solar paper

BUCK BOOST- BRIDGE PHOTOVOLTAIC MICROCONVERTER

P.HEMALATHA

Asst.Prof. ECE DEPARTMENT,

MUTHAYAMMAL ENGINEERING COLLEGE,

NAMAKKAL-637 408, TAMILNADU, INDIA.

Abstract-The buck boost -bridge photovoltaic microconverter is a

Series strings of photovoltaic modules with integrated dc-dc

microconverter. A multi-mode dc-dc converter as a candidate

microconverter topology for photovoltaic modules. The topology

constitutes a single inductor and four switching devices and can

function in either buck, boost or an intermediate bridge mode

based on the load. The proposed microconverters harvest more

energy Compared to conventional string-inverter architectures if

the arrays are partially shaded or the modules mismatched. The

inverter inverts the dc voltage of microconverter topology into ac

output. The proposed maximum power point tracking scheme is

capable of tracking the true maximum even in partially-shaded PV

modules. Matlab simulations are used to compare the efficiency of

each topology as well as evaluating the benefits of increasing cost

and complexity. The buck boost bridge photovoltaic

microconverters are shown to be the most efficient topologies for a

given cost, with the buck best suited for long strings and the boost

for short strings.

Index Terms— DC–DC converter (micro-converter), grid connected

photovoltaic module (PV), string inverter.

I. INTRODUCTION

The growing nationwide interest in photovoltaic power

systems has induced significant expansion and R&D efforts in

the PV field. Grid-tied Photovoltaic (PV) installations are

commonly built with arrays of PV modules series-connected to

string inverters. An emerging system architecture that

supplements the string-inverter paradigm involves dc-dc

converters (referred to here as microconverters) dedicated to

individual PV modules. Several advantages of microconverters

have been postulated and demonstrated [1]-[2]. In particular, conventional systems are known to under perform if individual

modules in a series string are partially shaded (due to cloud

cover or shadowing), illuminated non-uniformly (due to

different roof angles in residential settings), or mismatched (due

to aging or manufacturing differences). Reference [1] presents a

comparison of basic power converter circuits (buck, boost, buck-

boost, and Cúk) adopted as PV microconverters. Our work seeks

a topology and control technique that maximizes versatility and

efficiency. The chosen circuit extends the buck-boost power

stage presented in [2]. Synchronous rectification achieves

efficiency above 95%, while a high switching frequency of 250

kHz enables the use of small passive components, eliminating the need for electrolytic capacitors and guaranteeing a compact

form factor.

II. SERIES CONNECTED PV PANELS

These dc energy sources are all series and parallel connections of a basic ―cell.‖ These cells all operate at a low dc

voltage, ranging from less than 1 V (PV cell) to 3 or 4 V (Li –Ion

cell). These low voltages do not interface well to existing higher

power systems, so the cells are series connected to create a

battery, a fuel cell stack, or a PV module or panel with a higher

terminal voltage. (The term PV panel rather than PV module will

be used in this paper to avoid confusion with the proposed

attached power electronic modules.) For example ―12 -V‖ PV

panels have 36 solar cells with a maximum power point (MPP)

of approximately 16–17 V under standard test conditions. These

system voltages are appropriate for lower power systems, but beyond powers of a few hundred Watts (W), these panels

themselves are placed in series strings to maintain lower currents

and higher efficiencies. These long strings of panels (and hence

cells) bring with them many complications. PV panels in a string

are never exactly identical. Because PV panels in a series string

are constrained to all conduct the same current, the least efficient

panel, and indeed cell, sets this string current. The overall

efficiency of the array is reduced to the efficiency of this cell.

This also means that PV panels in a string must be given the

same orientation and be of identical size .

Fig. 1. Micro converter System Architecture

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INTERNATIONAL JOURNAL OF EMERGING TRENDS IN ENGINEERING AND DEVELOPMENT

Issue 2, Vol.6.(SEPTEMBER-2012) ISSN 2249-6149

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III. BUCK/BOOST BRIDGE POWER STAGE

The power stage, illustrated in Fig.2, comprises buck side

switches, S1-S2, boost-side switches, S3-S4, an inductor, L, and

input and output capacitors, Ci and Co, respectively .In addition,

there are low-side current sensing resistors and buffer amplifiers

to enable the acquisition of input and output voltage and current

as well as 5V and 12V on-board house-keeping supplies

powered by the PV source. The power stage is intended to be

compatible with a wide variety of PV sources and a string

inverter loads. In addition, it is expected to harvest energy from partially-shaded PV sources. To accomplish these goals, the

topology must operate in buck mode or boost mode.

The power stage is designed to operate at a nominal input

maximum power point (MPP) of about 40 V and 5 A (IM, VM).

The inductance is 40 μH. At the MPP, continuous current mode

(CCM) buck operation is guaranteed when the input current

exceeds approximately 300 mA for any buck load current.

Similarly, in boost mode, CCM is guaranteed when the input

current exceeds 2 A. PWM constraints are imposed by minimum

switch ON times of 100 nsec (S2 and S4) and 133 nsec (S1 and S3) and a dead time of 150 nsec at all switch transitions.

Consequently, the buck duty cycle, Dbu (the fraction of S1 ON-

time) cannot have a value between 0.9 and 1.0. Likewise, the

boost duty cycle, Dbo (the fraction of S3 ON-time) cannot exist

between 0 and 0.033. The PWM method described below

provides a smooth transition between the buck and boost modes

as load current increases while adhering to all switching

constraints.

The ideal dc gain of the converter is given by

Buck mode switching, where Dbo = 0, is used for 0<G≤0.9,

where the minimum S2 ON-time is required. Likewise, boost

mode switching, where Dbu = 1, is used for G>1.034. The duty

cycle resolution is 0.00375% (150 psec steps).To obtain similar

resolution in the buck-to-boost transition range, 0.9 < G < 1.034,

bridge switching is\employed. The bridge mode is divided into

two regions, br-A and br-B, as shown in Fig. 3, where Dbu and Dbo are plotted as a function of converter gain. At the low-gain

end of br-A, S3 is switched on for its minimum allowable time,

133 nsec, corresponding to Dbo = 0.033. At the same time, Dbu

= 0.875, which results in a gain of 0.9052. To increase the gain

within the br-A region, Dbu is increased up to a maximum of 0.9

(again limited by S2 minimum ON-time), corresponding to a

converter gain of 0.9310. In the br-B region, Dbo is varied while

holding Dbu = 0.9. The high-gain end of br-B, 1.033, is reached

when a smooth transition to boost mode can be made employing

the minimum S3 ON time. It is worth noting that, given the

minimum and dead time switching constraints, this strategy achieves the minimum possible average inductor current at all

values of gain and therefore minimizes the conductive losses in

the inductor and the switches.

In bridge operation, the relative phase of S1 and S3 switching

is chosen to minimize ripple current. The ON time of switch S1

(S3) is symmetrical about the beginning (middle) of the 4 μsec switching period.. Note that in the br-A and br-B modes, outside

the brief shaded time intervals, the voltage across the inductor is

approximately (or in some cases exactly) zero. Thus, the ripple

current is very small, further reducing the conductive power loss

in those switching modes.

Fig. 2. Micro-converter Power Stage Topology

Fig. 3. Dbu and Dbo in the Buck-To-Boost Transition Region

IV. PV MODEL DESIGN

Based on the parameters, it is easy to formulate a simulation

model with most computer simulation tools. The model shown

in Fig.4 is designed with the Sirnulink" software package. The

photovoltaic output current is represented as a function of

voltage , and voltage-current characteristics configured inside

the box is influenced by insulation and temperature. Look-up

tables are used to represent the relationship between temperature

and parameters (A or R,).

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Fig. 4.simulink model of PV with temperature and irradiance.

(a)

(b)

Fig. 4.(a)simulation waveform of current (I) and voltage(V)

And (b) is simulation waveform of power and voltage of the PV

module

In PV panel design characteristics based on the I-V model

for various kinds of solar arrays. The short circuit voltage (Iscn)

and open circuit current (Vocn) values are 8.21 and 32.9

respectively. Solar array voltage and current at maximum power

point tracking is Imp=7.61, Vmp=26.3.The no of series and

parallel modules are Nss ad Npp.

Calculating power by using maximum power point tracking

Newtons Raphson Method.

Power using the I-V equation

P = (Ipv-Io*(exp((V+I.*Rs)/Vt/Ns/a)-1)-

(V+I.*Rs)/Rp).*V.

The current will be affected by the temperature and irradiance is dT = T-Tn;

the power and voltage calculated by the equation. The matlab script file is used to design the PV module (TABLE1).

V. BUCK AND BOOST OPERATION

The buck and boost operation of converter configuration is

designed based on the operation in buck operation the given

input voltage approximately given as 20 the output voltage is

reduced the current will be increased under the efficiency. In the

same given input the boost operation voltage is increased curren t

value decreased.

Fig.5.Buck Operation of Micro-Converter

The DC input voltage of microconverter module is buck or

boost the voltage based on the operation. In boost operation

input is changed to A1,B1, buck operation input is A,B.Under

the nominal temperature boost operation is required.

The simulation model of microconverter system connected

with inverter to convert the DC voltage into required AC voltage. The final efficiency of the microconverter was over

95% over the range of 20-70W. A string of buck converters

requires many more panels, but can always deliver any

combination of panel power. The buck converter will be the

most efficient topology for a given cost.

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TABLE1

MATLAB SCRIPT FILE

clear all

clc

% You may change these parameters to fit the I-V model % to other kinds of solar arrays.

Iscn = 8.21; %Nominal short-circuit voltage [A]

Vocn = 32.9; %Nominal array open-circuit voltage [V]

Imp = 7.61; %Array current @ maximum power point [A]

Vmp = 26.3; %Array voltage @ maximum power point [V]

Pmax_e = Vmp*Imp; Array maximum output peak power [W]

Kv = -0.123; %Voltage/temperature coefficient [V/K]

Ki = 3.18e-3; %Current/temperature coefficient [A/K]

Ns = 54;

%Number of series cells

%% Array with Nss x Npp modules Nss = 15;

Npp = 2;

%% Constants

k = 1.3806503e-23; %Boltzmann [J/K]

q = 1.60217646e-19; %Electron charge [C]

a = 1.3; %Diode constant

%% Nominal values

Gn = 1000; % Nominal irradiance [W/m^2] @ 25oC

Tn = 25 + 273.15; % Nominal operating temperature [K]

%% Adjusting algorithm

% The model is adjusted at the nominal condition

T = Tn; G = Gn;

Vtn = k * Tn / q; %Thermal junction voltage (nominal)

Vt = k * T / q; %Thermal junction voltage (current temperature)

Ion = Iscn/(exp(Vocn/a/Ns/Vtn)-1);

% Nominal diode saturation current

Io = Ion;

% Initial guesses of Rp and Rs

Rp = Rp_min;

Rs = 0;

tol = 0.001; % Power mismatch Tolerance

P=[0]; error = Inf; %dummy value

% Iterative process for Rs and Rp until Pmax,model =

Pmax,experimental

while (error>tol)

% Temperature and irradiation effect on the current

dT = T-Tn;

Ipvn = (Rs+Rp)/Rp * Iscn; % Nominal light-generated current

Ipv = (Ipvn + Ki*dT) *G/Gn;% Actual light-generated current

Isc = (Iscn + Ki*dT) *G/Gn; % Actual short-circuit current

% Increments Rs

Rs = Rs + .01;

% Parallel resistance Rp = Vmp*(Vmp+Imp*Rs)/(Vmp*Ipv-

Vmp*Io*exp((Vmp+Imp*Rs)/Vt/Ns/a)+Vmp*Io-Pmax_e);

% Solving the I-V equation for several (V,I) pairs

clear V

clear I

V = 0:.1:35; % Voltage vector

I = zeros(1,size(V,2)); % Current vector

for j = 1 : size(V,2) %Calculates for all voltage values

% Solves g = I - f(I,V) = 0 with Newton-Raphson

g(j) = Ipv-Io*(exp((V(j)+I(j)*Rs)/Vt/Ns/a)-1)-(V(j)+I(j)*Rs)/Rp-I(j);

while (abs(g(j)) > 0.001)

g(j) = Ipv-Io*(exp((V(j)+I(j)*Rs)/Vt/Ns/a)-1)-(V(j)+I(j)*Rs)/Rp-

I(j);

glin(j) = -Io*Rs/Vt/Ns/a*exp((V(j)+I(j)*Rs)/Vt/Ns/a)-Rs/Rp-1;

end

end % for j = 1 : size(V,2)

plott = 1; %Enables plotting during the algorithm execution

% Calculates power using the I-V equation

P = (Ipv-Io*(exp((V+I.*Rs)/Vt/Ns/a)-1)-(V+I.*Rs)/Rp).*V;

if (plott)

%Plots P x V curve plot(V,P,'LineWidth',2,'Color','k')

%Plots the "remarkable points" on the power curve

plot([0 Vmp Vocn],[0 Vmp*Imp

0],'o','LineWidth',2,'MarkerSize',5,'Color','k')

end % if (plott)

end % while (error>tol)

%% Outputs

% I-V curve

figure(1)

grid on

hold on , title('Adjusted I-V curve'); xlabel('V [V]'); ylabel('I [A]');

xlim([0 Vocn+1]); ylim([0 Iscn+1])

plot(V,I,'LineWidth',2,'Color','k')

plot([0 Vmp Vocn ],[Iscn

Imp0],'o','LineWidth',2,'MarkerSize',5,'Color','k')

% P-V curve

figure(2)

grid on

hold on , title('Adjusted P-V curve');

xlabel('V [V]'); ylabel('P [W]');

xlim([0 Vocn+1]); ylim([0 Vmp*Imp+1]);

plot(V,P,'LineWidth',2,'Color','k') plot([0 Vmp Vocn ],[0 Pmax_e 0],'o','LineWidth',2,'MarkerSize',5,'Color','k')

Fig.6. DC output of Buck-Boost Bridge Photovoltaic Micro-Converter and required inverter AC output

Page 578



V. CONCLUSION

New residential scale photovoltaic (PV) arrays are commonly

connected to the grid by one of two approaches—a single dc–ac

inverter connected to a series string of PV panels, or many small

dc–ac inverters which connect one or two panels directly to the

ac grid. In buck boost bridge photovoltaic micro-converter PV

panels are integrated with micro-converter blocks. A micro-

converter approach offers many advantages including

1) Better protection of PV sources and redundancy in the case of

source or converter failure.

2) Easier and safer installation and maintenance. 3) Better data gathering.

In this micro-converter system buck and boost the required

voltage. The efficiency of the system is improved above 95%

under wide load range conditions. The string inverter with

micro-converter system obtain the maximum voltage even the

PV modules are partially shaded.

APPENDIX MATLAB SCRIPT FILE

This Matlab script file (see Table I) is used to design PV

module. The output voltage and power characteristics are

obtained.

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