FIELD: electric engineering, in particular, relay protection of power supply systems of plants. SUBSTANCE: method involves setting levels of static and dynamic stability of load unit, detection of symmetric constituents of residual voltage, comparison of said constituents against levels of static stability of load unit, detection of time delay for switching load unit off. Load unit is switched off under condition that values of symmetric constituents are kept outside of levels of static stability for time which is determined by dynamic stability level. Corresponding device has three-phase voltage detector 1, which output is connected to input of unit 2 which detects levels of direct and reverse voltage direction. Output of unit 2 is connected to input of unit 3 which calculates level of static stability. Output of unit 3 is connected to first input of testing unit 4 and unit 5 which sets time delay. Output of testing unit 4 is connected to second input of time delay setting unit 5 and timer 6, which is connected to one input of comparison unit 7, which another input is connected to output of time delay setting unit 5. Output of comparison unit 7 is connected to first input of output relays unit 8, which second input is connected to output of reset unit 9. Output signal is sent to circuit for control of input switch of load unit. EFFECT: increased reliability of protection due to elimination of unnecessary switching unit off. 2 cl, 2 dwg
