Phase Locked Loop(PLL) for 3 phase grid connected inverter | MATLAB Simulation.

Summary

This video discusses the implementation of a Phase Locked Loop (PLL) for a three-phase grid-connected inverter, explaining both open-loop and closed-loop methods. The importance of generating a reference signal in phase with the grid voltage to send active or reactive power is highlighted. The closed-loop PLL method is favored for handling issues like harmonics and surges. A MATLAB simulation demonstrates how to transform ABC voltages to DQ components, align voltage vectors, and generate unit vectors for active and reactive power control, ensuring stability and accuracy under critical grid conditions.

Takeaways

• 🔄 PLL (Phase-Locked Loop) is essential for aligning current with grid voltage to ensure proper power transmission.
• ⚡ For active current transmission, the reference signal must be in phase with the grid voltage, and its magnitude between 1 and -1.
• 💡 The first PLL method involves converting three-phase voltages (ABC) into two-phase (alpha-beta) and using an inverse cosine to generate angle information, but it has limitations.
• ❌ The first method can lead to instability under critical grid conditions, such as harmonics, surges, and noise.
• 🔒 A closed-loop PLL system is used to address the limitations of the open-loop system, providing better stability under adverse conditions.
• 📐 In the closed-loop method, the three-phase signals are transformed into alpha-beta and DQ voltages, where VQ is controlled to zero to align VD with the D-axis.
• 🔧 The PI controller is used to keep the Q value at zero, improving phase alignment and ensuring proper generation of active and reactive components.
• 📊 The simulation demonstrates how PLL aligns the active component with the alpha waveform and the reactive component with the beta waveform.
• ⚙️ The final output of the closed-loop PLL system generates unit vectors for both active and reactive components, ensuring stability and performance.
• 🖥️ MATLAB simulation is used to validate the PLL operation, showing correct phase alignment and expected waveforms for both alpha-beta and DQ transformations.

Q & A

What is the primary function of a Phase-Locked Loop (PLL) in a grid-connected inverter?

The primary function of a PLL in a grid-connected inverter is to generate a reference signal that is in phase with the grid voltage, ensuring that the active current sent to the grid is synchronized with the grid voltage.

Why is it important for the current sent to the grid to be in phase with the voltage?

It is important for the current to be in phase with the grid voltage to ensure efficient power transfer. When the current and voltage are synchronized, active power can be transmitted without power loss due to phase differences.

What are the two main methods of PLL implementation discussed in the script?

The two main methods of PLL implementation discussed are an open-loop algebraic method and a closed-loop control method. The closed-loop control method is more robust and is preferred in situations with harmonics, surges, noise, and spikes.

What are the key problems associated with the open-loop PLL method?

The key problems with the open-loop PLL method are that it is a simple algebraic system without feedback, making it prone to instability under critical grid conditions such as harmonics, surges, noise, and spikes. This can result in incorrect angle information.

How does the closed-loop PLL method improve stability?

The closed-loop PLL method improves stability by using feedback control, specifically a PI controller, to adjust the system based on the current grid conditions, ensuring that the generated signal remains accurate and stable even under challenging conditions like noise or harmonics.

What is the role of the alpha-beta to DQ transformation in the PLL process?

The alpha-beta to DQ transformation is used to convert the grid's three-phase voltages into a two-phase system. This simplifies the process of extracting phase information, as it aligns the D axis with the voltage, allowing for easier control of the active and reactive power components.

What is the significance of making the VQ value zero in the PLL process?

Making the VQ value zero aligns the D axis with the grid voltage, which ensures that the system is correctly synchronized with the grid. This is essential for accurate phase locking and for generating the correct active and reactive power signals.

How does the PI controller function in the closed-loop PLL system?

The PI controller in the closed-loop PLL system is responsible for minimizing the VQ value, ensuring that the voltage is aligned with the D axis. Its output is used to adjust the system’s phase angle (Omega T), which helps maintain synchronization with the grid.

What is the purpose of generating sine and cosine functions in the PLL block diagram?

The sine and cosine functions are generated to create the reference signals for active and reactive power. These signals ensure that the current components are correctly aligned in phase (for active power) or 90 degrees out of phase (for reactive power) with the grid voltage.

What result should be expected from the MATLAB simulation of the closed-loop PLL system?

The MATLAB simulation should show that the active component aligns with the alpha waveform and the reactive component aligns with the beta waveform, both forming unit vectors. This indicates that the PLL is working correctly and the system is synchronized with the grid.