05-11-2012, 05:32 PM
400 KV SUB STATION
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Types of circuit breakers
An air circuit breaker for low voltage (less than 1000 volts) power distribution switchgear
Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.
Low voltage circuit breakers
Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, and include:
MCB(Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
Low voltage power circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear cabinets.
Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which lets the contacts open by spring action. Some magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring restrains the core until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.
High-voltage circuit breakers
Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. Insubstations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish the arc.
Bulk oil
Minimum oil
Air blast
Vacuum
SF6
Sulfur hexafluoride (SF6) high-voltage circuit-breakers
A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.
Corona discharge
corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish (or other color) glow in the air adjacent to pointed metal conductors carrying high voltages. Spontaneous corona discharges are undesirable where they waste power in high-voltage systems or where the high chemical activity in a corona discharge creates objectionable or hazardous compounds, such as ozone. Controlled corona discharges are used in a variety of filtration, printing and other processes.
Water
It is most important factor influencing the life of organism. In fact, life on earth originated in water and is unsustainable without water. Its ability is so limited in desert that only special adaptations make it possible to live there. The productivity and distribution of plants is also heavily dependent on water. You might think that organism living in ocean lakes and rivers should not face any water related problems, but it is not true. For aquatic organism the quality of water becomes important. The salt concentration, is less than five in inland,30-35 in the sea and > 100% in some hyper saline lagoons, some organisms are tolerant of wide range of Salinities but other are restricted to a narrow. Many freshwater animals cannot live for long in sea water and vice versa because of osmotic problem, they could face.
Foreword
The national curriculum framework, 2005, recommends that children’s life at school may be linked to their life outside the school. This principal marks a departure from the legacy of the bookish learning which continue to shape our system and cause to gap between the school, home and community. They syllabi and text book developed on the basis of NCF signify an attempt to implement to basic idea. They also attempt to discourage rote learning and maintenance of sharp boundaries between different subject areas. We hope these measure will take us significantly further in direction in child-centred system of education outlined in the national policy on education (1986).
biotechnology
ever since the days of rene descartes . the French philosopher mathematician and biologist of seventeenth century. all human knowledge especially natural science were directed to develop technologies which add to the creature comforts of human lives, as also value to human life. The whole approach to understanding natural phenomenon became anthropocentric physics and chemistry gave to rise to engineering, technologies and industries which all worked for human comforts and welfare. the major utility of the biological word is a source of food . Biotechnology, the twentieth century off- shoot of modern biology, change our daily life as its products brought qualitative improvement in health and food production. the basic principal underlying biotechnological process and some applications are highlighted and discussed in this unit.