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EDIT:

It seems like your results are correct and PSSE is giving wrong result in governor step response simulations.

STRT/RUN will give correct results regarding the swing equation if Network frequency dependence (NETFRQ) is activated. If NETFRQ is not used, the accelerating torque is wrongly calculated at off-nominal speeds, and the results will be too optimistic. It is therefore a good rule to always have NETFRQ activated in dynamic simulation options, even if it makes the dynamic simulation to take longer time.

In the simulation below (using STRT/RUN), the blue curve is with NETFRQ, giving correct results. The green curve is without NETFRQ, leading to wrong result. Please note that there is no network in this simulation and the difference comes from the generator modelling itself. A sudden load increase for a single generator/single bus system is simulated.

Governor step response test with GSTR/GRUN is not using NETFRQ during the step response test (even if NETFRQ is activated in dynamic simulation options!) and thereby the swing equation is wrongly calculated and the results are not correct.

I have reported this bug to Siemens PTI, but they are reluctant to correct GSTR/GRUN, because "that it was a conscious decision by our founding fathers for whatever reasons". At this moment PTI is hesitating to correct the flaw of GSTR/GRUN that has existed in PSSE for around 40 years!

Consequently, governor response tests with GSTR/GRUN are not 100% accurate!