Tapping protective relays for power quality information

Protective relays are vital components of power systems. They are deployed on every piece of major equipment and are attached to every circuit breaker in the system. As microprocessor technology continues to expand, microprocessor-based protective relays will enjoy enhanced functionality. Load profiling, fault oscillographic waveform capture, and metering are some of the enhanced functionality available in microprocessor-based protective relays. This article shows how existing signal processing capabilities of protective relays can be used to intelligently monitor power quality (PQ). The definition of power quality, the available functions, and their application to utilities are examined. In particular, it is noted that those PQ events that industrial account managers are interested in (voltage sags, swells, interruptions) and additional events PQ engineers need (waveform capture, harmonics) can be gathered by protective relays. The article concludes with some field results from a microprocessor-based protection unit. While this article discusses the integration of power quality monitoring into protective relaying specifically, PQ monitoring is also integrated into other intelligent electronic devices (IEDs) that have access to the system voltages and currents such as switch controllers and meters.