Simulation of H6 full bridge Inverter for grid connected PV system using SPWM technique

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 01 | Jan -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1451
Simulation of H6 full bridge Inverter for grid connected PV system
using SPWM technique
K. Raghava Reddy 1, M. Mahesh2, M. Vijaya Kumar 3
1Student, Dept. of Electrical & Electronics Engineering, JNTUA, Anantapuram, A.P., India
2AdHoc lecturer, Dept. of Electrical & Electronics Engineering, JNTUA, Anantapuram, A.P., India
3Professor, Dept. of Electrical & Electronics Engineering, JNTUA, Anantapuram, A.P., India
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - Transformer less inverter is widely used in grid-
tied photovoltaic (PV) generation systems, due to the benefits
of achieving high efficiency and low cost. Various transformer
less inverter topologies have been proposed to meet the safety
requirement of reducing leakage currents. In the proposed
paper, a family of transformer less H6 invertertopologieswith
low leakage currents is proposed, and the intrinsic
relationship betweenH5Invertertopology, highlyefficient and
reliable inverter concept (HERIC) topology, and the proposed
H6 Inverter topology has been discussed well. One of the
proposed H6 inverter topologies is taken as an example for
detail analysis with operation modesandmodulationstrategy.
The proposed H6 Inverter topologies have the following
advantages and evaluated by simulation results: The
conversion efficiency of the novel H6 Inverter topology is
better than that of the H5 Inverter topology, and its thermal
stress distribution is better than that of the H5 Inverter
topology. The leakage current is almost the same as HERIC
Inverter topology, and meetsthesafetystandard. Theexcellent
DM performance is achieved like the isolated full-bridge
inverter with unipolar SPWM. Therefore, the proposed H6
Inverter topologies are good solutions for the single-phase
transformer less PV grid-tied inverters.
The MATLAB R2010a version is used to simulate the system
results and to validate the concept proposed.
1.INTRODUCTION
Technologies available to grid-tie inverters include
newer high-frequency transformers, conventional low-
frequency transformers, or they may operate without
transformers altogether.Insteadofconvertingdirectcurrent
directly to 120 or 230 volts AC, high-frequencytransformers
employ a computerized multi-step process that involves
converting the power to high-frequency AC and then back to
DC and then to the final AC output voltage. Transformerless
inverters, lighter and more efficient than their counterparts
with transformers, are popular in Europe. However,
transformer less inverters have been slow to enter the
market over concerns that transformer less electrical
systems could feed into the public utility grid without
galvanic isolation between the DC and AC circuits that could
allow the passage of dangerous DC faults to be transmitted
to the AC side. However, since 2005, the NFPA's NEC allows
transformer less (ornon-galvanically)invertersbyremoving
the requirement that all solar electric systems be negative
grounded and specifying new safety requirements. From the
safety point of view, most of the PV grid-tied inverters
employ line-frequency transformers to provide galvanic
isolation in commercial structuresinthepast. However,line-
frequency transformers are large and heavy, making the
Fig.1.1 Leakage current path for transformerless PV
inverters
whole system bulky and hard to install. Compared with line-
frequency isolation, inverters with high-frequency isolation
transformers have lower cost, smaller size and weight.
However, the inverters with high-frequency transformers
have several power stages, which increase the system
complexity and reduce the system efficiency [3]–[6]. Thus,
the transformerless PV grid-tied inverters, as shown in
Fig.1.1, are widely installed in the low-power distributed PV
generation systems.
Unfortunately, when the transformer is removed,
the common-mode (CM) leakage currents (ileakage) may
appear in the system and flow through the parasitic
capacitances between the PV panels and the ground [7], [8].
Moreover, the leakage currents lead to serious safety and
radiated interference issues [9]. Therefore, they must be
limited within a reasonable range [10]. As shown in Fig.1.1,
the leakage current iLeakage is flowing through the loop

consisting of the parasitic capacitances (CPV1 and CPV2),
bridge, filters (L1 and L2), utility grid, and ground
impedance Zg.The leakage current path is equivalent to an
LC resonant circuit in series with the CM voltage [11], and
the CM voltage VCM is defined as
VCM = (VAN + VBN)/2 + (VAN - VBN ) ………. Eqn. (1.1)
Where VAN is the voltage difference between points
A and N, VBN is the voltage difference between points B and
N. L1 and L2 are the output filter inductors. To eliminate
leakage currents, the CM voltage must be kept constant or
only varied at low frequency, such as 50 Hz/60 Hz.
In the full-bridge inverters the filter inductors L1
and L2 are usually with the same value. Thus, Eqn. (1.1) is
simplified as
VCM = (VAN + VBN)/2 ………………………………………Eqn. (1.3)
Many solutions have been proposed to realize CM
voltage constant in the full-bridge transformerlessinverters
A traditional method is to apply the full-bridge inverterwith
the bipolar sinusoidal pulse width modulation (SPWM). The
CM voltage of this inverter is kept constant during all
operating modes. Thus, it features excellentleakagecurrents
characteristic. However, the current ripples across the filter
inductors and the switching losses are likely to be large.
The full-bridge inverters with unipolar SPWM
control are attractive due to the excellent differential-mode
(DM) characteristics such assmaller inductorcurrent ripple,
and higher conversion efficiency. The CM voltage is kept
constant by these full-bridge topologies with unipolar
modulation methods. Another solution is to disconnect the
dc and ac sides of the full-bridge inverterinthefreewheeling
modes. Various topologies have been developed and
researched based on this method for keeping theCMvoltage
constant to eliminate leakage currents.
Eliminating the leakage current is one of most
important issues for transformer less inverters in grid
connected photovoltaicapplications.Thetechnical challenge
is how to keep the common mode voltage constanttoreduce
the leakage current. For this purpose, an improved single
phase transformer less inverter is proposed. It has two
additional switches connected in the dc side. The PWM
pulses for those switches are given in such a way that the
condition for making the common mode voltage constant is
completely met. The common mode voltage can remain a
constant during all the modes in the improved inverter. By
maintaining common mode voltage as constant in all modes
of operation the leakage currents can be effectivelyreduced.
2.EXISTING INVERTER TOPOLOGIES
H5 and HERIC Inverter topologies are already
existing models. H5 Inverter circuit configuration is as
shown in fig 2.1. which employs an extra switch on DC side
of the Inverter.HERIC Inverter circuit configuration is as
shown in fig 2.2. which employs two extra switchesatthe AC
side of the Inverter. Both Inverter topologies have four
modes of operation. They are:
1. Active mode in the positive half cycle
2. Freewheeling mode in the positive half cycle
3. Active mode in the negative half cycle
4. Freewheeling mode in the negative half cycle
Fig.2.1 Circuit structure of H5 topology
Fig.2.2 Circuit structure of HERIC topology
Each operation mode of operation of the two
Inverter topologies has been explained well in reference
paper [1]. In this paper a new H6 full bridge Inverter is
proposed with unipolar SPWM (sinusoidal pulse width
modulation) to reduce common mode leakage currentmuch
less compared to H5 and HERIC Inverter topologies.
3.PROPSED H6 INVERTER TOPOLOGY
3.1 Condition of eliminating common mode leakage
current
The following assumptions are made for deriving the
condition of eliminating the common mode leakage current.
 Filter inductors used LA and LB are assumed to be
of same value.
 Common mode voltage, µcm is expressed as
µcm = (µAN + µBN) / 2
 The switches and diodes are ideal and the dead
time between the switches are neglected.
 Inductors are ideal without any internal
resistance.

3.2. Parasitic capacitance and leakage current
PV panels are manufactured in many layers and the
junction of these layers is covered by grounded metallic
frame. A parasitic capacitance (stray capacitance) is formed
between the earth and the metallic frame.Itsvalueisdirectly
proportional to the surface area of the PV panel. Dangerous
leakage currents (common mode currents)canflowthrough
the large stray capacitance between the PV array and the
ground if the inverter generates a variable common mode
voltage. These leakage currents have to be eliminated or at
least limited to a safe value.
3.3 Condition of eliminating the common mode
leakage current
The ground leakage current that flows through the
parasitic capacitance of the PV array is greatly influence on
the common mode voltage generated by a topology.
Generally, the utility grid does not influence the common
mode behavior of the system. The common-mode voltage
can be defined as the average of the sum ofvoltages between
the outputs and the common reference. In this case, the
common reference is taken to be the negativeterminal ofthe
PV. The differential-mode voltageisdefinedasthedifference
between the two voltages.
µcm = (µAN + µBN) / 2…Eqn (3.1)
µdm = μAB = µAN - µBN…. Eqn (3.2)
From the above two equations
µAN = µcm + ….Eqn (3.3)
µBN = µcm - …Eqn (3.4)
Fig.3.1. Model Showing the Common-Mode and
Differential-Mode Voltages
Using Thevenin’s theorem in the above circuit the
model can be simplified. By applying Kirchhoff’s voltage law
in the Fig 3.1.
Fig.3.2 Model to find out the Equivalent Common-Mode
Voltage
To find out the current,
-µcm - – iLA – iLB + µcm - = 0… Eqn (3.5)
-µdm - iLA – iLB = 0 …………………….Eqn (3.6)
-µdm = i (LA + LB) ......................Eqn (3.7)
i = ………………..Eqn (3.8)
To find out the equivalent common mode voltage (µecm)
-µcm - – iLA + µecm = 0 …………..…Eqn (3.9)
µecm = µcm + + iLA ……………. Eqn (3.10)
µecm = µcm + + LA …Eqn(3.11)
µecm = µcm + …. … Eqn (3.12)
The simplified equivalent model of the common-mode
resonant circuit has been derived in as shown in Figure 3.3,
where CPV is the parasitic capacitor, LA and LB are the filter
inductors, icm is the common-mode leakagecurrent.And,an
equivalent common-mode voltage µecm is defined by,
µecm = µcm + ……………...Eqn (3.13)
3.4 Proposed H6 Inverter circuit topology
From the analysis, an extra switch S6 is introduced
into the H5 inverter topology between the positive terminal
of the PV array and the terminal (B) to form a new current
path [1]. Thus, a novel H6 transformer less full-bridge
inverter topology is derived, as shown in Fig.3.4. Similarly,
the extra switch S6 can be introduced into the H5 inverter
topology between the positive terminals of the PV array and
the terminal (A) to form a new current path as well, as
shown in Fig.3.5. Therefore, a new circuit structure of novel
H6 inverter is presented. Thus, the conduction loss of the
proposed H6 topologies is higher than HERIC topology and
less than H5 topology. The two possible configurationsofH6
inverter are as shown in figure below.
Fig 3.4 Proposed H6-type inverter topology (structure A)

Fig 3.5 Proposed H6-type inverter topology
(structure B)
3.5 Operation mode analysis
The circuit structure of proposed novel H6 inverter
topologies shown in Fig.3.4 is taken as an example to
analysis. PV grid-tied systems usually operate with unity
power factor. The waveforms of the gate drivesignalsforthe
proposed novel H6 topology are shown in Fig.3.6, where vg
is the voltage of utility grid. iref is the inductor current
reference. vgs1 to vgs6 represent the gate drive signals of
switches S1 to S6, respectively.
Fig.3.6 Schematic of gate drive signals with unity power
factor
There are four operation modes in each period of
the utility grid, where VAN represents the voltage between
terminal (A) and terminal (N) and VBN representsthevoltage
between terminal (B) and terminal (N). VAB istheDMvoltage
of the topology, VAB = VAN – VBN. The CM voltage VCM =0.5(VAN
+ VBN). The four operation modes in each period of utility
grid are
1. Active mode in the positive half cycle
2. Freewheeling mode in the positive half cycle
3. Active mode in the negative half cycle
4. Freewheeling mode in the negative half cycle
3.5.1 Active mode in the positive half cycle
Mode I is the active mode in the positive half period
of the utility grid voltage, as shown in Fig.3.7. S1, S4 and S5
are turned ON, and the other switches are turned OFF. The
inductor current is flowing through S1, S4 and S5.
During this mode of operation
VAN = UPV
VBN = 0
Thus, VAB = UPV
And the CM voltage VCM = (VAN + VAN)/2 = 0.5 UPV.
Fig.3.7 Active mode in the positive half cycle
3.5.2 Freewheeling mode in the positive half cycle
Mode II is the freewheeling modeinthe positivehalf
period of the utility grid voltage, as shown in Fig.3.8. S1 is
turned ON; the other switches are turned OFF.As the
inductor does not allow sudden changes of current through
it so inductor current is freewheeling through S1andthe ant
paralleled diode of S3.
VAN = VBN ≈ 0.5 UPV
Thus, VAB = 0
And the CM voltage VCM= (VAN + VBN)/2 ≈ 0.5 UPV.
Fig.3.8 Freewheeling mode in the positive half cycle
3.5.3 Active mode in the negative half cycle
Mode III is the active mode in the negative half
period of the utility grid voltage, as shown in Fig.3.9. S2, S3,
and S6 are turned ON; the other switches are turned OFF.

The inductor current is flowing through S2 and S6. Although
S3 is turned ON, there is no current flowing through it, and
the switch S3 has no conduction loss in this mode.
Nevertheless, in the H5 topology, the inductor current flows
through S2, S3, and S5. Therefore, the conduction loss of
proposed topology is less than that of H5 topology.
VAN = 0
VBN = UPV
Thus, VAB = − UPV
And the CM voltage VCM = ( VAN + VBN)/2 = 0.5 UPV.
Fig.3.9 Active mode in the negative half cycle
3.5.4 Freewheeling mode in the negative half cycle
Mode IV is the freewheeling mode in the negative
half period of the utility grid voltage, as shown in Fig.3.10.
S3is turned ON, and the other switches are turned OFF.As
the inductor does not allow sudden changes of current
through it so inductor current is freewheeling through S3
and the anti-paralleled diode of S1.
VAN = VBN ≈ 0.5 UPV
Thus, VAB = 0
And the CM voltage VCM = (VAN + VBN)/2 ≈ 0.5 UPV
From all modesofoperation, itisconcluded
that the common mode voltage is constant in each mode of
operation. By maintaining so the leakage currents can be
reduced. The CM voltage of the proposed topology in each
operation mode is equals to 0.5 UPV, and it results in low
leakage currentcharacteristicoftheproposedH6topologies.
Fig.3.10 Freewheeling mode in the negative half
cycle
Based on the fore mentioned analysis, the PV array
can be disconnected from the utility grid when the output
voltage of the proposed H6 inverter is at zero voltage level
and the leakage current path is cut off. The CM voltage of the
proposed topology in each operation mode is equals to 0.5
UPV, and it results in low leakage current characteristicof the
proposed H6 topologies. The proposed H6 topology with
unipolar SPWM method not only can achieve unity power
factor, but also can control the phase shifts between voltage
and current waveforms. The modulationstrategyisshownin
Fig.3.11. The drive signal is in phase with the grid-tied
current. Therefore, it has the capability of injecting or
absorbing reactive power, whichmeetsthedemandforVDE-
4105 standard.
The schematic of gate drive signals with power
factor other than unity are as shown in fig.3.11. From figure,
it is observed that the switches S4 & S5 conducts
simultaneously while the switches S2 & S6 areinoff position
and vice versa. The switches S1 & S2 will conduct for more
than 50% of the duty cycle.
Fig.3.11 Schematic of gate drive signals with power factor
other than unity
4. COMPARISON OF THE PROPOSED H6 INVERTER
WITH EXISTING TOPOLOGIES
Table 4.1 Comparison of Existing and proposed Inverter
topologies
H6 TOPOLOGY H5 TOPOLOGY HERIC
TOPOLOGY
Includes two
extra switches on
the DC side of the
Inverter
Includes two
extra switches on
the DC side of the
Inverter
AC side of the
Inverter have
two extra
switches
Total device
number is six
Total device
number is five
Total device
number is six

Device cost is
same as that of
HERIC
Have lowest
device cost
Device cost is
same as that of
H6
H6 topology has
four modes of
operation
H5 topology has
four modes of
operation
HERIC topology
has four modes of
operation
Conduction
losses higher
than HERIC
Have highest
conduction loss
Conduction loss
less than H6
Thermal stress
distribution is
between H5 and
HERIC
Worst thermal
stress
distribution
Good thermal
stress
distribution
Leakage current
characteristic is
like that of HERIC
topology
Best leakage
current
characteristic
Occupies second
place in case of
leakage current
characteristic
Switching losses
are same as that
of H5 and HERIC
Switching losses
are same as that
of H6 and HERIC
Switching losses
are same as that
of H5 and H6
Diode
freewheeling loss
is same as that of
H5 and HERIC
Diode
freewheeling loss
is same as that of
H6 and HERIC
Diode
freewheeling loss
is same as that of
H5 and H6
European
efficiency is
about 97.09%
European
efficiency is
about 96.78%
European
efficiency is
about 97%
5.SIMULATION RESULTS
5.1 simulation model
The simulation model of the H6 full bridge Inverter
circuit fed from PV panel feeding the grid through filter
inductors is as shown in the figure below. The parasitic
capacitances appearing between PV panel and ground are
also represented. Unipolar SPWM technique is used as a
modulation strategy of switches. By simulating the circuit
the common mode voltage is maintained as constant in all
modes of operation. And from the results we can prove that
H6 full bridge Inverter provides good Differential Mode
characteristics with low leakage current characteristic.
Fig.5.1. Simulation Model of H6 inverter using SPWM
technique
5.2 Output Voltage
Fig.5.2 Output voltage waveform Differential mode
voltage (VAB) = VAN-VBN.
5.3 Common mode voltage
Fig.5.3 common mode voltage waveform

5.4 Leakage current characteristic in H6 topology
Fig.5.13. Leakage current in H6 topology
6.CONCLUSIONS
Common mode leakage current problem in
transformer less inverter is solved using the improved
transformer less inverter. The proposed H6 topology is
explained in detail with each mode of operation with
relevant waveforms showing common mode voltage,output
voltage, leakage current,differential modecharacteristicand
voltage stress across the switches. The improved H6
topology has two additional switches connected in the dc
side of the inverter. By employing sinusoidal pulse width
modulation technique to the H6 topology thecommonmode
leakage current is kept constant throughout all modes of
operation. By maintaining so, the leakage current path is
cutoff from the PV panel to the load during freewheeling
mode of operation both in positive and negative half cycles.
Thereby the common mode leakage current problem in
transformer less inverter is solved using the improved
transformer less inverter.
ACKNOWLEDGEMENT
I wish to express my gratitude to my beloved guide
Dr. M. VIJAYA KUMAR M.Tech, Ph.D, Professor, Electrical
Engineering Department, J.N.T.U.A. College of Engineering
(Autonomous), Anantapur, for his valuable guidance in the
successful completion of this dissertation work. I am very
much indebted to him for suggesting this topic and helping
me at every stage for its successful completion.
I express my profound thanks to Dr.R.KIRANMAYI
M. Tech, Ph. D, Professor and Head of the Electrical Engineering
Department, who hasencouragedmetocompletetheproject
by providing all necessary facilities to carry out the work in
the college.
REFERENCES
[1] Li Zhang, Kai Sun, Yan Xing, Mu Xing “H6 transformerless full bridge PV
grid tied Inverters”, IEEE Transactions on Power electronics, vol.29, No.3,
March 2014.
[2] Y. Gu, W. Li, Y. Zhao, B. Yang, C. Li, and X. He, “Transformerless inverter
with virtual DC bus concept for cost-effective grid-connected PV power
systems,” IEEE Trans. Power Electron., vol. 28, no. 2,pp.793–805,Feb.2012.
[3] L. Zhang, K. Sun, L. Feng, H. Wu, and Y. Xing, “A family of neutral point
clamped full-bridge topologies for transformerless photovoltaic grid-tied
inverters,” IEEE Trans. Power Electron., vol. 28, no. 2, pp. 730–739, Feb.
2012.
[4] B.Yang, W.Li, Y.Gu, W. Cui,andX.He,“Improvedtransformerlessinverter
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] H. Xiao and S. Xie, “Transformerlesssplit-inductorneutralpointclamped
three-level PV grid-connected inverter,”IEEETrans.PowerElectron.,vol.27,
no. 4, pp. 1799–1808, Apr. 2012.
[6] T. Kerekes, R. Teodorescu, P. Rodriguez, G. Vazquez, 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.
[7] W. Cui, B. Yang, Y. Zhao, W. Li, and X. He, “A novel single-phase
transformerless grid-connected inverter,” in Proc. IEEE IECON, 2011,
pp.1067–1071.
[8] W. Yu, J. Lai, H. Qian, andC. Hutchens, “High-efficiency MOSFET inverter
with H6-type configuration for photovoltaic nonisolated ac-module
applications,” IEEE Trans. PowerElectron.,vol.26,no.4,pp.1253–1260,Apr.
2011.
[9] VDE-AR-N 4105: Power Generation Systems Connected to the Low-
Voltage Distribution Network—Technical Minimum Requirements For the
Connection to and Parallel Operation with Low-Voltage Distribution Net-
works, DIN_VDE Normo, 2011–08.
[10] O. Lopez, F. D. Freijedo, A. G. Yepes, P. Fernandez-Comesana, J. Malvar,
R. Teodorescu, and J. Doval-Gandoy, “Eliminating ground current in a
transformerless photovoltaic application,” IEEE Trans. Energy Convers.,vol.
25, no. 1, pp. 140–147, Mar. 2010.
[11] H. Xiao and S. Xie, “Leakage current analytical modeland applicationin
single-phase transformerless photovoltaic grid-connected inverter,” IEEE
Trans. Electromagn. Compat. vol. 52, no. 4, pp. 902–913, Nov. 2010.
[12] S. V. Araujo, P. Zacharias, andR. Mallwitz, “Highlyefficientsingle-phase
transformerless inverters for grid-connected photovoltaic systems,” IEEE
Trans. Ind. Electron., vol. 57, no. 9, pp. 3118–3128, Sep. 2010
[13] Y. L. Xiong, S. Sun, H. W. Jia, P. Shea, and Z. J. Shen, “New physical
insights on power MOSFET switching losses,” IEEE Trans. Power Electron.,
vol. 24, no. 2, pp. 525–531, Feb. 2009.
[14] T. Shimizu and S. Iyasu, “A practical iron loss calculation for AC filter
inductors used in PWM inverter,” IEEE Trans.Ind.Electron.,vol.56,no.7,pp.
2600–2609, Jul. 2009.
[15] B. Sahan, A. N. Vergara, N. Henze, A. Engler, and P. Zacharias, “A single
stage PV module integratedconverter basedonalow-powercurrentsource
inverter,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2602–2609, Jul. 2008.
[16] Q. Li and P. Wolfs, “A review of the single phase photovoltaic module
integrated converter topologies with three different dc link configuration,”
IEEE Trans. Power Electron., vol. 23, no. 3, pp. 1320–1333, May 2008.
[17] R.Gonzalez, E.Gubia, J.Lopez, and L.Marroyo, “Transformerless single-
phase multilevel-basedphotovoltaicinverter,”IEEETrans.Ind.Electron.,vol.
55, no. 7, pp. 2694–2702, Jul. 2008.

Simulation of H6 full bridge Inverter for grid connected PV system using SPWM technique

More Related Content

What's hot (20)

Similar to Simulation of H6 full bridge Inverter for grid connected PV system using SPWM technique (20)

More from IRJET Journal (20)

Recently uploaded (20)

Simulation of H6 full bridge Inverter for grid connected PV system using SPWM technique