SYMMETRICAL HYBRID MULTILEVEL DC

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SYMMETRICAL HYBRID MULTILEVEL DC-AC CONVERTER IN CASCADE
Héctor Vergara*
Miguel Lopez*
Samir Ahmad Mussa**
Domingo Ruiz-Caballero*
René Sanhueza*
Marcelo L. Heldwein**
Member
Member
*Pontificia Universidad Católica de Valparaíso
School of Electrical Engineering – EIE
Power Electronic Laboratory – LEP
Av. Brasil 2147, P.O. BOX 4059,
Valparaíso, CHILE.
e-mail: [email protected]
Abstract—This paper studies the cascading of Symmetrical
Hybrid Multilevel Inverter based on the cell of three-levels
(TC) of voltage. The inverter operates with DC power sources
of equal value and with two technologies switches. Besides, this
inverter has as great advantage to be completely modular,
which makes it very reliable because it can be designed with
redundancy in the modules. The modulation strategy for
driving the switches is based on the sinusoidal PWM technique
known as Phase-Shifted Disposition (PSD). By means of
Fourier series are represented the output voltages for N cells
connected in cascade. To validate the operating principle a
single-phase low power prototype has been built, operating at
1.5 kHz for the fast switches and 50 Hz for the slow switches.
Index Terms: DC-AC converters, Hybrid Inverters,
Symmetrical Multilevel Inverters, Cascaded Inverters.
I. INTRODUCTION
The late technology improvements regarding switched
power capacity and operating frequency for turn-off high
power semiconductor devices (IGBT's, GTO's, IGCT's and
others), has consolidated static converters based on
multilevel topologies. The advances in this field make the
medium and high voltage applications increasingly
common, especially in the ac motor drives [3], [4]. Another
prominent field of application is the high resolution
converters [15], which typically are implemented through
voltage source multilevel inverters that generate output
voltage waveforms from a large set of dc voltage sources and
are capable of synthesizing waveforms that approximate sine
waves. This goal is achieved by applying suitable multilevel
topologies, increasing the switching frequency to the limits
of the semiconductor technology and applying proper
modulation strategies to meet a compromise between
reduction of total harmonic distortion, switching losses
minimization, common mode voltage mitigation, among
other requirements [1-15].
Different multilevel converter topologies have been
**Federal University of Santa Catarina
Department of Electrical Engineering/Power Electronics
Institute – INEP
P. O. BOX 5119 – 88040-970
Florianopolis – SC – BRAZIL
e-mail: [email protected]
introduced and reviewed [5,8] in the literature, presenting
advantages and disadvantages depending on application,
power and voltage levels, need for modularity, insulation
and other requirements. In this context, one of the first
multilevel topologies is the symmetrical cascaded H-bridge
converter [2,5,10]. This is a multilevel topology that profits
from the series connection of several three-level H-bridge
converters in order to achieve a high number of voltage
levels at its output voltages. Asymmetrical cascaded
converters have been proposed [11] as alternatives to this
type of converter, presenting a lower number of
semiconductor devices with unbalanced stresses in the
power semiconductors. Hybrid multilevel converters [13,14],
i.e., integrating different circuit topologies present less
modularity, but advantages in other characteristics.
This work introduces the cascading of symmetric
multilevel cells based on the hybrid multilevel inverter
topology presented in [1]. The presented study generalizes
the topology to accommodate any number of three-level cells
(TC). Thus, a high number of voltage levels are generated
and waveforms with reduced harmonic contents are
produced. Furthermore, a proper fixed frequency PWM
modulation scheme is applied and the theoretical voltage
waveforms are derived for the Cascaded Symmetrical Hybrid
Multilevel Inverter based on Three-Level Cells. Finally,
experimental results are presented in accordance to the
performed theoretical analysis.
II.
CASCADED SYMMETRICAL HYBRID MULTILEVEL
INVERTER BASED ON TC CELLS
The basis for the proposed topology is the hybrid
multilevel inverter topology presented in [1] and shown for
clarity in Fig. 1(a). The number of levels that can be
synthesized through this DC-AC converter is five, as
exemplarily drawn in Fig. 1(b) for a single pulse modulation
scheme that leads to the following voltage levels for a full
modulation cycle: 0, E, 2E, E, 0, E, 2E, E and 0.
Another characteristic of such a converter is that the threelevel cell (TC) comprising switches Sj, with j=1..4, operates
twice per modulation period switching at the PWM carrier
frequency, while the inverting bridge consisting of Si , with
i=5..8, switches at the modulation frequency. In this
converter, the TC switches can be implemented with faster
semiconductors, for instance IGBTs switching at some
kilohertz, while the inverting bridge requires devices with
twice the voltage ratings that can be slow, for instance
GTOs or IGCTs switching at frequencies up to 60 Hz.
Based on the aforementioned multilevel topology, two
cascading strategies are possible in order to increase the
number of output voltage levels. The first strategy is the
direct series connection of the five-level topology as seen in
a cascaded H-bridge converter. This leads to a highly
modular design where the circuit shown in Fig. 1(a) is
simply replicated as many times as desired, employing
points a and b as input and output terminals. This approach
is suitable for high power/voltage converters since voltage
levels are reduced in all semiconductors.
Another approach for a cascaded converter is to series
connect only the TC cells, while a single inverting bridge is
responsible for the low frequency inversion. This strategy is
introduced in Fig. 1(c) and presents a clear advantage on the
lower number of insulated gate drive circuits that are
required, whereas the inverting bridge switches, even
though in lower quantity, must withstand the sum of all
input dc voltage sources. This approach can be taken when
the main goal of the multilevel converter is to generate a
near-sinus waveform, i.e., in the case of high a high
resolution converter where the voltage across the switches is
not the main limitation. The topology drawn in Fig. 1(c)
presents N cascaded TC cells.
Both approaches lead to a high number of output voltage
levels and, with a proper modulation scheme, reduce the
total harmonic distortion (THD) of the output voltage.
Redundancy can be applied to both approaches as well,
where a higher number of cascaded cells still guarantees that
in case of a given number of cells fails, these are shorted and
the others continue to operate accordingly.
E
NL
4N
1,
(1)
and, conversely, for a given number of voltage levels, the
number of cascaded TC cells is
N
NL 1
.
4
(2)
Each TC cell requires two series connected dc sources
x
S5
S3
S7
E
S6
S2
(a)
S8
y
N
A
S4
Load
v ab
vab
2E
E
(b)
S1
S4
S2 S1 S2
S3 S3 S4
S5
S8
S2 S1 S 2
S4 S3 S 3
S2 S 1 S2
S4 S 3 S3
S 1 S2
S 3 S4
S6
S7
E
2E
t
S1
S4
S11
E
S12
Cell 1
E
S13
S14
E
S21
S22
Inverting bridge
Cell 2
E
S23
S5
S24
E
E
S7
Sj1
S6
vab
a
(c)
b
Load
S8
Sj2
Cell j
Sj3
Sj4
A. The Hybrid Cascaded TC Cells Topology
Five voltage levels can be generated for each of the TC
cells in a cascaded TC cells topology (cf. Fig. 1(c)). Twocascaded cells produce nine levels, three cells generate
thirteen distinct voltage levels and so on. From this rule, the
total number NL of synthesizable voltage levels for a generic
number of cascaded TC cells N is given by,
TC cell
Inverting bridge
S1
SN1
E
SN2
Cell N
E
SN3
SN4
Fig. 1 – (a) Single-phase symmetrical hybrid multilevel
inverter [1]; (b) an example of its output voltage, and; (c)
single-phase multilevel symmetrical hybrid circuit obtained
with cascading N TC cells.
that must be insulated from the dc sources of other cells.
Thus, the total number of insulated dc sources is half of the
total number of dc sources F, which is
F
2N
NL 1
.
2
(3)
v t1
+
vM
v t2
from which, the total number of high frequency switches
STC, i.e., switches at the TC cells is found with,
v t2’
4N
S bridge
4 N
1
NL
3 ,
+
v t1’
(4)
S total
S11
S12
S14
S13
S21
S22
S24
+
ST C
1 .
NL
v tj
Modulation Strategy
NL 1
.
2
2N
(6)
Considering that the number of carriers depends on the
number of cascaded cells, the displacement angle between
carriers P is found with
2
P
P
NL
1
360
.
NL 1
(7)
As an example, four carriers with a 90° phase-shift
between them are required to implement the modulation for
two cascaded cells. The inverting H-bridge is driven by the
comparison of the modulating sinusoidal signal with level
zero. Thus, the inverting H-bridge switches at the
fundamental frequency of the modulating waveform.
C.
Output Voltage Time Behavior
The generated output voltage of a Five-Level SinglePhase Symmetrical Hybrid Multilevel Inverter [1]
employing the PSM-PWM is obtained through from the
pulse signal that represent switching the switches, which is
given in Fourier series form by
e
t
E m i sin
n 1
t
2E
sen n m i sin
n
t
cos m f n t
,
(8)
where mi is the modulation index, given by
mi
Sj1
Sj2
Sj4
Sj3
Vm V p ,
mf is the frequency index, defined with
(9)
+
v tj’
v tN
v tN’
abs
+
Ts
vM
S5 , S 8
0
SN3
vM
vM
vtj vtj’
SN1
SN2
SN4
+
The chosen modulation strategy for driving the switches
is based on the sinusoidal pulse width modulation (PWM)
technique known as Phase-Shifted Disposition (PSD). The
switching signals for each TC cell are derived from the
comparison of a modulating signal vm with two triangular
carrier signals vtj and vtj’, with j=1..N (cf. Fig. 2) displaced
rad from each other. Therefore, the required total number of
triangular carriers P to implement Phase-Shifted Disposition
modulation for the proposed cascaded converter is
P
S23
(5)
+
B.
4 N
+
+
The total number of switches (and diodes) is given by the
four switches in the inverting bridge (Sbridge = 4) summed to
four more switches for each cascaded TC cell. Thus, the
total number of switches for the hybrid cascaded TC cells
inverter is
S6 , S 7
Fig. 2 – Implemented modulation based on the PSD-PWM, where
the inverting bridge switching signals are generated by the
comparison of the modulating signal to zero.
mf
f p fm .
(10)
Vm is the amplitude of the sinusoidal modulating signal, Vp
is the peak-to-peak amplitude of the triangular carrier
signals, fm is the modulating signal frequency (the
fundamental frequency), and fp is the frequency of the
triangular carrier signals. The generalized Fourier series for
the output voltage of the cascaded converter with N cells is
found employing equation (7) and considering the carrier
signals phase-shift given by (6). The resulting output voltage
vab is
vab
t
n 1
2Nm i E sin
t
2E
sin 2N m i n sin
n
t
cos 2N m f n t
. (11)
Equation (11) is employed to generate the waveforms
shown in Fig. 3 for a frequency index mf = 20, a modulation
index mi = 0.94 and a total number of harmonics nh = 1000.
Fig. 3(a) shows the output voltage for a single Five-Level
Single-Phase Symmetrical Hybrid Multilevel Inverter [1],
i.e. N=1. Three cascaded cells (N=3) produce the voltage
plotted in Fig. 3(b), while six cells (N=6) generate the
waveform in Fig. 3(c). The harmonic contents are clearly
reduced for higher number of cascaded cells, as the
synthesized output voltage approximates a pure sinusoid
with more voltage levels. The number of voltage levels is
NL=5, 13 and 25 for N=1, 3 and 5, respectively, which
N=1
N=1
N=1
(a)
(a)
N=3
N=3
N=3
(b)
(b)
N=6
N=6
N=6
(c)
Fig. 3 – Normalized output voltage waveforms (vab/E) for N=1, 3
and 6 cascaded TC cells, computed with equation (11) for mi=0.94
and mf =20.
(c)
Fig. 4 – Output voltage waveforms (vab) for N=1, 3 and 6 cascaded
TC cells, obtained from numerical simulations (PSIM©).
Simulation conditions: E=760 V, mi=0.94 and mf =20.
matches the value predicted with equation (1).
Computer simulations for a converter based on the
following specifications were carried out in order to verify
the output voltage waveforms theoretical analysis for the
same conditions employed in the calculations for Fig. 3.
Converter: E = 760 V; mi = 0,94; fm= 50 Hz; mf = 20.
Load: R = 21,33 ; L = 51 mH; cos( )= 0,8.
The results obtained with the computer simulations for
N=1, 3 and 6 are shown in Fig. 4 and closely follow the
theoretically derived voltages.
number of harmonics n = 1000. Fig. 3(a) shows the output
voltage for a single Five-Level Single-Phase Symmetrical
Hybrid Multilevel Inverter [1], i.e. N=1. Three cascaded
cells (N=3) produce the voltage plotted in Fig. 3(b), while
six cells (N=6) generate the waveform in Fig. 3(c). The
harmonic contents are clearly reduced for higher number of
cascaded cells, as the synthesized output voltage
approximates a pure sinusoid with more voltage levels. The
number of voltage levels is NL=5, 13 and 25 for N=1, 3 and
5, respectively, which matches the value predicted with
equation (1).
III. OUTPUT VOLTAGE TOTAL HARMONIC DISTORTION
An important figure of merit for the analysis of high
resolution multilevel inverters is the output voltage total
harmonic distortion THDv%. Equation (11) is employed to
generate the waveforms shown in Fig. 3 for a frequency
index mf = 20, a modulation index mi = 0.94 and a total
The THDv% is defined with
V h2
T HDv %
100%
h 2
V1
,
(12)
which requires the rms value of each generated voltage
N=1
(a)
Fig. 6 – Derived relation between THDv% and the modulation
index for N=3 cells, mi=0.94 and mf =20, curve fitted through
minimum squares regression.
N=3
vab
t
2Nm i sin
t
n 1k 0
1
sin 2Nm f n
2
(b)
harmonics. In this equation, h represents the order of the
corresponding harmonic, while the sub-index 1 corresponds
to the fundamental frequency. As harmonics are generated
at the multiples of the switching frequency with side-bands
influenced by the modulation frequency, the switching
frequency impacts the THDv%. This is supported by the
theoretical analysis of the output voltage considering
equation (11) through the use of
k 0
2
J
n 2k
,
sin 2Nm f n
2k
1
t
TABLE I
OUTPUT VOLTAGE FREQUENCY COMPONENTS (MAGNITUDE)
Fig. 5 – Output voltage frequency spectra for N=1, 3 and 6
cascaded TC cells, obtained from numerical simulations (PSIM©).
Simulation conditions: E=760 V, mi=0.94 and mf =20.
2
t
2 Nm i n
(14)
(c)
t
1
1
from where the magnitudes are explicitly given in Table I.
The harmonic components according to Table I show that
the high frequency components of the output voltage are
shifted up by N when compared to a single five-level
inverter. Furthermore, only odd harmonics are observed.
N=6
sin 2 Nm in sin
2k
4
J
n 2k
1
2 Nm i n sin 2k
1
t
(13)
and other trigonometric identities, where Jx is the Bessel
function of first kind and order x. Thus, the magnitude of
each harmonic component can be directly derived from
rewriting equation (11), which leads to
Components
Amplitude
Fundamental
2N m i E
Harmonics (n,k)
n=1,2,3...
k=0,1,2...
4E
J 2k
n
1
Frequency
1
2Nm f n
2k
1
2Nm n
2k
1
1
2N m in
Applying the Fast Fourier Transform (FFT) to the
simulated voltages shown in Fig. 4 and limiting the
calculation bandwidth to 2 MHz lead to the frequency
spectra shown in Fig. 5 for inverters with N=1, 3 and 6
cascaded TC cells and modulation index mi =0.94 and
frequency index mf =20. The frequency spectra present only
odd harmonics and consist of sidebands around multiples of
2.N.mf. Computing the THDv% for different numbers of
cascaded cells leads to the values presented in Table II. The
total harmonic distortion reduces with increasing number of
cascaded cells as expected by the visual inspection of, both,
time domain waveforms and frequency spectra. A
relationship between THDv% and N could be derived even
considering that N is a natural number. This relationship
would be of help when a system is designed with redundant
cells that could be excluded in order to evaluate the impact
of this measure to the voltage quality.
TABLE II
THDV% FOR DIFFERENT SIMULATED CASCADED CONFIGURATIONS
N
1
2
3
4
5
6
mi
0.3
0.4
0.6
0.7
0.8
0.94
THDv%
30,92
15,97
10,68
7,96
6,31
5,199
THDv%
35.55
24.58
16.94
13.27
12.49
10.68
Another parameter that strongly influences the THDv% is
the modulation index. During the normal inverter operation,
the modulation index varies depending on load condition
and input voltage regulation. This impacts the output
voltage distortion and a mathematical expression that relates
the THDv% to the modulation index is of help. This
relationship is exemplarily obtained in the following for a
multilevel inverter composed of N=3 cascaded TC cells with
a frequency index mf =20. The inverter circuit is simulated
for several values of mi and the computed FFT of its output
voltage is recorded, from where a minimum squares
regression is performed. The fitted curve is
T HDv %(m i )
9.693m i
1.0448
,
(15)
and the computed THDv% values are given in Table III.
The fitted THDv% function presents a mean square error
R2=0.9927, which is reasonable for this type of function.
The exponent –1.0448 approaches unity, from where the
total harmonic distortion is inversely proportional to the
modulation index, as observed in Fig. 6.
IV. EXPERIMENTAL RESULTS
The proposed inverter topology has been implemented
in a N=2 laboratory prototype employing dc bus voltages
E=100 V; a total output power Po=500 W; switching
frequency fs=1.5 kHz; and, modulation frequency fo=50 Hz.
In addition, a small LC filter with L=8 mH and C=8 µF was
included at the load. The applied modulation strategy is
based on the aforementioned Phase-Shifted Disposition
(PSD) PWM. This is implemented on a DSP TMS320F2812
that generates the open loop switching signals required for
correct inverter operation.
Computer simulations were carried out with these
design specifications, obtaining the results shown in Fig. 7,
where the output PWM voltage generated by the inverter,
the load voltage and current are shown.
Fig. 8 shows the experimentally obtained results
TABLE III
DEPENDENCE OF THE THDV% FOR N=3 TO THE MODULATION INDEX
Fig. 7 – Simulation results: multilevel inverter output voltage vab
(upper trace), load voltage (larger sinusoid) and current (smaller
sinusoid). Conditions: N=2 cascaded cells, E=100 V, mi=0.94 and
mf =20.
Fig. 8 – Experimental results: multilevel inverter output voltage
vab (channel 3 – 250 V/div), load voltage (channel 2 – 250 V/div)
and current (channel 1 – 5 A/div). Conditions: N=2 cascaded
cells, E=100 V, mi=0.94 and mf =20.
Fig. 9 – Simulation results: multilevel inverter output voltage vab
frequency spectrum with a computed THDv%=15.93%.
Conditions: N=2 cascaded cells, E=100 V, mi=0.94 and mf =20.
Fig. 10 – Experimental results: multilevel inverter output voltage
vab frequency spectrum with a computed THDv%=18.11%.
Conditions: N=2 cascaded cells, E=100 V, mi=0.94 and mf =20.
Fig. 11 – Simulation results: voltages across the inverting bridge
switches (upper traces) and, across the switches of a TC cell
(lower traces). Conditions: N=2 cascaded cells, E=100 V, mi=0.94
and mf =20.
employing the same circuit specifications. Here, the PWM
inverter output voltage (channel 3), the load voltage
(channel 1), which is measured after the LC filter, and
finally, (channel 2) the current through the load are
presented. Good agreement is observed by comparing these
results with those obtained by simulation.
Fig. 12 – Experimental results: voltages across the inverting
bridge switches (channels 2 and 3 – 100 V/div) and, across the
switches of a TC cell (channels 1 and 4 – 100 V/div). Conditions:
N=2 cascaded cells, E=100 V, mi=0.94 and mf =20.
Fig. 9 shows the computed frequency spectrum from the
inverter output PWM voltage, whereas Fig. 10 presents the
calculated experimental spectrum for the same voltage. The
spectra are highly correlated, validating the theoretical
analysis. However, the digital switching signals generation
creates asymmetries and more spread side-bands when
compared to ideally simulated conditions. Another
observation is that the high frequency harmonics present
lower values for the experimental results, but this comes
from the fact that the simulation results show peak values,
while the experimental results represent rms values.
Considering all differences, the voltage THDv% presents a
difference lower than 9%.
Fig. 11 and Fig.12 show, respectively, simulation and
experimental results for voltages across the multilevel
inverter switches. Maximum voltage across the high
frequency (TC) switches (channels 1 and 4) is close to 100
V, while the inverting bridge switches (channels 2 and 3)
block 400 V as expected. Finally, a photograph of the built
prototype is shown in Fig. 13.
V.
CONCLUSIONS
This work introduced two ways of generating a Cascaded
Symmetrical Hybrid Multilevel Inverter based on ThreeLevel Cells, namely: the cascading of complete five-level
inverter cells and, the cascading of the TC cells with a
single inverting bridge. The second topology has been
theoretically analyzed with respect to the total number of dc
voltage sources, semiconductors and their respective
characteristics, and the total number of output voltage levels.
The analyzed topology presents advantages regarding the
total number of semiconductors required to generate a given
number of output voltage levels, since only four
Fig. 13 – Photograph of the implemented lab prototype.
semiconductors are employed to invert the cascaded
multilevel voltages, even though, these four switches block
the sum of the voltages of all dc sources. In this sense, the
topology is well adapted for applications that require high
resolution converters, such as, switch-mode amplifiers.
A modulation strategy based on the Phase-Shifted
Disposition PSD-PWM for the switches of the TC cells and
on the direct comparison of the modulation signal with zero
to drive the inverting bridge switches. It has been
demonstrated that the proposed topology is capable of
synthesizing the expected theoretical number of output
voltage levels, where an increased number of cascaded TC
cells creates a higher number of output voltage levels and,
thus, reduced harmonic contents. The influence of the
number of cascaded cells and of the modulation index has
been studied, where the total harmonic distortion is reduced
with increasing the number of cascaded cells and is
increased with reduced modulation index.
Finally, experimental results on a nine-level inverter
validated the theoretical analyzes and proved that the
converter is feasible.
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