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Droop in governers

Speed droop is the decrease in engine speed as the engine load it raised. It is a property of many mechanical governors. Speed droop can vary and is often adjustable

Synchronous generators operating in parallel must meet four conditions for synchronous operation - Phase sequence, phase angle, voltage, and frequency.

When load is added to the generator, the speed “droops” or slows down because of the load.

The frequency in a synchronous generator is controlled by the grid it is connected to. The initial speed of the prime mover is set somewhat higher than the RPM required to match the generator frequency with that of the grid. When the load is added, the generator slows in response to the added load and if all the calculations were correct, the generator frequency will match the grid frequency.

This is significant because if the generator runs slower than the grid, then it will act like a motor as the grid tries to bring it up to speed. If it is faster than the grid, then the grid tries to slow it down. Both of these conditions may cause significant damage to the generator stator. In some cases it would destroy the stator.

Therefore, it is imperative that droop is accounted for to prevent damage to the generators as they come on line.

The calculation is simple.

Droop % = (No load rated speed - full load rated speed) / No Load Rated Speed

4% is a common droop percentage. If the full load rated speed is 1800 RPM, the no load rated speed will be 4–5% higher which is about 1875 RPM.

When the grid is already powered and another generator is brought online to meet demand, we must make certain of the four requirements. Voltage and frequency must be as close to the grid as possible. The phases are connected in the same sequence.

The generator is operating at no load speed which produces a slightly higher frequency than the grid. The synchroscope indicator will rotate slowly clockwise indicating the frequency is slightly faster than the grid. As it reaches the 12:00 position, we close the generator breaker which adds load to the generator and slows it down.

If all the calculations were correct, the generator frequency is closely matched to the grid and it generates power rather than consuming power as a motor. We can now tweak the throttle on the generator to resolve any remaining gap in the phase angle between the grid and generator by slightly slowing it or speeding it up to bring the phase angles into perfect alignment. If we start out close then all is well. Too far apart and the smoke will indicate we really messed it up.


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