Abhijit V. Padgavhankar and Sharad W.Mohod

Solar Energy System with Digital Controller for Grid Connected Systems

Abhijit V. Padgavhankar and Sharad W.Mohod

i) Converting a DC input voltage into a DC output voltage.

ii) Regulating the DC output voltage against the load and line variations.

iii) Reduceing the AC voltage ripples in the DC output voltage below the required level.

iv) Providing the isolation between the input source and the load.

v) Protecting the supplied system and the input source from electromagnetic interference (EMI); and satisfying various international and national safety standards.

As shown in Fig. 1, there are two stages. First is the automatic boost converter which contains the PV module, DC-DC boost converter, and feedback circuit with a comparator, and PIC microcontroller. The PV module generates the DC voltage according the sun light intensity. Due to atmospheric changes there is changes in the intensity of the sun light which falls on the PV module so as to change the output of PV module. For the ideal condition of atmosphere, the PV module gives the maximum output voltage approximately 17.6 V, but due to changes of the sun light this voltage varies from 8 V to 17.6 V. This varying voltage is applied to the DC-DC boost converter, which levels up the voltage to 24 V. The feedback circuit uses the voltage divider network to detect the output voltage of the PV module. The detected voltage is applied to the comparator, which compares the detected voltage with a reference voltage. Thus the comparator generates the error voltage which is applied to the analog to digital converter (ADC) of the PIC microcontroller which is programmed in such a way where it will send the pulse width modulated pulses to the switch according to the error signal from the comparator. Finally, the output of the DC-DC boost converter is maintained constantly, and can be used for the DC load or battery charging or DC/AC grid.

The second stage is the inverter, which converts the constant DC voltage into the AC voltage. For this purpose, the MOSFET H-bridge is used, which contains four MOSFETs. For switching these MOSFETs controlled by the PIC microcontroller, the switching pattern for these MOSFETs is: At a time one MOSFET is ON and the other MOSFET is OFF in the same limb. Here, SPWM is used to control the H-bridge for getting the AC voltage which can be used for AC loads or AC grids.

The paper is organized as follows. Section 2 gives the operating configuration. Section 3 proposes the Proteus based implementation. The simulation and experimental results are presented in Section 5 and Section 6 draws the conclusions.

2. Operating Configuration

2.1Solar Cell Module

A solar PV module consists of the number ofNssolar cells in series. The equivalent circuit of the physical model of the solar cell is given in Fig. 2.

Fig. 2. Equivalent circuit diagram of solar cell.

Solar cells are made from semiconductors cells and are usually arranged in modules. There are different types of solar cells available in the market and under development, such as sensitized NAND-crystalline cells for a high efficiency and low cost. The datasheet parameters of physical PV module are shown in Table 1.

Table 1: Specifications of PV module

The equations describe the inverse relationship between currents and voltages in a solar cell as follows:

whereiis the current flowing out of the positive terminal of the solar module;iphis the PV current;idis the diode current;iris the resistance current;Iscois the short circuit current of each solar cell at a reference temperature;Sis the light intensity input;Sois the standard light intensity;Ctis the constant temperature;Tis temperature;Trefis the reference temperature, which is 25°C here;Iois the output current;qis the electron charge (q=1.6×10-19C);vdis the diode voltage;Akis the ideality factor;Rshis the shunt resistance;vis the voltage across the entire solar module;Nsis the number of cell in solar module;Rsis the seriesresistance;Tais the ambient temperature input;ksis the Boltzmann constant (ks=1.3806505×10-23).

The PV module provides power to the load, which often operates away from the maximum operating point of the module. The characteristics curves for the simulated solar cell module are shown in Fig. 3.

Fig. 3. Characteristics of current versus voltage and power versus voltage.

2.2DC-DC Boost Converter Configuration

The boost converter with the assumed continuous conduction operation mode is shown in Fig. 4.

Fig. 4. Boost converter circuit diagram.

The following parameters are necessary for describing the power stage: 1) Input voltage ranges fromVin(min)toVin(max), 2) Nominal output voltageVout, and the maximum output currentIout. The first step to calculate the switch current is to determine the duty cycle for the minimum input voltageVin(min), which leads to the maximum switch currentIout:

where η is the efficiency of the converter andDis the duty cycle.

The efficiency is added to the duty cycle calculation, because the converter also has to deliver the energy dissipated.

whereLIΔ is the change in load current,sfis the switching frequency, andLis the selected inductor value:

whereVinis the input voltage andLΔ is the change of the inductance. To reduce losses, the Schottky diode is used, so

whereIfis the diode forward current,is the maximum output current,is the minimum value output capacitor, andis the change of output voltage.

The designed parameters for the boost converter described in the above equations are shown in Table 2.

Table 2: Designed parameters for boost converter

3. Proteus Based Implementation

Proteus VSM has the microcontroller programming tool and environment with many software features libraries and hardware options. Many researchers and engineers use Proteus for testing and rapid prototyping simulation.

The controller tends to maximize the output power from the PV module by adjusting the duty cycle so that the solar cell module will always be at its maximum power point. Digital controller automatically increase or decrease the duty cycle of the converter according to the PV module output voltage. The maximum power point loop is used to set the corresponding set point voltage to the input and the voltage regulator loop is used to regulate the solar output voltage, which is set at the maximum operating point, according to the set point voltage.

The complete circuit diagram of the designed boost converter with a programmed PIC microcontroller is shown in Fig. 5. For sensing the voltage of the solar panel, the voltage divider network is used. The sensed voltage is compared with the reference voltage through the comparator. The error voltage is generated by the comparator, which is applied to the ADC of the PIC microcontroller. The output voltage of the solar PV module is sensed by the resistive divider network, which is applied to the comparator input at the inverting terminal and the reference voltage is applied at the non-inverting terminal to the comparator.

To get the maximum power point, many methods havebeen proposed. The program is developed according to the flow charts which are shown in Fig. 6 (a) and (b). Fig. 6 (a) shows the flowchart of the MPPT algorithm and Fig. 6 (b) shows the SPWM inverter control program logic.

Fig. 5. Implementation in Proteus.

Fig. 6. Program flow charts: (a) MPPT algorithm and (b) SPWM inverter control.

In this paper a single phase full bridge is connected with the grid system. To generate SPWM signals for switching the single phase full bridge, the CCP (capture, compare, PWM) module from the microcontroller is used. This allows a much greater flexibility in the microcontroller section. Generated SPWM signals are sent to the required MOSFETs through the MOSFET driver circuit as shown in Fig. 7.

Fig. 7. Inverter circuit diagram in Proteus.

The implemented hardware is shown in Fig. 8, containing the solar panels, implemented hardware, and output on a digital storage oscilloscope (DSO).

Fig. 8. Experimental setup.

4. Simulation and Experiment Results

The boost converter with a microcontroller PIC16F877A is designed for compatible loads to achieve the maximum output power from PV modules. The role of the DC-DC boost converter using the digital controller is to maintain a constant voltage. The boost converter experiment results are shown in Table 3. The input voltageVinis varied in accordance with the light intensity.Vois the output voltage andIois the output current.

Table 3: Results of boost converter (RL=40 Ω)

As shown in Table 3, Fig. 9, and Fig. 10, which are the Proteus simulation results, it is clear that with the change inVin, the output voltage maintains constant approximately 24 V andIoutis 0.60 A. The time scale is in milliseconds.