The article considers a new formulation of the state estimation problem, i.e., calculating the electric power system operation mode based on remote measurement data. The suggested approach implies the possibility of computing the electric operation mode parameters in a three-phase formulation for the grid parts equipped with three-phase measurement instruments. As regards the grid parts that are not fitted with such instrumentation, but in which single-line measurements are available, or which are characterized by an insignificant imbalance of operation mode parameters, their state is estimated in the single-line formulation. A distinguishing feature of such problem formulation is that the AC power flows in the grid circuit branches are used as the sought variables. The use of this approach essentially simplifies the problem and brings about additional advantages that are especially important for distribution networks. A hybrid three-phase and single-line operation mode analysis model for solving the state estimation problem is proposed and described. Owing to the use of AC power flows in the grid circuit branches as independent variables (a flow model), it becomes possible to use the phase or line form of the described model depending on the available measurement information in the grid part or component and the requirements posed to the problem being solved. The change over from one system of coordinates to another is made without making essential changes in the analysis model itself. The model errors were analyzed on a test network circuit. The analysis results have shown that the proposed model can be used for calculating the electric operation mode if the phase voltages at the interface between the three-phase and single-line grid submodels do not differ from each other by more than 10%.