THE DESIGN AND IMPLEMENTATION OF A VERSATILE MICROPROCESSOR-BASED DIRECTIONAL OVERCURRENT RELAY

Abstract

Due to advancements in the field of electronics, relay designers are able to implement practical microprocessor-based relay designs which are being used more and more by utilities. With the increasing number of digital overcurrent relays being developed for use in electrical power systems need has arisen for the development of directional relays. In many situations, the magnitudes of fault currents do not provide enough information for correctly discriminating faults in a power system where the lines form loops, rings or grids. As power systems have become increasingly complex and interconnected, this scenario occurs more often. Directional relays can sense the direction of power flow and often act as sensors and logical switches for activating other types of relays. An example of this type of monitoring is the directional overcurrent relay. This thesis is concerned with the design and implementation of a versatile microprocessor-based directional overcurrent relay for phase and ground fault detection. The user is able to specify various design parameters at the time of commissioning the relay so that it may be used in distribution networks, transmission networks or other power flow applications. Some of these parameters are the maximum torque angles, torque offsets, directional sensing voltage-current pairs, pickup settings, time multiplier settings, current-time characteristics and configuration of the relay. The relay design is implemented on an Ariel Corporation's PC-C25 DSP Coprocessor Board which contains a Texas Instruments TMS320C25 digital signal processor. The relay's software structure is divided into a main program and numerous support functions. The main program and several functions are written in TMS320C25 C to take advantage of the flexibility of the C programming language. The C code is cross-compiled into TMS320C25 assembly language and linked with other developed assembly language support functions. The majority of time critical code is directly implemented in assembly language and some code employs floating point arithmetic instead of fixed point. The capabilities of the designed relay are demonstrated by including cases of transmission line shunt faults. These tests were performed off line using (i) data generated by the Electro-Magnetic Transients Program using a six-bus model of the Saskatchewan Power Corporation's (SPC) transmission network and (ii) fault data recorded by SPC.