TRANSFORMERS

Introduction
For transmission and distribution networks to transfer large amounts of alternating current electricity over long distances with minimum losses and least cost, different voltage levels are required in the various parts of the networks. For example, the transfer of electricity efficiently over a long transmission line requires the use of high voltages. At the receiving end where the electricity is used, the high voltage has to be reduced to the levels required by the consumer. Transformers enable these changes in voltage to be carried out easily, cheaply and efficiently. A transformer used to increase the voltage is called a "step up" transformer, while that used to decrease the voltage is called a "step down" transformer.

Theory of the Transformer
The operation of a transformer is based on two principles:

1. A voltage is induced in a conductor when the conductor passes through a magnetic field. The same effect is produced if the conductor is stationary but the magnetic field in which it is located varies; and
2. A current passing through a conductor will develop a magnetic field around the conductor.
Note: In this discussion on transformers, the term magnetic "flux" will usually be used instead of magnetic "field". A magnetic field is the space or region surrounding a magnet or a current carrying conductor, in which magnetic effects can be detected. The strength of the magnetic field is generally expressed in terms of magnetic flux density (magnetic flux per square meter). Magnetic flux refers to the magnetic lines of force.

A transformer consists of two coils electrically separate but linked by a common magnetic circuit of low reluctance formed by a laminated soft iron core. If one coil (the primary coil) is connected to an AC supply, an alternating magnetic flux is set up in the iron core. This alternating magnetic flux passes through the secondary coil and induces and alternating voltage in the secondary coil. The magnitude of the secondary voltage is directly proportional to the ratio of the number of turns in the secondary and primary windings and to the primary voltage.

Construction of a Large Transformer
The iron core, which forms a complete magnetic circuit, is made up of laminated strips of special steel having low hysteresis loss and high electrical resistivity. The lamination of the core reduces the eddy-current loss.

For the average transformer used in a power station, the conductor used for the windings consists of paper insulated copper bar or wire. In assembling the transformer, great care is taken to ensure windings are well insulated both from the iron core and from each other.

The basic construction of a core type transformer consist of the iron core, then a cylinder of insulation, followed by the low voltage winding, then a further insulating cylinder and then the high voltage winding. Clamps are used to hold the assembly in place. These basic components are shown on the attached diagram and are also shown in the attached part cross-section of a very large transformer.

The assembled transformer has its winding and iron core assembly usually contained in a tank and immersed in transformer oil. The oil is used for further insulating purposes plus the removal of heat from the windings. The assembly of the windings on the core allows gaps to enhance the oil circulation around the windings. The tank is constructed with fins or tubes to allow better circulation of the oil and to provide a greater surface area for contact with the cooling air. Very large transformers have banks of fans to provide greater air-cooling and are operated in conjunction with temperature sensors. Some transformers also have forced oil circulation using a pumping system and an oil cooling circuit. In installations where the use of transformer oil needs to be avoided, the cooling medium used can be gas (nitrogen is often used).

Small transformers are often solely air-cooled. Large transformers that are of open construction so that cooling is provided by direct contact with the surrounding air are being developed for indoor use.

Most distribution type transformers have a tap changer, which is a selector switch that allows the voltage ratio of the transformer to be changed by increasing or decreasing the turns of the winding. The different coils of the transformer winding are brought out and connected to the selector switch to allow the additional turns to be brought into or taken out of circuit. In some distribution transformers, the tap changer switch is an off load manual switch, while in others, the tap changer is an on-load automatic switch. In a generator transformer, the tap changer is a very sophisticated device that is automatically operated on load by the system control.

Devices on a transformer normally protect against overload, earth fault, pressure and temperature. For these protection systems, current and voltage transformers are built into the transformers. In large transformers, current transformers are provided in conjunction with the insulated terminals.

Testing

Manufacture
To ensure that the manufacturing process is proceeding as per the design program, a number of tests will be required. The most important of these are:

1. Core plate checks - Incoming core plate is checked for thickness and quality of insulation covering;
2. Core frame insulation resistance - This is checked by megger and by application of a 2 kV RMS or 3 kV DC test voltage on completion or erection of the core and again following replacement of the top yoke after fitting of the windings;
3. Core loss measurement - This is carried out by application of a few temporary turns of cable before the windings are fitted and the core excited to normal flux density; and
4. Tank tests - The first tank of any new design is checked for stiffness and vacuum withstand capability. Tanks are also checked for leak tightness by filling with a fluid of lower viscosity than transformer oil and pressurising for a period of time.
Prior to final testing, the assembled core and windings are heated to between 850°C and 1200°C for a length of time. The time taken can be as long as weeks for very large transformers or a few days for medium sized transformers. The transformer windings will be considered dry when plotted values of power factor drop to a minimum value and the insulation resistance increases rapidly.

It is then best to immerse the transformer winding in the transformer oil while the windings are hot because they tend to absorb the oil. Before the final tests are carried out the transformer, is left to stand for several days to let any remaining air bubbles become absorbed by the oil.

Final Testing
The final works test on a transformer fall into three categories:

Tests to prove that the transformer has been built correctly - These include ratio; polarity; resistance and tap change operation tests;
Tests to prove guarantees - These are losses; impedance; temperature rise and noise levels tests; and
Tests to prove that the transformer will be satisfactory in service for at least thirty years - The test in this category include dielectric or overload and load current runs. For the first transformer of a new design, impulse tests including chopped waves to simulate lightning strikes and withstand capability are usually required.
Transformer Losses
Losses in a transformer are known as 'Iron losses' and Copper Losses'.

Iron Losses
Iron losses are due to hysteresis and eddy-current loss produced by the alternating magnetic flux in the iron core. The iron losses are almost independent of the load and thus are considered to be constant at all loads.

In order to determine the iron loss, one winding of the transformer (whichever is the most convenient) is open circuited. A voltage is applied to the other winding and the power (watts) in this circuit is measured. This power represents the iron losses. Copper losses under these circumstances are negligible.

Copper Losses
Copper losses are the heat losses in the windings due to the electrical resistance of the windings. The copper losses are proportional to the square of the current and therefore to the kVA output. These losses can be calculated from the design data but can also be measured by a test. This test is known as the short circuit test for copper losses.

The short circuit test is carried out by short circuiting one winding, thus causing the transformer to behave like a coil having a leakage impedance equal to that of both windings. A low voltage is applied to the open winding sufficient to circulate full load current through the open winding due to transformer action in the short-circuited winding. Under these conditions, the flux set up in the core is so small that iron losses can usually be neglected and the wattmeter would give the total copper loss.

Maintenance
Large transformers are usually very reliable and efficient. To ensure their continued reliable and efficient operation, the maintenance of the average oil filled type transformer requires a check on the following:

1. Oil quality;
2. The condition of the tank in regard to leaks and cleanliness;
3. A check on the insulating terminals for condition of insulation (breaks or cracks and cleanliness) and oil leaks;
4. Condition of the pressure and temperature elements, and indicators;
5. A check on the connections and the current and voltage transformers where fitted;
6. A check on the breather and the condition of its silica gel;
7. A check on the tap changer;
8. A check on the explosion vent (if fitted);
9. A check on the condition of the connecting cables and connections in the terminal boxes; and
10. A check on the insulation of the transformer windings.

If considered necessary, the internals may have to be removed from the tank and a check made on the condition of the windings and their bracing. This is usually only necessary if the transformer has been subjected to unusual circumstances, such as an external fault, or if the transformer has registered high temperatures or excessive noise.

If the oil does not meet its quality standards, it will have to be changed or filtered. Maintenance procedures may require the oil to be filtered on a regular basis.

Reference WebSite :
http://www.energy.qld.gov.au/electricity/infosite/index.htm