A question of compensation in high and medium voltage networks with the help of the change of the node reactive power is considered in the present work. The aim of the work is to find and create a computational model, sufficiently simplified for practical use which determines the nodal sensitivity of the complex high-voltage electrical networks when switching single compensatory powers. This paper shows, that this model can be used as a basis for creating practical criteria for one-valued choice using compensatory devices, it is necessary for steady-state conditions voltage control in two situations: first – voltages in the permissible area returning; second – electrical power losses decrease staying in the permissible range of voltage. Using a real part of a power system, questions of its sensitivity determining relation to the voltage and losses switching the single reactive power using a simplified computational model, which is built on the basis of a linear transformation of the circuit layout is considered in the paper.