Is voltage waveform distortion the culprit for equipment malfunction or overloaded power sources? Find out how to extend your equipment's life.
The scene is all too common. Technicians and electricians investigating voltage quality problems uncover distorted voltage waveforms at individual power sources or at the end-use equipment locations. These waveforms often have a distinctive flattopping characteristic: where the waveform appears cut off at the positive and negative peaks. What's behind the invisible culprit?
Typical causes. Voltage waveform distortion typically relates to electronic equipment, which has an internal switch-mode power supply (SMPS) that draws a nonlinear current waveform. Whereas a linear load produces a sine wave current, the SMPS draws current pulses at only one portion of the applied voltage waveform. This nonlinear current increases with each added device. The nonlinear current, combined with the impedance of the circuit conductors and the power source, creates a voltage drop you see at the peak portions of the voltage waveform. The heavier the loading, the greater the root-means-square (rms) voltage drop. Recent changes in SMPS design and efficiency decreased the amplitude of current and, therefore, the amount of voltage waveform distortion.
Effects on equipment. Most equipment can operate properly with little voltage waveform distortion and still maintain a regulated DC output. However, with greater distortion, efficiency of the power system and loads decreases. Feeder and branch-circuit lengths also affect the amount of distortion. Overloaded branch circuits and associated wiring can't accommodate increased current flow.
Investigation techniques. How do you know if voltage waveform distortion is the culprit for equipment malfunction or overloaded power sources? One of the first items to investigate is the AC electrical panelboard feeding the affected equipment.
1. Check for tight connections. Surprisingly, this simplistic solution is one of the most overlooked.
2. Perform true rms current measurements around each of the phase conductors. Make sure there are no overloaded conductors. Verify there is not a significant imbalance between any of the phase conductors. Also, make sure the individual phase conductor currents do not exceed 20% of the average phase current.
3. In instances where significant waveform distortion exists, you'll find the neutral conductor current equal to or greater than the average phase current. Here's a general rule of thumb: Verify the neutral conductor current is at least 15% less than the average phase current.
4. Compare the phase-neutral voltages at the panelboard with phase-neutral voltage measurements made at the branch circuit. If the voltage drop is greater than 3% across the branch circuit, then it's possible you've overloaded a particular circuit, or a loose connection exists on the hot or neutral conductors.
5. Always make sure and measure the voltage across the circuit breaker contacts. This is a good indication of the unit's integrity. If a voltage drop of more than 50mV exists across the contacts, replace the breaker.
6. Use a harmonic analyzer to determine the total harmonic distortion (THD) for the various areas of the facility. Using IEEE Std. 519 as a guide, verify the voltage THD does not exceed 5%. Also make sure the phase currents do not exceed 20% THD.
Even though Std. 519 deals specifically with the point of common coupling (PCC), it's good practice to carry over the same logic to the secondary of isolation transformers.
Solutions. Consider the following approaches to address and troubleshoot voltage waveform distortion problems.
• Reduce transformer loading. Where a transformer provides power to nonlinear loads, you should not load it to more than 60% of its kVA nameplate rating. This practice reduces distortion and prevents overheating.
• Increase the size of the neutral conductor. This reduces the impedance of the neutral conductor path. A lower neutral conductor impedance decreases the voltage drop and reduces waveform distortion. Unfortunately, you can only make this type of wiring modification when you first install the electrical distribution.
• Limit the length of feeder circuits to less than 200 ft. This reduces the overall impedance of the power system.
• Limit the length of branch circuits to less than 50 ft.
• Eliminate shared neutral conductors and "daisy-chained" circuits. When wiring for electronic equipment, it's best to wire no more than five outlets off of a single circuit breaker.
• Reduce the amount of load on a branch-circuit panelboard. Voltage waveform distortion exists on every site. If it's a cause of equipment malfunction or overloaded power sources, the above recommendations should improve the efficiency of your system and extend the life of your equipment.