Modelling and Performance of transmission lines.pptx

To model and check the performance of the transmission lines

1. UNIT - II Modeling and Performance of Transmission Lines Performance of Transmission lines - short line, medium line and long line – equivalent circuits, phasor diagram, attenuation constant, phase constant, surge impedance - transmission efficiency and voltage regulation, real and reactive power flow in lines – Power Circle diagrams – Ferranti effect - Formation of Corona – Critical Voltages – Effect on Line Performance.

2. Course Outcome Evaluate the performance of the transmission lines.

3. Classification of Overhead Transmission Lines Short transmission line : upto about 50 km, <20kV Medium transmission line : about 50 km to 150 km, (>20 kV < 100 kV) Long transmission line : more than 150 km (> 100 kV)

4. Important Terms Voltage regulation : The difference in voltage at the receiving end of a transmission line between conditions of no load and full load is called voltage regulation and is expressed as a percentage of the receiving end voltage. Obviously, it is desirable that the voltage regulation of a transmission line should be low i.e. the increase in load current should make very little difference in the receiving end voltage

5. Important Terms Transmission efficiency : The ratio of receiving end power to the sending end power of a transmission line is known as the transmission efficiency of the line.

6. Performance of Single Phase Short Transmission Lines

7. Performance of Single Phase Short Transmission Lines

8. Performance of Single Phase Short Transmission Lines

9. Performance of Single Phase Short Transmission Lines

10. Effect of Load Power Factor on Regulation and Efficiency

11. Effect of Load Power Factor on Regulation and Efficiency

12. Performance of Medium Transmission Lines End condenser method Nominal T method Nominal π method

13. End condenser method

14. End condenser method

15. End condenser method

16. End condenser method

17. End condenser method

18. Nominal T Method

19. Nominal T Method

20. Nominal T Method

21. Nominal π Method

22. Nominal π Method

23. Nominal π Method

24. Long Transmission Line

25. Long Transmission Line

26. Rigorous Method

27. Rigorous Method

28. Rigorous Method

29. Rigorous Method

30. Rigorous Method

31. Generalized circuit constants of a transmission line

32. Generalized circuit constants of a transmission line

33. Determination of Generalized circuit constants for transmission line

34. Determination of Generalized circuit constants for transmission line

35. Determination of Generalized circuit constants for transmission line ……..vi ……vii I c

36. Determination of Generalized circuit constants for transmission line

37. Determination of Generalized circuit constants for transmission line

38. Determination of Generalized circuit constants for transmission line

39. Surge Impedance Surge Impedance  is the characteristic impedance  of a lossless transmission line. It is also called Natural  Impedance   because this impedance  has nothing to do with load impedance . Since line is assumed to be lossless, this   means  that series resistance and shunt conductance is negligible i.e. zero for power lines . Z = Characteristics impedance is defined as the square root of the ratio of series impedance to shunt admittance.  

40. Surge Impedance The surge impedance is defined as the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Its SI unit is ohm. It is purely real with no reactive component. The approximate value of surge impedance of overhead lines is 400 Ω while typically it is in the range of 400 Ω to 600 Ω .

41. Surge Impedance Loading (SIL) SIL of a line is the power delivered by a line to a purely resistive load equal to its surge impedance. The line is assumed to have no resistance. SIL = V R 2 /Z C . SIL is also called natural power of line. Sometimes it is convenient to express the power transmitted by a line in terms of per unit of SIL which is the ratio of the power transmitted to the surge impedance loading.

42. Surge Impedance Loading (SIL) The permissible loading of a transmission line may be expressed as a fraction of its SIL and SIL provides a comparison of load carrying capabilities of lines.

43. Ferranti Effect It can be seen that for a long transmission line under no load conditions, the voltage at receiving end is more than at sending end because of the effect of the line capacitance. This is called Ferranti effect

44. CORONA When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage , the conductors are surrounded by a faint violet glow called corona.

45. CORONA The phenomenon of corona is accomplished by a hissing sound, production of ozone, power loss and radio interference . The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and the radio noise. If the applied voltage is increased to breakdown value, a flash-over will occur between the conductors due to breakdown of air insulation.

46. CORONA Discharges The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona. If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter .

47. CORONA Discharges Factors Affecting Corona: Atmosphere .(stormy weather) Conductor size . (shape and condition, rough and irregular surface) Spacing between conductors . Line voltage.

48. CORONA Discharges

49. CORONA Discharges

50. CORONA Discharges

51. CORONA Discharges

52. CORONA Discharges

53. CORONA Discharges

54. CORONA Discharges

55. CORONA Discharges It has been seen that intense corona effects are observed at a working voltage of 33 kV or above . Therefore, careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33 kV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment.

56. CORONA Discharges Methods of reducing corona: