As a power quality detective, you get to see it all — from flickering lights to overheated transformers and tripped drives. But humming conductors was a first for me! I'm not talking about the low, faint hum you might hear at a transformer. In this historic building, you could hear a loud harmonic oscillation from the basement to the attic on all six floors. To make matters worse, the conduit closets on each floor were located in the offices of some of the most influential senators in our nation's capitol.

The building's vintage 1940 elevator system had just undergone major renovations. However, little did anyone know that using state-of-the-art 12-step silicon-controlled rectifier (SCR) controls would create such a headache. Each time an elevator would jump to life and draw current on the 250 MCM conductors, the resulting harmonics riding on the 300-ft conductor made an awful sound that lasted for 20 sec or more. With the cycle of the elevators starting and stopping, the office workers got quite a tune.

Strange things can happen when you update old facilities with new technology. Wiring and voltages for buildings of this vintage are typically 208V or 220V. Yet manufacturers design today's systems and controls for 480V (as in this case). The building needed step-up transformers to match the two systems. Basement panels supplied the feeders for the elevators at 208V and stepped the voltage up to 480V using transformers located in the attic next to the elevator control rooms. The problem was obvious, but the toughest question was where to begin.

Drawing from experience, I knew how important it is to understand the power distribution system before you begin any troubleshooting work. This usually means locating or creating a one-line diagram for the building's electrical system. However, relying on someone else's one-line diagram can cause problems. Often, someone rearranges or adds loads without properly documenting this on the drawings. Therefore, I prefer to walk through the building and create my own one-line diagram as I go. Even if you think you already know all the particulars of a building, it's still a good idea to put them down on paper. With interacting loads and cyclic events, it always helps to understand the big picture. In fact, this provides a great opportunity to write down all your observations and concerns. With these things in mind, I began my power quality investigation.

Starting at the load itself, I began the process of collecting data at the line side to the step-up transformer for the elevator controllers. This included voltage, current, and peak currents, as well as harmonic content. I documented readings for each of the three phases and compared for imbalances between phases. I also took the same readings at the source down in the basement and compared them to the previous readings. Armed with this information, a picture began to form in my mind. Phase C had the lowest voltage (200V on a 230V system) and a total harmonic distortion of more than 30% (5 ^th and 7 ^th harmonics). That sent up a red flag. If you have current harmonic distortion of 20% or more, you should pay close attention to the system. If it's above 40%, you should consider corrective action.

The firing of the SCRs at the elevator controller generated large current spikes of a nonsinusoidal nature. Coupled with the long distance of the conductors from their source and the low voltages present, these factors combined to create a resonant frequency (distortion). After completing my investigation, I made the following recommendations to the facility's maintenance staff:

1. Replace the conductors due to age and the new stresses placed upon them. The newer insulation materials allowed for extra room in the conduits, which lowered the temperature inside the conduit.

2. Locate the step-up transformer in the basement at the source and power the long conductor run at 460V, at reduced currents.

3. If the staff detects further noise, place a line reactor midstream of the conductor run, and allow it to act as a wave trap.

4. Complete a separate harmonic survey once all elevators are retrofitted with the new controls to determine the effect on the standby generator.

All power quality problems are not as mysterious as the one described here, so it's important to remember some basic troubleshooting tips. If you suspect harmonics problems, you can use these techniques to your advantage: balance loads (if necessary, redirect loads with large harmonic content); oversize conductors and add neutral conductors for individual loads; derate transformers feeding harmonic loads; specify harmonic-rated breakers, transformers, and motors; and — most importantly — perform yearly power quality studies to check the amount/sources of harmonics in your building.

Turkel is a certified power quality engineer and consultant in Baltimore .

To gain quick insight into a potential harmonics problem, you can take an easy and fast measurement of some of your loads without investing thousands of dollars in specialized test equipment. Just remember: This test only gives you an indication that harmonics are a possibility. You should always conduct further testing.

Here's the equipment you will need for this quick test:

• One true RMS clamp-on ammeter. This meter must be capable of reading AC current that is distorted. This meter will be labeled as a true rms type meter and usually costs $200 to $400.

• A second clamp-on meter that is not true rms, but is an averaging type. This meter will not give an accurate reading of a distorted current wave shape and can be off by as much as 40% — depending on the amount of harmonic distortion. This type of meter will cost under $100. A digital type is recommended.

The first meter will give an accurate reading in spite of the harmonic content, because it is a true rms type. The second meter will give you an incorrect reading because of the harmonic content. Then, you can compute the difference between the two measurements to give you an approximate percentage of total harmonic distortion (THD) present on your system.

Once you have the two measurements, divide the average reading by the true rms reading. For example, 40A taken with the average type meter divided by 52A taken with the true rms meter gives a ratio of .769 or a harmonic distortion of 23.1%. As a rule, watch percentages greater than 20%; those over 40% require immediate action.

If any of these problems sounds familiar to you, then trouble could be brewing at your site.

1. Are circuit breakers tripping without overloaded conditions (false trips)?

2. Have you seen an increase in the number of motor failures?

3. Do you see an increase in operating temperatures of transformers or conductors, especially neutral conductors?

4. If you use power factor correction, have you had any unexpected trips or failures of this equipment?

5. Has any part of your power distribution system experienced unaccounted failures or trips?

6. Are conductors humming a tune?