Table 5.1 Droop definitions
Droop name Description
Governor or AVR droop The droop setting of the governor or AVR.
Speed droop
This droop value is related to the engine speed. Since engine speed and the generator frequency
are proportional, you can also use the generator frequency to calculate speed droop.
Voltage droop The droop value that is related to the generator voltage.
The droop for every genset in the system must be the same. The shaft generator must also have the same voltage droop as
the gensets.
When the droop is the same, the system is more stable, because the generators will naturally tend to share the load and the
reactive power equally.
If the speed droop is not the same, then, in the absence of any control signals, the generator with the lower droop tends to
take more of the load.
If the voltage droop is not the same, then, in the absence of any control signals, the generators have different excitation
levels. Circulating currents flow from the generators with lower droop to the generators with higher droop. These circulating
currents could be higher than the full load current, and can stop the generator from supplying the full load.
The following sections describe how to measure the speed droop and voltage droop for each generator. The droop must be
adjusted so that it is the same for each genset.
See
Regulation in the Designer's handbook for more information about droop.
5.4.3
Checking the system stability by running under switchboard control
You can check whether the system is stable with the preconfigured settings by running it under switchboard control.
For example:
1. Start two gensets with switchboard control activated.
2. Adjust each genset's speed to synchronise it to the busbar, and close the breaker.
3. When both gensets are connected, check whether the system's frequency and voltage are stable.
4. Increase the load on the system. Within seconds, the active load and reactive load should be shared evenly. The load
supplied by each genset should be stable. The system frequency and voltage should still be stable.
• If the active and reactive loads are not shared evenly, or stable, then the gensets probably have different droops.
5. Decrease the load on the system. Within seconds, the active load and reactive load should be shared evenly. The load
supplied by each genset should be stable. The system frequency and voltage should still be stable.
• If the active and reactive loads are not shared evenly, or stable, then the gensets probably have different droops.
INFO
If the system is inherently stable under switchboard control, it is not necessary to measure and adjust the droop.
PPU 300 Commissioning guidelines 4189341106 UK
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