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.

Outlines

00:00

🔍 Introduction to Phase Locked Loop (PLL) for Three-Phase Grid Connected Inverters

This section introduces the Phase Locked Loop (PLL) and its importance in grid-connected inverters. PLL is essential for sending active current to the grid by ensuring the current is in phase with the grid's voltage. It generates a reference signal aligned with the actual voltage, with a magnitude between 1 and -1. The signal helps in the implementation of current controllers for both active and reactive power transmission. The paragraph also highlights the use of PLL to generate signals 90 degrees out of phase with the grid voltage when transmitting reactive power.

11:03

⚙️ Basic Open-Loop PLL Method for Three-Phase Inverters

The paragraph explains the first and simplest method of implementing PLL for a three-phase grid-connected inverter. The three-phase ABC voltages are transformed into two-phase alpha-beta voltages. From the inverse cosine of V alpha over the square root of V alpha squared plus V beta squared, angle information is derived. This angle is used to generate current references for active and reactive power. However, this open-loop method has limitations, including instability under grid disturbances, and it struggles with noise, surges, and harmonics, making it unsuitable for certain conditions.

🔄 Introduction to Closed-Loop PLL with Control Mechanisms

This section introduces the closed-loop PLL system that overcomes the limitations of the previous open-loop method. It begins similarly with transforming ABC signals into alpha-beta and DQ voltages. The goal is to align the D-axis with the grid voltage, and this is achieved by making the Q-axis voltage equal to zero. The paragraph includes a phasor diagram to visualize the process and explains how Omega T (the angle between the alpha component and D axis) is generated using a PI controller. The resulting Omega T value helps create active and reactive components for the inverter.

🛠️ MATLAB Simulation of Closed-Loop PLL

This paragraph marks the transition to MATLAB simulation for implementing the closed-loop PLL. The necessary blocks for constructing the PLL are outlined, and the simulation begins by displaying the input AC voltages with an RMS value of 400. The simulation confirms the accuracy of the PI controller by showing a Q-axis value of zero, aligning with the reference. Alpha-beta waveforms are generated with a 90-degree phase difference, and their magnitudes match the original AC waveforms.

📊 Output Analysis of PLL in Simulation

The final section presents the results of the PLL simulation. The alpha waveform and the active component of the PLL are shown to be in phase, with the active component having a magnitude of 1, representing a unit vector. Similarly, the beta waveform and the reactive component are aligned in phase, also forming a unit vector. The paragraph concludes by thanking the audience and closing the presentation.

Keywords

💡Phase-Locked Loop (PLL)

A Phase-Locked Loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. In the context of this video, it is used to synchronize the inverter's output current with the grid voltage. The PLL generates reference signals that ensure the current sent to the grid is in phase with the grid voltage, making it crucial for grid-connected inverters.

💡Three-Phase Grid-Connected Inverter

A three-phase grid-connected inverter converts DC power to AC power and feeds it into the grid. The video focuses on how PLL is implemented in this type of inverter to ensure that the generated current is in phase with the grid voltage, facilitating effective power delivery.

💡Active Power

Active power is the real power that performs useful work in an electrical system. In the video, active power refers to the current that is in phase with the grid voltage. The PLL helps to generate a reference signal that aligns the inverter's output current with the grid voltage for active power delivery.

💡Reactive Power

Reactive power is the power that oscillates between the source and the load without performing any useful work. It is associated with the current that is 90 degrees out of phase with the voltage. The video explains how PLL can generate a signal that is out of phase with the grid voltage to send reactive power.

💡Alpha-Beta Transformation

Alpha-beta transformation refers to converting three-phase voltages into two-phase voltages, simplifying control in a PLL system. In the video, it is the first step in both the open-loop and closed-loop PLL methods for generating the necessary phase information for synchronization.

💡DQ Transformation

DQ transformation converts the two-phase alpha-beta components into a rotating reference frame (d-axis and q-axis) that aligns with the grid voltage. The video highlights this transformation as critical for aligning the inverter's voltage and current with the grid voltage for precise control in the PLL system.

💡Open-Loop System

An open-loop system is a type of control system that does not use feedback to adjust its performance. The video mentions an open-loop method for PLL, which is algebraically simple but susceptible to instability in critical grid conditions like harmonics, surges, or noise.

💡Closed-Loop System

A closed-loop system uses feedback to adjust its operation and maintain stability. The video explains the superiority of a closed-loop PLL over an open-loop system for grid-connected inverters, as it can handle disturbances and maintain proper phase alignment under various grid conditions.

💡PI Controller

A Proportional-Integral (PI) controller is a control loop mechanism that uses both proportional and integral terms to correct errors in system performance. In the video, the PI controller is used to ensure that the Q-axis voltage is zero in the PLL, aligning the D-axis voltage with the grid voltage for synchronization.

💡Unit Vector

A unit vector is a vector with a magnitude of one. In the video, the unit vector is used to represent the active and reactive components of the PLL output. The alpha and beta waveforms, corresponding to the active and reactive components, are normalized to a magnitude of one to simplify control.

Highlights

Introduction to Phase Locked Loop (PLL) and its necessity in three-phase grid-connected inverters.

The current sent to the grid needs to be in phase with the voltage, and a PLL helps generate the reference signal for this.

PLL generates a signal for reactive power by producing a 90-degree out-of-phase signal with the voltage.

Explanation of the first and simplest method of PLL implementation using three-phase ABC voltages converted into two-phase alpha-beta voltages.

Using an inverse cosine of V alpha divided by the square root of V alpha squared plus V beta squared gives the angle information.

This open-loop method is not suitable for handling harmonics, surges, noise, and spikes, leading to incorrect angle information under critical grid conditions.

Introduction to the closed-loop Phase Locked Loop system, which offers a more stable solution.

In the closed-loop method, ABC voltages are first converted into alpha-beta voltages, followed by DQ transformation.

Explanation of the phasor diagram, showing that D and Q axis voltages are not aligned with grid voltage, leading to non-zero values.

By setting VQ to zero, the system aligns the VD with the D-axis, stabilizing the system and improving accuracy.

PI controller is used to maintain the VQ value at zero, and the output is integrated to determine the Omega T angle.

Omega T, derived from VQ=0, is used to generate active and reactive components for the grid-connected inverter.

MATLAB simulation demonstrates the effectiveness of the PLL system, starting with a face-to-face voltage with an RMS value of 400V.

The Q output of the alpha-beta to DQ transformation block confirms that the PI controller is working correctly by maintaining a zero Q value.

The alpha-beta waveforms show a 90-degree phase difference, and the active and reactive components align with their respective alpha and beta waveforms.

Transcripts

[Music]

today we are discussing phase lock loop

for a three-phase grid connected

inverter and it's MATLAB simulation

first let me give you a brief

introduction to PLL

why do we need a PLL the answer is very

simple suppose we want send an active

current to the grid first I will mark

the grid side voltage now the current

which has to be sent should be in phase

with this voltage in order to send this

current we need to generate a reference

signal and that signal should be in

phase with the actual voltage and it's

magnitude should be in between 1 and

minus 1 so PLL is used to generate the

signal and the signal is used as the

reference for the implementation of

current controller in a grid connected

inverter similarly when we need to send

reactive power to the grid we use PLL to

generate a signal which is 90 degree out

of phase with the actual voltage

here I will explain two methods of PLL

implementation this is the first method

and the simplest method for a

three-phase grid-connected inverter you

can see this method explained in many

literature's here three-phase ABC

voltages are first converted into two

phase alpha beta voltages from the alpha

beta voltages taking the inverse cosine

of V alpha divided by root of v alpha

square plus v beta square will give us

the angle information and from this

angle information we generate the

current reference for active and

reactive power but there are few

problems with this method and hence this

method is not used in many situations

the major problem is that this is merely

an algebraic method with simple

mathematics involved and it's just an

open-loop system so the system can go to

an unstable situation under critical

grid conditions

so this PLL cannot withstand conditions

like harmonics surges noise and spikes

due to this the output of PLL will give

wrong angle information we can get rid

of all these issues by using a closed

loop phase locked loop that I will

explain now this is the complete block

diagram of phase locked loop with closed

loop control similar to the previous

method here also we start with

transforming the ABC signals to alpha

beta signals let me also draw the phasor

diagram I have marked alpha beta and

grid voltage now we convert the alpha

beta voltages to DQ voltages now let me

mark the d axis and Q axis in the phasor

diagram from the phasor diagram it is

clear that the axis is not aligned with

the grid voltage so we will have nonzero

value of D axis and Q axis voltages I

will mark that also in the phasor

diagram I will also mark Omega T which

is the angle between V axis and alpha

component now by using some kind of

control mechanism we are making the

value of V Q as equal to 0 when we do

that the phasor diagram will get

modified in such a way that the VD will

get aligned with the D axis let me draw

the new phasor diagram so it's pretty

clear from the phasor diagram that VD is

now aligned with the axis and the value

of V Q becomes 0 and Omega T which is

the angle between alpha component and D

axis has now changed to it new value so

this Omega T which we got after making Q

value 0 can

used for generating the active and

reactive component now going back to the

PLL block diagram you can see API

controller which I have used to make the

cue value 0 so I have given VQ reference

as 0 the output of the P I control er is

given to an integrator to find the Omega

T and finally we use sine and cosine

functions to generate the active and

reactive components and that is the

final output of PLL this is how we make

a PLL for three-phase grid connected

inverter now we will do the simulation

using MATLAB

these are the blocks required to make

the PLL

now we can start connecting all

components

you

we have completed connecting all the

components now it's time to run the

simulation

first we shall see the input AC voltages

this is the face-to-face voltage with

rms value 400

now we shall see the Q output of alpha

beta to DQ transformation block

the Q value we are getting is zero which

is same as the reference value so P I

controller working fine now we shall see

the alpha beta waveforms

these are the alpha-beta waveforms as we

expected there is a 90 degree phase

difference between these two waveforms

magnitude of these waves are same as the

original AC waveforms

now we shall see the output of PLL first

we see the alpha wave form an active

component of PLL the active component

should align in phase with the alpha

wave form

you

as we expected alpha wave an active

component are aligned in phase with each

other and the magnitude of active

component as one so we call it as unit

vector now we shall see the beta wave

form and reactive component of PLL the

reactive component should align in phase

with the beta wave form

as we expected beta wave and reactive

component are also aligned in phase with

each other and the magnitude of reactive

component as one so this also called as

unit vector this is the end of this

presentation thank you

[Music]