Variable Frequency Transformer

8/12/2019 Variable Frequency Transformer

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Modeling, Simulation, and Analysisof

Variable Frequency Transformers

Brian C. RaczkowskiPeter W. Sauer


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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 whileenergized

Requires additional circuitry

Phase shifting transformer

Addition of 90out of phase voltage

Useful for controlling real power

Most cases there is a fixed range


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Another Kind of Transformer

(cont.) Doubly-Fed Induction Machine (DFIM)

Rotor end not shorted

Wound rotor machine with access to rotorwindings

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 powerlosses

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 transmissiondevice with ability to transfer power

between asynchronous networks


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Worlds First VFT Hydro-Quebecs Langlois substation

Exchange +100MW to -100MWbetween power grids of Quebec(Canada) and New York (USA)

Closed Loop Control System to increase

or decrease power delivery to maintainstability


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VFT Ideal Use

Drive Motor

Control

System

PowerSystem

Area

#2

PowerSystem

Area

#1

Variable Frequency

Transformer


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Model Derivation The machine is assumed to be a two-

pole three phase machine with an a:1

turns ratio

X

X

X

X

X

X

as

bs

cs

ar

brcr

as

bs

cs

ar

br cr

tm

mrs


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Final Equations

)(2

3)

2

3(

rmsssmslsssss ILjILLjRV

)(23)

23(

smsrrmslrrrrr ILjILLjRV

r

s

mslrrms

msmslss

rr

ss

I

I

LLjRLj

LjLLjR

V

V

)2

3(

2

3 2

3)

2

3(

)(

lsls

LX lrlr

LX msms LX

2

3


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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 PowerSystem from -21.9to +30

Pin

Pload

Pline2

Ploss

Pline3

Pline1


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Experimental System Setup

GE I689, 7.5 hp, 3, 6-pole inductionmachine

2.93:1 turns ratio

SLACK

1

52 3 VFT

1:93.2 je

6

WALL S R

1:2.93

Line 2

Line 1

Pin

Pline2

Pline1

Pstator Pload


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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 12.8

Source had 10A fuses

1mechanical was 3

electrical

Verification in PowerWorld Simulator

Voltage - 1000x

Power1e6x


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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

121.5F to each phase at Bus 3

Current reduced from 7.03Arms

to2.45Arms

Needed 61.32V to achieve 7.05Arms For comparative purposes V

in=20.4V

rms


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Test System Resultswith 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 powerflow

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??

Variable Frequency Transformer

Documents

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