they must be viewed as Ultra-Large-Scale Systems, with the attendant characteristics and implications. What is, sometimes, dismissed is that everything in the current structure of the electricity grid is designed to operate at a certain frequency, and really bad things can happen when there is a deviation from that frequency. As the energy system ‘decarbonizes,’ an increasing amount of our electricity will be generated by intermittent renewable sources such as wind and solar. The importance of energy storage in a renewables-intensive energy system is often talked about. What is discussed less often is the need for frequency stability in the alternating current (AC) supplied. Maintaining a consistent frequency is critical for the safe and reliable operation of the infrastructure that supplies electricity as well as for the equipment attached to it.
Our current national grids rely heavily on the inertia of the large rotating turbines and generators in conventional power stations to provide this frequency stability. As these sources are replaced by renewables that lack this rotating inertia, the inertia created from conventional fossil fuels, nuclear, and hydropower generators that was abundant and thus taken for granted in the planning and operations of the grid system, will be significantly decreased with increasing penetrations of inverter-based resources e.g., wind, solar photovoltaics, and battery storage. These resources do not inherently provide inertia, so important questions have emerged about the need for inertia and its role in the future grid. Doubtless, alternative methods of maintaining frequency stability will be required. Changing the voltage within the grid is particularly important for efficient transmission and distribution of electrical energy. https://www.nrel.gov/docs/fy20osti/73856.pdf
The most important reason for regulating the frequency of the AC supply is that if there are different frequencies within the grid, it will damage equipment. Frequency regulation is, therefore, not so much about achieving an accurate frequency as it is about synchronizing all the equipment so that it operates smoothly together. This is of great importance to the generators themselves, but industrial motors and many other pieces of equipment are also affected. As wind, solar and other distributed and renewable sources are beginning to replace large, centralized power stations, it is becoming more difficult to achieve frequency stability. There are two major reasons for this. First, there is now a far larger number of small generators, many of which are not directly operated by the grid. This makes controlling them far more complex. Second, most of these small generators provide no inertia, meaning that a much more rapid control response is required to effectively maintain frequency stability. Although generator designs typically produce AC, it would be just as easy for generators to be designed to produce direct current (DC). AC is useful for power transmission, but in many other areas it is simply an inconvenience, especially with modern electronics and semiconductor devices. Virtually all electronic devices require DC, as does LED lighting and the small electric motors used in household appliances. This means that all of these devices require not only a transformer to step down from mains voltage; they also need an AC-to-DC converter. This conversion is carried out by a ‘rectifier’. Rectifiers let you convert from alternating current (AC) to direct current (DC). AC is current that switches between flowing backwards and forwards at regular intervals while DC flows in a single direction. They generally rely on a bridge rectifier or a rectifier diode — these diodes take advantage of the positive and negative (p-n) junction to convert AC to DC as a sort of electric ‘switch’ that lets current flow in either the forward or reverse direction based on the p-n junction direction. https://www.engineering.com/story/grid-frequency-stability-and-renewable-power
Although wind turbines have physical inertia, they are not coupled to the grid frequency in the same way as other power generators and therefore do not provide inertia for frequency stability. They do provide some frequency control capability since wind output can be regulated down or held back. Solar panels can be rapidly switched on and off to provide good frequency regulation. However, because solar panels are dispersed very widely and are not directly operated by the grid operator, achieving this kind of control is especially challenging.
Renewables-intensive energy systems will require different types of energy storage that are able to buffer supply and demand over different time periods. These can broadly be categorized as frequency regulation, daily or weekly fluctuations, and seasonal variation. There is, however, significant synthesis between these provisions. For example, battery storage is likely to play a significant role in buffering daily or weekly fluctuations but, importantly, it can also provide frequency stability. https://www.engineering.com/story/grid-frequency-stability-and-renewable-power
Grid architecture was developed by the Pacific Northwest National Laboratory (PNNL) for the Department of Energy as a framework the industry can use to solve its most pressing problem of transitioning to a cleaner, cheaper, and more sustainable grid. PNNL defines the concept of “grid resilience” as the ability to avoid or withstand grid stress events without suffering operational compromise or to adapt to and compensate for the resultant strains to minimize compromise via graceful degradation. In other words, resilience is the ability to avoid or limit operational degradation to the grid or electricity in the event of an unexpected incident. More specifically, 1) absorb stresses and maintain function in the face of external stresses imposed upon it by climate change and 2) adapt, reorganize, and evolve into more desirable configurations that improve the sustainability of the system, leaving it better prepared for future climate change impacts.” 3) Resilience of the grid is, therefore, the ability of the grid to absorb stresses and maintain function in the face of external stresses imposed upon it and the ability to evolve to more desirable configurations in the face of changing threats. https://www.pnnl.gov/grid-architecture
Grid architecture is the specialization of system architecture for electric power grids. As such, it includes not just information systems, but also industry, regulatory, and market structure; electric system structure and grid control framework; communications networks; data management structure; and many elements that exist outside the utility but that interact with the grid, such as buildings, merchant distributed energy resources (DER), and microgrids.
System architecture in general and grid architecture specifically make use of a set of architectural principles, or rules, to guide architecture development and aid in evaluation. Where possible, system architecture also makes use of rigorous bases for architectural structure, thus minimizing the “artistic” aspects of the architecture.
For grid architecture, the rigor issue is crucial, because managing and changing the grid necessarily cuts across multiple disciplines such as control engineering, market operations, and industry structure. Grid architecture starts (as any architecture does) with the needs of the end users of the grid. These are shaped by public policy and that combination leads to a set of desired grid qualities. The architecture development process flows from this point.
The inherent dangers of not using grid architecture are:
- Increasing risk of creating unintended consequences detrimental to resilient operations, such as those emerging at the interaction of certain grid functions previously considered in isolation;
- Increasing risk of massive stranded investments in infrastructure, such as have already happened;
- Blockage of energy innovation and resultant value streams associated with new products and services; and
- The mismatch of policy directives and operational realities associated with the grid, which have emerged in the context of certain early market approaches. https://www.pnnl.gov/grid-architecture
Hand in glove with grid architecture and ability of a transmission grid to accept “variable energy resources” (VER) is the level of reliability and the actual resiliency of a transmission grid. For decades, reliability has been the watchword for electric utilities, but now there’s a focus on a related concept: Resilience. It has gained notice as planners began thinking about increased natural disasters brought on by climate change, man-made interference due to malicious cyberattacks, and the instability brought about by adding large quantities of renewable energy. Resilience has become a legitimate field of study involving industry, academia, and government labs, complete with experts in the field. According to Mike Bryson, senior vice president for operations at PJM Interconnection, “The concept of resilience involves protecting against multiple scenarios that are beyond reliability criteria.” These are very low frequency events that have a high impact on society. Whereas maintaining reliability involves dealing with discrete failures as they occur, (think of the downed power lines after a storm); Bryson sees resilience as the means to take on multiple, simultaneous challenges to the grid. https://insidelines.pjm.com/reliability-and-resilience-different-concepts-common-goals/
The hope by those who champion DERs, such as smart microgrids with sensors embedded throughout the system, is that these localized generation resources might be more resistant to failure and easier to bring back online than large, multi-state electric grids. But the emerging smart grid, together with distributed renewable energy such as rooftop solar, presents a new set of challenges to resilience. Mike Bryson continues saying, “The smart grid involves more distributed energy down to the home level. That kind of penetration adds a level of vulnerability to a cyber threat. We certainly have to pay attention to that as the grid gets smarter.” The increasing penetration in the PJM region of wind and solar and other types of DERs, are attaching an even greater importance to reliability issues, including the combination of distributed energy storage and distributed solar, which are reversing the power flow, allowing customers and communities to generate most of their energy at home or nearby.
The grid of the future will be more distributed than centralized as it will involve millions of new participants affecting power supply and demand. And it will convey more and more electricity from solar and wind energy sources, which are inherently intermittent and difficult to predict. This is the challenge for America’s transmission grid.
Finally, maintaining grid stability, reliable energy supplies and affordability will require solutions in technology, public policy, markets, data communications and public understanding. It is increasingly clear the electricity grid will be unable to meet the demands of an ever-growing digital society or the expansion of renewable energy without dramatic change. The inexorable push by some special interest groups demanding change sooner rather than later, has a ‘cart before the horse’ look to it. Attempting to load the current vertically integrated transmission grid with VERs will have real and problematic consequences. Enlightened and prudent approaches must be the rule, especially given the nascent stage of many of the attributes discussed including control systems, market designs, architectures, related products, and the importance of grid security.
The following link is a quite high level Lawrence Berkeley National Laboratory report on “Solar-to-Grid Trends in System Impacts, Reliability, and Market Value in the United States.” It can be a worthwhile resource. https://emp.lbl.gov/sites/default/files/solar-to-grid_technical_report.pdf