Power Quality Analyzer: Class A, Harmonics and Flicker

What it is and where it goes

A power quality analyzer is an instrument that records, with standardized methods, how the voltage departs from an ideal sine wave and how a load’s current contributes to it. It can be portable, for measurement campaigns of days or weeks, or fixed, permanently installed at a point of common coupling, in a substation or in an industrial panel.

At medium and high voltage it connects to the secondaries of current transformers and voltage transformers; at low voltage, directly or with current clamps.

Main parts

• Voltage inputs (normally four, with neutral) and current inputs (clamps, flexible probes or 1 A and 5 A inputs).
• Analog-to-digital conversion synchronized with the grid frequency.
• Processor that calculates each parameter according to IEC 61000-4-30.
• Memory for trends, events and waveforms.
• Clock with external synchronization, by GPS or NTP.
• Software for download, analysis and reports.

How it works

Measurement intervals. The basic Class A value is calculated over 10 cycles at 50 Hz or 12 cycles at 60 Hz, about 200 ms, with a window that follows the actual frequency. These values are aggregated into 150/180-cycle intervals (about 3 s), 10-minute and 2-hour intervals; the 10-minute and 2-hour ones are aligned with the clock. Fluke states that the required time uncertainty is ±20 ms at 50 Hz and ±16.7 ms at 60 Hz, which requires external synchronization.

Parameters. IEC 61000-4-30 covers frequency, voltage magnitude, flicker, sags and swells, interruptions, transients, unbalance, harmonics and interharmonics, rapid voltage changes, mains signaling and, for current, magnitude, harmonics and unbalance.

• Harmonics and interharmonics per IEC 61000-4-7, with the same 200 ms window as the other Class A measurements.
• Flicker per the standardized flickermeter of IEC 61000-4-15, which gives the short-term index Pst and the long-term index Plt (2-hour interval).
• Sags, swells and interruptions: the RMS value is calculated over one full cycle and updated every half cycle. Each event is described by its depth or magnitude and its duration.
• Unbalance: ratio of the negative-sequence (and zero-sequence) components to the positive-sequence component.

Classes. Class A (“advanced”) is for contractual measurements and for comparing instruments; Class S (“surveys”) is for statistical surveys. The 2025 edition of IEC 61000-4-30 removed the annex for the former Class B. The tests to verify that an instrument meets its class are in IEC 62586-2.

Variants and selection criteria

• Portable Class A for studies and compliance verification.
• Fixed Class A at points of common coupling, substations and plants with sensitive or disturbing loads.
• Multifunction meter with quality functions: useful for continuous monitoring, but check which parameters it really meets at the stated class.
• Disturbance recorder with quality functions, which shares hardware with the fault recorder.

To choose, define the purpose (compliance, diagnosis or monitoring), the parameters, the number of channels, the memory, the synchronization and the voltage level of the point.

Key specification data

• Class per IEC 61000-4-30, by parameter.
• Number of voltage and current channels, ranges and sensor type.
• Maximum harmonic order and interharmonic measurement.
• Waveform recording capacity and configurable triggers.
• Time synchronization.
• Measurement category and insulation for the connection site.

Integration with protection and control

The analyzer does not protect or trip. Its data serve the power quality study and, in fixed installations, are sent to the control system. IEC 61850-7-4 has logical nodes for these quantities: MHAI (harmonics and interharmonics), MFLK (flicker), MSQI (sequences and unbalance), QVVR (voltage variations) and QVUB (voltage unbalance).

IEEE 519 is the most widely used reference for evaluating harmonics at the point of common coupling. It sets voltage distortion limits that fall as the voltage of the point rises, and current limits that depend on the ratio between the short-circuit current and the user’s load current. The evaluation is statistical, using percentiles of the measured values. Use the current edition for the exact values.

What a recording is compared against

Measuring well is half the work; the other half is knowing which value to compare against. IEC 61000-4-30 defines how to measure, not what limit to require. The limits come from another document, depending on the purpose:

• Characteristics of the supplied voltage. In Europe, EN 50160 describes the voltage characteristics of public low- and medium-voltage networks and evaluates the 10-minute values over one week. For low voltage, the technical references agree on these values. The voltage magnitude must stay within ±10% of nominal for 95% of the week. Long-term flicker (Plt) must be 1 or less for 95% of the week. Voltage unbalance must be up to 2%, also for 95% of the week. Total harmonic distortion must be up to 8% for 95% of the values. Frequency must be 50 Hz ±1% for 99.5% of the year. Use it as a reference for what counts as a normal supply, bearing in mind that 60 Hz systems follow their own national regulation.
• Local grid code and regulation. Each country or regulator sets its own power quality standards, usually with steady-state voltage bands (often around ±10% of nominal) and time limits. Because regulators revise these rules, a report should cite the document and the version that was applied.
• Contract or point of connection. Harmonic limits at the point of common coupling come from IEEE 519, and those for an industrial customer come from the contract with the utility.

The same instrument, properly configured, supplies the data for all three comparisons; what changes is the evaluation window and the percentile. That is why the objective of the study must be written down before starting the campaign.

How a measurement campaign is planned

• Duration. A full week is the usual minimum when evaluating against a weekly statistical criterion, such as that of EN 50160, because it captures the load cycles from Monday to Sunday. For a harmonic study of a load with monthly cycles, the campaign extends to that cycle.
• Measurement point. Measure at the point of common coupling to evaluate compliance, and at the feeder of the suspected load to find the source. A single point answers only one of the two questions.
• State of the network during the campaign. Note the switching operations, capacitor banks, motor starts and utility tests; without that log, an anomaly has no explanation.
• Quality of the instrument transformers. At medium and high voltage, protection or metering CTs and VTs limit the bandwidth and accuracy of high harmonics. For fine harmonic analysis, first ask whether their frequency response is adequate; the IEEE 519 harmonic study starts from that question.
• What to deliver. A report that states the instrument class, the aggregation interval, the evaluation window, the criterion used and the out-of-limit measurements with date and time, together with the operating events at that moment.

For specific cases see voltage sags in industry, harmonics of variable-frequency drives and power quality in industry.

Commissioning and maintenance tests

• Connection: each voltage matched to its current, clamp direction, and the CT and VT ratio configured.
• Phasor diagram when the recording starts.
• Time synchronization verified.
• Event thresholds configured according to the purpose of the study.
• Periodic metrological verification in a laboratory for Class A instruments.

Some portable analyzers have their own guide: the Metrel MI 2892 and MI 2893 (Class A), the Metrel MI 2885 and MI 2883 (Class S) and the Janitza MRG Flex and Unipower Unilyzer.

Common faults and what to check first

• Negative power: reversed clamp.
• High voltage distortion on one phase only: check the cable and connection of that phase first.
• Thousands of sag events: threshold too close to nominal voltage, or reference voltage declared wrongly.
• Gaps in the records: memory full or power supply interrupted.
• Different results between two instruments: one is not Class A for that parameter or is configured differently.

Power quality with Tiza Group

Do you have trips or equipment failures that you suspect are caused by power quality? At Tiza Group we review the records and the relay and meter configuration with you remotely through our remote relay configuration. For the on-site measurement campaign with Class A analyzers and the IEEE 519 report, see our power quality study, for substations and industry, in Colombia or elsewhere on request.

References

• IEC 61000-4-30: power quality measurement methods (Classes A and S).
• IEC 61000-4-7: harmonic and interharmonic measurement.
• IEC 61000-4-15: flickermeter.
• IEC 62586-2: functional tests and uncertainty of power quality instruments.
• IEEE 519: harmonic control in power systems.
• IEEE 1159: monitoring electric power quality.
• IEC 61850-7-4: logical nodes MHAI, MFLK, MSQI, QVVR and QVUB.

Frequently asked questions