Harmonic Analysis of a DFIG for a Wind Energy Conversion System

Lingling Fan, Ph.D., P.E.Assistant ProfessorDept. Electrical EngineeringUniversity of South FloridaTampa, FL 33620Linglingfan@usf.edu

April 20, 2010

2010 IEEE Transmission & Distribution Conference and Exhibition New Orleans, LA

http://www.usf.edu/

Outline

Objective Principle Case studies

– Rotor injection– Unbalanced stator conditions

Conclusion

Objective

Develop a steady-state circuit to give quantitative analysis for harmonics in DFIG

The work is useful for understanding of– DFIG behavior during non-sinusoidal rotor

injection– DFIG under unbalanced grid conditions (fault

ride through)

Doubly Fed Induction Generator

5

fm

RSCAC/DC

Pr = sPs

Ps

GSCDC/AC

A generalized circuit for harmonic analysis in DFIG

Case study 1 – non-sinusoidal rotor injection

Frequency components in rotor voltage and stator current

Stator currents– (6n+1)fr+fm, -(6n-1)fr+fm,

Electromagnetic torque Interaction of stator and rotor currents

Stator Is1 (60 Hz) Is2(-24*5 +36 Hz) Is3 (24*7 +36 Hz)

Rotor Ir1(24 Hz) Ir2 (-24*5 Hz) Ir3 (24*7 Hz)

Ex: Is1, Ir2 torque (60- (-120 +36)) = 144 Hz = 6*24 Hz 6fr

Is1, Ir3 torque (60- (168 +36)) = -144 Hz = -6*24 Hz 6fr

Torque

+

http://www.usf.edu/

Case study 2 – unbalanced stator conditions Rotor injection – programmable power

source –sinusoidal 3-phase Stator phase a resistance is reduced

Analysis

13

Unbalanced stator currents

Positive sequence(fe)

Negative sequence(-fe)

Zero sequence

Ir: fe-fm=sfe

Rotor currents

Ir: -fe-fm=-(2-s)fe

s: slip = 1-fm/fe

fm: electric frequency corresponding to rotating speed.

fe: nominal frequency

60 Hz (stator) – 50 Hz (rotating speed) = 10 Hz

-60 Hz (stator) – 50 Hz (rotating

speed) = -110 Hz

Torque

Positive Negative

Stator Is1 (+60 Hz) Is2 (-60 Hz)

Rotor Ir1 (10 Hz) Ir2 (-110 Hz)

1. Te1 Te2 – dc components2. Te3, Te4 – pulsating components 120 Hz

Reference frames

15

Pos. Neg. Sequences can be separated by a dc filter!

Extraction scheme

16

Lab setup

http://www.usf.edu/

http://www.usf.edu/

Case 3 – Grid interconnected DFIG

Simulation results

Confirm simulation results with the analysis

Conclusion

This paper develops– A generalized steady-state DFIG circuit for

harmonic analysis– A systematic method to compute torque by

computing the interactions of stator and rotor currents

– The sequence network based on DFIG pos, neg circuits which facilitates the analysis under unbalanced stator conditions.