https://www.svri.nl/en/isochronous-vs-droop-control-for-generators/

29 July 2021

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Isochronous Vs Droop Control For Generators

What is isochronous control? And droop control? When should you use isochronous speed control and when droop? What is reactive droop compensation? In this post I will answer these questions.

Why should you use droop control?

Suppose that a generator is paralleled with the utility grid. If it is considered as an infinite bus, the utility grid will determine the generator frequency (speed) and voltage. When the speed (and frequency) setpoint  is slightly lower, the speed governor will fully close the fuel supply in attempt to lower the frequency. Is the setpoint slightly higher, the speed governor will fully open the fuel supply. For the automatic voltage regulator and the reactive power something similar happens. To solve these problems, we use droop controls.

Isochronous and Droop Speed Control

In the isochronous speed control mode the speed will return to the original speed setpoint after a load has been applied or rejected. However in parallel operation this would cause instability as explained above.

In the droop speed control mode the speed will decrease by a fixed percentage when the generator is loaded from no-load to full load. This provides a stable working point for each load in case of parallel operation.

Isochronous Speed Control – speed and frequency return to speed setpoint after a load change.

Droop Speed Control – speed and frequency will decrease when the load increases.

The droop slope is a fixed setting mostly between 3 and 5%. But the operator or power management system can adjust the speed setpoint in order to:

• return to the rated frequency in stand-alone operation
• change the generator active power (MW) in parallel operation

Constant Voltage Control and Reactive Droop Compensation

In the constant voltage control mode the voltage will return to the original voltage setpoint after a load has been applied or rejected. However in parallel operation this would cause instability as explained above.

In the droop voltage control or reactive droop compensation mode the voltage will decrease by a fixed percentage when the generator is loaded from no-load to full load. This provides a stable working point for each load in case of parallel operation.

Constant Voltage Control – voltage returns to the voltage setpoint if reactive load changes.

Droop Voltage Control – voltage will decrease if the reactive load increases.

The droop slope is a fixed setting mostly between 2 and 4%. But the operator or power management system could adjust the voltage setpoint in order to:

• return to the rated voltage in stand-alone operation
• change the generator reactive power (MVAr) in parallel operation