What is the difference between droop and isochronous operation?

Many gas turbines manufactured by General Electric afford the operator 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 turbine control system works 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.

Figure 1 Speed Droop Curves for Gas Turbine Governor Using Typical Speedtronic™ System
Example:
Assume that all generators on a power grid are operating in the droop mode with the same 4 percent speed regulation. Refer to Figure 1 below. Assume also that one of the generators is rated at 50 megawatts (call it Unit #1) and is synchronized on a grid whose total generating capacity is 8000 megawatts. The speed governor for Unit #1 will take 50 ÷ 8000 or .625% of any load demand changes that should occur. For example, assume that Unit #1 is currently generating 37 MW. If the grid is operating at 60.00 Hz and an increase in demand of 5 MW occurs, Unit #1 will increase its power output by: (.00625) (5) = .03125 MW. Unit #1 will then be generating 37.03125 MW. The other generators, with their own 4 % droop characteristic, will share proportionally the remainder of the load change (that is, 5 MW minus .03125 = 4.96875).

Figure 1: Speed Droop Curves for Gas Turbine Governor Using Typical Speedtronic™ System.
In the above example, something happens to grid frequency as well. Assume that the frequency is 60.00 Hz when the additional load of 5 MW came on the grid. In this example, the system frequency would droop the following amount: 60.00 - [(.04) (60) (5) ÷ 8000] = 60.00 - .0015 = 59.9985 Hz.
If the operator increases the setpoint on Unit #1 as the other governor setpoints remain steady, the frequency will return to 60.00 Hz and all of the new load of 5 MW will be transferred to Unit
#1.

Below is a simplified sketch of a droop style governor for a gas turbine. The two input signals are the actual turbine speed (called NHP) and the load setpoint (called DSP). The "feedback" signal from the amplifier output is called VCE. If DSP is held constant and NHP reacts be decreasing (speed droops because of the increase in load, see above), the _VCE value must increase to balance the operational amplifier (Op Amp). Thus, with the DSP constant, the governor will respond to generate 37.03125 MW and droop with the rest of the grid to 59.9985
Hz.

                                                       Figure 2: Speed Droop Mode

In some small power grids (like on Caribbean islands), automatic frequency control is accomplished often by operating the largest turbine/generator in the grid in the isochronous mode. With one unit in the isoch mode, any changes in load demand will try to reduce system frequency. This attempt to "droop" will be first "noticed" by the turbine operating with an isochronous governor (call it Unit #2). This isoch governor will immediately notice a slight decrease in speed (frequency) and increase output (VCE here) to increase fuel flow to generate more power. The isoch machine will "pick up" all of the new load demand (5 MW in our example above). This happens before the droop governors can react. In our example, the power output from Unit #1 would remain steady, at its current setpoint of 37 MW, but the isochmachine would increase its output by 5 MW.

Below is a simplified schematic which shows the result of switching to the isochronous mode. Assume that the Digital Setpoint (DSP) is set to generate a particular load by a droop governor. A switch is thrown that opens the _ VCE feedback and also temporarily disables the DSP signal (at its existing value), putting Unit #2 in the isochronous mode. Any load change thereafter that tries to cause speed (frequency) to change will not be allowed. The isoch machine will respond to sustain system frequency at 60.00 Hz.

Later, 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.