Rapid troubleshooting for power quality anomalies
02 April 2012
Power quality problems are notoriously difficult to troubleshoot, as they often occur intermittently and may be result of a wide range of causes. Fortunately, there are instruments available that can help. A power analyser from Outram Research, for example, has helped a major US power utility to resolve a power quality problem for a residential customer quickly and with minimal effort.
Southern Company is a premier energy supplier serving the South Eastern USA. It recently received complaints of flickering lights from a residential customer with concerns that her electrical installation was at fault. Southern responded initially by installing a standard meter socket voltage recorder, but failing to find anything out of the ordinary using this device, the problem was referred to the company's specialist Power Quality group.
The instrument chosen to undertake measurements at the site was an Outram Research PM7000 power quality analyser. This uses a patented Single Cycle Adaptive Store data management technique to search for, and record, those waveforms most likely to be the cause of power quality problems.
The instrument was used to zoom in on the exact times that the flickering occurred, and to identify sags in the voltage that were followed, surprisingly, by very short periods of oscillation to even lower voltages. This detail illuminated the very small anomalies in an otherwise normal looking recording that enabled the source of the problem to be identified - a nearby sewage treatment plant.
This plant was using a solid state starter to apply 'waves' of reverse power in order to bring a large pump motor gradually to a standstill, thus preventing water hammer. Once the problem was identified, the residential customer was assured that her electrical installations were safe and that corrective action could be taken. Southern Company's Patrick Coleman takes up the story:
"A conventional recorder would have to be set on cycle store to get this level of detail, and we only set them to cycle store when we already suspect something like this is happening. Starting from scratch, we would have spent weeks zeroing in on something like this with a conventional recorder, but the PM7000 caught it on the first try, with no special set-up."
Outram believes that even those recorders that meet the full rigour of IEC61000-4-30 would not have captured this detail. This standard requires a record longer than 200ms and the voltage excursions measured by the PM7000 would have been much too small to constitute a dip. The evaluation method fundamental to IEC61000-4-30 would have filtered out and eliminated the very effect that needed to be spotted in this particular case.
Outram's Adaptive Store automatically records the chosen parameters in great detail and at high sampling rates when anomalies and deviations from the predicted trend occur. By automatically adjusting the thresholds distinguishing the anomalies from the trend as signal dynamics change or the available memory becomes full, the PM7000 ensures that less significant phenomena can be summarised and greater detail recorded for abnormal behaviour.
The instrument can record this level of detail over long periods - indefinitely if set in 'First In First Out' mode - and the recording sessions are automatically downloaded to a USB stick. Adaptive Store is considerably faster and simpler than setting and adjusting trigger levels, which is the norm with other power quality monitors. Basing his methodology on this process might see an operator taking weeks to 'zero in' on the problem while having to wade through much unnecessary data.
By taking quick action using the PM7000, Southern Company was able to identify the cause and resolve the problem in a short time frame, minimising the inconvenience to its customer.
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