On load tap changer

On Load Tap Changer (OLTC) is used in "High Power Transformers" to control output voltage, when electric load on transformers get increase the output voltage get decrease due to internal voltage drop inside winding, change in tap is required to maintain output voltage. OLTC is a device which perform tap changing in High Power Transformers during On Load conditions and is powered by a motor.

1. On Load Tap Changer (OLTC) As the name suggests, permit tap changing and hence voltage regulation with the transformer during on-load.

2. “On Load Tap Changer” is used to compensate the drop in output voltage of high power transformer, drop in output voltage from no load to full load in know as voltage regulation. Flow of current in “LV Side” cause available voltage to drop due to inherent resistance of the winding. Quick Review of Voltage Regulation On Load Tap Changer (OLTC)

3. Quick Review of Voltage Regulation On Load Tap Changer (OLTC) Load on consumer end of transformer is never fixed, it varies at random, its higher value is matter of concern.

4. On Load Tap Changer (OLTC) Quick Review of Voltage Regulation Light Load; Load may be low due to fact that much of the time not all the connected load remain “on”. Though at low load there is slight drop in voltage at the load end but not matter of concern up to certain level.

5. On Load Tap Changer (OLTC) High Load; when so many connected load gets on same time high current is drown out from transformer to drive all. Heavy magnitude load current cause considerable voltage drop in internal resistance of transformer windings. Considerable drop in voltage affect all the acting load, output of each load get reduces due to availability of less voltage then desired. Quick Review of Voltage Regulation

6. 1 2 3 Quick Review of Voltage Regulation Tap changer is used to compensate for this, usually connected on the HV winding  for two reasons: Because the currents are lower, the tap changer contacts, leads, etc., can be smaller. HV winding is wound outside the LV winding, it is easier to get the tapping connections. On Load Tap Changer (OLTC)

7. 1 2 3 Drop in voltage on LV side is compensated by reducing number of turns of HV side, selector arm is moved from tap 1 to tap 2 to reduce number of turns. Turn reduction increase the per turn voltage of HV side results in increase of LV terminals voltage. Quick Review of Voltage Regulation On Load Tap Changer (OLTC)

8. 1 2 3 Tap changer is used to compensate for this, usually connected on the HV winding  for two reasons: Because the currents are lower, the tap changer contacts, leads, etc., can be smaller. HV winding is wound outside the LV winding, it is easier to get the tapping connections. Quick Review of Voltage Regulation On Load Tap Changer (OLTC)

9. Selector switches Bypass switch Vacuum switch Reactor coil Detail Insight of OLTC 1. Selector Switches These switches select the physical tap position on the transformer winding. 2. Reactors During each tap change, there is an interval where two voltage taps are spanned. Reactors (inductors) are used in the circuit to increase the impedance of the selector circuit and limit the amount of current circulating due to this voltage difference. 3. Vacuum Switch During each tap change the duty of circuit breaker is performed by vacuum switch. 4. Bypass Switch This switch operates during the tap changing sequence but, at no time,  does it make or break load current. High Voltage Winding On Load Tap Changer (OLTC)

10. Selector switches Bypass switch Vacuum switch Reactor coil Detail Insight of OLTC Change in tap position during on load is not a quite simple process, proper steps must be followed to do this without causing sever transient and damage to the windings High Voltage Winding On Load Tap Changer (OLTC)

11. Detail Insight of OLTC Step 1 tap changer selected to tap 1, all switches in home position. On Load Tap Changer (OLTC)

12. Step 2 “Selector Switch” arm moving from Tap-1 to Tap-2. “Vacuum switch” open, makes right part of selector arm isolated. Due to moment of selector arm the “Bypass Switch” stay at home position and get only in contact with left part of selector arm, isolating right part. Detail Insight of OLTC On Load Tap Changer (OLTC)

13. Detail Insight of OLTC Step 3 Isolated part of “selector arm’ comes in contact with tap-2 which is relatively at lower voltage than tap 1. “Vacuum switch” remain open till this position. “Bypass switch” remain only in contact with left part of “selector arm”. On Load Tap Changer (OLTC)

14. Detail Insight of OLTC Step 4 “Vacuum Switch” get closed, tap 1 and tap 2 get short. Current circulates through closed loop formed by “Selector Switch” arms due to voltage difference, instant rush of current is counter by reactor coils in both arm . “Bypass switch” remain only in contact with left part of “selector arm”. On Load Tap Changer (OLTC)

15. Detail Insight of OLTC Step 5 “Vacuum Switch” remain closed, voltage of turns from tap-1 to tap-2 ceased at line voltage. “Bypass Switch” switch now easily slide from left part to right part. No spark/discharge occur as vacuum switch is closed in parallel. On Load Tap Changer (OLTC)

16. Detail Insight of OLTC Step 6 “Bypass Switch” gets completely in contact with right part of “Selector Switch”. On Load Tap Changer (OLTC)

17. Detail Insight of OLTC Step 7 “Vacuum Switch” get opened, right part get isolate. Current continues through right part in to the main winding, Main winding’s turns between tap-1 and Tap-2 gets out of context. On Load Tap Changer (OLTC)

18. Detail Insight of OLTC Step 8 Left part of “selector switch” can left tap-1 without facing discharge/spark. On Load Tap Changer (OLTC)

19. Detail Insight of OLTC Step 8 “Selector Switch” completely rest on Tap-2 after leaving Tap-1. This way tap changing happens, following a right sequence, same sequence must be followed for selection of next tap i.e Tap-3, tap. Tap-4 can’t be selected by jumping over Tap-3. Reverse sequence must be followed to when moving in reverse direction i.e. from 4 to 1 On Load Tap Changer (OLTC)

20. Detail Insight of OLTC On Load Tap Changer (OLTC)

21. Detail Insight of OLTC On Load Tap Changer (OLTC)

22. Detail Insight of OLTC On Load Tap Changer (OLTC)

23. Detail Insight of OLTC On Load Tap Changer (OLTC)

24. Detail Insight of OLTC On Load Tap Changer (OLTC)

25. Detail Insight of OLTC On Load Tap Changer (OLTC)

26. Detail Insight of OLTC On Load Tap Changer (OLTC)

27. Detail Insight of OLTC On Load Tap Changer (OLTC)

28. Detail Insight of OLTC On Load Tap Changer (OLTC)

29. Detail Insight of OLTC On Load Tap Changer (OLTC)

30. Detail Insight of OLTC On Load Tap Changer (OLTC)

31. Detail Insight of OLTC On Load Tap Changer (OLTC)

32. Detail Insight of OLTC On Load Tap Changer (OLTC)

33. Detail Insight of OLTC On Load Tap Changer (OLTC)

34. Detail Insight of OLTC On Load Tap Changer (OLTC)

35. Detail Insight of OLTC On Load Tap Changer (OLTC)

36. OLTC Connections On Load Tap Changer (OLTC) R Y B A B C High Voltage Winding connection without OLTC

37. OLTC Connections On Load Tap Changer (OLTC) R Y B A B C High Voltage Winding connection with OLTC Connections of winding A and B with OLTC and Bushings

38. OLTC Connections On Load Tap Changer (OLTC) A B C R Y B High Voltage Winding connection with OLTC Connections of winding B and C with OLTC and Bushings

39. OLTC Connections On Load Tap Changer (OLTC) A B C R Y B High Voltage Winding connection with OLTC Connections of winding C and A with OLTC and Bushings

40. OLTC Connections On Load Tap Changer (OLTC)

41. OLTC Connections On Load Tap Changer (OLTC) Thank You