1. EEE223 Energy Conversion II Md. Asif Uddin Khan Lecturer, EEE BRAC University 1

2. Operation and Analysis of Synchronous Generator Materials taken from: Stephen J. Chapman: Electric Machinery Fundamentals, McGraw-Hill, 5 Th Edition Chapters: 3 and 4 2

3. Speed: 3

4. The Internal generated voltage of a synchronous generator: 4

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6. The following information is known about the simple two-pole generator in Figure 4--16. The peak flux density of the rotor magnetic field is 0.2 T, and the mechanical rate of rotation of the shaft is 3600 r/min. The stator diameter of the machine is 0.5 m, its coil length is 0.3 m, and there are 151lU1ls per coil. The machine is V-connected. (a) What are the three phase voltages of the generator as a function of time? (b) What is the rms phase voltage of this generator? (c) What is the rms terminal voltage of this generator? 6

7. The Equivalent circuit of a Synchronous Generator: 7

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9. Self-inductance and Resistance of the Armature Coils 9

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12. Phasor Diagram of Synchronous Generator: 12 For unity power factor For lagging power factor For, leading power factor

13. Relationship between induced toque and electrical power output through simplified phasor diagram: 13

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15. Measuring Model Parameters of Synchronous Generator: To measure synchronous reactance and armature of a synchronous machine, the following tests are required: Open Circuit Test Short Circuit Test DC Test 15

16. Open Circuit Test: Steps: 1.Generator is rotated at the rated speed. 2.No load is connected at the terminals. 3.Field current is increased from zero to maximum. 4.Record values of the terminal voltage and the field current value. 16 At First the curve is almost perfectly linear, until some saturation is observed at high field currents. The unsaturated iron in the frame of the synchronous machine has a reluctance several thousand times lower than the air-gap reluctance. So at first almost all the mmf is across the air-gap, and the resulting flux increase is linear. When the iron finally saturates, the reluctance of the iron increases dramatically and flux increases much more slowly with an increase in mmf. The linear portion of an OCC is called the air-gap line of the characteristic.

17. Short Circuit Test: Steps: 1.Generator is rotated at rated speed 2.Adjust field current to zero 3.Short circuit the terminals 4.Measure armature current or line current as the field current is increased 17 Since B S almost cancels B R, the net magnetic field Bnet is very small and hence the machine is unsaturated and the SCC is also linear.

18. Determining X S ignoring R A : 18

19. Problem with this method of determining Xs: E A is taken from the OCC whereby the core would be partially saturated for large field currents while I A is taken from the SCC where the core is never saturated at any field current. Therefore E A value taken from the OCC may not be the same E A value in the SCC test. Hence the value of X S is only approximate. 19 Remedy: To gain better accuracy, the test should be done at low field currents which correspond to the linear region of the OCC.

20. SCR (Short Circuit Ratio): 20 SCR

21. DC Test: 21 For Y-connected stator (armature), = 2 For Δ-connected stator (armature), = (2/3) This technique is not perfectly accurate, since the ac resistance will be slightly larger than the dc resistance (as a result of the skin effect at higher frequencies).

22. A 200kVA, 480V, 50Hz, Y ‐ connected synchronous generator with a rated field current of 5A was tested, and the following data were taken: 1. V T,OC at the rated I F was measured to be 540V 2. I L,SC at the rated I F was found to be 300A. 3. When a dc voltage of 10V was applied to two of the terminals, a current of 25A was measured. Find the values of the armature resistance and the approximate synchronous reactance in ohms that would be used in the generator model at the rated conditions. 22

23. The Effect of Load Changes on a Synchronous Generator Operating Alone: Assume a generator is connected to a load. Load increase: An increase of load at a constant power factor causes an increase in real and reactive power drawn from the generator. Such an increase in load increases the load current drawn from the generator. Assumptions: Field resistor has not been changed, field current is kept constant, hence flux (ф ) is constant. Generator rotor speed ( Ѡ ) is maintained constant. Therefore E A = K ф Ѡ is constant. If E A is constant, what actually varies with a changing load i.e. Changing armature current ? 23

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29. Parallel operation of synchronous generators: Reasons for operating in parallel: Handling larger loads. Maintenance can be done without power disruption. Increasing system reliability. Increased efficiency. This means a single generator of large size will be relatively inefficient if that is not operated near full load. On the other hand a number of small generators can be running in parallel to operate near the respective full load and hence more efficiently. 29

30. Conditions for paralleling two generators: 30 Paralleling 2 or more generators must be done carefully as to avoid damage to generators or other system components. Conditions are as follows: a) RMS line voltages must be equal. b) The generators to be paralleled must have the same phase sequence. If the phase sequence is different (as shown above), then even though one pair of voltages (the a phase) is in phase, the other 2 pairs of voltages are 120° out of phase. If the generators were connected in this manner, there would be no problem with phase a, but huge currents would flow in phases b and c, damaging both machines. c) Generator output phase angles must be the same. d) The oncoming generator (the new generator) must have a slightly higher operating frequency than that of the running generator or system.

31. This is done so that: The phase angles of the oncoming machine will change slowly with respect to the phase angles of the running system. This will then allow observation of phase angles between corresponding voltages on an oscilloscope and a closure of the switch exactly when the phase angle difference becomes zero. Also due to having a slightly higher frequency the oncoming generator will deliver power to the load i.e. not take power from the running generator or system. Once paralleled, the oncoming generator will share the load with the running generator at a common frequency. 31

32. General Procedure for Paralleling Generators: (For smaller generators these steps are followed by the operators manually) 1.Using Voltmeters, the field current of the oncoming generator should be adjusted until its terminal voltage is equal to the line voltage of the running system. 2.Check and verify phase sequence to be identical to the system phase sequence. There are 2 methods to do this: i.Alternately connect a small induction motor to the terminals of each of the 2 generators. If the motor rotates in the same direction each time, then the phase sequence is the same for both generators. If the motor rotates in opposite directions, then the phase sequences differ, and 2 of the conductors on the incoming generator must be reversed. 32

33. ii.Another way is using the 3 light bulb method, where the bulbs are stretched across the open terminals of the switch connecting the generator to the system (as shown in the figure beside). As the phase changes between the 2 systems, the light bulbs first get bright (large phase difference) and then get dim (small phase difference). If all 3 bulbs get bright and dark together, then the systems have the same phase sequence. If the bulbs brighten in succession, then the systems have the opposite phase sequence, and one of the sequences must be reversed. 33

34. 3.Check if phase angles between two machines corresponding voltages are the same using any of the two methods. i.In the three bulb method when all the three bulbs get dark at the same time the phase angles are same. ii.Using a Synchroscope – a meter that measures the difference in phase angles (it does not check phase sequences but checks only one phase’s angle). When the synchroscope needle is in the vertical position, the voltages are in phase. 34

35. 4.Adjust oncoming generator frequency to be slightly higher than the system frequency. This is done by watching a frequency meter until the frequencies are close. 5.Once the frequencies are nearly equal, the voltages between two generators will change phase with respect to each other very slowly. The phase changes are observed as in step 3, and when the phase angles are equal, the switch connecting the oncoming generator is closed. In large power plants the whole process of paralleling an oncoming generator with the line or grid bus is automated using a computer and PLC system. 35

36. Frequency-Power and Voltage-Reactive Power Characteristics of a Synchronous Generator: 36

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38. Terminal voltage versus reactive power for a synchronous generator: 38

39. Frequency‐power, voltage‐reactive power characteristics of an infinite bus (large power system or grid): 39

40. In conclusion, for a single generator: Real and reactive power to be supplied should be equal to the amount demanded by the load attached to the generator. For any given real power, the governor set points control the generator operating frequency. For any given reactive power, the field current controls the generator’s terminal voltage. So if the load (lagging) of an isolated generator increases, its governor set point and the field current have to be increased to maintain the original frequency and terminal voltage else these will be falling with increase in load. 40

41. Figure below shows a generator supplying a load. A second load is to be connected in parallel with the first one. The generator has a no ‐ load frequency of 61.0 Hz and a slope S P of 1 MW/Hz. Load 1 consumes a real power of 1000kW at 0.8 PF lagging, while load 2 consumes a real power of 800kW at 0.707 PF lagging. a) Before the switch is closed, what is the operating frequency of the system? b) After load 2 is connected, what is the operating frequency of the system? c) After load 2 is connected, what action could an operator take to restore the system frequency to 60Hz? 41

42. Operation of Generators in Parallel with Large Power Systems: 42

43. Effects of changing governor set point i.e. to change no‐load speed when excitation and speed are kept constant: 43 As SP↑ P gen ↑ P bus ↓ δ↑ I A ↑ and becomes leading while f, V T and E A remains constant. If SP so increased that Pgen > Pload then extra power flows into infinite bus.

44. Effects of changing field current at a fixed set point p of the governor i.e. constant power output from the generator: 44 As I f ↑ Q gen ↑ E a ↑ δ ↓ I A ↑ and becomes lagging while f, V T and Pgen remains constant.

45. Hence, for a generator operating in parallel with an infinite bus (large system): Frequency and terminal voltage of generator are controlled by the large system. Changes in generator’s governor set points will control real power to be supplied to the system. Changes in generator field current will control the amount of reactive power to be supplied to the system. 45

46. For a generator paralleled with another generator: In this system, the basic constraint is that the sum of the real and reactive powers supplied by the two generators must equal the P and Q demanded by the load. The system frequency is not constrained to be constant, and neither is the power of a given generator constrained to be constant. 46

47. 47 The house diagram at the moment G2 is paralleled with the system As SP2↑ P G2 ↑ P G1 ↓ f ↑As I f2 ↑ QG2 ↑ QG1↓ V T ↑

48. Shifting power sharing without affecting system frequency while the total load remains constant: 48 SP1 ↓ and SP2↑ together so that P G1 ↓ P G2 ↑ and f remains unchanged

49. Shifting system frequency without affecting power sharing while the total load remains constant: 49 SP1 ↑ and SP2↑ together so that f ↑ but P G1 and P G2 remain unchanged

50. Shifting reactive power sharing without affecting terminal voltage while the total reactive load remains constant: 50 I f1 ↓ and I f2 ↑ together so that Q G1 ↓ Q G2 ↑ and V T remains unchanged

51. Shifting terminal voltage without affecting reactive power sharing while the total reactive load remains constant: 51 I f1 ↑ and I f2 ↑ together so that V T ↑ but Q G1 and Q G2 remain unchanged

52. Two synchronous generators with flat frequency-power characteristics. A very tiny change in the no load frequency of either of these machines could cause huge shifts in the power sharing. 52 Any synchronous generator intended to operate in parallel with other machines must have a drooping frequency ‐ power characteristic. So, to ensure good control of power sharing between generators, they should have speed droops in the range of 2 ‐ 5%.

53. Figure below shows two generators supplying a load. Generator 1 has a no- load frequency of 61.5 Hz and a slope Sp1 of 1 MW/Hz. Generator 2 has a no- load frequency of 61.0 Hz and a slope Sp2 of 1 MW/Hz. The two generators are supplying a real load totaling 2.5 MW at 0.8 PF lagging. The resulting system power-frequency or house diagram is shown. (a) At what frequency is this system operating, and how much power is supplied by each of the two generators? (b) Suppose an additional 1-MW load were attached to this power system. What would the new system frequency be, and how much power would G1 and G2 supply now? (c) With the system in the configuration described in part b, what will the system frequency and generator powers be if the governor set points on G2 are in creased by 0.5 Hz? 53