When an isolated synchronous grid is initialized in steady state, the reference angle remains undefined. In order to make the angles well defined, this ReferenceBus component should be used in the position where the power flow problem has the slack node.
The purpose of this component when the grid is initialized in steady state is twofold:
UStart and phase UPhaseStartPSlack and reactive power QSlack that allow to balance the power flows at the nominal value of frequency. These two values correspond to the active and reactive power flows into the slack node of the power flow model. PSlack and QSlack then remain constant throughout the simulation. If the power flow is correcty balanced, those two values are nearly zero, so the Reference Bus is not absorbing any significant active or reactive power during the simulation.setPhaseOnly is set to true, then only the initial phase of the bus voltage is set to the same value of the power flow, i.e., UPhaseStart. The initial voltage magnitude is computed to ensure zero reactive power flow QSlack.