IJDI-ERET
INTERNATIONAL JOURNAL OF DARSHAN INSTITUTE ON
ENGINEERING RESEARCH & EMERGING TECHNOLOGIES
Vol. 3, No. 2, 2014
www.ijdieret.in
(Research Article)
Design and Implementation of Leakage Current Minimization
Technique for Single phase Grid Connected Transformer-less PV
Inverter
R. Ramaprabha1*, A. Arrul Dhana Mathy2
1*
Associate Professor, Department of Electrical & Electronics Engineering, SSN College of Engineering, Chennai, Tamil Nadu, INDIA
2
PG-Scholar, Department of Electrical & Electronics Engineering, SSN College of Engineering, Chennai,Tamil Nadu, INDIA
Abstract
Leakage current is an important parameter to be considered in grid connected transformer-less photovoltaic (PV) inverter.
This paper presents a comparative study of H5 topology of transformer-less inverter which employs galvanic isolation
technique (dc-decoupling) and H-bridge zero voltage state rectifier (HBZVR) topology of transformer-less inverter which
combines both galvanic isolation technique (ac decoupling) and common mode voltage clamping (CMV). The sine carrier pulse
width modulation (SPWM) is applied. By using SPWM, the total harmonic distortion can be reduced and higher fundamental
voltage is obtained with reduced filter size.Leakage current, CMV, output voltage and current of both topologiesare compared in
this paper. It is found that the performance of HBZVR is superior to H5 topology.
Keywords:Common mode voltage, galvanic isolation, leakage current, photovoltaic, H5topology, HBZVR.
1. Introduction
The traditional natural resources like fuel and coal are
becoming scarce and energy demand is increasing due to
rapid increase in population, fast growing industries, etc. So
in order to meet the increasing energy demand alternate
energy resources like water, wind and solar are considered.
Solar power is considered to be the best solution for the
increasing energy demand because it is abundant in nature,
free, inexhaustible and pollution free.
Grid connected PV inverters can be with or without
transformer. In grid connected PV inverters with transformer,
the transformer provides galvanic isolation between the PV
and the grid, thus eliminating leakage current. But, the use of
transformer in system reduces its efficiency and also they are
bulky, costly and heavy. Hence to improve the efficiency of
the system and to reduce its cost transformer-less inverter are
used nowadays[1]-[3].
of high leakage current[1]-[2]. In topologies like H5 and
highly efficient and reliable inverter concept (HERIC),
galvanic isolation is provided by employing dc decoupling
and ac decoupling but these topologies do not neglect the
effect of common mode voltage clamping [4-5]. It is reported
in the literature that the topologies like H6 and HBZVR
neglect the effect of common mode voltage clamping [5-8]. It
is required to completely eliminate the leakage current in
both galvanic isolation and common mode voltage clamping
to avoid shock when contact with PV panels.
In this paper, H5 topology which employs only galvanic
isolation and HBZVR topology which employs both galvanic
isolation and CMV are analyzed in terms of leakage current,
common mode voltage, and total harmonic distortion (THD)
using MatLab-Simulink. The block diagram is shown in
Figure 1.
In transformer-less inverter because of absence of galvanic
isolation, leakage current flows from the PV to the grid.
Moreover,the stray capacitance due to common mode voltage
fluctuationscharges and discharges. This leads to generation
*
Corresponding Author: e-mail: ramaprabhar@ssn.edu.in
ISSN 2320-7590
2014Darshan Institute of Engg.& Tech., All rights reserved
Figure 1.Block diagram of the grid connected PV inverter
with leakage current minimization technique
International Journal of Darshan Institute on Engineering Research and Emerging Technology
Vol. 3, No. 2, 2014, pp. 20-24
2. Common mode voltage and leakage current
Generally, transformers provide galvanic isolation
preventing the flow of current between the grid and the PV.
In grid-connected transformer-less inverter galvanic
connection exists and a resonant circuit is formed when the
transformer is removed. The resonant circuit consists of PV
panels, grid, filter inductors and stray capacitances [1]-[2].
The power converter block shown in the Figure 2(a)
represents the various inverter topologies. The dc voltage
source is connected the terminals P and N, and the grid is
connected to the output terminals A and B via the filter. As
seen from the grid, the power converter block can be
considered as two voltage sources VANand VBNas shown in
the Figure 2(b).
Figure 3.H5 topology converter structure
Figure 4.Switching sequence of H5 topology
Figure 2. (a) Common-mode full model for single-phase
grid-connected inverter (b) Simplified model
2.2 HBZVR topology: In this topology, switches S1 to S4 form
full bridge inverter. Switch S 5 and anti-parallel diodes D1 to
D4 provides galvanic isolation by introducing low loss ac
decoupling. Diodes D5 and D6 form the clamping branches
[11-13]. The output voltage has three levels as +V dc, 0 and –
Vdc. During the positive half cycle, switches S 1 and S4 are ON
while switches S2, S3 and S5 are OFF. During the zero voltage
vectors, switch S5 is ON while all other switches are OFF and
the current freewheels through diodes D1 to D4. During the
negative half cycle, switches S2 and S3 are ON while switches
S1, S4 and S5 are OFF. The switching pulse generation for
HBZVR is shown in Figure 6.
Common mode voltage and differential mode voltages are
represented by (1) and (2) [ref]. From these, the output
voltages are derived as (3) and (4).
V
+V
VCM = AN BN
2
V𝐷𝑀 = 𝑉𝐴𝑁 − 𝑉𝐵𝑁
𝑉
V𝐴𝑁 = 𝑉𝐶𝑀 + 𝐷𝑀
(1)
(2)
(3)
V𝐵𝑁 = 𝑉𝐶𝑀 −
(4)
2
𝑉 𝐷𝑀
2
The leakage current depends on the common mode voltage.
Different topologies have different VANand VBN, so
accordingly different common mode voltageVCM. Thus the
leakage current varies for different topologies [1]-[4].
2.2 H5 topology: The H5 topology (Figure 3) consists of an
extra switch S5 connected on the dc side of the H-bridge
inverter structure. This switch S5 provides galvanic isolation
by introducing dc decoupling to disconnect the PV and the
grid [7-10]. The inductors Lf and the capacitor Cf acts as the
filter and is coupled to the grid. The switching sequence is
shown in Figure 4. The output voltage has three levels as
+Vdc, 0 and –Vdc. During the positive half cycle, switch S5
and S4 commutates with switching frequency. During the
zero voltage vectors, S5 and S4 are turned-off and the
freewheeling current flows through S1 and the anti-parallel
diode of S3. In the negative half cycle, S5 and S2 are
commutates with switching frequency and the freewheeling
current flows through S3 and the antiparallel diode of S1[8].
Figure 5.HBZVR topology converter structure
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International Journal of Darshan Institute on Engineering Research and Emerging Technology
Vol. 3, No. 2, 2014, pp. 20-24
Figure 6.Switching sequence of HBZVR topology
The simulation of both topologies is carried out and the
results are presented in the next section.
3. Simulation results
The H5 and HBZVR topologies of grid- connected
transformer-less inverter are simulated for the parameters
given in Table 1 using MatLab [14].
Table 1. Simulation parameters
Parameters
Value
Input voltage
110 Vdc
Load
30 Ω
Switching frequency
4 kHz
Filter inductors
3 mH
Filter capacitor
10 µF
Stray capacitors
100 nF
Frequency
50 Hz
The waveforms of the inverter voltage before and after
filtering, grid current, VAN, VBN, common mode voltage (VCM)
and leakage current for both the topologies are shown in the
Figure 7 and Figure 8.
From the waveforms of VAN, VBN and CMV shown in
Figure 7, it is observed that the common mode voltage is not
clamped in H5 topology. This leads to high leakage current
flow between PV and the grid (Figure 8). For HBZVR
topology, this problem is eliminated which is illustrated in
waveforms of VAN, VBN and CMV shown in Figure 8. Here
the CMV is almost constant. The leakage current waveform is
shown in Figure 9 for this case.
Figure 7.Simulated Results forH5 topology
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International Journal of Darshan Institute on Engineering Research and Emerging Technology
Vol. 3, No. 2, 2014, pp. 20-24
Figure 10.Leakage current of HBZVR topology
It is clear from the waveforms shown above, that the
leakage current is reduced in HBZVR topology compared to
the H5 topology. The comparisons between the topologies are
shown in Table 2.
Table 2.Comparative analysis
H5
Parameters
topology
No. of switches conduct in each
3
conduction period
VLL
Unipolar
CMV
Floating
Load current THD (%)
4.89
HBZVR
topology
2
Unipolar
Constant
1.64
4. Conclusion
In this paper, the H5 topology and HBZVR topology of
transformer-less inverter are modeled and simulated in terms
of inverter output voltage before and after filtering, grid
current, common mode voltage and leakage current. Even
though H5 topology provides galvanic isolation via dc
decoupling, the leakage current is not completely eliminated
due to the improper clamping of CMV. This problem is
eliminated in HBZVR topology by using ac decoupling for
galvanic isolation. It is inferred that the losses are less when
ac decoupling is used and the leakage current is completely
eliminated.
Nomenclature
Cf
CMV
CPV
HBZVR
HERIC
Lf
PV
Figure 8.Simulated Results forHBZVR topology
-
Filter capacitor
-
Common Mode Voltage
Stray capacitor
H-bridge zero voltage state rectifier
Highly efficient and reliable inverter concept
Filter inductor
Photovoltaic
Acknowledgement
The authors wish to thank the management of SSN College
of Engineering, Chennai for providing all the computational
facilities to carry out this work. This project is supported
through student internal funding by SSNCE.
Figure 9.Leakage current of H5 topology
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International Journal of Darshan Institute on Engineering Research and Emerging Technology
Vol. 3, No. 2, 2014, pp. 20-24
8.
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Biographical notes
Dr. R.Ramaprabha is an Associate Professor in SSN College of Engineering, Chennai, TamilNadu, India. She obtained B.E and M.E.
degrees from Bharathidasan University in 1997 and 2000 respectively. She obtained PhD degree from Anna University in the area of solar
PV systems. She has been working in the teaching field for about 14 Years. She has published 26 papers in National conferences, 57
papers in referred International conferences and 44 papers in international journal in the area of solar photovoltaic and power electronics &
drives. She is a life member in ISTE and member in IEEE. Her areas of interest include Solar PV Systems, Power conversion techniques
for renewable energy sources.
A.ArrulDhanaMathyis a Postgraduate student in the department of EEE at SSN College of Engineering, Chennai.She has received B.E.
in Electrical and Electronics Engineering from Anna University in 2013. She is doing project in the area of transformer-less grid connected
PV system.
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