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Dear Jose,

The governor models in the nine bus system (IEEE9) are instable when performing long simulation.

For example, a 5% governor step test results in the following behaviour:

The speed signal is here recalculated to Hz. Initially, Pmech decreases instead of increase, it takes 40 s before Pmech is higher than the initial value (0.6 pu).

The instability is most likely caused by extreme values for the K2 and K3 fractions and long time constants.

This has created problems for some users of the IEEE9 test system.