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Dithering Digital Ripple Correlation Control for Rapid Photovoltaic Maximum Power Point Tracking

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  1. Dithering Digital Ripple Correlation Control for Rapid Photovoltaic Maximum Power Point Tracking Christopher Barth and Robert Pilawa-Podgurski University of Illinois at Urbana-Champaign This work was supported by the Illinois Center for a Smarter Electric Grid (ICSEG)

  2. Outline • Motivation • Maximum power point tracking, PWM resolution • Dithering digital ripple correlation control (DDRCC) • ADC measurement windowing • DDRCC experimental Results • Conclusions

  3. What’s Here for Me? • Necessity of maximum power point tracking. • DDRCC is a scalable approach to MPP tracking which optimizes both tracking speed and tracking efficiency • Demonstrated 10X improvement. • Digitally assisted windowed measurements can be used to increase measurement precision in PV MPPT applications. • 8X precision improvement demonstrated.

  4. Motivation – Solar Challenges • Cost disadvantage compared to fossil fuels. • Initial capital requirements large. • Grid parity achieved in parts of US. • Hawaii, California • Intermittency • Daily fluctuations • Seasonal fluctuations Credit: Inter Solar

  5. Outline • Motivation • Maximum power point tracking, PWM resolution • Dithering digital ripple correlation control (DDRCC) • ADC measurement windowing • DDRCC experimental Results • Conclusions

  6. Maximum Power Point Tracking • Voltage and current cannot be maximized simultaneously. • Power maximized at the maximum power point (MPP). • Maximum power point (MPP) changes. • With solar exposure • With temperature • With panel age • MPP must be tracked continuously. Current Voltage Current [A] Power [W] Load Voltage [V] Current [A] Irradiance = Solar exposure

  7. Maximum Power Point Tracking • Power converters are used to track the MPP. • Perturb and observe (P&O) • Duty ratio is repeatedly adjusted and power measured. • Maintains close proximity to the MPP. • PWM resolution can restrict tracking efficiency. Current Load PV Panel Voltage Load Power Converter

  8. Maximum Power Point Tracking

  9. Maximum Power Point Tracking • Power converters are used to track the MPP. • Perturb and observe (P&O) • Duty ratio is repeatedly adjusted and power measured. • Maintains close proximity to the MPP. • PWM resolution can restrict tracking efficiency. Current Voltage Buck Converter: Ipanel∝ D

  10. PWM Resolution and Dithering • Low duty ratio resolution limits tracking efficiency.

  11. PWM Resolution and Dithering • Average resolution can be increased by dithering between duty ratios. • Converter operates between native duty ratios.

  12. PWM Resolution and Dithering • Additional convergence time required when dithering. • Standard PWM control requires multiple switching cycles for the converter to converge to steady state. • Dithering requires multiple dithering waveforms for the converter to converge to steady state. Is it necessary to wait for steady state?

  13. Outline • Motivation • Maximum power point tracking, PWM resolution • Dithering digital ripple correlation control (DDRCC) • ADC measurement windowing • DDRCC experimental Results • Conclusions

  14. Dithering Digital Ripple Correlation Control • Based on P-I relationship at MPP. Average duty ratio for operation at panel MPP

  15. Panel V Principles of RCC • Integral can be simplified into a sign evaluation. • Current increasing during high duty ratio. • Difference in power determines direction of average duty change. Panel I Low Duty I increasing on interval t=0 t=RdTd t=Td High Duty Low Duty

  16. Operation below the MPP • Below the MPP power increases with increasing current. Low Duty Ratio Panel I vs. P + Slope Panel V Panel P + Slope Panel I t=RdTd t=0 t=Td

  17. Operation above the MPP • Above the MPP power decreases with increasing current. High Duty Ratio Panel I vs. P High Duty Ratio - Slope Panel Power Panel V Panel P Panel I + Slope Panel I Panel V t=RdTd t=0 t=Td

  18. Operation at the MPP • At the MPP Power stays relatively constant. MPP Duty Ratio Panel I vs. P Panel V Panel P Panel I t=RdTd t=0 t=Td

  19. Dithering Digital Ripple Correlation Control • DDRCC can be used to track the MPP faster than perturb and observe. • Relatively small changes in current and voltage must be measured.

  20. Dithering Digital Ripple Correlation Control • Figure shows measurement sweep with noise at 5 Amp scale without converter running. • 25 mA fluctuation (10 bit ADC) • DDRCC not feasible

  21. Outline • Motivation • Maximum power point tracking, PWM resolution • Dithering digital ripple correlation control (DDRCC) • ADC measurement windowing • DDRCC experimental Results • Conclusions

  22. Measurement Windowing • Measurement precision can be improved by windowing. • Window is shifted as DC signal changes.

  23. Measurement Windowing • Measurement precision can be improved by windowing. • Window is shifted as DC signal changes. • MPP application allows this technique

  24. Measurement Windowing • Basic analog components used for amplification Buck converter with controls Conceptual depiction of biasing circuit

  25. Measurement Windowing • Relative significance of noise is reduced

  26. Outline • Motivation • Maximum power point tracking, PWM resolution • Dithering digital ripple correlation control (DDRCC) • ADC measurement windowing • DDRCC experimental Results • Conclusions

  27. DDRCC Tracking Results • Using windowed measurements DDRCC has been shown to converge quickly to the MPP. • Achieved a 3.5X reduction in tracking losses over undithered P&O at 26 watts using identical hardware. • Tracking efficiency of 99.3% for DDRCC compared to 97.4% for P&O.

  28. What did we accomplish? • High PWM resolution with 8 MHz controller clock speed. • Excellent MPP tracking with fast convergence for dithered applications. • A windowed measurement approach which allows DDRCC to be performed at typical panel power levels. • Windowed measurements can be applied in other MPPT techniques to improve noise-free resolution. • Hardware requirements are minimal. Successfully demonstrated on a low cost ultra-low power microcontroller! • Selected References: • C. Barth and R. C. N. Pilawa-Podgurski, “Dithering digital ripple correlation control with digitally-assisted windowed sensing for solar photovoltaic mppt,” in Proc. Twenty-Ninth Annual IEEE Applied Power Electronics Conference and Exposition (APEC), vol. 29, IEEE, • 2014. C. Barth and R. C. N. Pilawa-Podgurski, “Implementation of dithering digital ripple correlation control for pv maximum power point tracking,” in The 14th IEEE Workshop on Control and Modeling for Power Electronics (COMPEL), 2013 (Prize Paper Award) • J. W. Kimball and P. T. Krein, “Discrete-time ripple correlation control for maximum power point tracking,” IEEE Transactions on Power Electronics, vol. 23, no. 5, pp. 2353–2362, 2008.

  29. Questions?

  30. P&O vs. Speed Comparison • Convergence frequency is 1/tconverge from 0.9 Voc.

  31. Test setup

  32. Measurement Windowing • Dividing the measurement range into multiple windows allows for higher noise-free code resolution • Ideal resolution scales linearly with the number of windows Should I keep this? It needs to be chanted

  33. PWM Resolution and Dithering • Average PWM resolution scales with the number of cycles the converter dithers over.

  34. Three-way Dithering

  35. Measurement Windowing • Basic analog components used for amplification Current measurement biasing circuit Voltage measurement biasing circuit

  36. Measurement Windowing • Full signal measurement is needed.

  37. Measurement Windowing • Basic analog components used for amplification Current measurement biasing circuit Voltage measurement biasing circuit

  38. Measurement Windowing • Noise reduces usable resolution. • Noise-free code resolution defines the highest DC measurement precision • Noise-free resolution is DC equivalent of ENOB for AC • RMS value of the measurement noise equal to standard deviation assuming noise source near Gaussian

  39. Measurement Windowing • Windowing compresses the noise into a smaller range and reduces the small signal error caused by noise

  40. Measurement Windowing • Measurement precision is improved by restricting ADC measurement range to a single window • 8X improvement in noise-free resolution • Application allows for decreased DC accuracy