DAVID SADEY, NASA

Operation and Control of a Three-Phase

Megawatt Class Variable Frequency (VF)

Power Generation and Distribution System

ILLUSTRATIONS BY WILLIAM CUTTER, VPL

OUTLINE

• Fundamental Operation of a Doubly Fed Induction

Generator (DFIG)

• Terrestrial Application of the DFIG as a Frequency

Converter

• Standard Operation

• Paralleling Procedure

• Implementation of a 12MW VF Power System

• Comparison vs. Standard VFDs

• Conclusion

Fundamental Operation of a DFIG

• DFIG is a Wound Rotor Induction Machine (WRIM)

• Fed Mechanical Shaft HorsePower (Hp) on the Rotor

• Fed Electrical Power on the Rotor

• Converts Both Rotor Power Quantities to Stator Power

• Direction of Power Flow Can Vary Depending on Application

Fundamental Operation of a DFIG

• DFIG can be used as a Frequency Converter

• Direct Control of the Shaft Speed allows for DFIG to act as

a Frequency Transformer

• Shaft Speed can be Controlled via DC Motor

Fundamental Operation of a DFIG

• Frequency Converter Examples (2-Pole)

Rotor (Mech, RPM) Rotor (Elec, Hz) Stator (Elec, Hz)

0 RPM 60Hz CW 60Hz CW

3600RPM 60Hz CW 120Hz CW

CW,(60Hz)

3600RPM 60Hz CW 0

CCW,(60Hz)

Fundamental Operation of a DFIG

• Rotor is Excited at a Constant Volts-per-Hertz (V/F)

• Constant Flux ϕ in the Machine

• Stator Output Voltage and Frequency Relationship

Remains Constant over All Frequencies 𝑉𝑆1𝑓𝑆1

=𝑉𝑆2𝑓𝑆2

=𝑘𝑉𝑅𝑓𝑅

∝ ϕ

VOLTS

Hz

Terrestrial Application of a DFIG as a Variable

Frequency Drive • DFIG Fed 60 Hz Grid Power on the Rotor

• DC Drive Motor Supplies Mechanical Shaft Hp and Regulates Rotor

Speed

• Process Load Machine is Speed Regulated by Frequency Regulation of

the VF Bus.

VFD LOAD

Power Capability of Terrestrial Frequency

Converter • Power Levels at the MW Level and Higher can be

Obtained by Paralleling Multiple DFIGs

• DFIGs Must Be of Equivalent Characteristics

• Paralleling Achieved by Synchronization and Load Balance

Procedures

Synchronizing Parallel Frequency

Converters • Master DFIG is Selected, E.g. Master ‘A’

• Master Sequentially Drives Remaining ‘Slaves’ as Motors on VF Bus

• DC Motors Speed Regulate Rotors for Synchronization on 60Hz Grid

Side. Allows For Industrial Synchronizers to Be Used.

• Synchronization Occurs When Voltage, Phase, and Frequency are

Equal Across the Slave Synchronization Breakers.

Balancing Parallel Frequency Converters

• Synchronizing DFIGs does Not Guarantee Load Sharing

• To Equally Share Load Among Generators, Armature

Currents of the DC Drive Motors must be Equalized

• Balanced within 1% of Master Rated Armature Current

• Balancing Achieved by Bumping Slave Rotor(s)

Accordingly

• Armature Currents of Slaves are Actively Balanced at All

Times after Initial Synchronization

Physical Implementation of a 12MW

Class VF Power System • NASA Glenn Research Center at Lewis Field has a 12MW Class VF

DFIG Based Power System

• System Consists of 10 1.2MW DFIGs which can be Run Individually

or in Parallel

• System Consists of Five Process Load Machines of Varying Hp

System One-Line Diagram

Variable Frequency Characteristics • *Converter Speed is Limited to -1100 RPM (5 Hz)

• **Zero RPM (60 Hz) is Not Allowed

Loaded Test Results • Eight Machines Were Paralleled to Drive the Partially

Loaded 15,000 Hp Machine

DC Armature Current Balance • DC Armature Currents of all Eight Machines Were

Demonstrated to be in Balance During Operation

Rotor Current Balance • Acceptable Rotor Current Balance was Demonstrated

Stator Current (Output) Balance • Stator Current Balance was Demonstrated

Challenges and Limitations

• Synchronizing and Paralleling Multiple DFIGs

• Special Instrumentation is Needed In Certain Areas

• Commercial Instrumentation has Bandwidth Limitations of 40-80Hz

• Applies to Protective Relaying as Well

• Low Frequency Machine Instability Limits System

Frequency to 5Hz on Low End

Comparison with Traditional VFDs

• Pros vs. Traditional VFDs

• System is Highly Configurable

• Can Efficiently Run Multiple Sized Loads

• Can Run Multiple Loads at any Given Time

• System is Easily Expandable

• Produces Pure, Three-Phase Power

• No Harmonics on the VF Bus

• Reduces Excess Heat and Torque Pulsations on Loads

• Not Susceptible to rapid dV/dT and Wave Reflection Phenomenon

Comparison with Traditional VFDs

• Cons vs. Traditional VFDs

• DFIG Based System is More Complex

• Higher Maintenance and Operating Costs

• Larger Footprint

• V/F Ratio is Constant and Cannot be Altered

• Fault Conditions Must Be Considered on Rotor Windings as Well

as DC Drive Side

Conclusion and Future Work

• Unique Alternative to Standard VFD Technology

• Can be Implemented for MW Class Systems and Higher

• Expansion Easily Achieved by Adding Further DFIGs and by using

Described Synchronizing and Paralleling Procedures

• Effective for Systems with Multiple Large Hp Loads

• Possible Alternative to Future Work on High Power Hybrid

Electric Aircraft Systems

Questions?

• Thanks to Bill Cutter and Don Brown for Their Support

and Contributions.