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IJSRD - International Journal for Scientific Research & Development| Vol. 5, Issue 01, 2017 | ISSN (online): 2321-0613
All rights reserved by www.ijsrd.com 1294
Transformer-less Inverter for Photovoltaic Grid Connected Power
System
Ms. Priyanka S. Patil1 Dr. Anwar M. Mulla2 1PG Student 2Principal
1Department of Electrical power system 1,2ADCET, Ashta
Abstract— This paper presents newly developed
transformer-less single phase inverter for photovoltaic (PV)
power system. When grid connected photovoltaic system is
transformer-less, a galvanic connection between the grid and
PV array exists. In these conditions, common mode leakage
currents can appear through the stray capacitance in between
PV array and the ground. In order to avoid these leakage
current, different inverter topologies are used which
generate constant common mode voltages. The two different
control techniques are used to eliminate common mode
leakage current. These current increase harmonics injected
in the utility grid and losses.
Key words: Total Harmonics Distortion (THD), Photo
Voltaic (PV), sinusoidal pulse width modulation (SPWM)
I. INTRODUCTION
Grid connected photovoltaic (PV) system include a line
transformer in between conversion stage and grid. The
transformer create galvanic isolation between grid and PV
system, thus there is no safety issue. However, because of its
use a system becomes bulky and expensive. More to the
point, it reduces the overall efficiency of conversion stage.
Because of cost and size reduction and overall efficiency
improvement, the interest on transformer less topologies is
growing. If the system is transformer-less the inverter must
cover purpose of transformer [1]. The galvanic connection
between the grid and PV Array result in common mode
leakage current flows through the parasitic capacitor appear
between the PV array and ground. As shown in fig 1.These
common mode leakage current cause severe electromagnetic
interferences, grid current distortion, and additional losses in
the system. To avoid these common mode leakage current, it
is necessary to use conversion topologies which do not
generate variable common mode voltages [4]. There is
several conversion topologies used to like half bridge
topology, Full bridge inverter topology with unipolar or
bipolar sinusoidal pulse width modulation (SPWM). Half
bridge topology requires a high input voltage which is
greater than 700Vfor 220V-ac applications. As a result,
large number of PV modules in series involved which
increase the cost of system. Full bridge inverter just need
half of the input voltage demanded by the half bridge
topology, which is about 350Vfor 220-application [3], But
full-bridge inverter can only employ for the bipolar SPWM
strategy which induces high current ripple, low system
efficiency.
Fig. 1: Leakage current in transformer less Inverter.
Furthermore, many advanced inverter topologies
developed for transformer-less PV application such as H5
inverter, HERIC inverter etc. as shown in fig2. This system
used for both the unipolar SPWM and bipolar SPWM with
three level output can be obtained in the presented inverter.
Unipolar have higher efficiency same as bipolar technique
but bipolar strategy have higher efficiency and lower current
ripple. A smaller filter inductor can be employed to reduce
harmonic and THD of the output current are reduced
greatly, and grid current quality is improved accordingly. In
this paper an improved grid connected inverter topology for
transformer-less PV system is presented, which can sustain
the same low input voltage as the full bridge inverter and it
also have guarantee not to generate the common mode
leakage current.
(a) Half bridge inverter
(b) Full bridge inverter
Transformer-less Inverter for Photovoltaic Grid Connected Power System
(IJSRD/Vol. 5/Issue 01/2017/353)
All rights reserved by www.ijsrd.com 1295
(c) H5 inverter
(d)HERIC inverter
Fig. 2: Advanced Inverter topologies for PV applications.
A. Analysis of Common Mode Leakage Current
In order to derive common mode leakage current
consider N be the reference point of solar panel. How to
eliminate a leakage current derived below in stepwise,
1) Step1.
Consider equivalent Circuit as below,
Fig. 3: Equivalent circuit of leakage current analysis
The common mode voltage defined as,
Vcm=(VAN+VBN)/2
The differential mode voltage,
Vdm=VAN-VBN
2) Step 2.
Equivalent circuit simplified as,
Fig. 4: Equivalent circuit of leakage current analysis
modified
3) Step3.
The switching frequency of IGBT is much higher than that
of gird hence effect of grid on variation of common mode
voltage is ignored[5].
Fig. 5: Equivalent circuit of leakage current analysis
modified
4) Step4.
Under balance condition, if LA=LB
Fig. 5: Equivalent circuit of leakage current analysis
modified
Thus from equivalent circuit shown in fig. 5 the
common mode current Icm is,
Icm = Cpv dVcm /dt
Thus it is derived that the common mode leakage
current is depend on variation of common mode voltage. So
for eliminating leakage current, it is necessary to keep
common mode voltage constant.
II. IMPROVED INVERTER TOPOLOGY
Fig. 6: Improved inverter topology
Fig.6. shows an improved grid connected inverter topology,
which eliminate common mode leakage current by keeping
common mode voltage constant. There are two strategy used
to keep common mode voltage constant, 1) Unipolar
SPWM, 2) Bipolar SPWM sinusoidal pulse width
modulation. This technique is used to achieve a three level
output and lower current ripple [6]. The improved inverter
Transformer-less Inverter for Photovoltaic Grid Connected Power System
(IJSRD/Vol. 5/Issue 01/2017/353)
All rights reserved by www.ijsrd.com 1296
topology is same as to full bridge inverter but one difference
is that, two additional switches are connected towards dc
side for decoupling purpose.
A. Unipolar SPWM Technique
There are four operation mode in unipolar SPWM which
generate the voltage stages as +Vdc, 0, -Vdc as below,
Fig. 7.Mode1 under unipolar SPWM
1) Mode 1:
When S4 and S5 are ON, VAB = +Vdc and the inductor
current increases through the switches S5, S1, S4, and
S6.Common mode voltage,
Vcm = (VAN+VBN)/2 = (Vdc+0) = Vdc/2
2) Mode 2:
When S4 and S5 are turned OFF, the voltage VAN falls and
VBN rises until their values are equal, and the antiparallel
diode of S3 conducts. Therefore, VAB =0V and the inductor
current decreases through the switch S1 and the antiparallel
diode of S3
The common-mode voltage isVcm,
Vcm = (Vdc/2+Vdc/2)/2= Vdc/2
Fig. 8: Mode 2 under unipolar SPWM
3) Mode3:
When S3 and S6 are ON, VAB =Vdc and the inductor current
increases reversely through the switches S5, S3, S2, and S6.
The common mode voltage Vcm,
Vcm= (0+Vdc)/2= Vdc/2
Fig. 9: Mode 3 under unipolar SPWM
4) Mode 4:
When S3 and S6 are ON, VAB =Vdc and the inductor current
increases reversely through the switches S5, S3 , S2 , and S6.
The common mode voltage Vcm,
Vcm=(Vdc/2+Vdc/2)/2=Vdc/2
Fig. 10: Mode 4 under unipolar SPWM
5) Operation Mode under Unipolar SPWM
Fig. 10: Ideal waveforms of the improved inverter with
unipolar SPWM
From above we can see that common mode voltage remain
constant during all mode of operation.
Mode Switches Voltage
Vab
Voltage
vcm ON OFF
1 S4andS5 +Vdc Vdc/2
2 S4and S5 0V Vdc/2
3 S3and S6 -Vdc Vdc/2
4 S3and
S6 0 Vdc/2
Table 1:
Transformer-less Inverter for Photovoltaic Grid Connected Power System
(IJSRD/Vol. 5/Issue 01/2017/353)
All rights reserved by www.ijsrd.com 1297
B. Bipolar SPWM Technique
The improved inverter can also operate with the Bipolar or
double frequency SPWM strategy to achieve a lower ripple
and higher frequency of the output current. In this situation,
both phase legs of the inverter are modulated with 180©
opposed reference waveform and the switches S1-S4 all
acting the switching frequency. Two additional switches S5
and S6 also commutate at the switching frequency
accordingly. In positive half cycle mode 1, mode2, mode5
continuously rotate to generate +Vdc and zero states. In
negative half cycle mode3, mode4 and mode 6 continuously
rotate to generate –Vdc and zero states.Thus there are six
mode of operation continuously rotate with double
frequency to generate +Vdc and zero states or –Vdc and
zero states.
Modes 1 to Mode 4 are same as unipolar SPWM remaining
two modes are as below,
Mode 5:
Fig. 11: Mode5under bipolar SPWM
WhenS1 and S6 are turned OFF, the voltage VAN
fall and VBN rises until their values are equal, and anti-
parallel diode of S2 conducts. Therefore, VAB = 0V and
the inductor current decreases through switch S4 and anti-
parallel diode of S2. The common mode voltage Vcm ,
Vcm= (VAN+VBN)/2 =(Vdc/2+Vdc/2)/2 =Vdc/2
Mode 6:
Fig. 12: Mode6 under bipolar SPWM
When S1and S 6are turned OFF, voltage VAN falls
and VBN rises until their values are equal, and the anti-
parallel diode of S2 conducts. Therefore VAB=0Vand
inductor current decreases through the switch S4 and the
anti-parallel diode of S2. The common mode voltage Vcm,
Vcm= (Vdc/2+Vdc/2)/2=Vdc/2
Thus, under bipolar SPWM strategy the common
mode voltage can keep a constant Vdc/2 in the whole
switching process of six operation modes. Furthermore, the
lower current ripple, higher grid current quality and lower
THD achieved.
Fig. 13: Ideal waveforms of the improved inverter with
bipolar SPWM.
III. SIMULATED RESULT UNDER UNIPOLAR SPWM
Fig. 14: Simulated results of (a) Output voltage (b) Grid
voltage (c) Grid current
Fig. 15: Simulated results of (a) common mode voltage (b)
Leakage current.
Fig. 16: THD with Unipolar SPWM
Transformer-less Inverter for Photovoltaic Grid Connected Power System
(IJSRD/Vol. 5/Issue 01/2017/353)
All rights reserved by www.ijsrd.com 1298
From above, it is clear that grid connected current is highly
sinusoidal and synchronized with grid voltage and common
mode leakage current is eliminated. The harmonic profile
for grid connected current with unipolar SPWM technique is
shown which indicates that THD of grid connected current
with unipolar SPWM technique is 2.48%
IV. SIMULATED RESULT UNDER BIPOLAR SPWM
Fig. 17: Simulated results of (a) Output voltage (b) Grid
voltage (c) Grid current
Fig. 18: Simulated results of (a) common mode voltage (b)
Leakage current
Fig. 19: THD with bipolar SPWM
From above, it is clear that grid connected current is highly
sinusoidal and synchronized with grid voltage and common
mode leakage current is eliminated. The harmonic profile
for grid connected current with bipolar SPWM technique is
shown which indicates that THD of grid connected current
with bipolar SPWM technique is 1.99%.
So it is clear that, THD of grid connected current
under double frequency SPWM technique is lower than the
unipolar SPWM technique.
V. CONCLUSION
This paper present the performance of transformer less
inverter with different control strategy to achieve three level
output. The bipolar technique achieves lower THD than
unipolar technique. This paper also deals with analysis of
common mode leakage current in transformer less inverter
in PV application. The simulated result shows that as the
common mode voltage is kept constant, the leakage current
is completely eliminated.
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