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Design and Simulation of Grid-connected HybridPhotovoltaic/Battery Distributed Generation System

Students:

Orkhan Baghirli

email : [email protected]

Supervisors:

Dr. Akshay Kumar Rathore

email: [email protected]

National University of Singapore

Department of Electrical Engineering

5/8/2013


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Abstract - This paper presents the design and simulation of grid connected photovoltaic system with

battery backup. All these models are simulated by using PowerSim. The main purpose of this

system is to distribute demanded power between loads and battery by controlling current, voltage

and power flow. System is composed of PV panel, boost converter, buck converter, single phase

inverter, LC filter, Grid/AC load, DC load and battery. The presented control strategy manages the

power flow between the converters and the loads in order to maintain the power balance in the

system and enable the battery to support the PV array when the available PV power is insufficient

to meet the load.

1. System Architecture

PV array provides the maximum power to the system by setting output current and voltage to their

reference values such that maximum power extraction is possible. Boost converter is connected to output

terminal of PV array and regulates the voltage and current at the DC link .Single phase inverter and buck

converter are connected to this DC link.

- Inverter changes the DC power into AC power at the grid frequency. LC filter at the end of

inverter smoothens the AC power by eliminating the higher harmonics resulted from inverter

switching. As a result, noise free AC power is delivered to the AC load and the grid.

- Buck converter steps down the DC link voltage to feed the DC load and the battery.

Figure 1: Overall system schematic


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2. System Requirements

Maximum DC load power usage should not exceed 240W.

DC link voltage should be stable at 48 V.

Power supplied to AC load/grid should be at grid frequency (50Hz for this design)

Priority should be given to supplying power to DC load.

Extra power should flow through AC load/grid and battery.

Battery should not be charged more than its maximum limit of SOC (state of charge %).

Battery should supply power to DC load during power shortage in the system.

System should be shut down if input power is less than demanded DC power and SOC of battery

is at its lower limit of operation.

3.

PV array

Simple approach is proposed to model PV array in order to avoid complex calculations by incorporating

dc voltage source and current controller to simulate the constant power flow into the system. This way,

input power can set to any value by changing the current reference of the current controller of boost

converter. To utilize current controller, relationship between inductor current and duty ratio of boost

converter has been derived as in figure 2.

Figure 2: current controller equation

This means, changing the duty ratio will result in a change in inductor current, so input power can be set

to its desired value.


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4.

Boost converter

Boost converter is a switch mode power supply that has an output voltage higher than its input voltage.

The switching in a boost converter is done through a MOSFET or IGBT.Boost converter is designed

based on the power and voltage requirements of this system. Input power supplied by PV array is set to

240 W (24 V, 10 A). Ignoring the switching losses, same amount of power is expected at the end of boost

converter. Converter parameters are chosen such that, 48 V DC link is maintained on the output capacitor.

Boost converter design equations are mentioned in Appendix A.

Figure 3: Boost converter with control

5. Inverter and LC filter

Single phase inverter that consists of 4 ideal IGBT is connected to DC link to invert the DC voltage to ACvoltage to supply AC load and grid. A Sinusoidal Pulse Width Modulation (SPWM) approach is

implemented to control the inverter to get 50Hz sinusoidal wave with 48V amplitude at the output.

LC filter is connected to inverter clears the higher harmonics to produce the noise free waves.

To see derivation of filter parameters, please refer to Appendix A.


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Figure 4: Inverter with its control and LC filter

6.

Buck converter

Buck converter connected to DC link steps down the input voltage by changing the duty ratio. To keep

output voltage of buck converter constant at 24V, controller is implemented such that changes duty ratio

in response to the change in the input voltage to maintain the stable output voltage.

Figure 5: Control equation of buck converter

The output of buck converter is connected to DC load and battery. Power consumed by DC load can be

calculated as: V_OUT ^2 / R_OUT. By changing the value of the output resistance, power consumption

can be changed. Converter design equations are mentioned in Appendix A.


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Figure 6: Buck conventer with its control

7.

Battery

As mentioned above, battery has several functions such as storing excess power in the system and

delivering power to DC load. Basic model of battery is proposed in this paper which only consists of

single capacitor with initial voltage. SOC indicates the charge level of battery. Battery should not be

charged more than its maximum limit of SOC and should not be let to drop below its minimum limit of

SOC. Battery voltage and SOC has linear relationship, so controlling the voltage within its prescribed

operation range will ensure the battery safety. Since frequent charging and discharging reduces the battery

Figure 7: Battery model

life, operation range is set to be between 20 -24V. This means battery should not be charged output

voltage is more than 24V and should not be discharged if it is less than 20V.


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7.1 Charging

Conditions that enable the battery charge up:

Input power is more than demanded DC power

SOC of battery is less than its maximum charge level (24V for this system)

Figure 8: charging controller

Since very high charging currents may damage the battery, 3.5A current limiter is set to define the

maximum current allowance to charge the battery. Battery charging current is calculated as:

I_bat = ( P_inP_dc_load ) / V_buck

and limited to 3.5A. If the difference between input and demanded power results in a higher current than

3.5A, then excess power (P_cur_lim_bat) will be sent to AC load/grid after battery absorbs maximum

possible power:

P_cur_lim_bat = (I_bat -3.5A)*V_buck (if I_bat is bigger than 3.5A)


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7.2 Discharging

Conditions that enable the battery discharge:

Input power is less than demanded DC power

SOC of battery is more than its minimum charge level (20V for this system)

Figure 9: Discharging controller

Algorithm to derive the discharge current is the same as of charging.

8. AC_load/Grid

AC load/Grid is modeled as a nonlinear resistor that absorbs the remaining power in the system (P_var)

after feeding DC load and battery. Voltage on this resistor is kept constant at 48V_peak; therefore current

passing through the resistor is a function of P_var.

P_var = P_in( P_dc_load +P_bat) + P_cur_lim_bat

Figure 10: AC_load/Grid controller


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9. System Shut Down

System has shut down mechanism to avoid any damage to PV array in case power shortage. If the

extracted power from PV array is less than demanded dc power and SOC of battery is 0 %, which

corresponds to 20V, system should shut itself down. This is modeled by placing a switch in front of the

PV array.

Figure 11: Shut down system and its control


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10.

Simulations

Test conditions:

Input power is 240 W

DC power demand is 60 W

Battery has 22V initial voltage (50% SOC)

Simulation 1.

As it is seen from the graphs, input power is divided between DC, AC loads and battery. After battery

gets fully charged up, it stops absorbing power from the system which results in an increase in power

extracted to AC load. This is consistent with power distribution priorities of design.


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Test conditions:




Simulation 2

Since I_bat is still more than 3.5A (120W/24V = 5A), battery absorbs maximum possible power

(3.5A*24V = 84W) and the rest of the power is sent to AC load. The overall P_ac went down since

P_dc_load increased from 60W to 120W.


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Test conditions:




Simulation 3.

This is the test case that input power is equal to the P_dc_load. Therefore, zero P_ac_load and P_bat is

expected. As it is seen from the graphs, P_bat is zero, while P_ac_load has noise ranging between +-

0.0001W which is ignorable.


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Test conditions:




Simulation 4.

This is the case which input power is less than demanded dc power; therefore battery is expected to

discharge to overcome the power shortage. However, after battery reaches its lowest possible SOC, it

stops supplying power to DC load in order to avoid battery damage. In this situation, system should be

shut down. The graphs present the same scenario.


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Test conditions:


Absence of DC load


Simulation 5.

In this case, since there is no dc load connected to the system, input power will feed only AC load after

battery gets fully charged.


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Test conditions:



Absence of AC load and Grid


Simulation 6.

Since there is no AC power consumption in the system (just ignorable noise), all of the input power will

be forwarded to battery and dc load.


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Conclusion

This paper presented design procedure and control techniques of Grid connected PV system with batterybackup. Converter and battery controllers are developed tosmoothly manage the power flow within thesystem. Several test cases are simulated under different operation points. Simulations verify that input

power extracted from PV arrays is reasonably divided between loads and battery in terms of system

priorities.


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Appendices

A: Design Calculations

Boost design

D=0.5

Fs=100 kHz

Ton=1/fs=10 us

IL=P_in/V_in=240 W / 24 V = 10 A

dIL < 0.1*IL (10% of IL) = 1A

dIL = ((V_inIL*RL)*T_on ) / (L)

L_min = ((V_inIL*RL)*T_on ) / (dIL) = 54.4 uH

dV < 0.04V

I_out=P_out / V_out

C_min = (I_out * D) / (fs *dV) = 671 uF

R_crit = (2*L) / (D*(1-D)^2) = 87 ohm

Buck design

D=0.5

Fs=100 kHz

Ts=1/fs=10us

dV


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B: Current and voltage contr ollers

Controller designs have been implemented as it is proposed in this paper:

http://www.ece.nus.edu.sg/stfpage/akr/controlboost.pdf

Matlab code is written to derive the PI controller values Kp and Tc.

function[KP,KI,TC]=currentloop(fc)% Given parametersL =2*10^(-3);C =4*10^(-3);R = 9;wc=2*pi*fc;Vin = 24;Vout = 48;I_L=10.4;D = 0.5;%--------------

s=1i*wc;r=0; % Ki/Kp%----------------while // phase marginwhile(r


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function[KP,KI,TC]=buck_voltage(fc)% Given parametersL =480*10^(-6);C =15*10^(-6);R = 4.8;wc=2*pi*fc;Vin = 48;Vout = 24;D = 0.5;%--------------s=1i*wc;r=0; % Ki/Kp%----------------while // phase marginwhile(r


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References

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[2] Hisham Mahmood, Dennis Michaelson, and Jin Jiang, Control Strategy for a Standalone PV/BatteryHybrid System, The University of Western Ontario

[3] Fei Ding, Peng Li, Bibin Huang, Fei Gao, Chengdi Ding, Chengshan Wang , Modeling andSimulation of Grid-connected Hybrid Photovoltaic/Battery Distributed Generation System , 2010 ChinaInternational Conference on Electricity Distribution

[4] Yann Riffonneau, Seddik Bacha, Member, IEEE, Franck Barruel, and Stephane Ploix, OptimalPower Flow Management for Grid Connected PV Systems With Batteries, IEEE TRANSACTIONS ON

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[6] John A. Shaw, The PID Control Algorithm, Process Control Solutions December 1, 2003

[7] M.MAKHLOUF, F.MESSAI, H.BENALLA, MODELING AND CONTROL OF A SINGLE-PHASE GRID CONNECTED PHOTOVOLTAIC SYSTEM, Journal of Theoretical and AppliedInformation Technology 31st March 2012. Vol. 37 No.2

[8] Milan Prodanovic, Student Member, IEEE, and Timothy C. Green, Member, IEEE , Control andFilter Design of Three-Phase Inverters for High Power Quality Grid Connection, IEEETRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 1, JANUARY 2003

[9] Wang Fa-Qiang and Ma Xi-Kui, Transfer function modeling and analysis of the open-loop Buck

converter using the fractional calculus, Chin. Phys. B Vol. 22, No. 3 (2013)

[10] Akshay Kumar Rathore, Assistant Professor,Two Loop Average Current Control of Boost

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[11] Ankur Bhattacharjee, Design and Comparative Study of Three Photovoltaic Battery ChargeControl Algorithms in MATLAB/SIMULINK Environment,International Journal of AdvancedComputer Research (ISSN (print): 2249-7277 ISSN (online): 2277-7970) Volume-2 Number-3 Issue-5

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[12] Shen Guo , The Application of Genetic Algorithms toParameter Estimation in Lead-Acid BatteryEquivalent Circuit Models, University of Birmingham

[13] Nazih Moubayed , Janine Kouta, Ali EI-AIi, Hala Dernayka, Rachid Outbib, PARAMETERIDENTIFICATION OF THE LEAD-ACID BATTERY MODEL, Lebanese University - Lebanon

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