Download Dithering Digital Ripple Correlation Control for Maximum Power

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)
Outline
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Motivation
Maximum power point tracking, PWM resolution
Dithering digital ripple correlation control (DDRCC)
ADC measurement windowing
DDRCC experimental Results
Conclusions
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What’s Here for Me?
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Necessity of maximum power point tracking.
DDRCC is a scalable approach to MPP tracking which
optimizes both tracking speed and tracking efficiency
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Demonstrated 10X improvement.
Digitally assisted windowed measurements can be
used to increase measurement precision in PV MPPT
applications.
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8X precision improvement demonstrated.
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Motivation – Solar Challenges
 Cost disadvantage compared to fossil fuels.
 Initial capital requirements large.
 Grid parity achieved in parts of US.
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Hawaii, California
 Intermittency
 Daily fluctuations
 Seasonal fluctuations
Credit: Inter Solar
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Outline
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Motivation
Maximum power point tracking, PWM resolution
Dithering digital ripple correlation control (DDRCC)
ADC measurement windowing
DDRCC experimental Results
Conclusions
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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
𝑷𝑷𝑷𝑷𝑷 = 𝑪𝑪𝑪𝑪𝑪𝑪𝑪 ∗ 𝑽𝑽𝑽𝑽𝑽𝑽𝑽
 MPP must be tracked continuously.
Voltage
Load
Voltage [V]
Irradiance = Solar exposure
Power [W]
Current [A]
Current
Current [A]
Maximum Power Point Tracking
 Power converters are used to track the MPP.
 Perturb and observe (P&O)
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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
Maximum Power Point Tracking
Maximum Power Point Tracking
 Power converters are used to track the MPP.
 Perturb and observe (P&O)
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Duty ratio is repeatedly adjusted and power measured.
Maintains close proximity to the MPP.
PWM resolution can restrict tracking efficiency.
Voltage
Current
Buck Converter: Ipanel ∝ D
PWM Resolution and Dithering
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Low duty ratio resolution limits tracking efficiency.
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PWM Resolution and Dithering
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Average resolution can be increased by dithering
between duty ratios.
Converter operates between native duty ratios.
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PWM Resolution and Dithering
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Additional convergence time required when dithering.
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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?
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Outline
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Motivation
Maximum power point tracking, PWM resolution
Dithering digital ripple correlation control (DDRCC)
ADC measurement windowing
DDRCC experimental Results
Conclusions
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Dithering Digital Ripple Correlation Control
 Based on P-I relationship at MPP.
Average duty ratio for
operation at panel MPP
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Principles of RCC
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Integral can be simplified into a sign evaluation.
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Current increasing during high duty ratio.
Difference in power determines direction of average
duty change.
High Duty
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Low Duty
Operation below the MPP
 Below the MPP power increases with increasing current.
Low Duty Ratio
Panel I vs. P
+ Slope
Panel V
Panel I
Panel P
+ Slope
t=0 t=RdTd t=Td
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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
Panel I
+ Slope
Panel V
t=0 t=RdTd t=Td
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Operation at the MPP
 At the MPP Power stays relatively constant.
MPP Duty Ratio
Panel V
Panel I vs. P
Panel P
Panel I
t=0 t=RdTd t=Td
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Dithering Digital Ripple Correlation Control
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DDRCC can be used to track the MPP faster than
perturb and observe.
Relatively small changes in current and voltage must
be measured.
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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
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Outline
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




Motivation
Maximum power point tracking, PWM resolution
Dithering digital ripple correlation control (DDRCC)
ADC measurement windowing
DDRCC experimental Results
Conclusions
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Measurement Windowing
 Measurement precision can be improved by windowing.
 Window is shifted as DC signal changes.
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Measurement Windowing
 Measurement precision can be improved by windowing.
 Window is shifted as DC signal changes.
 MPP application allows this technique
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Measurement Windowing
 Basic analog components used for amplification
Buck converter with controls
Conceptual depiction of biasing circuit
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Measurement Windowing
 Relative significance of noise is reduced
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Outline






Motivation
Maximum power point tracking, PWM resolution
Dithering digital ripple correlation control (DDRCC)
ADC measurement windowing
DDRCC experimental Results
Conclusions
26
DDRCC Tracking Results
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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.
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Tracking efficiency of 99.3% for DDRCC compared to 97.4%
for P&O.
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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.
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Questions?
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P&O vs. Speed Comparison
 Convergence frequency is 1/tconverge from 0.9 Voc.
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Test setup
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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
𝑆𝑆𝑆𝑆𝑆𝑆 𝑟𝑟𝑟𝑟𝑟
𝑆𝑆𝑆𝑆𝑆𝑆 𝑟𝑟𝑟𝑟𝑟
Ideal Resolution =
Ideal Resolution = 𝐴𝐴𝐴 𝑏𝑏𝑏𝑏
2𝐴𝐴𝐴 𝑏𝑏𝑏𝑏
2
∗ 𝑤𝑤𝑤𝑤𝑤𝑤𝑤
PWM Resolution and Dithering
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Average PWM resolution scales with the number of
cycles the converter dithers over.
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Three-way Dithering
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Measurement Windowing
 Basic analog components used for amplification
Voltage measurement biasing circuit
Current measurement biasing circuit
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Measurement Windowing
 Full signal measurement is needed.
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Measurement Windowing
 Basic analog components used for amplification
Voltage measurement biasing circuit
Current measurement biasing circuit
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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
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Measurement Windowing
 Windowing compresses the noise into a smaller range
and reduces the small signal error caused by noise
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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
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