Power Quality Basics

What “Power Quality” Means

“Power quality” is the voltage and current waveform delivered to a load relative to a perfect 60 Hz sine wave at nominal voltage. Real power systems deliver something less than perfect — and modern electronic loads are increasingly sensitive to those imperfections.

The classifications below come from IEEE 1159, the standard that defines and measures power-quality phenomena. Each phenomenon has a different cause, different symptom, and different mitigation.

1. Voltage Sags (Dips)

A sag is a short-duration decrease in RMS voltage to 10–90% of nominal, lasting from half a cycle to one minute. Anything longer is an “interruption.”

Causes

• Faults on adjacent feeders (most common — 60–80% of sags).
• Large motor starts (the inrush dip lasts 5–30 cycles).
• Transformer energizing inrush.
• Weather events affecting the utility grid.

Symptoms

• VFDs trip on undervoltage protection.
• Computer power supplies brown out and reset.
• Contactors drop out, taking process equipment offline.
• Lighting flickers visibly.

Mitigation

• Ride-through capability — modern VFDs and UPS systems include configurable ride-through; tune to your facility’s typical sag profile.
• Constant-voltage transformers (CVTs) — protect single critical loads up to ~25 kVA; energy is buffered in a saturated core.
• Dynamic voltage restorers (DVRs) — inject voltage during sags; protect MV-fed industrial processes.
• UPS — full battery-backed protection for everything downstream.

2. Voltage Swells

A swell is the opposite of a sag — a short-duration increase to 110–180% of nominal. Less common than sags but more damaging when they occur.

Causes

• Single-line-to-ground faults causing voltage rise on un-faulted phases.
• Sudden load shedding upstream.
• Switching of large capacitor banks.

Symptoms & Mitigation

Equipment damage from over-voltage stress on insulation, electronics, and capacitors. Mitigated by surge protective devices (SPDs), voltage regulators, or UPS double-conversion topology.

3. Transients (Surges)

A transient is a very brief voltage spike — microseconds to milliseconds — but with peak voltages 2–10× nominal.

Causes

• Lightning (induced surges, not direct strikes).
• Switching transients (capacitor banks, large motors, power factor correction).
• Switch-mode power supply commutation.

Symptoms

• Electronic component failure (semiconductors, capacitors).
• Burnt circuit board traces.
• Random equipment failures with no clear cause.

Mitigation

• Surge Protective Devices (SPDs) per IEEE C62.41 and UL 1449. Coordinated SPDs at service entrance (Type 1), distribution panels (Type 2), and equipment (Type 3).
• Lightning protection per NFPA 780 for buildings in lightning-prone areas.
• Properly sized grounding — surges find a path to ground; make sure that path doesn’t go through your equipment.

4. Harmonic Distortion

Steady-state distortion of the current or voltage waveform from non-linear loads. See our harmonics & K-factor article for the full treatment.

Quick Summary

• Total Harmonic Distortion (THD) measures how much harmonic content is in the waveform. IEEE 519 limits voltage THD at the point of common coupling to 5% for general distribution systems.
• K-factor transformers handle the heating effect on the supply side.
• Active or passive harmonic filters reduce harmonic content flowing into the system.
• 12-pulse or 18-pulse rectifiers on large drives reduce harmonics at the source.

5. Voltage Flicker

Repeated voltage variations that cause visible light fluctuation. Different from a sag — flicker is repetitive, typically 1–25 Hz.

Causes

• Arc furnaces (the worst offender).
• Large welders.
• Motor cycling on large compressors.
• Wind turbines (gust-induced).

Mitigation

• SVC (Static Var Compensator) — fast-acting reactive power compensation.
• STATCOM — IGBT-based reactive compensation; faster than SVC.
• Dedicated transformer for the flicker source, isolating it from sensitive loads.

6. Voltage Imbalance

The condition where the three phase voltages are not equal in magnitude or evenly spaced 120° apart. Measured as a percentage per NEMA MG-1: max deviation from average × 100% / average.

Causes

• Unbalanced single-phase loads on a three-phase system.
• Open neutral or open phase upstream.
• Asymmetric impedance in long feeders.

Symptoms

• Three-phase motors derate (severe — 5% imbalance can derate a motor by 25%).
• Excessive heating in one or two windings.
• VFDs trip on input phase imbalance protection.

Mitigation

• Balance single-phase loads across phases.
• Verify utility supply quality at the meter.
• Use phase-imbalance monitors on critical motors.

The Power-Quality Survey

If you suspect a power-quality problem but don’t know which one, install a power-quality monitor for 1–2 weeks at the suspected location. Modern PQ meters (Fluke 1750, Dranetz HDPQ, Schneider PowerLogic, Eaton iQ Analyzer) capture every event with timestamps and waveform data.

Designing for Power Quality

Three principles for new construction or major renovations:

1. Separate sensitive loads

Run a dedicated transformer for sensitive loads (servers, lab equipment, medical imaging). Isolation from heavy industrial loads (motors, welders, chillers) reduces sag and transient propagation dramatically.

2. Coordinate SPD ratings

Type 1 SPD at the service entrance (rated 100+ kA), Type 2 at distribution panels (rated 40–80 kA), Type 3 at the equipment (rated 10–20 kA). The cascade absorbs the surge progressively.

3. Don’t undersize the transformer

A transformer running at 80%+ load has degraded voltage regulation under transient loads. Size for actual demand × 1.25 minimum, with K-factor headroom for non-linear content.

Quick Reference: What’s Causing Your Problem?