The Adaptive Current Protection of Electric Motors at a Digital Substation

Abstract

To protect high-voltage electric motors (EM) from phase-to-phase short circuits, current cut-offs are widely used. The current cut-offs main disadvantage is the low sensitivity due to the tripping current fixed value. The current cut-offs sensitivity can be increased by giving it adaptive properties. To perform adaptive protection, it is necessary to exchange information between intelligent devices of different cells: EM cells, input switch cells and voltage transformer cells. At a classical substation, the organization of such an exchange of information leads to a complication of secondary circuits and a loss of the transmitted data accuracy. Therefore the implementation of adaptive protection became a reality only with the advent of digital substations. The article considers the change in working conditions, the scheme and properties of adaptive protection at a digital substation. In the adaptive current cut-off the tripping current is adjusted from the inrush current that would have been when the EM was started in the current network mode and which is determined by the measured bus voltage and input current in real time. At the same time, the main feature of adaptive protection is that both the tripping current and the sensitivity coefficient are determined at the same parameters of the electrical network. A mathematical model of the network, current cut-off and electric motor has been developed for the study of adaptive protection. The current cutoff efficiency was estimated by the protected zone length at internal short circuits in the stator winding.