REPORT ON MANUFACTURING & TESTING OF POWER TRANSFORMERS

THE IDEAL TRANSFORMER VOLTAGE RATIO

A power transformer normally consists of a pair of windings, primary and secondary, linked by a magnetic circuit or core. When an alternating voltage is applied to one of these windings, generally by definition the primary, a current will flow which sets up an alternating m.m.f. and hence an alternating flux in the core. This alternating flux in linking both windings induces an e.m.f. in each of them. In the primary winding this is the ‘back-e.m.f.’ and, if the transformer were perfect, it would oppose the primary applied voltage to the extent that no current would flow. In reality, the current which flows is the transformer magnetising current. In the secondary winding the induced e.m.f. is the secondary open-circuit voltage. If a load is connected to the secondary winding which permits the flow of secondary current, then this current creates a demagnetising m.m.f. thus destroying the balance between primary applied voltage and back-e.m.f. To restore the balance an increased primary current must be drawn from the supply to provide an exactly equivalent m.m.f. so that equilibrium is once more established when this additional primary current creates ampere-turns balance with those of the secondary. Since there is no difference between the voltage induced in a single turn whether it is part of either the primary or the secondary winding, then the total voltage induced in each of the windings by the common flux must be proportional to the number of turns. Thus the well-known relationship is established that:

E1/E2=N1/N2

And, in view of the need for ampere-turns balance:

I2/I1=N1/N2

Where E, I and N are the induced voltages, the currents and number of turns respectively.

E2/E1=N2/N1=K

This constant K is known as voltage transformation ratio.

(a).If N2>N1 i.e. K>1, then the transformer is called as STEP UP TRANSFORMER.

(b).If N2<N1 i.e. K<1, then the transformer is called as STEP DOWN TRANSFORMER.

Again for ideal transformer Input=Output

(V1)(I1) = (V2)(I2) [neglecting Iu ]

I2/I1=V1/V2=1/K

Where I1 and I2 are primary and secondary currents.

Hence the currents are in the inverse ratio of the transformation ratio.

STAGES OF POWER TRANSFORMER MANUFACTURING

1. CORE BUILDING

2. UNLACING OF CORE

3. FITTING BOTTOM INSULATION

4. CORE COIL ASSEMBLY AS PER DRAWING / ELECTRICAL SPECIFICATION

5. FITTING OF TOP INSULATION

6. RELACING OF CORE

7. TERMINAL GEAR MOUNTING

8. VAPOUR PHASING PROCESSING

9. FINAL TANKING AND OIL FILLING

10. CASE FITTING

11. TESTING

12. DESPATCH

TYPES OF WINDING

The following are the types of winding used in manufacturing of power transformer:

1. R-S COIL :

This is also known as the reverse section coil. One section is given the reverse winding while one section is given the forward winding. This can be manufactured in various sizes say five parallel or two parallel conductors as suggested by the designer. The layer depth or LD of the winding is defined as the number of turns to the number of segments available in the section. Once the number of turn is given rest of the segment in that section is provided with packing to maintain LD. Generally one to eight numbers of conductors are used in this kind of winding.

2. HELICAL COIL :

This kind of coil can be said to be winded in shape of a spring. Here the numbers of conductors are more in numbers than others and all are insulated from each other in order to reduce EDDY current losses. Transposition in this kind of winding is done to make the conductors equal in length so that no possibility of spark should be there due to differences of voltages induced due to length of conductors. Two kinds of transposition are usually done in this kind of winding:

a) Section wise transpose

b) Three numbers of transpose are provided in between conductor turns

3. SPIRAL COIL :

This is the simplest kind of winding the conductors are simply wounded on the base. The side from where the winding is started is known as the start lead end where it is finished is known as the finish lead.

4. HALF SECTIONAL COIL:

These kind of coil can be classified on basis of winding direction if the winding is started by rotation of conductors in anticlockwise direction it is known as standard half sectional coil whereas if the rotation is clock wise it is termed as non-standard half sectional coil. The brazing leads required in some cases of the transformer are also provided in this kind of winding.

5. INTERLEAVED COIL:

In some cases the conductors are wounded in such a fashion that the interleaved winding current rotates in same conductor. When inter leaved the number of conductor gets doubled than initial number. After that section is made the conductors are connected using entries.

6. COMPOSITE COIL: In this kind of winding more than one conductor are winded with each other on the same machine. These windings are generally used for the purpose of tapped coils.

CORE

CRGO (Cold rolled grain oriented silicon steel) is used to build the core. The grain of CRGO is oriented in the direction of rolling. The purpose of using CRGO is to reduce the Hysteresis Losses. The thicknesses of these sheets are available either in the dimension of 0.23mm or 0.27mm. The lamination on these sheets minimizes the eddy current losses.

For core material, high-grade, grain oriented silicon steel strip is used Connected by a core leg tie plate fore and hind clamps by connecting bars. As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force.

In large-capacity Transformers, which are likely to invite increased leakage flux, nonmagnetic steel is used or slits are provided in steel members to reduce the width for preventing stray loss from increasing on metal parts used to clamp the core and for preventing local overheat. The core interior is provided with many cooling oil ducts parallel to the lamination to which a part of the oil flow forced by an oil pump is introduced to achieve forced cooling. When erecting a core after assembling, a special device shown in Fig. (8) Is used so that no strain due to bending or slip is produced on the silicon steel plate

BURR LEVEL: The sheets undergo proper cutting and is then available in many shapes like Trapezoidal, octagonal and hexagonal e.t.c. but while cutting the edges of these sheets, there is some generation of rough surfaces, these are known as the burr level. The formation of these levels should be avoided as because they produces air gap which increase the losses. To control the burr level cutting of CRGO is done with the help of CNC (computer numeric control) machines.

Core Building under Progress

Core building done on Three 65 T cradle, which ensures minimum jerk during lifting of core

CORE BUILDING

CORE LAYOUT : The base is made up of frames on which core is mounted as shown in the figure ,the top and end frame and bottom end frame are connected as shown the level of the end frame from the ground the limbs on which core is mounted is also given mechanical support

- Transformer Core

Construction in which the iron circuit is surrounded by windings and forms a low reluctance path for the magnetic flux set up by the voltage impressed on the primary.

The steel strip surface is subjected to inorganic insulation treatment.

All cores employ miter-joint core construction. Yokes are jointed at an angle of 45° to utilize the magnetic flux directional characteristic of steel strip. A computer-controlled automatic machine cuts grain-oriented silicon steel strip with high accuracy and free of burrs, so that magnetic characteristics of the grain-oriented silicon steel remains unimpaired. Silicon steel strips are stacked in a circle-section. Each core leg is fitted with tie plates on its front and rear side, with resin-impregnated glass tape wound around the outer circumference. Sturdy clamps applied to front and rear side of the upper and lower yokes are bound together with glass tape.

And then, the resin undergoes heating for hardening to tighten the band so that the core is evenly clamped. Also, upper and lower clamps are connected by a core leg tie plate; fore and hind clamps by connecting bars. As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force.

0When erecting a core after assembling, a special device is used so that no strain due to bending or slip is produced on the silicon steel plate.

CORE-COIL ASSEMBLY

After the unlacing of core is done i.e., the top yoke is removed, the core is made to stand erect and then the coils are mounted on the core.

The coils as specified in the design may be of following types:

1. L.V COIL: This is known as low voltage coil. These coils are often referred to as the primary coil for step down transformer. These coil are made in Order to allow the flow of large current through it and thus the cross

Sectional area of the conductor used in this kind of coil is larger and the numbers of turns per conductor are few, also less number of conductors is used in L.V coil.

2. H.V COIL: This is known as the high voltage coil. These coils are often referred as the secondary coil of a step down transformer. These are made in order to allow high voltages and hence small amount of current through it. So the conductors used are smaller in size and number of turns per conductor

is more in numbers. Also the number of conductor in this kind of coil increases.

3. T.V COIL: This is known as the tertiary voltage coil.

4. M.V COIL: This is known as the medium voltage coil.

As required or specified in the design at the bottom of the core an insulator circular in shape is provided with blocks made of wood attached on it . This is known as the block washer assembly the wood attached on the waddman insulator material serves ducts, which help in circulation of transformer oil and thus better cooling of transformer is achieved. The core is also given a surrounding of a layer of waddman insulating material on which spacers are provided which serves the purpose of creating ducts for oil circulation as well as it gives support to the coil wounded on it. Generally the L.V coil is mounted as the first layer after the spacers on insulation material thereafter the coil is again shielded with the insulating material and the spacers on which the H.V coil is mounted and this way the process is carried on based on the design.

After the L.V and H.V coil are mounted on the core, the top of the core where the mounting ends again the layer of wad man insulation material the block washer assembly is provided. Now the job is taken for relacing.

PROCESSING AND DRY-OUT

The paper insulation and pressboard material, which make up a significant proportion by volume of transformer windings, have the capacity to absorb large amounts of moisture from the atmosphere. The presence of this moisture brings about a reduction in the dielectric strength of the material and also an increase in its volume. The increase in volume is such that, on a large transformer, until the windings have been given an initial dry-out, it is impossible to reduce their length sufficiently to fit them on to the leg of the core and to fit the top yoke in place.

The final drying out is commenced either when the core and windings are placed or when they are fitted into their tank, all main connections made, and the tank placed in an oven and connected to the drying system. The tapping switch may be fitted at this stage, or later, depending on the ability of the tapping switch components to withstand the drying process.

VAPOUR PHASE DRYING

The main difference between conventional vacuum drying and vapour phase drying is that, in the latter process the heat carrier is a vapour of low viscosity solvent more like kerosene, with a sufficiently high flash point instead of air . The vapour is condensed on the transformer and then re-evaporated in the plant. For this reason, vapour phase installations include an evaporator and condenser system in addition to the vacuum equipment and vacuum vessels associated with conventional drying system which is applicable to transformers dried in the vacuum vessel as well as in their own tank. The solvent heat conveyer system consists of storage, evaporation, condensation, filteration, solvent feedback and control arrangement.

HEAT CARRIER:

The solvent used should posses the following properties for effective and efficient drying.

1. Vapour pressure must be distinctly below that of water, so that a large pressure difference assists efficient water diffusion from the beginning of the heating phase.

2. Evaporation heat should be as high as possible.

3. The presence of small amounts of the heat carrier in the solid or oil insulation must have no effect on their ageing or general properties.

4. High ageing stability, allowing practically unlimited use in the drying process. A solvent storage tank is normally required to be refilled after few years.

5. Flame point should be above 55⁰C.

The following solvents meet the above characteristics and are normally used in vapour phase drying systems.

1. Shellsol H (Shell)

2. Somenter T (Esso)

3. Varsol 60

4. Varsolene 60

5. Essovarsol 60 E

PHYSICAL HYDROCARBON AIR

PROPERTIES SOLVENT

1. Specific density 0.785g/m ³ 1.25kg/m³

(Liquid) (Gaseous)

2. Molecular weight 160 29

3. Heat of vaporization 306ҳ10³ Ws/kg ------

4. Specific heat 2.09ҳ10³Ws/kg⁰C 1ҳ10³Ws/kg⁰C

(Liquid) (Gaseous)

5. Inlet temperature in 130⁰C 110⁰C

Vacuum vessel

6. Outlet temperature from 90⁰C 90⁰C

Vacuum vessel at the start of

Heating

7. Vapour pressure at 130⁰C 140 torr -----

8. Energy provided per mole 62.7ҳ 10³Ws 581ҳ10³Ws

9. Energy provided per mole 179m³ at 130°C 31.4m³ at 110°C

10. Energy released per m³ 351ҳ10³Ws 18.4ҳ10³Ws

The drying process takes place in four stages as mentioned

1. Preparation

2. Heating up and drying

3. Pressure reduction

4. Fine vacuum

1. PREPARATION (setting up): The entire evaporator and condenser system is first evacuated to an absolute maximum pressure of 5 torr by leakage air vacuum pump, before drawing the solvent into the evaporator and heating it to the required temperature of 130⁰C.The vacuum vessel valves remain closed during this operation.

In parallel with the above preparatory steps, the vacuum system evacuates the vacuum vessel containing the core and windings assembly to approximately to 5 torr. For draining the condensate from the diverter switch oil compartment of on load tap changer, wherever it is processed along with the transformer, and the drain plug in the bottom of the compartment is opened. Vessel floor is at a descending slope of1:100 towards the drainage system, so that no condensed solvent remain inside the vessel. If tank containing core and windings assembly is loaded into the vessel on a horizontal trolley, tank is also kept at a slope of 1:100 to drain out the solvent from the tank.

2. HEATING UP AND DRYING: After the vessel is evacuated, vessel heating is started and this heating is continued till the end of fine vacuum phase. The vessel valves are opened at this stage, admitting vapours of heat carrier in the vessel, most of which condense on which on cold surfaces of the transformer. The condensed heat carrier is pumped back to the evaporator through a filter. Heat released by the condensation gradually warms up the insulation and mass component. Heat carrier vapour pressure in the vessel and insulation moisture vapour pressure both increase with rising temperature. As the water vapour pressure is considerably higher than that of heat carrier, insulation moisture starts to evaporate at a low insulation temperature. This produces mixtures of water vapour, leakage air and heat carrier vapour inside the vessel, which is conveyed back to the condenser via the vapour return in the condenser, whereas the leakage air discharges to the atmosphere through the vacuum pump. The condensed water and heat carrier mixture is sent into the collecting tank in which its component settle out under gravity. The water gets collected in the bottom of tank due to higher specific weight, which is measured periodically and drained off. Final insulation drying temperature of 120-125°C is maintained for the time required, to ensure full moisture evaporation from the deeper insulation layers. Longer the heating phase, the shorter is the fine vacuum phase.

3. PRESSURE REDUCTION: The vapour supply remains closed during this stage in which most of the heat carrier absorbed by the insulation re-evaporates, condense out in the condenser and is finally returned to the evaporator. This phase is terminated when an absolute pressure of 15 to 20 torr is reached in the vessel.

4. FINE VACUUM: This is the final drying stage, which comes immediately after the pressure reduction phase. It is same as conventional vaccum drying. The vessel is evacuated by the main vaccum system to a pressure not exceeding 0.1 torr. This phase is terminated when water extraction rate is below the desired level and insulation resistance and dissipation factor of windings become constant.

After drying of insulation by solvent vapours, other activities like oil impregnation, soaking, draining, retanking redrying, completion of fittings, and oil circulation follow in the same manner as for conventional drying with the following exceptions.

i. Since the transformer is at a temperature of 125⁰C at the end of V.P.D., it is cooled to a temperature depending upon the pressure in the vessel, such that oil is neither vaporized nor oxidized during oil impregnation.

ii. If only core and windings assembly is dried without tank, the assembly is taken out from the vessel and is immediately loaded into the tank. The tank is again kept in vessel and oil is filled after evacuating vessel to the desired level removing any moisture absorbed by the insulation due to exposure to the atmosphere.

OIL IMPREGNATION

RECONDITIONING OF OIL:

Transformer oil is dehydrated and de-aerated in the oil de-aeration plant. Regular sampling of transformer oil shall be done from vessel for measurement of break down voltage.

The value should be as following:

BDV

Below 72.5 kv 40 kv (minimum)

(72.5-170) kv 50 kv (minimum)

170 kv & above 245 kv 60 kv (minimum)

The oil flow rate shall be such that the pressure in the vessel at the end of oil impregnation doesn't exceed to more than double of pressure at start of impregnation.

SOAKING:

Oil impregnated transformer shall be kept under vacuum for a minimum of twelve hours for transformer of voltage class above 145 kv and up to and including 245 kv. For 145 kV the transformer is kept under atmospheric pressure for minimum of twelve hours. So that insulation is completely soaked with oil.

DRAIN OUT:

After the above process the oil is pumped out completely from the transformer to oil de-aeration plant.

ACCEPTANCE CRITERIA:

Rate of condensed water for six consecutive measurements taken at an interval of one hour shall be within the limits given below:

For Transformers condensate rate (liter/hour)

(Above 145kv-245kv) 0.05

(Up to 145 kV) 0.10

FINAL TANKING/CASE FITTING

The job is finally put into a tank in which various points are taken care of such as:

1. Dimension: the proper tank dimension is achieved as specified in the design.

2. Weld leakage test: this test is performed on the tank to check that whether any kind of leakage is present in the tank or not.

3. Vacuum and pressure tests are per formed on the tank to check its endurance.

Testing of Power Transformer

Tests during manufacture

As part of the manufacturer’s QA system some testing will of necessity be carried out during manufacture. These are:

Core-plate checks: Incoming core plate is checked for thickness and quality of insulation coating. A sample of the material is cut and built up into a small loop known as an Epstein Square from which a measurement of specific loss is made. (According to BS 6404-IEC 404)

Core-frame insulation resistance: This is checked by Megger and by application of a 2 kV R.M.S. test voltage on completion of erection of the core. These checks are repeated following replacement of the top yoke after fitting the windings. A similar test is applied to any electrostatic shield and across any insulated breaks in the core frames.

Core-loss measurement: If there are any novel features associated with a core design or if the manufacturer has any other reason to doubt whether the guaranteed core loss will be achieved, then this can be measured by the application of temporary turns to allow the core to be excited at normal flux density before the windings are fitted.

Winding copper checks: If continuously transposed conductor is to be used for any of the windings, strand-to-strand checks of the enamel insulation should be carried out directly the conductor is received in the works.

Tank tests: The first tank of any new design should be checked for stiffness and vacuum-withstand capability. For 132 kV transformers, a vacuum equivalent to 330 mbar absolute pressure should be applied. This need only is held long enough to take the necessary readings and verify that the vacuum is indeed being held for hours. After release of the vacuum, the permanent deflection of the tank sides should be measured and should not exceed specified limits, depending on length. Following this test, a further test for the purpose of checking mechanical withstands capability should be carried out. Typically a pressure equivalent to 3 mbar absolute should be applied for 8 hours.

The tank is provided with an adequate number of smaller removable covers, allowing access to bushing connections, winding temperature CTs, core earthing links, off-circuit tapping links and the rear of tapping selector switches.

Tanks must be provided with valves for filling and draining, and to allow oil sampling when required. These also enable the oil to be circulated through external filtration and drying equipment prior to initial energization on site, or during service when oil has been replaced after obtaining access to the core and windings. Lifting lugs or, on small units, lifting eyes must be provided, as well as jacking pads and haulage holes to enable the transformer to be maneuver on site.

Bushing connections

A bushing is a means of bringing an electrical connection from the inside to the outside of the tank. It provides the necessary insulation between the winding electrical connection and the main tank which is at earth potential. The bushing forms a pressure-tight barrier enabling the necessary vacuum to be drawn for the purpose of oil impregnation of the windings. It must ensure freedom from leaks during the operating lifetime of the transformer and be capable of maintaining electrical insulation under all conditions such as driving rain, ice and fog and has to provide the required current-carrying path with an acceptable temperature rise. There is a current-carrying stem, usually of copper, and the insulation is provided by a combination of the porcelain shell and the transformer oil. Under oil, the porcelain surface creepage strength is very much greater than in air, so that the ‘below oil’ portion of the bushing has a plain porcelain surface. The ‘air’ portion has the familiar shedded profile in order to provide a very much longer creepage path, a proportion of which is ‘protected’ so that it remains dry in rainy or foggy conditions.

At 33 kV and above, it is necessary to provide additional stress control between the central high-voltage lead and the external, ‘earthy’ metal mounting flange. This can take the form either of a synthetic resin-bonded paper multifoil capacitor or of an oil-impregnated paper capacitor of similar construction. This type of bushing is usually known as a condenser bushing.

CONCLUSION

To conclude power transformers are extensive device in today’s world for transmission and distribution systems. A device which could take the high-current, relatively low-voltage output of an electrical generator and transform this to a voltage level which would enable it to be transmitted in a cable of practical dimensions to consumers. BHEL is one who’s manufacturing the transformers. Power transformer undergoes several stages for manufacturing process. Tests are done to ensure the status and reliability of the power transformer during as well as after manufacturing.

These include some major tests like:

1. Impulse Tests

2. Temperature Rise Tests

These four weeks helped me a lot in gaining the knowledge of power transformer. It helps me to learn the manufacturing process of power transformer. There are many auxiliary types of equipment which are also used with transformer during operation or at site which are also tested in BHEL.

It includes:

1. Motor Drive Unit

2. RTCC panels