COMPUTER RELAYING SCHEME FOR PROTECTION OF LARGE MOTORS

Abstract

The present day power plants and industrial units utilize larger motors than the plants of late 1970s. Motors power the vital fuel, air, gas, water and coolant systems in power plants and drive varied loads in industries. To ensure reliable and smooth operation of these plants, it is necessary to reduce the outage time of these motors to minimum. The safe working of motors can be endangered by a number of abnormalities that may appear during motor operation. During these abnormalities, some preventive measures have to be taken to prevent the motor from being damaged. This is achieved by providing automated relaying which isolate the motor from the supply when any abnormality occurs. In this thesis, the various abnormalities that may occur in induction and synchronous motors and the conventional protection provided to safeguard the motor against them have been reviewed. Comprehensive protection scheme for induction motors with details of such a relay have also been reviewed. A digital relaying scheme based on monitoring of motor winding and bearing temperatures along with sequence components of stator current, for the pro-tection of large induction motors has been developed and implemented on a 8085 based microprocessor. To incorporate the concept of an independent back up protection, the schemes has been divided into two parts - slow protection and fast protection and implemented on two separate microprocessors. The details of the digital relaying scheme both hardware and software are presented. The results of test conducted on it are also reported.