Gunnam Durga Devi* et al. / (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH
Volume No. 1, Issue No. 5, August -September 2013, 438 -443.
GUNNAM DURGA DEVI
PG Scholar
NCET, Vijayawada.
SK GOUSE BASHA
Assistant Professor
NCET, Vijayawada.
FIROZALI MOHAMMED
Associate Professor
NCET, Vijayawada.
ANYAM NARESH
PG Scholar
NCET, Vijayawada.
Abstract - Multi-level converters have tremendous applications in the power industry and specially cascaded Hbridge (CHB) converters have a vital role in high power applications. They present a new set of features that are
well suited for real and reactive power compensation. The distinctive feature of the multilevel inverters is to provide
high switching frequencies with low switching losses. In this paper, power balance analysis of single phase
cascaded H bridge converter is presented. Power delivering from source to load or load to grid is analyzed by
required ac output voltage and from de link voltages. Detailed power balance analysis of this converter has
been carried out u s i n g MATLAB/SIMULINK and simulation results are shown.
Keywords- cascaded hybrid bridge inverters, two cell bridges
I.
INTRODUCTION
Converter here is combination of both rectifier
and inverter. Rectifier converts AC input voltage
into DC output voltage, whereas an Inverter
converts DC input Voltage into Ac Output
Voltage of variable magnitude and frequency. The
basic Bridge converters suffer from many
drawbacks like they have high ripple content in
both input and output. The series parallel
operation of the switching devices for high power
and high voltage applications is a not easy task.
Fabrication is difficult for higher rating
switching devices. Total harmonic distortion
(THD) and harmonic content is high.
Multilevel Inverters defeat the many of the above
mentioned drawbacks. In high power application
systems, the multilevel converters can suitably
substitute the existing system without the use of
transformers. The main features of a multi level
structure are, as the number of levels increases
the harmonic content decreases, switching stress
and EMI are low thus reduces the filtering
equipment and requirements. Their efficiency
is very high (>98%) because of the
minimum switching frequency. They can
advance the power quality and dynamic
stability for utility systems. Because of their
modular and simple structure, they can be
stacked up to an almost unlimited number of
levels. The switching devices do not encounter
any voltage sharing problems. Because of these
many advantages, multilevel converters are easily
be applied for high power applications such as
renewable energy integrated systems like photo
voltaic or fuel cell systems, large motor drive
systems and utility supplies.
II.
STRUCTURE DESCRIPTION
A single-phase 2C-CHB power converter is shown
in Fig 1. The system is connected to the grid
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through a smoothing inductor L. Load behavior
depends on load currents iL1 and iL2
connected to each dc-link capacitor C1 and C2
respectively. The system parameters and variables
are described
Figure.1. A single-phase 2C-CHB power converter
L
smoothing reactor
C1 & C2
dc link capacitors
Vs
grid voltage
Is
grid current
Vab
Converter output voltage
Each cell output voltages,
vm1 = δ1 vdc1
vm2 = δ2 vdc2
where δ is the control signal
Total power
Pt=P1+P2
P1= vdc1 iL1
P2= vdc2 iL2
The behavior of 2C H bridge inverter is
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Gunnam Durga Devi* et al. / (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH
Volume No. 1, Issue No. 5, August -September 2013, 438 -443.
characterized by inductor currents and dc capacitor
voltages.
When the converter is operated as inverter mode
(power flows from the dc side to the ac side), it
may be sure that power drawn from each dc
source should be shared equally. For example, if
batteries are using as dc sources, then they will
discharge at same rate. When operating as a
rectifier at high power, then this strategy is
pointing out to get the dc voltages across the dc
side by using capacitors. However, this can
become difficult if the loads connected to the cells
in the converter are not equal, as may be the case
when the loads are power converters connected in
a multilevel structure.
Another aim of this converter is to keep the
switching frequency of the system as low as
possible. This is desirable as a result of working at
high power since losses due to switching will
increase with switching frequency. Since each HB
cell has a single-phase input, real power flowing
into the converter has ripple contents as twice the
supply side frequency; this problem can be very
high when the input system is loaded with output
inverter cells. The real power ripple will result in
the dc voltage at each cell, also rippling at the
same frequencies. This ripple may result in a third
harmonic entering in to the current demand signal
in some control topologies which could cause
unwanted supply harmonic currents to flow if
modulated. This can be improved by using large
magnitude dc side capacitors to filter the ripple to
an acceptable level or by using advanced control
schemes to ensure that this ripple does not further
cause problems into the system.
III.
POWER BALANCING ANALYSIS
To evaluate the steady-state power balance in
the cells of a cascaded converter, t h e c e l l s
h a v e b e e n replaced by voltage sources with
values v1 and v2 that are equal to the rms values
of the fundamental harmonic of the voltages
current. The capability to be supplied with active
power from the grid or to deliver active power to
the grid in each cell depends on the value of the
capacitor voltage in the cell and the voltage that
should be modulated by the 2C-CHB. In what
follows, the three possible cases are described and
in all cases, it is assumed that vab can be
modulated by the converter, i.e.,
vc1 + vc2 ≥ vab .
A. vc1 ≤ vab and vc2 ≤ vab
In this case, it is necessary to use both cells to
generate the output voltage vab. Fig 2 shows in a
marked area the possible points to achieve the
desired output voltage. Any point outside of this
region makes the system unstable because the
output voltage cannot be modulated with those
values of the dc-link capacitor voltages. In
addition, as shown in the figure, the projection of
v1 over is is always positive, and the same occurs
for v2; as a consequence, the active power values in
both cells are positive, meaning that the grid
supplies active power to both cells simultaneously.
Moreover, it is not possible to find a point where
the grid supplies active power to one cell and, at
the same time, the other cell delivers active power
to the grid. In addition, this situation means that it
is not possible to have the grid supplying active
power only to one cell or to have only one cell
delivering active power to the grid. On the other
hand, it can be observed that the reactive power
exchanged with the inductor is supplied by the
cells. There is no restriction to the reactive power
sign contributed by each cell. This means that the
reactive power in each cell can be different; even
in one cell, the reactive power can have a
capacitive nature while the other has an inductive
nature.
Py = vy iy cos θy
Qy = vy iy sin θy .
Here vy and iy are the rms values of the voltage
and current respectively, and θy represents the
angle between them. Moreover, Py is the
active power which is equal to the mean value
of the instantaneous power Py and Qy is the
reactive power.
In the analysis it is assumed that Is is calculated in
such a way that it is in phase with Vs. On the other
hand same active power is deliver or supplied to
the cells from the grid. The only difference is
reactive power exchange between grid and a cell
depends on shift angle between voltage and
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Fig 2. Stable control area when vc1 ≤ vab &
vc2 ≤ vab
B. vc1 > vab and vc2 ≤ vab
In this case, the desired output voltage can be
attained by using both cells or just using the cell
with the higher dc voltage. Active powers in
each cell have different signs. Thus, the cell with
the higher dc voltage is supplied from the grid,
while the other cell is delivering active power to
the grid. Under this situation, the values for the
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Gunnam Durga Devi* et al. / (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH
Volume No. 1, Issue No. 5, August -September 2013, 438 -443.
maximum active power that can be delivered to
cell 1 i s v max and the minimum active power
that has to be supplied to cell 2 is v min are to
be defined. Moreover, the maximum active power
supplied to cell 1 is higher than the total amount
of active powers delivered from the grid. This
means that the active power delivered from the
second cell is going into the first cell
C. vc1 > vab and vc2 > vab
Fig.3. Evolution in time of dc voltages
In this case, the output voltage can be modulated
using both cells or just using one of them. As a
result, three possible power balance situations in
the cells are under concern. The reactive power is
exchanged between the smoothing inductor and
the cells without restrictions in the sign of the
reactive power of each cell. To achieve a stable
operation, for a given total amount of active
powers consumed by the loads connected to the
converter, under this situation, the minimum active
power that has to be supplied in each cell is
negative; thus the cell is delivering active power.
Meanwhile the maximum active power that can be
consumed by the loads connected to the cell is
higher than the total active power supplied to the
converter; thus part of the energy utilized in this
cell draws from the other cell and not from the
grid.
IV.
SIMULATION
In this section, simulation results are shown to
corroborate the analysis for this purpose, a singlephase 2C-CHB converter prototype has been used.
The electric parameters of the prototype are
summarized in Table II.
Table II. Electric Parameters
A. Stable Operation With Vc1 ≤ Vab and Vc2 ≤
Vab
In this case, both cells have to be supplied or to
deliver active power from the grid simultaneously.
To illustrate this operation, a resistor of 60 Ω is
connected to each dc link as a load. Several dc
voltage step references are applied to show the
behavior of the 2C-CHB. Initially, the dc voltage
commands are set to 200 V. When the actual dc
voltages achieve their references, the loads are
connected. Approximately 2 s later, the voltage
command for the first cell is changed to 300 V,
and then, after 1 s, a new reference for the second
cell of 100 V is established.
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Fig.4(a) . Evolution in time of grid current.
Fig.4 (b). Evolution in time of grid current.
Fig 3 shows the behavior of the dc voltages and
Fig 4(a) & 4(b) shows the evolution of the input
current and voltage. In the graphs, it can be
noticed that the dc output voltage references
are achieved without difficulties. Moreover, Is
grows or decreases in accordance with the output
load value variations. In addition, the input current
is almost in phase with the input voltage and
presents a low total harmonic distortion (THD),
and this can be seen in Fig 5, where a detail of Vs
and Is is shown. This input current has a PF of
0.99 and a THD of 2.5%. This THD value can be
calculated up to the 50th harmonic order.
Fig 5. Detail of grid voltage and current.
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Gunnam Durga Devi* et al. / (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH
Volume No. 1, Issue No. 5, August -September 2013, 438 -443.
B. Unstable Operation With vc1 ≤ vab and vc2
≤ vab
In this section, the behavior of the 2C-CHB
converter, when it is operated in a point outside
the stable region, is shown. For a fixed total
amount of active power exist the minimum
active power values that have to be consumed by
the loads connected in each cell and the maximum
active power values that can be supplied to the
cells. In this experiment, the output dc voltage
commands are established to 200 V, and the total
active power consumed by the converter is set to 2
kW. The minimum and maximum active power
values under these working conditions can be
calculated using the below formulae.
max
P1
min
P2

V1max

( Vs  V 2max )
Vs
Pt
Vs
However, some criteria have to be taken into
account to achieve a stable converter operation.
This is accomplished by applying the required
levels of power into each cell to ensure that they
are constantly balanced with the connected loads.
In this paper, the power balance limits in the cells
of a single-phase 2C-CHB power converter have
been addressed. These limits depend on the dclink voltage values. It is shown that, under
certain conditions, it is possible to have
opposite sign active power values simultaneously
in both cells. Moreover, to have a stable
operation, it is necessary to ensure that, for a total
amount of active power supplied to the 2C-CHB,
both cell loads are between the maximum and
minimum allowed. Finally, simulation results are
introduced, validating that the presented analysis
is an appropriate tool to establish the design
criteria for the 2 cell cascaded bridge converter
using dc voltages.
Pt
V max
max
 2
P2
Pt
Vs
( Vs  V1max )
min
Pt

P2
Vs
Fig 6 shows the behavior of load compensation. It
can be noticed that, for the first load configuration,
the converter achieves a stable operation, the dc
voltages are stable in the reference commands, and
the input current is established in agreement with
the output load. When the load step is applied, the
converter tries to follow the references; however,
as it is working outside the stable region, it is
not possible to achieve the commands and the dc
voltages change without control. Finally, the
converter has to be stopped to avoid a malfunction
caused by the input current or by a high output
voltage value.
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Fig. 6. Evolution in time of dc voltages.
V.
CONCLUSION
A CHB power converter is a suitable strategy to
be used when two or more independent dc voltage
sources are needed in an industrial applications,
synchronous rectifier or back-to-back application.
ISSN 2320 –5547
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AUTHORS’ PROFILE
Gunnam Durga Devi Pursuing M.Tech in Nimra
College of Engineering and Technology,
Vijayawada. Her specialization is Power &
Industrial Drives. She graduated in Electrical and
Electronics Engineering from Regency Institute of
Technology, Pondicherry University.
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Gunnam Durga Devi* et al. / (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH
Volume No. 1, Issue No. 5, August -September 2013, 438 -443.
SK Gouse basha is currently working as a
Assistant Professor in Electrical and Electronics
Engineering Department, Nimra Vijayawada. He
obtained his M.Tech Degree in JNTU University
and graduated in Electrical and Electronics
Engineering from NCET, Vijayawada. His research
interest includes Power Electronics, drives and
control.
Firozali Mohammed is currently working as a
Associate Professor in Electrical and Electronics
Engineering Department, Nimra College of
Engineering and Technology (NCET), Vijayawada.
He obtained his M.Tech Degree in Power
Electronics from Sathyabama University, Chennai.
He received B.Tech degree in Electrical and
Electronics Engineering from NCET, Vijayawada.
His research interest includes Power Electronic
Converters and applications.
Anyam Naresh Pursuing M.Tech in Nimra
College of Engineering and Technology,
Vijayawada. His specialization is Power &
Industrial Drives. He graduated in Electrical and
Electronics Engineering from Regency Institute of
Technology, Pondicherry University. His research
interest includes Industrial drives and applications.
ISSN 2320 –5547
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