Power Transformer

Transformer is one of the most vital and important electrical machinery. The development of the present day power system is very much attributable to the large number and types of transformer that are in operation in the system, such as, generator transformers, step-up transformers, step-down transformers, interlinking transformers, power transformers & distribution transformers etc. Being a static machine, it is inherently reliable compared to other machines,. Distribution transformers are a important link between the power
system and millions of electricity consumers. Any failure of this important equipment, apart from adversely affecting the consumers, will also mean considerable financial loss to the electricity undertaking. It is therefore of important that utmost care is taken in the design, manufacture, testing, installation, and maintenance of transformers.

A transformer consists of a magnetic core made out of insulated silicon steel laminations. Two distinct sets of windings, one called primary and other called secondary winding, are wound on such core. The transformer helps in converting low voltage into high voltage or visa-versa and accordingly the transformer is termed step-up or step-down. The winding to which the voltage is applied is called primary winding, where as the winding to which the load is connected is called secondary winding. The transformer works on the principle of electro-magnetic induction. Such phenomena can take place in a static device, only, if the magnetic flux is continually varying. It is therefore clear that static transformers can only be used with alternating currents only. When an alternating EMF is applied to the primary winding of a transformer with the secondary winding open circuited, a small current flows in the primary winding which serves to magnetize the core and to feed the iron losses of the transformer. As primary and secondary windings are wound on the same core, the magnetizing flux is the same for both the windings. The magnetizing flux corresponds to the magnetizing current in the primary and the number of turns of the primary winding. Primary and secondary windings are wound on the same core, hence the induced voltage per turn is the same for both primary and secondary winding. Also the absolute value of induced voltage in the primary and secondary windings is proportional to the number of turns in the respective windings.

TRANSFORMER CONSTRUCTION DETAILS : 

The main parts of a transformer are;

1- Transformer core

2- Transformer Windings.

3- Transformer Tank and Radiators.

Transformer core : 

Every transformer has a core, which is surrounded by windings. The core is made out of special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of core material  which result in low eddy currents. In order to reduce eddy current losses, the laminations are

kept as thin as possible. The thickness of the laminations is usually around 0.27 to 0.35 mm. The transformer cores construction are of two types, viz, core type and shell type. In core type transformers, the windings are wound around the core, while in shell type transformers, the core is constructed around the windings. The shell type transformers provide a low reactance path for the magnetic flux, while the core type transformer has a high leakage flux and hence higher reactance.

The ideal shape for the section of the core is a circle, as this would mean no wastage of space between the core and windings, except the space taken by the insulation between laminations. A perfectly circular section of core would mean varying dimensions for each successive lamination, which may not be economical. A compromise is therefore struck and a stepped core (four or six steps) construction is normally preferred. The net sectional area is calculated from the dimensions of the various sections and giving due allowance for the insulation thickness. The yoke section is arranged similar to the limb section. To make the best use of the grain oriented silicon steel it is necessary that the flux run parallel to the direction of the rolling for as much of the magnetic path as possible. This is achieved by selecting identical cross-section and shape for core and yoke sections and having mitered corners. The materials used are such as to give low hysteretic losses, for a particular flux density. These are dependant on weight of material used and design flux density. In case a low flux density is employed, the weight of material increases, which in turn also leads to increase in length of mean turn of transformer coil. Both these aspects result in increase in losses. Similarly, the eddy current loss depends on the quality of material thickness of laminations and the flux density employed.

The limb laminations in small transformers are held together by stout webbing tape or by suitably spaced glass fiber bends. The use of insulated bolts passing through the limb laminations has been discontinued due to number of instances of core bolt failures. The top and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes are held together by vertical tie bolts.

TRANSFORMER WINDINGS : 

1- Cross over Type.

2- Helical Type.

3- Continuous Disc Type.

Cross-over type winding is normally employed where rated currents are up-to about 20 Amperes or so. In this type of winding, each coil consists of number of layers having number of turns per layer. The conductor being a round wire or strip insulated with a paper covering. It is normal practice to provide one or two extra lavers of paper insulation between lavers. Further, the  

insulation between lavers is wrapped round the end turns of the lavers there by assisting to keep the whole coil compact. The complete windings consists of a number of coils connected in series. The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are provided between adjacent coils to ensure free circulation of oil.

In helical winding, the coil consists of a number of rectangular strips wound in parallel racially such that each separate turn occupies the total radial depth of the winding. Each turn is wound on a number of key spacers which form the vertical oil duct and each turn or group of turns is spaced by radial keys sectors. This ensures free circulation of oil in horizontal and vertical direction. This type of coil construction is normally adopted for low voltage windings where the magnitude of current is comparatively large.

The continuous disc type of windings consists of number of Discs wound from a single wire or number of strips in parallel. Each disc consists of number of turns, wound radically, over one another. The conductor passing uninterruptedly from one disc to another. With multiple-strip conductor. Transpositions are made at regular intervals to ensure uniform resistance and length of conductor. The discs are wound on an insulating
cylinder spaced from it by strips running the whole length of the cylinder and separated from one another by hard pressboard sectors keyed to the vertical strips. This ensures free circulation of oil in horizontal and vertical direction and provides efficient heat dissipation from windings to the oil. The whole coil structure is mechanically sound and capable of resisting the most enormous short circuit forces.

The windings coils after manufacture are subjected to drying out in an oven by circulation of hot air at around 80 degree centigrade. The pre drying and shrinking of coils is achieved in this process. The coils are further dried un-till the required insulation resistance is achieved. In case of larger distribution and power transformers, the assembled core and windings are further subjected to drying out at about 100C and 730mm absolute pressure to drive out water vapor and gas from the windings. Appropriate clamping arrangements in the form of rings are provided on the windings to adjust for any shrinkage of insulation. The clamping rings could be either metallic with suitable earthing arrangements or of insulating material.

The insulation of the windings comprises of insulating cylinders between LV windings and core and between HV winding. Also insulating barriers are provided where necessary, between adjacent limbs, in some cases and between core yoke and coils.

The leads from top and bottom end of windings and from such tapings, as may be provided, are brought out to a few centimeters length only. The electrical connection from these leads to the terminals or bushings consist of either copper rod or strips depending on the current to be carried. Copper rods are insulated with bakelite tubes and supported by cleats. Which in turn are supported from the vertical tie rods passing through the top and bottom yoke clamps. When copper strips are used for low voltage leads no insulation need to be provided, except the cleats, which hold the strip in position. The strips are however wrapped with linen or varnish cloth at the point where it passes through the leads. Leads from tapings are brought out to a point just
below the top oil and so arranged that tapings may be readily changed by means of off load Tap changer.

TRANSFORMER TANK : 

Transformer tanks commonly used are of the following types : 

1- Plain sheet steel tank.

2- Sheet steel tank with external cooling tubes.

3- Radiator tanks.

4- Tanks with corrugated wall panels.

Plain sheet steel tanks are used where the size of the tank provides adequate cooling surface to dissipate the heat generated on account of losses inside the transformer. Normally transformers up-to 50KVA could be manufactured without external cooling tubes. For transformers of higher rating, tanks are constructed with external cooling tubes to provide additional surface for heat dissipation. The cooling tubes could be circular or elliptical. Elliptical tubes with smaller width are employed where one of the sides of the transformer is fully occupied by on load tap changer. This ensures more tubes on the given surface thereby providing more area for heat dissipation. In larger tanks, stiffeners are also provided on the sides of the tank to prevent bulging of the tank under oil pressure. The tubes are welded on the inside of the tank, while all other joints are welded both, inside and outside.

Large size transformers, above 5 MVA rating are normally provided with detachable Radiator banks to provide required cooling surface. The radiator bank consists of series of elliptical tubes or a pressed steel plate assembly welded into top and bottom headers. The radiator bank is bolted on to the tank wall and two isolating valves are fitted into the oil inlet and outlet. In case of very large transformers, even detachable radiator banks mounted onto the tank walls do not provide adequate cooling surface. IN such cases, separate self supporting coolers are provided which are connected to the main transformer through large detachable pipes. This type of arrangement is good for naturally cooled transformers, as well as, for forced cooled transformers. Forced air cooling could be provided by means of suitable fans located below the cooler banks. Similarly, forced oil cooling could be provided by installing an oil pump in the return cold oil pipe connecting the main transformer tank to the cooler bank. For outdoor transformers, the transformer has to be water-tight. For this purpose, the cover bolts are closely spaced and a substantial tank flange of ample width is provided. Further a Neoprene bonded cork gasket is provided between the tank flange and the cover. The bushing insulators are selected considering the maximum system voltages encountered in the system and pollution conditions prevailing at site. The joints are made water-tight by use of Neoprene bonded cork gaskets.

Transformers of rating 1 MVA or more are also normally provided with a conservator tank connected to the main tank. The conservator tank has a capacity of about 10% of the oil content of the main tank.