1. 1. GENERATING EQUIPMENT Prepared by: LENY A. ETCOY BSEE-5B
2. 2. LEAKAGE REACTANCE AND ARMATURE - Two different result produced in an alternator due to the armature windings carrying current.
3. 3. Indicates the conditions existing between the armature and the field when the armature current is in phase with the generated voltage.
4. 4. Indicates conditions when the current in the armature lags the generated voltage by 90 electrical degrees.
5. 5. Indicates when the current in the armature leads the generated voltage by 90 electrical degrees.
6. 6. LEAKAGE REACTION The alternating current in the armature windings will set up a certain magnetic flux, which will encircle each conductor or group of conductors. ARMATURE LEAKAGE FLUX -the flux that distinguish it from flux that crosses the air gap and is available as useful flux to generate the alternator voltage. E e.i. = -L(di/dt)
7. 7. ARMATURE REACTION The resultant m.m.f. produced by a three-phase alternator armature when carrying current is of practically constant magnitude but revolves at synchronous speed; hence it is fixed in position relative to the field m.m.f.
8. 8. THREE CONDITIONS: 1. When the armature current is in phase with the generated voltage, the armature m.m.f. is cross- magnetizing 2. At zero percent power factor leading current the armature m.m.f. is magnetizing 3. At zero percent power factor lagging current the armature m.m.f. is demagnetizing in its effect upon the field m.m.f.
9. 9. It=Eg/( 3 Xt) Where: It=initial current (A) Eg= the generated voltage per line, or normal no- load voltage Xt= transient reactance in ohms, approximately equal to the leakage reactance
10. 10. M.m.f. net = m.m.f. f m.m.f. d Where: m.m.f. d =is the portion of the total armature m.m.f. which is effective in demagnetizing the field poles.
11. 11. Ix=Ex/( 3 Xt) Where: Ix = current flowing into the short circuit, at x sec. after the short circuit has been produced. Es = generated voltage at x sec. after the short circuit has occurred. Xt = transient reactance of the alternator The current flowing into the short circuit will become steady at a value that will produced sufficient armature- demagnetizing m.m.f. to limit the flux crossing the air gap to a value just large enough to generate the necessary to send the current through the armature-leakage reactance.
12. 12. Xs= Eg/( 3 Is) Where: Eg = no-load voltage Is = sustained short-circuit current Xs = synchronous reactance, (ohms)
13. 13. MAGNETIC-FLUX DISTRIBUTION IN THE AIR GAP OF ALTERNATORS AT FULL LOAD TWO TYPES: A) Nonsalient-pole Machines B) Salient-pole Machines
14. 14. ALTERNATOR VECTOR DIAGRAM AT FULL LOAD A) Nonsalient pole Machines Assume an alternator supplying a load whose power factor is practically 80 percent lagging current. B) Salient-pole Machines As stated in art. 41 it is not correct to represent the field or the net m.m.f. acting across the air gap at full load by vectors in the case of salient-pole machine; hence a salient-pole machine cannot have a space vector diagram of m.m.fs.
15. 15. ALTERNATOR CHARACTERISTICS CURVE A) No-load Saturation Curve B) Full-load Saturation Curve 3 Eo = 3 I Zs And I Zs/ Eo = 1 or 100 percent
16. 16. VOLTAGE REGULATION The voltage regulation of alternators is found in the same manner as for direct-current generators (art. 25)
17. 17. LOW SHORT-CIRCUIT CURRENT versus GOOD REGULATION In art 24 it was stated that direct-current generators used for lighting should not have a voltage regulation higher than 2 percent. In contrast to such value the regulation of an alternator at 8O percent power factor may be high as 42 percent (art. 44)
18. 18. PARALLEL OPERATION OF ALTERNATORS Before an alternator can be connected in parallel with another machine that is supplying a load, it is necessary that the incoming machine have the same voltage and frequency, and also be in phase with the operating machine; and before the switches are closed, it is necessary that the polarities and phase sequence of the machine be identical.
19. 19. I = E / AA + XB Where: I = the circulating current (A) E = resultant voltage, causing current I to flow AA and XB = synchronous reactance of the alternators
20. 20. LOSSES IN ALTERNATING-CURRENT GENERATORS The losses of alternating-current generators are essentially of the same nature as those for direct- current generators given in art. 26.
21. 21. VENTILATION OF ALTERNATORS The rating of any piece of equipment is dependent upon the temperature rise of the different parts of the equipment and, hence, upon the rate at which the losses, which appear as heat, can be radiated from the different surfaces of the equipment.
22. 22. TWO METHODS: 1. Increasing the radiating surface 2. Increasing the amount of ventilation
23. 23. This method is the same as is used for water-wheel units, except that, on amount of the much larger volume of air needed, independent motor-driven fans are always supplied to maintain a higher air velocity through the machine.
24. 24. It differs from the simple radial system in that air from the end bells is carried axially across the back of the core in passages provided in the frame, passes radially into the frame through radial vent ducts in parts of the core, and travels axially through the gap and out to the back of the core through radial vent ducts in other sections of the core.
25. 25. Air delivered by the fans between and around the stator coils into a chamber formed by the end housings.
26. 26. HYDROGEN COOLING The used of hydrogen as the cooling medium for electrical machinery has been introduced by manufacturers in the last few years, and it has so far with considerable favor in the operation of the certain types of machines.
27. 27. ADVANTAGES OF HYDROGEN OVER AIR: 1. The density of hydrogen is about one- fourteenth that of air 2. The heat conductivity through loose types of insulation is about 25 percent better with hydrogen 3. The heat conductivity across iron laminations is about three times as good with hydrogen DISADVANTAGE: 1.When hydrogen and air mixed in proper proportions will form an explosive gas; however, if the hydrogen content is above 70 percent, there seems to be little danger of an explosion.
28. 28. INDUCTION GENERATOR Is obtain by driving an induction motor at a speed above synchronous speed, in which case the machine can be considered as receiving from the line the necessary exciting current and supplying to the line the power or energy current.
29. 29. THEORY- the torque produced by an induction motor depends on the magnetic flux that crosses the air gap, the current, and the power factor of the rotor.
30. 30. MISCELLANEOUS GENERATING EQUIPMENT
31. 31. MOTOR-GENERATOR SETS 1. Transforming from direct current to direct current at different voltage. 2. Transforming from alternating current to direct current or vice versa. 3. Transforming from alternating current to alternating current at different frequency.
32. 32. INDUCTION MOTOR-GENERATOR SETS A. CONSTRUCTION- an induction motor-generator set consists of an induction motor direct-connected to a direct-current generator. B. APPLICATIONS- induction motor generators are used to supply direct current for lighting and general power up to medium capacities.
33. 33. SYNCHRONOUS MOTOR- GENERATOR SETS A. CONSTRUCTION- as the name implies, such a motor-generator set comprises a synchronous motor connected to the same shaft of a direct-current generator. B. AVANTAGES and DISADVANTAGES- the main advantage of the synchronous motor-generator set is the power-factor corrective effect which can be accomplish by properly adjusting the field current of the synchronous motor.
34. 34. C. APPLICATION 1. lighting service 2. industrial service 3. electromechanical service 4. railway service 5. storage-battery charging
35. 35. D. STARTING OF SYNCHRONOUS MOTOR- GENERATOR SETS -the most common method used in starting such units is from low-voltage taps on transformers or autotransformers, by means of the torque produced by squirrel-cage windings which are placed in the pole shoes.
36. 36. FREQUENCY CHANGERS A. CONSTRUCTION- frequency changers consist of an alternating-current generator direct-connected to the shaft of an alternating-current motor. B. APPLICATION 1. as a means of interchanging energy between two systems of different frequencies. 2. to supply certain types of loads at a frequency different from the frequency of the system.
37. 37. C. PARALLEL OPERATION D. ELECTRONIC FREQUENCY CHANGERS- heavy power electronic frequency changers may soon replace the rotating type changers. Alternating-current power of any frequency is first converted to direct-current power through a typical rectifier circuit and then converted back to alternating-current power of any second frequency through an inverted rectifier circuit or inverter.
38. 38. ROTARY CONVERTERS A. CONNECTIONS- the armature winding is an ordinary direct-current winding, which may be either series or multiple (art. 20) B. VOLTAGE RATIOS- the theoretical voltage ratios of rotary converters are definitely fixed by the number of phases and the method by which the transformer terminals are connected to the slip rings. C. CURRENT RATIOS- the current ratios of a converter are not so definitely fixed as the voltage ratios, but are different for different power factors.
39. 39. D. HEATIGN AND CAPACITY- the effective current in each part of a converter-armature winding is the difference between the instantaneous values of alternating-current input and direct-current output. F. VOLTAGE VARIATION- it is evident that the ratio between the alternating and direct-current voltages of an elementary converter is a nearly fixed quantity and remains constant within a very few percent from no load to full load.
40. 40. 1. SYNCHRONOUS-BOOSTER CONVERTER- is a rotary converter with a mechanically connected alternating-current generator, the armature winding of which is connected in series relation with the armature winding of the rotary converter so that the voltage generated in it, whenever its held is excited, either adds to or subtracts from the voltage supplied to the converter and affects the direct-current voltage accordingly. 2. REGULATING TRANSFORMER OR POTENTIAL REGULATOR- consist of a transformer secondary with a large number of taps and some switching device to change the converter connections from tap to another.
41. 41. 3. DIRECT-CURRENT BOOSTER- a dc generator can be connected in series with the direct-current side of the converter, and by varying the field of this generator the dc current can be varied. 4. AUTOMATIC COMPOUNDING- with this method of voltage variation, it is necessary that there be a certain amount of reactance in the supply lines to the converter, which may be reactance coils or embodied in the supplying transformers.
42. 42. G. THREE-WIRE SYNCHRONOUS CONVERTERS- in art. 28 it was shown that, by connecting a high- reactance and low- resistance coil between diametrically opposite points of a direct-current generator armature, a neutral line could be obtained by bringing out a tap from the middle point of the reactance coil, thereby obtaining a three-wire generator. F. PARALLEL OPERATIONS- converters can be operated in parallel, the division of load being determined by the induced voltage of the individual. H. HUNTING- hunting of synchronous converters may be caused by the periodic variation of the supply frequency, by sudden changes of load, or by excessive line drop.
43. 43. J. STARTING OF SYNCHRONOUS CONVERTERS 1. alternating-current self-starting method 2. alternating-current motor-starting method 3. direct-current self-starting method
44. 44. CONVERTERS versus MOTOR- GENERATOR SETS 1. Reliability 2. Voltage regulation 3. Power-factor corrective effect 4. Efficiency 5. Cost 6. Parallel operation 7. Starting
45. 45. MERCURY-ARC RECTIFIERS A. glass-bulb rectifiers B. Polyphase rectifiers C. Power rectifiers