Topic 1 : Magnetic Concept and Transformer

1. Topic 1 : Magnetic Concept and Transformer

2. Two winding transformers Construction and principles Equivalent circuit Determination of equivalent circuit parameters Voltage regulation Efficiency Auto transformer 3 phase transformer Introduction

3. Different variety of transformers Introduction

4. Introduction

5. The word “Transformer” means an electromagnetic device which transforms electrical power from one end to another at different voltages and different currents keeping frequency constant. Unlike motor and generator it is static machine with different turns ratio of primary and secondary windings through which voltage/current is changed. The transfer of energy takes place through the magnetic field and all currents and voltages are AC. The rating of transformer is either in kVA or MVA because load to be connected is unknown Introduction

6. Transformers are adapted to numerous engineering applications and may be classified in many ways: Power level (from fraction of a volt-ampere (VA) to over a thousand MVA), Application (power supply, impedance matching, circuit isolation), Frequency range (power, audio, radio frequency (RF)) Voltage class (a few volts to about 750 kilovolts) Cooling type (air cooled, oil filled, fan cooled, water cooled, etc.)-ONAN, ONAF Purpose (distribution, rectifier, arc furnace, amplifier output, etc.). Introduction

7. Examples of transformer classifications: Power Three phase transformers (Step up) used for transmission of power (3-phase) at a distance Distribution transformer (Step down) used for utilization of power 3-pahse/1-phase Instrument Transformer (VT & CT) used for measurement/practical Auto Transformer (Single limb, electrically connected) used for measurement, practical, supply/utilization Isolation Transformer (having winding ratio of 1:1) used for safety of human and equipment for sensitive appliances or practical purpose Introduction

8. The invention of transformer caused transmission of heavy AC electrical power possible thus plays important role in electrical power technology Functions of transformer: Raise or lower voltage or current in AC circuit Isolate circuit from each other Enable to transmit electrical power energy over large distances at about >1200kV Provides electrical power according to the utilization needs Introduction

9. Transformer- Introduction • Power transmission

10. A typical power system consists of generation, transmission and distribution. Power from plant/station is generated around 11-13-20-30kV (depending upon manufacturer and demand). This voltage is carried out at a distance to reach for utilization through transmission line system by step up transformer at different voltage levels depending upon distance and losses. Its distribution is made through step down transformer according to the consumer demand. Here again at this stage, a transformer play an important role to reduce the voltage to suit the consumer need. Introduction

11. Power Transmission Introduction

12. Transformer is a device that makes use of the magnetically coupled coils to transfer energy. It is typically consists of one primary winding coil and one or more secondary windings. The primary winding and its circuit is called the Primary Side of the transformer. The secondary winding and its circuit is called the Secondary Side of the transformer. A magnetic circuit provides the link between primary and secondary. Introduction

13. When an AC voltage Vp is applied to the primary winding of the transformer, an AC current Ip will result. Ip sets up a time-varying magnetic flux Ф in the core. A voltage Vs is induced in the secondary circuit according to the Faraday’s law. Introduction

14. The magnetic (iron) core is made of thin laminated steel sheet. to minimize the eddy current loss by reducing thickness. There are two common cross section of core square or (rectangular) for small transformers circular (stepped) for the large and 3 phase transformers. Construction

15. Core (U/I) Type: Constructed from a stack of U and I shaped laminations. The primary and secondary windings are wound on two different legs of the core. Shell Type: Constructed from a stack of E and I shaped laminations. The primary and secondary windings are wound on the same leg of the core, as concentric windings, one on top of the other. Construction

16. Construction

17. Construction

18. Construction

19. Construction

20. Winding resistances are zero, no leakage inductance and iron loss Magnetization current generates a flux that induces voltage in both windings Current, voltages and flux in an unloaded ideal transformer Ideal Transformer

21. Currents and fluxes in a loaded ideal transformer Ideal Transformer

22. Turn ratio If the primary winding has N1 turns and secondary winding has N2 turns, then: The input and output complex powers are equal Ideal Transformer

23. Functional description of a transformer: a = 1 Isolation Transformer | a | < 1 Step-Up Transformer Voltage is increased from Primary side to secondary side | a | > 1 Step-Down Transformer Voltage is decreased from Primary side to secondary side Ideal Transformer

24. Transformer Rating Practical transformers are usually rated based on: Voltage Ratio (V1/V2) which gives us the turns-ratio Power Rating, small transformers are given in Watts (real power) and Larger ones (Power Transformers) are given in kVA (apparent power) Ideal Transformer

25. Example 1 Determine the turns-ratio of a 5 kVA 2400V/120V Power Transformer Turns-Ratio = a = V1/V2 = 2400/120 = 20/1 = 20 This means it is a Step-Down transformer Ideal Transformer

26. Example 2 A 480/2400 V (r.m.s) step-up ideal transformer delivers 50 kW to a resistive load. Calculate: the turns ratio, (0.2) the primary current, (104.17A) the secondary current. (20.83A) Ideal Transformer

27. Exercise 1 A 250kVA, 1100V/400v, 50Hz single-phase transformer has 80 turns on the secondary. Calculate: the approximate values of the primary and secondary currents (227A, 625A) the approximate number of primary turns (220) the maximum value of the flux (22.5mWb) Ideal Transformer

28. Nameplate of a transformer Ideal Transformer

29. Equivalent circuit of an ideal transformer Ideal Transformer

30. Equivalent circuit of an ideal transformer Transferring impedances through a transformer Ideal Transformer

31. Equivalent circuit when secondary impedance is transferred to primary side and ideal transformer eliminated. Equivalent circuit when primary source is transferred to secondary side and ideal transformer eliminated. Ideal Transformer

32. Practical Transformer

33. In a practical magnetic core having finite permeability, a magnetizing current Im is required to establish a flux in the core. This effect can be represented by a magnetizing inductance Lm. The core loss can be represented by a resistance Rc. Equivalent Circuit

34. Rc :core loss component Xm : magnetization component Equivalent Circuit

35. Phasor diagram of an unloaded transformer Equivalent Circuit

36. Winding resistance and leakage flux The effects of winding resistance and leakage flux are respectively accounted for by resistance R and leakage reactance X (2πfL). Equivalent Circuit

37. Rc :core loss component Xm : magnetization component R1 and R2 are resistance of the primary and secondary winding X1 and X2 are reactance of the primary and secondary winding Equivalent Circuit

38. Phasor diagram of a loaded transformer (secondary) Equivalent Circuit

39. Phasor diagram of a loaded transformer (primary) Equivalent Circuit

40. Since no load current is very small(3-5% of full load), the parallel circuit of Rc and Xm can be moved close to the supply without significant error in calculation. Calculations becomes easier Approximate Equivalent Circuit

41. Calculations will be much more easy if the primary and secondary circuit are combined. Transfer the secondary circuit to the primary circuit Approximate Equivalent Circuit

42. Phasor diagram of a loaded transformer (primary) Approximate Equivalent Circuit

43. For convenience, the turns is usually not shown The resistance and reactance can be lumped together We can also transfer the primary circuit to the secondary circuit Approximate Equivalent Circuit

44. Example 3 A 100kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistance are 0.3 ohm and 0.01 ohm respectively and the corresponding leakage reactances are 1.1 ohm and 0.035 ohm respectively. The supply voltage is 2200V. Calculate: the equivalent impedance referred to the primary circuit (2.05 ohm) the equivalent impedance referred to the secondary circuit (0.082) Approximate Equivalent Circuit

45. The equivalent circuit model for the actual transformer can be used to predict the behavior of the transformer. The parameters Rc, Xm, R1, X1, R2, X2 and N1/N2 must be known so that the equivalent circuit model can be used. These parameters can be directly and more easily determined by performing tests: No load test (or open circuit test) Short circuit test Transformer Test

46. No load/Open circuit test Provides magnetizing reactance (Xm) and core loss resistance (Rc) Obtain components are connected in parallel Short circuit test Provides combined leakage reactance and winding resistance Obtain components are connected in series Transformer Test

47. Equivalent circuit for open circuit test, measurement at the primary side Simplified equivalent circuit Transformer Test – Open Circuit

48. Open circuit test evaluation . . Transformer Test – Open Circuit

49. Short circuit test Secondary (normally the LV winding) is shorted, that means there is no voltage across secondary terminals; but a large current flows in the secondary. Test is done at reduced voltage (about 5% of rated voltage, with full-load current in the secondary. Hence the induced flux are also 5%) The core losses is negligible since it is approximately proportional to the square of the flux. So, the ammeter reads the full-load current; the wattmeter reads the winding losses, and the voltmeter reads the applied primary voltage. Transformer Test – Short Circuit

50. Equivalent circuit for short circuit test, measurement at the primary side Simplified equivalent circuit Transformer Test – Short Circuit