My past two columns covered monitoring from the service entrance and down into the facility to record what happened and to determine what caused it. At last, we get down to probably the most important aspect: monitoring at a load to determine why it is not operating properly.
Problems continue to grow as we become more dependent on electrically powered equipment to carry out our daily tasks at work, home and play. Most electrical and electronic equipment is susceptible to power quality phenomena of one kind or another. It’s important that the characteristics of the power required by the load equipment is compatible with the characteristics or quality of the power being supplied. The corollary is that the wrong amount of energy at the wrong time can result in equipment malfunction or even destruction.
The figure at right (which comes from the IEEE Std 1346 Electric Power System Compatibility with Electronic Process Equipment) shows examples of other types of equipment, where you can see there is a significant difference in what magnitude and duration would cause problems. The graph shows various types of equipment and their susceptibility to voltage sags that occur to the right and below of the magnitude and duration lines.
Of course, the discussion so far has only been about rms variations. Power quality phenomena also include impulsive and oscillatory transients, notching, noise, harmonics, interharmonics, voltage fluctuations and rapid voltage changes, and frequency variations. In the end, we can’t cover all types of equipment and their susceptibility in just a short article; the table shown at right gives a summary of often-found, power quality-sourced problems of some common equipment found in a facility. The wide range of sources reinforces the need to monitor for just about all kinds of power quality phenomena at the load, just like at the service entrance and distribution panels.
Symptoms of Equipment
|False trip of GFI protective device||Electrical interference or harmonic distortion confuses the GFI controller electronics.|
|Clock, or processes controlled by such, running fast||Multiple zero-crossings from harmonics or notching|
|Bearing failures on motors||Harmonic currents traveling through uninsulated bearings to ground|
|Adjustable speed drive tripping offline||Positive “kick-back” transient from PF capacitor switching in|
|Enclosure of PF capacitors bulging||Excessive harmonics, especially due to resonant conditions of current harmonic frequencies matching that of the circuit impedance|
|Insulation failure of motor windings||Notching from the commutation period in the rectifying circuit of adjustable speed drives|
|Humming sound in telecommunications equipment||Harmonic currents carried through wires running closely parallel to communications cables|
|Desktop computer, printer or fax machines rebooting when lights don’t also blink||Neutral-to-ground voltage swells result in the line-to-neutral voltage acting like a sag|
|TVSS or other surge protection equipment failing||Sustained over-voltage condition, resulting in the peaks of the voltage waveform being near the clamping voltage of the devices, causing them to clamp every cycle|
|Transformers overheat though rms current level is below nameplate rating||Harmonic currents, particularly higher order ones, causing excessive eddy current losses in the core|
|Annoyance or nausea from lights flickering||High current loads (such as arc furnaces or welders) with interharmonics and subharmonics result in voltage fluctuations in the 1–15 Hz region.|
|Frequent operation of uninterruptible power supply (UPS) for short duration||Cycling load (such as motor starts), intermittent high-impedance fault on utility distribution system (such as contact with tree branch in wind), or faulting equipment (such as defective regulator or tap-changer)|
|Motor life well below rated life expectancy||Harmonic or unbalanced voltages can increase the heating of the motor, shortening life of windings and/or bearings.|
|Dielectric breakdown of power supply capacitors||Transients that are punching holes through the dielectric material over time or in one large catastrophic event|
|Neutral (grounded) conductor overheating||Triplen harmonic currents (3rd, 6th, 9th, …) in a wye circuit that are additive, rather than canceling out|
|Process interruptions in general||More often than not, voltage sags are the source.|
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.