Phase Locked Loop(PLL) for 3 phase grid connected inverter | MATLAB Simulation.
Summary
TLDRThis 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
- π PLL (Phase-Locked Loop) is essential for aligning current with grid voltage to ensure proper power transmission.
- β‘ For active current transmission, the reference signal must be in phase with the grid voltage, and its magnitude between 1 and -1.
- π‘ 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.
- β The first method can lead to instability under critical grid conditions, such as harmonics, surges, and noise.
- π A closed-loop PLL system is used to address the limitations of the open-loop system, providing better stability under adverse conditions.
- π 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.
- π§ The PI controller is used to keep the Q value at zero, improving phase alignment and ensuring proper generation of active and reactive components.
- π The simulation demonstrates how PLL aligns the active component with the alpha waveform and the reactive component with the beta waveform.
- βοΈ The final output of the closed-loop PLL system generates unit vectors for both active and reactive components, ensuring stability and performance.
- π₯οΈ 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 systemβs 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
π 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.
βοΈ 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.
π 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.
π οΈ 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.
π 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.
Mindmap
Keywords
π‘Phase-Locked Loop (PLL)
π‘Three-Phase Grid-Connected Inverter
π‘Active Power
π‘Reactive Power
π‘Alpha-Beta Transformation
π‘DQ Transformation
π‘Open-Loop System
π‘Closed-Loop System
π‘PI Controller
π‘Unit Vector
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]
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