Bendable Differential Power Processing

Converter for Curved Photovoltaic Panels

Ibaraki University

Keito Aikawa, Hayato Sato, Masatoshi Uno

1/22

■Propose novel differential power processing converter (DPP) using

the flexible transformer

■Eliminated the mismatch issues due to curved photovoltaic panels

■Improved extractable power by 4.7% with proposed DPP converter

Highlights

Background

Characteristics and mismatch issues

of curved photovoltaic panels

Conventional DPP converter

Proposed DPP Converter

Features and operation

Flexible Transformer

Design method and evaluation

Experimental Results

Conclusion

Outline

ICPE 2019 : Keito Aikawa

Flexible panel

Prototype

2/22

Background

3/22

Characteristics and Issues of Curved Photovltaic Panels

Mismatched substring characteristics

due to uneven irradiance

Configuration of PV panels

Bypass diode

Cell Substring

Panel

front rear

① ② ③

①

③

②

• Bendable, lightweight

• Wide range of applications

ICPE 2019 : Keito Aikawa 4/22

Curved solar roofs

（PHEV）Flexible panel

ICPE 2019 : Keito Aikawa 5/22

• Significant power reduction

• Occurrence of multiple maximum power points (MPPs)

Po

wer

Normal

Panel characteristics

I pa

nel

Vpanel

Substring characteristics

Ipanel

Cu

rren

t

Voltage

Normal

Low-irradiance

PV2

PV1

PV3

Substring

Bypass Diode

Low-irradiance

Characteristics and Issues of Curved Photovltaic Panels

Differential Power Processing (DPP) Converter

• Complex system

• Collective power

conversion loss

ICPE 2019 : Keito Aikawa 6/22

Low-irradiance

• Simply system

• High power

conversion efficiency

Adjacent substring-to-substring Panel-to-substring

Low-irradiance

Conventional DPP Converter

ICPE 2019 : Keito Aikawa

• Combination of LLC resonant inverter and voltage multiplier (VM)

• Automatic current supply to low-irradiance substrings

M. Uno and A. Kukita: “Two-switch voltage

equalizer using an LLC resonant inverter

and voltage multiplier for partially-shaded

series-connected photovoltaic modules,”

IEEE Trans. Ind. Appl., vol. 51, no. 2, pp.

1587–1601, Mar/Apr. 2015.

Low-irradiance

7/22

Issues and Solutions of Conventional DPP Converter

ICPE 2019 : Keito Aikawa

Transformer

Impaired PV panel’s shape

8/22

G.K.Y. Ho, C. Zhang,

B.M.H. Pong, and S.Y.R.

Hui, “Modeling and

analysis of the bendable

transformer,” IEEE Trans.

Power Electron., vol. 31,

no. 9, pp. 6450–6460, Sep.

2016.

Flexible transformer

Larger copper losses

due to increased iLmg

Novel DPP converter using a flexible transformer

• Operation at high frequency

• Low voltage stress of transformer

Proposed DPP Converter

9/22

提案補償器（提案補償器（提案補償器（提案補償器（1/2分圧分圧分圧分圧LLC共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路））））

• LLC resonant VM with a voltage divider (VD)

• Similar the basic operation principle to that of

conventional DPP converter

Proposed DPP Converter

ICPE 2019 : Keito Aikawa 10/22

提案補償器（提案補償器（提案補償器（提案補償器（1/2分圧分圧分圧分圧LLC共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路））））Features

ICPE 2019 : Keito Aikawa 11/22

Allowing of using flexible transformer

• Transformer operates at doubled switching frequency fs

• Halved voltage stresses of switches and transformer

• ZVS operation

Key operation waveforms

������

2

2fs

180°

Mode 1

提案補償器（提案補償器（提案補償器（提案補償器（1/2分圧分圧分圧分圧LLC共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路））））Operation under Mismatched Conditions

ICPE 2019 : Keito Aikawa 12/22

Lmg

LrCr

C1

C2

Cout1

Cout2

PV1

PV2

D4

D3

D2

D1

Cq4Dq4

Cin2

Q4

Cin1

Cq3Dq3

Q3

Cq2Dq2Q2

Cq1Dq1Q1

C3

Cout3

PV3

D6

D5

C4

Cout4

PV4

D8

D7

Vres

vDS4

vDS3

vDS2

vDS1

Mode 1

Lmg

LrCr

C1

C2

Cout1

Cout2

PV1

PV2

D4

D3

D2

D1

Cq4Dq4

Cin2

Q4

Cin1

Cq3Dq3

Q3

Cq2Dq2Q2

Cq1Dq1Q1

C3

Cout3

PV3

D6

D5

C4

Cout4

PV4

D8

D7

Vres

vDS4

vDS3

vDS2

vDS1

Mode 2

������

2

2fs

Mode 2

提案補償器（提案補償器（提案補償器（提案補償器（1/2分圧分圧分圧分圧LLC共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路共振形倍電圧回路））））Operation under Mismatched Conditions

ICPE 2019 : Keito Aikawa 13/22

Lmg

LrCr

C1

C2

Cout1

Cout2

PV1

PV2

D4

D3

D2

D1

Cq4Dq4

Cin2

Q4

Cin1

Cq3Dq3

Q3

Cq2Dq2Q2

Cq1Dq1Q1

C3

Cout3

PV3

D6

D5

C4

Cout4

PV4

D8

D7

Vres

vDS4

vDS3

vDS2

vDS1

Mode 3

Lmg

LrCr

C1

C2

Cout1

Cout2

PV1

PV2

D4

D3

D2

D1

Cq4Dq4

Cin2

Q4

Cin1

Cq3Dq3

Q3

Cq2Dq2Q2

Cq1Dq1Q1

C3

Cout3

PV3

D6

D5

C4

Cout4

PV4

D8

D7

Vres

vDS4

vDS3

vDS2

vDS1

Mode 4

ZVS

2fs

Mode 3

Mode 4

Flexible Transformer

14/22

Design Method

Design Constraint

• Size limitation : 150 mm×100 mm

• Upper frequency limitation due to using silicon MOSFETs

• Influence of skin effect and proximity effect

ICPE 2019 : Keito Aikawa

P : Pattern width

S : Interval

between patterns

T : copper foil

thickness

Numerical analysis using FEMTETⓇ

15/22

Prototype Evaluation

Theoretical Measured

Lmg [mH] 10.6 11.5

Lr [mH] 0.83 1.48

RTp [W] 2.83 2.68

RTs [W] 0.77 0.67

Key parameters

• Made of copper foil and

insulted plastic

• Operating frequency of

transformer : 600 kHz

150 mm

100

mm

Prototype of flexible transformer

ICPE 2019 : Keito Aikawa 16/22

Primary Secondary

P [mm] 2.0 2.0

S [mm] 1.5 1.3

T [mm] 35 35

Number

of turns 13

7

(2 parallel)

Total thickness [mm] 300

Outline of prototype

Bending of the Flexible Transformer

L : Arc length

r : Radius

q : Bending angle

0°(Flat) 30° 45°Lmg [mH] 11.5 11.4 11.3

Lr [mH] 1.48 1.46 1.45

RTp [W] 2.68 2.70 2.73

RTs [W] 0.67 0.68 0.70

Measured key parameters

Independent on bending angles

Styrene foam supporters

ICPE 2019 : Keito Aikawa 17/22

45°30°

Experimental Results

18/22

18 V

18 V

Measured key waveforms

Output Characteristics

Prototype

71.7%

Power conversion efficiency

ICPE 2019 : Keito Aikawa 19/22

Experimental setup

Vin = 36 V

(MPP voltage)

fs = 300 kHz

Adverse effects due to Joule

loss of winding resistance

Laboratory Experiment

Eliminated the mismatch issues

with the proposed DPP converter

ICPE 2019 : Keito Aikawa 20/22

I-V characteristics of substrings

under mismatched condition

P-V characteristics

with/without DPP converter

12.2%1 A

The ideal extractable

power = 84.0 W

The utilization ratio = 92.6%

ICPE 2019 : Keito Aikawa 21/22

Field Test

Confirmed the effectiveness

of proposed DPP converterI-V characteristics of substrings

under mismatched condition

The ideal extractable

power = 88.9 W

30°

P-V characteristics of panel

4.7%

The utilization ratio = 95.2%

Field test setup(Ibaraki, Japan, at 11:50 on February 24th)

Conclusion

• The bendable DPP converter using flexible transformer

has been proposed to eliminate mismatch issues in

curved PV panels

• The proposed DPP converter can flexibly be applied to

curved PV panels

• Experimental results demonstrated increased energy

yield, and local MPPs disappered

ICPE 2019 : Keito Aikawa 22/22

Future works

Employing GaN devices to increase the switching

frequency and miniaturize the prototype