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Three Phase Grid Connected Solar PV and Battery System

Three Phase Grid Connected Solar PV and Battery System


Simulink Model Explanation

This model is developed for testing the three-phase grid-connected solar PV and battery systems. It consists of a solar PV system, battery storage systems, and a three-phase inverter connected to the grid.



Solar PV Design and Boost Converter

We are using a 250-watt panel connected in series, with each string having 15 modules and two parallel strings. The voltage rating for a single panel at maximum power point condition is around 30.7V, and the current is 8.15A. The total maximum power for different irradiation conditions is 7506 watts at 1000 watts per meter square, with the voltage at maximum power point being 460.5V.

The solar PV is connected to a DC bus, requiring a boost converter design. The input voltage is fixed at 460.5V, and the output voltage, based on grid line-to-line voltage (400V), is set at 700V. The design includes calculating inductor (L) and capacitor (C) values using specific equations for the boost converter and input filter.



Battery Converter Design

A bi-directional converter connects the battery, allowing it to act as both a source and a load. The battery, with a rating of WT35, is connected in series with 35 modules, and the design parameters are based on the same PV power rating. The L and C values for the bi-directional converter are calculated similarly to the boost converter.

Filter Design for Three-Phase Inverter

The three-phase inverter is connected to the grid via an LC filter to ensure proper waveform for voltage and current. The power rating for the filter is 8000 watts, slightly more than the solar PV rating of 7500 watts. The DC bus voltage is maintained at 700V, and specific equations are used to calculate the L and C values for the AC filter.

Control Logic for Three-Phase Inverter

The control logic for the three-phase inverter involves a current control algorithm using ABC to DQ transformation. Voltage across the capacitor and current through the inductor are measured, and a PLL concept generates sine and cosine waveforms. The inverter's operation is controlled by fixing reference currents based on PV current and battery state of charge (SOC). The inverter can either send power to the grid or receive power from the grid, depending on the conditions.

Incremental MPPT for PV System

An incremental conductance MPPT (Maximum Power Point Tracking) method is employed to maximize the power extracted from the PV panel. The duty cycle is adjusted based on changes in voltage and current to maintain the PV operation at its maximum power point.

Voltage Control of Battery Converter

The bi-directional DC-DC converter's voltage control maintains the DC bus voltage at 700V. The measured DC bus voltage is compared with the reference voltage, and a PI controller generates the duty cycle to regulate the converter.

Simulation Results

Battery SOC at 50% with Varying Irradiance

Initially, the PV reaches maximum power, and the system supplies power to both the DC load and the grid. The power output changes with varying irradiance, and the battery switches between charging and discharging modes based on PV power availability.

Battery SOC at 9% with Varying Irradiance

With a lower SOC, the battery initially charges when PV power is available. As the irradiance changes, the battery switches to discharging mode when PV power is insufficient, drawing power from the grid to maintain the system's operation.

THD Analysis

The Total Harmonic Distortion (THD) of the inverter current is analyzed during both power intake from the grid and power delivery to the grid. The THD values are found to be within acceptable limits, ensuring compliance with IEEE standards.

Conclusion

This is the working principle of a three-phase grid-connected solar PV and battery system. We demonstrated the control logic, design parameters, and simulation results for different battery SOC conditions and varying irradiance levels.

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