Make sure your power quality measurements are top of the class!

Poor power quality can lead to inefficient and potentially dangerous operation of electrical systems and may also cause equipment damage. Therefore, it is increasingly important not only to measure and monitor power quality, but also to be sure that the measurements are reliable and have been carried out using approved methods. Julian Grant of Chauvin Arnoux discusses IEC 61000-4-30, a standard that addresses these issues.

In a perfect world, your network operator would provide you with an electrical supply at a constant voltage, fixed frequency, and with perfectly sinusoidal voltage and current waveforms that have no nasty ‘spikes’ on them. If it’s a three-phase supply, you would expect the voltages of the three phases to be exactly the same. This would be perfect power quality. However, as we don’t live in a perfect world, your supply may not actually meet these requirements and, even if it meets them at the point it enters your premises, it may well become degraded as it passes through your site’s electrical installation.

It is, therefore, important to monitor power quality. The results can help you to improve energy efficiency, prolong the life of your electrical equipment, reduce failures, and even reduce the risk of fire. To meet the requirement for monitoring, numerous manufacturers developed power quality instruments but, each manufacturer used its own measurement parameters and methodologies. This created an element of uncertainty about the reliability of the results and has also made comparing results from different analysers difficult.

IEC 61000-4-30, which was published in 2003 and amended in 2021, resolves this problem by prescribing specific requirements for power quality, a broad term which covers the voltage, frequency and waveform of an electrical supply, and also by specifying methodologies for measuring power quality parameters.

For power quality analysers, the standard defines three performance classes:

  • Class A – These must meet the highest performance and accuracy levels to deliver reproducible results that can be reliably compared with those from other instruments.
  • Class S – Accuracy levels are less strict. Class S power quality analysers can be used for statistical surveys and contractual applications for which comparison of measurements is not required.
  • Class B – this class was introduced in the 1st and 2nd editions of the standard to avoid making existing instrument designs obsolete. In this class, the standard requires the measurement method and the accuracy to be specified by the manufacturer in the instrument’s technical data sheet. In Edition 3 of the standard, this performance class has been moved to an Appendix.

When buying a power quality analyser, users should look carefully at their operational requirements, and choose an instrument from the appropriate class to meet these requirements.

The IEC 61000-4-30 standard further defines the measurement methods, aggregation periods, and accuracy, for each of the key power quality parameters, which include:

  • Network frequency
  • Amplitude of the supply voltage
  • Amplitude of the current
  • Flicker (IEC 61000-4-15)
  • Dips and swells
  • Voltage interruptions
  • Voltage unbalance
  • Current unbalance
  • Voltage harmonics (IEC 61000-4-7)
  • Current harmonics (IEC 61000-4-7)
  • Voltage interharmonics (IEC 61000-4-7)
  • Current interharmonics (IEC 61000-4-7)
  • Mains signals
  • Rapid voltage changes (RVCs)
  • Current and voltage recording during events

RMS values are measured and calculated using multiple test methods and measurement durations. There are too many methods to include all of them in this article, but these are examples:

RMS values refreshed every half-period

This involves voltage (or current) values measured over one period, beginning with a zero crossing of the fundamental component and refreshed every half-period. Each measurement channel is independent and, for polyphase networks, this technique will produce RMS values at successive instants on each channel. This method is only used for detecting and assessing voltage dips, temporary overvoltages at system frequency, outages and rapid voltage changes (RVC).

Measurement over 10/12 periods (for 50 or 60 Hz supplies, respectively) corresponds to an aggregation of the measurement time intervals. The values over 10/12 periods are collected at three additional intervals:

  • 150/180 periods, or 3 seconds,
  • 10 minutes,
  • 2 hours for Plt measurements (flicker), aggregated from twelve 10-minute intervals.
Synchronization of the aggregation intervals for Class A, Source: IEC 61000-4-30

Harmonics and Interharmonics

In relation to harmonics and interharmonics, IEC 61000-4-30 is complemented by IEC 61000-4-7. This standard is applicable to instrumentation intended for measuring spectral components in the frequency range up to 9 kHz which are superimposed on the fundamental of the power supply system.

Values are calculated on 10/12-period windows, with a resolution (bins) of 5 Hz. These are harmonic subgroups, and between adjacent harmonic subgroups there is an interharmonic subgroup.

Illustration of subgroups Source: IEC 61000-4-7

The measurements must be performed at least once up to the 50th order. An interharmonic centred subgroup without discontinuities, Yisg,h., must be measured over 10/12 periods.

More details on events, flagged data, flicker, unbalance and mains signalling voltages can be found at www.cauk.tv/qualistar-8345-case-study/.

IEC 61000-4-30 is an important step forward in the measurement of power quality. It ensures that measurements taken by various instruments provide consistent, dependable results that can be readily and reliably compared. A measuring instrument can be designed to measure all or some of the parameters identified in the IEC 61000-4-30 standard and should ideally fall into the same performance class for all measurements.

Before stating that their instruments comply with IEC 61000-4-3, manufacturers must perform the tests laid down in the IEC 62586 standard. This rigorous testing has been carried out by Chauvin Arnoux on its new CA 8345 power quality analyser which complies fully with IEC 61000-4-30 Class A.

Equally well suited for diagnostic fault finding on electrical installations, auditing electricity consumption and validating the quality of electrical supplies, the CA 8345 combines exceptional flexibility with ease of use and extensive communication options. The CA 8345 sets new standards for versatility and convenience, and is suitable for use on all types of single- and three-phase systems up to 1,000 V.

www.chauvin-arnoux.co.uk