Superconductors

Superconductivity materials lose all resistance and exclude external magnetic fields when they become superconducting. The temperature at which they become superconducting is known as transition or critical temperature Tc. At temperature above Tc the resistance of the material is not zero.

In like manner the magnetic field strength at which a superconducting material loses its superconductivity is the critical field. A useful superconductor has a high critical temperature and field. In January 1987 superconductivity critical temperature of a compound of yttrium, barium, and copper oxide was found to be about 90o K. This is above the temperature of liquid nitrogen, which boils at 77o K. Higher critical temperatures have been reported, but have not been reliably reproduced yet. The new superconducting compounds are brittle, but have been drawn into fine wires. Superconducting materials found so far show promise for electronic devices. Much work remains to be done before they can be used for superconducting transmission lines.
Short experimental superconducting lines have been built and operated, but have not yet been shown to be economical.

November 22, 2007 | Filed Under Engineering | Leave a Comment

Introduction to HVDC transmission

The invention of effective dc air blast and SF6 circuit breakers and lightning arresters made the use of HVDC transmission possible. Today there are over 40 HVDC lines throughout the world.
Both ac and dc lines of same voltage level have the same transmission capability and can transmit the same amount of power. However, the dc line has two conductors rather than three in the case of ac line and thus requires two-thirds as many insulators. Therefore, the required towers and right-of-way are narrower in the dc line than the ac line. Also the power losses due to corona phenomena are smaller for dc than that for ac lines. Underground cables used for ac transmission can also be used for dc and they can normally carry more dc power than ac due to the absence of capacitive charging current and better utilization of insulation and less dielectric wear.

The major advantages are:

If the cost of converter station is excluded, the dc overhead lines and cables are less expensive than ac lines and cables.
A dc link is asynchronous.
The corona loss and radio interference are less.
For dc line, reactive power compensation is not needed. However, reactive power support will be required at both ends as explained later.
The line length is not restricted by stability.
The interconnection of two separate ac systems via a dc link does not increase the short-circuit capacity, and thus the circuit breaker ratings of either system.
The dc line loss is smaller than for the comparable ac line.

The major disadvantages are:

The converters generate harmonic voltages and currents on both dc and ac sides and therefore filters are needed.
The converter consumes reactive power.
The dc converter stations are expensive.
The dc circuit breakers are difficult to design.

November 22, 2007 | Filed Under Engineering | Leave a Comment

Conductor and insulator types

In the early days, copper conductors were usually used, but aluminum conductors have almost replaced copper for overhead lines because of low cost and lightweight compared to copper of same resistance. Also, because of the higher size of the aluminum conductor, the corona effect is reduced. The conductors used in modern overhead power transmission lines are bare aluminum conductors, which are classified as follows:

AAC – all aluminum conductor
AAAC – all-aluminum-alloy conductor
ACSR – aluminum conductor steel reinforced
ACAR – aluminum conductor alloy reinforced

November 22, 2007 | Filed Under Engineering | Leave a Comment

Overhead and Underground systems

Most of the transmission systems use overhead lines, but the power distribution within the city or crowded places is done using underground cables. Less than one percent of the total transmission lines are placed underground. Although underground ac transmission would present a solution to the environmental and aesthetic problems involved in overhead lines, there are technical and economical reasons that make the use of underground ac transmission prohibitive. For low voltage distribution applications there are no technical problems in using cables. The aesthetic and safety requirements override the economical considerations in most of the distribution systems and hence the underground cables are invariably used. Read more

November 22, 2007 | Filed Under Engineering | 1 Comment

Transmission and distribution voltage levels

Power systems includes a generating system, a transmission and distribution system and loads. Transmission and distribution system is an important connecting link between the bulk power generating stations and the load centers. The generation plants are normally located in place where the resources are available to generate power economically. The recent trend in power system practice is to locate the generation plants away from the heavily populated areas. The power is then transmitted to the load by transmission lines and distribution networks. Read more