MATLAB Implementation of a Single-Stage Three-Phase Grid-Connected PV System
Designing the PV Array for a 100 kW System
The first step in the implementation is determining the specifications for the PV array. We aim for a 100 kW grid-connected PV system, and this requires calculating the number of solar panels and how they should be connected.
Each PV panel operates at a maximum power point (MPP) with the following characteristics:
Voltage at MPP: 29V
Current at MPP: 7.35A
To design the array, we need to calculate how many panels should be connected in series and in parallel:
Series-connected modules: By dividing the inverter voltage (900V) by the voltage of a single panel (29V), we find that 31 series-connected modules are required.
Parallel strings: The number of parallel strings is determined by the desired power output. The formula to calculate this involves the panel's voltage and current values, and the result shows that 15 parallel strings are needed to achieve the 100 kW target.
Maximum Power Point Tracking (MPPT) with P&O Algorithm
The next important step is to ensure that the PV system operates at its maximum power point (MPP). For this, we use the Perturb and Observe (P&O) Maximum Power Point Tracking (MPPT) algorithm. The P&O algorithm dynamically adjusts the reference voltage to maximize the power extracted from the PV array.
Key steps in implementing MPPT include:
Measuring the voltage and current of the PV panel.
Providing reference values for maximum voltage (open-circuit voltage) and minimum voltage (voltage at charge circuit current).
Setting initial conditions for the reference voltage and power.
By continuously adjusting the reference voltage based on real-time measurements, the system ensures that the PV array operates at its optimal power point.
Connecting the PV Array to the Inverter
Once the PV array is designed and operating at its maximum power point, the next step is to connect the array to an inverter. The inverter converts the DC power from the PV array into AC power suitable for grid connection.
A few important components are used in this stage:
RLC Branch Filter: This is used to smooth the DC output from the PV array before it reaches the inverter.
IGBT and Diodes: These components are used to ensure efficient power conversion from DC to AC.
LCL Filter: After the inverter, an LCL filter ensures that the output AC power matches the grid’s voltage and frequency specifications.
Voltage and current measurements are taken both at the inverter output and the grid side to monitor the system’s performance.
Control Logic for Inverter Operation
The key to efficient integration with the grid is the inverter control logic. This involves managing the inverter’s voltage and current in real-time to match grid conditions and ensure minimal harmonic distortion.
The control system consists of:
Phase-Locked Loop (PLL): A PLL is used to synchronize the inverter’s output frequency with the grid’s frequency.
Decoupling Control: A decoupling control strategy is used to separate the direct axis (Id) and quadrature axis (Iq) components of the inverter's output, allowing independent control of voltage and current.
PID Controllers: PID controllers are used to regulate the voltage and current to maintain stable operation.
The inverter control ensures that the PV system provides a stable output to the grid while minimizing power losses and harmonic distortion.
Simulation and Tuning of Controllers
Once the system design is in place, it is essential to simulate the entire model to verify its performance and fine-tune the controllers. This simulation process helps identify any oscillations or inaccuracies in the system's output.
The PID controllers are initially tuned to minimize any oscillations, but some adjustments are made using the PD tuning method. This process is done iteratively, and the parameters are refined until the system achieves optimal performance with stable voltage and current outputs.
Power Output Monitoring and Performance Evaluation
With the system fully designed and controllers optimized, it’s time to test the performance under various conditions. The system’s performance is evaluated based on:
Power Output: The power generated by the PV system is monitored, with particular focus on how it varies with changes in irradiation.
Grid Current: The current flowing from the inverter to the grid is also monitored to ensure it meets grid requirements.
Harmonic Distortion: The Total Harmonic Distortion (THD) is calculated to ensure that the system meets the acceptable limits for grid integration.
In the simulation, as irradiation increases, both the power output from the PV system and the current sent to the grid increase. Even with high power levels, the system maintains a THD of less than 5%, which is within acceptable limits for grid-connected systems.
Conclusion: Efficient Grid Integration of PV Power
The final step in the process involves verifying the system’s performance under varying irradiation conditions. The system is tested with different levels of solar radiation, and the results show that as irradiation increases, the power output from the PV array increases accordingly, providing up to 100 kW of power to the grid.
The system's efficiency is confirmed with stable operation and minimal harmonic distortion. This demonstrates the effectiveness of the design, the use of MPPT for optimal performance, and the control strategies that ensure smooth integration with the grid.
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