Regulation
Regulation is one of the ancillary services for which storage is especially well-suited. Regulation involves managing interchange flows with other control areas to match closely the scheduled interchange flows and momentary variations in demand within the control area. The primary reasons for including regulation in the power system are to maintain the grid frequency and to comply with the North American Electric Reliability Council’s (NERC’s) Real Power Balancing Control Performance (BAL001) and Disturbance Control Performance (BAL002) Standards.
Regulation is used to reconcile momentary differences caused by fluctuations in generation and loads. Regulation is used for damping of that difference. Consider the example shown in Figure 3 The load demand line in Figure 3 shows numerous fluctuations depicting the imbalance between generation and load without regulation. The thicker line in the plot shows a smoother system response after damping of those fluctuations with regulation.
Generating units that are online and ready to increase or decrease power as needed are used for regulation and their output is increased when there is a momentary shortfall of generation to provide up regulation. Conversely, regulation resources’ output is reduced to provide down regulation when there is a momentary excess of generation.
An important consideration in this case is that large thermal base-load generation units in regulation incur significant wear and tear when they provide variable power needed for regulation duty.
Two possible operational modes for 1 MW of storage used for regulation and three possible operational modes for generation used for regulation are shown in Figure 4. The leftmost plot shows how less-efficient storage could be used for regulation. In that case, increased storage discharge is used to provide up regulation and reduced discharge is used to provide down regulation. In essence, one-half of the storage’s capacity is used for up regulation and the other half of the storage capacity is used for down regulation (similar to the rightmost plot, which shows how 1 MW of generation is often used for regulation service). Next, consider the second plot, which shows how 1 MW of efficient storage can be used to provide 2 MW of regulation – 1 MW up and 1 MW down – using discharging and charging, respectively.
When storage provides down regulation by charging, it absorbs energy from the grid; the storage operator must pay for that energy. That is notable – especially for storage with lower efficiency – because the cost for that energy may exceed the value of the regulation service.
Technical Considerations
Storage System Size Range: 10 – 40 MW
Target Discharge Duration Range: 15 minutes to 60 minutes
Minimum Cycles/Year: 250 – 10,000
The rapid-response characteristic (i.e., fast ramp rate) of most storage systems makes it valuable as a regulation resource. Storage used for regulation should have access to and be able to respond to the area control error (ACE) signal or an automatic generation control (AGC) signal if one is available from the Balancing Authority in which the storage system is located, as opposed to conventional plants, which generally follow an AGC signal. The equivalent benefit of regulation from storage with a fast ramp rate (e.g., flywheels, capacitors, and some battery types) is on the order of two times that of regulation provided by conventional generation, due to the fact that it can follow the signal more accurately and thus reduce the total wear and tear on other generation.
Figure 5 shows two plots to illustrate the storage response for a regulation requirement. The upper plot is an exaggerated illustration of the generation variance in response to fluctuating loads. The lower plot shows storage either discharging or charging to inject or absorb the generation as needed to eliminate the need for cycling of the generation units.
