Power Quality, Harmonics, and Predictive Maintenance

Background

Power quality (PQ) problems can be defined as any power problem manifested by voltage, current, and frequency deviations that result in failure or misoperation of load or equipment. PQ problems may be classified into two categories: (1) conducted low-frequency phenomena and (2) radiated frequency phenomena. The conducted low-frequency phenomenon is characterized by the following types of PQ problems.

• Overvoltage, undervoltage, transients including sags and swells

• Voltage fluctuations (flicker)

• Voltage dips and interruptions

• Voltage imbalance or unbalance

• Power frequency variations

• Induced low-frequency voltages

• Harmonics, interharmonics, and harmonic resonance

The radiated frequency phenomenon known as noise is characterized by the following types of PQ problems.

• Magnetic and electric fields (electromagnetic interference, EMI)

• Radio frequency interference (RFI)

Further, the addition of nonlinear (digital and electronic) loads in industrial and commercial power systems have lead to problems in the quality of power that is being delivered to a site. A nonlinear load is defined as that which draws a nonsinusoidal current wave when supplied by a sinusoidal voltage source. PQ problems can produce results that range from erratic equipment behavior to complete shut down of a facility. In some cases, the shut down may be accompa- nied by a catastrophic failure costing millions of dollars in some cases. Thus, it is important to understand and solve PQ problems. But, catastrophic failure is just one possible outcome. PQ problems can creep along—silently consuming maintenance resources for trouble shooting PQ anomalies and at the same time increasing the cost of electrical energy (utility bills) because of inefficient use. Things may appear to be normal, but that is only because of a lack of understanding of PQ problems and power anomalies. Like the old saying, what you do not know cannot hurt you. PQ covers a wide range of issues, from volt- age disturbances like sags, swells, outages and transients, to voltage and current harmonics, to performance of wiring and grounding. The concept of load and source compatibility is not new. The need to provide power with steady voltage and frequency was recognized since the early days of the electrical power. Some of the early concerns were flicker of light bulbs due to voltage fluctuations and overheating of motors due to voltage waveform distortion (harmonics). More recently, transient voltage disturbances associated with lightning and power system switching have emerged as a major concern to manufacturers and users of electronic equipment. The issue of grounding, and how to deal simultaneously with surges, lightning protection (i.e., known as RFI, EMI or noise), and safety is a complicated task because of conflicting philosophies advocated by different professionals. Today’s PQ problems are far more complex. They cannot be handled so easily because of a multitude of different causes and a variety of specific sensitivities in the end user equipment that is most affected. Because both the causes and consequences of PQ prob- lems are so diverse, they are not amenable to a single solution. The symptoms of poor PQ include intermittent lockups and resets, corrupted data, premature equipment failure, overheating of components for no apparent cause, nuisance tripping of relays and protective devices, etc. The ultimate cost is in downtime, decreased productivity and frustrated personnel.

In this chapter we attempt to provide an understanding of the fundamentals of PQ, harmonics, and their effects on electrical equipment. More importantly, we are going to approach PQ issues from the prospective of predictive maintenance so that maintenance personnel can be vigilant and alert in identifying and resolving actual and potential PQ problems before they become a major source of trouble.

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