Power quality describes how well the electricity supply suits the equipment that uses it. A power quality problem is any deviation in voltage, current or frequency that makes customer equipment misoperate or fail. Common power quality events include voltage sags (also called dips), swells, interruptions, flicker, harmonic distortion and phase unbalance. A voltage sag is a sudden drop in voltage while the load stays connected. Faults on the power system usually cause sags, but starting large motors can also cause them. A rapid voltage change is a quick shift in rms voltage between two steady states that stays inside the sag and swell thresholds. Switching, inrush currents and energizing a transformer can all cause one. Power quality problems can come from the utility, from your own site's equipment, or from a neighbouring facility. Equipment that is sensitive to disturbances can also be the source of them. Revenue and protection meters, such as Class A meters built to IEC 61000-4-30, log sags, swells, flicker and harmonics for this kind of analysis. Power quality analysis is also part of PV commissioning.
<!-- review notes
related_toc1 drafted · lead Point of common coupling proposed
Rapid voltage change and flicker limits are judged at the PCC, and the utility may record flicker at or near the PCC.
sec=h16 h=1976bab3fb7csec=7.2.1 h=6405266c22c2c2 drafted · lead Harmonic distortion
Harmonics and THD are tracked as power quality parameters.
sec=h51 h=7ab6616df5desec=h5 h=6e1fc8814236c3 drafted · lead Phase unbalance
Voltage and current unbalance are logged as power quality events or parameters.
sec=h51 h=7ab6616df5desec=h5 h=6e1fc8814236tbl=1.r18 h=9f0ccabe45a2c4 drafted · lead Load inrush current
Start-ups, inrush currents and the switching of capacitor banks or transformers are listed as causes of rapid voltage changes.
sec=G.2 h=1a7557497dc3c5 drafted · lead IEEE 519 proposed
One utility points to IEEE 519 as guidance when it reviews flicker data.
sec=h16 h=1976bab3fb7cc6 drafted · lead Power quality analyzer
Where flicker is a concern, the utility may install a recording power quality meter.
sec=h16 h=1976bab3fb7cc7 drafted · lead SEL-735 Power Quality and Revenue Meter proposed
This meter complies with IEC 61000-4-30 Class A and measures flicker, sags, swells and interruptions.
sec=h5 h=6e1fc8814236sec=h73 h=7d6fa37db3a8tbl=2 h=6fc453723b2ec8 drafted · lead Accuenergy Acuvim II power meter
It logs power quality events such as sags and swells, each with a timestamp.
sec=h12 h=059240ab1b55related_to c5related_to c6