Electrical Machines

1. Electrical Machines LSEGG216A 9080V

2. Parallel Operation&Auxiliary equipment

3. Introduction • Identify

4. Parallel Operation Why not get a bigger one? • Can carry many of the same size so maintenance is easier & cheaper • If one fails can still supply some load • Can turn one off if load reduces

5. Parallel Operation Both transformers must have: • Same voltage • Frequency • Roughly the same VA rating • Same polarity

6. Parallel Operation Single Phase High circulation currents will flow Incorrect Polarity

7. Parallel Operation Single Phase Correct Polarity

8. Parallel Operation Testing for Polarity Place a voltmeter across the transformer terminal and the supply line Correct polarity = Incorrect polarity = 0 Volts Transformer voltage or higher

9. 3 Phase Transformers Types PRIMARY SECONDARY Star Star No Phase Shift Reference

10. 3 Phase Transformers Types PRIMARY SECONDARY Delta Delta No Phase Shift Reference

11. 3 Phase Transformers Types PRIMARY SECONDARY Star Delta V2 Leads V1 by 30 Reference 30

12. 3 Phase Transformers Types PRIMARY SECONDARY Delta Star V2 Lags V1 by 30 30 Reference

13. Paralleling 3 Phase Transformers Must be from the same vector group Must have the same effect on voltage: -Δ paralleled with -Δ  Δ- paralleled with Δ-  Provided that the line voltages are equal Δ- paralleled with -Δ 

14. Paralleling 3 Phase Transformers Must be from the same vector group Must have the same effect on voltage: -Δ paralleled with -Δ  Δ- paralleled with Δ-  Δ- paralleled with -Δ  Δ- -30 & -Δ +30 The voltage phase difference would be 60

15. Auxiliary Equipment • Protection • Voltage adjustment • Over temperature • Over pressure • Low oil level • Oil quality • Reduction of water • Tap changers

16. Transformer Construction • Most large transformers are oil filled • The oil level must be high enough to cover the transformer • Oil must be kept dry and clean

17. Transformer Construction Conserver Tank Sealed Tank Tank always stays full

18. Transformer Construction Buchholz relay monitors oil between transformer and conserver tank Pressure relief valve allows over pressure oil to escape Over temperature thermostat trips circuit breaker if transformer gets too hot

19. Buchholz relay Monitors • Presence of air bubbles due to faulty initial filling or faulty seal of gaskets • Formation of gas due to burning of oil or solid insulation • Lowering of oil level due to leak of the tank • Oil flow from tank to conservator due to explosion or short circuit

20. Buchholz relay On a slow accumulation of gas, due to slight overloadaccumulates in the top of the relay and forces the oil level down. A float operated switch in the relay is used to initiate an alarm signal

21. Buchholz relay rapidgas accumulation, flow of oil operates a switch attached to a vane Test port to allow the accumulated gas to be withdrawn for testing

22. Buchholz relay Dry air test Gas testing Oil to conserver Oil & Gass from Tx Oil for testing

24. Tap Changers • Two types: • Off Load • On Load Can be located on the primary or secondary of the transformer

25. Off Load • Basically a multi-position switch • Open circuits output between positions • Contacts not rated to open circuit under load

27. Transformer Open circuited

29. On Load • Manual • Automatic Sliding switch Transformer windings Common terminal

30. On Load Inductors carry current when contact is between taps

31. On Load Contact moves to new tap