This article about the reactive power in the alternating current network, why it to compensate and what devices for this is used. In the work it is shown that to compensate reactive power is possible not only in order to support the proper condition for the electrical devices work. In many cases, the reactive power compensation allows to increase in the operation effectiveness of electrical networks. But this means that, with the same consumption, it is possible to decrease the active power generation. The 118 bus IEEE test case is used for study and it is shown that the reactive power effective compensation can decrease the losses in the network without the maximum voltages increase.