Friday, March 10, 2017
Thursday, March 9, 2017
EEE General Knowledge- What is electricity
What is electricity:
All are made of atoms. In central atom is the nucleus may protons and neutrons. Tiny atom also has electrons spin around the nucleus. Electrons have a negative charge,and have positively charged protons.Electrons do not stay in one place. They move around different atoms so that some atoms has more protons, some have more Electrons. Sealed with more protons is positively charged. With more electrons Is negatively charged. When electrons move from one atom to another, it creates electric current.
Nuclear Power Plant Types Advantages and Disadvantages
TYPES OF REACTOR:
Figure-1 :Typical Diagram of a MAGNOX Reactor
Figure 1 shows,
MAGNOX- Magnesium Alloy Graphite Moderated Gas Cooled Uranium Metal Reactor
For magnox reactors, uranium ore is chemically processed to yield uranium metal that is formed into fuel rods which are inserted into cans made of an alloy consisting mostly of magnesium with small amounts of aluminium and other metals. Heat generated due to fission is extracted by CO2 gas that is circulated from the reactor core to the steam generators. Graphite which is the moderator is, in the form of rectangular blocks, placed in a steel pressure vessel. Fuel rods are placed inside the graphite blocks such that coolant can flow through the annular space between the fuel rod and the moderator. Control rods made of Boron are inserted from the top. The whole assembly is housed inside a concrete containment vessel. The steam generator is placed outside the containment. Gas circulator, turbo alternator and the feed pump are also shown. Stand pipes, which are used for fuel loading and unloading, are inserted from the top. Fueling machine can operate from the top when the reactor is on power.
Advantages:
- Natural uranium is used as fuel. So no enrichment is necessary.
- Fuel loading and unloading including small repair is possible when the reactor is power. Such feature allows high level of availability.
- Track record of safety is excellent.
Heavy water is water in which the hydrogen is replaced by its heavier isotope, deuterium, 2H.
The term light water designates water containing the protium isotope (contains no neutrons)
Figure-2 : Typical Diagram of a Pressurized Water Reactor (PWR)
Figure-2 shows a Pressurized Water Reactor (PWR). The containment structure includes Reactor Pressure Vessel, Steam Generator, Pressurizer and the Primary Pump. Steam turbine, generator and condenser are situated just like any other conventional thermal power station. It may be seen that the control rods are inserted from the top. Fuels which are slightly enriched ( 3% to 4%) uranium dioxide are in the form of fuel bundles canned in Zircalloy. Light water is moderator as well as coolant. High pressure is maintained inside the reactor vessel so that water does not boil in the core. This is known as primary circuit. Steam is generated at a separate steam generator which is known as secondary circuit. Incidentally, Three Mile Island Reactor where accident occurred in 1979 was of this type.
Advantages:
- Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.
- In case of any accident radioactive fall out is contained inside the containment, it cannot spread to surrounding area. It was amply proved in case of Three Mile Island Accident in 1979.
- Loading and unloading cannot be done when the reactor is on power. Reactor has to be shut down and then the same is affected.
- Since enriched fuel is used, enrichment services are necessary.
Figure 3: Typical Diagram of a Boiling Water Reactor (BWR)
Figure-3 shows a Boiling Water Reactor (BWR). Unlike the PWR it does not have a separate steam generator or in other words, the Primary and the Secondary circuit is same. Water is boiled inside the pressure vessel. Hence, steam generated is radioactive. Fuel elements, canning material, moderator and coolant are exactly same as those of PWR. The distinguishing feature of BWR is that the control rods are, on control point of view, inserted from the bottom. Loading and unloading is exactly similar to PWR.
Advantages:
- Radioactive fall out from the pressure vessel is contained in the containment.
Disadvantages:
- Loading and unloading cannot be done when the reactor is on power. Reactor has to be shut down and then the same is affected.
- Turbine hall is also radioactive zone.
- Since enriched fuel is used, enrichment services are necessary.
CANDU Reactor: which is heavy water moderated and cooled reactor. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad .The reactors are used in nuclear power plants to produce nuclear power from nuclear fuel.
The primary loop is in yellow and orange, the secondary in blue and red. The cool heavy water in the calandria can be seen in pink, along with partially-inserted shutoff rods.
The CANDU reactor is conceptually similar to most light water reactors, although it differs in the details.
Why transformer oil is used in transformer & its types.
The core-coil of transformer carry high voltage and current. The surrounding construction is isolated from the core-coil. A certain distance is maintained from the core-coil which is called “Electrical Clearance”. Plus the surrounding medium is maintained electrical isolated (dielectric) that charge can not pass through the medium.
As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by self convection.
Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings.
Core-coil of transformer is immersed in Oil
most important and their high cost can be justified, for example, in traction (railroad) transformers.
Transformer manufacturers in the U.S. are now investigating the use of natural esters obtained from
vegetable seed oils.
With air as surrounding dielectric medium the electrical clearance must be high. Thus a transformer become large and bulky. Plus with heavy stresses like in fault condition air might loose its dielectric property becoming ionized or electrically conductive.
On the other-hand both the dielectric and cooling property of liquid mineral or transformer oil is much higher than the air. Thus for compact design transformer oil is a better choice for transformer dielectric medium.
On the other-hand both the dielectric and cooling property of liquid mineral or transformer oil is much higher than the air. Thus for compact design transformer oil is a better choice for transformer dielectric medium.
What is dielectric
A dielectric material (or in short dielectric) is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material.Why transformer oil is used in transformer
Mineral oil surrounding a transformer core-coil assembly are called “Transformer oil” enhances the dielectric strength of the winding and prevents oxidation of the core. Dielectric improvement occurs because oil has a greater electrical withstand than air and because the dielectric constant of oil is closer to that of the insulation.As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by self convection.
Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings.
Transformer oil types
Below are the coolant being used now-a-days.01. High-Temperature Hydrocarbons
High-temperature hydrocarbons (HTHC), also called high-molecular-weight hydrocarbons. These coolants are classified by the National Electric Code as “less flammable” if they have a fire point above 300˚C. The disadvantages of HTHCs include increased cost and a diminished cooling capacity from the higher viscosity that accompanies the higher molecular weight.02. Esters
Synthetic esters are being used in Europe, where high-temperature capability and biodegradability aremost important and their high cost can be justified, for example, in traction (railroad) transformers.
Transformer manufacturers in the U.S. are now investigating the use of natural esters obtained from
vegetable seed oils.
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