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2019 21st International Middle East Power Systems Conference (MEPCON), Tanta University, Egypt
Cascaded Multilevel Split-Source Inverters:
Analysis and Modulation
Mahmoud AbdulSalam, Sherif M. Dabour, Essam M. Rashad
Power Electronics and Electric Drives (PEED) Research Lab
Faculty of Engineering, Tanta University, Tanta, Egypt
Abstract—A new cascaded multilevel inverter topology is
proposed in this paper. This topology extends the idea of the
recently introduced Split-Source Inverter (SSI) to the cascaded
Multi-Level Inverter (MLI) configurations. This structure
provides the boosting capability in conjunction with the dc to ac
conversion in a single unit with reduced passive elements.
Therefore, it represents a good alternative of the well-known Z-
Source MLIs. Two different configurations of the proposed
inverter are introduced based on the configuration of each SSI
unit. The topological structure, operating principle and
modulation techniques for each configuration of the proposed
inverter are presented. The validity of the analysis and the
performance of the presented topology are investigated via
simulation study.
Keywords—Cascaded multilevel inverter, Split-source
inverter, phase-shift carrier modulation, level-shifted carrier
modulation
I.
INTRODUCTION
RID-CONNECTED PV farms have different power
converter connections and configurations, centralized,
string, and microinverters [1]. Centralized converters market
share is lowered by time due to high voltage connections,
shading, partial shading, and power devices rating, cost, and
size problems. String and microinverters shipment are
increased a year by year and expected to reach 9GW by 2021.
Microinverters succeeded in the market as modularity, and
scalability of the systems is achieved, this leads to easy, and
fast maintenance in zero outage time. As the price of PV
panels cost is lowered, power converter manufacturers also
need to enhance efficiency, reduce the size, and lowering the
cost. Performance enhancement of power converter can be
achieved by using Sic or Gan devices, but it’s still not
preferred as a cost-wise [2].
Multilevel inverters (MLI) are the standard solution of the
PV applications if low devices’ ratings are needed [3].
Different technologies are available to use for grid-connected
systems [4]. Among the multilevel inverters, the cascaded H-
Bridge topologies has the advantages of modularity. Because
it can be constructed from multiple units of single-phase H-
bridge modules as shown in Fig. 1. To achieve a module-level
monitoring, and control such as MPPT, each PV panel is
connected to microinverter that is connected in series with
each other to get required system voltage. Microinverter
consists of the dc-dc stage called power optimizer (MPPT)
and dc-ac stage. In this case, each module of the cascaded MLI
is constructed as shown in Fig. 2(a) from front-end boost
converter fed H-bridge inverter. On the other hand, each unit
could be a single-stage power converter (SSC) such as in Fig.
2(b), which uses a single-phase impedance source inverters
(ZSI). The SSC reduces the system cost, size, and enhances
the performance, and efficiency. The ZSI utilizes four passive
elements in the impedance network and suffers from [5]
discontinues input current, besides
using additional switching states.
Utilizes four passive elements
A modified ZSI that overcomes discontinuity of input
current called quasi-ZSI (QZSI) suffers from pulsating
inverter bridge dc voltage that arises switches voltage stresses
[6]. Another SSC suggested technology is the split-source
inverter [7]. The SSI consists of a single inductor and a diode
for each inverter leg. It has two different topologies termed in
this paper by P-type and N-type as shown in Fig. 2(c) and (d).
The SSI offers many advantages compared to other SSC [8]
Same number of switches
The lowest number of passive elements
Continuous supply current and lowered bridge
capacitor voltage stress
Use the same switching states and modulation
techniques of the VSI
No special added switching states
The SSI has been used as a single-stage two-level inverter
and can be extended to multi-level inverters [9], [10]. Split-
source H-bridge (SSHB) is a perfect submodule configuration
to be a power conditioning stage between PV panels and load,
or grid terminals. Cascaded H-bridge can be built using the
aforementioned microinverter as it achieves module-level
control.
This paper proposes a new cascaded multilevel inverter
topology based on split-source H-bridge submodules. The
proposed topology improves the performance of the
conventional single-stage and multilevel boosting topologies
in terms of reducing the number of passive elements in each
H-bridge unit. The operating principles and modulation
techniques of the proposed inverter are introduced in this
paper. Finally, a seven-level topology is selected as a case
study to verify the presented analysis via simulation results
using Matlab/Simulink.
Fig. 1. Schematic diagram of the conventional CHB inverter.
978-1-7281-5289-9/19/$31.00 ©2019 IEEE
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(a) front-end two-stage boost H-bridge inverter (b) Quasi-Z Source single-stage H-bridge inverter
(c) Split-Source single-stage H-bridge inverter (N-type) (d) Split-Source single-stage H-bridge inverter (P-type)
Fig. 2. Configurations of two and single stage boost inverter topologies.
II.
ANALYSIS OF SPLIT-SOURCE H-BRIDGE INVERTERS
The power circuit topology of SS-HB is composed of
conventional H-bridge inverter, which is fed from a unique
impedance network. The impedance network consists of
single inductor, single capacitor, and two diodes, each diode
is connected to the midpoint of a leg. Based on the method of
connecting the dc-supply to the impedance network, the SS-
HB inverters are classified into two types as shown in Fig. 3
[11]. The first one is called N-type SS-HB in which the dc-
supply is connected between the inductor and the common
negative dc-rail of the inverter. The topology is called P-type
SS-HB, where the dc-supply is connected between the
common positive-dc rail of the inverter and the inductor. Both
two types can be used as a submodule to build up the cascaded
SSI.
To better understand the differences exhibited by the
aforementioned types, the following subsections give details
about the operation and modulation of both types.
A.
Modes of Operation
1)
N-type: In this type, the negative terminal of the dc-
supply is connected to the common negative dc-rail of the
inverter, while the positive terminal of the dc-supply is
connected to the inductor. In addition, the second terminal of
the inductor is connected to the anodes of the diodes, while
diode cathodes are connected to the H-bridge midpoints as
shown in Fig. 2. (c). On the other hand, the upper and lower
switches of each leg in the H-bridge are operated in
complementary manner. Therefore, the switches of the H-
brige has four different switching states as shown in Fig. 3(a).
Faced by the aforementioned switching states, the
inductor is charging by turrning ON any of the lower
switches. However, the inductor is discharge if both upper
switches are operated. The upper part of Table 1 summarize
these states and the modes of operation. Consequently, the
state (1,1) is mandatory for boosting operation of the N-type
SS-HB inverter.
TABLE I.
MODES OF OPERATIONS OF SS MLI TOPOLOGIES
Switching State
(0,0)
(0,1)
(1,0)
(1,1)
N-type
charging
Inductor
discharging
P-type
Inductor
discharging
Inductor charging
2)
P-type: Unlike the N-type of SS-HB inverter, the
positive terminal of the dc-supply is connected to the
common positive dc-rail of the inverter, while the negative
terminal of the dc-supply is connected to the inductor as
shown in Fig. 2(d). In addition, the second terminal of the
inductor is connected to the common-cathods of the diodes,
while their anodes in this case are connected to the H-bridge
midpoints as shown in Fig. 2. (d). As a result of this
configuration, the parasitic inductance in the commutation
path of these diodes is minimized.
In this case, the inverter bridge still operates with the same
manner of the N-type SS-HB inverter. As a result, the
switches of the H-brige has the same four different switching
states of the N-type, but with other charging and discharging
modes as shown in Fig. 3(b).
In this case, the inductor is charging by turrning ON any
of the upper switches. However, the inductor is discharge if
both lower switches are operated. The lower part of Table 1
summarize these states and the modes of operation.
Therefore, the state (0,0) is mandatory for boosting operation
of the P-type SS-HB inverter as concluded from Fig. 3(b).
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(a) N-type SS-HB modes of operation
Fig. 3. Modes of operation of SS-HB inverters.
B.
Modulation
(a) P-type SS-HB modes of operation
߶
𝑐𝑟
ൌ
͵ͲΤ
ൌ
𝑚
ൌ
ͳ
ൌ
(1)
There are different modulation schemes have been
introduced in the literature for MLI. Each of them has different
advantages and drawbacks. The Selective Harmonic
Elimination (SHE) and stair case modulation are fundamental
switching frequency modulation techniques. However, the
Space Vector Modulation (SVM) is a medium switching
frequency one. Both SHE and SVM are not preferred for MLI
as they require a lot of calculations and need some
modifications for different number of levels. Most used
modulation technique is the Carrier-Based (CB) modulation
technique due to simplicity, and their capability to scaled for
any number of levels [3].
Generally, there are two basic CB modulation schemes for
MLIs [12]. These schemes are 1) Phase-Shifted Multicarrier
Modulation and 2) Level-shifted Multicarrier Modulation. In
this subsection, the suitability of their applications for the
proposed inverter are investigated.
1)
Phase-shifted multicarriers modulation scheme: In
this scheme, (𝑚ൌ ͳ) times of triangular carriers are required
for 𝑚 times of voltage levels, where all carrier waves have
the same amplitude and frequency, but with a phase-shift
For the sake of example, Fig. 4 shows the generation of the
gating pulses for the upper SS-HB inverter unit of phase-𝑎 in
a seven-level cascaded SS-MLI using this approach. As can
be observed from Fig. 4 that, six triangular carriers shifted by
60 degrees are utilized. Another note is raised here that the
both discharging states of the P-type and the N-type of the SS-
HB inverters are employed in the switching sequence of this
modulation scheme. Therefore, the phase-shifted multicarriers
modulation scheme is suitable for both P-type and N-type SS
MLI topologies.
2)
Level-shifted multicarriers modulation scheme:
Similar to the phase-shited multicarriers scheme, (𝑚ൌ ͳ)
times of triangular carriers are required for 𝑚 levels MLI.
Howevre, in this scheme, the carriers are vertically disposed.
As an illustration, Fig. 5 shows the gating signals generation
using this scheme for seven level MLI, where the six carrier-
waves are level shifted with in-phase disposition (IPD).
Alternative phase dispositions are also available. According
to the waveforms of Fig. 5, a unique discharging state, {0,0}
is valid in this modulation scheme. Therefore, the boosting
between them. The phase-shift between the carrier, ߶ is capability is valid in the level-shifted multicarriers scheme
given by
𝑐𝑟
for the P-type SS-MLI.
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Fig. 4. Phase-shifted multicarriers scheme for a seven-level SS-HB
inverter
Table II summarize the conclusions about the aforementioned
discussion about the suitability of the conventional
modulation schemes for the proposed SS-MLI topologies.
Fig. 5. Level-shifted multicarriers scheme for a seven-level SS-HB
inverter
TABLE II.
SUITABILITY OF CONVENTIONAL MODULATION SCHEMES FOR THE
PROPOSED SS MLI TOPOLOGIES
Modulation schemes
Phase-shifted multicarriers
modulation
Level-shifted multicarriers
modulation
N-type
ξ
ൌ
P-type
ξ
ξ
III.
CASCADED SPLIT-SOURCE H-BRIDGE INVERTER
A cascaded split-source MLI consists of a series of split-
source H-bridge inverter units, which are described in the
aforementioned section. The main function of this topology is
to shape the output voltage waveform from several separate
dc supplies, which can be obtained photovoltaic panels. For
example, the basic structure of a seven-level single-phase
cascaded SSI can be obtained by the building block for three
SS-HBs, where the output terminals of the different SS-HBs
are connected in series.
It is important to note that, while discussing the operation
of P-type and N-type SS-HB inverters in Section II, a general
observation is noted that both topologies need different
switching states to discharge the inductor through the
capacitor. This operation is necessary for the boosting action.
In the P-type SS-MLIs, both modulation strategies (phase-
shifted and level-shifted) can be utilized, while in the case of
N-type SS-ML only the phase-shifted multicarriers strategy is
used.
IV.
SIMULATION RESULTS
In order to verify the validity of the proposed three-phase
cascaded split-source multilevel inverter topologies, three
different models using MATLAB/Simulink platform are
simulated. The first model for N-type cascaded SS-MLI
topology, which is modulated by phase-shifted multicarriers
approach, while the other two models for the P-type topology
with both phase-shifted and level-shifted modulation
schemes. Table III lists the simulation parameters of all
models, while the modulation indexes are adjusted to give the
same output voltage magnitudes in all schemes. The proposed
inverters has been used to supply an inductive load, while the
switches and passive components are assumed to be ideal.
The two possible configurations with the proposed
modulation schemes are tested and the simulation results are
shown in Figs. 6 and 7. It can be observed that the boosted
output phase voltages produced by the phase-shifted
multicarriers modulation scheme are almost identical for both
P-type and N-type except the N-type has an increased THD as
listed in Table V. However, the voltage produced by the other
method is differ in its waveform.
TABLE III.
SIMULATION PARAMETERS
Parameters
Values
Supply dc-voltage
20V for each SS-HB inverter
Desired output voltage
220V per phase
Output frequency
50 Hz
Switching frequency
1 kHz
Modulation index
0.955 for Phase-
shifted carriers
0.81 for Level-shifted
carriers modulation
Load parameters
R=100 ohm
L= 0.25 H
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(a) Output phase-voltage (f) Output phase-voltage (k) Output phase-voltage
(b) Output Line-voltage (g) Output Line-voltage (l) Output Line-voltage
(c) output currents (h) output currents (m) output currents
(d) Capacitor voltages of the cascaded
submodules (i) Capacitor voltages of the cascaded
submodules (n) Capacitor voltages of the cascaded
submodules
(e) Inductor currents of the cascaded submodules (j) Inductor currents of the cascaded submodules (o) Inductor currents of the cascaded submodules
Fig. 6. Simulation results: (a)-(e) for P-Type cascaded SS-HB MLI using Level-shifted multicarriers, (f)-(j) for P-Type cascaded SS-HB MLI using Phase-
shifted multicarriers, and (k)-(o) for N-Type cascaded SS-HB MLI using Phase-shifted multicarriers
The line voltage waveforms are also looks like and it
contains seven voltage levels due to the application of high
modulation index. A summary of the simulation results of the
inductor currents and capacitor voltages in a SS-HB inverter
unit for both topologies are given in Table IV. It can be
obvious from the waveforms and Table IV that, there are
significant diversity between the inducer currents and
capacitor voltages in the P-type topology with level-shifted
carrier modulation. This results in high stresses on the
switching devices. However, in the other cases, there are near
agreement between the results.
V.
CONCLUSIONS
This paper proposes two different cascaded split-source
multilevel inverter topologies, which are termed by P-type and
N-type in this study. These topologies has some advantages
compared with the conventional cascaded multilevel inverters
and the other boosting single-stage inverters. The multicarrier
based modulation techniques have also been successfully
extended for the proposed topologies. For the P-type topology,
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