The main difference between droop and isochronous control modes lies in their relationship to frequency. In isochronous mode, generators maintain a constant frequency, whereas droop mode allows for changes in frequency in response to changes in load. This guide explains the implications of these differences and applications of the two modes.
Because droop mode accommodates changes in frequency, it allows multiple generators to work 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 frequency decreases, output increases. If a generator has a 5% droop setting, 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%.
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 nominal 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.
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.
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 of 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 working synchronistically on the same grid, using droop control mode is preferable.
Petrotech’s Isochronous Loadshare control algorithm allows multiple generators to operate in isochronous control on a common bus. This is advantageous because the bus load can vary from 0 to the total generating capacity of all the units on the bus without deviations in bus frequency or units tripping off line. The generators will pick up and reject load quickly to maintain frequency while the Loadshare controller trims them back to equal loads.
At Petrotech, our goal is to help companies produce energy safely and effectively. Whether you’re looking for a new application or have an older system that requires retrofitting, we’ll work to increase its efficiency, cut downtime, and protect your staff. If you’re interested in our services, request a quote today.