While we’ve enjoyed enormous benefits from the evolution of solid state technology, the fact is that the microelectronics at the heart of that technology requires clean power. Faster speeds and lower voltages mean that there is less and less tolerance for anything less than quality power. Power Quality (PQ) covers a wide range of issues, from voltage disturbances like sags, swells, outages and transients, to current harmonics, to performance wiring and grounding. The symptoms of poor PQ include intermittent lock-ups and resets, corrupted data, premature equipment failure, overheating of components for no apparent cause, etc. The ultimate cost is in downtime, decreased productivity and frustrated personnel. This application note gives you information on how to troubleshoot PQ problems. It also gives you information on how to start fixing those problems. But before grabbing that meter, please read the following cautionary notes:
1. Suggested measurements should only be made by qualified personnel who have been trained to make these measurements in a safe manner, using proper procedures and test tools rated for work on electrical power circuits.
2. To the best of our knowledge, recommended solutions are consistent with the National Electric Code (NEC), but in any case, NEC requirements must not be violated.
3. We have tried to make the information accurate and current, but it is not intended to be a substitute for the specialized knowledge and experience of professional power quality practitioners.
What this application note offers is a “starter kit”, not the final word on PQ troubleshooting.
START AT THE SCENE OF THE CRIME
To troubleshoot PQ problems, one approach is to start as close to the “victim load” as possible. The “victim load” is the sensitive load, typically electronic, that is somehow malfunctioning. Poor PQ is suspected, but part of your job is to isolate PQ as a cause from other possible causes (hardware, software?). Like any detective, you should start at the scene of the crime. This bottom-up approach can take you a long way. It relies on making use of a sharp eye and on taking some basic measurements.
An alternative is to start at the service entrance, using a three-phase monitor, and work back to the “victim load”. This is most useful if the problems originate with the utility. Yet survey after survey has concluded that the great majority of PQ problems originate in the facility. In fact, as a general rule, PQ is best at the service entrance (connection to utility) and deteriorates as you move downstream through the distribution system. That’s because the facility’s own loads are causing the problems. Another illuminating fact is that 75% of PQ problems are related to wiring and grounding problems!
For this reason, many PQ authorities recommend that a logical troubleshooting flow is to first diagnose the electrical infrastructure of the building, then monitor if necessary. Our bottom-up troubleshooting procedure is designed to help you do this detective work.
1. Make a map: Obtain or create a current one-line It’s tough to diagnose PQ problems without having a working knowledge of the site being investigated. You can start by locating or reconstructing a one-line diagram of the site. The one-line will identify the AC power sources and the loads they serve.
If you work on-site, the map might already exist in your head, but it will be a big help to everyone, including yourself, if it’s on paper. If you’re coming to a work site for the first time, getting an up-to-date one-line means identifying new loads or other recent changes in the system.
Why go to this effort? Systems are dynamic; they change over time, often in unplanned and haphazard ways. Furthermore, while some problems are local in origin and effect, there are many problems that result from interactions between one part of the system and another.
Your job is to understand these system interactions. The more complete your documentation, the better off you’ll be. It’s true, however, that the sites that need the most help are the ones least likely to have a good record of what’s going on in their system.
Many a consultant has earned his fee by upgrading the documentation handed him with what actually exists on-site. So the simple rule is, at this point in the investigation, do the best you can to get good documentation, but don’t count on it being available.
From: Power Quality & UPS Handbook, Vol 11, The Electricity Forum