Transposition of Conductors

          Transposition of Conductors refers to the exchanging of position of conductors of a three phase system along the transmission distance in  such a manner that each conductors occupies the original position of every other conductor over an equal distance. 

          When conductors are not transposed at regular intervals, the inductance and capacitance of the conductors will not be equal.

              When conductors such as telephone lines are run in parallel to transmission lines, there is a possibility of high voltages induced in the telephone lines.  This can result in acoustic shock or noise.  Transposition greatly reduces this undesired phenomenon.  

            In practice, however, conductors are not transposed in the transmission lines.  The transposition is done in the switching stations and the substations. 

XLPE Cables

XLPE stands for Cross linked Polyethylene.  It is a long chain polymer whose chains are cross-linked.  XLPE is a widely used material for use in cable insulation.  XLPE can be used from LV cables to cables with voltages upto 275 kV. 

The power factor of XLPE cable is very close to one.  XLPE cables have smaller charging currents and lesser dielectric loss.

XLPE cables do not require the protective metallic sheath.  They are lighter as compared to other cables and are easier to lay.

Tin Plated Copper

Copper used in bus bars is sometimes plated with Tin.  This is because copper oxidises in the presence of air.  The oxides of copper fall off the surface, this exposes more copper and further oxidation takes place.

These oxides increase the contact resistances in electrical contacts causing localized heating.  This heating causes further oxidation.  Thus, the copper is steadily eroded.

Tin plating is used protect the copper from corrosion.   Tin does not react easily with either air or water.  Tin is a soft metal which can be easily polished to give a shiny finish.   Tin Plated contacts are also used in the contacts of  isolators and switches.

Flexible Busbars

Flexible bus bars are used in Low voltage installations.  The are used in place of cables, particularly where parallel runs of cables need to be drawn to supply equipment in a particular location. 

Flexible bus bars are made of flexible strips of electrolytic copper which is usually tin-plated.  These strips are enclosed in a PVC insulation.  These bus bars can slide over each other which makes them flexible.  These bus bars can be bent to a considerable extent.  The elimination of parallel runs of cables reduces costs. 

Flexible bus bars have better heat dissipation than conventional rigid bus bars.  Hence, they can carry higher currents for the same cross section. 

Since these bus bars are insulated, the clearance from the ground can be lesser as compared to normal bus bars. 

Transformer Oils.

Oil is an important component of most transformers.  Oil serves to dissipate the heat generated in the core.  It also serves to provide dielectric isolation between the windings and the transformer body.  Transformer oils need to be stable at high temperatures with excellent dielectric properties.  

Traditionally,  mineral oils have been used in transformers. The downside of mineral oil is that they are easily combustible causing transformer fires. 

Polychlorinated Biphenyls, a synthetic compound was used as a replacement for mineral oil as it is not inflammable.  However, it is highly toxic and carcinogenic, not biodegradable and can cause damage to the environment if leaked.   The use of PCBs is now banned in many countries. 

Research has led to the development of many types of transformer oil which are fire-resistant and non-polluting.  Some of these oils are based on esters which are naturally available in vegetable oil or on synthetic esters.  These oils though expensive are environment-friendly, fire resistant and have the require dielectric properties.  Their high cost is offset by the elimination of the need for building expensive vaults for the transformer to contain any fire. 

Book of the Week

Electric Motors and Drives: Fundamentals, Types and Applications (3rd Edition)
by Austin Hughes
ISBN-10: 0750647183 | ISBN-13: 978-0750647182

This is a useful and interesting book which covers fairly all the topics in the field of motors and drives. The concepts are explained clearly and the mathematics is kept to a minimum. Topics start from the basics and and cover almost all aspects of the topic. The topics covered include dc motors, induction motors theory and types as well as stepper motors. Starting methods for various motors are also adequately addressed. The book also covers drives for dc and ac motors. It also includes a selection guide for drives. Though there pictures and diagrams, more pictures could have been added. 

In all, a comprehensive book which covers all the important topics at a relatively low price. This book would be ideal for industry professionals as well as students of electrical engineering.

Permanent Magnet Synchronous Generators

Permanent Magnet Synchronous Generators are becoming the generators of choice in the wind turbine industry.  They are being increasingly used in place of induction generators (induction machines which run above the synchronous speed).  They are advantageous over induction generators as they have a higher efficiency.   Besides, they do not need a magnetizing current which needs to be fed from the grid.

The normal synchronous generator has a wound rotor with poles.  These poles in the rotor are excited by means of a dc current.  In a permanent magnetic Synchronous generator, the wound rotor poles are replaced with permanent magnets.  

The machine is often directly coupled to the wind turbine or through gears.  The AC output of the synchronous generator which has a variable frequency depending on the turbine speed is converted into DC.  This DC is converted into a sinusoidal AC voltage at the system frequency.  This voltage is then fed to the grid.

Permanent Magnet Synchronous Generators (PMSG) are costlier as the permanent magnets are made of rare earth metals.  NeFeB, an alloy of Neodymium, Iron and Boron is used to make these magnets.  The use of permanent magnets in the rotors minimizes the losses in the rotor and improve efficiency.