At the recent Power Quality World Conference, I chaired a session on “Specifying and Purchasing PQ Equipment.” Afterward, I met one of the presenters from an electric utility who said he planned to bring his lawyer with him to the session the next day.
I thought he was joking, until he arrived the next morning, lawyer in tow. I have been on some panels that covered controversial topics before, and seen people get upset by a point raised by a presenter. But none has ever required that an attorney be present, either during or after the session.
As it turns out, it was one of the most informative presentations on power quality that I have heard in a long time. The gist of the presentation by both the lawyer and utility engineer was the different legal obligations of either being the manufacturer or the user of power quality monitors or mitigation equipment, and what legal rights a “recipient” of the information or results of such equipment has.
In one of the cases cited, there was a data processing facility that was having trouble with keeping equipment running and productivity high enough to be profitable. The facility manager called in consultants, including a representative from the local utility. An off-hand remark by the utility person about the quality of the supply provided by the electric utility became the basis of a $42 million lawsuit when the company went out of business. They claimed poor power quality caused the equipment problems and low productivity, bankrupting the company. The improper comment by the utility person was at the center of their case.
Fortunately for them, the expertise of their legal counsel was able to show that the company’s problems were not really PQ-related, and the suit was rejected (though it is still in appeal).
One of the key points that the presentation brought out is not to make judgements in haste, nor comments (no matter how casual) outside of one’s area of expertise. First, get all the facts from the facility people, and then repeat the facts as you understand them back to the facility people, to make sure you have it right.
Next, make sure that the instrument is properly set up to record the data accurately. One of the most common errors that I hear is claims of excessive current THD, only to find out that they were measuring less than 1 percent of the full-scale rating of the CT. Clamp-on CTs typically require a certain amount of current just to energize the probe, which in many cases is 10 percent of the full scale value before the accuracy numbers are real.
Similarly, having 0.5A of harmonic current out of 1A total on a 30A circuit is rarely a harmonic problem. To tell the facility manager that they better install harmonic filters because they have 50 percent would be doing a disservice to everyone involved.
Knowing the wiring configuration that you are connecting to and connecting the measurement probes accordingly is another common mistake that leads to erroneous power measurements. A delta circuit will have a 30-degree phase shift between voltage and current even with a resistive load. That could be interpreted as a Power Factor of 0.866 if one wasn’t monitoring the proper wiring configuration.
Another one is having the wrong CT associated with the voltage channel; once the watts are calculated on a sample-by-sample basis, they can’t be accurately post-corrected, even if the problem is discovered later. Sizing transformers and distribution feeders based on data that is off by the square root of three or more isn’t good advice, legal or otherwise.
Assuming the data has recorded correctly, now it must be interpreted properly.
Reviewing the voltage and current data can often indicate the direction of the PQ phenomena (source or load) of whether a sag, swell, transient or harmonic. But the rules aren’t always foolproof, and can be influenced by a variety of factors that may not be obvious at first glance. For example, harmonic directivity can often be determined at a single feeder by the phase angle between the harmonic voltage and harmonic current. If it is greater than 90 and less than 270 degrees, this would usually indicate that the power is flowing in the opposite direction of the arrow on the CT, which should be point downstream or toward the load. Hence, it is assumed that to be from the source in that case.
However, at the point-of-common-coupling or when multiple feeders are connected at a substation, these rules can break down, making the direction usually undeterminable.
Explaining what the data shows and what the logic is being your interpretation or opinion of the problem and its source to the facility person can go a long way to prevent misunderstandings and legal hassles later on. You aren’t being called on to be an expert witness; you are providing your interpretation based on your PQ knowledge.
Obviously, the more you learn, the more likely your opinion would be accurate. The more accurate your opinion, the less likely that you may find yourself on the witness stand, trying to explain to a judge just what you meant. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.