With 'system.sim = steady' transients are suppressed and only slow dynamics, imposed by the source-voltage is resolved.
This approximation corresponds to an infinitely fast response of the system.
The example uses the dqo-representation, but is valid for abc too.
See for example:
meter.p[1] active power in pu, changing from 0.5 (1000 MW) to 0.25 (500 MW) meter.p[2] reactive power in pu, from 0.25 (500 MW) to 0.125 (250 MW)and other meter-signals.