 Open Access
 Total Downloads : 2157
 Authors : S. Siva Ramakrishna Reddy, S.Sateesh
 Paper ID : IJERTV1IS6513
 Volume & Issue : Volume 01, Issue 06 (August 2012)
 Published (First Online): 30082012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Comparison of the Performance of Three Level Inverter Based STATCOM with Sinusoidal and Space Vector PWM Techniques
S. Siva Ramakrishna Reddy PG Student
MIC College of Technology
Email: seelam.reddy246@gmail.com
S.Sateesh Associate Professor
MIC College of Technology
Email: sateeshsukhavasi@gmail.com
Abstract
This paper compares the performance of diode clamped three level inverter based STATCOM with sinusoidal and space vector PWM techniques. Space Vector Pulse Width Modulation (SVPWM) technique is the most advantageous technique than the conventional sinusoidal PWM technique for a multilevel inverter. The space vector pulse width modulation technique is an advanced, computationintensive PWM method and is possibly the best among the all other PWM techniques. Unlike the conventional PWM method the space vector pulse width modulation technique produces lesser amount of harmonics. The total harmonic distortion (THD) of the output waveform is reduced by 47% than the sinusoidal PWM. The switching losses are reduced by 30%. The modelling of complete system is developed in MATLAB Simulink.
Keywords STATCOM, SVPWM, PWM, THD, IGBT
Introduction
The output obtained from a twolevel inverter is not a pure sinusoidal waveform. It is due to the presence of harmonics in the inverter output voltage which may cause heavy losses and may lead to low efficiency of the induction motors or any other applications which may take the supply from the inverter. So, there is a need for us to reduce these harmonics. The harmonics in a two level inverter is reduced by increasing the switching frequency. But the switching frequency is restricted by the switching losses in high power applications. In such applications multilevel inverters have been widely used in recent years for the advantage of low harmonic output at low switching frequency. At the same time low blocking in the switching devices can be achieved. The more the number of levels of the output voltage the lesser will be the harmonic content. Multi level inverters have advantages of good power quality, good electromagnetic compatibility, low switching losses, high voltage capability. These multi
level inverters are used in the active rectifiers and the FACTS applications. The multi level inverters synthesize several voltage levels from the various levels of the DC input. A near sinusoidal voltage waveform can be generated from the various levels of the DC input. They have become attractive in the high power and high voltage applications. By using the multilevel inverters the stress on each device is reduced proportional to the number of the output levels present. With several levels in the output waveform the switching dv/dt stresses are reduced, and hence the lifetime of motor and cables are increased. By using a multilevel inverter the power rating of the equipment can enhanced without any dangerous consequences.
1. Diode Clamped ThreeLevel Inverter
Fig. 1 Power Circuit for ThreePhase ThreeLevel Inverter
Fig. 1 shows the basic circuit for the diode clamped threelevel inverter. The circuit employs 12 power switching devices (Sa1Sa4) and 6 clamped diodes (D1 D6). And the dcbus voltage is split into threelevel by two seriesconnected bulk capacitors can be defined as the neutral point O. as the result of the diodeclamped the dcbus voltage Vdc/2. Thus, the voltage stress of the switching device is greatly reduced. The output voltage Vao has three different states: +Vdc/2, 0, Vdc/2. Here takes phase A as an example for voltage. For voltage Vdc/2, Sa3 and Sa4 need to be turned on. We can define these states as 2, 1, and 0, respectively. Then, the switching variable Sa is shown in table1. be similar to threephase twolevel inverter, the switching states of each bridge leg of threephase threelevel inverter is described by using switching variables Sa, Sb and Sc. Whereas the difference is that, in threelevel inverter, each bridge leg has three different switching states.
Table 1 Switching Variable of Phase A
Vao 
Sa1 
Sa2 
Sa3 
Sa4 
Sa 
+Vdc/2 
ON 
ON 
OFF 
OFF 
2 
0 
OFF 
ON 
ON 
OFF 
1 
Vdc/2 
OFF 
OFF 
ON 
ON 
0 
Using switching variable Sa and DC bus voltage Vdc, the output phase voltage Vao is obtained as follows:
Van = (Sa1)*Vdc/2 (1)
And the output line voltage of phase A and B can be expressed as follows.
Vab = Vao Vbo = 1/2*Vdc (SaSb) (2)

Static Compensator(STATCOM)
The STATCOM has a characteristic similar to the synchronous condenser, but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways, such as better dynamics, a lower investment cost and lower operating and maintenance costs. A STATCOM is build with Thyristors with turnoff capability like GTO or today IGCT or with more and more IGBTs. The static line between the current limitations has a certain steepness determining the control characteristic for the voltage.
The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to the SVC. This means, that even during most severe contingencies, the STATCOM keeps its full capability.
Figure 2. STATCOM structure and voltage / current characteristic
STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either GateTurnoff Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices. The STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es then lagging or inductive VARS are produced.

Controlling Methods for an Inverter
In many industrial applications, it is often required to vary the output voltage of the inverter due to the following reasons:

To compensate for the variations in the input voltage.

To compensate for the regulation of the inverters.

To supply some special loads which need variation of voltage with frequency, such as an induction motor.
The inverter output voltage can be controlled by various following techniques.

Single pulse width modulation

Multi pulsewidth modulation.

Minimum ripple current modulation

Sinusoidal pulse width modulation (SINE
subject of intensive research during the last few decades. Especially, the spacevector PWM is used for threephase converter applications. Here we mainly consider the carrier based PWM approaches that are often applied to the single phase applications.
Figure 3 is a general scheme of PWM modulation. In order to produce a sinusoidal voltage at desired
PWM).

Selected harmonic elimination PWM (SHE
frequency, say f1, a sinusoidal control signal V
at
control
PWM). the desired frequency (f1) is compared with a triangular

Space vector pulse width modulation (SVPWM).
waveform V
carrer
as shown in Fig. 3(a), at each compare
match point, a transition in PWM waveform is

Single Pulse Width Modulation
generated as shown in Fig. 3(b). When V
control
is greater
In singlepulsewidthmodulation control, there is
than V , the PWM output is positive and When
carrier
only one pulse per half cycle and the width of the pulse
is varied to control the inverter output voltage. Here the
V
control
is smaller than V
, the PWM waveform is
carrier
gating signals are generated by comparing a rectangular reference signal of amplitude (Ar) with a triangular carrier wave of amplitude (Ac). The ratio of Ar to Ac is a control variable and is defined as the amplitude modulation index or modulation index M.

Multi Pulse Width Modulation
The harmonic contents can be reduced by using several pulses in each halfcycle of the output voltage. The generation of gating signals for turning on and off of the switching device is made by comparing a reference signal with a triangular carrier wave. The modulation index controls the output voltage. This type of modulation is also known as uniform pulsewidth modulation (UPWM).
The number of pulses per half cycle is found from p=Fc/2Fo= mf/2(mf is the frequency modulation ration or carrier ratio).

Minimum Ripple Current Modulation
One of the disadvantages of the harmonic injection PWM is that the elimination of lower order harmonics considerably boosts the next higher level of harmonics. Since the harmonic loss in a machine is dictated by the ripple current, it is this parameter that should be minimized instead of emphasizing the individual harmonics.
negative.
Fig. 3(a) Reference and carrier wave Fig. 3(b) Pulses generated on comparison
The frequency of triangle waveform V
carrier
establishes the inverters switching frequency fs. We define the modulation index mi as follows:
mi = Vcontrol / Vtri (3)
where V
control
is the peak amplitude of the control signal
and V
tri
is the peak amplitude of the triangle signal

Sinusoidal Pulse Width Modulation
Sinusoidal Pulse width modulation (PWM) techniques are effective means to control the output voltage frequency and magnitude. It has been the
(carrier). Also the frequency modulation ratio is defined as
mf = fs / f1 (4)
mf is the ratio between the carrier and control frequency. The fundamental component (Vout)1 of the H bridge output voltage (Vout)1 has the property as depicted in equation below in a linear modulation region:
(Vout) 1 = mi * Vd mi 1.0 (5)
The equation (3) shows that the amplitude of the fundamental component of the output voltage varies linearly with the modulation index. The mi value from zero to one is defined as the linear control range of sinusoidal carrier PWM.

Selected Harmonic Elimination PWM ( SHE PWM)
In the selected harmonic elimination PWM method the undesirable lower order harmonics of a square wave can be eliminated and the fundamental voltage can be controlled .

Space Vector Pulse Width Modulation
Space Vector Pulse Width Modulation (SVPWM) technique is the most advantageous technique than the conventional sinusoidal PWM technique for a multilevel inverter. The space vector pulse width modulation technique is an advanced, computation intensive PWM method and is possibly the best among the all other PWM techniques. Unlike the conventional PWM method the space vector pulse width modulation technique produces lesser amount of harmonics. The total harmonic distortion (THD) of the output waveform is reduced by 47% than the sinusoidal PWM. The switching losses are reduced by 30%. The output crest voltage is increased by 1.115 times than that of the conventional sinePWM method. If the inverter is used for the drives then this method gives higher torque and higher efficiency for the motors. It gives 15% better utilization of the DC bus voltage. It also reduces the switching as well as the commutation losses. It gives the maximum output voltage at the rated frequency. It also limits the duty cycle to the two thirds than the sinePWM. On the whole it improves the inverter efficiency. The maximum phasetocenter voltage by sinusoidal and space vector PWM are respectively;
Vmax = Vdc/2 for Sinusoidal PWM Vmax = Vdc/3 for Space Vector PWM where Vdc is DCLink voltage.
This means that Space Vector PWM can produce about
15 percent higher than Sinusoidal PWM in output voltage. The Figure 2 shows the variation of the output voltage with sinePWM and SVPWM techniques.
Fig. 4 Comparison of SinePWM and SVPWM
The basic principle of SVPWM depends on synthesizing the reference voltage vector by time averaging of the three nearest vector produced by the inverter. The reference voltage vector is the required command voltage which should be given to the application as required. The state of the inverter is nothing but a condition of the devices of the inverter which are whether on/off. Space vector pulse width modulation technique is based on the approximation of a rotating reference voltage space vector. The rotating reference voltage space vector in question represents the spatial vector sum of the threephase voltage.
The implementation of the SVPWM technique involves many steps. They are mainly

Transformation of 3phase to 2phase.

Calculating the space vector voltage.

Identifying the three nearest vectors

Calculation of the dwelling times on the three nearest vectors.

Determination of the switching instants.

Giving the pulses to the inverter devices.



Sinusoidal PWM For a ThreeLevel
Inverter
If the fundamental output voltage and corresponding power level of the PWM inverter are to be increased to a high value, the dc link voltage Vdc must be increased and the devices must be connected in series. By using matched devices in series, static voltage sharing may be somewhat easy, but dynamic voltage sharing during switching is always difficult. The problem may be solved by using a multilevel inverter or neutral point clamped (NPC), inverter..
Since the operation of all the phase groups is essentially identical, consider only the operation of the halfbridge for phase A. A pair of devices with bypass diodes is connected in series with an additional diode connected between the neutral point and the center of the pair as shown. The devices Sa1 and Sa2 function as main devices(like a twolevel inverter), the Sa2 and Sa3 function as auxiliary devices which help to clamp the output potential to the neutral point with the help of clamping diodes D1 and D2. All the PWM techniques discussed so far can be applied to this inverter. The main devices (Sa1 and Sa4) generate the Va0 wave, whereas the auxiliary devices (Sa3 and Sa2) are driven complementary to the respective main devices. With such control, each output potential is clamped to the neutral potential in the off periods of the PWM control. Evidently, the positive phase current +ia will be carried by devices D1 and Sa2 at the neutral clamping condition. On the other hand, negative phase current ia will be carried by D1 and D2 when Vao is positive, by Sa3 and Sa4 when Vao is negative, and by Sa3 and diode at the neutral clamping condition. This operation mode gives three voltage levels (+0.5Vdc, 0, and – 0.5Vdc) at the Vao wave as shown in the figure of phase voltage below. Like wise the wave forms for all the other phases are generated and the resultant line line voltages are obtained.
The implementation of a threelevel inverter by sinePWM is carried out by the same principle as that of a twolevel inverter. Here we have two carrier waves are these two carrier waves are compared with the single sinusoidal wave and corresponding pulses are generatedwhich are to be supplied to the inverter gate devices. And for the other phases the sinusoidal wave is displaced by an angle 2/3 and 4/3.

Space Vector Pulse Width Modulation For ThreeLevel Inverter
There are altogether 27 switching states in a diode clamped threelevel inverter. They correspond to 19
voltage vectors (V0 to V18) whose positions are fixed. These space voltage vectors can be classified into 4 groups: large voltage vectors (V13, V14, etc…), medium voltage vectors (V7,V8,etc..) small voltage vectors (V1,V2, etc..), and zero voltage vectors (V0).
Fig. 5 Space vector hexagon for threelevel inverter
The plane can be divided into 6 major triangular sectors (16) enclosed by solid lines by the large voltage vectors and zero voltage vector. Each major section represents pi/3 of the fundamental cycle. Within each major sector, there are 4 minor triangular sectors (enclosed by the dotted lines). There are totally 24 minor sectors in the plane. And the vertices of these sectors represent the voltage vectors. Notice table 2, each small voltage vector and zero voltage vector have 2 and 3 redundant switching states, respectively. This will be analyzed in the later section.
In threephase threelevel inverter, when the rotating voltage vector falls into one certain sector,
SWITCHING STATES 
Sa 
Sb 
Sc 
VOLTAGE VECTORS 
Table 2. 27 states for a threelevel inverter
S27 
2 
0 
2 
V18 
S1 
0 
0 
0 
V0 
S2 
1 
1 
1 
V0 
S3 
2 
2 
2 
V0 
S4 
1 
0 
0 
V1 
S5 
1 
1 
0 
V2 
S6 
0 
1 
0 
V3 
S7 
0 
1 
1 
V4 
S8 
0 
0 
1 
V5 
S9 
1 
0 
1 
V6 
S10 
2 
1 
1 
V1 
S11 
2 
2 
1 
V2 
S12 
1 
2 
1 
V3 
S13 
1 
2 
2 
V4 
S14 
1 
1 
2 
V5 
S15 
2 
1 
2 
V6 
S16 
2 
1 
0 
V7 
S17 
1 
2 
0 
V8 
S18 
0 
2 
1 
V9 
S19 
0 
1 
2 
V10 
S20 
1 
0 
2 
V11 
S21 
2 
0 
1 
V12 
S22 
2 
0 
0 
V13 
S23 
2 
2 
0 
V14 
S24 
0 
2 
0 
V15 
S25 
0 
2 
2 
V16 
S26 
0 
0 
2 
V17 
adjacent voltage vectors are selected to synthesize the desired rotating voltage vector based on the vector synthesis principle, resulting in threephase PWM waveforms. By the examination of the phase angle and the magnitude of a rotating reference voltage vector V*, the sector wherein V* resides can be easily located.
8. Simulation Results
Fig. 6 shows the simulink diagram of considered system without STATCOM. Three phase source is connected Fixed RL Load through three phase VI measurement block. Three phase fault is created during the time interval 0.1 to 0.2 sec.
Fig. 6 Simulink Diagram of Considered System
Three phase output voltages and currents at source and harmonic analysis of currents are shown in Fig. 7 and Fig. 8 respectively.
Fig. 7 Three phase voltages and currents at source
Fig. 8 Harmonic analysis of sourece three phase
currents
Three phase output voltages and currents at source and at STATCOM and harmonic analysis of currents with sinusoidal pulse width modulation are shown in Fig. 9 , Fig. 10 and Fig. 11 respectively.
Fig. 9 Three phase voltages and currents at source
Fig. 10 Three phase voltages and currents at STATCOM
Fig. 11 Harmonic analysis of sourece three phase
currents
Three phase output voltages and currents at source and at STATCOM and harmonic analysis of currents
with sinusoidal pulse width modulation are shown in Fig. 12 , Fig. 13 and Fig. 14 respectively.
Fig. 12 Three phase voltages and currents at source
Fig. 13 Three phase voltages and currents at STATCOM
Fig. 14 Harmonic analysis of sourece three phase
currents

Conclusions
The space vector pulse width modulation technique is an advanced, computationintensive PWM method and is possibly the best among the all other PWM techniques. Unlike the conventional PWM method the space vector pulse width modulation technique produces lesser amount of harmonics. The total harmonic distortion (THD) of the output waveform is reduced by 47% than the sinusoidal PWM. The switching losses are reduced by 30%.Without FACTS DEVICES, the lineline voltages drooped from 300Vphph to nearly 5Vphph and current reached from about 10A to 20A. With FACTS DEVICES, the lineline voltages maintained 300Vphph. Excess current and voltage generated and absorbed by STATCOM.

References

Acha, V.G. Agelidis, O. AnayaLara and T.J.E. Miller, Power Electronic Control in Electrical Systems, England, Newnes, 2002.

Ghosh and G.Ledwich, Power quality enhancement using custom power devices, London, Kluwer Academic Publishers, 2002.

V.R. Dinavahi, M.R. Iravani and R. Bonert, Realtime digital simulation and experimental verification of a D STATCOM interfaced with a digital controller, International Journal of Electrical Power & Energy Systems,Vol. 26, No. 9, Nov. 2004, pp. 703713.

Akagi, Y. Kanazawa and A. Nabae, Generalized theory of the instantaneous reactive power in threephase circuits, in Proc. of IEEE IPEC, 1983, pp. 821827.

D.M. Divan, S. Bhattacharya and B. Banerjee, Synchronous Frame Harmonic Isolator using Active Series Filter, in Proc. of European Power Electronic Conference, 1991, pp. 30303035.

G.D. Marques, A comparison of active power filter control methods in unbalanced and nonsinusoidal conditions, in Proc. of IEEE IECON, Vol. 1, 1998, pp. 444 449.

V. Soares, P. Verdelho and G. D. Marques, Active Power Filter Control Circuit Based on Instantaneous Active and Reactive Current idiq Method, in Proc. of IEEE PESC, 1997, pp. 1061101.

P. K. Dash, D.P. Swain, A.C. Liew and S. Rahman, An adaptive linear combiner for online tracking of power system harmonic, IEEE Trans. on Power ystems, January 1996, pp. 2125.

N. Pecharanin, An application of neural network to harmonic detection in active filter, in Proc. of WCCI ICNN94, 1994, Vol. 6, No 684, pp. 3756.

Pechanranin, H Uitsui and M. Sone, Harmonic detection by using neural network, in Proc. of IEEE International Conference on Neural Network, 1995, Vol. 2, pp. 923926.

B.N. Singh, A. Chandra and K. AlHaddad, DSPbased indirectcurrent controlled STATCOM. I. Evaluation of current control techniques, IEE Proceedings Electric Power Applications, Vol. 147, no 2, March 2000, pp.107 – 112.