INTRODUCTION

The electric power industry is going through significant changes due to the increased requirements for improved quality of power supplied by the utility in order to avoid costly interruptions of manufacturing or other processes caused by voltage sags, swells or unbalanced conditions when a short circuit fault occurs in the distribution system.

The improvement of power quality during short circuit faults can be achieved in several different ways. Like any other problem that has to be solved, we need first to understand the nature of the problem and its effect on sensitive users. The most common short circuit faults in the system - single-phase to ground faults - are characterized by the fact that they introduce a voltage sag in the faulted phase, and at the same time they result in a voltage swell in the two healthy phases. This is clearly seen in Figure 1 that shows the recorded waveform for a single-phase to ground fault.

The effects of voltage sags and swells on sensitive equipment have been studied for many years by industry organizations such as Computer and Business Equipment Manufacturers' Association (CBEMA). They record both characteristics of this power quality event - the depth of the sag and its duration. Figure 2 shows a plot of depth vs. duration known as the ITIC (Information Technology Industry Council) or CBEMA graph.

The first characteristic of voltage sags - the depth - is a function of the type of fault, fault location and the system configuration. This is something that we can not control. The second characteristic of voltage sags - the duration - is the parameter that we can control by applying the advanced features of multifunctional protection relays. Monitoring the changes of the power system configuration and adapting the relay to these changing conditions can further improve the performance of the relays and reduce the effect of short circuit faults on sensitive equipment or processes.

The effects of short circuit faults on the voltage profile across the distribution system may result in the shutting down of sensitive industrial processes. The performance of typical distribution feeder or substation protection systems historically has not been considered as critical as the behavior of transmission line protection devices. This is changing now, resulting in a new look at the requirements and functionality of distribution protection relays.

The effect of changes of the system configuration on the performance of the protective relays is another factor that needs to be considered. Using all available protection and programmable logic functions in multifunctional protective IEDs can help us significantly reduce the effect of short circuit faults on sensitive loads supplied from the distribution substation. Adaptive protection based on detected changes in the system configuration, combination of instantaneous, definite time and inverse time-delayed phase, ground and negative sequence elements, will shorten the fault duration, resulting in changes in the voltage level/time characteristics of the fault condition and reduced probability for the costly interruption of voltage sensitive processes.