Microprocessor protection of electrical motors against phase failure in external power supply network

Abstract

Nonsymmetrical modes in the external power supply network during operation can be caused by several reasons: phase failure due to strong wind or glaciation of overhead power lines, wire burnout due to persistent short circuits, open phase automatic reclosing. In case of open phase condition, the external power supply network, synchronous and asynchronous electric motors are powered along the two phases that are in operation. It leads to the negative sequence current, which poses maximum danger for electric motors, since it causes additional heating and, as a result, premature damage. Therefore, the currently important goal of the project is to study transient processes of the motor load when an open phase mode occurs in the external power supply network and to design microprocessor protection of electrical motors against phase failure in the supply network. To achieve the goal, a computer simulation method is used, based on mathematical models of the elements of electrical network presented as the systems of differential equations in a three-phase coordinate system. The regularities of the flow of transient processes in synchronous and asynchronous electric motors in case of open phase modes in both the internal and external power supply networks have been established. Thus, it is possible to develop an algorithm of the operation of microprocessor protection of electrical motors against phase failure in the external power supply network. The proposed algorithm is based on monitoring the negative sequence current level and the absence of current in one of the phases at the section input. The obtained results show that the proposed algorithm to protect electrical motors against open phase modes in an external power supply network has a high selectivity. Its use as a starting element of the fast-acting automatic transfer switch device allows you to provide uninterrupted power supply for enterprises with a continuous technological process.