Test & measurement - Power quality measurement...What does Class A mean to me?

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Power quality measurement is still a relatively new and quickly evolving field. Whereas basic electrical measurements like RMS voltage and current were defined long ago, many power quality parameters have not been previously defined, forcing manufacturers to develop their own algorithms.

There are now hundreds of manufacturers around the world with unique measurement methodologies. With so much variability between instruments, technicians must often spend time trying to understand the instrument’s capabilities and measurement algorithms instead of concentrating on the quality of the power itself.
The IEC 61000-4-30 CLASS A standard defines the measurement methods for each power quality parameter to obtain reliable, repeatable and comparable results. It also defines the accuracy, bandwidth, and minimum set of parameters. Going forward, manufacturers can begin designing to Class A standards, giving technicians a level playing field to choose from and increasing their measurement accuracy, reliability, and efficiency on the job.
IEC 6100-4-30 Class A standardises measurements of:
• Power frequency
• Supply voltage magnitude
• Flicker, harmonics, and inter-harmonics (by reference)
• Dips/sags and swells
• Interruptions
• Supply voltage unbalance
• Mains signalling
• Rapid voltage changes.
Examples of Class A requirements:
• Measurement uncertainty is set at 0.1% of declared input voltage. Low cost power quality measurement systems with uncertainties greater than 1% can erroneously detect dips at -9% when the threshold is set at -10%. With a Class A certified instrument, a technician can confidently classify events with internationally accepted uncertainty. This is important when verifying compliance to regulations or comparing results between instruments or parties.
Dips, swells and interruptions must be measured on a full cycle and updated every half cycle, enabling the instrument to combine the high resolution of half-cycle sampled data points with the accuracy of full-cycle RMS calculations.
• Aggregation windows – A power quality instrument compresses acquired data at specified periods which are called aggregation windows. A Class A instrument must provide data in the following aggregation windows:
- 10/12 cycle (200ms) at 50/60Hz, the interval time varies with actual frequency
- 150/180 cycles (3s) at 50/60Hz, the interval time varies with actual frequency
Harmonics must be measured with 200ms intervals according to the new standard, IEC 61000-4-7 / 2002. The old standard allowed 320ms intervals which cannot be synchronised with the 200ms aggregation windows of other Class A measurements.
Using 200ms intervals allows harmonic calculations to be synchronous to all the other values like RMS, THD, and unbalance.
The Harmonics FFT algorithm is specified exactly such that all Class A instruments will arrive at the same harmonic magnitudes. The FFT methodology allows for infinite algorithms that can result in vastly different harmonic magnitudes. By standardising on 5Hz bins and summing the harmonics and inter-harmonics according to specific rules, Class A instruments will be consistent and comparable.
• External time synchronisation is required to achieve accurate timestamps, enabling accurate correlation of data between different instruments. Accuracy is specified with ±20 ms for 50Hz and ± 16.7ms for 60Hz instruments.
• 10 min interval sync to clock
• 2 h interval sync to clock.
Latest product developments
There have been a number of significant introductions to the market in the past 12 months of power quality analysers offering compliance with IEC 61000-4-30 CLASS A. These new products include both handheld devices and those designed for leaving in a fixed location for a time period set by the user. They will log a large number of parameters at user chosen time intervals for later analysis by a PC. Thus there is a choice of products, offering different capabilities, from which a technician can choose the most appropriate tool for the job.
These new tools are designed for ease-of-use to uncover intermittent and hard-to-find power quality issues. Suitable handheld analysers will provide on-screen display of trends and captured events even while background recording continues. Some can be used to analyse disturbances, to validate incoming power compliance, for capacity verification before adding loads, and for energy and power quality assessment before and after improvements. The best tools provide powerful reporting software to enable rapid assessment of the quality of power at the service entrance, a substation or at the load according to EN50160 standards. The software can quickly analyse trends, create statistical summaries and generate detailed graphs and tables.

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