Modeling, Simulation, and Analysis of Variable Frequency Transformers

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Modeling, Simulation, and Analysis of Variable
Frequency Transformers

• Brian C. Raczkowski
• Peter W. Sauer

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Overview

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

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Ways to control power flow

• Prime mover and excitation control of generators
• Open and Close Breakers
• Reactive Power Compensation

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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

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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

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Drawbacks of These Methods

• Set minimum and maximum constraints
• Fixed change
• Power transfer frequency requirement
• Harmonics

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Another Kind of Transformer

• Induction machine
• Squirrel cage rotor
• Conducting bars laid in slots and shorting rings
• Wound rotor
• 3F windings with mirror images of windings on
  stator

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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)

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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

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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

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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

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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

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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

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VFT Ideal Use
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Model Derivation

• The machine is assumed to be a two-pole three
  phase machine with an a1 turns ratio

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Starting Equations
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Final Equations
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VFT Model (per phase)
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Small Power System Case
Glover and Sarma example
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Small Power System with 3 VFTs
Line 1
Line 2
Line 3
Just by inserting VFTs, the flows have changed
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Power Flows in Small Power System from -21.9 to
30
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Experimental System Setup

• GE I689, 7.5 hp, 3F, 6-pole induction machine
• 2.931 turns ratio

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Experimental System Notes

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

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Test System Results - No Caps
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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 Vin20.4Vrms

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Test System Results with Caps
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Interesting Cases

• Results verified in Power World Simulator

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Circulating Real Power
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VFT Conclusions

• Alternative method to control power flow
• Easy model
• Use in small power system case
• Use in experimental power system case

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Future Work

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

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Questions

• Questions??