7/25/2019 What is the Difference Between Droop and Isochronous Operation

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What is the difference between droop and isochronous operation?

Many gas turbines manufactured by General Electric afford theoperator two choices: droop or isochronous operation. There is

often a selector switch or operator interface screen allowing the

operator to choose either governing mode.

The differences between these two control modes should be

understood. In the droop mode (which is typical for smaller gas

turbine generators operating on large power grids) the turbinecontrol system wor!s in concert with the other on"line governors

on the system to share proportionally load demand changes. This

sharing is done based upon the base load rating of each

generator to the overall capacity of the grid.

7/25/2019 What is the Difference Between Droop and Isochronous Operation

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#igure $ %peed &roop 'urves for Gas Turbine Governor sing Typical %peedtronic %ystem

E*ample:

+ssume that all generators on a power grid are operating in the

droop mode with the same , percent speed regulation. -efer to

#igure $ below. +ssume also that one of the generators is rated

at / megawatts (call it nit 0$) and is synchroni1ed on a gridwhose total generating capacity is 2/// megawatts. The speed

governor for nit 0$ will ta!e / 3 2/// or .456 of any load

demand changes that should occur. #or e*ample assume that

nit 0$ is currently generating 78 M9. If the grid is operating at

7/25/2019 What is the Difference Between Droop and Isochronous Operation

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4/.// 1 and an increase in demand of M9 occurs nit 0$

will increase its power output by: (.//45) () ; ./7$5 M9.

nit 0$ will then be generating 78./7$5 M9. The other

generators with their own , 6 droop characteristic will shareproportionally the remainder of the load change (that is M9

minus ./7$5 ; ,.

7/25/2019 What is the Difference Between Droop and Isochronous Operation

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#igure 5: %peed &roop Mode

In some small power grids (li!e on 'aribbean islands)

automatic fre=uency control is accomplished often by operating

the largest turbinegenerator in the grid in the isochronous mode.9ith one unit in the isoch mode any changes in load demand

will try to reduce system fre=uency. This attempt to CdroopC will

be first CnoticedC by the turbine operating with an isochronous

governor (call it nit 05). This isoch governor will immediately

notice a slight decrease in speed (fre=uency) and increase output

(D'E here) to increase fuel flow to generate more power. The

isoch machine will Cpic! upC all of the new load demand ( M9in our e*ample above). This happens before the droop governors

can react. In our e*ample the power output from nit 0$ would

remain steady at its current setpoint of 78 M9 but the

isochmachine would increase its output by M9.

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@elow is a simplified schematic which shows the result of

switching to the isochronous mode. +ssume that the &igital

%etpoint (&%B) is set to generate a particular load by a droop

governor. + switch is thrown that opens the D'E feedbac! andalso temporarily disables the &%B signal (at its e*isting value)

putting nit 05 in the isochronous mode. +ny load change

thereafter that tries to cause speed (fre=uency) to change will not

be allowed. The isoch machine will respond to sustain system

fre=uency at 4/.// 1.

Hater if desired the isoch machine can return to the droop mode

by returning the selector switch to this mode of operation. Then

all governors will be operating again in the droop mode and

sharing load changes in proportion to their base load rating.