AN IMPROVED TRANSFORMER LESS INVERTER TOPOLOGY FOR
COST EFFECTIVE PV SYSTEMS
1
GEETHU CHACKO, 2RIYA SCARIA
1
Student ,M-tech,Power Electronics and Power Systems, FISAT; Assistant
2
Professor, Department of Electrical and Electronics, FISAT
Abstract- This paper presents an improved transformerless inverter with common mode leakage current elimination for a
photovoltaic grid connected power system . To eliminate the common-mode leakage current in the transformerless
photovoltaic grid-connected system, an improved single-phase inverter topology is presented. The improved transformerless
inverter can sustain the same low input voltage as the full-bridge inverter and guarantee to eliminate common-mode leakage
current. The inverse sine carrier pulse width modulation (ISPWM) control strategy can be applied to implement the presented
inverter. The lower total harmonic distortion and higher fundamental output voltage are obtained by using the inverse sine
carrier pulse width modulation (ISPWM). The maximum power point tracking (MPPT) is used to extract the maximum power
form PV panel. The simulation result of the proposed topology using MATLAB/SIMULINK is presented.
Index Terms- Common mode leakage current, Inverted sine pulse width modulation, Transformerless Inverter, Virtual DC
bus.
which is greater than, approximately, 700V for
220-V_ac applications. As a result, either large
numbers of PV modules in series are involved or a
boost dc/dc converter with extremely high-voltage
conversion ratio is required as the first power
processing stage. The full-bridge inverter just needs
half of the input voltage demanded by the half-bridge
topology, which is about 350V for 220-V_ac
applications. But the main drawback is that the full
bridge inverter can only employ the bipolar SPWM
strategy with two levels, which induces high current
ripple, large filter inductor, and low system efficiency.
In this paper, an improved grid-connected inverter
topology for transformerless PV systems is presented,
which can sustain the same low input voltage as the
full-bridge inverter and guarantee not to generate the
common-mode leakage current. The inverse sine
carrier pulse width modulation(ISPWM) can be
applied in the presented inverter. The lower total
harmonic distortion and higher fundamental output
voltage are obtained by using the inverse sine carrier
pulse width modulation (ISPWM). Therefore, a
smaller filter inductor can be employed and the
harmonic contents of the output current are reduced
greatly, and the grid-connected power quality is
improved accordingly.
I. INTRODUCTION
Now a days, the grid-connected photovoltaic (PV)
systems, especially the low-power single-phase
systems, call for high efficiency, small size, light
weight, and low-cost grid connected inverters. Most of
the commercial PV inverters employ either
line-frequency
or
high-frequency
isolation
transformers. However, line-frequency transformers
are large and heavy, making the whole system bulky
and hard to install. Topologies with high-frequency
transformers commonly include several power stages,
which increases the system complexity and reduces
the system efficiency . Consequently the transformer
less configuration for PV systems is developed to over
the advantages of high efficiency, high power density,
and low cost. Unfortunately, there are some safety
issues because a galvanic connection between the grid
and the PV array exists in the transformer less systems.
A common-mode leakage current flows through the
parasitic capacitor between the PV array and the
ground once a variable common-mode voltage is
generated in transformer less grid-connected inverters.
The common-mode leakage current increases the
system losses, reduces the grid-connected current
quality, induces the severe conducted and radiated
electromagnetic interference, and causes personal
safety problems.
II. LITERATURE REVIEW
To avoid the common-mode leakage current, the
conventional solution employs the half-bridge inverter
or the full-bridge inverter with bipolar sinusoidal
pulse width modulation (SPWM), because no variable
common-mode voltage is generated. However, the
half-bridge inverter requires a high input voltage
Ideal transformer less inverter generates constant
common mode voltage. However, if the voltage varies
with time, then a leakage current is produced. For the
sake of minimizing this leakage current, different
topologies were studied in details . Among these are
the full bridge with bipolar PWM, the half bridge,
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
170
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
HERIC, H5, H6 and NPC all of which experience
certain drawbacks which are discussed next.
= constant.
A. Common Mode Leakage Current
If the transformer is omitted, the common mode (CM)
ground leakage current may appear on the parasitic
capacitor between the PV panels and the ground. The
existence of the CM current may reduce the power
conversion efficiency, increase the grid current
distortion, deteriorate the electric magnetic
compatibility, and more importantly, give rise to the
safety threats. The CM current path in the
grid-connected transformer less PV inverter is
illustrated in figure. It is formed by the power switches,
filters, ground impedance$ ZG and the parasitic
capacitance Cpv between the PV panels and the
ground.
One of the filter inductors LA and LB is commonly
zero. The condition of eliminating common-mode
leakage current is accordingly met that
=
= constant(
= constant (
As a result, the condition of eliminating
common-mode leakage current is met that
= constant (
B. State of art topology
One of the way to realize this goal is to use full bridge
inverter with the bipolar sinusoidal pulse width
modulation (SPWM). Though the unipolar SPWM
has better performance when compared to bipolar
SPWM, it cannot be used directly for the full bridge
inverter because it generates switching frequency CM
voltage.
Figure 1.Common mode leakage current path
The simplified equivalent model of the common mode
resonant circuit has been derived in as shown in the
Figure where Cpv is the parasitic capacitor, LA and
LB are the filter inductors, icm is the common-mode
leakage current.
For this reason, some state-of-the-art topologies, such
as the H5 inverter, the HERIC inverter, etc., have been
developed based on the full-bridge inverter, to keep
vCM constant when the unipolar modulation is used.
By inserting extra switches into the full bridge inverter
either on the dc or ac side, the dc bus can be
disconnected from the grid when the inverter output
voltage is at zero voltage level, so that the CM current
path is cut off. Such solutions need two filter inductors
with independent iron cores, which may lead to a rise
in the size and cost. Moreover, the dc and ac sides
cannot be perfectly disconnected by the power switch
because of the switch parasitic capacitance, so the CM
current may still exist .
Figure 2. Equivalent Circuit
An equivalent common-mode voltage Vecm is defined
by
Another kind of solution is to use the half-bridge
inverter with the grid neutral line directly connected to
the midpoint of the dc bus,. In this way, the voltage
across the parasitic capacitor is clamped to be constant
by the dc bus capacitor. However, this method has an
important disadvantage that the required dc bus
voltage should be doubled compared with the
full-bridge topologies. For the 220 Vac system, it can
be as high as 700V. Although the three-level neutral
point clamped (NPC) circuit can help improve the
performance of the half-bridge inverter, the dc bus
voltage is still high. There are other topologies
proposed in recent literature works..The Karschny
where Vcm is the common-mode voltage, Vdm is the
differential mode voltage, VAN and VBN are the
output voltages of the inverter relative to the negative
terminal N of the dc bus as the common reference.
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
171
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
inverter and the paralleled buck inverter are derived
from the buck–boost and buck circuits, respectively.
These solutions have high reliability, but are not
capable of supplying the reactive power to the grid.
the conventional method. By employing this new
modulation technique it has been proved that the
fundamental voltage is improved throughout the
working range and is greater than the voltage obtained
using conventional method which employs triangular
carriers for modulation.
III. PROPOSED TOPOLOGY
Figure 3.Block diagram of proposed topology
An improved transformerless inverter with common
mode leakage current elimination for a photovoltaic
grid connected power system by using inverse sine
carrier pulse width modulation (ISPWM). To
eliminate the common-mode leakage current in the
transformerless photovoltaic grid-connected system,
an improved single-phase inverter topology is
presented. The improved transformerless inverter can
sustain the same low input voltage as the full-bridge
inverter and guarantee to eliminate common-mode
leakage current. The inverse sine carrier pulse width
modulation (ISPWM) control strategy can be applied
to implement the presented inverter. The lower total
harmonic distortion and higher fundamental output
voltage are obtained by using the inverse sine carrier
pulse width modulation (ISPWM). The maximum
power point tracking (MPPT) is used to extract the
maximum power from PV panel.
Figure 4. Generation of switching pulses
B. Boost converter and maximum power point
tracking
The boost converter has one controlled semiconductor
switch and it is controlled by applying appropriate
gating pulses. The turn off resistance of the switch is
very much higher than the turn on resistance. Thus by
varying the duty cycle of the gating pulse, the effective
resistance offered by the circuit is varied. The boost
converter is placed right between the inverter and the
PV panel to ensure maximum power transfer. The
resistance of the circuit as seen from the PV panel
must be equal to the internal resistance of the PV
module for maximum power transfer. The duty cycle
of the boost converter is adjusted in such a way that
maximum power is transferred from the module to the
output terminal. The performance of the PV panel
depends highly on then environmental conditions
which vary throughout the day. The efficiency of the
PV panel is very less and hence it becomes necessary
to extract the maximum power from the panel by
shifting the operating point to the maximum power
point.
A. ISPWM Technique.
The modulation strategy employed in this paper is the
inverted sine PWM (ISPWM) technique. In the
conventional PWM method, triangular wave is used as
carrier wherein they are replaced by inverted sine
carrier waves in this model. The inverse sine carrier
pulse width modulation (ISPWM) technique has a
better spectral quality and a higher fundamental
component compared to the conventional sinusoidal
PWM without any pulse dropping. Also, there is a
reduction in the total harmonic distortion (THD).An
inverted sine wave of high switching frequency is
taken as a carrier wave and is compared with that of
the reference sine wave.
The operating point of the PV panel is fixed by the
load resistance. Perturb and observe (P&O) algorithm
is adopted in this work due to its simplicity. In this
algorithm, a perturbation is made on the PV panel
operating point to force tracking in the direction
towards maximum power point.
The pulses are generated whenever the amplitude of
the reference sine wave is greater than that of the
inverted sine carrier wave . The total harmonic
distortion for the different values of switching
frequencies is obtained and is found to be lesser than
Boost converter.
The maximum power point tracking is basically a load
matching problem. In order to change the input
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
172
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
resistance of the panel to match the load resistance (by
varying the duty cycle), a DC to DC converter is
required.
Maximum power point tracking
The photovoltaic (PV) system is one of the renewable
energies that attract the attention of researchers in the
recent decades. The PV generators exhibit nonlinear
I–V and P–V characteristics. The maximum power
produced varies with both irradiance and temperature.
Since the conversion efficiency of PV arrays is very
low, it requires maximum power point tracking
(MPPT) control techniques. The maximum power
point tracking (MPPT) is the automatic control
algorithm to adjust the power interfaces and achieve
the greatest possible power harvest, during moment to
moment variations of light level, shading,
temperature, and photovoltaic module characteristics.
The purpose of the MPPT is to adjust the solar
operating voltage close to the MPP under changing
atmospheric conditions.
It has been studied that the efficiency of the DC to DC
converter is maximum for a buck converter, then for a
buck-boost converter and minimum for a boost
converter but as we intend to use our system for tying
to a grid which requires 230 V at the output end, so
we use a boost converter.
A typical solar panel converts only 30 to 40 percent of
the incident solar irradiation into electrical energy.
Maximum power point tracking technique is used to
improve the efficiency of the solar panel. According
to Maximum Power Transfer theorem, the power
output of a circuit is maximum when the Thevenin
impedance of the circuit (source impedance) matches
with the load impedance. Hence our problem of
tracking the maximum power point reduces to an
impedance matching problem. In the source side we
are using a boost convertor connected to a solar panel
in order to enhance the output voltage so that it can be
used for different applications like motor load. By
changing the duty cycle of the boost converter
appropriately we can match the source impedance
with that of the load impedance.
Figure 5.Boost converter
Mode 1 operation of the Boost Converter:
When the switch is closed the inductor gets charged
through the battery and stores the energy. In this mode
inductor current rises (exponentially) but for
simplicity we assume that the charging and the
discharging of the inductor are linear. The diode
blocks the current flowing and so the load current
remains constant which is being supplied due to the
discharging of the capacitor.
Perturb & Observe
Perturb & Observe (P&O) is the simplest method. In
this we use only one sensor, that is the voltage sensor,
to sense the PV array voltage and so the cost of
implementation is less and hence easy to implement.
The time complexity of this algorithm is very less but
on reaching very close to the MPP it doesn’t stop at the
MPP and keeps on perturbing on both the directions.
When this happens the algorithm has reached very
close to the MPP and we can set an appropriate error
limit or can use a wait function which ends up
increasing the time complexity of the algorithm.
However the method does not take account of the rapid
change of irradiation level (due to which MPPT
changes) and considers it as a change in MPP due to
perturbation and ends up calculating the wrong MPP.
Figure 6.Mode 1 operation
Mode 2 operation of the Boost Converter
In mode 2 the switch is open and so the diode becomes
short circuited. The energy stored in the inductor gets
discharged through opposite polarities which charge
the capacitor. The load current remains constant
throughout the operation.
The Perturb & Observe algorithm states that when the
operating voltage of the PV panel is perturbed by a
small increment, if the resulting change in power P is
positive, then we are going in the direction of MPP
and we keep on perturbing in the same direction. If P
is negative, we are going away from the direction of
Figure 7. Mode 2 operation
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
173
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
MPP and the sign of perturbation supplied has to be
changed.
the parasitic capacitor is clamped to zero. As a result,
the
CM
current
is
eliminated
completely.Meanwhile,the virtual dc bus is created to
help generate the negative output voltage.The
required dc bus voltage is still the same as the full
bridge, and there is not any limitation on the
modulation strategy since the CM current is removed
naturally by the circuit structure.In this way,the
advantage of the full bridge and half bridge based
solutions are combined together.
virtual dc bus concept
Figure 8. Solar panel characteristics showing MPP and
operating points A and B.
Figure shows the plot of module output power versus
module voltage for a solar panel at a given irradiation.
The point marked as MPP is the Maximum Power
Point, the theoretical maximum output obtainable
from the PV panel. Consider A and B as two operating
points. As shown in the figure above, the point A is on
the left hand side of the MPP. Therefore, we can move
towards the MPP by providing a positive perturbation
to the voltage. On the other hand, point B is on the
right hand side of the MPP. When we give a positive
perturbation, the value of P becomes negative, thus it
is imperative to change the direction of perturbation to
achieve MPP. The flowchart for the P&O algorithm is
shown in figure The voltage and current of the PV
panel are measured after one perturbation and the
power is calculated. This is then compared with the
previous value of power and the difference ΔP (ΔP =P
k –Pk-1) is calculated. If ΔP is positive, perturbation is
continued in the same direction. For negative values of
ΔP, the direction of perturbation is reversed.
Figure 10. Virtual DC bus
By connecting the grid neutral line directly to the
negative pole of the PV panel, the voltage across the
parasitic capacitance Cpv is clamped to zero. This
prevents any leakage current flowing through it. With
respect to the ground point N, the voltage at midpoint
B is either zero or +Vdc, according to the state of the
switch bridge.
The purpose of introducing virtual DC bus is to
generate the negative output voltage, which is
necessary for the operation of the inverter. If a proper
method is designed to transfer the energy between the
real bus and the virtual bus, the voltage across the
virtual bus can be kept the same as the real one. The
positive pole of the virtual bus is connected to the
ground point N, so that the voltage at the midpoint C is
either zero or −Vdc.
The dotted line in the figure indicates that this
connection may be realized directly by a wire or
indirectly by a power switch. With points B and C
joined together by a smart selecting switch, the voltage
at point A can be of three different voltage levels,
namely +Vdc, zero and –Vdc. Since the CM current is
eliminated naturally by the structure of the circuit,
there’s not any limitation on the modulation strategy,
which means that the advanced modulation
technologies such as the unipolar SPWM or the double
frequency SPWM can be used to satisfy various PV
applications.
Figure 9.Flowchart P&O
Improved transformerless inverter topology
Here ,a novel topology generation strategy called the
virtual dc bus concept is proposed for the
transformerless grid connected pv inverter. In this
solution, the grid neutral line is connected directly to
the negative pole of the dc bus, so that voltage across
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
174
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
Derived Topology And Modulation Strategy
For all of the four operation states, there is no
limitation on the direction of the output current igrid,
since the power switches with antiparallel diodes can
achieve bidirectional current flow. Therefore, the
proposed topology has the capability of feeding
reactive power into the grid to help support the
stability of the power system.
The proposed topology is also immune against
transient overvoltage of the grid. During the mains
positive voltage spikes, the voltage at point A is
clamped at Vdc by C1 and the antiparallel diodes of S1
and S4 . Similarly, during the negative voltage spikes,
the voltage at point A is clamped at −Vdc by C2 and
the ant parallel diodes of S2 and S5 . Therefore, the
mains transient overvoltage does not pose a safety
threat for the inverter.
Figure 11. Proposed topology
It consists of five power switches S1~S5 and only one
single filter inductor Lf. The PV panels and capacitor
C1 form the real DC bus while the virtual DC bus is
provided by C2. With the switched capacitor
technology, C2 is charged by the real DC bus through
S1 and S3 to maintain a constant voltage.
Operating states for proposed topology
IV. EXPERIMENTAL RESULTS.
The simulation and analysis of circuits are done using
MATLAB-SIMULINK.
A. Simulink model of overall system
State 1
B. Simulation Results.
P-V characteristics of PV module.
(b) State 2
State 3
State 4
Figure 12. Operating states of proposed topology
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
175
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
I-V characteristics of PV module
Leakage current waveform
Output voltage and current
Output voltage of PV panel
CONCLUSION
This paper presented an improved grid-connected
inverter topology for transformerless PV systems. The
concept of the virtual DC bus is proposed to solve the
CM current problem for the transformer less
grid-connected PV inverter. By connecting the
negative pole of the DC bus directly to the grid neutral
line, the voltage on the stray PV capacitor is clamped
to zero. This eliminates the CM current completely.
The inverse sine carrier pulse width modulation
control strategy can be applied to implement the
presented inverter, which can guarantee not to
generate the common-mode leakage current because
the condition of eliminating common-mode leakage
current is met completely. Moreover, the lower total
harmonic distortion(THD) and higher fundamental
output voltage are obtained by inverse sine carrier
pulse width modulation(ISPWM). The smaller filter
inductors are employed and the copper losses and core
losses are reduced accordingly. The software tool used
in this project is MATLAB 2007b.
Switching pulses to inverter switches
Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
176
An Improved Transformer less Inverter Topology for Cost Effective PV Systems
REFERENCES
[1]
Y. Gu, W. Li, Yi Zhao, Bo Yang, C. Li and X. He,”
Transformerless Inverter With Virtual DC Bus Concept for
Cost-Effective Grid-Connected pv power Systems,” IEEE
Transactions on Power Electronics, vol. 28, no. 2, Feb. 2013
[4]
W. Cui, B. Yang, Y. Zhao, W. Li, and X. He, “A novel
single-phase transformerless grid-connected inverter,” in Proc.
37th Annu. Conf. IEEE Ind. Electron. Soc., Nov. 7–10, 2011,
pp. 1126–1130
[2]
B. Yang, W. Li, Y. Gu, W. Cui, and X. He, “Improved
transformerless inverter with common-mode leakage current
elimination for a photovoltaic grid-connected power system,”
IEEE Trans. Power Electron., vol. 27, no. 2, pp. 752–762, Feb.
2012.
[5]
T. Kerekes, R. Teodorescu, P. Rodr´ıguez, G. V´azquez, and E.
Aldabas, “A new high-efficiency single-phase transformerless
PV inverter topology,” IEEE Trans. Ind. Electron., vol. 58, no.
1, pp. 184–191, Jan. 2011.
[3]
H. Xiao and S. Xie, “Transformerless split-inductor neutral
point clamped three-level PV grid-connected inverter,” IEEE
Trans. Power Electron., vol. 27, no. 4, pp. 1799–1808, Apr.
2012
[6]
T. Kerekes, R. Teodorescu, P. Rodr´ıguez, G. V´azquez, and E.
Aldabas, “A new high-efficiency single-phase transformerless
PV inverter topology,” IEEE Trans. Ind. Electron., vol. 58, no.
1, pp. 184–191, Jan. 2011.

Proceedings of 07th IRF International Conference, 22nd June-2014, Bengaluru, India, ISBN: 978-93-84209-29-2
177