Power Quality, Part One – What Is It?

"I have sensed a powerful disturbance in the force."<br><br>Sorry about 
that! I just finished watching a "Star Wars" marathon. I don't know how 
many times I heard that phrase… often preceding some cataclysmic and 
universe-altering event. Siemens

Steve Maurer, IME

"I have sensed a powerful disturbance in the force."

Sorry about that! I just finished watching a "Star Wars" marathon. I don't know how many times I heard that phrase… often preceding some cataclysmic and universe-altering event.

But honestly, that's not too far off when describing power quality problems. Things are flowing along quite nicely when, suddenly, there is a disturbance in the grid. Sometimes, these disturbances can have equally cataclysmic consequences.

That's reason enough to ensure that your powered equipment, particularly devices and systems controlled by electronics, are guaranteed a safe, consistent and reliable power supply.

There a many terms used to describe these "disturbances" in the grid. They range from spikes and blackouts to brownouts and flickers. Most of these, however, refer to specific durations, not actual quality definitions.

The best source of information is the Institute of Electrical and Electronics Engineers, or IEEE. They define terminology for eight power quality problems. I highly recommend you refer to their standards for guidance, in particular but not limited to IEEE Standard 1100 – Powering and Grounding Electronic Equipment.

Here are eight problems they identify:

Sags — these are sudden decreases in voltage that last less than one minute. This is the equivalent to what many call "brownouts."

Swells — Swells are sudden increases in voltage, again, lasting less than one minute.

Overvoltage — This describes the condition where supplied voltage is higher than the voltage a connected device or circuit is designed to operate on.

Undervoltage — The opposite of overvoltage, supplied voltage is less that what a device, component or machine needs for optimum operation. One common cause is an extension cord that is so long that the resulting voltage drop overheats the device or tool connected to it.

Harmonics — Not the musical terminolgy. Harmonics are defined as current flow in your power system operating at frequencies other than the normal 60 hertz. Do you have fuses blowing or circuit breakers tripping for no apparent reason? Harmonics could be the culprit.

Transients — These are brief voltage fluctuations, normally less than one cycle in duration. They can be either positive or negative. Large fluctuations, even though brief, can damage solid-state electronic equipment.

Noise — If the unwanted signal is none of the above, it must be noise. It can cause problems. Any distortion in the grid can have damaging effects.

Grounding — While grounding itself doesn't cause problems, improper grounding does. It the resistance to ground is not low enough, faults may not be properly dissipated. The IEEE recommendation is 1 ohm for substations and 2-5 ohms for commercial and industrial services.

Facilities with high ground resistance are asking for power quality problems. High resistance grounds can induce large voltages into your system.


Photo courtesy of Siemens