Droop Mode vs Isochronous Mode

 

When to use Droop Control Mode

·         Droop mode accommodates changes in frequency, it allows multiple generators in tandem by dividing loads in proportion to their power. It is useful when employed in grids with multiple generators and when dealing with loads with higher degrees of variance.

·         In droop mode, a generator’s output and frequency are inversely proportional. When the frequency decreases, output increases. If a generator has a droop setting of 5%, for example, then a 5% decrease in frequency will increase the unit’s power output by 100%. If, on the other hand, the frequency rises by 1%, the unit will decrease its power output by 20%

Disadvantages of Droop Control Mode

·         Problems will occasionally arise when a generator in droop control mode encounters a large load. If the load is tripped, the frequency will reach a value above its normal value when it settles. If, on the other hand, a large generating unit shuts down, the frequency will settle at a steady-state value below its nominal value. Both cases require secondary and tertiary frequency controllers to return the frequency to its nominal value

When to use Isochronous Control Mode

·         Isochronous mode is typically used when a generator either stands alone or is the largest unit on a grid. In this mode, the energy admitted to the prime mover is regulated very tightly in reaction to load changes, which, in droop control mode, would cause changes in frequency.

·         In droop mode, load increases cause the frequency to decrease, but because energy is quickly supplied to the prime mover in isochronous mode, the frequency remains constant. Likewise, Load decreases cause frequency to increase in droop mode, but because the energy directed to the prime mover is quickly reduced in isochronous mode, the frequency is maintained.

Disadvantages of Isochronous Control Mode

·         In isochronous mode, the generator maintains a constant speed regardless of the load. Issues arise when multiple generators in isochronous mode are operating on the same grid (or parallel to each other) and the load changes. If all the units are in isochronous mode, they will start competing to respond first. In the case of parallel generators, one will assume the entire load, while the other will receive none of it. When multiple generators are synchronistically on the same grid, using droop control mode is preferable.