Droop Control

Droop control is a decentralized, proportional control strategy in which a generator or inverter automatically adjusts its active power output in response to measured frequency deviations, or its reactive power output in response to voltage deviations. The defining feature is that the power-frequency (or reactive power-voltage) characteristic has a fixed slope, called the droop setting, which determines how much the unit responds to a given deviation.

Key Aspects of Droop Control:

• Frequency Droop (P-f): When frequency drops below nominal, the governor increases mechanical power to the generator (or the inverter increases its active power setpoint). The droop percentage, typically 4–5% for synchronous generators, means that a 4–5% change in frequency produces a 100% change in power output from zero to full load.
• Load Sharing Without Communication: The key advantage of droop control is that multiple generators can share load changes proportionally to their ratings without needing any communication link between them. Each unit responds independently to the same frequency signal, and the steady-state load sharing is determined by the relative droop settings.
• Voltage Droop (Q-V): An analogous droop characteristic relates reactive power output to voltage magnitude. Generators and inverters with Q-V droop increase reactive output when voltage drops and decrease it when voltage rises, enabling automatic voltage support from multiple sources.
• Steady-State Frequency Offset: A well-known limitation of droop control is that it does not restore frequency to the nominal value; it only arrests the decline and stabilizes frequency at a new steady-state offset. Returning frequency to exactly 50 or 60 Hz requires secondary control (AGC), which adjusts the droop curve setpoints.
• Inverter-Based Resources: Grid-forming inverters implement droop control in their firmware to emulate the behavior of synchronous machines. This allows wind, solar, and battery inverters to participate in primary frequency response and to operate stably in parallel with other sources, including in island or microgrid configurations.