Impact of frequency deviations protection functions

Before 2003 blackout, protection engineers didn´t pay much attention to this phenomenon; it was not even part of relay acceptance testing. However, multiple nuisance trips during this blackout, forced system operators to look at this problem and re-evaluate relay performance for such conditions. In fact, Federal Energy Regulatory Commission (FERC) has appealed to the IEEE Power System Relaying Committee to evaluate system relay protection response to off-nominal frequency conditions and come up with a report and recommendations to avoid misoperations during major system disturbances. As it was demonstrated in this paper, practically all protection elements are impacted by off-nominal frequency conditions, some to a greater extent than others. This paper has explained how the microprocessor relay adjusts sampling frequency to the system frequency to minimize errors. It also has given some insights to the protection engineer on how to test in order to evaluate a particular relay response during such conditions. The potential Impact on major protection functions was revealed. It is clear that different relay designs may be more or less vulnerable to off-nominal frequency conditions.