Integrated metering and protective relay systems-large drive motor applications

The process of converting sampled analog signals to digital numbers using a low-cost microprocessor and an A/D converter has been found to open unique possibilities for the field of protection. Digital filtering extracts phasor components while suppressing DC offset and unwanted harmonics. These components, used in real-time computations and stored in memory for later communication, make metering, event recording and fault locating an integral part of the protective relay system. Systems employing these techniques provide a range of protection functions in maintainable software that includes self-diagnostics. The systems use unified hardware and provide a significant reduction in the cost per function. These attributes are discussed in terms of the field experience with three microprocessor-based protective relay systems covering distribution, advanced motor protection, and general switchgear applications. Distribution fault records, relay performance with extremely saturated low ratio circuits, and advanced methods of protecting high-inertia drive motors are covered.<>