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Electrical Connection: Case study, harmonic distortion of current

 

Case Study 5 - Harmonic Distortion of Current

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Environment

Office building with personal computers, terminals, copiers and other electric office equipment supplied by three-phase wye service.

 

Problem

Facility engineers at this site experienced repeated problems with the failure of electrical distribution equipment. A distribution transformer overheated and failed, circuit breakers were tripping and electrical connectors were burning out. These problems are all symptomatic of overload conditions.

However, initial measurements of phase currents using a true RMS ammeter showed current readings of 257 to 298 amps. These values did not exceed equipment ratings.

 

Measurements

The real problem started to become apparent when readings were taken of the current in the common neutral conductor. The neutral was carrying 229 amps, nearly equal to the phase currents, even though the phase loads were well balanced.

Further analysis was performed using a power monitor. Figure 1 shows the wave form of Phase A current. The non-sinusoidal shape is due to harmonic currents typical of switching mode power supplies which are used in the majority of modern office automation equipment. These are nonlinear loads. The peak current shown here is 475 amps. If the wave form was sinusoidal, its peak current would be only 363 amps. As shown in the plot of Figure 2 total harmonic distortion is about 32 percent, of which the third harmonic contributes about 31 percent.

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Figure 1. Non-sinusoidal Phase A current wave form caused by nonlinear loads.

 

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Figure 2. Plot of Phase A current harmonics showing high third harmonic content.

 

When phase currents are distorted to this extent, the normal three-phase cancellation, which results in near zero neutral current, does not take place. The odd harmonics produced at 180 Hz, 300 Hz and higher frequencies in the phase conductors result in large currents being carried by the neutral at predominantly 180 Hz. This is shown in Figure 3.

 

The net effect on facility wiring is that the common neutral conductor will frequently be carrying current beyond its rated capacity. In severe cases this can well exceed phase currents. These high frequency currents can be damaging to transformers and other devices designed to operate at 60 Hz.

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Figure 3. Common neutral current wave form showing predominance of 180 Hz, modulated by 60 Hz fundamental.

 

Solution

In the short term these problems can be addressed by over-sizing neutral conductors and de-rating transformers to a more conservative value of 60 percent.

 

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Electrical Resources

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Courses and Training

Products

Miscellaneous

 

IEEE Kansas City Section and Society Chapter Meetings:

 

Need to pass an electrical exam?

So, you need a state license and must pass a code exam to get it. You can choose between taking the test and hoping for the best (the usual approach), or preparing for the exam and passing with flying colors the first time.

Let’s assume you don’t like to waste time and money, and don’t want to wait six months or longer for a retest. That assumption means you must know how to prepare for the exam.

To prepare for electrical code exams, click here.

Step 1: Buy and review a current code book. You can’t know the code unless you read it.

  • See how it’s laid out. Notice which sections provide the definitions, purpose, limitations, and applicability of the code. Then, notice which sections apply to general circumstances and which to special circumstances.
     
  • Carefully read the definitions. Make it a point to carefully read one definition each day between now and the test day.
     
  • Note the major divisions, categories, or chapters. You may find it beneficial to read a separate book on those general topics you have no knowledge of—don’t count this toward your exam study time, but do it if you need to. If time is short, skim the supplemental books—read the headings, then read the first three chapters. That’s generally all you’ll need to do to get a general understanding of the topic.

Step 2: Study

  • Obtain an exam prep course (we offer these for the National Electrical Code).
  • Set aside 6 hours per week to study. One way to divide up the 6 hours is to use one hour every night except Wednesday, and then study for 2 hours on Saturday. Whatever regimen you set up, make sure you stick with it. Provided you have several months prior to the exam, it will not be necessary to study more than the 6 hours. If you have less time than that, schedule two or three sessions with someone who can tutor you, and interleave these with your self-study. Trying to proceed with too difficult a study program leads to burnout.
     
  • Don’t have a supply of chips and other junkfood on hand. Declare your study time a "no food zone." Drinking water is fine—nothing else, though. The last thing you want to do is emerge from your study efforts with two additional inches of waistline. Absolutely no food or water while practicing taking the exam.

Step 3: Practice.

  • Work all the practice questions that come with the exam materials.
     
  • Make a photocopy of the practice exam that was in your exam prep materials. If you have no such exam, prepare your own. A few weeks before the exam, take your sample exam once, and then carefully research your answers. Study again wherever you had difficulty. This differs from taking a previous exam, because in this case you have the answers. If you do develop your own, try simply modifying the previous exam and coming up with answers. When you take the exam as practice, do so under the exam time limits.
     
  • Two weeks before the real exam, take the sample exam again. Study where you had weaknesses.

Passing a code exam is tough for most folks, but easy for some. Follow these tips and you’ll be in that second group.

 

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