DESIGN AND EVALUATION OF MODULAR RESONANT SWITCHED CAPACITORS

Ben-Gurion University of the Negev – Power Electronics Laboratory

DESIGN AND EVALUATION

OF A MODULAR RESONANT

SWITCHED CAPACITORS EQUALIZER

FOR PV PANELS

Shmuel (Sam) Ben-Yaakov, Alon Blumenfeld, Alon Cervera, and Michael Evzelman

Power Electronics Laboratory

Department of Electrical and Computer Engineering

Ben-Gurion University of the Negev

September 20, 2012 1


The Shading Problem in Serially Connected Arrays

• Shading strongly affects the MPP current

• Panels with different light exposures connected in series can’t all be in MPP 0

50100150200250

300350400

450

0 20 40 60 80

3 Serial Connected PVs With Bypass Diodes

𝑉𝐿 [𝑉]

𝑃𝐿 [w]

A PV Panel’s I-V characteristics for various Insolation levels


150W


Existing Solutions


Local Modules Central Current Compensation Local MPPT Implemented by DC-DC converters or part-time bypass circuitry

Local MPPT Creates a parallel power source, fed from the main BUS


Current Bypass– Overview


Using

1 − 𝐷 ⋅ 𝑉𝑎𝑣1 = 𝐷 ⋅ 𝑉𝑎𝑣2

𝑛 = 1

Bypass route can only be an energy source!

𝑉 ⋅ 𝐼 > 0

𝑉1𝑎𝑣 = 𝑛 𝑉2𝑎𝑣

Local Current Transfer Current Distribution

𝑉𝑎𝑣2𝐼𝑎𝑣2

=𝛼 00 𝛼

𝑉𝑎𝑣1𝐼𝑎𝑣1


Voltage Equalizing – The Concept

MPP for different shadings share approximately the same voltage


A PV panel’s I-V and P-V characteristics for various Insolation levels


Objective

• Evaluate a cost-effective shading problem solution

• Use a simple implement SCC modules

• Achieve high efficiency with voltage equalization

• Provide design guidelines



Basic Implementation

Equalizing SCC modules Average model for the EQSCC



Hard Switched Capacitor Average Model

𝑅𝑒 =1

2𝑓𝑆𝐶1⋅ coth

𝛽12

𝑅𝑒1

+ 1

2𝑓𝑆𝐶2⋅ coth

𝛽22

𝑅𝑒2

, 𝛽𝑖 =𝑡𝑖𝑅𝑖𝐶𝑖

≈1

2𝑓𝑠𝑅𝑖𝐶𝑖𝑖 = 1,2

• A good 𝛽 is around 1

September 20, 2012 8 September 20, 2012 8

1 102

4

6

8

10

fs

tti

ti i

0.1

eRi

*

*

P.C

𝑅𝑒𝑖∗ → 2

N.C

C.C

𝑡𝑖 ≫ RiCi 𝑡𝑖 ≈ RiCi 𝑡𝑖 ≪ RiCi

𝑅𝑒𝑖∗ =

𝑅𝑒𝑖𝑅𝑖

𝑓𝑠∗ = 𝑓𝑠𝑅𝑖𝐶𝑖

𝑅𝑖 − charge/discharge Ohmic loop resistance 𝐶𝑖 − charge/discharge loop capacitance


𝜔0𝑖 =1

𝐿𝑖𝐶𝑖; 𝑸𝒊 =

𝝎𝟎𝒊𝑳𝒊

𝑹𝒊=

1

𝑅𝑖

𝐿𝑖𝐶𝑖

𝜙𝑖 =𝜋

2 ∙ 4𝑄𝑖2 − 1

; 𝑖 = 1,2


Q i

0 5 102

2.5

3

3.5

1 3

*eR i

Soft Switched Capacitor Average Model

A good Q factor is around 1

𝑅𝑒𝑖∗ =

𝑅𝑒𝑖𝑅𝑖

𝑅𝑖 − charge/discharge Ohmic loop resistance 𝐶𝑖 − charge/discharge loop capacitance 𝐿𝑖 − charge/discharge loop inductance 𝑅𝑒 = 4𝑄1

2𝑅1 ⋅ 𝜙1 ⋅ tanh 𝜙1𝑅𝑒1

+ 4𝑄22𝑅2 ⋅ 𝜙2 ⋅ tanh 𝜙2

𝑅𝑒2


Peristaltic Relations


(3)

(2)

(1)

Current is delivered from adjacent panel and from neighbouring EQSCC

The peristaltic process is then formed from the whole chain


Peristaltic Relations

Assuming panels’ voltages is approximately equal:

• 𝐼𝐷 𝑘=

𝐼𝑂−𝐼𝑆

𝑁𝑘 , 𝑘 < 𝑆

𝐼𝑂−𝐼𝑆

𝑁𝑁 − 𝑘 , 𝑘 ≥ 𝑆

• 𝐼𝐿 =𝑁−1 𝐼𝑂+𝐼𝑆

𝑁


-2.00 -1.00 0.00 1.00 2.00

I(D1)

I(D2)

I(D3)

I(D4)

I(D5)

I(D6)

I(D7)

50% shaded


Power losses and Efficiency


Power extraction efficiency for a chain of length with one shaded PV in the center

With EQSCC

Shaded PV is in short-

circuit

Insolation ratio

𝜂 =𝑃𝑜𝑢𝑡

𝑃𝑙𝑜𝑠𝑠 + 𝑃𝑜𝑢𝑡 𝑃𝑙𝑜𝑠𝑠 = 𝐼𝐷𝑖

2 𝑅𝑒𝑖𝑖

Is/IO=0.625

90

92

94

96

98

100

2 5 8 11 14

η [%]

n


Power losses and Efficiency


Power extraction efficiency for a chain of length with one shaded PV in the center

With EQSCC

Shaded PV is in short-

circuit

Insolation ratio

𝜂 =𝑃𝑜𝑢𝑡

𝑃𝑙𝑜𝑠𝑠 + 𝑃𝑜𝑢𝑡 𝑃𝑙𝑜𝑠𝑠 = 𝐼𝐷𝑖

2 𝑅𝑒𝑖𝑖

Is/IO=0.125

90

92

94

96

98

100

2 5 8 11 14

η [%]

n


Prototype Power Stage Diagram


DC Restorers

Drivers SCC


Design Considerations – Resonant SCC

Rtotal - Designed according to maximum allowable power loss:

Rtotal ≤𝑃𝑙𝑜𝑠𝑠𝑚𝑎𝑥

5 𝐼𝐷2𝑚𝑎𝑥

L – Chosen or estimated according to switching frequency, providing 𝑄 ≈ 1:

𝐿 =𝑅

2𝜋𝑓𝑠 or 𝑓𝑠 =

𝑅

2𝜋𝐿

C – Was chosen providing desired resonant frequency:

𝐶 ≈1

4𝜋2𝑓𝑠2𝐿

CBulk – Was chosen according to maximum allowable voltage ripple:

𝐶𝐵 ≈𝐼𝐷𝑚𝑎𝑥

2𝑓𝑆𝑉𝑟𝑝𝑝


Ben-Gurion University of the Negev – Power Electronics Laboratory September 20, 2012 16

Experimental Results – Differential Current


1st Generation EQSCC

2nd Generation EQSCC


Experimental Results – Differential Current


Discharging

Charging

ID

Charging and Discharging Current to Average Differential Current


Simulation – Power Improvment

Using only bypass diodes: – Complicate MPPT implementation (Multiple Power Points) – Lower Maximum Power Point

Using the EQSCC: – The Multiple Power Point problem is solved, with a higher MPP


0

100

0 25 50

150 W

105 W

105 W

Po [W]

Vo [V] Theoretical and experimental Pout curves for 2 panels, one with about 50% shade


Experimental Results – Power Improvment

Using only bypass diodes: – Complicate MPPT implementation (Multiple Power Points) – Lower Maximum Power Point

Using the EQSCC: – The Multiple Power Point problem is solved, with a higher MPP


0

100

0 25 50

150 W

105 W

105 W

Po [W]

Vo [V] Theoretical and experimental Pout curves for 2 panels, one with about 50% shade


Simulation – Efficiency

The EQSCC increases efficiency Up to 50% September 20, 2012 20

60%

80%

100%

0 0.25 0.5 0.75 1

95%

With EQSCC

η

Irradiance Ratio

𝜂𝑟 =𝑃𝑚𝑝𝑝 𝑙𝑜𝑎𝑑

𝑃𝑚𝑝𝑝 𝑝𝑣1 + 𝑃𝑚𝑝𝑝 𝑝𝑣2⋅ 100%

Theoretical and experimental efficiency curves for 2 panels, one with irradiation swept from 0% to 100%

66%


Experimental Results – Efficiency

The EQSCC increases efficiency Up to 50% September 20, 2012 21

60%

80%

100%

0 0.25 0.5 0.75 1

95%

66%

97%

With EQSCC

η

Irradiance Ratio 65%

78%

𝜂𝑟 =𝑃𝑚𝑝𝑝 𝑙𝑜𝑎𝑑

𝑃𝑚𝑝𝑝 𝑝𝑣1 + 𝑃𝑚𝑝𝑝 𝑝𝑣2⋅ 100%

Theoretical and experimental efficiency curves for 2 panels, one with irradiation swept from 0% to 100%


Conclusions

The EQSCC processes only the differential power

Voltage equalization implies low voltage stress on switches

Power losses match the theoretical analysis

Smaller loop resistance will lead to higher efficiency

System can be embedded in to PV Panel



Thank You for Your Attention!



Driver Design Approach

• R-C considerations for the DC Restorer:

– 𝐶𝑏𝑢𝑓𝑓 ≫ 𝐶𝑔𝑎𝑡𝑒

– 𝐶𝑏𝑢𝑓𝑓 ⋅ 𝑅𝑏𝑙𝑒𝑒𝑑 ≫1

𝑓𝑆

• An added loop capacitor minimizes the ground loop impedance.


N-type circuit

P-type circuit


IMPP as a Function of VMPP

V.V.R. Scarpa, G. Spiazzi, and S. Buso, "Low complexity MPPT technique exploiting the effect of the PV cell series resistance," Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition, (APEC 2008), pp. 1958-1964, 24-28 Feb. 2008.



Central Current Feedback

Y. Nimni and D. Shmilovitz, "A returned energy architecture for improved photovoltaic systems efficiency," Proceedings of 2010 IEEE International Symposium on Circuits and Systems (ISCAS 2010), pp. 2191-2194, May 30 2010-June 2 2010.



Central Current Feedback

T. Shimizu, M. Hirakata, T. Kamezawa, and H. Watanabe, "Generation control circuit for photovoltaic modules," IEEE Transactions on Power Electronics, vol. 16, no. 3, pp. 293-300, May 2001.


Circuit configuration of GCC based on a dc/dc converter.

Circuit configuration of GCC based on a multistage chopper.


Buck-Boost Implementation Example

P.S. Shenoy, B. Johnson, and P.T. Krein, "Differential power processing architecture for increased energy production and reliability of photovoltaic systems," Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition, (APEC 2012), pp. 1987-1994, 5-9 Feb. 2012.


Differential power converters using a buck-boost topology connected to neighboring nodes. Differential power processing architecture.

DESIGN AND EVALUATION OF MODULAR RESONANT SWITCHED CAPACITORS

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