60

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.