Power quality describes how close the voltage a plant receives is to an ideal sine wave, at nominal value and frequency, without interruptions. The most common problems in industry are harmonics, voltage sags, transient overvoltages, phase unbalance, flicker and low power factor. Some come from the grid and others are produced by the plant itself; telling them apart requires measuring with a Class A analyzer at the right points.
What power quality is
Plant equipment is designed for a voltage with a defined shape, value and frequency. Power quality measures how far the real voltage departs from that ideal, and how the currents the plant draws distort it.
A power quality problem is recognized by its effects: drives that trip for no reason, PLCs that reset, motors that overheat, capacitors that fail, lights that flicker, noisy transformers or a bill with reactive power charges. It is almost always diagnosed late, because the symptoms are blamed on the equipment and not on the power feeding it.
The problems, one by one
| Phenomenon | What it is | Typical cause | Symptom in the plant |
|---|---|---|---|
| Harmonics | Components at multiples of the fundamental frequency | Drives, rectifiers, UPS, electronic lighting | Heating of transformers and cables, failing capacitors, hot neutral |
| Voltage sags | Brief drops in RMS voltage | Faults on the utility or plant network, large motor starts | Process stops, contactors dropping out, drives tripping |
| Interruptions | Total loss of voltage | Grid openings, reclosing | Total shutdown |
| Transient overvoltages | Spikes of microseconds to milliseconds | Lightning, switching of capacitor banks and breakers | Damaged electronic boards, insulation failures |
| Slow voltage variations | High or low voltage for minutes or hours | Weak grid, transformer taps, variable load | Hot motors, equipment out of range |
| Unbalance | Different phase voltages | Poorly distributed single-phase loads, a phase with a problem | Hot motors, unbalance trips |
| Flicker | Fast voltage fluctuations that make light flicker | Arc furnaces, welders, rapidly changing loads | Complaints about lighting from staff or neighbors |
| Low power factor | Much reactive power compared with active | Motors, transformers at no load, inadequate compensation | Reactive power charges, capacity occupied |
Each phenomenon has its own guide:
- VFD harmonics and THD: total harmonic distortion.
- Voltage sags in industry and drive that trips.
- Transient overvoltages in the plant.
- Voltage unbalance.
- Power factor in industry.
Who produces them
Power quality is a shared responsibility. The utility must deliver a voltage within certain limits, and the plant must not distort it beyond what is allowed with its own loads.
- They come from the grid: sags from faults on other circuits, interruptions, overvoltages from lightning on the lines, voltage variations and part of the background harmonic distortion.
- The plant produces them: harmonics from its drives, flicker from its fluctuating loads, unbalance from its single-phase distribution, uncompensated reactive power, sags from starting its own motors.
Utilities and regulators set their own power quality rules, and we adapt the work to the local code and the utility requirements that apply to your project. Typically they measure voltage harmonic distortion against IEEE 519 or EN 50160, unbalance as the ratio of negative-sequence to positive-sequence voltage, sags and swells under IEC 61000-4-30 Class A, and flicker under IEC 61000-4-15. When a deficiency is found, they usually give the responsible party a period to correct it. To identify the source, the utility can install recorders on the network or at the customer boundary and require remedial measures to be designed.
Typical reference values
Reference values depend on the voltage level and on the document that applies. Two widely used references:
| Phenomenon | Typical reference |
|---|---|
| Steady-state voltage | ANSI C84.1 Range A: ±5 % of nominal at the service point. EN 50160 (low voltage): ±10 % for 95 % of the 10-minute values over a week |
| Voltage THD (IEEE 519-2022, at the PCC) | 8 % up to 1 kV; 5 % from 1 kV to 69 kV; 2.5 % from 69 kV to 161 kV; 1.5 % above 161 kV |
| Flicker | Short-term severity Pst measured over 10 minutes, calculated under IEC 61000-4-15 |
| Sags and swells | Classified under IEEE 1159 by duration: instantaneous (0.5 to 30 cycles), momentary (30 cycles to 3 s) and temporary (3 s to 1 min) |
The IEEE 1159 classification distinguishes, for example, a sag of 0.5 to 30 cycles with a typical magnitude of 0.1 to 0.9 per unit from one of 3 seconds to 1 minute with the same magnitude. For that reason an event record must include its duration and not only its depth.
Reference standards
- IEEE 1159: classifies the phenomena (transients, short- and long-duration variations, unbalance, distortion, fluctuations) and gives guidance to monitor them.
- IEC 61000-4-30: defines how they are measured. A Class A analyzer gives results that are comparable and repeatable between instruments from different manufacturers. It is the one used when a measurement may end in a dispute with the utility.
- IEEE 519: harmonic limits at the point of connection to the grid; see harmonic study under IEEE 519.
- EN 50160: voltage characteristics delivered by public networks in Europe.
- IEC 61000-2-4: compatibility levels in industrial networks.
What a Class A measurement does
IEC 61000-4-30 defines the measurement methods and classifies them by performance class. In Class A, the base measurement is made in windows of 10 cycles (50 Hz grid) or 12 cycles (60 Hz), about 200 ms, which are aggregated into intervals of 150/180 cycles (about 3 s) and 10 minutes. The 10-minute interval starts at an absolute clock boundary, and the long-term flicker interval is formed from twelve 10-minute intervals. A separate test standard, IEC 62586-2, covers conformity with Class A.
One principle of the standard is event flagging. When a sag occurs, the 10-minute values that contain it are flagged, and the other quantities, such as harmonics, unbalance and flicker, must reflect the normal condition of the grid. This way a single event is not counted twice in different parameters. Keep that flagging in mind when reading reports from a Class A analyzer, such as the Metrel MI 2892 and MI 2893.
How it is diagnosed
- Symptoms. What fails, when, on which equipment and since when
- Measurement. Class A analyzer at the boundary and at the suspect loads
- Correlation. Quality events against failures and plant operation
- Origin. Grid or plant, and which loads
- Solution. Filters, compensation, backup, protection or settings
The measurement is made with a power quality analyzer over a period that captures all shifts and days of the week, usually at least one week, at the grid boundary and at the suspect boards or loads. The most useful thing is an event log synchronized with what failed in the plant: if a drive tripped at 10:42, the analyzer must show what happened to the voltage at that time. To dig deeper into how to classify and monitor power quality phenomena, read power quality monitoring under IEEE 1159.
The most common mistake is to measure for a while with a clamp meter and conclude. Power quality problems are intermittent: a sag lasts tenths of a second and may happen twice a month.
How we help
Our team carries out the power quality study of your plant. We define where and how long to measure, analyze the records and events against your failures, identify the origin and design the solution, from filters and compensation to protection settings and backup for sensitive loads. We do the analysis and design remotely, worldwide, with your measurements. In Colombia, or elsewhere on request, we also measure on site. See our power quality service and write to us telling us what is failing.
Related guides
- Harmonic study under IEEE 519
- VFD harmonics
- Drive that trips
- Transient overvoltages in the plant
- Power factor in industry
- Active vs passive harmonic filter
- Energy under-metering in the plant
- Metrel MI 2892 / MI 2893 Class A Analyzer: What It Measures
- Power Quality Analyzer: Class A, Harmonics and Flicker
- THD: What Total Harmonic Distortion Is
- Voltage Sags in Industry: Why the Plant Stops
- Voltage Unbalance: Effect on Motors and Measurement
- Capacitor Bank Resonance: Why Banks Fail
- Power Quality Monitoring: IEEE 1159
References
- IEEE 1159, Recommended Practice for Monitoring Electric Power Quality.
- IEC 61000-4-30, Testing and measurement techniques – Power quality measurement methods.
- IEC 61000-4-15, Flickermeter.
- IEEE 519-2022, Standard for Harmonic Control in Electric Power Systems.
- EN 50160, Voltage characteristics of electricity supplied by public electricity networks.
- ANSI C84.1, Electric Power Systems and Equipment – Voltage Ratings (60 Hz).
Frequently asked questions
What are the most common power quality problems in an industrial plant?
Harmonics produced by drives and rectifiers, voltage sags from grid faults, transient overvoltages from lightning and switching, phase unbalance, flicker caused by fluctuating loads and low power factor. Each has different symptoms and solutions.
How do I know if the problem comes from the grid or from my plant?
By measuring at the same time at the grid boundary and at the suspect loads. If a disturbance appears first or stronger at the boundary and unrelated to what the plant runs, it comes from the grid; if it follows the start of certain machines, the plant produces it. The direction of harmonic power flow and the event logs help confirm it.
Which standard governs power quality?
IEEE 1159 classifies the phenomena and gives monitoring guidance; IEC 61000-4-30 defines how they are measured (Class A); IEEE 519 sets harmonic limits at the point of connection; EN 50160 describes the voltage characteristics of public networks in Europe. Grid codes and utility rules add the limits that apply locally.