5/12/061 Modeling, Simulation, and Analysis of Variable Frequency Transformers Brian C. Raczkowski Peter W. Sauer Slide 2 5/12/062 Overview Power Flow Control Langlois Converter Project Derivation of Model Small Power System Case Experimental Case Future Work Slide 3 5/12/063 Ways to control power flow Prime mover and excitation control of generators Open and Close Breakers Reactive Power Compensation Slide 4 5/12/064 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 Slide 5 5/12/065 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 Slide 6 5/12/066 Drawbacks of These Methods Set minimum and maximum constraints Fixed change Power transfer frequency requirement Harmonics Slide 7 5/12/067 Another Kind of Transformer Induction machine Squirrel cage rotor Conducting bars laid in slots and shorting rings Wound rotor 3 windings with mirror images of windings on stator Slide 8 5/12/068 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) Slide 9 5/12/069 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 Slide 10 5/12/0610 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 Slide 11 5/12/0611 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 Slide 12 5/12/0612 Worlds First VFT Hydro-Quebecs 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 Slide 13 5/12/0613 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 Slide 14 5/12/0614 VFT Ideal Use Slide 15 5/12/0615 Model Derivation The machine is assumed to be a two- pole three phase machine with an a:1 turns ratio Slide 16 5/12/0616 Starting Equations Slide 17 5/12/0617 Final Equations Slide 18 5/12/0618 VFT Model (per phase) Slide 19 5/12/0619 Small Power System Case Glover and Sarma example Slide 20 5/12/0620 Small Power System with 3 VFTs Line 1 Line 2Line 3 Just by inserting VFTs, the flows have changed Slide 21 5/12/0621 Power Flows in Small Power System from -21.9 to +30 Slide 22 5/12/0622 Experimental System Setup GE I689, 7.5 hp, 3, 6-pole induction machine 2.93:1 turns ratio Slide 23 5/12/0623 Experimental System Notes Variac used to match odd turns ratio Slack Bus was the standard wall outlet Load is purely resistive 12.8 Source had 10A fuses 1 mechanical was 3 electrical Verification in PowerWorld Simulator Voltage - 1000x Power 1e6x Slide 24 5/12/0624 Test System Results - No Caps Slide 25 5/12/0625 Make Things Better System is already inherently lossy Add a capacitor bank to cut reactive losses 121.5F to each phase at Bus 3 Current reduced from 7.03A rms to 2.45A rms Needed 61.32V to achieve 7.05A rms For comparative purposes V in =20.4V rms Slide 26 5/12/0626 Test System Results with Caps Slide 27 5/12/0627 Interesting Cases Results verified in Power World Simulator Slide 28 5/12/0628 Circulating Real Power Slide 29 5/12/0629 VFT Conclusions Alternative method to control power flow Easy model Use in small power system case Use in experimental power system case Slide 30 5/12/0630 Future Work Larger Test Systems Higher Voltage Torque Analysis Multiple Frequencies Stability of the System Economical Impact Slide 31 5/12/0631 Questions Questions??
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