Measurement method

A very important criterion for the correct and repeatable measurement of power quality is compliance with standards for the measurement procedure. These are not to be confused with power quality compliance standards.

In addition, there are striking differences to measuring instruments for power quality (PQI), electrical safety in low-voltage networks (PMD) and measuring transducers (Transducer).

The Classical Differentiation Matrix of Measuring Instruments in the Context of Application

Standard for the measurement procedure according to IEC 61000-4-X

  • IEC 61000-4-30 Ed. 3
    Method for measuring the power quality of the network.
    According to chapter 5.9.1 “Measurement method”:
    Measurement up to the 50th harmonic (bandwidth of 2.5 kHz at 50 Hz, required sampling rate of at least 4.5-5 kHz).

Evolutionary steps of the editions

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Innovations in IEC 61000-4-30 Ed. 3

  • The current measurement is binding for class A devices
  • Recording of level, unbalance, harmonics and interharmonics of the currents in the same interval as the associated voltage channels
  • Measurement method for rapid voltage changes (RVC) added

Innovations in IEC 61000-4-30 Ed. 4

  • High sampling rates (bandwidth coverage from 2.5kHz – 150kHZ)
  • Real-time capability
  • Fast transient measurement
  • PTP time synchronisation
  • The introduction of the standard is planned by the IEC for the next 2-3 years.

METAS (Swiss Federal Institute of Metrology) provides a measurement and testing infrastructure for Phasor Measurement Units (PMUs) on power quality quantities. As a result, METAS is one of only a few laboratories worldwide that can calibrate, test and certify PMUs according to IEEE C37.118 and power quality measuring instruments according to IEC 62586. The PMU measuring station allows voltage and current signals to be generated UTC-synchronised and is traceable to the SI International System of Units through calibration.

Definition of classes

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Class A (PQI)

Measuring instruments according to IEC 61000-4-30 Class A provide measured values that are comparable across measuring instruments and manufacturers. Measured values from class S measuring devices are no longer to be considered comparable.

  • IEC 61000-4-7
    Guide for measurement of harmonics / interharmonics
  • IEC 61000-4-15
    Flickermeter design specification

For legal certainty in terms of comparability of measurement data, it is recommended to use metrologically certified measuring instruments. Another advantage is that the relevant measurement data are “quasi-calibrated” by the certified type test.

Class S (PQI)

IEC 61000-4-30 class S power quality analysers are intended for basic / advanced power quality analysis and provide useful monitoring data. Instruments that meet Class S performance requirements are used for statistical power quality surveys and contractual applications. There are no potential disputes to be suspected. Therefore, comparability according to class A is not mandatory. The accuracy and performance requirements for Class S are less stringent than for Class A. As a result, they are also set at a lower price. As a result, they are also set at a lower price. Class S measuring instruments are often used in industrial and supply engineering (DSPQ).

Electrical safety in low voltage distribution systems – Equipment for testing, measuring or monitoring of protective measures according to IEC 61557 (PMD)

This part of IEC 61557 specifies requirements for combined performance measuring and monitoring devices (PMD) that measure and monitor electrical parameters in electrical distribution systems. These requirements also specify the operating behaviour in single-phase and three-phase AC or DC systems with rated voltages of up to AC 1,000 V or DC 1,500 V. These devices are either permanently installed or portable. They may be intended for indoor and/or outdoor use.

Measurement of power quality in power supply systems according to IEC 62586 (PQI)

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IEC 62586-1

IEC 62586-1 specifies product and performance requirements for equipment whose functions include measuring, recording and possibly monitoring power quality parameters in power systems and whose measurement methods (Class A or Class S) are defined in IEC 61000-4-30 .These requirements apply to single-phase, two-phase (split) and three-phase AC supply systems with 50 Hz or 60 Hz.

IEC 62586-2

IEC 62586-2 specifies functional tests and uncertainty requirements for equipment whose functions include measuring, recording and possibly monitoring power quality parameters in power systems and whose measurement methods (Class A or Class S) are defined in IEC 61000-4-30. This document applies to IEC 62586-1 power quality equipment. This document may also be referenced by other product standards (e.g. digital fault recorders, MV or HV protective relays) that specify equipment in which Class A or Class S power quality functions are embedded in accordance with IEC 61000-4-30.

Electrical devices for the conversion of alternating current quantities into analog or digital signals according to IEC 60688 (measuring transducers)

This International Standard applies to transmitters (transducer) with electrical inputs and outputs to measure AC or DC electrical quantities. The output signal can be an analog DC current, an analog DC voltage or it can be in digital form. In the latter case, the part of the transmitter used for communication purposes must be compatible with the external system. This standard applies to transmitters used to convert electrical quantities such as current, voltage, active power, reactive power, power factor, phase angle, frequency, harmonic or harmonic content (distortion), apparent power into an output signal.

This standard is not applicable to instrument transformers conforming to the IEC 60044 series of standards, transducers for industrial process control conforming to the IEC 60770 series of standards, and instruments for measurement and monitoring of operational behavior (PMD) conforming to the IEC 61557-12 standard.

Advantages of metrological certification for standard-compliant measuring instruments

Although the requirements for a power quality device are precisely defined both in terms of measurement methods (IEC 61000- 4-30), device characteristics (IEC 62586-1) and testing of compliance with the standards (IEC 62586-2), there are nevertheless differences between manufacturers. In particular, providers are often unable to demonstrate why their analyser complies. So why should it measure correctly.

Proof of a truly correct measurement is only possible via an independent certification agency. In the best case, by a metrological institute. Non-certified test bodies or even self-declarations of the manufacturers cannot replace metrological certificates. Therefore, these should also be viewed critically.

All instruments of Camille Bauer Metrawatt AG are metrologically certified according to METAS

Examples (association):

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Only calibration and traceability to the SI unit can ensure that 1 kg is really 1 kg and is displayed in this way.

The LINAX PQ3000 power quality meter shows 237.4 V. Due to the independent metrological certification, you can also be sure that exactly 237.4 V is applied.
The certification of a power quality meter according to IEC 62586-2 requires more than 150 tests, some of them complex. For this purpose, an elaborate testing infrastructure, traceable to the International System of Units SI by calibration, is necessary.

Note

The above-mentioned measurement methods and procedures according to the IEC standards are often integrated and requested in tenders. This article should help to provide some clarity in this area. However, this chapter should certainly not be regarded as conclusive.

Special note: Please do not disregard the necessary sensors when integrating PQI. The measurement can only be as accurate as the accuracy of the worst link in the measurement chain allows (e.g. current transformer, Rogowski coils).