Renewables in Bulk Electric System Planning
As we look ahead and anticipate what electric grid performance and planning issues are coming, renewable generation operating characteristics become increasingly interesting. For this week’s post we’ve called on David Hilt, former NERC VP of Compliance, and current resource for HSI, to help us understand some of these changes to the energy landscape. These changes include challenges for system planners and operators as renewables become a larger percentage of the generation mix.
Generators and their operating characteristics are the basis for predictive models designed to capture how interconnected high voltage grids perform in both near- and long-term scenarios. Load, transmission, and generation must work in harmony – matching load with supply – or the system collapses.
Changes to transmission are primarily happening at the distribution level in smart grid projects and behind-the-meter small scale generation like rooftop solar. However, as more utility scale inverter-based generation is added to the system, managing these resources and their variability while modeling their controls, will require an acute understanding of how they differ from more traditional steam turbine driven machines. Large rotating machine inertia (kinetic energy) provides instantaneous response to system disturbances. While controls for inverter-based systems can be fast, at present they do not provide the inertia or stabilizing torque to the system that large rotating machines do.
One critical area in grid planning and management is modeling the availability of generators and calculating reserve margins based on the reliability level society depends on. Simply put, traditional generation supports an availability rating, or generation available to run, of 85% to 90%. Solar-powered resources without storage batteries have an availability of around 25% and wind’s availability is roughly 40%. Current utility-scale battery storage presently can produce at their full capacity for three to four hours. Using a simple calculation, if 100 MW of storage had to be relied on for a 12-hour period, three 100 MW battery facilities would be needed. Dispatched sequentially, they may well be located at a different interconnection points hundreds of miles away from one another. That option solves the gross capacity problem, but three different energy flows will have to be managed.
Blackstart also needs to be addressed to truly rely on non-carbon producing resources such as wind and solar versus fast-start natural gas fired units. Off-site power restoration for baseload nuclear generation also needs to be defined as nuclear is not blackstart.
Reserves include demand side response, pumped storage, remedial action schemes, and other established technologies. They will continue to be a critical piece of the planning puzzle. But we can expect additional data requests and coordination from generators on what they can do and when they can do it to make the planning models accurate.
None of the issues are inherently bad for reliability as long as we’re prepared to make the changes required. The growing impact of renewable generation on the transmission and distribution systems requires a renewed focus on electric grid performance and planning issues. Expect to see more questions, comments, and information around load management, generation stabilization, reserve margins, and blackstart as renewable percentages on the grid increase.