THREE PHASE INVERTER USING COUPLED INDUCTOR FOR
GRID – CONNECTED PHOTOVOLTAIC SYSTEM
G.KANIMOZHI.ME.,Mrs.S.RAKKAMMAL.ME.,
Mail id:gkmozhi1@gmail.com
Mail id:rakkammalram@yahoo.com_
9159719678
8124408556
M.A.M COLLEGE OF ENGINEERING, M.A.M COLLEGE OF ENGINEERING,
SIRUGANUR,SIRUGANUR,
TRICHY.TRICHY.
Abstract—This letter presents a modulation technique for
themodified coupled-inductor single-stage boost inverter (CLSSBI)-based grid-connected photovoltaic (PV) system. This
technique can reduce the system leakage current in a great deal
and can meet the VDE0126-1-1 standard. To maintain the
advantages of the impedance network, only a diode is added in
the front of the original topology, to block the leakage current
loop during the active vectors and open-zero vectors. On the
other hand, the near-state pulse width modulation (NSPWM)
technique is applied with one-leg shoot-through zero vectors in
order to reduce the leakage current through the conduction path
in the duration of changing from and to open-zero vectors.
Simultaneously, the leakage current caused by other transitions
can also be reduced due to the fact that the magnitude of
common-mode voltages is reduced. Simulation re-sults of the
transformerless PV system are presented in two cases: modified
CL-SSBI modulated by maximum constant boost (MCB) control
method and NSPWM. Experimental results for both CL-SSBI
topology modulated by the MCB control method and mod-ified
CL-SSBI topology modulated by NSPWM are also obtained to
verify the accurateness of theoretical and simulation models.
Index Terms—Leakage current, photovoltaic (PV) power system, shoot-through zero vector, single-stage boost inverter, width
modulation.
I. INTRODUCTION
Thetransformerless photovoltaic (PV) power system has
been attracting more and more attention for its lower
cost,smaller volume, as well as higher efficiency, compared to
the ones with transformer [1]–[15]. One of the technical
challenges is the safety issue of the leakage current caused by
the common-mode voltages (CMV), conducting in the loop
with parasitic capacitors between the solar panel and the
ground. For single-stage boost inverter transformerless PV
systems, such as the Z-source inverter [7]-based systems, the
modulation strategy is carefully designed to maintain the
constant CMV to reduce
the leakage current. But the OPWM or EPWM method uses
only odd or even active vectors to synthesize the output reference voltage, leading to only 57.7% of the maximum
magnitude compared to SVPWM, and also to worsen
harmonic distortion of the output waveforms.
A coupled inductor single-stage boost inverter (CL-SSBI) is
proposed in [16], which introduced an impedance network, including coupled inductor in the front-end of the inverter bridge.
The structure is simple, while LCD can be viewed as a snubber.
The converter uses shoot-through zero vectors [17] to store and
transfer energy within the unique impedance network, to step up
the bus voltage. Turns ratio of the coupled inductor within the
impedance network can also be designed to improve the boost
gain. So the ac output voltage can be regulated in a wide range
and can be stepped up to a higher value. Higher power loss and
lower efficiency would be unavoidable if higher boost gain is
required, which is the disadvantage of inverters of this type. As
shoot-through zero vectors evenly distributed among the three
phase legs during a switching period [17], the equivalent switching frequency viewed from the impedance network can be six
times the switching frequency of the inverter bridge, which will
greatly reduce the power density and cost of the inverter.
This letter presents the method to reduce the leakage current of
the transformerless grid-connected PV system based on CLSSBI. A diode is added in the front of the topology to block the
leakage current loop when in the active vectors and open-zero
vectors. In addition, the near-state PWM (NSPWM) technique is
used with one-leg shoot-through zero vectors to reduce the
leakage current caused in the transient states of changing from
and to open-zero vectors. And the leakage current caused by
other transitions can also be reduced due to the fact that the
magnitude of CMVs is reduced. Note that the leakage current can
be reduced effectively without lowering the maximum magnitude of the output reference voltage, for the modulation index
of NSPWM stays in the high modulation section.
II. PROPOSED TRANSFORMERLESS GRID-CONNECTED PV
SYSTEM BASED ON CL-SSBI
The modified CL-SSBI is shown in Fig. 1. Only a diode is
added in the front of the topology compared to the original
structure, to block the leakage current loop during the active
vectors and open-zero vectors, of which the CMV vC M is
defined as [4]
v
vC M =
3
+v +v
aN
bN
cN
.
(1)
TABLE II
SIMULATION AND EXPERIMENTAL PARAMETERS OF CL-SSBI AND CL-SSBI-D
TRANSFORMERLESS PV SYSTEM
Fig. 1.Transformerless grid-connected PV system based on CL-SSBI with an
additional diode.
TABLE I
COMMON-MODE VOLTAGESvCM , VOLTAGESv N nANDvP nIN DIFFERENT
SPACE VECTORS (a) FOR CL-SSBI-D (b) FOR CL-SSBI
In section A1, V1,V2,V0 , and V7 are used to synthesize the
output reference voltage and Vshoot is inserted in open-zero
vectors to realize the boost characteristics. Fig. 2(b) and (c)
illustrates that the magnitude of CMV of CL-SSBI-D is lower
than that of CL-SSBI, which indicates that the magnitude of the
leakage current can be also reduced.
III. MODULATION TECHNIQUE TO REDUCE
LEAKAGE CURRENT
The CMV of CL-SSBI-D changes in a maximum step value
when the active vectors Vo dd convert to open-zero vector V 0 ,
and changes in a relatively high step value when the open-zero
vector V0 convert to shoot-through zero vectors Vshoot , as
shown in Fig. 2(c), which will induce high spikes in the leakage
current due to the parasitic capacitor path. Therefore, open-zero
vectors are the key to be considered to reduce the magnitude of
the leakage current.
The voltage between positive P or negative N solar panel and
One possible technique is the NSPWM control, which omits
grounded neutral n can be expressed as
the open-zero vectors and employs three adjacent voltage vecv
+v +v
aN
bN cN
tors to synthesize the output reference voltage. Vshoot can still
vN n=−
= −vC M
(2)
be inserted to boost the output voltage. The utilized voltage vec3
◦
tors are changed every 60 throughout the space, as shown in
vP n=vP N+vN n=vP N−vC M .
(3)
Fig. 3(a). Compared to the MCB control method [see Fig. 2(a)],
◦
the sections rotate 30 in clockwise. Moreover, only one-leg
Because shoot-through of the inverter bridge becomes a
normal operation state, the possible switching states inshoot-through vectors are used in order to reduce switching
◦
clude six active vectors (V1–V6 ), two open-zero vectors events, and are changed every 120 to assure equal
current stress
a
(V0, V7 ), and seven shoot-through zero vectors including oneof each leg during shoot-through zero vectors, that is V
for
a
◦
◦ b
◦
◦
◦
b
c
through (V
,V
,V
), two-legs shoot
30 to 150 , V
for 150 to 180 and − 180 to − 90◦, and
leg shoot ab
b
◦
◦
ac
bc
through (V
,V
,V
) and three-legs shoot through V
for − 90 to 30 .
ab c
shoot
shoot
shoot
shoot
(Vshoot ). For all the odd active vectors (V1, V3, V5 ), all the
The voltage utilization level can be indicated by the modulashoot
shoot
shoot
shoot
even active vectors (V2,V4,V6), all the open-zero vectors
shoot
tion index mi(mi=Vm /(2Vb /π), where Vm is the magnitude
(V0, V7 ), and all the shoot-through zero vectors, the common-
of the reference voltage vector). Modulation index within the
mode voltages (vC M) and vol tages (vP n, vN n ) of C L-S SBI and
linear area for N SPWM contro l is mi
π 3√3, π 2√3 =
∈
CL-SSBI with an additional diode (CL-SSBI-D) can be derived
from (2) and (3), as shown in Table I.
For convenience, supposing the turns ratio N of the coupled
inductor is 2.5, shoot-through zero duty cycle D0 is 0.17, and
then boost factor B is 3, according to the bus voltage expression
[16], and using the maximum constant boost
(MCB) control method realized by space vector-based PWM
control [18], the switching pattern and CMV of CL-SSBI and
CL-SSBI-D in section A1 [see Fig. 2(a)] can be obtained, as
shown in Fig. 2(b) and (c), in which Ts is defined as a switching
period.
of Vb=BvP N
[0.61, 0. 907]. Therefore,mistays in the high modulation in-
∼
dex section, leading to lower harmonic distortion of the output
waveforms than the remote-state PWM (RSPWM) control [19],
which include OPWM and EPWM control.
Under the same circuit conditions from Section II and by
using the NSPWM control, the switching pattern and CMV of
CL-SSBI-D in section B1 and B2 can be obtained, as shown
in Fig. 3(b) and (c), in which Tsh is defined as a shoot-through
period.
From Fig. 3(b) and (c), changes of CMV should result in eight
spikes in the leakage currents per switching cycle, corresponding
Fig. 2. (a) Voltage space vectors of a three-phase inverter; switching pattern and CMV of (b) CL-SSBI in section A1, and of (c) CL-SSBI-D in section A1.
Fig. 3. (a) Voltage space vectors of NSPWM definition; switching pattern and CMV of CL-SSBI-D in (b) section B1 and (c) section B2.
to 1600 spikes in the leakage current per fundamental cycle
(Ts= 100 μs, 50 Hz grid). Nevertheless, the magnitude of the
leakage current is lower than that of CL-SSBI with the MCB
control method.
It is important to note that leakage current occurs from
CMV only in the duration of transiting from or to shootthrough zero vectors with NSPWM control, when open-zero
vectors are omit-ted. And the magnitude of CMV is also
reduced, which leads to lower leakage current.
IV. SIMULATION AND EXPERIMENTAL RESULTS
In order to validate the theoretical analysis, the simulation
and experimental tests of the transformerless grid-connected
PV system constructed by CL-SSBI and CL-SSBI-D are
carried out, respectively. The PV frame and the neutral point
of the grid are grounded. The simulation and experimental
parameters are shown in Table II.
Fig. 4 shows the simulation results of the grid-connected
CL-SSBI system modulated by MCB control. The three-phase
Fig. 4. Simulation waveforms of transformerless grid-connected PV system based on CL-SSBI with MCB control: (a) grid currents; (b) CMV vCM ; (c) leakage
current.
Fig. 5. Simulation waveforms of transformerless grid-connected PV system based on CL-SSBI-D with MCB control: (a) grid currents; (b) CMV v CM ;
(c) leakage current, and with NSPWM control: (d) grid currents; and (e) CMV vC M ; and (f) leakage current.
currents as shown in Fig. 4(a) present high ripple due to the
high leakage current [see Fig. 4(c)]. CMV vcm shown in Fig.
4(b) has four different levels and changes eight times. The
magnitude of the leakage current is 1.5 A, and its RMS is
calculated as 0.96 A, which is well above the 300 mA
threshold level stated in the VDE0126-1-1 standard [20].
Fig. 5 shows the simulation results of the CL-SSBI-D gridconnected system modulated by the MCB control method and by
NSPWM, respectively. The magnitude of the leakage current is
0.45 A, and its RMS is calculated as 0.28 A of CL-SSBI-D
modulated by the MCB control method. While the magnitude of
the leakage current is 65 mA, and its RMS is calculated as
27 mA of CL-SSBI-D modulated by NSPWM, which is below
the threshold level of VDE0126-1-1 standard. The three-phase
currents of both control methods have lower ripple than that of
Fig. 4(a) due to the lower leakage current. CMV vcm of CLSSBI-D modulated by NSPWM has three different levels which
lower than that of Fig. 4(b) and Fig. 5(b), and similar to Fig. 3(b).
Fig. 6 shows the experimental results for CL-SSBI-D topology modulated by MCB control with the same parameters of
simulation. The shoot-through duty cycle is 0.17, and boost factor is 3 if the coupled inductor is fully coupled. The diodes D1 ,
D2 , D3 , and D4 are 600 V/30 A fast-recovery diodes. Due to the
proper regulation of the shoot-through zero vectors and
Fig. 6. Experimental waveforms of transformerless PV system based on CL-SSBI with MCB control: (a) bus voltage vb , voltage vN n , leakage current ile a k ;
(b) phase current ia , leakage current ile a k , and its FFT analysis.
design of the coupled inductor of impedance network, the dcbus voltage is stepped up to about 400 V when the input dc
source equals to 150 V. The magnitude of the leakage current
is 0.4 A, of which 40 mA at the point of switching frequency
and 25 mA at shoot-through frequency by FFT analysis.
Fig. 7 shows the experimental results for CL-SSBI-D topology modulated by NSPWM with the same parameters of
simu-lation. The magnitude of the leakage current is 80 mA,
of which 10 mA at the point of switching frequency and 4 mA
at shoot-through frequency by FFT Analysis. The output
phase current iahas low ripple and smooth waveform.
V. CONCLUSION
This paper has presented a transformerless grid-connected PV
system based on a coupled inductor single-stage boost three
phase inverter. Diode D 4 is added in the front of the topology
together with D1 , to block the leakage current loop during the
active vectors and open-zero vectors. The leakage current caused
in the transient states of changing from and to shoot-through zero
vectors is also reduced by using the NSPWM technique with oneleg shoot-through zero vectors, when open-zero vectors are
omitted. Simultaneously, the leakage current caused by other
transitions can be further reduced due to the magnitude reduction
of the CMV. The CMVs and the caused leakage currents are
compared between CL-SSBI with MCB control and CL-SSBI-D
with NSPWM. According to the simulation and experimental
results, the amplitude and RMS value of the leakage current can
be well below the threshold level required by the VDE0126-11 standards, indicating an effective leakage current reduction.
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