BY Troy Miller, S&C Electric Company
Electric power utilities are working diligently to meet renewable electricity mandates enacted by many governments around the world. This massive undertaking demands more widespread deployment of grid-scale and distributed renewable resources.
Wind and solar energy generation, however, poses significant integration challenges. Wind and solar energy are intermittent resources that can cause serious power grid reliability and stability issues while impacting electric service quality for businesses and consumers. Our interconnected, energy-dependent lifestyles require reliable energy sources, so utilities must find an effective way to ensure power quality and reliability while fulfilling renewable electricity mandates.
THE CHALLENGE: UNPREDICTABLE OUTPUT, TWO-WAY POWER FLOWS
When intermittent renewable energy resources compose a significant portion of overall generation, utilities begin to see more distribution system problems that impact grid stability. Variable output from solar and wind generation produces local voltage swings that can impact power quality for customers, including voltage excursions beyond accepted power quality standards. This output variation also might contribute to distribution feeder-level voltage swings, which can occur in seconds. Traditional distribution voltage regulation equipment typically is not suited to respond to such rapid, highly variable fluctuations.
Variation in the output of renewable energy resources also can prove problematic if it is not sufficient to meet demand. Particularly when it comes to distributed renewable energy resources on the customer side of the meter, utilities do not always have visibility into the output or availability of these resources. As a result, utilities need to ensure that additional spinning reserve (and peaking plants) is available to meet unexpected demand caused by a sudden drop in distributed generation output. This, in turn, increases costs.
Bidirectional power flows can impact grid stability and reliability further. Electric power systems were built to handle one-way power flow from centralized generators down transmission and distribution lines to loads. Distributed renewable energy sources, however, are creating power flows in two directions that traditional power systems were not built to handle, which makes the power grid more vulnerable to disruptions and extended outages.
THE SOLUTION: SMART GRID TECHNOLOGY
To manage the impact of renewable energy resources, one must look beyond 20th-century technology. Smart grid solutions offer the best, most realistic path to address the renewable integration challenge.
Reactive compensation solutions that provide rapid response, such as dynamic static compensators (DSTATCOMs), have proven effective at mitigating some of the impact of fluctuating renewable energy output on power systems. Such systems are typically applied at wind and solar plants larger than 10-15 MW. Transmission system operators in many areas of the world have developed strict interconnection requirements for renewable energy plants to protect transmission systems against such output variability. DSTATCOMs are effective at providing VAR support, but real power is needed to smooth electricity generated by photovoltaic (PV) panels and to manage PV ramping.
New volt/VAR optimization (VVO) technology also can help address the impact of renewable energy resources on power grids. The latest solutions can provide a comprehensive solution by optimizing and adjusting voltage profiles for all distribution feeders served by a substation, thus helping address the voltage swings that can result from varying renewable energy output. VVO solutions also can use data from advanced meters to provide better monitoring of voltage levels at the customer level to ensure levels remain within the appropriate band to provide desired power quality levels.
Advanced metering infrastructure and associated home energy management solutions also might play roles in aligning intermittent renewable energy supply with demand in real time. The idea is that advanced meters, when paired with the appropriate in-home technology, can enable consumers to use energy only when renewable energy supplies are generated and thus can match demand with renewable energy supplies.
This approach, however, remains a theory. Such capabilities aren’t available at this stage. Even if consumers could use this technology, it’s unclear whether they would be willing to shift their electricity usage, often inconveniently, unless they would save a significant amount on their electricity bills. The savings would have to make up for the inconvenience. Moreover, utilities still would need to maintain backup energy supplies to continually balance electricity supplies with demand in real time because such balance is crucial to ensure grid stability.
ENERGY STORAGE OFFERS KEY PIECE OF PUZZLE
One emerging technology, distributed energy storage, is demonstrating it can help significantly with renewable integration.
Distributed energy storage systems, particularly when close to renewable resources, are well-suited to address the key challenges associated with renewable energy supplies. Variability in output from wind and solar energy generation can create local and feeder-level voltage swings that occur rapidly, but distributed energy storage can provide fast response to help firm voltage levels and effectively fill gaps created by large voltage swings and fluctuations. Colocating storage and renewable energy resources gives utilities a particularly effective way to manage unwanted voltage changes and allows them to maintain grid stability while meeting power quality requirements.
Distributed energy storage systems also provide a dispatchable energy resource for utilities and gives them more control to ensure renewable energy supplies are available to meet demand. Renewable energy generated when demand is low then is stored to meet later demand. Distributed energy storage systems provide greater flexibility and faster response than using conventional generating plants, compensating for renewable generation output shortfalls.
Energy storage can be economically deployed in smaller capacity sizes, too, which can help avoid the need to invest in establishing a new plant. Distributed energy storage also supports other grid functions such as peak shaving, which can provide further savings to utilities by reducing the need to maintain conventional generation and maintaining grid capacity to meet peak demand.
Today, energy storage systems are available in smaller chunks than ever. One of the newest and most innovative storage systems is community energy storage (CES), which integrates lithium-ion battery-based storage at the edge of distribution system networks. CES systems are small, pad-mounted products that can be installed strategically along residential feeders. They are ideally suited to serve residential neighborhoods and small commercial buildings. One 25-kVA CES unit can supply one to three hours of battery storage for multiple residential or light commercial loads.
CES units easily can be colocated with renewable energy resources, including grid-tied rooftop PV panels and small wind turbines. With the growing adoption of small-scale renewable resources and plug-in electric vehicles, CES units can offer utilities a more effective way to control voltage on local feeders to sustain service quality. Fleets of CES units also can be managed and dispatched by central utility systems in such a way that they can function as a virtual power plant. Because they are close to loads, CES units can serve as backup power supply in the event of a disruption on the power grid or at a central generating plant.
CES systems offer additional benefits during outages. When energy storage is colocated with PV panels, it can prevent reverse current flow caused by excess generation during low demand. Storage reliably mitigates reverse current flow by quickly consuming real power.
OVERCOMING ROADBLOCKS TO TECHNOLOGY ADOPTION
Although smart grid technologies offer practical solutions for integrating renewable energy supplies at scale, the current U.S. regulatory environment limits adoption and investment in such systems. These technologies are new and lack sufficient regulatory history, so regulators are often uncertain about how to value the benefits of the systems. The lack of understanding about the solutions and benefits they offer means that the technologies—energy storage, in particular—are undervalued, and allowed returns on investment do not align with the benefits provided.
In the case of energy storage, utilities also face uncertainty about how regulators will treat such investments. For example, some states treat energy storage as a generation resource, which does not acknowledge the role storage can play in maintaining a stable power grid. The lack of clarity about how these investments will be treated and how costs will be recovered will continue to hold back needed spending on intelligent grid infrastructure. These problems must be addressed to drive greater investment in the systems that are critical to successfully integrate renewable energy sources to the grid.
PATH TO GREATER RENEWABLE ENERGY USE
Many technologies needed to integrate renewable energy to the grid already exist. These smart grid solutions can help maximize use of renewable energy resources while ensuring a reliable and stable electricity supply. Utilities continue to explore these new technologies, which will continue to improve. We must ensure our regulatory structures and processes adapt as technologies advance, too. A better regulatory environment that can keep up with a rapidly changing industry can help utilities meet and embrace the renewable energy challenge.
Troy Miller is a business development and marketing manager in the Power Quality Products Division at S&C Electric Company. He has more than 21 years of experience in the power engineering industry. Miller has a bachelor’s degree in electrical engineering from the Milwaukee School of Engineering.
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