Power quality describes how closely a building's electricity matches the steady, smooth supply equipment is designed for. Problems include sags, swells, surges, short interruptions, unbalance and harmonics, the distortion created when electronic equipment draws current in pulses. Most can be diagnosed by measuring them over time, and most have practical fixes once the cause is known.
Power quality problems are often reported as something else: computers that reboot, lights that flicker, drives that trip, motors that run hot, a transformer that hums more than it used to, or a neutral conductor that is warm to the touch on a thermographic scan. Because the symptoms are vague and intermittent, they are easy to blame on the utility or on the equipment. Measurement is what separates the two.
The main types of power quality problems
| Problem | What it is | Common causes |
|---|---|---|
| Sag | Voltage drops for a short time, from a fraction of a second to a few seconds | Large motors starting, faults on nearby circuits or the utility system |
| Swell | Voltage rises for a short time | Large loads switching off, some utility faults |
| Transient or surge | Very short, high voltage spike | Lightning, utility switching, motors and other inductive loads switching inside the building |
| Interruption | Brief or longer loss of power | Utility reclosing after a fault, outages |
| Unbalance | Phases at different voltages | Single-phase loads unevenly spread across phases |
| Flicker | Rapid, repeated small voltage changes | Welders, compressors and other loads that cycle often |
| Harmonics | Distortion of the wave shape | Electronic loads: LED drivers, computers, VFDs, UPS and chargers |
What harmonics are
Ordinary loads such as heaters and incandescent lamps draw current that follows the voltage wave smoothly. These are called linear loads. Most modern equipment is different. Computers, LED drivers, variable frequency drives, UPS systems and EV chargers convert the incoming AC to DC internally, and their power supplies draw current in short pulses near the peaks of the voltage wave. The resulting current is distorted, and engineers describe that distortion as a set of harmonics: extra currents at multiples of the 60 Hz fundamental frequency.
Why harmonics cause problems
- Overheated neutrals. In a three-phase, four-wire system, the ordinary 60 Hz currents of balanced phases cancel in the neutral. Certain harmonics, especially the third, add up in the neutral instead. With many single-phase electronic loads, the neutral can carry more current than the phase conductors.
- Transformer heating. Harmonic currents cause extra losses in transformer windings and cores, so a transformer can overheat at a load that would be fine for ordinary equipment.
- Voltage distortion. Distorted currents flowing through the system's impedance distort the voltage, which can upset other sensitive equipment on the same supply.
- Capacitor problems. Power factor correction capacitors can interact with harmonics and fail early or amplify the distortion.
- Nuisance tripping and meter errors. Some protective devices and older meters respond poorly to distorted current.
Guidance on acceptable harmonic levels at the point where a customer connects to the utility is given in IEEE 519. Utilities may set their own limits for large customers, especially those with many drives.
How problems are investigated
- Collect the symptoms. What happens, to which equipment, when, and how often. A log with dates and times is very valuable.
- Review the system. Single-line diagram, transformer ratings, neutral sizes, large motors and drives, and any capacitors.
- Measure. A qualified person installs a power quality recorder at the service or the affected panel, typically for a week or more so it captures a full operating cycle. It records voltage, current, events and harmonic content.
- Correlate. Compare recorded events with the symptom log, the building's operating schedule and utility events.
- Recommend and fix. Target the cause, then measure again to confirm the fix worked.
Connecting recorders to energized equipment carries shock and arc flash risk, so it is done by qualified electrical workers following your electrical safety program.
Common fixes
| Cause | Typical remedies |
|---|---|
| Harmonics from many single-phase electronic loads | Larger or separate neutrals, transformers rated for harmonic loads, harmonic-mitigating transformers |
| Harmonics from drives | Line reactors or DC chokes, passive or active filters, drives designed for low harmonics |
| Sags from motor starting | Soft starters or VFDs, moving sensitive equipment to a different circuit or transformer |
| Surges and transients | Surge protective devices at the service, panels and sensitive equipment |
| Short interruptions | UPS for critical equipment, ride-through settings on drives |
| Unbalance | Rebalancing single-phase loads across phases |
Designing for power quality from the start
Most power quality problems are cheaper to prevent than to fix. In new buildings and major renovations, a few design decisions make a large difference:
- Separate sensitive and disruptive loads. Elevators, large motors, welders and compressors go on different panels, and ideally different transformers, from computers, labs and medical equipment.
- Allow for harmonics in the distribution. Where electronic loads will dominate, specify transformers suited to harmonic loads, and size neutrals and neutral bus bars with that in mind.
- Specify drives carefully. Require harmonic mitigation on variable frequency drives as part of the mechanical specification, rather than adding filters later.
- Plan surge protection as a system. Coordinated devices at the service, distribution panels and critical equipment work better than plug-in strips alone.
- Get grounding and bonding right. Many problems blamed on power quality turn out to be loose neutrals, missing bonds or neutral-to-ground connections in the wrong place.
- Provide a way to measure. Revenue-grade or power quality metering at the service and major panels gives facility staff data instead of guesses when something goes wrong.
A typical case
An office floor renovated with LED lighting and dense workstations starts having trouble: the transformer serving the floor runs hot and a thermographic scan shows the panel neutral warmer than the phase conductors. A week of recording shows high third harmonic current in the neutral during working hours. The phase currents are within the transformer's rating, but the harmonic content makes it run hotter. The fix in a case like this is often to replace the transformer with one designed for harmonic loads and to check that the neutral conductors and neutral bus are adequate. A follow-up recording confirms the result.
How ehZee can help
We design new systems with harmonics and power quality in mind and investigate problems in existing buildings, working with qualified contractors for measurements and corrections. See our electrical design service, and read surge protection for buildings and motors, starters and VFDs.
Common questions
Is it the utility's fault?
Sometimes. Sags, swells and interruptions often start on the utility system. Harmonics and many transients usually come from inside the building. Measuring at the service shows which side the problem is coming from.
Will a UPS fix power quality problems?
An online UPS protects the equipment connected to it from most disturbances. It does not fix the cause, and it can add harmonics of its own to the building system.
Do LED retrofits cause harmonic problems?
Individual LED drivers draw small currents, but a whole floor of them adds up, especially with poor-quality drivers. Specifying drivers with low harmonic distortion and checking the neutral and transformer during a large retrofit avoids most issues.
How is harmonic distortion expressed?
As total harmonic distortion, or THD, a percentage that compares the harmonic content of a wave to its 60 Hz part. It is quoted separately for voltage and current.