You must have one and only one swing bus for each interconnected system in your power flow case. An interconnected system is defined as all buses that are electrically interconnected to ach other via ac connections, transmission lines or transformers. Sometimes, we represent multiple interconnected systems that might be interconnected via hvdc lines or back-to-back hvdc links, in which case we would have separate swing buses for each island or group of interconnected ac systems.

The fact that you can have one and only one swing bus is at the heart of the methods (and number of equations and number of variables) you use to obtain the power flow solution.

Now, could you please elaborate on why you have to have multiple swing buses?

I have check his case in DIgSILENT. He has not Swing bus as Utility but, he use two generators as a slack buses so when the load flow is run, one of generator has negative production of active P, so it means that there is a surplus of production in the system (with three subsystem).

Srete

Try to use real element Utility as SL bus two or three, with geiven 1 pu. voltage and 0 degree, P and Q can flow in borh direction.

Srete

Each system can be independently analyzed. One Slack Bus is used for each system. When the state is to use more than one Slack Bus , special programming should be used for it. This is because Slack Bus need special programming in load flow.

Prior to DIgSIELNT PowerFactory 2016 the software will crash even if you tried to create an outage on the generator that is connected to slack bus.

Samir M. Alhejaj, that is normal because system cannot work (power flow) without slack bus (swing bus) BTW How do you model outage of generator since there is no option in 15.2 version for entering failure rate and repair rate for generators. Maybe in newer versions 2016 2017 2018 generator is well modeled for reliability analysis (not generator assessment analysis with MC method)