DG FED MULTILEVEL INVERTER BASED D-STATCOM FOR VARIOUS LOADING CONDITIONS

International Journal on Cybernetics & Informatics (IJCI) Vol. 5, No. 1, February 2016
DOI: 10.5121/ijci.2016.5106 59
DG FED MULTILEVEL INVERTER BASED
D-STATCOM FOR VARIOUS LOADING
CONDITIONS
Naarisetti Srinivasa Rao1
and R. Kalyani2
1
Associate Professor, Department of Electrical & Computer Engineering
College of Engineering and Technology, Dilla University, Dilla, Ethiopia
2
Assistant Professor, Department of Electrical & Electronics Engineering, Mahaveer
Institute of Engineering and Technology, Bandlaguda, Telangana, India.
ABSTRACT
During the past few decades, power industries have proved that the adverse impacts on the PQ can be
mitigated or avoided by conventional means, and that technique using fast controlled force commutated
power electronics (PE) are even more effective. PQ compensators can be categorized into two main types.
One is shunt connected compensation device that effectively eliminates harmonics. The other is the series
connected device, which has an edge over the shunt type for correcting the distorted system side voltages
and voltage sags caused by power transmission system faults. The STATCOM used in distribution systems
is called DSTACOM (Distribution-STACOM) and its configuration is the same, but with small
modifications. Recent advances in the power-handling capabilities of static switch devices such as 3.3kV,
4.5kV, and 6.5kV Insulated Gate Bipolar Transistors (IGBTs) with voltage rating commercially available,
have made the use of the voltage source inverters (VSI) feasible for high-power applications. High power
and high-voltage conversion systems have become very important issues for the power electronic industry
handling the large ac drive and electrical power applications at both the transmission and distribution
levels. For these reasons, new families of multilevel inverters have emerged as the solution for working
with higher voltage levels. Multilevel inverters (MLI) include an array of power semiconductors and
capacitor voltage sources, the output of which generate voltages with stepped waveforms. These converter
topologies can generate high-quality voltage waveforms with power semiconductor switches operating at a
frequency near the fundamental. It significantly reduces the harmonics problem with reduced voltage stress
across the switch. This research work is mainly focusing on application of multilevel DSTATCOM for
power quality improvement in distribution system with integration of RES. Matlab/Simulink based model is
developed and simulation results are presented.
KEYWORDS
DSTATCOM, Cascaded H- Bridge Multilevel Inverter, Pulse Width Modulation, renewable energy sources
1. INTRODUCTION
"A power electronic based system and other static equipment that provide control of one or more
AC transmission system parameters to enhance controllability and increase power transfer
capability [3]." Among FACTS controllers, the shunt controllers have shown feasibility in term of
cost effectiveness in a wide range of problem-solving from transmission to distribution levels [1,
2]. For decades, it has been recognized that the transmittable power through
transmission/distribution lines could be increased, and the voltage profile along the
transmission/distribution line could be controlled by an appropriate amount of compensated
reactive current or power. Moreover, the shunt controller can improve transient stability and can

60
damp out power oscillation. Using a high-speed power converter, the shunt controller can further
alleviate or even cancel the flicker problem caused by electrical arc furnaces and regulate the
voltage levels within certain criteria [7].
In principle, all shunt-type controllers inject additional current into the system at the point of
common coupling (PCC). An impedance of the shunt controller, which is connected to the line
voltage, causes a variable current flow, and hence represents an injection of current into the line.
As long as the injected current is in phase quadrature with the line voltage, the shunt controller
only supplies or consumes variable reactive power. Among recently developed power converter
topologies, multilevel converters have become an important technology, and have been utilized in
high-power applications, particularly FACTS controllers [5, 6]. Several multilevel converter
topologies have been developed to demonstrate their superiority in such applications. With
converter modules in series, and with balanced voltage-sharing among them, lower-voltage
switches can possibly be used in high-voltage systems. Thus, the low-voltage-oriented insulated
gate bipolar transistor (IGBT) devices can be stacked for medium-voltage systems [3, 4]. For
higher-voltage applications, however, efforts have been made to use GTO-based devices for
multilevel converters.
In harmonic elimination, reactive power compensation, cascaded-multilevel VSCs with separated
DC capacitors are somewhat the most feasible topology for many reasons. The cascaded-
multilevel converter (CMC) is constructed with a number of identical H-bridge converters. This
modular feature makes the cascaded converter very attractive and simple in structure. The
cascaded converter topology not only simplifies hardware manufacturability, but also makes the
entire system flexible in terms of power capability. In addition, for the same power capability, the
cascaded converter requires fewer total components. Auxiliary components, such as the clamped
diodes and capacitors, are not required in the CMC topology [7, 8]. The CMC-based D-
STATCOM, however, challenges researchers to improve its dynamic responses and to balance its
excessive number of DC capacitor voltages. To date, several papers have discussed the
configurations and control strategies for the reactive power compensation systems that utilize
CMCs based on these previous works, an accurate model and an effective control technique
associated with the simple DC capacitor balancing strategy are elements important to achieving a
high-performance, stable, cost-effective CMC-based D-STATCOM [6, 8]. Finally the proposed
DSTATCOM is used for integrating the PV system.
2. CASCADED MULTILEVEL BASED VOLTAGE-SOURCE CONVERTERS FOR
D-STATCOM APPLICATION
An attractive to the seven-level converter is a multilevel VSC. Without semiconductor devices
connected in series, the multilevel converters show feasible capability of clamping the voltages
across individual devices below their limitations. This allows the recent semiconductor devices to
be utilized in higher-voltage applications without incurring voltage sharing problems. Another
significant advantage of the multilevel configuration is the harmonic reduction in the output
waveform with very low switching frequency, line frequency for example. Besides the high-
voltage capability, multilevel converters also provide other advantages over the seven-level
converters. The multilevel converter in the four following aspects: output voltage quality, DC
capacitance requirement, losses and power capacity. The schematics of the cascaded multilevel-
converter are shown in Fig.1. Respectively.
With its modularity and flexibility, the CMC shows superiority in high-power applications,
especially shunt and series connected FACTS controllers. The CMC synthesizes its output near
sinusoidal voltage waveforms by combining many isolated voltage levels. With a sufficiently
high number of voltage levels, a premium-quality output waveform with fast system response can

61
be achieved by switching the main power semiconductor devices only at the line frequency. This
naturally minimizes the entire system losses and the output filter requirement. In addition, by
adding more H-bridge converters, the amount of Var can simply increased without redesign
Vdca
+
-
Sa5
Sa8 Sa6
Sa7
Vdca
+
-
Sa1 Sa3
Sa4 Sa2
Vdcb
+
-
Sb5
Sb8 Sb6
Sb7
Vdcb
+
-
Sb1 Sb3
Sb4 Sb2
Vdcc
+
-
Sc5
Sc8 Sc6
Sc7
Vdcc
+
-
Sc1 Sc3
Sc4 Sc2
N
Va
Vb Vc
Vdca
+
-
Sa9
Sa12 Sa10
Sa11
Vdcb
+
-
Sb9
Sb12 Sb10
Sb11
Vdcc
+
-
Sc9
Sc12 Sc10
Sc11
Fig.1. Schematic Diagram of 7-Level Cascaded Multilevel Converter
the power stage, and build-in redundancy against individual H-bridge converter failure can be
realized. Moreover, a three-phase CMC topology is essentially composed of three identical phase
legs of the series-chain of H-bridge converters, which can possibly generate different output
voltage waveforms and offers the potential for AC system phase-balancing. This feature is
impossible in other VSC topologies utilizing a common DC link. By nature of this topology,
however, the CMC is impossible to apply in intertie or back-to back applications, such as
universal power flow controllers (UPFC). Fortunately, with energy storage systems, the CMC can
now successfully overcome this limitation. This combination also enhances recent low-voltage
energy-storage system technology for high-voltage applications. A great combination of the D-
STATCOM concept and the CMC topology is a promising controller in the modern FACTS
technology.
3. CONTROL STRATEGY
IRP theory was initially proposed by Akagi. This theory is based on the transformation of three
phase quantities to two phase quantities in α-β frame and the calculation of instantaneous active

62
and reactive power in this frame. A basic block diagram of this theory is shown in fig.2. Sensed
inputs are fed to the controller, and these quantities are
processed to generate reference current commands , , ), which are fed to a pulse width
modulation (PWM) signal generator to generate final switching signals fed to the D-STATCOM;
therefore this block works as controller for D-STATCOM.
abc
calculation
calculation
PI controller
HPF
LPF
Reference
current
Extraction
α,β
abc
Unipolar current
controlled PWM
controller
+
-
isa
isb
isc
vsabc
iLabc
From
Sensors
α,β
Vα,β
iα,β
P
Q
iα*
iβ*
isa*
isb*
isc*
Sa1 Sa8 Sb1 Sc1 Sc8
+
-
Vdc
Vdc*
Gating Pulses for Three Single
Phase Multilevel Voltage sources
converter
Sb8
Park’s
Transformation
Inverse Park’s
Transformation
Fig.2. Block Diagram of Proposed Control Strategy- Instantaneous Real & Reactive Power Theory
The system terminal voltages are given as
)
(1)
And the respective load currents are given as
(2)
In a-b-c coordinates, a, b, and c axes are fixed on the same plane, apart from each other by 2π/3.
The instantaneous space vectors va and iLa are set on the “a” axis, and their amplitude varies in
positive and negative directions with time. This is true for the other two phases also. These
phasors can be transformed into α–β coordinates using Park’s transformation as follows:
(3)
(4)

63
Where α and β axes are the orthogonal coordinates. Conventional instantaneous power for three-
phase circuit can be defined as
(5)
Where p is equal to conventional equation
(6)
Similarly, the IRP is defined as
(7)
Therefore, in matrix form, instantaneous real and reactive power are given as
(8)
The α-β currents can be obtained as
(9)
Where
(10)
Instantaneous active and reactive powers and can be decomposed into an average (dc) and an
oscillatory component.
(11)
Where and are the average (dc) part and and are the oscillatory (ac) part of these real and
reactive instantaneous powers. Reference source currents are calculated to compensate the IRP
and the oscillatory component of the instantaneous active power. Therefore, the reference source
currents and in α-β coordinate are expressed as
(12)
Theses currents can be transformed in a-b-c quantities to find the reference currents in a-b-c
coordinates using inverse transformation.
(13)
Where is the zero sequence components, which is zero in three- phase three wire system.
4. PHOTOVOLTAIC SYSTEM

64
In the crystalline silicon PV module, the complex physics of the PV cell can be represented by the
equivalent electrical circuit shown in Fig. 4. For that equivalent circuit, a set of equations have
been derived, based on standard theory, which allows the operation of a single solar cell to be
simulated using data from manufacturers or field experiments.
The series resistance RS represents the internal losses due to the current flow. Shunt resistance
Rsh, in parallel with diode, this corresponds to the leakage current to the ground. The single
exponential equation which models a PV cell is extracted from the physics of the PN junction and
is widely agreed as echoing the behavior of the PV cell
(14)
The number of PV modules connected in parallel and series in PV array are used in expression.
The Vt is also defined in terms of the ideality factor of PN junction (n), Boltzmann’s constant
(KB), temperature of photovoltaic array (T), and the electron charge (q). Applied a dynamical
electrical array reconfiguration (EAR) strategy on the photovoltaic (PV) generator of a grid-
connected PV system based on a plant-oriented configuration, in order to improve its energy
production when the operating conditions of the solar panels are different. The EAR strategy is
carried out by inserting a controllable switching matrix between the PV generator and the central
inverter, which allows the electrical reconnection of the available PV modules.

65
Fig.3.Schematic diagram of non-linear load with cascaded seven level inverter D-STATCOM
Fig.4 Equivalent electrical circuit of a PV module
5. DG INTEGRATION WITH DSTATCOM

66
Distributed generation (or DG) generally refers to small scale electric power generators that
produce electricity at a site close to customers or that are tied to an electric distribution system.
Distributed generators include, but are not limited to synchronous generators, induction
generators, reciprocating engines, micro turbines (combustion turbines that run on high energy
fossil fuels such as oil, propane, natural gas, gasoline or diesel), combustion gas turbines, fuel
cells, solar photovoltaic’s, and wind turbines. Globally, the long-term technical potential of wind
energy is believed to be five times total current global energy production, or 40 times current
electricity demand. This could require wind turbines to be installed over large areas, particularly
in areas of higher wind resources. Offshore resources experience average wind speeds of ~90%
greater than that of land, so offshore resources could contribute substantially more energy.
Vdca
+
-
Sa5
Sa8 Sa6
Sa7
Vdca
+
-
Sa1 Sa3
Sa4 Sa2
Vdcb
+
-
Sb5
Sb8 Sb6
Sb7
Vdcb
+
-
Sb1 Sb3
Sb4 Sb2
Vdcc
+
-
Sc5
Sc8 Sc6
Sc7
Vdcc
+
-
Sc1 Sc3
Sc4 Sc2
N
Va
Vb Vc
Vdca
+
-
Sa9
Sa12 Sa10
Sa11
Vdcb
+
-
Sb9
Sb12 Sb10
Sb11
Vdcc
+
-
Sc9
Sc12 Sc10
Sc11
PV
PV
PV
PV
PV
PV
PV
PV
PV
Fig.5.schematic diagram of DG integration with seven level inverter
6. MATLAB/SIMULATION RESULTS
Here the simulation is carried out by two cases
Case.1. Balanced Non-linear load with seven level cascaded multilevel D-STATCOM.
Case.2. Un-balanced Non-linear load with seven level Cascaded multilevel D-STATCOM.
Case1. Balanced Non-linear load with seven level cascaded multilevel D-STATCOM.

67
Fig.6.Matlab/Simulink model of balanced non-linear load with seven level cascaded multilevel D-
STATCOM
Figure 7: Source voltage, Source current, Load current
Figure 7 shows the source voltages, source currents and load currents respectively with balanced
non-linear load and Cascaded Multilevel Seven level.
Figure 8: Power Factor with balanced non-linear load
Figure 8 shows the power factor waveforms of the designed system with balanced non-liner load
seven level cascaded multilevel D-STATCOM .The waveform clearly shows that there is no unity
power factor.

68
Fig.9.Seven Level output voltage
Figure.9.Shows the Seven level output voltage, when system is connected to cascade seven level
multilevel D-STATCOM.
Figure 10: Harmonic spectrum of Phase-A Source current for seven level D-STATCOM
Figure10 shows the harmonic spectrum of Phase –A Source current for balanced non-linear load
with cascaded Multilevel Seven level D-STATCOM. The THD of source current is 5.66%.
Figure 11: Harmonic spectrum of Phase-A load current for seven level D-STATCOM

69
Figure11 shows the harmonic spectrum of Phase –A Load current for balanced non-linear load
with cascaded Multilevel Seven level D-STATCOM. The THD of source current is 30.26%.
Case2. Un-balanced Non-linear load with seven level Cascaded multilevel D-STATCOM.
Figure.12.Matlab/Simulink model of unbalanced non-linear load with seven level cascaded multilevel D-
STATCOM
Figure 13: Source voltage, Source current, Load current
Figure-13 shows the source voltages, source currents and load currents for unbalanced non-linear
load with Cascaded Multilevel Seven level.
Figure 14: Power Factor with unbalanced non-linear load

70
Figure 14 shows the power factor waveforms of the designed system with unbalanced non-liner
load seven level cascaded multilevel D-STATCOM.
Figure 15: Harmonic spectrum of Phase-A Source current for seven level D-STATCOM
Figure15 shows the harmonic spectrum of Phase –A Source current for unbalanced non- linear
load with cascaded Multilevel Seven level D-STATCOM. The THD of source current is 3.26%.
Figure 16: Harmonic spectrum of Phase-A Load current for seven level D-STATCOM
Figure16 shows the harmonic spectrum of Phase –A Load current for unbalanced non- linear load
with cascaded Multilevel Seven level D-STATCOM. The THD of load current is 16.71%.
7. CONCLUSION
Seven level cascaded multilevel voltage source inverter based D-STATCOM using instantaneous
real-power controller is found to be an effective solution for power line conditioning. D-
STATCOM with the proposed controller reduces harmonics and provides reactive power
compensation due to balanced non-linear and unbalanced non-linear load currents; as a result

71
source current(s) become sinusoidal and unity power factor is also achieved under both transient
and steady state conditions. The proposed instantaneous real-power controller uses reduced
computation for reference current calculations compared to conventional approach. The cascaded
inverter switching signals are generated using triangular-sampling current controller; it provides a
dynamic performance under transient and steady state conditions.
REFERENCES
[1] Ghosh and G. Ledwich, Power Quality Enhancement using Custom Power Devices, Kluwer
Academic Publisher, Boston, MA, 2002.
[2] Ghosh and G. Ledwich, “Load Compensating DSTATCOM in weak AC systems,” IEEE Trans.
Power Delivery, vol.18, No. 4, pp. 1302-1309, Oct. 2003.
[3] J. S Lai and F. Z. Peng, “Multilevel converters A new breed of power converters,” IEEE Trans.
Industry Applications, vol.32, no.3,
[4] F. Z. Peng., J. W. McKeever and, D .J. Adams, “A power line conditioner using cascade multilevel
inverters for distribution system,” IEEE Trans. Industry Applications, vol.34, no.6, pp. 1293-1298,
Nov.-Dec.1998.
[5] W. Liqiao, L. Ping, L. Jianlin and Z. Zhongchao, “Study on shunt active power filter based on
cascaded multilevel converters,” 35th IEEE Power Electr. Spec. Conf. (APEC), vol.5, pp. 3512-3516,
20-25 June 2004.
[6] H. Gruning,”Power Electronics Circuit Arrangement having Plural Power Converters”, U.S. Patent
No. 5,805,487, 1998.
[7] G. Carrara, S. Gardelta, M. Marchesoni,”A new multilevel PWM method: theoretical analysis”, IEEE
Trans. On power electronics Vol. 7. No. 3 , July, pp.497-505, 1992.
[8] V. Agelidis, M. Calais,” Application specific harmonic performance evaluation of multicarrier PWM
techniques”, IEEE-PESC’98 Conference Record, pp. 172-178, 1998.

DG FED MULTILEVEL INVERTER BASED D-STATCOM FOR VARIOUS LOADING CONDITIONS

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