transformers

1. Prepared by: DIANA ROSE L. MUYNA

2. Transformer A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.

3. The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.

4. A transformer carries the operations shown below: 1. Transfer of electric power from one circuit to another. 2. Transfer of electric power without any change in frequency. 3. Transfer with the principle of electromagnetic induction. 4. The two electrical circuits are linked by mutual induction.

5. Transformer Construction For the simple construction of a transformer, you must need two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated. In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.

6. In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included. The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated. By effectively laminating the core, the eddy-current losses can be reduced. The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.

7. GENERAL CLASSIFICATION Type of magnetic circuit Number of phases Arrangement of windings Methods of cooling Type of service Special features of construction

8. Types of Magnetic Circuit Laminated, or Punched type - type involves stacking individually punched sheet-steel laminations

9. Ribbon-wound type - made up of continuous strips steel wound into a tight coil.

10. Number of Phases A three-phase transformer generally has the three magnetic circuits that are interlaced to give a uniform distribution of the dielectric flux between the high and low voltage windings. The exception to this rule is a three-phase shell type transformer. In the shell type of construction, even though the three cores are together, they are non- interlaced.

11. The three-limb core-type three-phase transformer is the most common method of three-phase transformer construction allowing the phases to be magnetically linked. Flux of each limb uses the other two limbs for its return path with the three magnetic flux's in the core generated by the line voltages differing in time-phase by 120 degrees. Thus the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage.

12. The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced height. A shell-type transformers core materials, electrical windings, steel enclosure and cooling are much the same as for the larger single-phase types.

13. Arrangement of Windings a. Shell-type Transformers In shell-type transformers the core surrounds a considerable portion of the windings. The comparison is shown in the figure below.

14. The coils are form-wound but are multi layer disc type usually wound in the form of pancakes. Paper is used to insulate the different layers of the multi-layer discs. The whole winding consists of discs stacked with insulation spaces between the coils. These insulation spaces form the horizontal cooling and insulating ducts. Such a transformer may have the shape of a simple rectangle or may also have a distributed form.

15. Both designs are shown in the figure below:

16. A strong rigid mechanical bracing must be given to the cores and coils of the transformers. This will help in minimizing the movement of the device and also prevents the device from getting any insulation damage. A transformer with good bracing will not produce any humming noise during its working and will also reduce vibration.

17. Core- Type Transformers In core-type transformer, the windings are given to a considerable part of the core. The coils used for this transformer are form- wound and are of cylindrical type. Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type.

18. You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on. The general arrangement of the core-type transformer with respect to the core is shown below. Both low-voltage (LV) and high voltage (HV) windings are shown.

19. The low voltage windings are placed nearer to the core as it is the easiest to insulate. The effective core area of the transformer can be reduced with the use of laminations and insulation.

20. Methods of Cooling ONAN Cooling of Transformer This is the simplest transformer cooling system. The full form of ONAN is "Oil Natural Air Natural". Here natural convectional flow of hot oil is utilized for cooling. In convectional circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant place is occupied by cold oil. This hot oil which comes to upper side, will dissipate heat in the atmosphere by natural conduction, convection & radiation in air and will become cold. In this way the oil in the transformer tank continually circulate when the transformer put into load.

21. As the rate of dissipation of heat in air depends upon dissipating surface of the oil tank, it is essential to increase the effective surface area of the tank. So additional dissipating surface in the form of tubes or radiators connected to the transformer tank. This is known as radiator of transformer or radiator bank of transformer. We have shown below a simplest form on Natural Cooling or ONAN Cooling arrangement of an earthing transformer below.

22. ONAF Cooling of Transformer Heat dissipation can obviously be increased, if dissipating surface is increased but it can be make further faster by applying forced air flow on that dissipating surface. Fans blowing air on cooling surface is employed. Forced air takes away the heat from the surface of radiator and provides better cooling than natural air. The full form of ONAF is "Oil Natural Air Forced". As the heat dissipation rate is faster and more in ONAF transformer cooling method than ONAN cooling system, electrical power transformer can be put into more load without crossing the permissible temperature limits.

23. OFAF Cooling of Transformer In Oil Forced Air Natural cooling system of transformer, the heat dissipation is accelerated by using forced air on the dissipating surface but circulation of the hot oil in transformer tank is natural convectional flow. The heat dissipation rate can be still increased further if this oil circulation is accelerated by applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop of transformer tank by means of oil pumps.

24. OFAF means "Oil Forced Air Forced" cooling methods of transformer. The main advantage of this system is that it is compact system and for same cooling capacity OFAF occupies much less space than farmer two systems of transformer cooling. Actually in Oil Natural cooling system, the heat comes out from conducting part of the transformer is displaced from its position, in slower rate due to convectional flow of oil but in forced oil cooling system the heat is displaced from its origin as soon as it comes out in the oil, hence rate of cooling becomes faster.

25. OFWF Cooling of Transformer We know that ambient temperature of water is much less than the atmospheric air in same weather condition. So water may be used as better heat exchanger media than air. In OFWF cooling system of transformer, the hot oil is sent to a oil to water heat exchanger by means of oil pump and there the oil is cooled by applying sowers of cold water on the heat exchanger's oil pipes. OFWF means "Oil Forced Water Forced" cooling in transformer.

26. ODAF Cooling of Transformer ODAF or Oil Directed Air Forced Cooling of Transformer can be considered as the improved version of OFAF. Here forced circulation of oil directed to flow through predetermined paths in transformer winding. The cool oil entering the transformer tank from cooler or radiator is passed through the winding where gaps for oil flow or pre-decided oil flowing paths between insulated conductor are provided for ensuring faster rate of heat transfer. ODAF or Oil Directed Air Forced Cooling of Transformer is generally used in very high rating transformer.

27. ODWF Cooling of Transformer ODAF or Oil Directed Water Forced Cooling of Transformer is just like ODAF only difference is that here the hot oil is cooled in cooler by means of forced water instead of air. Both of these transformer cooling methods are called Forced Directed Oil Cooling of transformer

28. Instrument Transformer - may be classified as metering and relay transformers, and may be ether current or potential transformers. - are used for two reasons: (1) to protect station operators from contact with high-voltage circuits and, (2) to permit the use of instruments with a reasonable amount of insulation and a reasonable current- carrying capacity. - the function of the instrument transformers is to deliver to the instruments a current and voltage that shall always be proportional to the primary current and voltage and that does not exceed a safe potential above ground.

29. a. Voltage transformers - used with voltmeters, watt meters, watt-hour meters, power factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and the no-voltage and over-voltage trip coils of automatic circuit breakers.

30. b. Current transformers -used with ammeters, watt meters, power-factor meters, watt-hour meters, compensators, protective and regulating relays, and the trip coils of circuit breakers. One current transformer can be used to operate several instruments, provided that the combined burden does not exceed that for which the transformer is designed and compensated.

31. c. Through-type Transformers - this type have no primary winding but use the current carried by the cable or busbar to energize the core. - usually regarded as suitable for instrument use if the ratio is 500:5 amp., or larger

32. d. Bushing-type Transformer - a special form of through-type transformer. - made in form of a hollow cylinder, built up of ring-shaped iron punchings on which the secondary winding is wound. - is mounted over the terminal bushing of a circuit breaker to supply current for tripping coil or tripping relay.

33. e. Metering Outfits It is possible to combine the necessary current- and voltage-transformer elements, which are needed to measure the power flowing over a three-phase line, all in one tank, thereby simplifying the outside connections and installation very much.

34. Autotransformer - is built in the same general manner as any other transformer, but it has only one winding. - are used as motor starters, as balance coils systems at different voltage. -not adaptable to general distribution work, because for this type of service it is generally desired to keep the secondary and primary coils electrically insulated from each other.

35. Constant-current Transformer The constant-current transformer, usually called a regulator, has a movable secondary winding that automatically changes position to provide constant current for any load within its full-load rating. The balance point between coil weight and magnetic force may be adjusted to provide the desired output current.

36. Induction-voltage Regulators Induction regulators are nothing more than constant-voltage transformers, one winding of which can be moved with respect to the other, thereby obtaining a variable secondary voltage. They are used at the end of distribution lines to maintain constant voltage. The primary, or movable, coil is connected across the line, while the secondary, or stationary, coil is connected in series with the line.

37. There are two types of induction regulators: a. Single-phase Regulators b.Polyphase Regulators are wound with polyphase windings on both the rotor and stator in the same general manner as a wound-rotor induction motor.

38. Conservator-type Transformer Oil type Transformer had come to existence since 1892 or more than 100 years ago. In the beginning, the Oil type Transformers were "Open Tank" type. It had air inlet and outlet for the expansion of the oil volume. The Oil volume goes up and down according to the Temperature of the Oil. We can say that the Transformer is "Breathing".

39. The Open Tank type Transformers were improved in time. A small oil reserved tank is connected above the Transformer Main Tank. This is called Conservator type Transformer. In order to prevent the danger of moisture and oxygen come in contact with the oil, "Silica Gel", desiccant agents are connected to the conservator tank.

40. Oil will be filled in the Main Tank and outflow into the conservator tank. The oil level will not be more than half of the conservator tank. When Transformers are energized, the temperature of the windings will increase. The surrounding oil will be hot and oil volume will increase according to the increasing temperature. The oil will expand at the maximum of 7% of the total oil volume in the tank. The air in the conservator tank will be pushed out to the atmosphere by the increasing volume of oil.

41. When the transformer cooled down, the volume of oil will decrease. The outside air will be sucked into the conservator to balance the pressure. During the cool down (by the decreasing of the load, or the cooler ambient temperature, or by rain water), the moisture in the air will enter into the conservator. We need the Silica Gel to help prevent the moisture enter into the transformer, but cannot prevent totally. For Conservator Type Transformer, we recommend to test the oil at least once a year. The best time to get the oil sample for testing is after the Rainy Season.

42. In order to prevent moisture entering into the transformer totally, a Rubber Bag (Rubber Air Cell or Rubber Diaphragm) will be placed in the conservator tank. This Rubber Bag will act as a partition to prevent the oil come in contact with the air, but it is still flexible for the oil expansion. The Rubber Bag Conservator system usually will be used in large Power Transformer.

43. CENTRIFUGAL PURIFIER Centrifugal force is defined as that force which impels a thing (and any or all of its parts) outward from a center of rotation. Every time you lean in as you take a fast turn, you are counterbalancing centrifugal force. How far in you lean is determined by the amount of centrifugal force exerted in the turn. Most people do it automatically, for centrifugal force, along with gravity, is the most prevalent physical force exerted upon us and upon all matter.

44. The purpose of the centrifugal purifier(fig. 4-26) in the JP-5 filling and transfer system is to separate and remove water, solids, and emulsions from JP-5 during transfer from storage to service tanks. The disk bowl centrifuge is a constant efficiency type of separator; that is, it achieves the same degree of efficiency at the end of a run as at the beginning. The reason for the constant efficiency is that accumulated solids are stowed away from the separation zone. Separation occurs within the disk spaces, and the separated liquids are discharged from outlets that are removed from interference of the stowed solids.

transformers

Engineering

phase transformer construction

phase shell type transformer

punched type type

limb coretype

basic transformer

laminated steel core

assembled core

shell type of construction