Governor Droop Control

GOVERNOR DROOP CONTROL

Mobin Ahmed ETO

HOD – Electrical, BP Marine Academy

Governor Speed Droop - Speed Droop is a speed control mode of a governor of prime mover driving an alternator connected to bus. This is also known as Governor Droop. Speed droop is a governor function which reduces the governor reference speed as fuel position (load) increases. All engine controls use the principle of droop to provide stable operation. Governor droop allows the engine to run at lower speed when the load increases and at rated speed at no load.

Droop is expressed as a percentage of the original frequency (speed) setting from no load to full load.

% DROOP = (fNL – fFL) *100/fFL %

Where, fNL is No load frequency

fFL is Full Load frequency

Normal recommended percent of droop is 3 to 5 % of the reference frequency (speed) over the full range of the governor output. Thus a 4% governor droop is having a frequency of 62.4 Hz at no load will be reduce to 60 Hz at full load. Droop causes the governor speed reference or frequency to decrease as load increases. This allows the governor to vary the load since the frequency (speed) cannot change and remain constant (as shown in below fig. 1). This mode allows alternators to run in parallel, so that loads are shared among generators in proportion to their power rating.

In the below fig. 2 Alternator has a% droop. The Governor droop and fNL can be found out by similar triangle proportions and slope equation of straight-line y = mx + b.

Governor Droop (GD) = (fNL – fFL)/ PFL =

Also, GD = GSR*fFL/PFL

Where GSR is Governor Speed Regulation or Droop%

PFL is full load power at full load frequency (fFL)

Marine Alternators are of same make & model. So, their power ratings are always similar. Their Governor droop settings are always kept exactly same. Hence, by increasing and decreasing the governor speed reference setting (No Load frequency), active loads are shared among them proportionally to their governor speed reference setting. Alternator which is having more governor speed reference setting will share more load and less governor speed reference setting will share less load.

Active Power division between two Identical Alternators having same Governor Droop setting - We know that Speed is proportional to frequency (Ns . If speed increases frequency also increases and vice-versa. When two alternators running in parallel, the loads on an engine are determined by the reference speed setting of the droop governor. In fig. 3, the graph shows that the amount of loads are shared by alternators are where the droop line intersects (point M) the common frequency (PQ line) of both alternators. If the location of this line is moved (New intersects point R), either by changing the reference speed or the amount of droop (is seldom changed) in the unit, the amount of load shared will also be changed.

Increasing the reference speed setting of one governor cannot cause a change in the frequency (speed) of the Alternator, but it will cause a change in the amount of load the engine is carrying.

The load transfer is accomplished by adjusting their governor’s no-load settings with same droop slope. If load is increased in Alternator B, speed will decrease, hence the frequency will decrease. In same time load is decreased in Alternator A, so, the speed will increase, hence the frequency will increase. Now, the frequency constant (say 60 Hz) to be kept constant. To achieved this, Alternator B raises its governor characteristics by raising no-load frequency. And Alternator A lowers its governor characteristics by lowering no-load frequency (both characteristics shown by dotted lines in fig. 3)

Active Power division between two Identical Alternators having different Governor Droop setting - When two identical alternators operating in parallel having different droop characteristics, the active load proportion shared by each alternator will depend on the total load and the droop characteristics of each alternator. If both governor No load frequency is same, then flatter droop will assume more load than more droop governor as shown in fig. 4.

Notice that the amount of droop set in the governor has little effect on the ability of the governor reference speed setting to determine the amount of load the engine will carry. The greater the droop the less sensitive engine load will be to speed setting. However, excessive droop presents the possibility of overspeed should the engine be removed from the bus, thus becoming unloaded.

Identical engines can show different characteristics, if droop settings are not similar as shown in Fig. 4. An engine with more droop will require a greater change in the speed setting to accomplish a given change in load than will an engine with less droop in the governor.

If two Alternators’ governor characteristics as shown in Fig 5, are connected in parallel on the same bus, they must have the same frequency of operation, hence the operating point (PQ Line shown common frequency). we can see that Alternator A delivers twice the power of Alternator B. In order to change the power in a Alternator for a given frequency of operation (PQ Line), one has to change the prime mover (change the value of the no-load frequency, or Governor Reference Speed). Changing the governor will cause the characteristic to move up and down with the same slope in order to change the power.

Active Power division between One Isochronous Alternator and One Alternator with Governor Droop setting

Isochronous governors - This type of governor can maintain constant speed for any level of output power. It is also known as 0% droop mode governor.

For isochronous governors

The isochronous mode can also be used on one engine, running in parallel with any other engine. However, unless the governors have isochronous load sharing capabilities, no more than one of the engines running in parallel can be in the isochronous mode.

· Load sharing is changed by changing Alternator B’s characteristic.

· Changing Alternator A’s characteristic, changes system frequency.

· Isochronous machine takes load variation, while maintaining constant frequency (speed).

All engines in the system are operated in the droop mode except for one which is operated in the isochronous mode. In this mode, the droop machines will run at the frequency (speed) of the isochronous unit. The droop percentage and speed settings of each droop unit are adjusted so that, it generates a set amount of power. The output power of the Isochronous machine will change to follow variations in the load demand while maintaining constant frequency (speed) of the system. Maximum load for this type of system is limited to the combined output of the Isochronous machine and the total set power output of the droop machines. A load above this maximum will result in a decrease in frequency (speed). The minimum system load cannot be allowed to decrease below the combined output set for the droop machines. If it does, the system frequency will increase and the isochronous machine can be motorized.

Formula of Active Power Division of two Parallel Alternators