The last two articles have covered power quality phenomena that occur very quickly (impulsive and oscillatory transients, often measured in microseconds) and those that are somewhat slower but still brief (short-­duration rms variations such as sags and swells with a duration of 1–60 cycles). Both types can cause process interruptions or equipment damage. Long-duration rms variations (sustained under/over-voltages and interruptions) usually don’t need a PQ monitor to detect, though one can be useful to determine why they occurred.

There is another type of PQ disturbance in the non-steady-state realm called rapid voltage change (RVC). Other than lighting, these generally don’t have nearly as much of an effect on equipment. RVC events are when the rms voltage level quickly changes from one steady-state value to another, but not enough of a voltage change to be classified as a voltage sag or swell. Typically, voltage sag threshold is set to a 10% change (or threshold of 90% of nominal voltage). 

An RVC minimum voltage threshold is usually 3% of nominal voltage. A voltage sag often has a duration of 6–10 cycles and then the voltage returns close to the original nominal voltage. An RVC event must become steady-state before and after the voltage change for at least 1 second. Think of what happens when there is a tap change of a transformer to correct for large load current increases or decreases. Voltage is steady at 115 Vrms, then instantly 120 Vrms and steady. 

What about lighting?

The often-cited Table 4 in the IEEE 1159-2019 “Recommended Practice for Monitoring Electric Power Quality” still doesn’t address RVC events, though it covers nearly all other PQ phenomena. The formal definition was originally proposed by Norwegian regulators and then formalized in the international standard, IEC 61000-4-30. The primary concern was the effect on lighting, such as the voltage fluctuations that result in light flicker. IEEE and IEC standards had defined new measuring parameters, perceptibility short-term (Pst) and longer-term (Plt), which are mathematically intense algorithms to determine effects on incandescent lighting from voltage fluctuations, even as small as 0.25% of nominal voltage. 

With the change to newer lighting technologies, especially LED-based luminaires, this method had significant limitations. In addition, many people have experienced LEDs flickering that was not caused by any voltage disturbance.

IEEE 1547-2018 Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces addressed RVC from a different perspective. It limits RVCs as a power quality requirement for distributed energy resources, limiting voltage fluctuations at the point of common coupling caused by switching operations or inverter behavior. It mandates that DERs manage voltage impacts during synchronization, limiting step changes to avoid affecting other customers during normal daily operations and fault conditions. 

Why is monitoring this useful?

So how is monitoring for RVC events useful in a facility since the lighting may not be affected by them? In a proactive electrical and electronic equipment maintenance program, it’s like having an EKG machine running all the time, looking for smaller events that don’t cause problems on their own but are clues to what’s going on. For example, as equipment turns on in the morning, the increase in current will cause a change in the voltage, per Ohm’s and Kirchhoff’s Laws. One can correlate those RVC events to particular pieces of equipment, such as HVAC system, heat curing oven or even the lighting. 

If the trend of the magnitude of the RVC at 6:45 a.m. every day is getting larger, something in the equipment is changing. A motor may have higher resistance in itself, or an increase in friction on the mechanical system it is driving. When the lights in the entire facility are turned out at the end of the work day, the data can be there with the right threshold setting to determine what those lights actually consume. 

We had a sales person for an energy-­efficient lighting company come to our plant one day and tell the facility manager and accountant how much money would be saved by replacing the existing lighting with his brand. Since I knew precisely what the cost of the lighting load was, I told him it was highly unlikely that he could save more money that what the lighting cost us, unless his lights were also solar panels. 

Some PQ monitoring programs aren’t concerned with Pst and Plt or even RVCs, as there aren’t any complaints anymore about flickering lights caused by the voltage. And the addition of photovoltaic panels on the roof aren’t causing any obvious problems. But is there evil lurking? Monitoring your cholesterol numbers isn’t going to tell you when you will have a heart attack, but it can help prevent one.

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