From the aforementioned EN50160—Voltage Characteristics of Electricity Supplied by Public Distribution Systems comes the following definitions:
- Voltage variation: An increase or decrease of voltage normally due to variation of the total load of a distribution system or a part of it.
- Rapid-voltage change: A single rapid variation of the RMS value of a voltage between two consecutive levels that are sustained for definite but unspecified durations.
- Voltage fluctuation: A series of voltage changes or a cyclic variation of the voltage envelope.
Rapid-voltage change has become a measurement used to describe phenomena occurring between the voltage fluctuations that result in light flicker and the variations in the RMS voltage that are known as sags or dips. But before delving into rapid voltage change, lets review the other two “blinkers.”
The voltage fluctuations that result in the lights being perceived as “flickering” used to be measured by comparing the amplitude and frequency of the changes in the voltage levels and comparing them against the famous “GE Curve.”
That has been replaced in the past decade with a more scientific-based set of parameters: PST (perceptibility short term) and PLT (perceptibility long term). These parameters use complex mathematical algorithms for approximating the light output effect on the filaments of incandescent light bulbs from voltage fluctuations at different magnitudes and frequencies and comparing it against the average human eye-brain response to these fluctuations.
A PST of one or greater means that most people will perceive the light bulb as flickering; less than one, most people wouldn’t. Whereas it may take 1 percent amplitude variation at a 1 Hz rate, it only takes about one-quarter percent variation at around 9 Hz to result in a PST of one.
Sags are usually defined as variations in the RMS voltage that go below 90 percent of the nominal value, and lasting a half cycle or longer, and up to one minute (after that it is called a sustained under-voltage).
Either way, it is generally noticeable because the lights will blink, though it is not very often that the blinks come in rapid and continual succession, as with the voltage fluctuations associated with flicker. Since the change in the voltage amplitude is more severe, it often results in the misoperation of equipment, rather than the annoyance and possible nausea usually associated with flicker.
So, why the need for another parameter called rapid-voltage change? If you notice how the amplitude variations in the two parameters were defined, there is a gap between about 3 and 10 percent deviation from nominal. A rapid-voltage change is a 3 percent or greater step change in the voltage, provided that the percent change does not result in the voltage variation being classified as a dip or sag or swell.
This would usually be if the step change was greater than 10 percent, but the steady state voltage could have been running higher than the nominal and a 10 percent change would still be within the limits of sags and swells. The window of the step change is also defined as one second.
The 3 percent or greater change must occur in less than one second, and then the voltage must be stable again for longer than one second before another variation in the voltage could be classified as a rapid voltage change. This one-second value is also around the point of the 1 Hz modulation rate near the low end of the flicker curves.
Now, the standards have filled in the gaps of variations in the voltage. The figure shows examples. Event No. 1 is severe enough to be classified as a sag, and the resulting PST value is well above the perceptible limit of 1. Event No. 2 is a rapid-voltage change event that wouldn’t have been considered a “flicker event” nor a “sag event.”
Event No. 3 is a flicker event, with the PST above one, yet clearly not a sag or rapid-voltage change. Event No. 0 was a “non-event,” just marked to show you how the PST went nearly to zero, when the min/max deviation of the voltage also was close to zero. All three of the events would have blinked the lights, but would have been characterized as different type of events.
Next month will describe what caused these events and why do we care, beyond just a blink of the lights. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.