Load Balancing the Grid
The Intricate Dance of Power Grid Management
In the complex world of electricity distribution, maintaining a stable and reliable power supply is a delicate balancing act. As I covered earlier1 electricity supply must match demand at all times. However the intricacies of how this balance is achieved are less well-known. And renewables make this more complicated as well as make for a more nuanced answer to “are they helping?”.
The Balancing Authority
At the heart of grid management is the Balancing Authority (BA). These entities are responsible for maintaining the delicate equilibrium between electricity generation and consumption within specific geographic areas2. BAs continuously monitor power flows, adjusting generation or calling on reserves to ensure that supply meets demand in real-time. They also manage power exchanges with neighboring areas and implement emergency procedures when necessary to prevent system-wide failures.
The Critical Role of Inertia
One of the most crucial yet often overlooked aspects of grid stability is inertia. Inertia is the tendency of a rotating object to resist changes in its rotational speed.
In traditional power systems, inertia comes from the massive rotating generators in coal, nuclear, and hydroelectric plants. These spinning masses resist sudden changes in grid frequency, providing valuable seconds for control systems to respond to disturbances3.
Inertia is keeping the generated power at 60Hz. With hundreds of power generators providing base load in a synchronous 60Hz, any plant that is joined to the grid must be in sync or it will have such strong feedback on the power lines the turbine can literally shake itself to death4.
However, as we transition to more renewable energy sources, particularly wind and solar, maintaining adequate inertia becomes challenging. Wind turbines and solar panels are typically connected to the grid through power electronics, which don't inherently provide inertia5. This shift necessitates new approaches to ensure grid stability.
For base load power that provides essential inertia, hydroelectric and nuclear plants are ideal choices. They offer consistent, reliable power output and significant inertial mass. Combined Cycle Gas Turbines (CCGTs) can also provide base load power with some inertial benefits, though not as much as hydro or nuclear6.
Handling Minor Demand Changes
Power systems are designed with some inherent flexibility to handle minor fluctuations in demand. This is primarily achieved through the governor response of generators. CCGT7 and Hydroelectric8 plants excel at this type of rapid, small-scale adjustment. These technologies can quickly modify their output, via the grid inertia, to match small changes in system frequency, helping to maintain the balance between supply and demand without intervention from the BA.
Batteries are a great system for instantaneous changes. Unfortunately the total battery capacity available for this is minimal9. Most batteries at present are used in a scheduled manner to address the duck curve10.
Managing Larger Demand Swings
For more significant changes in demand, such as the evening peak when people return home from work, additional generation capacity must be brought online or taken offline. This is where peaking plants come into play. Single Cycle Gas Turbines (SCGTs) are the workhorses of peak demand management due to their ability to start up and shut down quickly11.
Wind and Solar are both very easy to take offline when the peak drops. And if the wind is blowing or the sun is shining, they are very easy to bring online. And this is done at times12. However, it’s rare.
While exact percentages can vary by region, a typical breakdown of peaking power sources might look like this:
SCGTs13: 60-70%
Hydroelectric (where available): 15-20%
Pumped storage: 10-15%
Batteries and other sources: 5-10%
The Role of Energy Storage
Energy storage systems are becoming increasingly important for grid balancing. Pumped hydro storage, the most mature technology, can store vast amounts of energy for extended periods. These systems typically operate on a daily cycle, pumping water uphill during low-demand hours and generating power during peak times14.
Battery storage systems, while currently less prevalent, are growing rapidly in importance. They excel at providing fast-response services, often used for frequency regulation on timescales of seconds to minutes. However, larger battery installations are now being used for longer-duration storage, typically in the 2-4 hour range15.
Both pumped hydro and battery storage are used in a combination of scheduled operations (e.g., daily cycling) and as-needed responses to grid conditions. The flexibility of these technologies makes them valuable tools for BAs in managing grid stability. However, at present, these systems are used primarily as scheduled operations.
The Intermittency Challenge
Intermittent power sources, such as wind and solar, add complexity to the balancing of the grid. These sources are dependent on weather conditions and cannot be dispatched on demand. As a result, the balancing authority must constantly monitor the forecast and adjust generation to compensate for fluctuations in renewable output16.
Distributed solar, particularly rooftop installations, adds another layer of complexity. These systems reduce visible load on the grid during sunny days but can't be directly controlled by grid operators. This "hidden" generation can make load forecasting more challenging and requires more sophisticated grid management techniques17.
Conclusion: Grid Stability is Complex & Hard
Maintaining grid stability is a complex task that requires a diverse set of tools and technologies. While SCGTs remain the primary source of peaking power due to their rapid response capabilities, the grid of the future will likely rely on an increasingly diverse mix of resources. This includes flexible conventional generation, energy storage systems, and advanced control strategies to integrate growing amounts of renewable energy.
If you remember one thing from this post it should be this - at present the primary source of peaking power is SCGT. And where the other sources are available, they tend to be used first. So when the BA needs to bring that last peak source online, it is almost always a SCGT.
Breakers will trip before that happens. In addition grid operators would never try to add a plant that was not in sync.
"Assessment of the Operational Flexibility of Combined Cycle Power Plants" by A. Garduno-Ramirez and K. Bialek (2014)
"Hydropower and the World's Energy Future" by International Hydropower Association (2019)
"Grid-Scale Energy Storage Systems" by U.S. Department of Energy (2013)
Discussion by me with BA operator
They do not use CCGTs for peak use because they are less efficient in that mode