1. 1 © ABB AB, 2008 2008-02-05 Application of Unit Protection Schemes for Auto-Transformers Zoran Gajić ABB AB Vasteras, Sweden Authors: Z. Gajić, ABB Sweden S. Holst, ABB Sweden

2. 2 © ABB AB, 2008 2008-02-05 An auto-transformer is a power transformer in which at least two windings have a common part Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer) In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower the zero sequence impedance for five- limb core constructions or for auto-transformers built from three single phase units Auto-Transformer

3. 3 © ABB AB, 2008 2008-02-05 Example Auto-Transformer

4. 4 © ABB AB, 2008 2008-02-05 Auto-Transformer has Dual Rating It can be shown that power is transferred in two different ways through an auto-transformer. One part of the power is transferred by galvanic connection and the other part is transferred via magnetic circuit (i.e. transformer action) Auto-transformer is cheaper that corresponding two/three winding power transformer design Possible problem with short circuit current withstand

5. 5 © ABB AB, 2008 2008-02-05 Auto-transformer Construction One three-phase unit Typically has five-limb core Three single-phase units connected to form three-phase group

6. 6 © ABB AB, 2008 2008-02-05 Possible CT Locations for Auto-Transformer

7. 7 © ABB AB, 2008 2008-02-05 Possible Differential Protection Principles Based on autotransformer ampere-turn balance 87T Based on the first Kirchhoff’s law between galvanically interconnected parts 87B

8. 8 © ABB AB, 2008 2008-02-05 Special Unit Protections Restricted Earth-Fault Zero-sequence current based Dedicated delta winding unit protections

9. 9 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1 and CT2 S Base =Throughput Power (400MVA) Tertiary delta winding can not be loaded Mandatory zero sequence current reduction

10. 10 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1 and CT2 Winding W1W2 Base Power [MVA] 400 Ph-Ph, No-Load Voltage [kV] 400231 Base Primary Current [A] 5771000 Vector Group Yy0 Zero Sequence Current Elimination Yes (Mandatory) Connected to CT (See Figure 3) CT1CT2 Base current on CT secondary side [A] 0.7210.833

11. 11 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2 and CT3 S Base =Throughput Power (400MVA) Tertiary delta winding can be loaded Mandatory zero sequence current reduction

12. 12 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2 and CT3 Winding W1W2W3 Base Power [MVA] 400 Ph-Ph, No-Load Voltage [kV] 40023110.5 Base Primary Current [A] 577100021994 Vector Group Yy0d5 Zero Sequence Current Elimination Yes (Mandatory) No / (Yes) Connected to CT (See Figure 3) CT1CT2CT3 Base current on CT secondary side [A] 0.7210.83315.71

13. 13 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2 and CT7 S Base =Throughput Power (400MVA) Tertiary delta winding can be loaded CT location within delta winding requires “special attention” Zero sequence current reduction not required if it is a five-limb or single-phase construction

14. 14 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2 and CT7 Winding W1W2W3 Base Power [MVA] 400 Ph-Ph, No-Load Voltage [kV] 400231 * Base Primary Current [A] 577100012698 Vector Group Yy0y0* Zero Sequence Current Elimination No / (Yes) Connected to CT (See Figure 3) CT1CT2CT7 Base current on CT secondary side [A] 0.7210.8333.175 * Influenced by CT location within tertiary delta winding

15. 15 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT5 and CT7 S Base =Magnetic Power (169MVA) Tertiary delta winding can be loaded CT location at neutral point and within delta winding requires “special attention” Zero sequence current reduction not required if it is a five-limb or single-phase construction MV (i.e. 220kV) bushings not protected!

16. 16 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT5 and CT7 * Influenced by CT7 location within tertiary delta winding Winding W1W2W3 Base Power [MVA] 169 Ph-Ph, No-Load Voltage [kV] 169231 * Base Primary Current [A] 5774225365 Vector Group Yy0y0* Zero Sequence Current Elimination No / (Yes) Connected to CT (See Figure 3) CT1CT5CT7 Base current on CT secondary side [A] 0.7210.8441.341 Influenced by CT5 location in neutral point

17. 17 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2, CT7 and CT8 S Base =Throughput Power (400MVA) Tertiary delta winding can be loaded Double CT location within delta winding requires “special attention” Relay with four restraint inputs required Mandatory zero sequence current reduction

18. 18 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT2, CT7 and CT8 * Influenced by double CT location within tertiary delta winding WindingW1W2W3 Base Power [MVA]400 Ph-Ph, No-Load Voltage [kV] 400231 * * Base Primary Current [A]577100025396 Vector GroupYy0 Zero Sequence Current Elimination No / (Yes) Connected to CT (See Figure 3) CT1CT2CT7CT8 Base current on CT secondary side [A] 0.7210.8336.349

19. 19 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT5, CT7 and CT8 S Base =Magnetic Power (169MVA) Tertiary delta winding can be loaded CT location at neutral point and within delta winding requires “special attention” Relay with four restraint inputs required Zero sequence current reduction not required if it is a five-limb or single-phase construction MV (i.e. 220kV) bushings not protected!

20. 20 © ABB AB, 2008 2008-02-05 87T Differential protection using CT1, CT5, CT7 and CT8 * Influenced by CT7, CT8 location within tertiary delta winding Influenced by CT5 location in neutral point Winding W1W2W3 Base Power [MVA] 169 Ph-Ph, No-Load Voltage [kV] 400231 * * Base Primary Current [A] 57742210730 Vector Group Yy0 Zero Sequence Current Elimination No / (Yes) Connected to CT (See Figure 3) CT1CT5CT7CT8 Base current on CT secondary side [A] 0.7210.8442.683

21. 21 © ABB AB, 2008 2008-02-05 87B Differential protection using CT1, CT2 and CT6 S Base =Throughput Power (400MVA) Zero sequence current reduction not required Tertiary delta winding can be loaded Not sensitive for winding turn to turn faults

22. 22 © ABB AB, 2008 2008-02-05 87B Differential protection using CT1, CT2 and CT6 Winding W1W2W3 Base Power [MVA] 400 Ph-Ph, No-Load Voltage [kV] 231 Base Primary Current [A] 1000 Vector Group Yy0 Zero Sequence Current Elimination No Connected to CT (See Figure 3) CT1CT2CT6 Base current on CT secondary side [A] 1.250.8332.00

23. 23 © ABB AB, 2008 2008-02-05 REF protection using CT1, CT2 and CT4 Base quantity is current (either CT2 or W2 rating) Tertiary delta winding can be loaded but it is not protected Operates only for phase to ground faults

24. 24 © ABB AB, 2008 2008-02-05 REF protection using CT1, CT2 and CT4 Winding W1W2Neutral Point Base Primary Current [A] 1000 Connected to CT (See Figure 3) CT1CT2CT4 Base current on CT secondary side [A] 1.250.8331.00

25. 25 © ABB AB, 2008 2008-02-05 Dedicated unit scheme for tertiary winding Use simple I> relay to provide only earth-fault protection for tertiary delta winding

26. 26 © ABB AB, 2008 2008-02-05 Field Recording External L2-Gnd fault which before clearing evolved into a L2-L3-Gnd fault CT1, CT2, CT4 and CT7 currents recorded CT5 current calculated Delta winding not loaded, thus CT7 currents are identical in all three phases CT1 CT2 CT5 CT4 & CT7

27. 27 © ABB AB, 2008 2008-02-05 Calculated Diff Currents from Field Recording

28. 28 © ABB AB, 2008 2008-02-05 Calculated REF Diff Current from Field Recording

29. 29 © ABB AB, 2008 2008-02-05 The following data are crucial for proper application of the selected differential protection scheme for Auto- transformer: Which base quantities (i.e. power, no load voltage and current) shall be used Which vector group shall be entered Whether or not zero sequence current elimination shall be enabled Conclusion

30. 30 © ABB AB, 2008 2008-02-05 Selection of unit protection schemes for particular auto-transformer application depends on: Available CTs User preference Previous experience Conclusion

31. 31 © ABB AB, 2008 2008-02-05 СПАСИБО ЗА ВНИМАНИЕ! THANK YOU!