4-Hour Rolling Dispatch Strategy for Evening Peak

Introduction#

A provincial grid dispatch center often encounters evening peak load conditions with low wind forecasts. During these periods, the planner must manage a 4-hour rolling horizon to ensure system stability and cost efficiency. This guide presents a systematic approach to coordinate thermal generation, utility-scale battery storage, and renewable energy.

The strategy relies on a rolling optimization loop. Every 15 to 60 minutes, the plan is refreshed with new forecast data and actual state measurements. For this case, assume an evening peak window from 16:00 to 20:00, with wind output declining sharply and load rising sharply after 17:30.

Scenario Context and Assumptions#

Wind forecasts are commonly subject to large errors, especially in ramp events. On this evening, the predicted wind production stays below 20 percent of installed capacity for all four hours. The storage plant is a modern lithium-ion battery system rated at 200 megawatts with four hours of duration, starting the horizon at a 60 percent state of charge.

Thermal plants include four coal-fired units. Two units are large and efficient but slow to ramp. Two units are smaller, faster, and useful for peak shaping. Each unit has a minimum output level and a maximum ramp rate that constrain the dispatch schedule. The goal is to decide each hour how much each unit should generate, how much storage should charge or discharge, and whether any wind curtailment is required.

Core Coordination Principles#

The first principle is predictability. Thermal units need advance notice before changing output, so the 4-hour horizon is long enough for commitment decisions. Battery storage, by contrast, can react within seconds. Therefore, the dispatcher should use storage for fast frequency support and for smoothing the net-load spikes caused by wind drops and load surges.

The second principle is economic ordering. In a system with no major transmission congestion, the most efficient thermal units should be loaded first, while storage is used only during the highest cost hours or when ramping limits prevent thermal units from following the load. Wind power, being zero marginal cost, should be absorbed whenever system security permits.

Step-by-Step 4-Hour Rolling Dispatch Framework#

Start with an initial schedule at 15:45 for the 16:00 to 20:00 interval. Use the latest wind forecast and a conservative load forecast. For each thermal unit, determine a minimum-load commitment level that avoids frequent on-off cycles. For example, keep the two large units online for the entire horizon and allow the two smaller units to be switched off only if the reserve margin remains above the regulatory threshold.

Next, calculate the net load by subtracting forecast wind and mandatory storage charge from the system load. Set storage discharge in the two hours of highest net-load growth or peak net load. Use a linear programming or heuristic algorithm to minimize total fuel cost while respecting ramp-up and ramp-down limits on thermal generators. In practice, this means loading the most efficient unit to its maximum capability before calling on the second unit.

Reoptimize at the top of each hour. At 17:00, replace the first 60 minutes of the old plan with actual measurements. Extend the horizon by one hour, so that the plan always covers the trailing four hours. If the wind forecast drops unexpectedly at 18:30, defer storage charging and increase the discharge from the fast-start unit, while ensuring the storage state of charge does not fall below 20 percent for regulation reserve.

Throughout the process, monitor the area control error and frequency. If a large thermal unit trips or the load spikes faster than forecast, use the battery to provide primary frequency response for 15 minutes while the automatic generation control adjusts slower units.

Balancing System Stability and Operating Cost#

Stability constraints take priority over cost. The dispatcher must keep a minimum regulating reserve, usually set to cover the largest single contingency. In this scenario, the largest supply loss is the largest thermal unit, so storage and any spinning reserve from other units must be able to replace that loss immediately.

Once stability constraints are met, optimize cost by shifting as much energy as possible to the most efficient thermal unit and by using storage to avoid starting an inefficient peaker. However, frequent storage cycling increases battery degradation. Therefore, the rolling plan should set a flat discharge price threshold. For example, the storage system discharges only when the marginal cost of the marginal thermal unit exceeds the battery degradation cost. This naturally balances daily operating cost against long-term asset life.

Wind curtailment is a last resort. If system load is low and thermal units cannot ramp down fast enough, the planner may request wind farms to reduce output temporarily. In the rolling schedule, these curtailment requests are planned one hour ahead and then updated each cycle, never relying on real-time action alone.

Conclusion#

A successful 4-hour rolling dispatch strategy for a low-wind evening peak requires tight coordination among thermal commitment, storage scheduling, and renewable awareness. The plan must be refreshed frequently because forecasts change and actual load often deviates from models.

The recommended approach starts with a cost-based thermal schedule, overlays a storage trajectory that explicitly protects reserve and ramp constraints, and finally adjusts renewable output through forecast-aware curtailment decisions. This process balances system stability with operating cost while providing enough flexibility to handle the uncertainty inherent in wind energy.