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rokie1432 (Electrical)
4 Jul 06 5:07
Hi,
Could anyone please explain in layman's terms what is droop and isoch control of GTGs on a Power Management System. Thanks!
Helpful Member!(3)  skm (Electrical)
4 Jul 06 6:42
i would put it as below:

Droop Mode: It is the mode of operation in which the machine will react to the load variation by changing its speed. It is utilised when multiple machine run in parallel so that load is shared between the machines. For this sharing to be equal the machine need to have same droop caharacteristic. Typicall if a machine has droop of 4% it means for a change of 1% in rated speed the machine takes 25% of its rated load. This is also referred to as load control mode of operation.
Isochronous Mode: In this mode the machine is not affected by  load and regardless of load it will maintain the frequency. This mode is also referred to as frequency control mode of operation. In case of systems not connected to grid it is required to run at leat one machine in this mode so as to take care of the load variation.
n1mr0d (Industrial)
4 Jul 06 7:51
Droop Mode: It is the mode of operation in which the machine will react to the load variation by changing its speed

Shouldn't the generor output frequency be fixed at 50 or 60Hz? Or are these changes in frequency very small?
Helpful Member!  byrdj (Mechanical)
4 Jul 06 8:02
first, need to know that generation must equal load exactly or the frequency will change.  load greater, frequency down.  
with that understanding, the turbine's governor is used to control system frequency by varing the generator's output.

In droop, the frequency will only be 60.000 Hz at SET output.  with a 5% droop setting, and the SET output at 50%, if the frequency increased to 61.5% due to load shedding, the turbine will back the generator output to 0%.  Droop is normally used when synched to a large grid.

In Isoc, the governor will try to maintain 60.000 Hz at all times.  thus if operating at 50% output and the frequency droops to 59.99, the turbine will go to 100% output.  in isoc, the stablity is determined by a time constaint, ie how fast will the unit change load as per the deviation.  Isoc is normally used when in island mode, ie small system and only one T/G

Since large grids are constaintly varing speed as every light is switched on or off, a unit in isoc would be swinging wildly.  And since the droop governor will not correct back to 60.000, dispatch watches a grid clock and changes the T/G SET output to correct to bring the daily average to 60.00 hz.  thus the dispatcher becomes the isoc governor

 

 
ScottyUK (Electrical)
4 Jul 06 12:07
If the normal conditions for a large interconnected grid apply and the generator rating is small relative to the grid, the grid frequency may change by an infinitesimal amount, but the significant change for the generator is its load angle, not its speed. Speed is essentially constant, whereas load angle is largely dependent upon the prime mover torque (and thus prime mover power output, assuming constant speed). Have a look at the vector diagram of a generator showing the internal EMF, terminal voltage, line current, IXd, IXq, etc. The relationship between power and load angle will be apparent.

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