1. Transformer Inrush Current Reduction Doug Taylor January 29, 2009

2. Overview What is Inrush Current? – Electromagnetic Phenomenon – Magnetic Properties – Causes of Inrush Thesis Work – Problem Statement – Experimental Measurements – Pre-fluxing – Future Work

3. Inrush Current Occurs when energizing transformers 5-6 times normal operating current Stresses insulation – Thermal Stress – Physical Stress Causes Protection/Power Quality Issues

4. Source: Wikipedia

5. Magnetic Hysteresis Source: http://hyperphysics.phy-astr.gsu.edu/Hbase/Solids/hyst.html Magnetic Permeability

6. Normal Transformer Operation Source: ABB App. Guide

7. Inrush Current Causes Based on two main factors: – Residual Flux – Energizing (or Switching) Instant Ideally switch when prospective flux equals residual flux Deviations from this instant result in inrush current

8. Inrush – Incorrect Switching Time Source: ABB App. Guide

9. No Inrush – Proper Switching Time Source: ABB App. Guide

10. Ideal Single Phase Switching – Demagnetized Case

11. Ideal Single Phase Switching – Demagnetized Case 2

12. Ideal Single Phase Switching – Magnetized Case

13. Worst Case Single Phase Switching

14. Ideal Three Phase, Three Pole Switching Φ r =87% of Φ max Φ r =-87% of Φ max Φ r =0% of Φ max

15. Problem Statement Set Residual Flux in Single-Phase Transformer – ‘Pre-fluxing’ – Set as high as possible Controlled Energization – Precise switching

16. Experimental Setup 55 kVA Transfomer – Primary/Secondary Winding – 230 V – Tertiary Winding – 25 V LabVIEW – Computer-based Measurement Software – Measure Flux Precision Switch – Allows Controlled Energization

17. LabVIEW

18. Experimental Hysteresis Measurements Nominal Winding Voltage - 25 V Sequence of plots – 8.5 V Applied – 17 V Applied – 19.5 V Applied – 25 V Applied

19. Hysteresis Loop Family Source: Wikipedia Nominal Operation

20. Tertiary Winding at 8.5 Vrms Flux Magnetizing Current Voltage

21. Tertiary Winding at 17 Vrms Flux Magnetizing Current Voltage

22. Tertiary Winding at 19.5 Vrms Flux Magnetizing Current Voltage

23. Tertiary Winding at Rated 25 Vrms Flux Magnetizing Current B r_max = 0.95 T Voltage

24. Inrush Current – Demagnetized Case Case 1: Unloaded Steady-State Operation Case 2: Proper Switching – No Inrush Case 3: Worst Case Switching – Maximum Inrush

25. Unloaded Steady-State Operation (230 V Winding) Voltage Magnetizing Current

26. Demagnetized – Proper Switching Time (~90 deg) Voltage Magnetizing Current

27. Demagnetized – Proper Switching Time (~90 deg) Voltage Magnetizing Current

28. Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

29. Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

30. Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

31. Prefluxing Send a pulse of energy to the transformer Circuit Used – Series capacitor and diode Fairly Simple Effective

32. Series Capacitor & Diode

33. Pre-Flux 9 mWb (75% of Φ r_max ) Flux Voltage Magnetizing Current

34. Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

35. Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

36. Future Work Sensitivity Testing – Non-Ideal Pre-Fluxing – Non-Ideal Switching – Keep Inrush To 1.5-2 p.u. – Increase Device Practicality

37. Ideal Three Phase, Three Pole Switching Φ r =87% of Φ max 210 Deg

38. Non-Ideal Pre-Fluxing Non-Ideal Pre-Fluxing/ Switching Φ r =50% of Φ max

39. Non-Ideal Switching– Demagnetized (114°, 24° error) Voltage Magnetizing Current

40. Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

41. Future Work (Con’t) Device Sizing – Dependent upon Transformer Size

42. Questions?