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 (CL-

SSBI)-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 sys-

tem, 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 refer-

ence 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, in-

cluding 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 switch-

ing 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 CL-

SSBI. 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 mag-

nitude 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]

vC M=

va N

+vbN

+vcN

. (1)

3

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 , VOLTAGESvN 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 V0 ,

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 vec-

vN n=−

va N

+vbN

+vcN

= −vC M (2) tors to synthesize the output reference voltage. Vshoot can still

be inserted to boost the output voltage. The utilized voltage vec-

3

vP n=vP N+vN n=vP N−vC M . (3)

tors are changed every 60◦ throughout the space, as shown in

Fig. 3(a). Compared to the MCB control method [see Fig. 2(a)],

Because shoot-through of the inverter bridge becomes a the sections rotate 30◦ in clockwise. Moreover, only one-leg

normal operation state, the possible switching states in- shoot-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

(V0, V7 ), and seven shoot-through zero vectors including one- of each leg during shoot-through zero vectors, that is Va for

through (Va

, Vb

, Vc

), two-legs shoot 30◦ to 150

◦, V

b

for 150◦ to 180

◦ and

180◦ to

shoot

leg shoot − − 90◦, and

through (Vab

, Vac

shoot shoot shoot

Vb

shoot

, Vb c ) and three-legs shoot through 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 modula-

even active vectors (V2,V4,V6), all the open-zero vectors 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 [0.61, 0.907]. Therefore,mistays in the high modulation in-

from (2) and (3), as shown in Table I. dex section, leading to lower harmonic distortion of the output

For convenience, supposing the turns ratio N of the coupled waveforms than the remote-state PWM (RSPWM) control [19],

inductor is 2.5, shoot-through zero duty cycle D0 is 0.17, and which include OPWM and EPWM control.

then boost factor B is 3, according to the bus voltage expression Under the same circuit conditions from Section II and by

of Vb=BvP N [16], and using the maximum constant boost using the NSPWM control, the switching pattern and CMV of

(MCB) control method realized by space vector-based PWM CL-SSBI-D in section B1 and B2 can be obtained, as shown

control [18], the switching pattern and CMV of CL-SSBI and in Fig. 3(b) and (c), in which Tsh is defined as a shoot-through

CL-SSBI-D in section A1 [see Fig. 2(a)] can be obtained, as period.

shown in Fig. 2(b) and (c), in which Ts is defined as a switching From Fig. 3(b) and (c), changes of CMV should result in eight

period. 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 shoot-

through 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 vCM ; (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 grid-

connected 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 CL-

SSBI-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 topol-

ogy modulated by MCB control with the same parameters of simulation. The shoot-through duty cycle is 0.17, and boost fac-

tor 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 dc-

bus 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 topol-

ogy 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 D4 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 one-

leg 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 VDE0126-1-1 standards, indicating an effective leakage current reduction.

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