A MICROPROCESSOR-BASED SYSTEM FOR PROTECTION OF POWER TRANSFORMERS

Abstract

Differential, overcurrent and ground fault relays are used for protecting transformers in electric. power systems. Several algorithms, that perform these functions and are suitable for implementation on microprocessors, have been proposed in the past. This thesis describes and evaluates an improved technique for modelling inverse-time overcurrent relay characteristics. This technique, which is used in a digital overcurrent relaying algorithm, is simple and requires a modest amount of computer memory. The proposed algorithm performs most computations in an off-line mode and, therefore, requires few on-line computations. The performance of the algorithm is evaluated using computer simulations. Some test results are reported in the thesis. Digital algorithms that can detect winding faults in power transformers are described in the thesis. The algorithms use non-linear models of a transformer to determine its health. The algorithms take the non-linearity and hysteresis of the transformer core into account, however, these do not explicitly become part of the algorithms. They are suitable for protecting transformers whose winding currents can not be measured at the terminals. The performance of the algorithms is studied for a variety of operating conditions simulated on a digital computer using the Electro-Magnetic Transient Program (EMTP). Some results of the simulation studies are reported in the thesis. The proposed algorithms for overcurrent relaying and transformer winding protection are implemented in a microprocessor-based system. The design implementation and, testing of the system are presented in the thesis. The system includes a man-machine interface for changing relay settings and relay software, and for uploading the relay signals for further analysis. The performance of the system was checked in the laboratory. The testing procedure and some test results are also presented.