Modeling, Simulation, and Analysis of Variable Frequency Transformers

# Modeling, Simulation, and Analysis of Variable Frequency Transformers

## Modeling, Simulation, and Analysis of Variable Frequency Transformers

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1. Modeling, Simulation, and Analysis of Variable Frequency Transformers Brian C. Raczkowski Peter W. Sauer

2. Overview • Power Flow Control • Langlois Converter Project • Derivation of Model • Small Power System Case • Experimental Case • Future Work

3. Ways to control power flow • Prime mover and excitation control of generators • Open and Close Breakers • Reactive Power Compensation

4. Ways to control power flow (cont.) • High Voltage DC (HVDC) • Rectifies AC to DC then inverts DC to AC • Economical for long distances • Harmonics • Isolation • Frequency

5. Ways to control power flow (cont.) • Transformers • Tap-Changing-Under-Load (TCUL) Transformers • Ability to change the ratio of transformation while energized • Requires additional circuitry • Phase shifting transformer • Addition of “90° out of phase” voltage • Useful for controlling real power • Most cases there is a fixed range

6. Drawbacks of These Methods • Set minimum and maximum constraints • Fixed change • Power transfer frequency requirement • Harmonics

7. Another Kind of Transformer • Induction machine • Squirrel cage rotor • Conducting bars laid in slots and shorting rings • Wound rotor • 3Φ windings with mirror images of windings on stator

8. Another Kind of Transformer (cont.) • Doubly-Fed Induction Machine (DFIM) • Rotor end not shorted • Wound rotor machine with access to rotor windings • Slip rings provide connection to rotor • Typically used to alter torque-speed curve • Same as Variable Frequency Transformer (VFT)

9. VFT Advantages • Continuous and no fixed set change points • Response for stability purposes • Simple model for power system use • HVDC alternative • Can transfer power at different frequencies • More control of the real power flow

10. VFT Disadvantages • Limits on maximum power flow capability • More lossy especially in reactive power losses • Works at low kV range so it needs step up/down transformers

11. Langlois Converter Project • GE investigated a new power transmission technology (2002) • Variable Frequency Transformer (VFT) • Controllable, bidirectional transmission device with ability to transfer power between asynchronous networks

12. World’s First VFT • Hydro-Quebec’s Langlois substation • Exchange +100MW to -100MW between power grids of Quebec (Canada) and New York (USA) • Closed Loop Control System to increase or decrease power delivery to maintain stability

13. General VFT • Core technology is rotary transformer with three phase windings on both rotor and stator • Continuously variable phase shifting transformer • Uses 2 transformers, a switched capacitor bank and a DC motor • Change rotor angle to change the power flow through the machine • Limits of the phase angle can be set as large as needed

14. VFT Ideal Use

15. Model Derivation • The machine is assumed to be a two-pole three phase machine with an a:1 turns ratio

16. Starting Equations

17. Final Equations

18. VFT Model (per phase)

19. Small Power System Case Glover and Sarma example

20. Small Power System with 3 VFTs Line 1 Line 2 Line 3 Just by inserting VFTs, the flows have changed

21. Power Flows in Small Power System from -21.9° to +30°

22. Experimental System Setup • GE I689, 7.5 hp, 3Φ, 6-pole induction machine • 2.93:1 turns ratio

23. Experimental System Notes • Variac used to match odd turns ratio • Slack Bus was the standard wall outlet • Load is purely resistive 12.8Ω • Source had 10A fuses • 1° mechanical was 3° electrical • Verification in PowerWorld Simulator • Voltage - 1000x • Power – 1e6x

24. Test System Results - No Caps

25. Make Things Better • System is already inherently lossy • Add a capacitor bank to cut reactive losses • 121.5µF to each phase at Bus 3 • Current reduced from 7.03Arms to 2.45Arms • Needed 61.32V to achieve 7.05Arms • For comparative purposes Vin=20.4Vrms

26. Test System Results – with Caps

27. Interesting Cases • Results verified in Power World Simulator

28. Circulating Real Power

29. VFT Conclusions • Alternative method to control power flow • Easy model • Use in small power system case • Use in experimental power system case

30. Future Work • Larger Test Systems • Higher Voltage • Torque Analysis • Multiple Frequencies • Stability of the System • Economical Impact

31. Questions • Questions??