A Novel design of Capacitive Transformer

ABSTRACT

The proposed micro transformer is based on polarization in dielectrics. It is a low expensive, small, lightweight, core less, heat less, high efficiency transformer. It has variable output voltages with good current output. It is just like the conventional transformers but the number of windings is less and core is not used. It has only two terminals like diode. The both primary and secondary winding wires are made up of copper and coated by silica dioxide. The area of conductor determines the voltage induction and the dielectric medium determines the current output. Here silica dioxide is used as a dielectric medium because it has good permittivity. In this transformer the electromagnetic field is not created. The novel theory behind it is that wherever an alternating potential presents, a region is developed around it. This phenomenon can be named as Electro Tele Field (ETF). The dipoles are rotated due to the application of alternative voltage. The alternative voltage consists of positive and negative half cycles. During the positive half cycle, dielectric medium is positively polarized, and during the negative half cycle, medium becomes negatively polarized. Hence dipoles get rotated continuously. A continuity tester can detect that field. ETF can flow through conductors and penetrate in insulators. The paper explains the novel effect of alternative voltage on a dielectric medium. The main application is that the emf can be induced from one coil to another coil without electromagnetic field. It is hoped that the micro transformer will make a turning point in transformer usage.

Necessity to work on this project topic : An accident made this innovation. one day i bought a continuity tester in front of the television picture tube, then the tester glows without any electrical contact. this incident makes me to do series of experiments on this process. i named it as Electro Tele Field [ETF]. The word Tele indicates television picture tube because very first time, this field was discovered in front of television picture tube so that the field is named as "tele field". At last i could find the reason behind that. By the application of this new theory, A novel core less micro transformer has made successfully.

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TRANSFORMER -- HOW IT'S DESIGNED?!!!

Transformer Construction

The construction of a simple two-winding transformer consists of each winding being wound on a separate limb or core of the soft iron form which provides the necessary magnetic circuit. This magnetic circuit, know more commonly as the “transformer core” is designed to provide a path for the magnetic field to flow around, which is necessary for induction of the voltage between the two windings.

However, this type of transformer construction were the two windings are wound on separate limbs is not very efficient since the primary and secondary windings are well separated from each other. This results in a low magnetic coupling between the two windings as well as large amounts of magnetic flux leakage from the transformer itself. But as well as this “O” shapes construction, there are different types of “transformer construction” and designs available which are used to overcome these inefficiencies producing a smaller more compact transformer.

The efficiency of a simple Transformer Construction can be improved by bringing the two windings within close contact with each other thereby improving the magnetic coupling. Increasing and concentrating the magnetic circuit around the coils may improve the magnetic coupling between the two windings, but it also has the effect of increasing the magnetic losses of the transformer core.

As well as providing a low reluctance path for the magnetic field, the core is designed to prevent circulating electric currents within the iron core itself. Circulating currents, called “eddy currents”, cause heating and energy losses within the core decreasing the transformers efficiency.

These losses are due mainly to voltages induced in the iron circuit, which is constantly being subjected to the alternating magnetic fields setup by the external sinusoidal supply voltage. One way to reduce these unwanted power losses is to construct the transformer core from thin steel laminations.

In all types of transformer construction, the central iron core is constructed from of a highly permeable material made from thin silicon steel laminations assembled together to provide the required magnetic path with the minimum of losses. The resistivity of the steel sheet itself is high reducing the eddy current losses by making the laminations very thin.

These steel transformer laminations vary in thickness’s from between 0.25mm to 0.5mm and as steel is a conductor, the laminations are electrically insulated from each other by a very thin coating of insulating varnish or by the use of an oxide layer on the surface.

Transformer Construction of the Core

Generally, the name associated with the construction of a transformer is dependant upon how the primary and secondary windings are wound around the central laminated steel core. The two most common and basic designs of transformer construction are the Closed-core Transformer and the Shell-core Transformer.

In the “closed-core” type (core form) transformer, the primary and secondary windings are wound outside and surround the core ring. In the “shell type” (shell form) transformer, the primary and secondary windings pass inside the steel magnetic circuit (core) which forms a shell around the windings as shown below.

Transformer Core Construction

 

In both types of transformer core design, the magnetic flux linking the primary and secondary windings travels entirely within the core with no loss of magnetic flux through air. In the core type transformer construction, one half of each winding is wrapped around each leg (or limb) of the transformers magnetic circuit as shown above.

The coils are not arranged with the primary winding on one leg and the secondary on the other but instead half of the primary winding and half of the secondary winding are placed one over the other concentrically on each leg in order to increase magnetic coupling allowing practically all of the magnetic lines of force go through both the primary and secondary windings at the same time. However, with this type of transformer construction, a small percentage of the magnetic lines of force flow outside of the core, and this is called “leakage flux”.

Shell type transformer cores overcome this leakage flux as both the primary and secondary windings are wound on the same centre leg or limb which has twice the cross-sectional area of the two outer limbs. The advantage here is that the magnetic flux has two closed magnetic paths to flow around external to the coils on both left and right hand sides before returning back to the central coils.

This means that the magnetic flux circulating around the outer limbs of this type of transformer construction is equal to Φ/2. As the magnetic flux has a closed path around the coils, this has the advantage of decreasing core losses and increasing overall efficiency.

Transformer Laminations

But you may be wondering as to how the primary and secondary windings are wound around these laminated iron or steel cores for this types of transformer constructions. The coils are firstly wound on a former which has a cylindrical, rectangular or oval type cross section to suit the construction of the laminated core. In both the shell and core type transformer constructions, in order to mount the coil windings, the individual laminations are stamped or punched out from larger steel sheets and formed into strips of thin steel resembling the letters“E’s”, “L’s”, “U’s” and “I’s” as shown below.

Transformer Core Types

 

These lamination stampings when connected together form the required core shape. For example, two “E” stampings plus two end closing “I” stampings to give an E-I core forming one element of a standard shell-type transformer core. These individual laminations are tightly butted together during the transformers construction to reduce the reluctance of the air gap at the joints producing a highly saturated magnetic flux density.

Transformer core laminations are usually stacked alternately to each other to produce an overlapping joint with more lamination pairs being added to make up the correct core thickness. This alternate stacking of the laminations also gives the transformer the advantage of reduced flux leakage and iron losses. E-I core laminated transformer construction is mostly used in isolation transformers, step-up and step-down transformers as well as auto transformers.

Transformer Winding Arrangements

Transformer windings form another important part of a transformer construction, because they are the main current-carrying conductors wound around the laminated sections of the core. In a single-phase two winding transformer, two windings would be present as shown. The one which is connected to the voltage source and creates the magnetic flux called the primary winding, and the second winding called the secondary in which a voltage is induced as a result of mutual induction.

If the secondary output voltage is less than that of the primary input voltage the transformer is known as a “Step-down Transformer”. If the secondary output voltage is greater then the primary input voltage it is called a “Step-up Transformer”.

Core-type Construction

The type of wire used as the main current carrying conductor in a transformer winding is either copper or aluminium. While aluminium wire is lighter and generally less expensive than copper wire, a larger cross sectional area of conductor must be used to carry the same amount of current as with copper so it is used mainly in larger power transformer applications.

Small kVA power and voltage transformers used in low voltage electrical and electronic circuits tend to use copper conductors as these have a higher mechanical strength and smaller conductor size than equivalent aluminium types. The downside is that when complete with their core, these transformers are much heavier.

Transformer windings and coils can be broadly classified in to concentric coils and sandwiched coils. In core-type transformer construction, the windings are usually arranged concentrically around the core limb as shown above with the higher voltage primary winding being wound over the lower voltage secondary winding.

Sandwiched or “pancake” coils consist of flat conductors wound in a spiral form and are so named due to the arrangement of conductors into discs. Alternate discs are made to spiral from outside towards the centre in an interleaved arrangement with individual coils being stacked together and separated by insulating materials such as paper of plastic sheet. Sandwich coils and windings are more common with shell type core construction.

Helical Windings also known as screw windings are another very common cylindrical coil arrangement used in low voltage high current transformer applications. The windings are made up of large cross sectional rectangular conductors wound on its side with the insulated strands wound in parallel continuously along the length of the cylinder, with suitable spacers inserted between adjacent turns or discs to minimize circulating currents between the parallel strands. The coil progresses outwards as a helix resembling that of a corkscrew.

Transformer Cores

The insulation used to prevent the conductors shorting together in a transformer is usually a thin layer of varnish or enamel in air cooled transformers. This thin varnish or enamel paint is painted onto the wire before it is wound around the core.

In larger power and distribution transformers the conductors are insulated from each other using oil impregnated paper or cloth. The whole core and windings is immersed and sealed in a protective tank containing transformer oil. The transformer oil acts as an insulator and also as a coolant.

Transformer Dot Orientation

We can not just simply take a laminated core and wrap one of the coil configurations around it. We could but we may find that the secondary voltage and current may be out-of-phase with that of the primary voltage and current. The two coil windings do have a distinct orientation of one with respect to the other. Either coil could be wound around the core clockwise or anticlockwise so to keep track of their relative orientations “dots” are used to identify a given end of each winding.

This method of identifying the orientation or direction of a transformers windings is called the “dot convention”. Then a transformers windings are wound so that the correct phase relations exist between the winding voltages with the transformers polarity being defined as the relative polarity of the secondary voltage with respect to the primary voltage as shown below.

Transformer Construction using Dot Orientation

 

The first transformer shows its two “dots” side by side on the two windings. The current leaving the secondary dot is “in-phase” with the current entering the primary side dot. Thus the polarities of the voltages at the dotted ends are also in-phase so when the voltage is positive at the dotted end of the primary coil, the voltage across the secondary coil is also positive at the dotted end.

The second transformer shows the two dots at opposite ends of the windings which means that the transformers primary and secondary coil windings are wound in opposite directions. The result of this is that the current leaving the secondary dot is 180o “out-of-phase” with the current entering the primary dot. So the polarities of the voltages at the dotted ends are also out-of-phase so when the voltage is positive at the dotted end of the primary coil, the voltage across the corresponding secondary coil will be negative.

Then the construction of a transformer can be such that the secondary voltage may be either “in-phase” or “out-of-phase” with respect to the primary voltage. In transformers which have a number of different secondary windings, each of which is electrically isolated from each other it is important to know the dot polarity of the secondary windings so that they can be connected together in series-aiding (secondary voltage is summed) or series-opposing (the secondary voltage is the difference) configurations.

The ability to adjust the turns ratio of a transformer is often desirable to compensate for the effects of variations in the primary supply voltage, the regulation of the transformer or varying load conditions. Voltage control of the transformer is generally performed by changing the turns ratio and therefore its voltage ratio whereby a part of the primary winding on the high voltage side is tapped out allowing for easy adjustment. The tapping is preferred on the high voltage side as the volts per turn are lower than the low voltage secondary side.

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Methods of Cooling Transformers

 When transformer supplies a load, two types of losses occur inside the transformer. The iron losses occur in the core while copper losses occur in the windings. The power lost due to these losses appears in the form of heat. This heat increases the temperature of the transformer.

Key Point : To keep the temperature rise of the transformer within limits, it is necessary to dissipate the heat developed to the surroundings.

      A suitable coolant and cooling method is necessary for each transformer to dissipate the heat, effectively to the surroundings.

       Basically there are two types of transformers, dry type transformers and oil immersed transformers. In dry type, the heat is taken to the walls of tank and dissipate to the surrounding air through convection. In oil immersed type, the oil is used as coolant. The entire assembly including core and windings is kept immersed in a suitable oil. The heat developed is transferred to the walls of tank by convection through oil. And finally heat is transferred to the surroundings from the tank walls by radiation.

      The various cooling methods are designated using letter symbols which depend upon :

i) Cooling medium used and ii) Type of circulation employed

      The various coolants used along with their symbols are,

1. Air - A,                    2. Gas - G,            3. Synthetic oil - L,

4. Mineral oil - O,        5- Solid insulation - S   and    6. Water - W

       There are two types of circulations which are,

1. Neutral - N  and  2. Forced - F

       In natural cooling, the coolant circulating inside the transformer transfers entire heat to the tank walls from where it is dissipated to the surroundings and transformers gets cooled by natural air circulating surrounding the tank walls.

      In forced cooling, the coolant circulating inside the transformer gets heated as it comes in contact with windings and core. The coolant partly transfers heat to the tank walls but mainly coolant is taken to the external heat exchanger where air or water is used in order to dissipate heat of the coolant.

1.1 Cooling Methods by Dry Type Transformers

The cooling methods of dry type transformers are classified as,

1. Air Natural (AN) :

       This method uses atmospheric air as cooling medium. The natural air surrounding the tank walls is used to carry away the heat generated, by natural convection. It is used for small voltage transformers. Due to the available insulating materials like glass and silicon resins now a days, the method can be used for the transformers upto ratings 1.5 MVA.

2. Air Blast (AB) :

       In large transformers, cooling by natural air is inadequate. In such cases, the transformer is located above the air chamber and a blast of compressed air is forced on core and windings with the help of blowers or fans. This improves the heat dissipation and hence higher specific loadings are allowed in dry type transformers. This reduced the size of transformers. The air supply must be property filtered to prevent accumulation of dust particles.

1.2 Cooling Methods for Oil Immersed Transformers 

The oil used as a coolant has following advantages,

1. It is good conductor of heat than air.

2. It has high coefficient of volume expansion. Due to this, adequate circulation is easily obtained.

3. The oil acts as an insulating medium, which increases the insulating strength.

       The only limitation of oil immersed transformers is that these transformers can not be used at places like mines where there are chances of fire hazard.

       The various cooling methods used for such oil immersed transformers are classified as, 

1. Oil natural (ON) :

       The transformer is immersed in oil so heat generated in core and windings is passed on to oil by conduction. The heated oil transfer heat to the tank wall from where it is taken away to the surrounding air. The assembly of oil immersed transformer is shown in the Fig. below

 The tubes are provided on the sides of a transformer tank. The oil in the tank is taken to the tubes. The circulation of oil through tubes causes the cooling.

        The temperature rise of a transformer can be reduced by,

1. Increasing the area of heat dissipation.

2. Decreasing the cooling coefficient.

       As the rating of transformer increases the plain walled tank can not be used. It is necessary to reduce the cooling coefficient. This is achieved by use of some improved methods of cooling.

       The transformers up to 30 KVA use plain walled tanks. But transformer with ratings higher than 30 KVA use corrugations, fins, tubes and radiator tanks. The Fig. below shows the fins and corrugations provided on tank walls.

The heat developed inside the transformer is taken outside with the help of oil. The oil is cooled with the help of fins, tubes or external radiations by natural circulation of air.

       Hence these methods are called Oil Natural and Air Natural (ONAN) methods. The tubes are used for transformers upto ratings 5 MVA.

2. Oil Natural Air Forced (ONAF) : 

       In this method, the tank is made hollow and compressed air is blown into the hollow space to cool the transformer. The oil circulating inside takes heat to the tank walls. The method is effective and can be used for large rating transformers. Another way to force air blast is to use elliptical tubes separated from tank walls through which air is forced by fans.

3. Oil Natural Water Forced (ONWF) :

        In this method, the copper cooling coils or pipes are fitted above the core but below the oil surface. The cool water is forced through these coils or pipes which provides the additional cooling where natural water head is available, this method is very cheap. The pipes are provided with fans to increase conduction of heat from oil to pipes. The major disadvantage of this method is, incase of leakage of water. the water can contaminate the oil reducing the dielectric strength of oil              

1.3 Oil Forced Methods With Heat Exchangers

In these method, forced circulation oil (OF) is the main feature. The motor driven pump is used to force the oil from top of transformer to the external heat exchanger. In the heat exchanger, the oil is cooled with some methods like use of air blast, water blast etc. The cold oil is circulated back to the transformer from the bottom.

      The oil forced methods are classified depending on how the oil is cooled in the heat exchangers. These methods are,

1. Oil Forced Air Natural (OFAN) :

      The oil is circulated with the help of pump and in the heat exchanger it is cooled with the help of natural air. This method is rarely used in practice.

2. Oil Forced Air Forced (OFAF) :

       In the external heat exchanger the compressed air is blasted with the help of fans to cool is the oil. The advantage of this method is at low loads when losses are less there is no need to use the fans to cool the oil. The natural air is sufficient. At higher loads, both fans and pump are switched on by sensing the temperature which improves the cooling. Hence efficiency of this system is higher. The scheme is shown in the Fig.below

3. Oil Forced Water Forced (OFWF) : 

      In this method, in the heat exchanger instead of air blast, water blast is used to cool the oil. The pressure oil is kept higher than water so oil mixes with water in case of leakage but water dose not mix with oil. Due to this method, smaller transformer size is sufficient as it is not necessary to employ water tubes inside the transformer tank. The method is suitable for transformers having ratings more than 30 MVA. The method is used for the transformers at hydroelectric stations as large water supply with appropriate water head is easily available. The scheme is shown in the Fig. below

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Heat Run Test on Three Phase Transformers

Heat run test is similar to that of back to back test which can be conducted on the transformers. In heat run test, the watt meters are not required which are used in back to back test. Only a voltmeter is used to measure the primary applied voltage and ammeter to measure current in the secondary side. The arrangement of the transformers in connection for heat run test is shown in the Fig.1.

The primary side is excited at normal voltage and frequency. The secondary side is connected in open-delta. It is provided with a circulating current from an auxiliary single phase supply of any convenient frequency. The method gives the same results as that of back to back connection without the requirement of two identical transformers. It is possible to apply the method whatever the normal internal connections if temporary alterations can be made where necessary.

       The heat run test is continued until the windings and the oil in the tank attain a steady temperature. This temperature rise must be within permissible limits which is set by designer.

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