Using Differential Power Processing Converters in Photovoltaic...

K. A. Kim, February 2014

Using Differential Power Processing

Converters in Photovoltaic Systems to

Improve Lifetime Energy Production

Katherine A. Kim

Advised by Philip T. Krein

Collaboration with Roy Bell, Jason Galtieri, Shibin Qin,

Robert Pilawa-Podgurski, Alejandro Domínguez-García

February 2014

Supported by:


PV Systems in Real-Life Environment

2


I. Mismatch in Photovoltaic (PV) Systems

II. Differential Power Processing (DPP) Converters

III. DPP Converter Power Rating

IV. Improvement with DPP Converters

V. Future Research Directions

VI. Conclusion and Contributions

Outline

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Outline

4


PV Panels Consist of Substrings

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[Image Source: GreenSourceGS.com]

n = 20 to 24 cells


PV Operation in Series - Ideal

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PV Operation in Series - Degradation

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PV Operation in Series - Degradation

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Mismatch causes power loss in strings


• External Variation

– Partial shading

– Dust accumulation

– Temperature differential

– Angle Differences

• Internal Variation

– Manufacturing

– Cell Degradation

Sources of Panel Mismatch

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• Temporary

• Difficult to predict

• Difficult to model

over lifetime

• Permanent

• Can be modeled

over lifetime


Wde HiPerforma™

240 W - 245 W

Vd SuperPoly

285 W - 290 W

PV Variation from Manufacture

10

Vd

275 W - 280 W

[Image Source: am.suntech-power.com]


• Laborious

• Adds Cost

• New panels have

some mismatch

PV Cell Binning

11

Wde HiPerforma™ Vd Vd SuperPoly


• Mean, μ

– Decreases over time

– 0.5-1% per year (Si)

• Standard Deviation, σ

– Increases over time

• Coefficient of Variation

PV Degradation Model

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[1] Vazquez and Rey-Stolle, 2008.


PV Degradation Field Study

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• Degradation linked to current characteristics

• Coefficient of variation (CV) over lifetime:

CV0 = 0.023, CV20 = 0.074, CV25 = 0.086

[2] C. Chamberlin, et al., 2011.








Outline

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• Panel-level

• Independent MPP control of each panel

• Processes 100% power

• Power rated for panel

• Maximum output is proportional to efficiency

Overcoming Mismatch – DC Optimizer

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[3] Walker and Sernia, 2004.

[4] Deline and MacAlpine, 2013.

[5] Pilawa-Podgurski andPerreault, 2013.


• Subpanel-level

• Independent MPP control of each string

• Processes fraction of power

• Power rating lower than subpanel string

• Higher output than dc optimizers

Overcoming Mismatch – DPP

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[6] Shenoy, et al., 2012.

[7] Stauth, et al., 2013.

[8] Olalla, et al., 2013.


PV-to-Bus PV-to-PV

DPP Architectures

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PV-to-Bus Flyback PV-to-PV Buck-Boost

DPP Converter Topologies

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Example: Mismatched PV Cells

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Total Power

194 W

50 W

52 W

42 W

50 W


Example: Series String

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Power Output: 183 W, 94.5%

Power processed: 0 W


Example: PV-to-Bus DPP Converter

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Power Output: 194 W, 100%

Power Processed: 10.0 W, 5.2%

Power (Current) Rating: ≥ 16%


Example: PV-to-PV DPP Converter

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PV-to-PV (>10% Rating)

Power Output: 194 W, 100%


Power (Current) Rating: ≥ 10%


Example: PV-to-Bus DPP Converter 8% Rating

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Example: PV-to-PV DPP Converter 8% Rating

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1. Determine appropriate DPP converter power rating for 25 years of operation

2. Evaluate performance improvement of DPP over series-string and dc optimizer architectures

Research Goals

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Outline

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• 15 PV substrings (5 PV panels)

• Monte Carlo Simulation

– Power variation: 1-20%

– 100 sets at each 0.5%

• DPP converters employ active bypass

• Assumptions

– Ideal converters

– MPP known

DPP Simulation Setup

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PV-to-Bus DPP Architecture

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Converter Ratings: 22% (75 percentile)

17% (50 percentile)

15% (25 percentile)


PV-to-PV DPP Architecture

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Converter Ratings: 33% (75 percentile)

23% (50 percentile)

18% (25 percentile)


Converter Ratings Scale with System

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Outline

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• Improvement Figure of Merit (IFoM)

– Power increase over series-string

– IFoM = 1 : Same as series-string performance

– IFoM > 1 : Better than series-string

Performance Improvement Metric

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DPP architecture

power output

Series-string

power output


PV-to-Bus DPP Performance

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PV-to-PV DPP Performance

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PV-to-Bus PV-to-PV

Number of DPP Modules Operating at MPP

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Improvement Distribution at 25-Year Variation

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• PV variation

0.023 CV for new panels

0.086 CV after 25 years of operation

• PV-to-bus converters rated at 15-17%

• PV-to-PV converters rated at 23-33%

• At 25 years, DPP converters provide 6% more power than series string

• Over 25 years, DPP converter harvest 2.8% more energy

Findings

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[9] Katherine A. Kim, Pradeep S. Shenoy, and Philip T. Krein. Converter rating analysis for photovoltaic

differential power processing systems. Submitted to IEEE Trans. Power Electron., 2014.








Outline

38


PV-Powered Electric/Hybrid Vehicles

PV-Powered Wearable Electronics

Power Electronics for Mobile PV Applications

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[Image Source: www.ecouterre.com]

[Image Source: wired.com]

[Image Source: www.Talk2MyShirt.com]


Uneven Illumination

• Mismatch among PV cells

– Angle differences

– Shading

– Temperature gradient

• Series-string does not handle mismatch well

Illumination Transients

• Extreme changes

• Frequent

• Traditional maximum power point tracking methods are slow

Challenges of Mobile PV Applications

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Uneven Illumination: Parallel PV DPP Converters

Illumination Transients: Voltage-Offset Resistive Control

Potential Solutions

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Outline

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1. Identified realistic CV value for PV variation over a 25-year lifetime

2. Outlined procedure to identify DPP converter power ratings for any size PV system

3. Identified 15-17% PV-to-bus and 23-33% PV-to-PV converters increase 25-year energy harvest by 2.8%

4. Proposed application of DPP converters and advanced control in mobile PV applications

Contributions

43

Questions ?

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Using Differential Power Processing Converters in Photovoltaic...

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