i NEWCASTLE UNIVERSITY SCHOOL OF ELECTRICAL, ELECRTONIC AND COMPUTER ENGINEERING I, HAMMED AWAD ALANZI , confirm that this report and the work presented in it are my own achievement. I have read and understand the penalties associated with plagiarism. Singed: ………………………………………………….. Date: ……………………………………………………..
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Abstract ................................................................................................................................ ii ACKNOWLEDGEMENTS.................................................................................................. iii List of Figures ...................................................................................................................... vi List of Tables ..................................................................................................................... viii CHAPTER1 Introduction ...................................................................................................... 1
2.5.1 Machine-Side Converter Control ........................................................................... 7 2.5.2 Line-Side Converter Control .................................................................................. 7
2.6 Control Schemes .......................................................................................................... 8 2.6.1 Current Control (CC) ............................................................................................. 9 2.6.2 Space-Vector (VS) Control .................................................................................... 9 2.6.3 Direct Power Control (DPC) ................................................................................ 11
Chapter 3 Simulation of the system ..................................................................................... 13 3.1 Simulation of sine-wave PWM control....................................................................... 13 3.2 Simulation of third harmonic injection PWM ............................................................ 16 3.3 Simulation of Space Vector Modulation (SVM) ......................................................... 19 3.4 Simulation of Phase Locked loop (PLL)..................................................................... 19
Chapter 4 control of the system ........................................................................................... 23 4.1 Introduction ............................................................................................................... 23 4.2 The PI controller ........................................................................................................ 29 4.3 The reference frame transform ................................................................................... 29 4.4 Voltage source converter connected to the resistive load ............................................ 33 4.5 Voltage source converter connected to the resistive load with control loop................. 35 4.6 Voltage source converter connected to the grid with controller .................................. 39
Figure 25: The output signal from abc to αβ transform....................................................31
Figure 26: Block diagram for transforming the αβ reference frame to the dq referenceframe.....................................................................................................................................31
Figure 27.: The output of the dq reference frame block...................................................... 32
Figure 28: Block in matlab/simulink for transforming from the dq to abc reference
In terms of percentage of early growth of installed capacity per technology source, the wind
energy was the fastest growing energy technology in the 90s [1]. Hence, this growing results
in significant proportion of consumer’s electrical power demands from wind energy [2]. For
example, “ The UK is the windiest country in Europe, with over 40% of the available
resources, and improvements in technology have resulted in the cost of wind power falling to
close to those of conventional sources of electricity” [3].
In recent years, several power converter topologies have been developed to incorporate with
the electrical grid, which allow variable speed operation of the wind turbine, and enhanced
power extraction as well. For this reason, designing variable speed turbine has the following
considerations: a control methods should be designed to extract the maximum power from the
turbine and provide a constant grid voltage and frequency [4].
In the sequel, enormous attentions, in terms of cost and complexity, have been moved
towards controlling techniques. This project investigates the Back-To-Back voltage source
converter (B-T-B VSC). Different topologies of B-T-B VSC generators, their advantages and
drawbacks, are studied. In addition, several control methods that have been used for both
Line-Side and Machine-Side converters are explained
1.2 Project objective
The main purpose of the project is the controlling of the grid current via the use of an appropriate scheme, afterwards the simulation of the voltage source converter between DC
link voltage and the grid is given more consideration.
1.3 Dissertation outline
The dissertation is divided into five chapters. First chapter covers the basic information about
importance wind energy. The second chapter reviews the wind turbine energy system along
with constant and variable speed turbines along with constant and variable speed turbines.
Chapter three outlines simulate of sine-wave PWM control, third harmonic injection PWM,
Space Vector Modulation (SVM) and Phase Locked loop (PLL). By the Proportional-
Integral (PI) controller, the DC link capacitor and the grid, is implemented and controlled in
chapter four. The fifth chapter conclude the project, the main conclusions is provided.
The Wind energy can be defined as the use of the wind to generate electricity. In other words,
wind energy system transforms moving energy of the wind into electrical energy that can beapplied for practical use. Occasionally, it can be more reasonable to obtain new power by
building a wind farm than by building a coal, natural gas, or other type of power plant,
particularly, in areas where an excellent wind can be founded. That is why recently wind
energy is considered to be a clean, safe, and renewable (inexhaustible) power resource [5].
The windmills have been used for hundreds of years in order to harness the wind’s energy.
On the contrary, nowadays wind turbines are efficient technology much more than windmills,
and usually, the horizontal-axis is used for this turbines [6].
Figure 1 shows simple wind energy generation system. The main elements that constitute
turbine are a rotor, hub, nacelle and tower. In order to generate the electrical energy, the wind
turns the three rotor’s blades around a central hub. As the nacelle houses the drive -train and
power converter, the electrical energy is generated by converting Kinetic energy into
electrical energy [5], [7].
To transmit the electrical current to the grid ensuring maximum productivity, in modern
system, mechanical drive systems cooperated with advanced generator. This generator
responsible to convert energy produced by mechanical parts into an electricity [4]. However,
different types of generator require different control methods, which dominate the flow of
energy form the wind turbine to the connected grid.
Furthermore, in term of turbine speed, two types, fixed and variable turbine speed have been
used. Next section explains the energy capturing, advantages and limitations for each turbine
type.
Fig.1. Simple Wind Turbine Energy Generation System [8]
As mentioned before, most of the converter configurations, like thyristor and hard-switching
converters, are limited in use due to the harmonic distortion, weak power factor, as well as
the system control complexity. Hence, for variable speed generator, back-to-back PWM
converter with a DC-link is an suitable solution, by which we can recognize advanced control,
and therefore achieving overall active and reactive power control [2]. However, new control
issues for both sides arise and the most important addition to control is the coordination
between the two PWM converters. For convenience, in this study, the two converters are
termed machine-side converter and line side-converter, respectively.
The VSC is not sued as a rectifier connected to generator solely, moreover, it can be used for
the inverter itself as well. However, in such scenario, VSC requires a minimum DC-link
voltage and sometimes need to DC/DC converter in order to increase the voltage level [11].
Likewise, to carry-out the optimum use of the VSC, it is important to select the proper
controlling and modulation schemes. In term of Pulse-Width Modulation (PWM), either the
current control or voltage control are commonly used [12].
2.5.1 Machine-Side Converter Control
In Variable Speed Constant Frequency (VSCF) generating system, the control schemes in the
machine-side are expected to perform the following objectives [13]: for maximum power
capturing, it is required to track a prescribed torque-speed curve, the voltage frequency of the
stator output must be constant, and then achieve flexible reactive power control.
It is emphasised by [11] that, in the machine-side converter, “it is essential to keep the DC-
link voltage constant regardless of the magnitude and direction of the rotor power”. To
achieve this objective, Current-Vector control approach has been developed.
2.5.2 Line-Side Converter Control
In the same way as in the machine-side converter, for which the optimal torque-speed profilecan be tracked, in line-side converter, the stator output reactive power control is the main
line-side converter objective. In addition, the DC link capacitor provides DC voltage to the
machine side converter, and store the active power in the capacitor, as a result the capacitor’s
voltage level will increases. One to ensure the consistent DC level of the DC-link, the power
flow of the converters should attempt to convene the following control objective [13]:
In contrast to conventional open-loop voltage PWM converters, the current-controlled PWM
(CC-PWM) have the following advantages [14]: “Control of instantaneous current wave form,
high accuracy, peak current projection, overload rejection, compensation of semiconductor
voltage drop of the inverter and compensation of the DC-link and AC-side voltage changes”.
The main task of the control scheme for CC-PWM converter is shown in figure 4. Three-
phase AC load can be supplied by the CC-PWM. Based on the switching states
(SA , SB , SC) for the converter power devices, and by comparing the three loads iA , iB , iC,current control can determine the current errors (εA , εB , εC), and obviously, can decrease
these errors.
Fig.5. Basic Block Diagram of CC-PWM Converter [14].
Recently, numerous methods have been developed for the CC. Paper in [15] have discussed
the basic CC techniques for the voltage source inverter. Examples for this are Hysteresis
Controller for three dependant and independent controllers. For independent one, threecontrollers each for one phase are constructed [14]. While in the dependant hysteresis, three
controllers are incorporated together to reduce the switching frequency when, at certain time,
zero-voltage vector is applied, and to limit the maximum current error [16].
2.6.2 Space-Vector (VS) Control
According to [17], Space-Vector modulation techniques based on PWM (SV-PWM) is
considered to be the most extended modulation strategies for three-phase converters. In this
Figure 9. Simulation model of sine-wave PWM control scheme
The right part of Figure 2 shows that the control scheme of the inverter. A repeating sequence
block is chosen to generate the PWM signal. In this case the PWM frequency is chosen to be
10K Hz. Of course, higher value of the PWM frequency means better performance of the
inverter. But considering the switching losses of the power stage could be higher as well, the
10k Hz seems to be reasonable. The reference signals given to the controller are three-phase
sine-waveforms with frequency of 50 Hz and are displaced 120° with each other. The
modulation index is chosen to be 1 to give the maximum dc link utilization in this case. The
result waveforms of the voltage between to common, the actual voltage over the coil, the
neutral voltage and the phase currents are shown in Figure 3.
As it can be seen in Figure 3, the peak to peak voltage of the voltage between to common
is 800V, which is equal to the DC link voltage. The phase voltage is shown as 400V, the
reason for this is because the neutral voltage of the inverter is 400V, so the maxim voltage foreach phase is 400V, which is half of the DC link voltage. The current waveform is not
balanced as it is shown in Figure 3. This happens because the inductance of the inverter is
fairly big, this results in a very long transient time. When the simulation is run for a longer
time, the balanced waveform can be seen in the scope. The peak current value can be
As it was concluded in the Literature Review, the SVM scheme is the most suitable strategy
for the project. However, simulation of SVM from scratch is quite difficult. Firstly, there’ssix parts of the hexagon, each part counts 1/6 of 1 power cycle; secondly, not like the control
schemes mentioned before, SVM looks at the invert as a whole; thirdly, how to pick up the 3
vectors each time, how to choose the zero space vector and make the right combination.
These three main issues become the obstacles of building SVM model, especially when the
time is limited. But substantial literature survey is been done in this area and hopefully the
obstacles will be conquered.
All these control schemes mentioned above are simple open loop control. But in reality,
there’s no such system without control loop. A good control loop not only copes with error
correction between the given value and the output value, but it gives the fast response as well.
Either current control or direct power control, a block called PLL is needed.
Phase Locked Loop (PLL)
PLL is widely used in three-phase closed loop system. The basic idea behind it is to make 3
to 2, 2S to 2r transformation, so that the ABC reference frame could be transformed into d-q-
o rotation reference frame. Because in natural reference frame, all the quantities we’ve got
are AC quantities. Both the voltage and current quantities vary all the time, not to mention the
power. Actually the real power is pulsating forward and reactive power is pulsating backward
and forward all the time. So it is quite useful to use controller like PI controller to track these
error signals. That’s why the phase locked loop plays an important role in reference frame
transformation. By changing the natural reference frame to rotational reference frame, all the
AC quantities become DC quantities. If it’s current control, then , , are changed into , in the d-q-0 co-ordinate. If it’s direct power control, then , , and
, , are changed into , . These signals will be compared with the given signals, and
then the error signals will be fed into the PI controller so that the PI controller could do a
good job.
3.4 Simulation of Phase Locked loop (PLL)
As mentioned above, the core of PLL is reference frame transformation. While 3s to 2s could
The wanted signal is determined and transmitted in the PI controller. The PI controllers
determined output signal is dependent upon on the proportional gain , the integral gain and error
. One of the above factors, which is the proportional gain
, changes the error
value proportional output and can make the system unstable if it’s value too high. In addition,
the controller is made less sensitive when a small value with a small output response has a
large input error. A signal cannot be driven at the demanded value if the proportional
controller is not suitable. A steady state error, named the offset, stays with the proportional
controller, thus the integral term is added to the controller, for the cancellation of the offset
effect [21].
The error signal’s magnitude and the length are both affected by the integral term. Theintegral gain is used to calculate the magnitude of the effect to the integral term of the
whole control action. In actual fact, the speed of the response of the controller is increased
and the residual steady state error is eliminated by the integral gain. The residual steady state
error occurs because of a pure proportional controller. Nonetheless, an overshoot can happen
in the output due to the integral control term. This happens because the response in the
integral term to the accumulated errors of the past [21].
The PI controller is good in the control of DC values; this is an important feature and needs to
be considered. And because of the above and because all the current and voltage variables
are AC, the transformation of the grid voltages and currents from the abc to the dq reference
frame, is needed for the use of the PI controller in this project. Thus, the variables are
converted to DC values, and can be more easily controlled by the PI controller [21].
4.3 The reference frame transform
As was previously mentioned for the use of the PI controller, the stationery frame (abc) needs
to be transformed to the synchronous frame (dq). An important concept of reference frame
theory has to be introduced for the sake of achieving this aim. First of all, the three phase
quantity needs to be transformed, as an example , , , into two vector, positive and
negative phase sequence vector. This, in a complex ref erence frame, is called the αβ frame.
The Clarke transform which is used, uses three time varying variables, which can be
demonstrated by a space vector, in the process of determining their two components in the
Figure (27): The output of the dq reference frame block
Additionally, the transforms’ output variable is clearly a DC value, the variable in the q axis
is 100 and the d axis variable is zero. Thus, when these two transforms are in a cascaded
format, the variable can be changed from an AC to DC signal, and this will enable the use of
the PI controller. However, the output of PI controller, which is a DC value, has to be
transformed to an AC value. And to do so, opposite measures to the one above are applied.
The inverse transform is presented by:
αβ = cos θ sin θ− sin θ cos θ de
And
=
1 0−1
2
32−1
2
− 32
[ ]
A Matlab/Simulink implemented block diagram for the dq to αβ to abc reference frametransformation is shown in figure (28). And it is noteworthy, that the Theta angle has not
changed.
Figure (28): Block in matlab/simulink for transforming from the dq to abc reference frame
In determining the filter capacitors’ value, a cut off frequency has to be chosen and
henceforth after using the inductor previously calculated the filter capacitors value can then
be calculated.
= 142 2
Figure (30): The transfer function model of the voltage source converter with the load
Thus, the value of filter capacitor is calculated to be 56.290 µF.
Table (4) shows a summary of the used values in the project.
Variable value
Three phase Power 60000 (W)
Frequency 50 (Hz)
The phase to phase voltage of grid 415 (V)
Carrier frequency 10000 (Hz)
Inductance of the filter 0.45 (mH)
Resistance of the filter 0.03 (ohm)Capacitance of the filter 56.290 (µF)
DC link voltage 720 (V)
Resistance of the load 3.239 (Ω) Table (4): Value of parameters when voltage source converter connected to the grid and load
After the application of unity modulation index to the voltage source converter mazimum
current flows in the load. Of course, the above means that 254.55V is the amount of rms
voltage applied. Figure(31) shows the converter current with unity modulation index. Thecurrent peak to peak amplitude is the same as previously calculated which is 111.11.
sake of producing more current to supply the load, the input voltage needs to be raised. This
can be done by increasing the modulation index.
In order to understand the systems operation, consider the voltage source converter which
supplies the load and the current of the load to reach the steady state value. After this, if the
output current is needed to increase, a current error signal is created which is a difference
between the actual output current and desired output current. The required current error signal
is specified by the error signal. Therefore, to compensate the error between the actual current
and the desired current, the quantity of the output voltage of converter needs to be increased.
The main aim is to achieve a zero error signal in the steady state mode by creating a relation
between the error signal and the modulation index of the voltage source converter.
A good current loop should be the first thing aimed for when designing the controller. This
means that the steady state current has to closely correspond with the current reference and
there should be fast and well damped transient response to step changes in the current
reference. The PI controller is needed to attain these objectives, the integral term in the PI
controller satisfies the first requirement, and the second is satisfied by an appropriate choice
of the proportional gain.
Because a three phase voltage source converter is connected to three phase load and becausethe voltage and current signals are sinusoidal, it is appropriate to use of abc to dq transform to
obtain a DC value for the current in dq reference frame. In addition, it is more suitable to use
the PI controller to compare the actual and desired values of the current. In another part of the
system the PI’s controller output as a voltage signal is abc reference frame transformed, in
order to be applied to the voltage source converter. In the figure (33) the voltage source
converter connected to load with current controller block diagram is shown.
Voltage source converter that is connected to a grid is analysed and modelled in this part. As
in the previous section, a transfer function of the system is implemented in Matlab/Simulink
first. A model of the transfer function without the control scheme is shown in figure (37).
Additionally, to investigate the transfer function model the modulation has to be set to unity
and the simulation has to run for a long time. The converter current is shown in figure (37)
when the modulation index is equal to 1, and the waveform of the current can be seen as a
perfect sinusoid after a transient time.
4.6 Voltage source converter connected to the grid with controller
The PWM previously described performs as an open loop or feed forward control of the
voltage source converters’ output voltages. The systems aim is the control of the output
currents demanded value, for this current sensor feedback must be present. A control system
compares between the actual currents with the reference currents and then produces the
appropriate modulation index for each phase. This results in the waveforms output current
following the reference waveforms.
The control scheme consists of a current loop and a feedback from the grid voltage. This
control scheme is used for the grid connected voltage source converter. This control scheme
is proposed in order to follow the three phase voltage of the grid, thus adding the grid
voltage’s feedback with a proportional gain to the current loop. Figure ( 37) shows the
proposed control scheme, which is used in this project.
Figure (37): transfer function model of the grid connected voltage source converter
It is obvious that the entire variable is first transformed to the dq reference frame, and afterthis the output waveform of the PI controllers is used as a reference value for the voltage loop.
This project deals with an investigation on the back to back voltage source converter which is
so much popular in the wind power industry these days. Firstly, various topologies of theback to back voltage source converter have been presented and their advantages and
limitations are described. Moreover, different control strategies for the generator side and the
grid side of converter are explained.
It is obvious that the performance of the converter is largely depends on the quality of the
control scheme which is employed. The space vector control scheme is the most suitable
strategy among other schemes, due to converter can achieve maximum output voltage, better
load controlling and low switching frequency.
Furthermore, due to the computational complexity of space vector control scheme, numerous
improvements have been done to achieve an excellent performance. This project attempts to
develop and model a control scheme to improve the performance and efficiency of the
converter based on the space vector control scheme.
In the simulation and modelling part, different control schemes have been simulated and
compared for the converter. A simple and excellent kind of direct power control scheme is
chosen as the main control scheme and the transient and steady state performance of a
voltage source converter have been investigated in different conditions. In every case, the
proposed control schemes have been shown to provide fast and satisfactory responses for the
voltage source converter. From the simulation we can deduce that the voltage source