Concepts

The Changing Power Grid

Wind-turbine-by-Tim-sm

The term “Renewable Integration” is a shorthand expression for “the design basis of the grid is changing.”

The electricity networks that exist today were designed on the basis of both load and generation being predictable and slow moving. Operators in the control room of ISOs had time to wait for the Energy Management System to identify the size of the change and, thus prompted, to manually dispatch the generation resources necessary to bring the system back into balance.

That is no longer the case. As larger amounts of Variable Energy Resources such as wind and solar PV are deployed, the underlying design basis of those networks is increasingly invalidated.

Scheduling of customer demand (load) and power generation is critical to the reliable operation of electric grids because it ensures that there is adequate transmission and energy available to balance the two.

Energy production of wind and solar PV is notoriously prone to unpredictable swings. This makes it difficult to schedule the output in the Day Ahead time frame. Moreover, the output of VERs, particularly for solar PV, experiences significant fluctuations in real time making it even more difficult to adhere to an hourly schedule. In organized markets, there are stiff penalties for deviations from schedule which is why there are workaround solutions for scheduling VERs and heated discussions about how to allocate such costs.

This has the potential to frustrate the public policy goals set for renewable energy.

“Recent wind integration studies and operating
experience demonstrate that at higher levels of penetration, wind generation may need to be curtailed
during certain periods, unless suitable flexibility is
designed into the bulk power system.” *

Energy production of VERs such as wind and solar PV is notoriously prone to unpredictable swings. This makes it risky indeed to schedule the output in the Day Ahead time frame as all Grid Operators require. Moreover, the output of VERs, particularly for solar PV, experiences large and small fluctuations in real time (2-3 second time frame) making it even more difficult to adhere to an hourly schedule. There are stiff penalties for deviations from schedule which is why there are workaround solutions for scheduling VERs (PIRP in California for example) and heated discussions about how to allocate such costs.

*Reference
NERC Special Report entitled “Flexibility Requirements and Potential Metrics for Variable Generation: Implications for System Planning Studies”, August 2010

Capability Gap

The gap between what the transmission grids must do to meet NERC reliability standards and the functionality of grids undergoing change has been growing in proportion to the level of renewable generation. The main driver for this widening gap is that wind and solar generation, despite all efforts, remain significantly unpredictable.

Scheduling has historically been a very effective way of making the day-to-day operation of grids and power markets predictable. This effectiveness is compromised by the uncertainty introduced into the scheduling process by forecast errors and the resulting schedule deviations.

Organized markets are similarly finding that their existing capabilities are not enough to counteract the flood of unpredictable wind and solar generation. This is because the new grid operates at much smaller time scales than before. Large changes in generation now take place in real time (4 seconds to a minute) whereas centralized markets may take up to 20 minutes to take corrective actions. That’s enough time for the Balancing Authority operating the grid and market to fail NERC compliance standards.


 

Closing the Gap

UniGen technology restores predictability to the new grid and allows renewable generation to be scheduled without exposure to financial penalties associated with missing schedule commitments. UniGen is a fast-acting distributed control system that automatically compensates for schedule misses using a dedicated mix of energy resources, such as a power plant and an energy storage system.