A Novel Three Phase Multi-string Multilevel Inverter with High DC-DC
Closed operation for Photovoltaic System
Abstract
This paper presents a novel three phase multi-
string multilevel inverter; this inverter reduces
number power devices and high performances.
Before this inverter provide a high step up DC-DC
converter with PI controller for better conversion
efficiency and to improve the output dc voltage of
varies renewable energy sources. This multi-string
multilevel inverter consists of six switches only
instead of eight switches in cascaded H-bridge
multilevel inverter in order to reduce conversion
losses. The main objective of this paper is to save
cost and size by removing any kind of transformer
as well as reducing the power devices .This multi-
string inverter topology have more advantages
such as better output waveforms ,lower
electromagnetic interference and low THD. Finally
this inverter connects to three phase induction
machine for analysis. Simulation and experimental
results show the effectiveness of proposed solution.
1. Introduction
In recent year’s electrical energy requirement very
high, because of different factors like raises
population, industries, colleges and hospitals, etc.,
conventional energy sources based on oil, coal and
natural gas have proven to be highly effectives
drives of economic progress, but at the same time
damaging to the environment and to human health.
Therefore the traditional fossil fuel based energy
sources are facing increasing pressure on a host of
environmental fronts, with perhaps the most serious
challenge confronting the future use of coal being
the greenhouse gas reduction targets. The potential
of renewable energy sources (RES) is enormous as
they can in principle meet many times the world’s
energy demand. Renewable energy sources such as
solar systems, fuel cells, micro-turbines and wind
has become a more issues for delivering premium
power
to loads with power quality, reliability and high
efficiency in converters of RES. In such systems,
RES usually supply a dc voltage that varies in a
wide range according to varies load conditions.
Thus, a dc/ac power converting processing
interface is required and is compliable with
residential, industrial, and utility grid standards [1]-
[2]. Various converter topologies have been
developed for RESs that demonstrate effective
power flow control performance whether in grid-
connected or stand alone operation. Among them,
solutions that employ high frequency transformers
or make no use of transformers at all have been
investigated to reduce size, weight, and expense.
For low-medium power applications, international
standards allow the use of grid-connected power
converters without galvanic isolation, thus allowing
so called “transformer less” architectures.
Furthermore, as the output voltage level increases,
the output harmonic content of such inverters
decreases, allowing the use of smaller and less
expensive output filters. As a result, various
multilevel topologies are usually characterized by a
strong reduction in switching voltages across
power switches, allowing the reduction of
switching power losses and electromagnetic
interference (EMI). A three-phase multi-string five-
level inverter integrated with an auxiliary circuit
was recently proposed for dc/ac power conversion.
This topology used in the power stage offers an
important improvement in terms of lower
component count and reduced output harmonics.
Unfortunately, high switching losses in the
additional auxiliary circuit caused the efficiency of
the multi-string five-level inverter to be
approximately 4% less than that of the
conventional multi-string three-level inverter. In
[3], a novel isolated single phase inverter with
generalized zero vectors (GZV) modulation scheme
was first presented to simplify the configuration.
However, this circuit can still only operate in a
limited voltage range for practical applications and
suffer degradation in the overall efficiency as the
duty cycle of the dc-side switch of the front-end
conventional boost converter approaches unity.
Furthermore, the use of isolated transformer with
Koppineni R N V Subbarao1
Asst.Prof in GIET Polyt College
Rajahmundry, AP, India
Atti V V Srinivas 3
Asst.Prof in GIET College
Rajahmundry, AP, India
D.Vani 2
Asst.Prof in GIET Polyt College
Rajahmundry, AP, India
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multi windings of the GZV based inverter results in
the larger size, weight, and additional expense [3].
In case of single phase to overcome the
aforementioned problem, the objective of this paper
is to study a newly constructed transformerless five
level multi-string inverter topology for RESs. In
this paper, the aforesaid GZV-based inverter is
reduced to a multi-string multilevel inverter
topology that requires only six active switches
instead of the eight required in the conventional
cascaded H bridge (CCHB) multilevel inverter. In
addition, among them, two active switches are
operated at the line frequency. In order to improve
the conversion efficiency of conventional boost
converters, a high step-up converter is also
introduced as a front-end stage to stabilize the
output dc voltage of each RES modules for use
with the simplified multilevel inverter. The newly
constructed inverter topology offer strong
advantages such as improved output waveforms,
smaller filter size, and lower EMI and total
harmonics distortion (THD). In this letter, the
operating principle of the developed system is
described, and a prototype is constructed for
verifying the effectiveness of the topology.
2. Photovoltaic System
A Photovoltaic (PV) system directly converts
sunlight into electricity. The basic device of a PV
system is the PV cell. Cells may be grouped to
form panels or arrays. The voltage and current
available at the terminals of a PV device may
directly feed small loads such as lighting systems
and DC motors. A photovoltaic cell is basically a
semiconductor diode whose pn junction is exposed
to light. Photovoltaic cells are made of several
types of semiconductors using different
manufacturing processes. The incidence of light on
the cell generates charge carriers that originate an
electric current if the cell is short circuited.
Figure1: Equivalent circuit of a PV device
The equivalent circuit of a PV cell is shown in
figure1. In the above diagram the PV cell is
represented by a current source in parallel with
diode, RS and RP represents series and parallel
resistance respectively. The output current and
voltages from PV cell are represented by I and V.
The V-I characteristic of PV cell is shown in
figure2. The net cell current I is composed of the
light-generated current Ipv and the diode current
Id.
Figure2: Characteristic V-I curve of the PV cell
I=Ipv – Id (1)
Where
Id = Io exp (qV/akT)
Io = leakage current of the diode
q = electron charge
k = Boltzmann constant
T = temperature of pn junction
a = diode ideality constant
The basic equation (1) of the PV cell does not
represent the V-I characteristic of a practical PV
array. The basic equation of PV array requires the
additional parameters as shown in figure.
I = IPV – [exp (V+RS/Vta) – 1] – (V+RS/RP) (2)
Where Vt = NSkT/q is the thermal voltage of the
array with NS cells connected in series.
3. Proposed Concept This topology configuration for single phase
consists of two high steps up dc/dc converters
connected to their individual dc-bus capacitor and a
simplified multilevel inverter. Input sources, PV
module 1, and PV module 2 are connected to the
inverter followed a linear resistive load through the
high step-up dc/dc converters. For three phases
consists of same as of single phase connection but
each phase connected by 1200 phase
difference.
The studied simplified five-level inverter is used
instead of a conventional cascaded pulse width-
modulated (PWM) inverter because it offers strong
advantages such as improved output waveforms,
smaller filter size, lower THD and EMI.
High step up converter introduced the output
voltage is compared with the reference value. The
error is given to the PI controller and the driving
pulses for the converter are generated. The
converter output voltage meets the reference value.
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The boosted DC voltage to Multi-string multilevel
inverter is shown in figure 3.
Figure 3: single phase multi-string five level inverter
3.1. High step-up converter stage
In this study, high step-up converter topology is
introduced to boost and stabilize the output dc
voltage of various RESs such as PV and fuel cell
modules for employment of the proposed
simplified multilevel inverter. The coupled
inductor of the high step-up converter in Fig. 4 can
be modeled as an ideal transformer, a magnetizing
inductor, and a leakage inductor. According to the
voltage–seconds balance condition of the
magnetizing inductor, the voltage of the primary
winding can be derived as
Vpri = Vin * (D/1-D)
Hence, the voltage conversion ratio of the high
step-up converter, named input voltage to bus
voltage ratio, can be derived as
𝑉𝑠𝑖
𝑉𝑝𝑟𝑖=
2 + 𝑁𝑠𝑁𝑝 ∗ 𝐷
(1 − 𝐷)
3.2. Multi-string Multilevel Inverter
This paper reports a new single-phase and block
diagram of three phase multi-string topology,
presented as a new basic circuitry in Fig. 3
Figure4. Basic Single Phase Multi-string Five level
inverter
Figure 5. Block diagram of Three Phase Multi-string
Multi Level Inverter
This three phase inverter consists of three (R, Y
and B) phase conductors connect to load and return
conductors connected to ground. In this three phase
inverter each phase conducts with 1200
difference.
For convenient illustration, the switching function
of the switch in Fig. 4 is defined as follows:
𝑆𝑎𝑗 = 1, 𝑆𝑎𝑗 𝑂𝑁
0, 𝑆𝑎𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3
𝑆𝑏𝑗 = 1, 𝑆𝑏𝑗 𝑂𝑁
0, 𝑆𝑏𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3
Table I lists switching combinations that generate
the required five output levels. The corresponding
operation modes of the multilevel inverter stage are
described clearly as follows.
1. Maximum positive output, 2VS: Active
switches Sa 2, Sb 1, and Sb 3 are ON; the
voltage applied to the LC output filter is
2VS.
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2. Half-level positive output, +Vs: This
output condition can be induced by two
different switching combinations. One
switching combination is such that active
switches Sa 2, Sb 1, and Sa 3 are ON; the
other is such that active switches Sa 2, Sa
1, and Sb 3 are ON. During this operating
stage, the voltage applied to the LC output
filter is +Vs.
3. Zero output, 0: This output condition can
be formed by either of the two switching
structures. Once the left or right switching
leg is ON, the load will be short-circuited,
and the voltage applied to the load
terminals is zero
4. Half-level negative output, −Vs: This
output condition can be induced by either
of the two different switching
combinations. One switching combination
is such that active switches Sa 1, Sb 2, and
Sb 3 are ON; the other is such that active
switches Sa 3, Sb 1, and Sb 2 are ON.
5. Maximum negative output, −2Vs: During
this stage, active switches Sa 1, Sa 3, and
Sb 2 are ON, and the voltage applied to the
LC output filter is −2Vs.
Figure 6. Modulation strategy for reference signal
Table1
Switching combination
In these operations, it can be observed that the open
voltage stress of the active power switches Sa 1 , Sa
3, Sb 1, and Sb 3 is equal to input voltage VS ;
moreover, the main active switches Sa 2 and Sb 2
are operated at the line frequency. Hence, the
resulting switching losses of the new topology are
reduced naturally, and the overall conversion
efficiency is improved.
The two input voltage sources feeding from the
high step up converter is controlled at 100V, i.e.
Vs1 = Vs2 = 100V. The switch voltages of Sa1, Sa2,
Sa3, Sb1, Sb2, and Sb3 are all shown in Fig. 6. It is
evident that the voltage stresses of the switches
Sa1, Sa3, Sb1, and Sb3 are all equal to 100V, and
only the other two switches Sa2, Sb2 must be 200V
voltage stress. For three phase inverter as similarly
as single phase inverter but each phase operates
with 1200 phase difference
3.3. Comparison with CCHB inverter
The average switching power loss Ps in the switch
caused by these transitions can be defined as
𝑃𝑠 = 0.5 𝑉𝑑𝑠 𝐼𝑜 𝑓𝑠[𝑡𝑐 𝑜𝑛 + 𝑡𝑐(𝑜𝑓𝑓)]
Where tc(on) and tc(off) are the turn-on and turn-
off crossover intervals, respectively; Vds is the
voltage across the switch; and Io is the entire
current which flows through the switch.
Figure 7. Five level inverter of CCHB
For simplification, both the proposed circuit and
CCHB inverter are operated at the same turn-on
and turn-off crossover intervals and at the same
load Io. Then, the average switching power loss Ps
is proportional to Vds and fs as shown in table2.
For three phases five level inverter of CCHB
compare with proposed topology required number
switches are reduced six switches and switching
loss is nearly half that of CCHB inverter.
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Table 2
Comparison of two multi level inverter for single phase
3.4. DC–DC Closed loop with PI
Controller
In the closed loop model, the simulation is carried
out to meet the reference value. The closed loop
model of the DC- DC step up converter is shown in
Figure 7.
Figure 7. Block diagram of DC-DC closed loop with PI
controller
The output voltage is compared with the reference
value. The error is given to the PI controller and the
driving pulses for the converter are generated.
Block diagram of PI controller as shown in figure
8. U* signal given to high step up converter switch,
the converter output voltage meets the reference
value. It can be seen that the output remains
constant. This constant voltage given to five level
inverter and load voltage is directly proportional to
inverter input.
Figure 8. Block diagram of PI controller
4. Simulation Results
4.1. Three Phase Resistor Load
Simulations were performed by using MATLAB/
SIMULINK to verify that the proposed inverter
topology. Three phase five level inverter with
resistor load is shown in figure 9. Each phase
voltage fed from two PV panels via high dc-dc
converter, this dc-dc converter operates with PI
controller.
Figure 9. MATLAB/SIMULINK model of Three Phase
Five Level Inverter with R-Load
A prototype system with a high step-up dc/dc
converter stage and the simplified multilevel dc/ac
stage are built with the specifications of the two
preceding high step-up dc/dc converters are 1)
input voltage 30V; 2) controlled output voltage
100V; and 3) switching frequency 2 kHz; 4) three
phase output voltage 200V.Simulation results of
three phase load voltage, single phase voltage
waveform and dc-dc output voltage are shown in
figure 10, figure 11 and figure 12.
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Figure 10. Simulation result of three phase load Voltage
waveform
Figure 11. Simulation result of single phase waveform
(200V Peak-Peak)
Figure 12. DC-DC output voltage waveform (100V).
This high DC-DC converter stage provide every
time 100V is shown in figure 12, the output voltage
of this converter compare with reference voltage
(100V) which gives error signal every time and this
error given to PI controller ( Kp = 0.001 and Ki =
0.01). After that driving pulses are generated by
triangular waveform with high frequency.
4.2. Three Phase Induction Motor Load
Figure 13. MATLAB/SIMULINK model of three phase
five level inverter with induction motor
For better analysis this new three phase multi-string
five level inverter connected to induction motor is
shown in figure 13. The analysis of three phase
induction motor results is shown in figure 14,
figure 15 and figure 16.
Figure 14. Simulation result of induction motor stator
current and voltage/phase
Figure 15. Simulation result of a induction motor voltage
and current
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Figure 16. Rotor speed and electromagnetic emf of
induction motor
The conversion efficiency of the implemented
inverter and THD of the output voltage measured
in this case are approximately 96% and 3%,
respectively. The studied multilevel inverter has
lower THD than the CCHB multilevel inverter.
5. Conclusion
In this paper modeling and simulation of a novel
three phase multi-string multi level inverter with
high dc-dc closed loop topology that produces a
significant reduction in the number of power
devices required to implement multilevel inverter.
This inverter topology has more advantages such as
improved output waveforms, and lower EMI and
THD. The proposed topology has minimum
number of switches compare than other
configuration.
6. Reference
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IJEERA
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