Rolling Dispatch Strategy for Power Grids

Introduction#

Modern power grids face increasing complexity as renewable energy sources like wind and solar become more prevalent. System operators must continuously balance supply and demand while minimizing costs and maintaining stability. A critical scenario arises when wind forecast output is low and the system enters the evening peak load period. During such times, the dispatch center must coordinate thermal generators, energy storage systems, and renewable resources to ensure reliable electricity delivery.

This guide provides a step-by-step approach to formulating a 4-hour rolling dispatch strategy. It focuses on practical decision-making for provincial grid dispatchers, emphasizing the trade-offs between system stability and operational cost. The strategy is designed to be adaptive, using updated forecasts and real-time data to adjust generation schedules every hour.

Assessing System Conditions and Forecasts#

Before any dispatch decisions are made, gather the latest wind power forecasts, load predictions, and state-of-charge (SOC) levels for energy storage. In the low-wind scenario, wind output may be 20-30% below typical levels. Simultaneously, evening peak load typically rises sharply between 6 PM and 10 PM. Use a rolling horizon of 4 hours, updating every hour, to capture changing conditions.

Evaluate the available thermal generation capacity, including minimum stable generation levels and ramp rates. Thermal plants, such as coal or natural gas units, require time to adjust output. Determine the cost curves for each unit, including fuel costs and startup/shutdown expenses. For energy storage, assess both power capacity (MW) and energy capacity (MWh), as well as round-trip efficiency. This data forms the basis for optimization.

Coordinating Thermal, Storage, and Renewable Resources#

The core of the strategy is to use thermal generation for base load and ramping, energy storage for fast response and peak shaving, and renewables as available. During low wind, thermal units must increase output to meet the evening peak. Start by scheduling thermal units to their most efficient operating points, then use storage to fill gaps and smooth transitions.

Energy storage plays a dual role: it can discharge during peak load to reduce reliance on expensive peaking units, and it can charge during off-peak periods or when wind output unexpectedly rises. In the 4-hour rolling window, set storage dispatch targets based on predicted net load (load minus wind). For example, if net load is expected to exceed thermal capacity, discharge storage. Conversely, if thermal output is high and wind is forecast to increase, store excess energy for later.

Balancing Stability and Cost#

System stability requires maintaining adequate reserve margins, voltage support, and frequency regulation. Thermal units provide inertia and reactive power, so at least a minimum number must remain online. Energy storage can provide fast frequency response, but its energy duration is limited. Ensure that the dispatch plan includes sufficient spinning reserve, typically 5-10% of load, to handle forecast errors or sudden outages.

Cost optimization involves minimizing total generation cost while respecting stability constraints. Use a security-constrained economic dispatch (SCED) model with a 4-hour look-ahead. The objective function includes fuel costs, storage degradation costs, and penalties for renewable curtailment. Solve the optimization problem every hour with updated data. If costs exceed a threshold, consider adjusting storage discharge or importing power from neighboring regions if available.

Implementation and Adjustment#

Implement the strategy by issuing setpoints to thermal plants and storage systems. Communicate ramp rates and expected output changes clearly. Monitor real-time frequency, voltage, and tie-line flows. If deviations occur, adjust storage output first, as it responds quickly. Thermal adjustments should be made gradually to avoid wear and tear.

After each hour, review performance and update forecasts. If wind output is higher than expected, reduce thermal output and charge storage. If load exceeds forecast, discharge storage more aggressively. Document lessons learned for future low-wind evening peak events. This iterative process ensures both stability and cost-effectiveness.

Conclusion#

A well-designed 4-hour rolling dispatch strategy is essential for managing low-wind evening peak scenarios. By integrating thermal, storage, and renewable resources, operators can maintain grid stability while minimizing costs. The key is to use forecasts, optimization, and real-time adjustments. With practice, dispatchers can handle such challenges efficiently, ensuring reliable power supply even under adverse conditions.