EET 208/3 ELECTRICAL POWER TECHNOLOGY

1. EET 208/3ELECTRICAL POWER TECHNOLOGY CHAPTER 4 TRANSFORMER (SINGLE PHASE) By: MOHD ASRI BIN JUSOH Email: asrijusoh@unimap.edu.my SCHOOL OF ELECTRICAL SYSTEM ENGINEERING

2. SINGLE PHASE TRANSFORMER • DESCRIBE & EXPLAIN: • Working Principle • Basic Structure • Magnetic Core • Primary & Secondary Windings • Emf equation of a Transformer • Transformer Ratio • Ideal Transformer & Equivalent Circuit of a Transformer • Open & Short Circuit Test • Voltage Regulation, Losses & Efficiency of a Transformer • Auto Transformer • Parallel Operation of Single Phase transformer

3. WORKING PRINCIPLE

4. WORKING PRINCIPLE Transformer is an AC static machine that: • Transfers electrical energy from one electric circuit to another • It does so without a change of frequency • It accomplishes by electromagnetic induction • Where two electric circuits are in mutual inductive influence each other

5. FUNCTION OF A TRANSFORMER • The main function of an electrical power transformer is to transfer electrical energy from one side (primary) to the other side (secondary). • The secondary current and voltage may or may not be at the same level as that of the primary current and voltage. • The energy is transferred by means of magnetic coupling. • The magnetic flux produced by the current in primary winding links the secondary winding. • Since the flux varies with time, this flux linkage results in an induced voltage in the secondary winding. • If the secondary winding is terminated with a load, the induced voltage will drive a secondary current through the load. CAUTION!!! • Transformer must not be connected to a dc source. If the primary winding of a transformer is connected to a dc supply, the flux produced will not vary but remain constant in magnitude and therefore no emf will be induced in the secondary winding, except at the moment of switching on. • There will be no back induced emf in the primary winding and therefore a heavy current will be drawn from the supply which may result in the burning out of the winding.

6. Other Important Information about Transformer • Transformers operate on mutual inductance. • A transformer has a primary winding and a secondary winding. The coil to which the source is applied is called the primary winding (primary coil). The coil to which the load is applied is called the secondary winding. • The coefficient of coupling is the portion of primary flux that links the secondary. • With 100% coupling, the turns-per-volt ratio is the same for all windings. • Transformers can have hysteresis, eddy current, and copper (I2R) losses. • Transformer losses can be reduced by using silicon steel cores, laminated cores, and small gage wires.

7. BASIC STRUCTURE

8. BASIC STRUCTURE OF A TRANSFORMER • The simple elements of a transformer consist of two coils having mutual inductance and a laminated steel core. • The two coils are insulated from each other and the steel core. • Other necessary parts are: • Suitable medium for insulating the core and its winding • Suitable bushing (porcelain/oil filled/capacitor) for insulating and bringing out the terminals of windings from the tank. • In all type of transformer, the core is constructed of transformer sheet steel lamination assembled to provide a continuous magnetic path with a minimum of air gap. • The steel used is high silicon content, sometimes heat treated to produce high permeability and low hysteresis loss at usual operating flux densities. • The eddy current loss is minimized by laminating the core. • Generally, transformer are divided into two types: • Core type • Shell type

9. BASIC STRUCTURE OF A TRANSFORMER  It was called core type because the winding surround a considerable part of the core while shell type transformer, the core surrounds a considerable portion of the windings.

10. MAGNETIC CORE

11. In both core and shell type transformers, the individual laminations are cut in the form of long strip of “L”, “E” and “I” as shown in below figure. • In order to avoid high reluctance at the joints, the laminations are “butted” against each other, the alternate layers are stacked differently to eliminate these joints.

12. CORE TYPE TRANSFORMERS • Below figure shows the single phase transformer cores. • In small size core type transformer, a simple rectangular core is used with cylindrical coils which are either circular or rectangular in form. • For large size core type transformers, round or circular cylindrical coils (because of their strength) are used which are so wound as to fit over a cruciform core section.

13. SHELL TYPE TRANSFORMERS • A shell type transformer may have a simple rectangular form as shown in figure (a) and (b). • A very commonly used shell type transformer is the one known as Berry Transformer (the name of designer). • The transformer core consists of laminations arranged in group which radiate out from the centre. • It may be pointed out that cores and coils of transformers must be provided with rigid mechanical baring in order to prevent movement and possible insulation damage. Good bracing will reduce vibration and noise during operation.

14. PRIMARY & SECONDARY WINDINGS

15. Below figure shows traditional winding which two windings were separately by distance on the core. • The other construction is the primary and secondary windings in a physical transformer are wrapped one on top of the other with the low voltage winding innermost. Such arrangement serves two purposes: • It simplifies the problem of insulating the high voltage winding from the core. • It results in much less leakage flux than would be the case if two windings were separately by a distance on the core.

16. As we can see from previous figure, the transformer has NP1 turns on its primary side and NS1 turns of wire on its secondary side . The relationship between the voltage VP(t) applied to the primary side of the transformer and the voltage VS(t) produced on the secondary side; “a” is defined to be turn ratio of the transformer • The relationship between the current iP(t) flowing into the primary side of the transformer and the current iS(t) flowing out of secondary side of the transformers is • Self induced emf is equal to V. It is also known as counter emfor back emfof the transformer.

17. EMF EQUATION OF A TRANSFORMER

18. Figure shows the flux increases from it zero value to maximum value Φm in one quarter of the cycle. • Form factor • Average rate of flux change@ e.m.f/turn • rms value of e.m.f/turn • rms value of induced e.m.f in whole primary & secondary winding

19. TRANSFORMER RATIO

20. Transformation ratio of a transformer @ “a” is defined to be turn ratio of the transformer • The relationship between the current flowing into the primary side of the transformer and the current flowing out of secondary side of the transformers is • Transformation ratio between voltage and current

21. A transformer has primary coil and secondary coil. The primary is connected to the source while secondary connected to the load. • During first half cycle, the flux builds up and collapse, this creates a half-cycle of induced voltage in the secondary. • During next half cycle, the flux builds up again and collapse, this creates another half-cycle of induced voltage in the secondary. • Notice that the primary and secondary voltages are out-of-phase.

22. Transformers are bidirectional devices. Step Down Transformer Step up Transformer • Both winding can be used as primary depend on their function.

23. TURNS PER VOLT RATIO • The 4 turns primary with a 1V source provides 4 turns per volt, therefore, each turn has 0.25 V across it. • Each turn will produces a specific amount of flux in the core. • This same flux will produce 0.25 V in each secondary turn. 1 turn secondary provides 0.25 V and a 3 turn secondary provides 0.75 V.

24. IDEAL TRANSFROMER & EQUIVALENT CIRCUIT

25. An ideal transformer is a lossless device with an input winding and an output winding. • The figure shows an accurate model of a transformer, it is not a very useful one. • To analyze practical circuits containing transformers, it is normally necessary to convert the entire circuit to an equivalent circuit at a single voltage level. • The equivalent circuit must be referred either to its primary side or to its secondary side in problem solutions.

26. (a) is the equivalent circuit of the transformer referred to its primary side . (b) is the equivalent circuit of the transformer referred to its secondary side .

27. OPEN & SHORT CIRCUIT TEST

28. The losses that occur in real transformers have to be countered for in any accurate model of transformer behavior. The major items to be considered in the construction of a such model are: • Copper losses (I2R) = the resistive heating losses in the primary and secondary windings of the transformer. • Eddy current losses = resistive heating losses in the core of the transformer. • Hysteresis losses = associated with the rearrangement of the magnetic domains in the core (they are complex). • Leakage flux = the fluxes which escape the core pass through only one of the transformer windings (rare).

29. In the open circuit test, transformer rated voltage is applied to the low voltage side of the transformer with the high voltage side let open. Measurement of power, current and voltage are made at the low voltage side. • Since the high voltage side is open, the input current IOC is equal to the excitation current through the shunt excitation branch. Because this current is very small, about 2-6% of rated current, the voltage drop across the low voltage winding and the winding copper losses are neglected. No Load Test

31. The measurement in the open-circuit test is normally done on the low voltage side of the transformer, since lower voltages are easier to work with. It is possible to determine the power factor of the input current and both the magnitude and the angle of excitation impedance. The conductance of the core loss resistor is given by The susceptance of the magnetizing inductor is given by The total excitation admittance is

32. The magnitude of excitation admittance can be found from the open-circuit test voltage and current. The angle of excitation admittance can be found from the knowledge of the circuit power factor. The power factor is given by the power factor angle is given by The admittance is ??

33. No Load Test • In the short circuit test, the low voltage side is short circuited, and the high voltage is connected to a variable low voltage source. • Measurement of power, current and voltage are made at the high voltage side. The applied voltage is adjusted until rated short circuit current flow in the winding. The voltage generally much smaller (3%-5%) then the rated primary voltage.

35. Since the input voltage is so slow during the short-circuit test, negligible currents flows through the excitation branch (excitation current is ignored), all the voltage drop in the transformer can be attributed to the series elements in the circuit. The magnitude of the series impedances referred to the primary side of the transformer is The power factor of the current is given by the current angle is given by

36. Therefore, the series impedance is series impedance is equal to ***Note The open-circuit test is usually performed on the low-voltage side ( RC & XM usually found referred to LV side) of the transformer, and the short-circuit test is usually performed on the high-voltage side ( Req & Xeq usually found referred to HV side). All the elements must referred to the same side (either High @ Low) to create the final equivalent circuit.

37. VOLTAGE REGULATION, LOSSES & EFFICIENCY OF A TRANSFORMER

38. Because a real transformer has series impedances within it, the output voltage of a transformer varies with the load even if the output voltage remains constant. • To conveniently compare transformers in this respect, it is customary to define a quantity called voltage regulation (VR). • Full-load voltage regulation is a quantity that compares the output voltage of the transformer at no load with the output voltage at full load. • Since at no load, VS = VP/a, the voltage regulation also can be expressed as

39. If the transformer equivalent circuit is in the per-unit system, then voltage regulation can be expressed as • Usually it is a good practice to have as small a voltage regulation as possible. For an ideal transformer, VR=0. • It is not always a good idea to have a low-voltage regulation, sometime high impedance and high voltage regulation transformers are deliberately used to reduce the fault current in the circuit.

40. Transformers are also compared and judged on their efficiencies. The efficiency of a device can be defined as • There are three types of losses present in transformer: • Copper losses • Hysteresis losses • Eddy current losses The efficiency of a transformer at a given load is defined as Pcu= I12 R1 + I22R2 = I12 R01 = I22 R02

41. Source provides 1640 W Transformer 90 W loss (heat loss) Load 1550 W consumed The transformer core and copper losses cause the transformer to heat up as electric energy is converted to heat energy. Efficiency = 1550 W / 1640 W = 0.945

42. AUTOTRANSFORMER

43. The polarity of a transformer can be shown by means of dots on the primary and secondary terminals. The terminal are designated by the symbols H1 and H2 for the high voltage (HV) winding and by X1 and X2 for the low voltage (LV) winding. By convention, H1 and X1 have the same polarity. • The polarity is known when the symbols H1, H2, X1, X2 are given. It is common practice to mount the four terminals on the transformer tank in a standard way so that the transformer has either additive or subtractive polarity. • If we know the power transformer has additive/subtractive polarity, we do not have to identify the terminals by symbols.

44. H1 X2 additive polarity subtractive polarity • Subtractive polarity is standard for all single phase transformers above 200 kVA, provided the high voltage winding is rated above 8660 V. • All other transformers have additive polarity. X1 H1 H2 X2 H2 X1

45. Standard 15 kVA, 600 V/120 V transformer Transformer reconnected as an autotransformer to give a ratio of 600 V/480 V Transformer reconnected to give a ratio of 600 V/720 V

46. PARALLEL OPERATION OF SINGLE PHASE TRANSFORMER

47. Objectives: • to increase power capability of transformer • for maintenance The conditions that must be satisfied • Primary windings of the transformers should suitable for the supply • system voltage and frequency. • The transformers should be properly connected with regard to polarity. • The voltage rating of both primaries and secondaries should be • identical. In other words, the transformers should have the same • transformation ratio. • The percentage impedances should be equal.

48. The current carries by each transformer The VA carries by each transformer

49. THANKS