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

Transcription

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 2

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. 3

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 4

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

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. PPPPP = CCCCCCC VVVVVVV Current Current [A] Voltage Load Power [W] 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 Voltage PV Panel Load 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: I panel D

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

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

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? 12

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

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

15 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. High Duty Low Duty 15

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

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

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=0 t=r d T d t=t d 18

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. 19

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 20

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

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

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

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

25 Measurement Windowing Relative significance of noise is reduced 25

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

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. 27

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, 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 ,

29 Questions? 29

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

31 Test setup 31

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 Ideal Resolution = Should I keep this? It needs to be chanted SSSSSS rrrrr 2 AAA bbbb Ideal Resolution = SSSSSS rrrrr 2 AAA bbbb wwwwwww

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

34 Three-way Dithering 34

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

36 Measurement Windowing Full signal measurement is needed. 36

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

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 38

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

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 40