POWER QUALITY AND UTILITY INTERFACE ISSUES

Irsad Tri Hartanto

Shandi Irawan

Untung Darmawan

Power Quality and Utility interface issues

The basicly term "power quality" means different things to different people. One definition is the relative frequency and severity of deviations in the incoming power supplied to electrical equipment from the steady 60 Hz, sinusoidal waveform of voltage or current.(IEEE) The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.

Quality is a term that means different things to different people, it all boils down to the objective of maintaining a set of electrical boundaries to allow a piece of equipment to function in its intended manner.

 All power factor correction devices used in the

industry, which perform the role of Utility Interface are designed with the goal of maintaining the voltage waveform as close as possible to the sine wave and frequency.

Generation of harmonics twists the entire issue owing to its unfavorable effects on the system. The presence of harmonics in a system also eflects back and causes problems to eighboring systems and the grid as a whole.

what are POWER QUALITY PROBLEMS ?

Power quality is affected from both sides of the grid, referring both to the quality and reliability of power as supplied by the utility and to the type and effect of customer  loads on the transmission system. While power supply is generally regular and reliable, it can be interrupted or altered by storms and overloading of the system. Conversely, an ideal load, with a power factor of 1 and no harmonic distortion--draws power that matches that supplied by the utility, an even "sinusoidal" voltage waveform, while loads that deviate from that form can strain and increase inefficiency in the transmission system.

Harmonics are a distortion of the utility- supplied waveform and are caused by "non-linear,“ loads, which include motor controls, computers, office equipment, compact fluorescent lamps, light dimmers, televisions and, in general, most electronic loads. High harmonics increase line losses and decrease equipment lifetime. Total harmonic distortion (THD) measures the degree to which the input is distorted, and is the relative value of all the harmonics combined, as a percentage of the fundamental current.

Transients : Commonly called swells, surges and spikes, transients are the most frequent types of power quality problems and often the easiest to fix. The difficulty with transients is in detection since they manifest only as a short -duration change in voltage.

Brownouts : Are hours-long voltage sags caused by system overload. U.S. utilities use rolling blackouts to avoid brownouts as brownouts tend to damage equipment, but such fluctuations are common in developing countries.

Reliability : Refers to the probability of maintaining a continuous supply of electricity without interruption. Utilities generally design systems to lose only one hour of service in a period of ten years giving reliability of 99.9999%.

Electrical loads that have a nonlinear relationship between the applied voltages and their currents cause harmonic currents in the power system. Passive electric loads consisting of resistors, inductors, and capacitors are linear loads. If the voltage applied to them consists of a single-frequency sine wave, then the current through them will be a single-frequency sine wave as well. Power electronic equipment creates harmonic currents because of the switching elements that are inherent in their operation. For example, consider a simple switched-mode power supply used to provide DC power to devices such as desktop computers, televisions, and other single-phase electronic devices.

Where Do Harmonics Come From?

Electrical loads that have a nonlinear relationship between the applied voltages and their currents cause harmonic currents in the power system. Passive electric loads consisting of resistors, inductors, and capacitors are linear loads. If the voltage applied to them consists of a single-frequency sine wave, then the current through them will be a single-frequency sine wave as well. Power electronic equipment creates harmonic currents because of the switching elements that are inherent in their operation. For example, consider a simple switched-mode power supply used to provide DC power to devices such as desktop computers, televisions, and other single-phase electronic devices.

SOURCES OF POWER QUALITY PROBLEMS

These problems can range from improper grounding and bonding to code violations and internally generated power disturbances. On their own, many of these problems may not have caused power quality problems in the past. The recent proliferation of highly sensitive computers, micro processor systems and power electronics in today’s commercial and industrial equipment has forever changed the nature of supply loading.

solve power quality issues can often become part of the problem, making power quality even more of a mystery. Among the types of equipment that both can cause power quality problems, and are susceptible to them, are Uninterruptible Power Supplies, Variable Frequency Drives, Battery Chargers, Large Motors During Startup, Electronic Dimming Systems, Lighting Ballasts (esp. Electronic), Arc Welders, and Other Arc Devices, Medical Equipment

SOLVING POWER QUALITY PROBLEMS

Double-Size Neutrals, or Separate Neutrals per Phase:

In most of the cases, harmonics can be easily handled by using double-size neutrals, as recommended by the IT Industry Council. Alternatively, separate neutrals can be used for each phase conductor. The additional cost of oversizing the neutral is minimal.

Harmonic Filters: Filters are sometimes most cost effective in an existing structure where rewiring is difficult or costly. But the filter design is dependent on the equipment on which it is installed, and may be ineffective if the particular piece of equipment is changed. Filtering characteristics need to be carefully designed for a given installation. Filters are also fairly expensive on a per-kVA basis. Shielded Isolation

Transformers: Shielded isolation transformers are filtering devices that lessen feed-through of harmonic frequencies from the source or the load. They are a plausible retrofit technique where power problems have already been encountered, but are also quite expensive per-kVA. K-Rated

Transformers: K-rated transformers have beefed-up conductors and sometimes cooling to safely handle harmonic loads. Alternatively, standard transformers are sometimes de-rated (up to 50%) to allow for the extra heating due to harmonics.A careful comparison of the relative costs of K-rated vs. de-rated standard transformers should be made.

Harmonic-Rated Circuit Breakers and Panels:

Overheating due to harmonics is the danger here, and beefed-up components used in these elements offer protection. Neutral buses should be rated for double the phase current.

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