# Variables in dynamic simulation I have a questions regarding the definitions of variables in dynamic simulation. It is a conceptual question not that relevant to PSS/E but I think it is helpful to know.

So my questions is: Since the simulation time step (delta t) in PSS/E dynamic simulation is 0.5 cycle, then why the voltage output is constant (say, voltage = 1.05 pu) during the simulation? To my understanding, the voltage simulation output should be sinusoidal, because the voltage is sinusoidal. Similarly, what is the definition of power (P) in the dynamic simulation? Is is the instantaneous value? Or is it a phasor value?

Thank you

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Sort by » oldest newest most voted Because PSSE does a DC approximation, you will not be able to observe any sinusoidal behaviour.

more In my humble opinion, the voltage output of PSSE is not instantaneous value, but approximation of Vrms (root mean square value). Also P,Q,I... are quadratic means. I haven't seen any channel that get instantaneous value.

more Hi Yu, that is an interesting question. It is true that we don't see the sinusoidal voltage in PSSE, as the output voltage when channeling out a voltage quantity is per unit (p.u.) value. The way power systems simulation tools such as PSSE, PSLF are written is different from other tools such as ETAP, PSCAD MATLAB Simulink etc. which are more focused towards phasor analyses, imbalanced networks simulations. PSSE and PSLF are good if you have to conduct simulations on large network ( could be thousands of buses ) to investigate system-wide behavior. PSSE/ PSLF use state estimation techniques ( "initializing" the case) and calculate variables much faster. Other software can work If you are interested in knowing localized impacts such as voltage waveform of each phase, using PSCAD , MATLAB Simulink etc would be recommended.

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The output file created after dynamic simulation has RMS quantity , which can be observed in PSSE Chop or output window.

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