Operational Changes Are Enhancing System Flexibility
Sub-hourly Scheduling and Dispatch
Grid integration studies show that sub-hourly scheduling and dispatch of generators (5- or 15-minute intervals) improves system efficiency, reduces the amount of reserves required to balance the system, and enables systems to integrate higher penetrations of variable renewable energy generation.
Faster dispatch can enable the system to access reserves from existing units at little or no extra cost. It can also reduce the need for regulation reserves, which are the most expensive types of reserves, because there are fewer minute-to-minute deviations between load and generation when the system is re-dispatched frequently.
For example, the Western Wind and Solar Integration Study indicates that sub-hourly scheduling is important for minimizing regulation requirements on the system. Scheduling resources every 5 or 15 minutes, rather than every hour, reduces the need to ramp units that provide load following. The study also found that wind and solar variability has a lesser impact on regulation requirements than hourly scheduling of generating units.
Expanded Balancing Footprint
Our studies have shown that increasing cooperation between balancing areas can reduce fluctuations in supply and demand and make it easier to maintain system balance. In addition, expanded balancing footprints can enhance the benefits of faster dispatch, reducing required regulation reserves and associated costs. The benefits of sub-hourly scheduling between balancing areas are greater with higher levels of variable renewable generation. Particularly where transmission constraints exist, faster scheduling across areas can allow variable generation to be more efficiently integrated through faster and coordinated dispatch with a neighboring market.
Advanced Variable Generation Forecasting
Improved forecasting is considered to be a key means of integrating wind and solar power efficiently and reliably. VG forecasts are more accurate the closer they are to real time, and many utilities have adopted practices that allow forecasts to be updated on a 5-minute basis.
VG forecast costs are dropping and system operators are becoming more confident in the forecasts, according to a 2014 survey of system operators in the Western Interconnection. Other survey highlights include:
Many system operators now regard VG forecasting as a cost-effective mechanism for maintaining electric reliability and scheduling resources efficiently. Several said the system savings are so great and the cost minimal enough that formal cost-benefit analysis is not needed.
Nearly all system operators surveyed—regardless of size, proportion of renewables, or average monthly load—use their wind forecasts for day-ahead unit commitment. Intra-day unit commitment and reserves planning are the next most common uses.
Wind forecasting accuracy continues to improve thanks to advanced forecasting techniques and models, seasoned vendors, and growing portfolio size. Wind forecast errors typically range from 3% to 6% of rated capacity one hour ahead and 6% to 8% a day ahead on a regional basis. For comparison, load forecast errors typically range from 1% to 3% day-ahead.
Solar forecasting is still in its infancy, but several system operators have recently begun working on in-house solar forecasts and at least one company is beginning to track solar forecast accuracy.