EnergyTech2015.com TERRESTRIAL POWER TECHNOLOGY II Track 2 Session 2 Moderator: Don Brown, NASA This session will continue the previous session’s exploration of technologies to improve terrestrial power systems including; power systems for building and industrial power, advanced generation, energy storage and smart grid developments. David J. Sadey: Paper 1: Operation and Control of a Three-Phase Megawatt-Class Variable Frequency Power Generation and Distribution System William Good: Paper 2: Modular Nuclear Power Josh Sparber: Paper 3: Effective Measures for Protection of US Power Grid Neil Tyrrell : Paper 4: Fast and flexible combined cycle gas turbines

1. 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

2. 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

3. 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

4. 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

5. 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)

6. 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
𝑉𝑆1
𝑓𝑆1
=
𝑉𝑆2
𝑓𝑆2
=
𝑘𝑉𝑅
𝑓𝑅
∝ ϕ
VOLTS
Hz

7. 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

8. 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

9. 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.

10. 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

11. 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

12. System One-Line Diagram

13. Variable Frequency Characteristics
• *Converter Speed is Limited to -1100 RPM (5 Hz)
• **Zero RPM (60 Hz) is Not Allowed

14. Loaded Test Results
• Eight Machines Were Paralleled to Drive the Partially
Loaded 15,000 Hp Machine

15. DC Armature Current Balance
• DC Armature Currents of all Eight Machines Were
Demonstrated to be in Balance During Operation

16. Rotor Current Balance
• Acceptable Rotor Current Balance was Demonstrated

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

18. 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

19. 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

20. 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

21. 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

22. Questions?
• Thanks to Bill Cutter and Don Brown for Their Support
and Contributions.