For a while in the world of power quality standards, there has been a characterization of RMS variations called sags (or dips) and swells. The generally accepted limit for a sag or swell was +/—10% of nominal voltage, respectively. For 120 VRMS nominal, a sag was often when the RMS voltage dropped below 108 VRMS. This limit was decided since equipment starts to misoperate below this voltage, though each type has its own limit.

A fluctuation of less than 3% of nominal voltage was labeled as flicker, as incandescent light bulbs do just that. The magnitude and frequency of the fluctuation determine whether the average human eye can perceive the flicker. It takes just 0.25% of nominal voltage modulating at 8.8 Hz.

Flicker was different than “blinks,” another complaint with incandescents. Blinks were caused by a variation in the RMS voltage that fell into the gap between flicker and sags, and could occur occasionally or every few seconds. PQ standards groups in the 1990s and early 2000s attributed them to the startup of heat pumps or other HVAC-type equipment. While they were working on ways to measure, quantify and put limits on flicker and sags, in many areas blinks remained mostly just a trouble report checkbox.

Norway was one of the exceptions.

Land of the midnight sun

The Norwegian Energy Regulatory Authority had a more significant blink problem, primarily due to the large number of aluminum smelters on their grid. The smelting process yielded lots of annoying blinks. The agency labeled them as “rapid voltage changes” and specified a measurement process in the NVE 2004 regulations, with limits on the number of blinks per hour, day and week. The IEC committee put a placeholder in the edition 3 revision of the IEC 61000-4-30 Testing and Measurement Techniques—Power Quality Measurement Methods, and added further details in subsequent revisions. Additionally, the IEEE Std 1453-2015, Recommended Practice for the Analysis of Fluctuating Installations on Power Systems, addressed rapid voltage changes (RVCs).

An RVC takes into account changes between two steady-state RMS voltage levels, where the change in the RMS voltage is larger than what typically results in light flicker (+/–3% of nominal), but less than the threshold limits for voltage sags or swells (+/–10% of nominal, respectively). There is also a 1-second window where the voltage must be stable at the new level before the RVC event is considered over, as shown in the figure above of a typical RVC event. The maximum deviation and the difference in voltage between original steady-state voltage and new steady-state voltage are defined in the RVC standards.

Our friends Ohm and Kirchhoff

Now that we have filled the RMS voltage variation gap between flicker and sags/swells, why do we care? (Apart from having a way to quantify blinks.) Does it even matter now that the newer lighting technologies, such as LEDs, don’t seem to respond to RVCs like incandescents? Although there is no definitive published study yet, it seems like LEDs would be less susceptible, since they have a storage capacitor in the rectifier circuit that would ride through this if the maximum deviation wasn’t too deep or long.

RVCs can be caused by various system events, such as switching capacitor banks, energizing power transformers and starting induction motors. What’s more, our friends Ohm’s and Kirchhoff’s laws apply to RVCs. A significant change in the current level of the circuit will change the voltage drop over the source impedance, leaving more or less voltage for load, depending on whether the current increased or decreased, respectively.

RVCs occur frequently at the facility level and on distribution systems. They can be harmful to some generators’ control systems and electronic equipment, although not as often as with sags. And, of course, they can cause annoying blinks. Many may be familiar with the voltage in the figure above, which is often caused by the inrush current of the startup of a large motor with a high HP.

RVC occurrence is expected to increase with the growing penetration of dynamic loads and distributed energy resources (DERs) in the smart grid. At a 2019 IEEE conference, the Electric Power Research Institute published a paper addressing this concern, as DERs continue to be integrated at transmission and subtransmission voltage levels.

Utility companies face a new situation now: regulating a source of generation rather than loads. This leads to a potential increase in RVCs. For example, having multiple 2-kVA to 4-kVA step-up transformers from a DER energizing at once is the equivalent of several thousand homes all having their HVAC compressors kick on at same time. Now those are going to be big blinks.