By Elaine K. Hart
In the United States, the growing share of renewable resources over the last decade has contributed to operational challenges in several regions, including the Pacific Northwest, Texas, and Hawaii. In California, policymakers and grid operators are already anticipating challenges in operation of the grid at 33% Renewable Portfolio Standard (RPS) and higher. With increased renewable penetrations come new challenges for integrating these resources onto the electricity system as well as new opportunities for technology and market solutions to promote more efficient integration.
Renewable integration challenges can be broadly classified into four categories: transmission interconnection, distribution system reliability, resource adequacy, and system-level operations. At the transmission level, renewable development tends to require transmission infrastructure investments to bring power from regions with strong wind or solar resources to load centers or to the nearest high voltage substation. When renewable resources are instead distributed, as is the case with rooftop solar PV, high penetrations may require utilities to upgrade distribution circuits to prevent voltage rise and to ensure that protections can accommodate reverse flows.
With respect to resource adequacy (ensuring that the grid has enough generating resources to meet load), renewables bring both benefits and challenges. In systems that are driven by cooling loads, like California, solar power is available in the hours when demand is highest, so it is capable of offsetting the need for conventional generating capacity to meet the peak demand. This capacity value ranges from 5% to about 50% of the installed capacity, depending on the renewable resource and the load pattern. For context, conventional generators provide 80-90% of their capacity because they are down only for maintenance or in the case of mechanical failure.
Renewables can also lower the cost of operations on the system. In hours with high electricity demand, traditional power systems tend to rely on the plants with the lowest efficiencies – these plants are cheaper to build but more expensive to operate, so they are important for reliability but are rarely dispatched. Renewable generation in these hours has the added benefit of reducing generation from the most costly and worst emitting sources. This conventional logic has led some utilities and utility commissions to calculate and assign an energy value to various renewable projects based on the ability to offset fuel burn and variable operations and maintenance costs. These energy values are then factored into renewable procurement decisions.
At higher penetrations, renewable resources pose a new set of operating challenges to the system. The primary technical challenge at high penetrations is that delivery of all (or almost all) of the renewable energy requires reducing generation from conventional power plants to lower levels than the grid has experienced in the past. According to conventional wisdom, operating at very low levels of conventional generation may compromise reliability for three reasons. First, voltages must be kept stable and within an acceptable range across the entire system. Voltages tend to be higher at points of power injection (power plants) and lower at loads, so some load centers must also have adequate generating resources nearby to prevent the voltage from dropping too low. This is a concern in areas like the LA basin, where once-through cooling natural gas plants have operated on the coast for several decades, but are now being retired for environmental reasons. In deciding how to replace both the energy and capacity provided by these resources, utilities and the California Independent System Operator (CAISO) are concerned in particular with maintaining voltages in the area.
Second, stability on an AC system depends on the physical inertia of all of the synchronous generators (with synchronized spinning rotors) on the system. This inertia resists the rapid drop in frequency that can be observed due to an unexpected generator or transmission line outage, providing the system with enough time to respond before shedding loads. Today, conventional resources provide this inertia and renewable resources do not. Despite the fact that wind turbines have spinning rotors, the rotor frequency is decoupled from the grid because the turbines utilize asynchronous generators and/or connect to the grid through power electronics.
Lastly, conventional power plants are physically limited in how much flexibility they can provide. Each unit has a minimum stable level below which the power plant cannot operate, a maximum ramp rate, and a minimum run time as well as a minimum down time. These operating constraints mean that a power plant that has been dispatched downward (or shut down) to allow more renewable generation on to the system may have difficulty being dispatched upward (or turned back on) to provide power when the renewable generation declines. These operational limitations have been studied using production simulation models in recent years by the CAISO, the National Renewable Energy Laboratory (NREL), Energy & Environmental Economics (E3), and others. In addition to these technical limitations, some plants face economic limitations to flexibility due to high maintenance costs associated with being turned on and off too frequently. Other plants have environment-based flexibility constraints, such as hydroelectric plants in the Pacific Northwest, which are operationally constrained by regulations intended to support the salmon population. Flexibility challenges can also be exacerbated by poorly-designed markets or scheduling practices.
As renewable penetrations continue to rise, attention must be turned to the implementation of solutions to the renewable integration challenges described above. Voltage solutions include utilizing smart inverters on PV systems to provide reactive power support (which is currently disallowed by some interconnection standards) and converting unneeded conventional capacity to synchronous condensers, which has been proposed as a solution for the LA basin. With regard to the inertia challenge, researchers (and increasingly system operators as well) have focused on the development of control systems for renewable resources that resist rapid drops in system frequency. These control systems, which are said to provide synthetic inertia, represent an important area of research and development for renewable integration.
Operational flexibility solutions range from new flexible power plants and energy storage to improved scheduling using updatable renewable forecasts and enhanced regional coordination. One challenge is that adoption of these solutions and acceptance of their costs will require determination of their value. In two recent publications, our renewable integration team at E3 proposed that a critical component of the value of these solutions is the ability to avoid renewable curtailment on the system.[5,6] While different systems will place different economic value on the delivery of renewable energy and hence the cost of curtailing these resources, all systems will benefit from the efficient integration of renewables to reduce both greenhouse gas and criteria pollution emissions from conventional thermal power plants.
Dr. Elaine Hart is a Consultant at Energy & Environmental Economics, Inc. (E3), where she specializes in electricity dispatch modeling for renewable integration and planning, valuation analysis for conventional generation, energy storage, and transmission assets, and multi-sector modeling of low carbon energy technologies. Dr. Hart holds both a PhD in Civil and Environmental Engineering and an MS in Materials Science and Engineering from Stanford University and a BS in Chemistry from Harvey Mudd College.
1. For further reading and an alternative taxonomy, see: Von Meier, A, “Integration of renewable generation in California: Coordination challenges in time and space,” 11th International Conference on Electrical Power Quality and Utilisation (EPQU), 2011, vol., no., pp.1,6, 17-19 Oct. 2011.
2. CAISO, “Operational Requirements and Generation Fleet Capability at 20% RPS,” 2010.
3. NREL, “Western Wind and Solar Integration Study,” 2010.
6. Hargreaves et al., “REFLEX: An Adapted Production Simulation Methodology for Flexible Capacity Planning,” IEEE Trans. Power Systems, 2014.