Looking beyond the symptoms for power quality problems, solutions

Sometimes, the reality of a situation is quite different from how it appears. The real problem may be cloaked by what appears to be symptoms of a completely different issue. Knowing how to look beyond the surface problems and having the tools to do so eliminates many headaches in the long run. This is particularly true when it comes to maintaining plant electrical systems.

02/01/2009


Sometimes, the reality of a situation is quite different from how it appears. The real problem may be cloaked by what appears to be symptoms of a completely different issue. Knowing how to look beyond the surface problems and having the tools to do so eliminates many headaches in the long run.

This is particularly true when it comes to maintaining plant electrical systems. A December 11, 2008 Webcast, hosted by Plant Engineering magazine and sponsored by Fluke Corp., focused on how to search for cures to power quality problems, instead of just doctoring the symptoms.

 

After the speakers concluded their presentations, a lively question and answer session ensued. Some of these questions touched on maintenance aspects of the power quality issues encountered in manufacturing plants.

 

The two panelists, Rudy Wodrich, P.Eng., MBA, director of the Power Quality Group, Schneider Electric; and Dan Carnovale, power quality solutions manager, Eaton Electrical, responded to these questions.

 

Q: What is the best type of instrument for power quality?

 

Wodrich: “It depends on what you think your problem is. Some problems are there all the time. Harmonics are steady-state phenomenon; if the load is running, harmonics are there. There is some variability to that, based on production variability and so on, so you might want to do some time-trending. Quite often, a portable instrument is suitable for capturing the steady-state types of phenomenon.

 

If you’re having a transient event, it’s a whole different story because you can’t predict when the next transient will occur, and what’s happening on the distribution system because the symptoms of a voltage sag can be similar to the symptoms of a voltage transient. You need instruments that are capable of very quick response and high sample rate. You may want to look at panel-mounted devices, rather than using portable instruments.

 

There are many makes and models of instrumentation available, both portable and permanently-mounted, that can do all of these things. It really is driven by what kind of problem you think you have and what you want to capture.

 

Carnovale: One of the things you should consider is whether the instrument monitors all the things you want to consider all the time; or whether you are trying to troubleshoot one thing at a time, harmonics or transients or sags. If you’re looking at a permanently-mounted instrument, my recommendation is that for high-speed transients, you should have a high-speed transient capture on the main incoming to pick up events such as lighting or other high-speed transients. If you have transients that are downstream, you may not want to invest that kind of money into the system. That’s more typical for a portable type instrument. But for sags on the main incoming, you want to be able to capture the magnitude and duration of those events.

 

Q: How important is it to keep phase rotation the same at each panel throughout the facility when using a three-phase system?

 

Carnovale: For single-phase systems, it’s not as critical. But, certainly on a three-phase system, consistency is important. While troubleshooting problems, I’ve been into panels with 16 different colors, and it’s very difficult to figure out what was going on originally when the system was set up. I think it’s very important to maintain consistency.

 

But as far as the loads are concerned, there won’t be a significant difference in the way that the harmonics or the power system, cancellation or other things occur, unless you use transformers between the two parts of the system, which will naturally make that happen.

 

Wodrich: From the load perspective, it’s not particularly critical. It does get to be a nuisance more than anything else when you are in troubleshooting mode and if you installed something, whether it’s a metering product or a harmonic mitigation product such as an active filter where you are using multiple CTs and inputs, and the phase rotation between the CTs and the active filter are critical, so there it becomes important. For motor loads, it’s not particularly important. I don’t think there’s any criticality for drives either.

 

Go to www.plantengineering.com to download the original Webcast “Power quality: Looking beyond the symptoms.”



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