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Modeling, Simulation, and Analysis of Variable Frequency Transformers

Modeling, Simulation, and Analysis of Variable Frequency Transformers. Brian C. Raczkowski Peter W. Sauer. Overview. Power Flow Control Langlois Converter Project Derivation of Model Small Power System Case Experimental Case Future Work. Ways to control power flow.

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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??

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