Understanding Power Quality.

Definition

Electric power quality has become an important part of power systems and electric machines. The subject has attracted the attention of many universities and industries, and a number of books have been published in this exciting and relatively new field. Despite important papers, articles, and books published in the area of electric power quality, its definition has not been universally agreed upon. However, nearly everybody accepts that it is a very important aspect of power systems and electric machinery with direct impacts on efficiency, security, and reliability. Various sources use the term “power quality” with different meaning. It is used synonymously with “supply reliability,” “service quality,” “voltage quality,” “current quality,” “quality of supply,” and “quality of consumption.” Judging by the different definitions, power quality is generally meant to express the quality of voltage and/or the quality of current and can be defined as: the measure, analysis, and improvement of the bus voltage to maintain a sinusoidal waveform at rated voltage and frequency. This definition includes all momentary and steady-state phenomena.

Causes of Disturbances in Power Systems

Although a significant literature on power quality is now available, most engineers, facility managers, and consumers remain unclear as to what constitutes a power quality problem. Furthermore, due to the power system impedance, any current (or voltage) harmonic will result in the generation and propagation of voltage (or current) harmonics and affects the entire power system. Illustrates the impact of current harmonics generated by a nonlinear load on a typical power system with linear loads.

What are the origins of the power quality problem? Some references divide the distortion sources into three categories: small and predictable (e.g., residential consumers generating harmonics), large and random (e.g., arc furnaces producing voltage fluctuations and flicker), and large and predictable (e.g., static converters of smelters and highvoltage DC transmission causing characteristic and uncharacteristic harmonics as well as harmonic instability). However, the likely answers to the question are these: unpredictable events, the electric utility, the customer, and the manufacturer.

The Electric Utility

There are three main sources of poor power quality related to utilities:

• The point of supply generation. Although synchronous machines generate nearly perfect sinusoidal voltages (harmonic content less than 3%), there are power quality problems originating at generating plants which are mainly due to maintenance activity, planning, capacity and expansion constraints, scheduling, events leading to forced outages, and load transferring from one substation to another. sinusoidal source voltage system impedance (e.g., cables, lines,& transformers ) harmonic voltage distortion at PCC due to propagation of harmonic currents through the system impedance nonlinear loads (e.g., switched-mode power supplies, AC drives, fluorescent lamps) drawing non sinusoidal currents from a perfectly sinusoidal voltage source linear loads customer with linear and nonlinear loads point of common coupling (PCC) customers with linear loads harmonic voltage distortion imposed on other customers Propagation of harmonics (generated by a nonlinear load) in power systems. Introduction to Power Quality 5

• The transmission system. Relatively few power quality problems originate in the transmission system. Typical power quality problems originating in the transmission system are galloping (under high-wind conditions resulting in supply interruptions and/or random voltage variations), lightning (resulting in a spike or transient overvoltage), insulator flashover, voltage dips (due to faults), interruptions (due to planned outages by utility), transient over voltages (generated by capacitor and/or inductor switching, and lightning), transformer energizing (resulting in inrush currents that are rich in harmonic components), improper operation of voltage regulation devices (which can lead to long-duration voltage variations), slow voltage variations (due to a long-term variation of the load caused by the continuous switching of devices and load), flexible AC transmission system (FACTS) devices and high-voltage DC(HVDC) systems ,corona , power line carrier signals , broadband power line (BPL) communications , and electromagnetic fields (EMFs).

• The distribution system. Typical power quality problems originating in the distribution system are voltage dips, spikes, and interruptions, transient overvoltages, transformer energizing, improper operation of voltage regulation devices, slow voltage variations, power line carrier signals, BPL, and EMFs.