The key to locating voltage sag sources in PQ data is the relationship between sag voltage and current. A high current, either from a system fault or excessive momentary load current (e.g. motor start) is the root cause of a voltage sag.
If the offending source causing the sag is downstream of the monitor, its high current will be included in the data along with all other downstream loads. Its current will appear as a spike in an RMS stripchart, exactly coincident in time with the voltage sag.
If the source is upstream, the voltage sag will be present, but the monitored current will only show other loads reacting to the sag, not the excessive spike causing the sag. These load reactions may be slight increases or decreases in current.
The sag depth is related to the system impedance, which is roughly constant at 60 Hz, so a comparison of a suspected sag-inducing load’s current with other similar current spikes and corresponding voltage changes can help reveal if an increase in current was the cause or effect of a sag.
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