Power Quality-Design

“Dirty power” is a phrase that is often used to explain unexpected behavior in the context of an electrical system. A noisy electrical system is painfully unpleasant for facility managers, owners, operators and customers. These difficulties are commonly communicated back to the electrical engineers who supported the design.

For many, the path of least resistance is to assign blame to the power utility company. In many cases, this snap judgment is inaccurate or neglectful. Careful analysis of the electrical system may identify opportunities to improve power quality at the facility.

Transient risks to power quality

A few suspects promptly come to mind when considering how a facility’s power source could be destabilized: lightning strikes, switching transients, starting large motors and failing equipment. Surge protection devices are designed to redirect transient overvoltages caused by lightning strikes and switching transients. Controllers for large motors are commonly designed and specified to minimize voltage sags. Protective devices and relays are designed to monitor system parameters and isolate components from faults due failing equipment.

Each of these transient conditions merits substantial individual engineering attention. This discussion, however, will focus on an expanding area of concern that is far more common and much less transient in nature.

Nonlinear load risks

Modern technology has changed the landscape of power distribution and control. We have all witnessed the market growth of battery–powered vehicles, adjustable speed drives and data centers. Battery chargers recharge the batteries that power electric cars. Variable frequency drives play a critical role in modern facilities and energy conservation measures. Today, servers and digital storage are components of nearly all modern buildings. These devices share a common thread — the power electronic devices with nonlinear load profiles.

The ideal sinusoidal waveform we all know so well is effectively mutilated by nonlinear load. Unlike a linear load, the impedance of a nonlinear load varies without a proportional relationship to the applied voltage. This unique impedance results in nonsinusoidal current. The choppy current waveform interacts with overall system impedance, which impacts voltage. As these currents make their way through the distribution network, the risk of operation beyond design constraints increases.

Sensitive equipment, such as computers, may not operate as intended or fail when supplied with a distorted source of power. Transformers, motors, generators, capacitors, conductors and computer equipment are all susceptible to harmonic distortion. Transformers, generators and conductors can overheat. Capacitors are vulnerable to harmonic resonance. Motor torque can oscillate, causing excessive vibration and strain on the motor shaft.

 

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