This paper presents the convergence analysis of Seidel method for solving power systems automatic tasks. In a condition of use of Programmable logical controllers with low capacity as a hardware, algorithm solution based on Seidel method have some advantages, such as memory saving. However, there are some limits for Seidel method convergence. If the electrical network condition is close to resonance there will be no assurance that Seidel method will converge. As a result of made experiments we can conclude that usage of relaxation method in conjunction with convergence control on the phase plane allows to solve network analysis problem with nonexpansible hardware.