Power System Flexibility
By Clark W. Gellings, Arshad Mansoor, and Ron Schoff
Increasingly, power systems will incorporate distributed “smart technologies,” flexible communication, a wide variety of digital devices on the power systems, and distributed command and control systems. In this new world, flexibility will be key—flexibility of generation resources, flexibility of the transmission and distribution system, flexibility at the consumer level, and flexibility of the market to incentivize the power system to account for variability.
In the last decade there has been a nine-fold increase in the global installed capacity of variable generation from wind and solar, which now comprise approximately 7 percent of total world capacity. In some countries such as Germany and Spain, variable generation comprises nearly 30 percent of installed capacity. At the same time, consumers are reaping the benefits of a connected lifestyle through end use technologies such as electric vehicles, consumer electronics and home appliances.
The grid is evolving to keep pace with those changes, as more intelligent electronic devices including sensors, data and communications technologies are deployed. But the changes on both the demand and supply sides represent a challenge to how the grid is managed. The industry is having to rethink how to match load with bulk power generation, and how best to monitor and possibly control both bulk and local variable generation and storage resources whose performance and availability is inherently difficult to forecast.
Increasingly, power systems will incorporate distributed “smart technologies,” flexible communication, a wide variety of digital devices, and distributed command and control systems. The integrated communications infrastructure will require hardening for cyber security to ensure reliable long-term operations of millions of nodes.
In short, the power system of tomorrow will look very different from today’s. In this new world, flexibility will be key—flexibility of generation resources, flexibility of the transmission and distribution system, flexibility at the consumer level, and flexibility of the market to incentivize the power system to account for variability.
What follows is an overview of some of the technology innovations in transmission and distribution and in energy utilization that could make the power system more flexible, with some emphasis on work done here at the Electric Power Research Institute (EPRI). It, of course, only scratches the surface; many more technology innovations are under development that could offer greater flexibility, and more will emerge as the industry evolves.
With the proliferation of new generation storage, and end-use technologies, the architecture of the power system will need to adapt. One concept that could support adaptation is EPRI’s service-marked ElectriNet, a combination of local energy networks that includes interconnected distributed end use, local generation, storage, and utility technologies at the building, community, or distribution level.
ElectriNet offers the potential for greatly enhanced flexibility through improvements in energy delivery and efficiency, power quality, reliability, and cost of operation for very concentrated and localized loads. To fully realize these benefits, however, it will be necessary to coordinate the control of complex local energy networks that may comprise several different kinds of local generation and storage systems, which may also be geographically dispersed.
The present grid operating system was not designed to offer that coordination and control. A new grid operating system, which we at EPRI refer to as Grid Operating System 3.0, could allow sufficient flexibility to facilitate high levels of security, quality, reliability, and availability of electric power; improve economic productivity and quality of life; and minimize environmental impact while maximizing safety. This new grid operating system will monitor, protect and automatically optimize the operation of its interconnected elements—from the central and distributed generator through the high-voltage network and distribution system, to industrial users and building automation systems, to energy storage installations, and to end-use consumers including their thermostats, electric vehicles, appliances, and other household devices.
Grid 3.0 must manage a two-way flow of electricity and information to create an automated, widely distributed energy delivery network. It must also incorporate the benefits of distributed computing and communications into the grid to deliver real-time information and to enable the near-instantaneous balance of supply and demand at the device level.
Grid 3.0 will enable additional innovations such as the use of dynamic protection. This concept builds on the laws of physics to develop protection approaches that do not depend on system studies. This new approach automatically adjusts protection to the situation through the use of high-speed data acquisition and basic power system analytics, eliminating one of the major causes of power system failures.
With the advent of photovoltaic (PV) and other distributed generation resources, consumers may now be served by a combination of grid-supplied energy services and power generated on-site. Availability of local generation and storage in combination with sophisticated end use devices such as plug-in electric vehicles offers inherent flexibility for consumers.
Plug-in electric vehicles, both all-electric and hybrid, could be used to supply energy to a home during an outage. Hybrid electric vehicles also could operate as a gasoline-fueled generator to provide additional standby power. Automakers are interested in the concept, but the technologies require further development. Nissan Motor Co., Ltd. recently unveiled a system that enables the Nissan Leaf to connect with a residential distribution panel to supply residences with electricity from its lithium-ion batteries. The batteries can provide up to 24 kWh of electricity, sufficient to power a household’s critical needs for up to two days.
Increasingly, consumers are installing rooftop PV systems to augment grid-supplied electricity. Usually limited by roof area and sized to meet an economically viable portion of the building’s electrical needs, these systems cannot supply 100 percent of a residence’s typical demand, nor do the systems, as currently configured, allow for operation as independent microgrids to supply part of a residence’s needs. EPRI assessments have identified inverter and control designs that could convert PV systems into self-sufficiency technologies, but few inverter manufacturers have stepped forward to serve this need.
The existing controls associated with PV arrays are not sufficiently functional to match the electrical demand of a residence without grid supply or local storage. Companies are developing residential circuit breaker panels that can control individual circuits and appliances. Control devices could be developed to weave these breaker panels into the PV system, so that when grid power is lost, load is automatically curtailed to balance supply and load for the residential microgrid. These systems also could manage the “ramps” that occur as the sun rises and sets, or as clouds block sunlight.
Solar and wind energy eventually will get a boost from evolving energy storage technologies, which help make variable generation dispatchable and can provide a temporary solution to overcome regional and local capacity shortages and localized transmission and distribution congestion. Advances in technology and expansion in production capacity have brought some storage technologies to the verge of cost-effectiveness, but their overall economics are still marginal. A broader range of benefits must be realized for these technologies to become cost-effective. The applications that contribute to the value of storage solutions have various requirements—meeting certain ramp rates, storage capacity, round-trip efficiency, and others—and these requirements have not yet been systematically developed, nor have the issues of allocating the costs and benefits across different portions of the power system.
Careful policy formulation, accelerated infrastructure investment, and greater commitment to public/private research, development, and demonstration can help overcome such barriers to grid modernization and provide the flexibility needed for optimal operation. As our power system becomes more variable on both the generation and consumer sides, the grid will need to act flexibly to maintain balance. Technology development should be a central component of the strategy to provide balancing resources as more variable generation is added.
Clark W. Gellings, a fellow at the Electric Power Research Institute, has had a long career in technical management at EPRI, serving in seven vice-presidential positions. He is a life fellow of IEEE and an honorary and distinguished member of CIGRE, the International Council on Large Electric Systems. He is a past-president of CIGRE’s U.S. National Committee.
Arshad Mansoor is Senior Vice President, Research and Development for the Electric Power Research Institute (EPRI). Previously he served as Vice President of EPRI’s Power Delivery and Utilization sector where he led research, development, demonstration and application of transmission and distribution and energy utilization technologies; as Vice President of the former EPRI subsidiary, EPRI Solutions; and as Vice President and Director of Engineering with the EPRI Power Electronics Application Center.
Ron Schoff is the manager of the Technology Innovation (TI) program at the Electric Power Research Institute (EPRI). The program’s portfolio of cross-cutting research, development and demonstration projects scouts, influences and builds on early-stage work across the global science and technology communities to capture innovations for application-oriented development and demonstration by EPRI.