Power Quality Article

ATSman

Senior Member
I found this article from EC&M magazine in my archives and thought it would
be a good topic for discussion on this forum:

I could not find a way to post the HTML link so here it is. Hope this does not violate the forum rules.

Taking the Mystery Out of PQ Problems
Stan Turkel, ATMS Technical Training Co.
Jun 01, 2000
Here's a true story from the files of a real power quality detective. Frank was just getting ready to sit down at his desk and enjoy his first cup of morning coffee when the door to the shop opens and in walks Jim, the third shift mechanic, with the bad news. "Good morning boss. How's it going?" he says. "You missed the show last night:: the fresh water booster pump in the mechanical room on the second

Here's a true story from the files of a real power quality detective.

Frank was just getting ready to sit down at his desk and enjoy his first cup of morning coffee when the door to the shop opens and in walks Jim, the third shift mechanic, with the bad news. "Good morning boss. How's it going?" he says. "You missed the show last night:: the fresh water booster pump in the mechanical room on the second floor burned up again. Security called the fire department. They thought we had a big one and were going to evacuate the whole building. Isn't that the same motor we replaced three weeks ago?"

Just as Frank begins to reply, his pager beeps. It's the vice president of the hospital. He probably wants to know what in the world happened last night.

Frank thinks back for a moment. How easy it was when he first started working at the hospital in 1970; long before strange things started happening to his facility. He remembered when a bank of high-hat lights blew out all at one time in the cafeteria. If that one didn't scare the cafeteria workers! That incident even led to rumors about dead patients haunting the hospital.

"No time to daydream," Frank mumbles to himself as he begins to pull out a purchase order to request another 7.5 hp motor. As he writes the order, he remembers to add a replacement 600A breaker for the penthouse substation. There's another strange situation. The breaker keeps tripping over the weekends, even though there's no overload. His maintenance crew put a recording amp meter on it, and everything checked out fine. "Either the breaker's just getting older like me, or ghosts are tripping breakers now," he muses.

Does this scenario sound familiar? Let's take a closer look at Frank's problems to find the real culprit.

When I got the call from Frank, I could tell he was desperate. When I told him my fee was $1200 a day, he simply asked how soon I could get there. I packed my bags that night, planning an early drive the next morning. I loaded up my equipment, which included two true rms digital multimeters, a true rms ammeter, a couple of handheld oscilloscopes, my handheld analyzer, a digital camera, and my safety gear. (Power quality work often requires you to work hot. That means making sure you're working safe. You don't get a lot of second chances in this line of work.)

As I walked into the lobby of the 500-bed hospital, I saw the plaque on the wall with the 1952 dedication ceremony listed. At this point, I knew I was taking a walk back in time. "This building probably had all of the original wiring," I thought.

As Frank and I began the power quality interview, I began to write down facts on my troubleshooting grid. Listening to Frank's story, I could tell he hadn't done his homework. When I asked for the motor shop's report on the cause of failure for the 7.5 hp pump motor (which he had rewound), he told me he didn't think to ask for one; same thing with the breaker in the penthouse. I asked for a logbook showing dates and times; again, he didn't have one. After a couple of hours of interviewing Frank, I had three troubleshooting grids: one for each complaint.

Using the troubleshooting grids, I associated each fact with a probable cause using a rating scale of 0 to 5. I rated the strongest relation as a 5 and no poss ible relation as a 0. Next, I summed the results for each probable cause and gave a ranking. The grid for the penthouse breaker suggested investigating harmonics first because it has a sum value of 21. The second area is a defective breaker, with a value of 7.

Upon further investigation of the breaker load, I learned the same substation fed the elevators and two new 125hp VFD drives for the chilled water pumps. The drives generated large amounts of harmonics back into the substation, which caused the 600A breaker to trip unnecessarily. But the big question still remained: "Why was the breaker tripping on the weekends only?"

During the week, the motor generator sets for the elevators absorbed the harmonics from the VFDs when they were online. In other words, the motor generator sets acted like line reactors by absorbing the harmonic currents generated by the drives. With some of the elevator banks turned off during the weekends, however, some motor generator sets were open and isolated from the station. This caused harmonic currents to travel a different route in the distribution system. Remember: The 7.5 hp motor also failed on the weekends. The harmonic currents traveled in that direction, taking out the motor on weekends.

Frank's power quality problems called for a rather simple solution. We sized and ordered line reactors for the VFDs. Until the hospital installed these units, we instructed the maintenance staff to keep all elevator motor generator sets on during the weekends.

"Back to business," says Frank to himself after implementing the power quality detective's recommendations. He sighs for a moment, thinking back on the probing questions and endless interviews of the past few days. But despite the recent chaos, Frank is optimistic for the future of his facility's operations. For now, he has conquered the invisible enemy. In the days to come, he plans to start his own log to help identify and track potential problems. So the next time ghosts decide to haunt his equipment, Frank knows he'll be ready. Feeling a true sense of accomplishment, Frank smiles and declares this case officially closed.

Turkel is a Senior Instructor, ATMS Technical Training Co., Owings Mills, M.D.


Sidebar: Just the Facts

Getting to the facts is always the hardest part of any power quality analysis. Too often, your client just wants to dump all this information on you, and half of it is typically just hearsay.

First, always ask, "How do you know?" when someone tells you something you need to rely on for evidence. For example, if someone tells you a breaker trips when a compressor starts, you might think that's a fact. But when you ask, "How do you know?" you might hear something like: "It sounds like the compressor," or "I think it's the compressor." Only troubleshoot with facts.

Second, find out, "What is it you want me to solve?" An open-ended statement like "Take care of my power problems" doesn't cut it. You need to know exactly what it is they're asking you to solve.

Third, always list each problem or complaint individually because you may have simultaneous problems. Besides, these problems may not directly relate to each other. Then, let the facts lead you to conclusions.
 

Besoeker

Senior Member
Interesting article. Resonates with what my line of work was. I've been associated with drives and power quality analysis all my working life.
Some quite large systems. But one system in particular sticks in my mind. And a tale I've told before.

We bid on a pumping station project with eight VFDs, the two larget being 200 kW IIRC. We made these 12-pulse to limit harmonic contribution.
The pumping station had a dedicated 11kV/400V transformer. The 11kV side was the point of common coupling with other consumers so that's where demonstration of compliance with the limits of G5/3 had to be done.

I had to do a before and after analysis. The result of the before one was a bit of a wake up call. I took a 24 hour recording. The results surprised me. The harmonic distortion exceeded G5/3 limits with none of our drives running. Puzzling. The pumping station was in the ancient city of St Albans in a residential area so no heavy industry. I reflected on this and was drawn to one conclusion. The growing number of domestic non-linear loads.

That was a valuable lesson from about 20 years ago.
 

Russs57

Senior Member
Got a kick out of that story because I'm a maintenance engineer at a hospital that opened on that date.

Don't mean to rant but I put forth considerable effort (without compensation) to try and stay current in my field and keep up with new technologies. I find those above me (and making a good deal more) generally don't. Yet they are the ones empowered to make decisions and often their ego prevents them from taking my counsel. Seems if some slick sales person assures them it is the right gizmo for their needs their butt is covered no matter how many problems this new gizmo causes. They just simply don't have the background to see why a line reactor, or a DC bus choke, might be needed.

Typically when stuff starts acting up, that had been reliable, it is a smart bet to look real hard at any changes that have recently been made. And in the average hospital changes are made every single day. The majority of them take place without your knowledge and without any engineering. Sad but true. You know, I had better stop posting the truth if I want to keep my job.

Across the pond, regulations are better, but I have seen no bigger offenders to power quality than the "personal computer".
 

junkhound

Senior Member
no bigger offenders to power quality than the "personal computer".

Back in early spring of 1990 or so, office building I worked in upgraded everyone's PC. No PFC front end in PC back then, straight full bridge rectifiers feeding a capacitor for the front end of 5V supplies.

Wonder of wonders, a few weeks later when weather warmed up a 50 kVA delta-wye transformer on the roof went up in smoke - PC triplet harmonics fried the delta winding.

edit to add - think the secondary neutral wire turned brown also :p
 
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