International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Three-Phase Eight Switch Inverter with Reduced
Common mode voltage for Transformerless
Photovoltaic Systems
Ahmad Syed
S.Tara Kalyani
Dept of Electrical and Electronics Engineering
Jawaharlal Nehru Technological University
Hyderabad, India
syedahmad2@gmail.com
Dept of Electrical and Electronics Engineering
Jawaharlal Nehru Technological University
Hyderabad, India
tarakalyani@jntuh.ac.in
Abstract— In grid connected photovoltaic (PV) systems
transformerless inverters are widely used due to reduced size,
weight and cost. However, common mode voltage reduction is
one of the most key issues for transformerless PV inverters. In
addition, several attractive single-phase transformerless inverter
topologies have been reported to eliminate the common mode
voltage based on converter topology and modulation strategy.
Moreover, in literature three-phase topologies with reduced
common mode voltage are not well investigated. In this paper, a
novel three phase eight switch inverter (H8) and a corresponding
modulation technique is proposed to reduce the common mode
voltage. It is derived from the standard three-phase (H6) inverter
but has different operating principles. Theoretical and
performance analyses are carried out via MATLAB/Simulink to
validate the effectiveness of the proposed H8 topology.
three level T-type inverters [13]. Recently Suan, et al propose
a new three phase converter with modified discontinuous
pulse width modulation (MDPWM) technique [14] and
X.Guo, et al propose a family of three-phase inverters based
on DC-decoupling topologies [17]. The main objective of this
paper is to introduce novel topology with suitable modulation
strategy. Based on the dc-decoupling single phase topologies,
a novel eight switch three phase inverter is proposed with
reduced common mode voltage and leakage current. This
paper is organized as follows. The common mode voltage
analysis in conventional three phase system is discussed in
section II, proposed topology and common mode evaluation is
presented in section III, simulation results and conclusion are
narrated in section IV and V.
Keywords—Common mode current, common mode voltage,
modulation index, transformerless inverter, three phase inverter.
II. COMMON MODE VOLTAGE ANALYSIS IN CONVENTIONAL
I. INTRODUCTION
Present trend needs more energy due to fast-growing
industries and human population. In renewable energy sources
photovoltaic energy plays a vital role because of unlimited and
pollution free [1]-[3]. In grid connected applications
transformerless PV inverters are attractive solutions compared
to transformer systems due to reduced size, weight and cost.
However, it has issues with common mode voltage and direct
current injection. In general, dc injection problems are
minimized by using advance sensors and control solutions. On
the other hand common mode voltage can be reduced with
novel topologies and control strategies.
In literature, several single phase topologies have been
reported to deal with the common mode voltage problems [4][10].But three phase topologies with reduced common mode
voltage are not well explored. A conventional three phase
topology and control strategy was proposed by Cavalcanti, et
al but it fails to reduce the common mode voltage and leakage
current or common mode current [11]. To solve these issues, a
novel space vector modulation technique along with three
phase configuration was proposed by Cavalcanti, et al [12].
On the other hand a new space vector modulation was
proposed by Lee, et al to reduce the common mode voltage in
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
THREE PHASE TOPOLOGY
The transformerless PV inverters are employed in grid
connected systems, as shown in Fig. 1(a) [9]. This section
explains on CMV issues associated with the conventional
three phase inverter, as shown in Fig. 1(b).
(a)
Cpv1
P
S1
S3
S5
RG1
A
Lf
B
Lf
grid
Cdc
grid
Lf
C
S2
S6
S4
RG2
Cpv2
N
(b)
grid
O
(c)
Fig.1 (a). Block diagram of the single phase grid connected system, (b)
conventional three phase inverter(H6) configuration, (c) Mode of operation
M4(011)
TABLE I. CONVENTIONAL THREE PHASE (H6) OPERATION MODES
AND CMV EVALUATION
Mode
VAn
VBn
VCn
Vcm
Vmax
Mode1(100)
Vd
0
0
Vd/3
Vd
Mode3(010)
0
Vd
0
Vd/3
Mode5(001)
0
0
Vd
Vd/3
Mode2(110)
Vd
Vd
0
2Vd/3
Mode4(011)
0
Vd
Vd
2Vd/3
Mode6(101)
Vd
0
Vd
2Vd/3
Mode7(111)
Vd
Vd
Vd
Vd
Mode8(000)
0
0
0
0
Vcmv_max
0
The Fig.1 (b) illustrates the conventional three phase
inverter, where Cpv is the stray capacitance between PV to
ground in series with ground resistance RG. Table I. shows the
list of eight modes by considering the complementary
operation of upper and lower switches in each phase, such as
Mode4 (011) refers to the upper switches S3, S5 of phase B
and C are turn on, while the switch S1 of phase-A is turn off.
In this case, the common mode voltage is Vcm=
(VAn+VBn+VCn)/3 =2Vd/3, where Vd is dc bus voltage.
Moreover, it can be observed that the common mode voltage
is varying and its maximum value (Vmax) is equal to the dc bus
voltage (Vd).
common mode voltage can be determined as 2Vd/3. Instead of
six modes, Mode7 (***1) is quite different, which is turn off
the all switches of Phase A-B-C S1-S6 and S7, S8 is turn on,
resulting CMV equal to Vd/2. Former six modes of operation
are similar to typical H6 configuration as shown in TABLE I
& II. The common mode voltage is greatly reduced by Vd/3,
by comparing the peak-to-peak values, Table I. The CMV is
mitigated and reduced leakage current flows, as per VDE
0126-1-1 standards. The operating principle of the proposed
eight switch topology is quite different from the conventional
three phase H6 topology. A novel modulation strategy is
presented in Fig. 2(b), which is implemented by the simple
logic functions, known as carrier modulation [14]-[17], as
shown in Fig.3. This approach is simple compared to
conventional space vector modulation.
TABLE II. PROPOSED TOPOLOGY (H8) OPERATION MODES AND
CMV EVALUATION
Mode
VAn
VBn
VCn
Vcm
Mode1(1001)
Vd
0
0
Vd/3
Mode3(0101)
0
Vd
0
Vd/3
Mode5(0011)
0
0
Vd
Vd/3
Mode2(1101)
Vd
Vd
0
2Vd/3
Mode4(0111)
0
Vd
Vd
2Vd/3
Mode6(1011)
Vd
0
Vd
2Vd/3
Mode7(***1)
Vd/2
Vd/2
Vd/2
Vd/2
Vmax
Vd
Vcmv_max
0
(a)
III. PROPOSED TOPOLOGY AND CMV EVALUATION
In order to reduce the CMV associated with the standard
H6 topology, a novel three-phase eight switch configuration is
proposed, as shown in Fig. 2(a). Note that, the proposed
topology derived from the standard three-phase six switch
(H6) inverter, with different operating principle as illustrated
in Table II.
Its mode of operations can be evaluated in terms of common
mode voltage such as Mode4 (0111) illustrates the switching
state, where the upper switches S3, S5 and S7, S8 are on,
while the lower switch S2 of phase A is turn on. Therefore
(b)
Fig.2 (a) Proposed H8 three-phase inverter (b) Carrier based modulation
strategy.
A. Logic functions of the Proposed H8 Inverter
600
O u tp u t V o lta g e (V )
400
200
0
-200
-400
-600
0
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
15
o u tp u t c u rre n t(A )
10
5
0
-5
-10
-15
0
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
(a)
500
400
C M V (V )
300
200
100
0
0
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
Le a k a g e C urre n t(A )
2
1
0
-1
-2
0
Fig.3. Presents the switching functions interms of logic gates (S1-S8)
To verify the effectiveness of the proposed modulation
technique and H8 configuration, the simulations are carried
out through MATLAB/SIMULINK. The system parameters
are as follows: DC bus voltage is 300V, parasitic capacitance
is 220nF, ground resistance is 11Ω, filter inductance is 5mH
and switching frequency is 10 kHz [17]-[18].
Fig.4 (a)-(b), 5(a)-(b) illustrates the performance
evaluations of the H6 and H8 topology interms of CMV,
leakage current, output voltage and output current. As per the
standard three-phase topology (H6) the CMV is varied
between 0-Vdc, as a result dangerous common mode current
flows via parasitic capacitance of the PV array, which fails to
meet the VDE 0126-1-1 standards such as below 300mA, as
shown in Fig. 4(b). On the other hand, proposed H8 topology
clearly shows that the common mode voltage can be reduced
and maintained between Vdc/3 to 2Vdc/3, from Fig. 5(b).
Consequently, the leakage current is suppressed below
300mA, which is specified in the VDE 0126-1-1 standard
[19].
(b)
Fig.4 Performance parameters of conventional three phase topology (H6) (a)
output voltage and output current (b) CMV and leakage current (top to
bottom)
600
400
O u tp u t V o lta g e (V )
SIMULATION RESULTS
200
0
-200
-400
-600
0
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
15
10
O u tp u t C u rre n t(A )
IV.
5
0
-5
-10
-15
0
(a)
technique good performance in terms of output current with
low THD. In addition, the leakage current is reduced
significantly with proposed H8 topology.
300
250
C M V (V )
200
150
V. CONCLUSION
100
50
0
0
0.01
0.02
0.01
0.02
0.03
Time (seconds)
0.04
0.05
0.06
0.03
0.04
0.05
0.06
L e a k a g e c u rre n t(A )
2
1
0
-1
-2
0
(b)
Fig.5. Performance parameters of Proposed H8 topology (a) output voltage
and output current (b) CMV and leakage current (Top to bottom)
Transformerless PV inverters are more popular in grid
connected applications due to small size & weight and low
cost. In this paper, a novel eight switch three-phase inverter is
proposed with suitable control strategy. The conventional six
switch inverter (H6) is modified using two additional switches
(S7 and S8) and two diodes (D7 and D8). In addition, a new
carrier based modulation technique is used with simple logic
functions instead of conventional space vector modulation,
which makes it easy to implement with the low cost analog
devices. Compared to conventional H6 topology, proposed H8
inverter significantly reduces the common mode voltage
without sacrificing the overall performance of the PV system.
As a result, the leakage current is reduced as per the standards.
The simulation results reveal that the proposed topology
together with new modulation strategy gives better
performance and makes it suitable for transformerless PV
applications.
Fundamental (50Hz) = 13.36 , THD= 4.07%
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3.5
3
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2.5
2
1.5
1
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10000
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20000
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10000
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20000
(b)
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