Purpose. Investigation of the nonequilibrium-compensation method for diagnosing the state of high-voltage devices with capacitor isolation, a description of the experience of using the method, finding the disadvantages and advantages of this diagnostic method, the development of improvements to the nonequilibrium-compensation method. Methodology. The study was carried out by the method of continuous monitoring under operating voltage (“online”) for bushings of a three-phase group of shunt reactors to a voltage class of 750 kV. Findings. As a result of the study, the authors developed an improved mathematical model for the nonequilibrium-compensation method, which takes into account the influence of the operating voltage, and received the final formula for determining the unbalance current, which can significantly increase the reliability of the diagnosis. The improved model is different in that it takes into account additional parameters - a working three-phase voltage, under the influence of which is the insulation of electrical devices. The values of the phase and amplitude of the unbalance current obtained using the model will not depend on the network operating modes, such as asymmetry, for example, and, accordingly, there is no need to set the overshot alarm settings and equipment shutdowns, unlike the “classical” method, since the parameters change unbalance current will be caused only by changes in active losses in isolation or partial breakdown of the insulation design of the device. Originality. The authors developed an advanced mathematical model for the diagnosis of high voltage apparatus with capacitor isolation under operating voltage. The result of the calculation of the new model does not depend on the effects of operating voltage (such as asymmetries of amplitudes and interphase angles). Practical value. The obtained model can be used in the production of systems and devices for continuous diagnostics (monitoring) of the main insulation of high-voltage bushings at voltage classes of 110 - 750 kV.