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CHAPTER 3
MITIGATION OF CURRENT HARMONICS USING
CONTROL STRATEGIES
3.1 INTRODUCTION
Filter designs for harmonic mitigation has become an active
research area. The present research work focuses on designing an efficient
filter which could perform well in non linear load conditions. A passive filter
has been an appropriate option for minimizing the currents harmonics because
of its low cost and high efficiency. However, the performance of the passive
filter design has certain limitations as the addition of the passive filter
interfaces with the system impedance and causes resonance with other
networks. Numerous active solutions, which are becoming more effective
means to meet the harmonic standards by overcoming the drawback of the
passive filter, have been discussed in Wei et al (2006).
The SAPF operates by injecting the reactive, unbalanced, and
harmonic load current components into the utility system with the same
magnitudes as the nonactive load currents demanded by a given nonlinear
load but with opposite phases as discussed in Shu et al (2008).
applications, the methods for extraction of the harmonic load currents and
determination of the filter reference current play an important and crucial role.
61
Indeed, the accuracy and speed of the SAPF response are related to this point
as discussed in Singh et al (2007).
The methods of reference current generation is categorized into two
main fields namely time-domain and frequency domain methods as discussed
in Shu et al (2008). Time-domain methods such as d q transformation (or
synchronous rotating reference frame), p q transformation (or instantaneous
reactive power), symmetrical components transformation, etc., are based on
the measurements and transformation of three-phase quantities as described in
Shu et al (2008).
The main advantage of these time-domain control methods,
compared with the frequency-domain methods, is the fast response obtained.
On the other side, frequency-domain methods provide accurate individual and
multiple harmonic load current detection. The compensation method
presented by Shu et al (2008) is the time-domain control type of
compensation, where all harmonic load current components are targeted and
compensated. The SAPF offers different options of compensation, such as
harmonic attenuation, load balancing, resonance elimination, and
displacement power factor improvement. Thus, the control strategy and the
method for extracting the nonactive load current references will depend on the
compensation objectives.
Although, the conventional linear controllers may fulfill certain
compromises between steady-state performance, and harmonic load current
compensation and dc bus voltage regulation, they remain unable to
compensate the inherent nonlinearity of such circuits, which is generated by
the switching process. This manifests with important overshoots and long
settling times, during transients from both the ac and dc side. On the other
62
hand, most of the techniques mentioned in the literature assume sinusoidal
supply voltages when compensating unbalanced nonlinear load currents.
However, in reality, the utility voltage available at the downstream end is
nonsinusoidal due to the harmonic load currents. A thorough investigation of
the experimental results reported in Rahmani et al (2006), reveals the fact that
the Total Harmonic Distortion (THD) in the supply currents cannot be
brought down below 5% to satisfy the IEEE-519 standard. This is due to the
presence of notches in the supply currents, whereas feed forward control
methods are used. The drawbacks can be eliminated by using the nonlinear
control theory, ideally without exaggerating computational and
implementation complexities. In addition, Youssef et al (2008) implemented
very useful advanced nonlinear control techniques to active rectifiers with
active filtering function. These control techniques can be applied to active
filtering technology. A nonlinear control strategy of an SAPF based on the
internal model principle has been presented.
This chapter presents an efficient SAPF based harmonic mitigation
approach based on control strategy and SVPWM technique with
interconnected RES.
3.2 SAPF WITH INTERCONNECTED RES
RES is generally combined at the transmission level. The service is
anxious due to the high dissemination level of intermittent RES in
transmission systems as it may cause a threat to network in terms of stability,
voltage regulation and THD issues. So, the transmission systems are
necessary to fulfill with strict technical and narrow frameworks to make sure
safe, reliable and efficient operation of overall network. Due to the
improvement of power electronics and digital control knowledge, the
63
transmission systems can now be keenly controlled to improve the system
operation. Though, the wide-ranging use of power electronics based apparatus
and non-linear loads at PCC produce harmonic currents, which may weaken
the quality of current as discussed in Guerrero et al (2004).
In modern years, a number of changes have been made in electrical
networks which inturn increases the contribution of the transmission system
in total energy production. In centralized systems, the features such as free
location in the network area with relatively small generated power and the
difference of generated power reliant on the flexibility and changeability of
primary energy as discussed in Wasiak & Hanzelka (2009).
In RES system, the inverter act as an active inductor for a certain
frequency to soak up harmonic current as studied in Borup et al (2001). In
real time process, the value of inductance of the network is difficult to
calculate and it could weaken the performance. Likewise, the shunt active
filters operate as an active conductance to moisture out the harmonics in
transmission network as studied in Jintakosonwit et al (2003).
An interfacing inverter based on the renewable energy system has
been discussed in Pinto et al (2007). In this method, the load and inverter
current sensing is required to balance the load current harmonics. The non-
linear load current harmonics may effect in voltage harmonics and can create
a serious problem in the electric network. Active power filters consumes
additional cost function to balance the load current harmonics and unbalanced
load. The main aim in the present research is to maximize the use of inverter
rating which is most of the time the resources are available or not due to
intermittent nature of RES.
64
The grid interfacing inverter has been utilized to perform functions
like transmission of active power yield from the renewable resources like
wind, solar, etc, current harmonics balance and current unbalance in case of
3-phase 4-wire system as discussed in Hanumantha & Bhanu (2012).
Furthermore, with sufficient control of grid-interfacing inverter, all the
objectives can be proficient either independently or concurrently.
The control diagram of grid- interfacing inverter for a 3-phase 4-
wire system shown in Figure 3.1 has been proposed by Singh et al (2011).
The current control technique used is the hysteresis controller. This approach
is considered as the motivation of the present research work and in order to
enhance the performance of the system, the present research work uses Space
Vector Pulse Width Modulation (SVPWM) as the contribution of the current
controller.
Figure 3.1 Block diagram representation of grid-interfacing inverter
control
LPF
PLL
Unit
Vector Template
Voltage Regulator
Hysteresis Current Controller
65
Thus, the present research work develops a SAPF interconnected
RES in transmission side with a SVPWM technique.
3.3 PULSE WIDTH MODULATION IN SAPF INTERCONNECTED
RES
Pulse-width modulation (PWM) is one of the widely used
controlling techniques in power electronics. An ideal PWM waveform with
zero rise time and fall time is a perfect means of operating the semiconductor
power devices. Instead of certain resonant converters, PWM signals have
been widely used in controlling most of the power electronic circuits. The
sudden rise and fall edges guarantee that the semiconductor power devices are
turned on or turned off as fast as possible to reduce the switching transition
time and the associated switching losses. Although other considerations, such
as parasitic ringing and Electromagnetic Interference (EMI) emission, may
require an upper limit on the turn-on and turn-off speed in practical
circumstances, the resulting finite rise and fall time can be eliminated in the
analysis of PWM signals.
The pulse frequency is one of the essential aspects of a PWM
approach. A Constant Frequency (CF) PWM signal can be generated through
comparing a reference signal, r(t), with a carrier signal, c(t), as depicted in
Figure. 3.2(a)
(a) Constant-Frequency (CF) PWM signal
r(t)
c(t)
66
(b) Sawtooth Carrier
Figure 3.2 (a) Constant-frequency PWM implemented by a comparator with different carrier signals (b) Sawtooth Carrier
The binary PWM output can be mathematically written as
(3.1)
Three types of carrier signals are
commonly used in constant-frequency PWM.
Figure 3.2(b) depicts the Sawtooth Carrier. The rising edge of
PWM arises at definite time instants while the falling edge is modulated as
the reference signal level varies. Hence, PWM method is also called constant-
frequency trailing-edge modulation.
Inverted Sawtooth Carrier is reported in Figure. 3.2 (c). The falling
edge of PWM arises at definite time instants while the position of the leading
edge is modulated as the reference signal level changes. The method is known
as constant-frequency leading-edge modulation.
Figure 3.2 (c) Inverted Sawtooth Carrier
67
Figure 3.2(d) is represented in Triangle Carrier. Both the leading
edge and the trailing edge of the PWM output are modulated. The rising and
falling edge of the triangle are generally symmetric, so that, the pulse is
centered within a carrier cycle when the reference is a constant. The method is
called constant-frequency double-edge modulation.
Figure 3.2 (d) Triangle Carrier
This PWM approach is the motivating factor in the present research
work to utilize the space vector pulse width modulation technique. The
principle idea is derived from the PWM technique and the limitations of the
PWM are overcome in the SVPWM technique used in the present research
work.
3.4 PROPOSED HARMONIC MITIGATION APPROACH
The present work utilizes a novel control strategy using active and
reactive current method ( ) and the SVPWM for improving the current
quality with lesser THD. The output of the ( ) control strategy is given
to the SVPWM block. Then, SVPWM produces the corresponding pulse
values. The Proposed Flow diagram of the SAPF design is shown in the figure
3.3.
68
Figure 3.3 Proposed Flow diagram of the SAPF design
The proposed architecture of SAPF with SVPWM and ( ) control
strategy is shown in Figure 3.4. In this research work, active and reactive
currents are separated based on the park transformation. Then, with )
control strategy, the reactive current is set to zero in order to minimize
the reactive current. Similarly, by controlling the dc link voltage error, active
current is attained. Thus, only the active currents will be available in the
system and when this active current is given to the SVPWM, it still eliminates
the 3rd and 5th harmonics which would result in better harmonic mitigation.
Grid
Non Linear Load
Control Strategy
Distorted Load Current
DC Voltage Regulator using
PI
Inverter
Supply Current ,
Reference SVPWM Current
If > required Power
Inverter Supply
PV Panel with Boost Converter
Grid Connection
Yes
No Resultant Source Current with Minimum
Harmonics
Injected Current
Supply Voltage
Reference Current ,
69
Figure 3.4 Proposed Architecture of Shunt Active Power Filter
3.4.1 Solar Array Characteristics
The solar array characteristics profoundly influence the converter
and control system as discussed in Bimal et al (1985). More generally, the
array cell static characteristics, as a function of light intensity and
temperature, are given by the following equations
(3.2)
Where
GRID
Control Strategies
Space Vector PWM
PV with Boost
converter
Bus Bus 2
Bus 3
Non Linear Load
A
B
C
a
b
c
70
(3.3)
(3.4)
(3.5)
All the symbols in Equations (3.2)-(3.5) can be defined as in the
following Table 3.1.
Table 3.1 Solar Characteristics
Symbols Explanation I cell output current V cell output voltage
cell saturation current T cell temperature in K
K/q
Boltzmann's constant divided by electronic 8.62 x 10-5 eV/K
TC cell temperature in °C K
short circuit current temperature coefficient 0.0017 A/°C
cell illumination (mW/cm2)
cell sort circuit current at 28°C and 100 m 2.52 A
light-generated current band gap for silicon = 1.11 eV
B = A ideality factors = 1.92 reference temperature = 301.18 K saturation current at = 19.9693 x 10-6 series resistance = 0.001
71
The converter which is connected at the array terminal can be
denoted by an equivalent resistive load at static condition. The intersection of
the load line with conductance slope G and the array VA - IA curve defines
the operating point and the corresponding dc power absorbed by the
converter.
For boosting up the voltage level from solar array, DC Voltage is
boosted up to 325 V ( ) to attain the peak voltage ( ) based on the
following formulation
(3.6)
Boost convertor is more preferable due to less number of devices
and simple control. Hence, in the present research work, boost converter has
been used.
3.4.2 Boost Converter Modelling and DC Link Capacitor Selection
Boost converter also called as high efficiency step-up converter
which has an output DC voltage greater than its input DC voltage. It consists
of two semiconductor switches and one storage element as discussed in
Kamatchi & Rengarajan (2013). Figure 3.5 shows the circuit diagram of the
boost converter. When the switch is closed, the inductor gets charged by the
PV panel and the energy is stored. The diode blocks the current flowing, so
that the load current remains constant which is being supplied due to the
discharging of the capacitor. When the switch is open, the diode conducts and
the energy stored in the inductor which in turn discharges and charges the
capacitor. Therefore, the load current remains constant throughout the
operation.
72
(3.7)
(3.8)
(3.9)
(3.10)
Figure 3.5 Circuit diagram of a boost converter
Inductance value
(3.11)
(3.12)
(3.13)
Where = Input Voltage
73
= Input Current
= Average Output Voltage
= Average Load Current
= Ripple Current of Inductor
= Ripple Voltage of Capacitor
The boost converter is used to maintain the constant output voltage
for all the conditions of temperature and variations in solar irradiance.
DC Link Capacitor Voltage Selection Formula
Where
should be more than the peak of the line voltage.
3.4.3 Shunt Active Power Filter Design
The active and the passive components are integrated to form active
filters and these filters needs an external power source as discussed in Sangu
et al (2011). Operational amplifiers are frequently used in the active filter
designs. These filters have high Q, and are able to attain resonance without
the use of inductors. Though, their higher frequency limit is restricted by the
bandwidth of the amplifiers used. Multiple element filters are typically built
designs of filters. Additional elements are required when it is desired to
74
develop some parameter of the filter such as stop-band rejection from pass-
band to stop-band.
A three-phase system provide for an inverter load has been chosen
to learn the operation of the APF system. From the experiential study, due to
the characteristics of non linear load of the power electronics loads, the THD
of the source current and the terminal voltage fall below the IEEE-519
standard. The principle of APF system is to infuse a current equal in
magnitude other than in phase opposition to harmonic current to get a purely
sinusoidal current wave in phase with the supply voltage as discussed in
Afonso et al (2001). Figure 3.6 shows the schematic diagram of three phase
four wire shunt active power filter with linear and nonlinear loads. Metal
Oxide Semiconductor Field-Effect Transistor (MOSFET) based on VSI is the
heart of APF system. A dc capacitor is used to distribute power for the VSI.
Intended for the successful operation of APF, capacitor voltage is supposed to
be 150 % of maximum line-line supply voltage. Since the PWM VSI is
assumed to be instant and considerably fast to track the compensation
currents, it is modeled as a current amplifier with unity gain.
Figure 3.6 Schematic diagram of three phase four wire shunt active power filter with linear & nonlinear loads
Nonlinear Load
Active Power Filter
75
Primary Design Considerations of SAPF
This section clearly discusses about the primary assumptions,
design consideration of SAPF.
1. The SAPF used in the present research is a three-phase AC/DC
converter, where the capacitor is the main energy storage
element and the inductors are used for the control of the filter
currents by means of the converter voltage. In figure
3.5, are the mains voltages, are the mains
currents are the load currents is the capacitor
voltage (DC BUS).
2. The mains voltages are co-sinusoidal of
frequency (50Hz or 60Hz) balanced and equilibrated
(3.14)
(3.15)
(3.16)
3. The sampling frequency and the PWM frequency are
supposed to be already chosen
4. The load currents are balanced and periodic of
frequency
(3.17)
76
where M represents the equal to infinity according to the
Fourier analysis. However, reduced number of harmonics is
considered for the compensation due to the limited bandwidth
of the controlled inverter.
5. Inductors are modeled as pure inductance .
6. The six-switches-bridge is supposed ideal.
7. The maximum current of the devices implementing the bridge
switches is . It is worth noting that several shunt active
filters can be parallel connected to the same load, providing a
appropriate coordinating strategy to increase the compensated
current.
8. The steady-state capacitor voltage must be kept inside the range
the upper bound depends on the kind of
capacitor chosen and on the number of series connected
capacitor banks. Hence, it can be assumed chosen before
starting the design procedure. The lower bound depends
on the controllability constraints.
9. The shunt active filter has to produce currents opposite to the
load distorted ones. It will be assumed that the control
techniques implemented are able to assure this behavior.
Shunt Active Filter model
The present research work utilizes the SAPF design model
discussed in Fabio & Andrea (2002). Let
77
be the arrays of, respectively : mains voltages, filter currents, voltages from
the node K to the half points of bridge legs , control inputs of the six-
switches bridge, . Then the filter equations can be
written, starting from inductor dynamics.
(3.18)
From the sum of the three scalar equations above, it can be found
that
(3.19)
That permits to define
(3.20)
can be assumed only 7 values at the time , which correspond to the
vertexes of the hexagon. The region included in this hexagon corresponds to
the that can be obtained as mean values in a PWM period as discussed in
Fabio & Andrea (2002). The status equations of the filter are:
(3.21)
78
(3.22)
It is useful to represent the model also in a d-q reference frame,
aligned to the mains voltage vector. In this frame mains voltages and load
currents can be written as
(3.23)
(3.24)
And the status of and becomes
(3.25)
(3.26)
Filter inductance design
The minimum value of inductance is evaluated based on the
methodology adopted in Fabio & Andrea (2002) as it is not evaluated with the
load. The utilization of the PWM techniques to obtain the reference values
causes current ripple, that must be kept less than a maximum value
in order to bound high frequency distortion.
(3.27)
(3.28)
79
Where the * indicates reference value and the ripple caused by
the PWM technique. Substituting these expressions in the equations (3.27)
(3.18) it will be obtained that
(3.29)
The ripple worst case is in the middle of a hexagon side. In
this circumstance, the peak to peak current ripple is considered in such a way
that the capacitor is constant in a PWM-period is given by
(3.30)
It must be less than the maximum ripple chosen
(3.31)
Unmodeled and uncertain dynamics in SAPF Design
Peak to peak current ripple, is based on the
PWM switching frequency , Maximum Voltage
. With higher switching frequency, THD can be reduced. Similarly for
the DC Link Capacitor Voltage Selection Formula, modulation index is
considered as 1.
3.5 CONTROL STRATEGY
Transformation of the phase voltages like , and the load
currents , , and - dinates are given in
80
equation (3.32 and 3.33). The main objective of active power filters is studied
in Montero et al (2007) is the harmonics there in the input currents. The
proposed structural design represents three phase three wire and it is
recognized with constant power control approach as discussed in Akagi
(2005).
(3.32)
(3.33)
In control strategy method, reference currents are attained
through instant active and reactive currents and of the non linear load. A
result follows alike the instant power theory, though load currents which
can be obtained from equation (3.33). Two stage transformations provide
relation between the motionless and rotating reference frame with active and
reactive current method. Mathematical relations is given below in equation
(3.34)
(3.34)
where , are the instantaneous - axis current references.
therefore this method makes the frequency autonomous by not including PLL
in the control circuit which is the main advantages of this control strategy. So,
synchronizing problems with disturbed and hazy conditions of main voltages
81
also avoided. Thus obtain large frequency operating limit
fundamentally by the cut-off frequency of Voltage Source Inverter (VSI) as
discussed in Soares et al (1997).
Figures 3.7 shows the park transformation and harmonic injection
circuit and control diagram for shunt active filter. The load currents and
are obtained from the park transformation and is facilitated to flow through
the high pass filter to eliminate the dc mechanism in the nonlinear load
currents. Butterworth filter and an alternative high pass filter (AHPF) are used
in the circuit. It can be attained through the low pass filter (LPF) of same
order and cut-off frequency merely difference between the input signal and
the filtered one. The frequency response of the Butterworth Filter
pass band is designed to have a frequency response which is as flat as
mathematically possible from 0Hz (DC) until the cut-off frequency at -3dB
with no ripples. Butterworth filters used in harmonic injecting circuit have
cut-off frequency equal to one half of the main frequency , with
this a small phase shift in harmonics and adequately high transient response
can be obtained. obtained from equation (3.6) is used as the zeroth frame.
82
Figure 3.7 Park Transformation and Harmonic Current Injection Circuit
3.6 DC VOLTAGE REGULATOR ( )
The process of voltage regulator on dc side is carried out by
Proportional Integral (PI) controller, inputs to the PI controller are, change in
dc link voltage ( ) and reference voltage ( ), on regulation of first
harmonic active current of positive sequence id1h+ it is feasible to control the
active power flow in the VSI and accordingly the capacitor voltage . The
Shunt Active Filter Control Circuit is shown in the Figure 3.8.
PI
Butterworth lowpass
filter
Butterworth lowpass
filter
Park transformation
abc-dq
Park
Transformation dq0-abc
+
-
+
+
+
+
+
- -
+
-1
SVPWM
83
Figure 3.8 Shunt Active Filter Control Circuit
Similarly, reactive power flow is controlled by first harmonic
reactive current of positive sequence iq1h+. Alternatively, the primary end of
the active power filters is to eliminate the harmonics caused by nonlinear
loads, so the current iq1h+ is always set to zero.
3.7 SPACE VECTOR PWM
The Space vector PWM is used for a two-level voltage source
inverter in linear region of operation as explained in Kerkman et al (1991). In
Voltage source inverter
SVPWM
Park transformation
DC Voltage Controller (PI)
Park transformation abc-dq and Harmonic
Current injection
AC mains
L
PV Panel with boost
Converter
, , ,
-
+
- +
+
-
Non Linear
84
space vector PWM six switching devices are present only three of them are
independent as the operation of two power switches of the same leg are
flattering. The grouping of these three switching states gives eight feasible
space voltage vectors.
The general output voltages of three-phase inverter using SVPWM
are given figure 3.9.
Figure 3.9 Output voltages of three-phase inverter
In the above figure, upper transistors are represented through S1,
S3, S5; lower transistors are represented through S4, S6, S2 and switching
variable vector: a, b, c.
S1 through S6 are the six power transistors that shape the output
voltage.
corresponding lower switch is turned Eight possible
combinations of on and off patterns for the three upper transistors (S1, S3, S5).
Line to line voltage vector [Vab Vbc Vca]t
+
-
a b c
b c
Point of Common Coupling (PCC)
85
(3.35)
Where switching variable vector
Line to neutral (phase) voltage vector [Van Vbn Vcn]t
(3.36)
The eight inverter voltage vectors (V0 to V7)
Figure 3.10 Eight inverter voltage vectors
The eight combinations, phase voltages and output line to line
voltages are shown in table. It is to be observed that the respective voltage
should be multiplied by .
86
Table 3.2 Switching Table of SVPWM
Voltage Vectors
Switching Vectors
Line to neutral voltage
Line to neutral voltage
a b c
0 0 0 0 0 0 0 0 0
1 0 0 2/3 -1/3 -1/3 1 0 -1
1 1 0 1/3 1/3 -2/3 0 1 -1
0 1 0 -1/3 2/3 -1/3 -1 1 0
0 1 1 -2/3 1/3 1/3 -1 0 1
0 0 1 -1/3 -1/3 2/3 0 -1 1
1 0 1 1/3 -2/3 1/3 1 -1 0
1 1 1 0 0 0 0 0 0
Principle of SVPWM
The space vectors form a hexagon with six definite hexagons. At a
given time, the inverter can produce only one space vector. In SVPWM, a two
active and a zero vectors can be selected to generate the preferred voltage in
each switching period.
Out of eight structures, six states generate a non-zero output voltage
and they are known as active voltage vectors and the remaining two structures
(states 0 and 7) generates zero output voltage and are known as zero voltage
vectors, different feasible switching states are shown in
Figure 3.11.
The space vector is a concurrent representation of all the three-
phase quantities as discussed in Kerkman et al (1991). It is a complex variable
and is function of time in contrast to the phasors. Phase-to-neutral voltages of
87
a star-connected load are most easily found by defining a voltage difference
between the star point n of the load and the negative rail of the dc bus N.
(3.37)
Since the phase voltages a star connected load sum to zero,
summation of Equation (3.37) yields
(3.38)
Substitution of (3.38) into (3.37) yields phase-to-neutral voltages of
the load in the following form:
Van = 2/3VaN - 1/3VbN - 1/3VcN (3.39)
Vbn = -1/3VaN + 2/3VbN - 1/3VcN (3.40)
Vcn = -1/3VaN - 1/3VbN + 2/3VcN (3.41)
The purpose of PWM is to control the inverter output voltage and
to reduce the THD. But, there are some drawbacks in PWM such as
Increase of switching losses due to high PWM frequency
Reduction of available voltage
EMI problems due to high-order harmonics
88
So, various PWM techniques have been developed to overcome the
above said drawbacks. In this research work, Space Vector PWM (SVPWM)
has been used.
Working of SVPWM in Harmonic Mitigation
SVPWM is used to reduce the current ripples which reduce the
Total Harmonics Distortion (THD). SVPWM considers the sinusoidal voltage
as a steady state amplitude vector at constant frequency. The PWM took
similar reference voltage Vref by a grouping of the eight switching patterns
(V0 to V7). A three-phase voltage vector is changed into a vector in the
motionless d-q coordinate frame which is equivalent to the spatial vector sum
of the three-phase voltage is called as coordinate transformation. The vectors
(V1 to V6) divide the plane into six sectors, each sector represents 60 degrees.
Basic switching vectors and Sectors
6 active vectors (V1,V2, V3, V4, V5, V6)
Axes of a hexagonal
DC link voltage is supplied to the load
Each sector (1 to 6): 60 degrees
2 zero vectors (V0, V7)
At origin
No voltage is supplied to the load
89
Figure 3.11 Basic switching vectors and sectors
The eight vectors as well as the zero voltage vectors can be
conveyed geometrically. All of the space vectors, in the diagram represent the
six voltage steps generated by the inverter with the zero voltages V0 (0 0 0)
and V7 (1 1 1) situated at the source. Space Vector PWM needs to average of
the adjacent vectors in each sector. Two adjacent vectors and zero vectors are
used to blend the input reference resolved for sector I. Using the suitable
PWM signals, a vector is produced.
Advantages of SVPWM
It produces less harmonic distortion in the output voltage or
currents in evaluation with sine PWM
It gives more efficient use of supply voltage in comparison with
sine PWM
Thus, the proposed approach uses SVPWM and ( ) control
strategy for reducing the ripple current in non-linear load conditions.
q axis
d axis
(010)
(100)
(011)
(001)
(101)
(000)
(111)
1
2 2
3
4
5
6
90
The overall algorithm of the proposed harmonic mitigation
approach is presented below.
) Control Strategy
From the main voltages, , is generated from Phase
Locked Loop (PLL) approach.
Due to the non linear load, load currents , , and are
generated.
Based on park transformation, actual main currents and
are generated.
Reference current is extracted from error control DC link
voltage regulator using PI controller.
The actual current and the reference current is
compared to extract the active voltage .
Similarly, actual current and the zero reference current
is compared to extract the reactive voltage
Inverse park transformation is applied to active voltage, ,
and to extract the reference voltages
The reference voltages are compared with
Reference currents are generated , , which are
given as input to SVPWM.
91
SVPWM
Then, from the reference currents, active voltage and
reactive voltage have been generated.
has been obtained.
Sector detection in SVPWM has been carried out
Switching table has been assigned based on the sector
detection
Then, the generated reference SVPWM voltage waveform is
compared with carrier triangular waveform
Then, the compared waveform has been fed to the inverter to
generate gate pulse.
This injected current will in turn assist the generation of
source current with minimized harmonic distortion.
3.8 SUMMARY
The present research work focuses on minimizing the harmonics
caused due to nonlinear loads. A novel control strategy for the SAPF has been
designed to improve the overall performance. The proposed algorithm uses
active and reactive current method ( ) control strategy and SVPWM.
The utilization of SVPWM helps in reducing the THD to a considerable
extent.
Recommended