SKIN EFFECT IN TRANSMISSION LINES CONDUCTOR


Skin Effect in AC circuit is the tendency of the current to crowd toward the outer surface. This results in a current density that is greater near the outer surface of the conductor.

Skin effect is more relevant in AC system since it involves frequency. All electric currents generate magnetic fields that in turn can affect the current (this is the principle behind electric guitar pickups). In a direct current case everything is constant and so nothing seems to happen. With an alternating current, however, there is a delay in the magnetic field's response to the change in current and the 'old' magnetic field tends to push the current towards the outside of the conductor.

Skin Effect causes the longitudinal element of the conductor near the center of the axis to be surrounded by more lines of magnetic force than near the rim. This results in an increase in inductance toward the center.

Skin Effect increases the AC resistance of the conductor relative to the DC resistance. As the voltage drop in all wires is the same, a larger portion of the current flows through the outer conductor layers causing an increase of the effective resistance.

The decreased area of conductance causes apparent increase in the resistance. At 60Hz, the phenomenon is negligible in copper sizes #2 AWG and aluminum sizes #1/0 AWG and smaller. As the conductor sizes increase, the effect becomes more significant.

As the frequency increases, so does the effect until at very high frequencies the entire current flows in a very narrow skin on the conductor--hence the name.

Skin Effect contribution for round conductors at 60 Hz can be approximated using the following formula:

Ycs = 11.18 / (Rdc^2 + 8.8)

Where              Ycs = Skin Effect expressed as a number to be added to the dc resistance
                        Rdc = dc resistance of the conductor in micro-ohms per foot at operating temperature

POWER TRANSMISSION LINES AND CANCER

Two words that in recent times have been linked. Some said that living near transmission lines have its ill effects. While other would debunk that, calling it a myth.

Transmission Lines, or power lines in general has been feared not only due its high voltage, but feared that it has bad effects on the human's body, particularly that could cause leukemia and any other forms of cancer.

That not much have been said about the relationship of Power Lines and Cancer in an official terms. This entry is about a collection of news, articles that may be of help in understanding this sensitive issue.

There is in no way that the site condemn or defend those in the Power Transmission Lines Business. Feel free to leave some comments, and suggest links similar to the ones provided below.

Power Transmission Lines and Cancer, and everything in between.


Do High-Voltage Power Lines Cause Cancer
(Studies link Electromagnetic Fields (EMFs) To Illness)
A growing body of scientific evidence suggests that invisible electromagnetic fields (EMFs) -- created by everything from high-voltage utility company lines to personal computers, microwave ovens, TVs and even electric blankets -- are linked to a frightening array of cancers and other serious health problems in children and adults.

Though it received scant attention from the mainstream press, a report leaked last October from the U.S. National Council on Radiation Protection said there is a powerful body of impressive evidence showing that even very low exposure to electromagnetic radiation has long-term effects on health. Read more...

FAQ for Health Effects of Transmission Power Line Magnetic and Electric Fields

This FAQ is designed to answer in a brief and readily accessible format questions frequently asked about the dangers associated with exposure to magnetic and electric fields emitted by transmission power lines.

The questions contained herein confined only to issues associated with the extremely low frequency electro magnetic fields (ELF-EMF) associated with transmission power lines.  Similar issues have been raised concerning the high frequency magnetic fields associated with cell phones. These high frequency fields have impacts that differ from those associated with power lines and, therefore, this FAQ does not apply to them. Read more...

Childhood Leukemia Risk Doubles Within 100 Meters of High Voltage Power Lines
This result from the OXFORD CHILDHOOD CANCER RESEARCH GROUP study, headed by Gerald Draper analysed and compared 33 years of data (from1962 to1995) on 35,000 children diagnosed with cancer, with their distance to the nearest electricity transmission line.

These latest findings from the Draper study of a direct effect on childhood leukaemia from U.K. power lines follow from the acknowledged International studies that the risk of childhood leukaemia is doubled for magnetic field exposures above 0.4 microtesla, well below that seen under high voltage powerlines. Read more...

Electromagnetic Fields (EMF) and Your Health
In brief, there is a suggestion which has appeared in many (but not all) studies of children living near power lines that such exposure can approximately double the risk of leukemia and/or brain tumors. Evidence is less clear for adults, but suggests the possibility of similar risks.

Risks of breast cancer, depression, and other negative health effects are based on much more limited evidence and are even more speculative. There is enough information to have some concern, but not enough to set exposure standards. There is also a lack of clear evidence from laboratory evidence from laboratory experiments to indicate what protective measures (if any) would be appropriate or to demonstrate to everyone's satisfaction a mechanism which could explain such an effect of powerline fields. Read more...

Health Risks Associated with Living Near High-Voltage Power Line
In 1995, the American Physical Society (APS) spoke out on the question of power-line EMFs and health effects. The APS policy statement reads, in part: "The scientific literature and the reports of reviews by other panels show no consistent, significant link between cancer and power line fields.

 While it is impossible to prove that no deleterious health effects occur from exposure to any environmental factor, it is necessary to demonstrate a consistent, significant, and causal relationship before one can conclude that such effects do occur. From this standpoint, the conjectures relating cancer to power line fields have not been scientifically substantiated." (See APS Policy Statement 95.2 reaffirmed in 2005.) Read more...

Myths and Realities: Do Power Lines Cause Cancer?
Although the Wertheimer study spurred the scientific community to publish hundreds of subsequent studies attempting to draw a correlation between EMFs and a variety of cancer types in differing age groups, the majority of studies have focused on hematologic malignancies in children.

 I am pleased to report that the literature has failed to demonstrate any consistent and statistically significant evidence to suggest a relationship. Most studies have been plagued by limitations in design including small sample size, unblinded investigators and the use of wiring codes as surrogates for EMFs. Detractors of the original study criticize Wertheimer’s incorrect assumption that the wiring code of a home could be used as a surrogate for EMF exposure and that EMFs levels were not directly measured. Read more...

Children Living Near a High-voltage Power Line at Birth at Increased Risk for Childhood Leukemia
Children who lived less than 200 meters away from a high voltage power line at birth were 70% more likely to develop leukemia than children who lived more 600 meters away at birth.

Children who lived between 200 and 599 meters away from a high voltage power line at birth were 23% more likely to develop leukemia than children who lived more than 600 meters away. The increased risk remained, even after the researchers took the children’s socioeconomic status into consideration. Read more...


Power Lines and Cancer: Nothing to Fear
The notion that electric power lines can cause cancer arose in 1979 with a single flawed epidemiogical study that created a stir. Subsequent epidemiologic and animal studies have failed to find a consistent and significant effect.

No plausible mechanism linking power lines and cancer has been found. In recent years, the verdict from large-scale scientific studies has been conclusively negative, and scientific and medical societies have issued official statements that power lines are not a significant health risk. In short, there is nothing to worry about. Read more...

EFFECTS OF VOLTAGE UNBALANCE ON MOTOR PERFORMANCE

 When the line voltage applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings will result. A small percentage voltage unbalance will result in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages.

Should voltages be unbalanced, the rated horsepower of the motor should be multiplied by a factor to reduce the possibility of damage to the motor. Operation of the motor above a five percent voltage unbalance condition is not recommended.

Alternating current polyphase motors shall operate successfully under running conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1 percent. Performance will not necessarily be the same as when the motor is operating with a balanced voltage at the motor terminals.

Medium Motor Derating Factor Due To Unbalanced Voltage

% Voltage Unbalance = 100 x ( Maximum Voltage Deviation from Average Voltage/ Average Voltage)

Reference: NEMA Standards MG 1-1

ELEMENTS OF TIDAL ELECTRIC ENGINEERING FREE EBOOK DOWNLOAD LINKS


As interest in tidal-electric power generation continues to grow in response to demands for renewable sources of energy, readers can now turn to Elements of Tidal-Electric Engineering for the first comprehensive treatment of the subject. The author, Robert H. Clark, a leader in the field for almost fifty years, has spearheaded several important research projects and consulted with governments and private industries around the world on tidal-electric issues.

The focus of this text is the feasibility study. Power engineers gain both the knowledge and the skills needed to accurately determine the feasibility of a proposed tidal power development plan, including:

* Major factors to consider in selecting a site for preliminary assessment
* Tidal power schemes and mode
* Hydraulic and mathematical models of estuaries to predict the estuary's response to physical changes and the effects caused by operation of the proposed plant
* Civil works required for tidal power development and the associated tidal generating equipment
* Procedures to optimize plant output
* Economic evaluation and risk assessment
* Environmental impact of proposed construction and operation

The book ends with an examination of commercially operating plants and a brief review of sites that have been the subject of investigation in the last half century. References and bibliographies direct readers to primary source material for further study.

Until publication of this text, power engineers have had to rely on random journal articles and anecdotal information to perform a feasibility investigation. With the publication of Elements of Tidal-Electric Engineering these engineers have a single, integrated source that methodically covers all the issues.

CONCEPTS IN ELECTRIC CIRCUITS FREE EBOOK DOWNLOAD LINKS



This is an amazing book on basic concepts of electric circuits. Itcovers fundamental aspects of electric circuits that form the core of many engineering disciplines.

In particular, basic circuit elements and their characteristics are presented when connected in an electric network. In addition, techniques to analyse and solve electric circuits are explained in a simple manner and examples are shown to demonstrate each strategy. 

DC as well as AC circuit analysis is provided with a brief account of filters in the end. Difficult electric circuit concepts such as sinusoids and phasors are explicated in an effective way so that basic understanding can be properly achieved.covers fundamental aspects of electric circuits that form the core of many engineering disciplines.

In particular, basic circuit elements and their characteristics are presented when connected in an electric network. In addition, techniques to analyse and solve electric circuits are explained in a simple manner and examples are shown to demonstrate each strategy. 

DC as well as AC circuit analysis is provided with a brief account of filters in the end. Difficult electric circuit concepts such as sinusoids and phasors are explicated in an effective way so that basic understanding can be properly achieved.