Uncategorized – APSEEE

Browsing: Uncategorized

Interconnected System

Interconnected System

The connection of many generating stations such as hydro power plant, Thermal power plant, Nuclear power plant etc running in parallel is called interconnected system. The interconnected system may be two types (i) Integrated and (ii) Uni-integrated. An integrated interconnection results in maximum overall economy. But most of the system interconnections are unintegrated. An unintegrated interconnection the identity of individual not lost. There is no central control office like integrated interconnection system.

Advantages of Interconnection or Interconnected System

The interconnected system has many advantages. There are given below.

  1. It increases the service reliability. Maintenance and replacement of any equipment is required during its operation so if the system is interconnected it will increase the reliability. An electrical energy is generated from different source, such as water, coal a nuclear etc. It the system is interconnected then we will choose that plant where will have to pay less cost. During peak load we can receive or transmit energy from large capacity plant.
  2. Load growth necessitates additional transmission facilities. If the system is interconnected them we have a provision to add and replace transmission. It makes the system more reliable.
  3. Reserve capacity required is reduced with interconnected system we do not need reverse capacity of plant.
  4. Reduction in total installed capacity. An interconnection decrease the installed capacity needed to meet the load requirement. Different areas have different demands of electrical energy. In areas required less energy and at the same other area required more electrical energy. If two such areas are interconnected, the diversity of load would cause the maximum combined demand to be less than the some of the individual maximum demands. In this way, diversity factor improves.
  5. The interconnected system improves the efficiency of plants.
  6. Economical operation of station is ensured. We can choose that plant which has lower running cost in this way the system become economical.
  7. With system interconnection capital and maintenance cost is reduced.

{ Add a Comment }

Electric Cells

Electric Cell

An Electric cell is source of electrical energy. The cell gives dc current. The emf developed and current supplied by a cell is very small. The electrical energy can be stored in cell.

Definition of Electric Cell

A cell is source of emf in which chemical energy is converted into electrical energy.

Forming of a Electric Cell

  • An electric cell basically consists of two electrodes of different material, so that different potentials are built up when chemical action takes place on them.
  • An electrolyte such as an acid, alkali or salt solution so that chemical action takes place between two electrodes. The solution must be capable to react chemically with the two electrodes.

electric cell

  • When two electrodes are immersed in the electrolyte, due to chemical action between electrodes and electrolyte, a potential difference established between two electrodes.

E.M.F developed in a Cell

The magnitude of emf developed in cell depends upon the following.

  • Nature or material of the plates used as electrodes of the cell.
  • Type of the electrolyte used in the cell.

Types of Cells

Electric cells can be divided into two types.

Primary Cells

The cell in which chemical action is not reversible, called primary cells. Voltaic cell, Danial cell, Silver Oxide cell, Dry cell etc are the examples of Primary Cell.

In these types of cells, once the cell is discharge it cannot be recharged because chemical action in this case is not reversible and the cells cannot be recharged. This makes Primary cells expensive source of electrical energy and that is why these cells are rarely used in commercial applications.

Secondary Cells

The cell in which chemical action is reversible, called secondary cell. Lead acid cell, Nickel-iron alkaline cell etc are the examples of secondary cells. These cells are rechargeable.

In these cells chemical action is reversible and cells can be recharged. While recharging, electrical energy is converted in the cell itself.

 

{ Add a Comment }

Semiconductor Physics and Materials

Semiconductor Physics and Materials

Semiconductor materials are those materials their conductivity lies in between the conductivity of conductors and insulators Germanium, Silicon, Carbon etc. are the semiconductor materials. Due to some useful properties of semiconductors they are extensively used in electronic circuits. Before the invention of semiconductors as an electronic components tube devices are used in the circuits, but now these days semiconductors components are extensively used because of their many advantages such as compact in size, cheap in cost, less maintenance etc. Now in these days the semiconductor devices occupies most important place in our daily life. Before studying the semiconducting materials we should about the behavior of the semiconductors.

What is an Atom?

An atom is tiny part of any substance which cannot be further subdivided. According to the BOHR’s ATOMIC MODEL (Neil’s Bohr is a Danish Physicist) an atom consists of a nucleus which contains neutrons and protons. Proton has positive charge whereas neutron has no charge. Extra nucleus contains electrons. The electrons revolve around the nucleus in different orbits. The charge on electrons is negative. An atom is electrically neutral because the number of electrons in an atom is equal to number of protons. The nucleus has a positive charge which attracts the electrons due to centrifugal force the electrons never stick on the protons.




IMG-20160818-WA0018

The electrons in the valence shell decide the behavior of a materials, whether the materials are conductors or semiconductors or insulators.

INSULATORS, SEMICONDUCTORS AND CONDUCTORS AND ENERGY BANDS

INSULATORS

Insulators are those materials which do not allow the passage of electric current through them. The valance band of the insulators is full, but conduction band is empty. So the forbidden energy gap is very large so there is not possibility of flow of electric current through an insulator, because the large amount of energy required pushing the valence electrons to the conduction band.

IMG-20160818-WA0016

SEMICONDUCTORS

Semiconductors are those materials whose conductivity lies in between conductor and insulators. In case of semiconductors the forbidden energy gap is more in case of conductor and less in case of insulator. It means valence electrons required least energy to enter the conduction band. Germanium, silicon carbon are the examples of semiconductor materials. Under normal condition, do not conduct current and behave as an insulator. When some energy (Heat of electric field) given to the valence electrons and some electrons cross over to the conduction band.

IMG-20160818-WA0020

CONDUCTORS

Conductors are those materials which allow the passage of current through them. The valance band of these materials overlap the conduction band due to this overlapping a large numbers of free electrons are available for conduction.

PROPERTIES OF SEMICONDUCTORS

  1. The resistivity of the semiconductors is less an insulators but more than a conductor. It means semiconductors neither a perfect insulator nor perfect conductor.

  2. The semiconducting materials have negative temperature coefficient of resistance. So that with increase in temperature on semiconductor the resistance is decreased and vice versa.

  3. When a suitable metallic impurity is added in the semiconducting materials, it changes the current conducting properties of the semiconductor appreciably.

COMMONLY USED SEMICONDUCTOR MATERIALS

There are large numbers of semiconductor materials are available such as, carbon, germanium, silicon etc. The minimum energy required for breaking the covalent bond in these materials is 7, 1.12, .75 etc. Carbon materials required large energy to break the covalent bond. In other words large energy is required to make conducting materials. Therefore, germanium and silicon are considered to be the most suitable semiconductor materials.

SILICON

The atomic number of silicon is 14. The silicon materials are obtained from silicon dioxide. To get pure silicon materials the silicon oxide is chemically treated. The number of electrons in the first orbit, second and third orbit is 2, 8 and 4 respectively. Silicon material is tetravalent because in valence shell of silicon has four electrons.

IMG-20160818-WA0019




GERMANIUM

It is an earth element recovered from the ash of certain coals or from the flue dust of zinc smelters. It was discovered in 1886. It is obtained from germanium oxide. Germanium oxide is chemically treated and it is then reduced to pure germanium. The atomic number is 32. Therefore it has 32 protons in the nucleus and 32 electrons in the different orbits. The number of electrons in the first, second, third and fourth orbit are 2, 8, 18 and 4 respectively. Four electrons in last orbit show the germanium is semiconductor.

IMG-20160818-WA0021

COMPARISON BETWEEN SILICON AND GERMANIUM

Silicon and germanium are must common semiconductors which are used to fabricate the semiconductor devices or components. But we prefer silicon over germanium. The question is here why silicon semiconductor preffered over germanium? The answer is, In case of germanium the forbidden energy gap between valance band and conduction band is less as comparatively silicon. So less energy is required to germanium to change its conducting properties because many free electrons are available at room temperature. On the other hand silicon has no free electrons at room temperature. For this reason we considered silicon as a best semiconductor for fabrications of electronic devices or components.

 




{ Add a Comment }

Fuse and Types of Fuses

INTRODUCTION

Electrical Fuse is simple and cheapest device which are used for interrupting an electrical circuit under abnormal conditions (such as short circuit conditions, overload conditions etc). Fuses are used upto 66kv called high voltage fuse and low voltage fuse upto 400V. In this article, we discuss about definition of fuse, working principle and fuse, fuse element materials and types of fuse.




Definition of Fuse

What is fuse?

A fuse is small piece of wire, inserted in the circuit which melts when the value of current plows more than the permissible limit through it and thus break the circuit. Fuse is always connected in series with the circuit.

The fuse element is usually made of materials having low melting point, high conductivity etc. The action of a fuse is based on the heating effect of the electric current when flows through a fuse element. When the current flows through a fuse is in safe value, the heat developed in fuse element rapidly dissipated into the surrounding air and therefore fuse element remains at a temperature below its melting point. However when fault occurs in the electric circuit, the current exceeds the safe value or limiting value. The heat is generated due to this excessive current cannot be dissipated fast enough and fusible element gets heated, melts and breaks the circuit. If the value of current is large enough, the more rapidly the fuse will blow i.e. the fuse has inverse time current characteristics.

untitled

ADVANTAGES AND DISADVANTAGES OF FUSE

ADVANTAGES

It is cheapest and simple form of protection.

It needs no maintenance.

It interrupts very large short circuit current without any noise, flame gas or smoke.

It operation is inherently completely. It means there is no required any more equipment for automatic action.

Its inverse time current characteristics enable. Its use for over load protection.

DISADVANTAGES

The fuse has following disadvantages: –

Considerable time is waste n rewiring or replacing a fuse after operation.

The current time characteristic of a fuse cannot always be correlated with that of the protected apparatus.

DESIRABLE CHARACTERISTICS OF FUSE ELEMENT

The fuse elements should have low melting point like tin and lead.

It should have high conductivity like silver and copper.

It should be free from deterioration due to oxidation. Silver material has such properties.

The fuse element should be cheap in cost like lead and tin.




FUSE ELEMENT MATERIALS

The materials used for fuse elements must be of low melting point, low ohmic loss, high conductivity, low cost and free from deterioration. A low melting point is available with a high specific resistance metal. The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For smaller value of currents tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For exceeding the current 15A this alloy is not used as the diameter of the wire will be lager so that beyond 15A rating circuits Copper wire fuses are employed. Zinc fuse is good if a fuse with considerable time-lag is required i.e. One which does not melt very quickly with a small overload.

The present trend is to use silver despite. Its high lost due to the following advantages.

It does not oxidized. It means it is free from oxidation.

It does not deteriorate when used in dry air. There is no effect of moist on the silver fuse element, when the silver surface is attacked and a layer of silver sulfide is formed at the surface of a fuse element which shield the silver fuse element from further attack.

The conductivity of silver fuse element is very high. Therefore, for a given rating of fuse element, the area of cross section of silver metal is required is smaller than that of other materials.

The conductivity of silver does not deteriorate with oxidation so that life of silver fuse element is long.

Owing to its high conductivity the mass of molten fuse element to be handled is minimum due to this property the operating speed is fast.

IMPORTANT TERMS OR DEFINITIONS

CURRENT RATING

It is the r.m.s value of current which the fuse element can carry without overheating. It depends upon the temperature rise of contacts of the fuse holder, fuse material and the surrounding area of the fuse.

FUSING CURRENT

It is defined as the minimum value of current at which the fuse element melts. It means the value of fusing current is always more than the value of rated current of the fuse element.

Approximated value of fusing current is given by

I = kd3/2

I = current

K is constant

d is diameter of the fuse element

The fusing current is depends upon the following factors: –

The fusing current the fuse element depend upon the type of materials used for making fuse element.

Cross-section of area of the fuse element whether it is round or rectangular.

Length the shorter length of the fuse element the grater the current can conduct heat easily.

Diameter of fuse wire.

Type of enclosure used.

FUSING FACTOR

It is the ratio of minimum fusing current to the current rating of the fusing element is called fusing factor. It is always greater than unity.

For a semi-enclosed or rewirable fuse which employs copper wire as the fuse element, fusing factor is equal to 2.0 and for cartridge fuses fusing factor is equal to 1.45.

BREAKING CAPACITY

Breaking capacity of a fuse is the r.m.s value of the AC component of the maximum prospective current with at rated system voltage.

PRE-ARCING TIME

When a fault occurs, the fault current rises sharply and heat is generated in the fuse element. At the fault current attains it cut-off value the fuse melts and an arc is initiated.

ARCING TIME

This is the time between the instant of arc initiation and the instant of arc being extinguished.

TOTAL OPERATING TIME

The sum of Pre-arcing time and arcing time is called total operating time.

TYPES OF FUSES

A fuse unit usually consists of a metal fuse element, a set of contacts between which it is fixed and a body to support the fuse element and isolate them.

How does a Fuse works?

Fuse is the simplest current interrupting device for protection of the electrical circuit against short circuit or overloads which results excessive current flow through the electrical circuit. Many types of fuses also incorporate means for extinguishing the arc that appears when the fuse element melts.

Different Fuse Types are given below

Low voltages fuses

High voltages

LOW VOLTAGE FUSES

Low voltage fuses are further divided into three types namely semi-closed or rewirable type and cartridge type.

REWIRABLE FUSES

Rewirable fuse is most commonly used fuse in house wiring and in small current circuits. It is simplest form of protection available. Semi-enclosed or Rewirable fuse is also sometimes called kit-kat type fuse. It consist a base and a fuse carrier. The fuse element is placed in fuse holder. Both are made of porcelain material.

When fault occurs, the value of current flowing through fuse element is increased which generated heat in the fuse element and fuse element melts.

CARTRIDGE FUSE

In case of cartridge fuse the fuse element is totally enclosed in a enclosed container and is provided with metal contacts on both sides. The cartridge fuse divided into two types (i) D-type and (ii) Link type.




{ Add a Comment }