IEEE Executive Project Expo 2024

                                                              Sustainable DC Microgrid: Solar-Battery Integration 

                                                                            with Bidirectional DC-DC Converter

Brief Summary:

This project involves implementing Maximum Power Point Tracking for a PV panel, boosting its output from 24V to 48V. It also includes designing a bi-directional DC-DC converter using embedded controllers for battery charging and discharging.

Aim: 

The project aims to design a DC Microgrid, consisting of a Boost Converter and Bidirectional Converter, where the Boost converter delivers power to the load and the  bi-directional converter  is used to charge or discharge a battery, depending on the PV panel output and load conditions.

Introduction:

A boost converter can deliver the needed power to a load from a given PV panel. However, as the PV panel depends on sunlight for power generation, there are times when it provides surplus power and times when it cannot provide any power at all. This greatly affects the efficiency of such systems, as power is not being utilized fully when there is surplus power, and when there is no sunlight, the load cannot be supplied with enough power.

This problem can be overcome by integrating a battery connected to a bidirectional converter parallel to the load. When surplus power is delivered to the load, the extra power can charge the battery. Then, when there is a power deficit from the PV panel, the battery can discharge to provide the remaining power to the load. 

The role of the bidirectional converter is to facilitate the bidirectional flow of current, that is, from the load to the battery and from the battery to the load.  This not only improves the efficiency of the system, but also optimizes it to our needs. 

Methodology:

Boost Converter:

DC-DC converter that steps up voltage levels from a lower to a higher value utilising an inductor, a switch (typically a transistor or MOSFET), a diode, and a capacitor. When the switch is on, energy is stored in the inductor, and when it's off, the stored energy is released to the output with the capacitor acting as an active low-pass filter to ensure constant voltage output. 

Fig1. Circuit Diagram of an Ideal Boost Converter (Source: ResearchGate)

Maximum Power Point Tracking:

Maximum Power Point Tracking (MPPT) is used in solar energy systems to extract the maximum available power from a solar photovoltaic (PV) panel. It continuously adjusts the electrical load or operating voltage of the PV panel to ensure that it operates at its maximum power point, where the product of voltage and current is maximized.

For the scope of this project, the Incremental Conductance algorithm is used. The operating point of a photovoltaic system is adjusted by comparing the instantaneous change in power with respect to voltage to the negative current ratio to voltage (i.e. ΔI/ΔV = -I/V). When this incremental conductance matches zero, the system is at the maximum power point, hence enabling optimized operation of the PV panel.

Fig2(a) P-V Characteristic Plot of a Solar Panel (Source: ResearchGate)

Fig2(b). Algorithm Flow of MPPT Control (Source: ResearchGate)

Bidirectional DC-DC converters: 

Converters facilitate energy transfer between two voltage sources, allowing power flow in both directions; i.e. they can act as buck and boost converters depending on the voltage required on either side of the converter. 

Fig3. Circuit Diagram of an ideal Bidirectional DC-DC Converter (Source: ResearchGate)

This bidirectional converter is connected to a battery and acts in two modes of operation : 

1. Charging Mode: When excess power is supplied to the DC bus, the extra power is used to charge the battery by bucking the voltage to the battery’s nominal voltage. Here, the power flow is from left to right. (Buck)

2. Discharging Mode: When there is a power deficit, the remaining power is drawn from the battery by boosting the voltage. Here, the power flow is from right to left. (Boost)

Control System:

A control system was used to maintain a constant voltage across the load and restrict the overflow of current through the inductor, which was part of the bi-directional converter, which helped determine the operational mode of the bidirectional converter. 

Simulation:

We have simulated the model using MATLAB Simulink. 

The PV panel has been modelled to the following parameters:

The MPP tracking for this PV panel has been achieved by using a function block in Simulink, which changes the duty cycle to give the appropriate Bus voltage. The MATLAB Function block contains the algorithm for Incremental Conductance MPP Tracking.

The load is initially connected to a Boost converter as the voltage produced from the PV panel has to be boosted to a higher level. The values for the Inductor and Capacitor for the Boost Converter are 2mH and 40uF, respectively.

The Load resistance value is  76.8Ohms, consuming 30W of Power (40W net consumption after accounting for circuit non-ideal characteristics losses).

This model is then connected in parallel to the Bidirectional converter with an Inductance of 6mH, Bus Voltage Capacitance of 40uF and Battery Capacitance of 40uF, which is connected to a battery with the following specifications: Nominal Voltage= 23.8V, 50% initial state of charge.

Fig5. Complete MATLAB Simulink Simulation, including all components.

Fig6. Boost Converter Simulation to boost PV panel voltage to DC bus voltage

Fig7. DC-DC Bidirectional Converter between Battery and DC Voltage Bus / Load

Fig8. MPPT Control Implementation to determine Duty Cycle for Boost Converter

MPPT Code is as follows:

function D = Duty_Cycle(V_PV, I_PV)

Di=0.5

dD=0.0001

persistent Vold Iold D_old;

dataType = 'double';

if isempty (Vold)

       Vold = 0;

       Iold = 0;

      D_old=Di;

end

dV=V_PV-Vold;

dI=I_PV-Iold;

if(dV==0)

     if (dI==0)

          D=D_old

else

if (dI>0)

     D=D_old-dD

else

    D=D_old+dD

end

end

elseif dI/dV==-I_PV/V_PV

      D=D_old

elseif dI/dV>-I_PV/V_PV

      D=D_old-dD

else

     D=D_old+dD

end

if D>=1 || D<=0

    D=D_old

end

D_old=D

Vold=V_PV

Iold=I_PV

Fig9. Voltage Control Loop for PWM input to MOSFET used for Bi-directional Converter

Fig10. Current Control Loop for PWM input for Bidirectional Converter

Results:

Fig11. Final Values of all circuit parameters in the DC Microgrid setup

Fig12. Output Voltage Plot of Boost Converter with Solar PV panel

Fig13. PV Panel Power Output at constant input of 1000 irradiance

Fig14. Load Power Output at a constant input of 1000 irradiance

Conclusion:

The Boost converter, along with the Bidirectional converter, has been implemented and tested. This project helps to highlight the advantages and the increase in efficiency brought by joining the Bidirectional converter to the existing PV system. 

References:

1. Mirza Mursalin Iqbal and Kafiul Islam,” Design And Simulation Of A PV System With Battery Storage Using Bidirectional DC-DC Converter Using Matlab Simulink”, International Journal of Scientific & Technology Research Volume 6, Issue 07, July 2017

2. K Pooja and Dr. A. Rubymeena, “Bidirectional DC-DC Converter for Solar Battery Backup Applications”, International Journal for Modern Trends in Science and Technology, Vol. 06, Issue 02, February 2020, pp.-47-52.

3. Krishna Kumar Pandey, Mahesh Kumar, Jagdish Kumar, Amita Kumari, “Bidirectional DC-DC Buck-Boost Converter for Battery Energy Storage System and PV Panel”