transformer-less inverter for photovoltaic grid connected ... · pdf filecommon mode leakage...

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)


(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,


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].


modified

4) Step4.

Under balance condition, if LA=LB


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




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


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


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:




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




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.

REFERENCES

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transformer-less inverter for photovoltaic grid connected ... · pdf filecommon mode leakage...

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