Speed Droop in a Governor

Droop refers to the change in speed from no load to full load, expressed as a percentage.

The formula is:

$$Droop\:\%=\:\frac{No\:load\:speed\:-\:Full\:load\:speed}{Full\:load\:speed}\times100\%$$

As shown in the graph, with an increase in load, the speed reduces.

• Coarse Droop: A larger droop percentage leads to slower stabilization after a load change.
• Fine Droop: A smaller droop percentage results in faster stabilization.

However, too little droop can cause instability, often observed as hunting, in response to load changes.

This droop characteristics of individual generators are important for load sharing as it must be similar for two generators to share proper load.

Main Engine Governor:

The governor fitted on a main engine is a "constant load governor." It maintains a constant load at varying speed settings by adjusting the fuel lever.

• As per IACS (International Association of Classification Societies) regulations, each main engine must be equipped with a speed governor adjusted to prevent the engine speed from exceeding the rated speed by more than 15%.
• There is no droop in this governor
• The set value of speed is not fixed, it is dictated by the fuel control lever
• The governor's characteristic is isochronous in nature. So reset action is incorporated, with a time constant.

Auxiliary Engine Governor:

The governor fitted on a generator set is a "constant speed governor." It maintains a constant engine speed despite load variations to ensure a stable frequency for the generator.

• To maintain a constant frequency, the prime mover (engine) must operate at a constant speed.
• As per IACS, each generator must have a speed governor capable of limiting transient variations in electrical network frequency to within ±10% of the rated frequency during load changes.
• Droop is incorporated in this governor whenever the auxiliary engine is meant for parallel operation
• The set value of speed is pre-determined according to the load, taking the droop into account from no-load to full load
• The governor's action is with a speed droop, droop increasing with the increase in load to facilitate equal land sharing (KW) between two generators running in parallel.

Load sharing refers to the proportional distribution of KW (active power) and KVAR (reactive power) loads between multiple generator sets operating in parallel.

When generator sets operate in parallel, the engine speed governor of each generator set determines the proportional sharing of the total active power (KW) required by the system. This is achieved by adjusting the fuel supply to each engine.

• If the governor characteristic is flat, as shown in Figure (i), the load will swing repeatedly between the two machines, resulting in unstable load sharing.
• The droop characteristics of the running and incoming generators must align for stable load sharing, as shown in Figure (ii).
• In Figure (iii):
  • Generator No. 1 has less droop, so it will take more load.
  • Generator No. 2 has more droop, so it will take less load.

  The droop allows the generators to have a definite intersection point on the droop line, enabling proportional load sharing.

  The amount of droop is a trade-off between frequency accuracy and system stability:

  • Large Droop: The system becomes more stable but experiences slight frequency variations as the KW load changes.
  • Small Droop: The system achieves greater frequency accuracy but may become unstable.

  Also, the amount of droop is a compromise between accuracy and stability in terms of frequency & voltage. i.e. if the governor droop is too large, then the system is stable, but the frequency will slightly change as the kw load changes, as shown below: