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Third Example of Droop Control in Power Systems

Imagine a scenario where we have three synchronous generators connected in parallel, each with its unique capacity and droop settings. Our goal is to understand how these generators work together to maintain a stable power supply to the grid. The generators have rated capacities of 300 MW, 180 MW, and 140 MW, with speed droops of 4%, 6%, and 0% for their respective isochronous governors. Currently, the generators are operating at 50% of their rated capacities, and the system frequency is stable at 60 Hz. But what happens when we alter the load or power requirements? To determine the frequency changes, we calculate the droop values for each generator by dividing the frequency changes by the total power. Multiplying the droop value with the initial power for each generator gives us the corresponding frequency change. For the first generator, with an initial power of 150 MW and a droop value of 0.008, the frequency change is 1.2 Hz, resulting in a no-load frequency of 61.2 Hz. Similarly, for the second generator with an initial power of 90 MW and a droop value of 0.02, the frequency change is 1.8 Hz, leading to a no-load frequency of 61.8 Hz. When we introduce an increase in the total system loading, the third generator, with an isochronous governor, delivers a maximum of 70 MW, while the remaining power is generated by generators 1 and 2. After increasing the load power by 100 MW, the new system frequency is calculated as 59.82857 Hz. To determine the power change for each generator, we consider their droop settings. Generators 1 and 2 generate an additional 21.42875 MW and 8.5715 MW, respectively, while the third generator continues to deliver 140 MW. If the extra power required is less than 70 MW, only the third generator with the isochronous governor will provide the power, keeping the system frequency at 60 Hz. Even if the load reduction is less than 70 MW, only the third generator will decrease its power output to maintain system frequency stability. In conclusion, for a load power between 240 MW and 380 MW, the system frequency will stay at 60 Hz, with generators 1 and 2 delivering 240 MW of power and the third generator supplying the remaining power. The coordination between the generators and their droop settings ensures a stable power supply to the grid.