Antenna gain explained

           This one stumps even some of the most advanced RF engineers, that is, the "gain" of an antenna. Even the law states that the "Effective Radiated Power (ERP) will not exceed..." and this is based on the input into the antenna multiplied by the antenna gain. There is this concept that, the moment they exhibit gain, antennas magically create power within themselves. Sadly, this is not the case. If one examines an antenna it will be noted it is constructed of basic materials, the best being gold, silver, copper, then aluminium following on. These materials in themselves cannot create power.

Before we go into any explanations there are some terms that need definition so-as to assist in the explanation of antenna gain.

decibel (dB): unit of measure of loss or gain. Gain has a positive value, loss has a negative value, and is equal to

10*log(Pout/Pin)

Antenna Gain: The relative increase in radiation at the maximum point expressed as a value in dB above a standard, in this case the basic antenna, a ½-wavelength dipole (as in Two-Poles) by which all other antennas are measured. The reference is known as 0dBD (zero decibel referenced to dipole). An antenna with the effective radiated power of twice the input power would therefore have a gain of 10*log(2/1) = 3dBD.

A note of warning: There is a second ´reference´ used in antenna gain figures but is used to simply give an antenna a higher gain figure than what is truly achieved. It is known as dBi and represents the gain of an antenna with respect to an imaginary isotropic antenna - one that radiates equally in a spherical pattern (equal in all directions). It increases the antenna gain figure by 2.14dB, this being the ´gain´ of a dipole over an isotropic antenna; But this is not a head start! This is covered more in the paper "Cheating with Antenna Gain"

Radiation Pattern: A graphical representation of the intensity of the radiation vs. the angle from the perpendicular. The graph is usually circular, the intensity indicated by the distance from the centre based in the corresponding angle.

All radiation patterns on this page are with the antenna element(s) mounted vertically, and viewed from the side (i.e. right-angles to the antenna) as seen alongside.

Radiation Angle: It has been generally accepted that beamwidth is the angle between the two points (on the same plane) at which the radiation falls to "half power" i.e. 3dB below the point of maximum radiation. Using anything other than 3dB does not do an antenna´s reputation any good as this could give the impression the antenna has a wider/narrower beamwidth and if a serious engineer looks at this he would, rightly so, discredit the design.

Coverage: The physical geological area where signal is still at a level which can be received, usually described as a radius distance from the antenna site.

To start with let us take a standard ½-wavelength dipole and "suspend" it in free space (i.e. ignore all possible surroundings e.g. the mounting pole etc. that could affect the antenna). The radiation pattern of this antenna is typically referred to as the "doughnut" as shown in the adjoining figure.

As the materials cannot create power the only other alternative is to focus the wasted energy, for example that which is going skywards, towards a more useful direction being on the horizontal plane. The result is shown in the adjoining picture. Here the shape of the radiation was changed such that the outer most energy was focused to compliment the middle half, the result being a doubling of the radiated energy along the required plane or effectively a 3dB gain.

This focusing can be even further intensified such that gains of 6dB (4 times) to 9dB (8 times) can be achieved. The resultant two patterns shown below.

As can be seen the method by which an antenna is made to have "gain" is merely to focus the radiation (i.e. taking the doughnut and flattening it into a pancake) thus intensifying the radiation along the horizontal. Antennas with omni-directional radiation and gains of beyond 9dB are impractical owing to the fact that the focusing is directly related to the length (in wavelengths) of the antenna. There is, however, one further method of focusing, to now intensify the radiation in only one direction.

If a reflector is placed next to a dipole all the energy that would have radiated in the direction of the reflector is now reflected back in the direction of the dipole. This makes all the energy appear in only one hemisphere and thus results in a doubling of radiated energy in this direction or 3dB gain.

Further focusing can be achieved with the use of "directors" and again, by making the angle smaller and smaller i.e. packing all the radiation into one direction, higher gain is achieved. Here it is practical to achieve gains as high as 20dB. The effective angle, however, of such an antenna is small (typically ±10 degrees).

With directional antennas, there is one further figure to bear in mind.

Front-Back Ratio: The driven element of most directional antennas is a dipole with the classic "doughnut" shape radiation pattern perpendicular to its axis. The idea, as shown, is to take this doughnut radiation pattern and squeeze it in to a beam off the front of the antenna. The reflector is usually just a single rod, maybe a collection of them. Even if a bunch, the reflector is not going to stop every scrap of energy from escaping between the cracks! Some will be radiated towards the rear (or, in the case of reception, bypass the reflector and be intercepted by the dipole). Remember, when in free space the dipole is just as sensitive to this direction as it is to the front of the antenna, and has a natural tendency to want to continue with the doughnut pattern.

Even a solid sheet of metal as a reflector will not completely isolate the front from the rear because of "diffraction". Yip, the very tips of the metal will cause some signal to "bend" on the edges of the reflector and toward the rear (or, in the case of reception, from the rear toward the dipole).

The ratio of this front-rear difference is defined with reference to the front (wanted) direction of the antenna, and is usually expressed in dB.

In Closing:

Antennas do not somehow magically create power but simply focus the radiated RF into narrower patterns such that there appears to be more power coming from the antenna in the required direction.

As can be seen, "gain" is also "loss". The higher the gain of an antenna the smaller the effective angle of use. This is the part people forget i.e. that they have robbed power from other directions and superimposed it on the radiation in the intended direction.

This directly impacts the choice of the antenna for a specific function. Choosing the correct antenna is dealt with in "Choosing the correct antenna".

Source: http://www.tp-link.us/FAQ-3.html

           http://www.antenna-theory.com/definitions/decibels.php

Why Ocean Water is Salty?

         There is no salt in fresh rain-water. River water tastes fresh, sea water salty. Yet the oceans are fed by the rivers that flow into them. Then where does the salt in the sea come from?

         When rain falls on the ground it soaks into the earth. In the earth are all sorts of minerals—salt, lime, magnesia, potash, sulphur, iron and many others. These are dissolved or melted and carried along by the water. Of all the minerals in the earth salt is most easily dissolved by water. So very often, we have salt springs. Rivers are fed by springs, and all of the minerals are in river water, but not enough so you can taste them. All the time this salt and other minerals are poured into the ocean by the rivers. When the sun takes vapor up into the rain clouds it takes only the water, leaving the minerals behind, just as lime is left in a teakettle. In this way the minerals in the sea, salt and everything else, slowly becomes greater in quantity as the centuries go by. About three and a half per cent of sea water is minerals, today. That is, if you put one hundred quarts of sea water in a tank and boil until the water is all boiled away, you will have three and a half quarts of dry salt, magnesia, lime, potash and other minerals. The greater part would be salt.

What would you think, then,, of water in which there are from fifteen to twenty quarts of salt and other minerals in every hundred quarts of water? The water of our Great Salt Lake and of the Dead Sea is four or five times as salt as the ocean. Like the ocean, they have no outlets in rivers. So they keep all the minerals that come into them. After ages and ages they will lose all their water, dry up and leave great salt beds behind. Do you think that could ever happen to the big oceans?

Source: http://chestofbooks.com/reference/The-New-Student-s-Reference-Work-Vol5/Why-Is-The-Water-Of-The-Ocean-Salt.html#.ViIKU_krLIU#ixzz3ooTIG3AH

Why Boiling point is low in Mountain Top...

                   If you boil water over a camp fire on the sea shore, you have to heat it to two hundred and twelve degrees. But on a mountain top, water boils before it gets as hot as that. This is because, on low ground, there is more air above the water than on land a mile or two higher. The lighter the air pressure the easier it is for water to expand into gas. Therefore, it takes less heat on a high mountain to make water boil. Of course, then, boiling water away up on the Alps isn't nearly as hot as boiling water on a sea beach. In some very high places boiling water should be just about right for a warm bath, and water there would escape in gas long before it was hot enough to boil an egg.

Source : http://chestofbooks.com/reference/The-New-Student-s-Reference-Work-Vol5/Why-Does-Heat-Make-Water-Boil.html#.ViII8_krLIU#ixzz3ooSDNLH1

Why An Iron Ship Does Not Sink...???

                If you put a nail or a lump of iron in a vessel of water it sinks at once. A piece of wood of the same size floats. So, until fifty years or so ago, people thought all boats and ships must be built of wood, or they would sink.

              Take a sheet-iron pan from the kitchen and put that on the water. It floats. It weighs just as much as the lump of iron that sinks, but the weight is spread or distributed over a larger volume of water That is all. It has been made lighter than the total amount of water it rests upon. A ship is just such a hollow vessel, whether made of iron or wood. When there is nothing in it, a ship stands high, almost on the surface of the water. As it is loaded with goods and people, it rides deeper. Load your sheet iron pan with a cargo of toys. Watch it go deeper. Don't fill it to the top. That would make it as heavy as if it were solid. Then it would sink.

            If you live in a lake or sea-port town, you will find that all ships have a water-line painted plainly around the hull. This is the safety loading line. No ship owners are allowed to load a vessel so heavily that that water line sinks below the surface of the water. Air spaces must be left, to keep the ship and its cargo lighter than the water that is beneath them. In the old days overloaded wooden vessels often sank. Today, iron ships ride the ocean safely.

Source: http://chestofbooks.com/reference/The-New-Student-s-Reference-Work-Vol5/Why-An-Iron-Ship-Does-Not-Sink.html#.ViIG-fkrLIU#ixzz3ooQryes9

What Is Buoyant Force...???

What Is Buoyant Force?

             Buoyant force is an upward force that fluids exerts on any object that is placed in them. The ability of fluids to exert this force is called buoyancy . What explains buoyant force? A fluid exerts pressure in all directions, but the pressure is greater at greater depth. Therefore, the fluid below an object, where the fluid is deeper, exerts greater pressure on the object than the fluid above it. You can see in theFigure below how this works. Buoyant force explains why the girl pictured above can float in water.  

Q : You’ve probably noticed that some things don’t float in water. For example, if you drop a stone in water, it will sink to the bottom rather than floating. If buoyant force applies to all objects in fluids, why do some objects sink instead of float?

A : The answer has to do with their weight.

Weight and Buoyant Force

Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure below . On the left, the object’s weight is less than the buoyant force acting on it, so the object floats. On the right, the object’s weight is greater than the buoyant force acting on it, so the object sinks.

Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and could easily pick up a friend or sibling under the water. Some of the person’s weight was countered by the buoyant force of the water.

Density and Buoyant Force

Density, or the amount of mass in a given volume, is also related to the ability of an object to float. That’s because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why the giant ice berg in the Figure below is floating in the ocean. You can see other examples of density and buoyant force at this URL:

http://www.youtube.com/watch?v=VDSYXmvjg6M

Q : Can you think of more examples of substances that float in a fluid because they are low in density?

A : Oil is less dense than water, so oil from a spill floats on ocean water. Helium is less dense than air, so balloons filled with helium float in air.

Summary

• Buoyant force is an upward force that fluids exert on any object that is placed in them. Buoyant force occurs because the fluid below an object exerts greater pressure on the object than the fluid above it.
• If an object’s weight is less than the buoyant force acting on it, then the object floats. If an object’s weight is greater than the buoyant force acting on it, then the object sinks.
• A given volume of a denser substance is heavier than the same volume of a less dense substance. Therefore, density of an object also affects whether it sinks or floats.

Source : ck12

Soldering Basics ...

Outline
This procedure covers the basic concepts for high quality soldering.

Minimum Skill Level - Intermediate
Recommended for technicians with skills in basic soldering and component rework, but may be inexperienced in general repair/rework procedures.

Acceptability References
IPC-A-610 3.0	Handling Electronic Assemblies
IPC-A-610 5.0	Component Installation
IPC-A-610 6.0	Soldering
IPC-A-610 7.0	Cleanliness

Procedure References
1.0	Foreword
2.1	Handling Electronic Assemblies
2.2	Cleaning
2.5	Baking And Preheating
7.1.2	Preparation For Soldering

Tools and Materials

Images and Figures

Figure 1: Wetting occurs when molten solder penetrates a copper surface, forming an intermetallic bond.

Figure 2: Minimal thermal linkage due to insufficient solder between the pad and soldering iron tip.

Figure 3: A solder bridge provides thermal linkage to transfer heat into the pad and component lead.

Figure 4: Solder blends to the soldered surface, forming a small contact angle.

Soldering Process
Soldering is the process of joining two metals by the use of a solder alloy, and it is one of the oldest known joining techniques. Faulty solder joints remain one of the major causes of equipment failure and thus the importance of high standards of workmanship in soldering cannot be overemphasized.  The following material covers basic soldering procedures and has been designed to provide the fundamental knowledge needed to complete the majority of high reliability hand soldering and component removal operations.

Properties of Solder
Solder used for electronics is a metal alloy, made by combining tin and lead in different proportions. You can usually find these proportions marked on the various types of solder available.
With most tin/lead solder combinations, melting does not take place all at once. Fifty-fifty solder begins to melt at 183 C -361 F, but it's not fully melted until the temperature reaches 216 C - 420 F. Between these two temperatures, the solder exists in a plastic or semi-liquid state.
The plastic range of a solder varies, depending upon the ratio of tin to lead. With 60/40 solder, the range is much smaller than it is for 50/50 solder. The 63/37 ratio, known as eutectic solder has practically no plastic range, and melts almost instantly at 183 C -361 F.
The solders most commonly used for hand soldering in electronics are the 60/40 type and the 63/37 type. Due to the plastic range of the 60/40 type, you need to be careful not to move any elements of the joint during the cool down period. Movement may cause what is known as disturbed joint. A disturbed joint has a rough, irregular appearance and looks dull instead of bright and shiny. A disturbed solder joint may be unreliable and may require rework.

Wetting Action
When the hot solder comes in contact with a copper surface, a metal solvent action takes place. The solder dissolves and penetrates the copper surface. The molecules of solder and copper blend to form a new alloy, one that's part copper and part solder. This solvent action is called wetting and forms the intermetallic bond between the parts. (See Figure 1) Wetting can only occur if the surface of the copper is free of contamination and from the oxide film that forms when the metal is exposed to air. Also, the solder and work surface need to have reached the proper temperature.
Although the surfaces to be soldered may look clean, there is always a thin film of oxide covering it. For a good solder bond, surface oxides must be removed during the soldering process using flux.

Flux
Reliable solder connections can only be accomplished with truly cleaned surfaces. Solvents can be used to clean the surfaces prior to soldering but are insufficient due to the extremely rapid rate at which oxides form on the surface of heated metals. To overcome this oxide film, it becomes necessary in electronic soldering to use materials called fluxes. Fluxes consist of natural or synthetic rosins and sometimes chemical additives called activators.
It is the function of the flux to remove oxides and keep them removed during the soldering operation. This is accomplished by the flux action which is very corrosive at solder melt temperatures and accounts for flux's ability to rapidly remove metal oxides. In its unheated state, however, rosin flux is non-corrosive and non-conductive and thus will not affect the circuitry. It is the fluxing action of removing oxides and carrying them away, as well as preventing the reformation of new oxides that allows the solder to form the desired intermetallic bond.
Flux must melt at a temperature lower than solder so that it can do its job prior to the soldering action. It will volatilize very rapidly; thus it is mandatory that flux be melted to flow onto the work surface and not be simply volatilized by the hot iron tip to provide the full benefit of the fluxing action. There are varieties of fluxes available for many purposes and applications. The most common types include: Rosin - No Clean, Rosin - Mildly Activated and Water Soluble.
When used, liquid flux should be applied in a thin, even coat to those surfaces being joined and prior to the application of heat. Cored wire solder and solder paste should be placed in such a position that the flux can flow and cover the joints as the solder melts. Flux should be applied so that no damage will occur to the surrounding parts and materials.
Soldering Irons
Soldering irons come in a variety of sizes and shapes. A continuously tinned surface must be maintained on the soldering iron tip's working surface to ensure proper heat transfer and to avoid transfer of impurities to the solder connection.
Before using the soldering iron the tip should be cleaned by wiping it on a wet sponge. When not in use the iron should be kept in a holder, with its tip clean and coated with a small amount of solder

Note
Although tip temperature is not the key element in soldering you should always start at the lowest temperature possible. A good rule of thumb is to set the soldering iron tip temperature at 260 C - 500 F and increase the temperature as needed to obtain the desired result.
Controlling Heat
Controlling soldering iron tip temperature is not the key element in soldering. The key element is controlling the heat cycle of the work. How fast the work gets hot, how hot it gets, and how long it stays hot is the element to control for reliable solder connections.
Thermal Mass
The first factor that needs to be considered when soldering is the relative thermal mass of the joint to be soldered. This mass may vary over a wide range.
Each joint, has its own particular thermal mass, and how this combined mass compares with the mass of the iron tip determines the time and temperature rise of the work.
Surface Condition
A second factor of importance when soldering is the surface condition. If there are any oxides or other contaminants covering the pads or leads, there will be a barrier to the flow of heat. Even though the iron tip is the right size and temperature, it may not be able to supply enough heat to the joint to melt the solder.
Thermal Linkage
A third factor to consider is thermal linkage. This is the area of contact between the iron tip and the work.
Figure 2 shows a view of a soldering iron tip soldering a component lead. Heat is transferred through the small contact area between the soldering iron tip and pad. The thermal linkage area is small.
Figure 3 also shows a view of a soldering iron tip soldering a component lead. In this case, the contact area is greatly increased by having a small amount of solder at the point of contact. The tip is also in contact with both the pad and component further improving the thermal linkage. This solder bridge provides thermal linkage and assures the rapid transfer of heat into the work.
Applying Solder
In general, the soldering iron tip should be applied to the maximum mass point of the joint. This will permit the rapid thermal elevation of the parts to be soldered. Molten solder always flows from the cooler area toward the hotter one.
Before solder is applied; the surface temperature of the parts being soldered must be elevated above the solder melting point. Never melt the solder against the iron tip and allow it to flow onto a surface cooler than the solder melting temperature. Solder applied to a cleaned, fluxed and properly heated surface will melt and flow without direct contact with the heat source and provide a smooth, even surface, filleting out to a thin edge. Improper soldering will exhibit a built-up, irregular appearance and poor filleting. For good solder joint strength, parts being soldered must be held in place until the solder solidifies.
If possible apply the solder to the upper portion of the joint so that the work surfaces and not the iron will melt the solder, and so that gravity will aid the solder flow. Selecting cored solder of the proper diameter will aid in controlling the amount of solder being applied to the joint. Use a small gauge for a small joint, and a large gauge for a large joint.
Post Solder Cleaning
When cleaning is required, flux residue should be removed as soon as possible, but no later than one hour after soldering. Some fluxes may require more immediate action to facilitate adequate removal. Mechanical means such as agitation, spraying, brushing, and other methods of applications may be used in conjunction with the cleaning solution.
The cleaning solvents, solutions and methods used should not have affected the parts, connections, and materials being cleaned. After cleaning, boards should be adequately dried.
Resoldering
Care should be taken to avoid the need for resoldering. When resoldering is required, quality standards for the resoldered connection should be the same as for the original connection.
A cold or disturbed solder joint will usually require only reheating and reflowing of the solder with the addition of suitable flux. If reheating does not correct the condition, the solder should be removed and the joint resoldered.
Workmanship
Solder joints should have a smooth appearance. A satin luster is permissible. The joints should be free from scratches, sharp edges, grittiness, looseness, blistering, or other evidence of poor workmanship. Probe marks from test pins are acceptable providing that they do not affect the integrity of the solder joint.
An acceptable solder connection should indicate evidence of wetting and adherence when the solder blends to the soldered surface. The solder should form a small contact angle; this indicates the presence of a metallurgical bond and metallic continuity from solder to surface. (See Figure 4)
Smooth clean voids or unevenness on the surface of the solder fillet or coating are acceptable. A smooth transition from pad to component lead should be evident.

Source : http://www.circuitrework.com/guides/7-1-1.shtml

What Is Flux? Why it is used in soldering ???

                      In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise

Advantages of HVDC over HVAC transmission ...

Advantages of HVDC over HVAC transmission (on photo: The valve hall of the HVDC system is where the power is made ready for transmission; by Siemens)

AC as preferred option

Despite alternating current being the dominant mode for electric power transmission, in a number of applications, the advantages of HVDC makes it the preferred option over AC transmission.
Examples include:

1. Undersea cables where high capacitance causes additional AC losses (e.g., the 250-km Baltic Cable between Sweden and Germany).
2. Endpoint-to-endpoint long-haul bulk powertransmission without intermediate taps, for example, in remote areas.
3. Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
4. Allowing power transmission between unsynchronized AC distribution systems.
5. Reducing the profile of wiring and pylons for a given power transmission capacity, as HVDC can carry more power per conductor of a given size.
6. Connecting a remote generating plant to the distribution grid; for example, the Nelson River Bipole line in Canada (IEEE 2005).
7. Stabilizing a predominantly AC power grid without increasing the maximum prospective short-circuit current.
8. Reducing corona losses (due to highervoltage peaks) compared to HVAC transmission lines of similar power.
9. Reducing line cost, since HVDC transmission requires fewer conductors; for example, two for a typical bipolar HVDC line compared to three for three-phase HVAC.

HVDC transmission is particularly advantageous in undersea power transmission. Long undersea AC cables have a high capacitance.

Example (VIDEO)

 

500 MW HVDC Light transmission interconnection

ABB has commissioned a 500-megawatt HVDC Light (VSC) transmission interconnection that links the Irish and U.K. grids, enabling cross-border power flows and enhancing grid reliability and security of electricity supplies.
The East West Interconnector includes a 262 km high voltage cable link of which 186 km runs subsea.

Consequently, the current required to charge and discharge the capacitance of the cable causes additional power losses when the cable is carrying AC, while this has minimal effect for DC transmission. In addition, AC poweris lost to dielectric losses.

In general applications, HVDC can carry more power per conductor than AC, because for a given power rating, the constant voltage in a DC line is lower than the peak voltage in an AC line.

This voltage determines the insulation thickness and conductor spacing. This reduces the cost of HVDC transmission lines as compared to AC transmission and allows transmission line corridors to carry a higher power density.
A HVDC transmission line would not produce the same sort of extremely low frequency (ELF) electromagnetic field as would an equivalent AC line. While there has been some concern in the past regarding possible harmful effects of such fields, including the suspicion of increasing leukemia rates, the current scientific consensus does not consider ELF sources and their associated fields to be harmful.
Deployment of HVDC equipment would not completely eliminate electric fields, as there would still be DC electric field gradients between the conductors and ground. Such fields are not associated with health effects.

Because HVDC allows power transmission between unsynchronized AC systems, it can help increase system stability. It does so by preventing cascading failures from propagating from one part of a wider power transmission grid to another, while still allowing power to be imported or exported in the event of smaller failures.

This feature has encouraged wider use of HVDC technology for its stability benefits alone. Power flow on an HVDC transmission line is set using the control systems of converter stations. Power flow does not depend on the operating mode of connected power systems.
Thus, unlike HVAC ties, HVDC intersystem ties can be of arbitrarily low transfer capacity, eliminating the “weak tie problem,” and lines can be designed on the basis of optimal power flows.
Similarly, the difficulties of synchronizing different operational control systems at different power systems are eliminated. Fast-acting emergency control systems on HVDC transmission lines can further increase the stability and reliability of the power system as a whole. Further, power flow regulation can be used for damping oscillations in powersystems or in parallel HVAC lines.
The advantages described above encourage the use of DC links for separating large power systems into several nonsynchronous parts.

Direct-Current (HVDC) Transmission Lines

For example, the rapidly growing Indian power system is being constructed as several regional power systems interconnected with HVDC transmission lines and back-to-back converters with centralized control of these HVDC elements (Koshcheev 2001).
Likewise, in China, ±800-kV HVDC will be the main mode used to transmit large capacity over very long distances from large hydropower and thermal power bases. Other applications involve long-distance transmission projects with few tie-ins of power supplies along the line (Yinbiao 2005).

Reference: Argonne National Laboratory – The design, construction and operation of long-distance high voltage electricity transmission technologies

Why are DC high-voltage transmission lines replacing AC high-voltage transmission lines at some places ???

         With AC systems the peak voltage is 2^0.5 (1.4142) times the RMS (nominal) voltage.  At transmission voltages that additional .4142 x voltage makes the insulation systems much more critical, lengthening the insulator strings and so increasing the cost of the insulators and poles required to separate the lines.

With DC systems the peak voltage is the nominal voltage, so the insulator strings need only deal with the nominal voltage reducing capital and maintenance costs (washing from helicopters etc.).

With very high voltage transmission where maximum voltage is limited by available technology, this fact is used to increase the nominal voltage on a given insulation type, resulting in a system which would be operated at 750,000 volts nominal (RMS) AC being used for 1,000,000 volts actual DC with 25% additional capacity for the same amperage (cable) design.

The other factors are

1. In HVDC no corona loss is there as incase of HVAC.
2. For longer distance,HVDC system is very economical as compared to the HVAC system.
3. Allowing power transmission between unsynchronized AC distribution systems.
4. HVDC increases the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
5. For  DC  frequency is 0.Therefor there is no inductive reactance drop, result of it is improve voltage regulation.
6. There is no problem of stability as in AC.
7. There is no skin effect in DC.
8. AC require 3 wire for transmission but DC require only 2 .
9. There is no ferranti effect in DC{it doesn't  mean that capacitance is absent in DC,capacitance act as open CKT for DC}

 

Why is electrical power transmitted at low frequencies and not at high frequencies?

GENERATION

• Power is generated from Synchronous machines (Alternators) which rotate at a particular speed called the synchronous speed given by

Ns= 120f /P
and the frequency is given by :
f= P*N /120

Now to produce voltage at a high frequency , two things have to be increased :

1. Number of poles in machine
2. Speed of machine

But these two factors contradict each other. Suppose for example you have a hypothetical machine of 50 poles , you cannot rotate it at higher speeds because of high centrifugal force , vibrational force and mechanical strength.

Since the number of poles in this Alternator have increased (compared to two pole  High speed turbogenerators rotating at 3000/3600 RPM at 50/60 Hz respectively) ,  the diameter of the machine will increase  which will make the size of machine large.

Now that to produce power at High frequency  , you would have to rotate the machine at high speeds which would require higher steam input for same amount of power at a higher frequency.

Higher input to alternator will deteriorate it mechanically. To paraphrase a high frequency alternator needs to be mechanically robust and thus will require a larger size and more material in construction.


TRANSMISSION

• Coming to your actual questions about transmission of power at high frequency , We need to realize the model of transmission line

A transmission line physically is a R,L,C,G circuit with R and L being series parameters . *R being frequency independent to be more specific*
While C and G are shunt parameters of line *G being frequency independent*

If the voltage at high frequency is used to transmit power

1. Drop across inductor will increase as V = I*X =I*2*3.14*f*L (f being the frequency).
2. A higher drop will cause the line voltage to sag.Hence the coveted flat voltage profile will not be obtained.
3. The voltage will decrease with increase in length and  it will be  less most at the receiving end (where it is needed the most).
4. Corona loss is directly proportional to frequency, hence the losses due to corona will increase
5. This will cause more interference with communication/telephone lines.
6. High losses in transmission wires due to skin effect.
7. Because of skin effect , the cost of conductor material will increase  as the conductor is not utilized fully at high frequencies.
8. Economy of transmission is a major point while designing transmission lines . increasing the cost of conductor will make the system uneconomical
9. Since the resistance due to skin effect increases ( assume Rac=1.6Rdc), line losses (I^2R ones) will increase and hence the power transmission capability will decrease
10. FACTS devices or compensators will be required for reactive power demand/supply making the system even more costly.

(Will add more if any other things come to mind)

DISTRIBUTION & UTILIZATION
Even in distribution , the high frequency AC will not be that efficient :

1. We use a lot of power electronic devices. A high frequency will cause large switching losses.
2. Harmonics of higher frequencies being fed back to grid , filter requirements might be there.
3. Poor voltage regulation
4. Possibility of  thermal noise/shot noise /white noise (high frequency ) interference with home appliances

Why do we hear a hissing noise while passing a high voltage line?

This is because of corona effect.

When an alternating current is made to flow across conductor of the transmission line, then air surrounding the conductor (composed of ions) is subjected to di-electric stress. At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. But when the potential difference is made to increase beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. This phenomena of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system which must be accounted for.

Corona Effect is the phenomenon of purple glow, hissing noise and production of ozone gas in a transmission line.