2.4 Speed governorConsidering the DG working with biogas, the most common topology is the use of a gas engine with a synchronous generator. For its proper operation, the speed of the synchronous machine has to be controlled. The synchronous machine can work attached to the distribution grid or islanded, and the speed governor must maintain a constant speed of the machine even in the case of islanding [22]. For this purpose, the two main methods used to control the speed of the generators, only based on local measurements, with no need of communication, are isochronous mode and droop mode. In the isochronous mode, the speed of the generator is kept constant from no load to full load, which is recommended for islanded operation. In case of parallel operation, if two generators are settled to isochronous mode, they will fight by the load and some of them will be switched off. In the droop mode, the speed is decreased as the load is increased in a proportional scale known as droop coefficient. For the operation parallel with the utility grid (infinite bus), the machines are usually set in droop mode. The usual speed droop coefficient is between 3 and 5% [8,23].For this simulation purpose, the model used for the speed governor is illustrated in Figure 9 as a droop control. As inputs of the controller, a summing block will compare the reference speed (wref) with the measured speed of the shaft of the generator (wm). A second adding block compares the measured active power (Peo) with the commanded power (Pcommand). So, the control will try to increase the speed of the shaft until the measured power matches the commanded power with an error defined by the droop characteristics. The speed error will be applied to the control block. The dynamics of the actuator and of the gas engine are described in ref. [8,9]. For simulation purposes, the models and parameters of the control, actuator, and engine were used as described in ref. [9]. The parameters are available in the Appendix.Some tasks to the students regarding speed governor can be to analyze the different speed droop settings as reference [24]. What would be the effects of a higher or lower droop setting? Another option to defy the students could be to improve the research about nonlinear methods to adjust the droop as developed in ref. [25]. As the isochronous mode is more appropriate for islanded operation and the speed droop mode more appropriate for grid tie operation, an automatic switching method is proposed in ref. [23]. As soon as an islanding event is detected, the generator is switched from droop mode to isochronous mode. Some research to improve this method can be proposed.