Half a Dirty Dozen: Common PQ phenomena can cause supply problems

Shutterstock / Cigdem
Shutterstock / Cigdem

movies titles with numbers reverberate in my head: ”The Magnificent Seven,” “12 Angry Men,” “Catch-22” and “The Dirty Dozen.” The power quality realm features a dirty half-dozen: the six most common PQ phenomena result in the majority of problems that cause disruptions, shutdowns or equipment damage. Once the PQ investigator has properly identified them through monitoring and recording, the next step is to mitigate and remove them. This can be an iterative process as, after one is fixed, a less significant phenomenon might still be present.

Voltage sags—We start with the most common disturbance for most electrical equipment problems. A reduction in the root mean square voltage, usually caused by a large increase in current somewhere on the circuit, means there is less energy available for the load. Computers, process control systems and most IT equipment will stop functioning when the magnitude and duration of the sag exceeds the holdup time of their power supply. This can also happen to the power converters in high-definition lighting and adjustable-speed drives. The most common solution is to have an uninterruptible power supply, such as batteries, that converts stored energy to fill in the missing energy. Power conditioners, such as constant voltage transformers, can also be effective. If the problem is within the facility, a cost-effective solution is to relocate large current-demanding loads from being on the same circuits as the tripping equipment.

Voltage transients—The destructive impulsive transient from lightning strikes isn’t the only concern. Smaller but repetitive ones from the commutation overlap in three-phase power converters or the oscillatory transient from switching of power-factor (PF) corrective capacitors may be the problem. Transient voltage surge suppressors are common in the power outlet strips used for PCs and other IT equipment and on a larger scale at the electrical service entrance. But TVSSs won’t fix all of the PF-cap-switching transient issues, as the initial transient is a negative one where energy is removed from the voltage waveform. That requires filling it back in with a power conditioner or having the PF cap switching mechanism changed to a zero-crossing type.

Interruptions—A voltage decrease that gets so low and lasts for a longer time than a sag is classified as a voltage interruption. This requires a more substantial energy backup. Larger banks of batteries are used in some situations. Usually, a motor-generator set provides power when it is needed for hours, not just minutes. The primary mover may be gasoline, diesel or natural gas. When using gasoline or diesel motors, be sure that your communication system with the fuel supplier still functions during the interruption—many facilities learned this lesson the hard way during the August 2003 Northeast blackout.

Harmonics—Though harmonics are common in most electrical systems today, whether or not they are an issue depends on their magnitude, frequency and the type of equipment. Harmonic problems often originate within the facility, as the majority of loads in commercial and some industrial facilities are nonlinear power supplies, which create harmonic currents. A properly designed filter at the offending equipment source can be the most cost-effective solution. Changing over to power converters that are 12- or 24-pole converters (versus the typical six from a full-wave, three-phase converter) can reduce the magnitude of the harmonic currents. The drawback is higher-order frequencies are present, which can cause greater losses in electromagnetic equipment such as motors and transformers. Additionally, when changing out equipment, watch out for resonant conditions where a particular harmonic can be amplified by the system’s harmonic impedances.

EMI/RFI noise—Signals transmitted in the air or conducted through the wires can disrupt communication systems and cause errors in transducer circuits. A simple solution is to separate the source of the interference—such as welders—physically and electromagnetically from the susceptible equipment. You must use properly shielded and grounded signal cables. In extreme cases, shielded enclosures or even Faraday cage rooms may be the solution.

Wiring and grounding—This extensive topic deserves an entire article. Some sources say 70% of power quality problems are caused by wiring and grounding. Maybe this was true 40 years ago, but I’m not sure if current research supports that claim. Regardless, there are plenty of issues to be addressed, including illegal neutral-to-ground bonds, undersized neutrals, inadequate or improper grounding, bad connections and mixing of incompatible loads on the same circuits. While many of these are best addressed during a facility’s design and construction, fixing them after commissioning and system changes is a common mitigation task.

Cleaning up the dirty half-dozen doesn’t require a big capital investment. Diligence and attention to detail during walk-through inspections, ongoing maintenance and infrastructure and equipment changes go a long way to keeping PQ out of your operations.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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