 Open Access
 Total Downloads : 106
 Authors : Dr. Deependra Singh , Rajat Sachan
 Paper ID : IJERTV6IS110126
 Volume & Issue : Volume 06, Issue 11 (November 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS110126
 Published (First Online): 27112017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Improved Transformerless Inverter with Commonmode Leakage Current Elimination for a Photovoltaic Energy Conversion System
Dr. Deependra Singh
Department of Electrical engineering Kamla Nehru Institute of Technology (KNIT)
Sultanpur, Uttar Pradesh
Rajat Sachan
Department of Electrical engineering Kamla Nehru Institute of Technology (KNIT)
Sultanpur, Uttar Pradesh
Abstract This paper represents the methods for elimination of common mode leakage current in the transformerless photovoltaic energy conversion system. An improved inverter circuitry is presented which works on low input same as full bridge inverter and insure the elimination of commonmode leakage current. MATLAB / SIMULINK model of both the control strategies unipolar sinusoidal pulsewidth modulation (SPWM) and double frequency SPWM is presented. The high efficiency and convenient thermal design is achieved with the help of two additional switches connected to the dc side, and also the higher frequency and lower current ripples are obtained by adopting the doublefrequency SPWM, and thus total harmonic distortion of the output current is greatly reduced. The performance comparison between conventional and improved technology is also mentioned.
Keywords Commonmode leakage current, junction capacitance, photovoltaic (PV) system, sinusoidal pulsewidth modulation (SPWM) strategy, transformerless inverter.

INTRODUCTION
In present scenario the gridconnected photovoltaic (PV) systems, especially the household singlephase systems, requires high efficiency, small size, light weight, and lowcost gridconnected inverters. Two configurations are used one employs line frequency transformer and other employs high frequency transformer [1].
First configuration have few drawbacks such as line frequency transformers are large in size and hard to install. Second configuration transformers are small in size because these transformers works on high frequency supply but few additional power stages are involved in these topologies which creates more complexity [1]. Consequently, the transformerless configuration for PV systems is developed to offer the advantages of high efficiency, and low cost. But this transformerless system have a safety issue of leakage current, because the removal of transformer from the gridconnected system results in direct connection between PV panel ground and grid ground. In electrical we cannot connect two circuits directly which have different ground potential we need some isolation for these type of connections. A commonmode leakage current flows through the parasitic capacitor between the PV array and the grid ground because a variable common mode voltage is generated in transformerless gridconnected inverters [2]. The commonmode leakage current increases the system losses, reduces the gridconnected current quality, induces electromagnetic interference, and causes personal safety problems [3].
Conventionally we use HalfBridge or FullBridge inverter with sinusoidal pulse width modulation to remove this commonmode voltage. No commonmode leakage current generate in absence of commonmode voltage.
Home Loads
Solar Panels
GridTie Microinverters
Breaker Panel
Grid
Transformer
Meter
Fig.1 Basic diagram of a grid connected PV system
However, the halfbridge inverter requires a high input voltage which is greater than, approximately, 700V for 220Vac applications [1]. That amount of voltage is generated either by applying large number of PV modules or with the help of Boost converters. The fullbridge inverter just needs half of the input voltage demanded by the halfbridge topology, which is about 350V for 220Vac applications [1]. However both the conventional techniques are able to resolve the common mode leakage current problem but they are unable to prevent the DC injection in the grid current. This DC injection reduce the grid current quality and generate electromagnetic interferences. For resolving both the problems we need an improved inverter circuitry which not only avoid commonmode leakage current but also prevent DC injection in the grid current, this DC injection may saturate the core of main grid transformer.
Home Loads
Solar Panels
GridTie Microinverters
Breaker Panel
Grid
Meter
Fig.2 Basic diagram for TRANSFORMERLESS grid connected PV system
These improve inverter circuitries need the same low input voltage as the fullbridge inverter and can adopt the unipolar SPWM and bipolar SPWM strategy with three levels. These topologies are able to tackle both the problems which are mentioned above.
In this paper, an improved gridconnected inverter topology for transformerless PV systems is presented, which can sustain the same low input voltage as the fullbridge inverter and guarantee not to generate the commonmode leakage current. Furthermore, both the unipolar SPWM and the doublefrequency SPWM with threelevel output can be applied in the presented inverter [1]. The high efficiency and convenient thermal design are achieved by adopting the unipolar SPWM [7]. Moreover, the higher equivalent frequency and lower current ripples are obtained by using the doublefrequency SPWM. Therefore, a smaller filter inductor can be employed and the harmonic contents and total harmonic distortion (THD) of the output current are reduced greatly, and the gridconnected power quality is improved accordingly [3].
uBN
ucm

udm
2
Fig.3 Model Showing the CommonMode and DifferentialMode Voltages
(4)


PARACITIC CAPACITANCE AND LEAKAGE REACTANCE
PV panels are manufactured in many layers and the
Using Thevenins theorem in the above circuit the model can be simplified. By applying Kirchhoffs voltage law in the Fig.3,
To find out current
junction of these layers is covered by grounded metallic frame. A parasitic capacitance (stray capacitance) is formed between the earth and the metallic frame. Its value is directly proportional to the surface area of the PV panel. Dangerous

ucm

udm iL
2 A

iLB

ucm


udm 0 2
leakage currents (common mode currents) can flow through the large stray capacitance between the PV array and the ground if the inverter generates a variable common mode

udm iLA iLB 0

udm iLA iLB
voltage. Range of the leakage current may vary depending upon the size of the PV panel but this current has value above the safe limits, this may affect the person working in that area.
i
udm
LA LB
(5)
These leakage currents have to be eliminated or at least limited to a safe value, and this can be done with the help of improved
To find out the uecm
inverter topology.


CONDITION OF ELLIMINATING COMMON

ucm

udm iL
2 A
u

uecm 0
MODE LEAKAGE CURRENT
The ground leakage current that flows through the parasitic capacitance of the PV array is greatly influence on
uecm
ucm

dm iL
2 A
u u
the common mode voltage generated by a topology. Generally the utility grid does not influence the common mode behavior of the system [14].
u u dm L dm
ecm cm A
2 LA LB
B A
u L L
The commonmode voltage can be defined as the average of the sum of voltages between the outputs and the

dm B A cm 2 L L
(6)
common reference. In this case, the common reference is taken
to be the negative terminal of the PV. The differentialmode voltage is defined as the difference between the two voltages.
The simplified equivalent model of the commonmode resonant circuit has been derived in as shown in Figure 4,
where CPV is the parasitic capacitor, LA and LB are the filter
ucm
u AN


uBN
2
(1)
inductors, icm is the commonmode leakage current. And, an equivalent commonmode voltage uecm is defined by,
udm uAB
uAN


uBN
(2)
From the above two equations,
u AN
ucm

udm
2
(3)
Fig.4 Simplified equivalent model of Commonmode Resonant Circuit
It is clear that the commonmode leakage current icm is excited by the defined equivalent commonmode voltage uecm. Therefore, the condition of eliminating commonmode leakage current is drawn that the equivalent commonmode voltage uecm must be kept a constant as follows,

Unipolar SPWM Strategy
In Unipolar SPWM the common mode voltage can remain constant during all the four modes of operation. Also the switching voltages of all commutating switches are half of the input voltage, so compared with the full bridge inverter topology the switching losses are reduced. Here the switches in one phase leg operating in grid frequency, switches in another phase leg operating in switching frequency and the additional switches are operating in grid frequency and switching frequency alternately. There are four modes of operation that generate the three level output.
In the positive half cycle switches S1 and S6 are always ON and switch S4 and S5 commutates at switching frequency. In the negative half cycle switches S2 and S5 are always ON and S3 and S6 commutates at switching frequency.
u u

udm LB LA
ecm
cm 2 L L
B A
uBN
u AN
2
uBN




u AN
2
LB LA LB LA
Constant (7)
In the fullbridge inverter family, the filter inductors LA and LB are commonly selected with the same value. As a result, the condition of eliminating commonmode leakage current is met that,
(a)
uecm
(LA LB )
ucm
u AN

uBN
2
Constant
(8)


IMPROVED INVERTER TOPOLOGY AND OPERATION MODES
Improved technique can meet the condition of eliminating commonmode leakage current. In this topology, two additional switches S5 and S6 are symmetrically added to the conventional fullbridge inverter, and the unipolar SPWM and doublefrequency SPWM strategies with threelevel output can be achieved.
Fig.5 Improved transformerless inverter system
(b)
(c)
(d)
Fig.6 Operating modes of improved transformerless Inverter (a) Mode 1. (b) Mode 2. (c) Mode 3. (d) Mode 4.
Mode 1: when S4 and S5 are ON, uAB = +Udc and the inductor current increases through the switches S5, S1, S4, and S6. The commonmode voltage is
ucm
1 u
2 AN

uBN
1 U
2 dc

0 U dc
2
(9)
Mode 2: when S4 and S5 are turned OFF, the voltage uAN falls and uBN rises until their values are equal, and the antiparallel diode of S3 conducts. Therefore, uAB = 0V and the inductor current decreases through the switch S1 and the antiparallel diode of S3. The commonmode voltage changes into
u 1 u

u 1 U dc U dc U dc
(10)
cm 2 AN
BN
2 2 2 2
Mode 3: when S3 and S6 are ON, uAB = Udc and the inductor current increases reversely through the switches S5, S3, S2, and S6. The commonmode voltage becomes
Fig.7 Ideal waveforms of the improved inverter with
ucm
1 u
2 AN


uBN
1 0 U
2 dc
U dc
2
(11)
Unipolar SPWM
Mode 4: when S3 and S6 are turned OFF, the voltage uAN rises and uBN falls until their values are equal, and the antiparallel diode of S4 conducts. Similar as to Mode 2, uAB = 0V and the inductor current decreases through the switch S2 and the antiparallel diode of S4.

DoubleFrequency SPWM Strategy
The improved inverter can also operate with the 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, respectively, modulated with 180 opposed reference waveforms and the switches S1S4 all acting at the switching frequency. Two additional switches S5
u 1 u
cm 2 AN


u 1 U dc U dc U dc
BN 2 2 2 2
(12)
and S6 also commutate at the switching frequency cooperating with the commutation orders of two phase legs. Accordingly, there are six operation modes to continuously rotate with double frequency and generate +Udc and zero states or Udc and zero states, as shown in Figs. 6 and 8.
Fig. 9 shows the ideal waveforms of the improved inverter with doublefrequency SPWM. In the positive half cycle, S6 and S1 have the same commutation orders, and S5 and S4 have the same orders. S2 and S3, respectively, commutate complementarily to S1 and S4. Accordingly, Mode 1, Mode 2, and Mode 5 continuously rotate to generate +Udc and zero states and modulate the output voltage with double frequency. In the negative half cycle, Mode 3, Mode 4 and Mode 6 continuously rotate to generate Udc and zero states with double frequency due to the completely symmetrical modulation.
(a)
(b)
Fig.8 Remaining two of six operation modes under doublefrequency SPWM.

Mode 5. (b) Mode 6.
Mode 5: when S1 and S6 are turned OFF, the voltage uAN falls and uBN rises until their values are equal, and the antiparallel diode of S2 conducts. Therefore, uAB = 0V and the inductor current decreases through the switch S4 and the antiparallel diode of S2. The commonmode voltage ucm keeps a constant Udc/2 referring to (10).
Under the doublefrequency SPWM strategy, the common mode voltage can keep a constant Udc/2 in the whole switching process of six operation modes. Furthermore, the higher frequency and lower current ripples are achieved, and thus, the higher quality and lower THD of the gridconnected current are obtained, or a smaller filter inductor can be employed and the copper losses and core losses are reduced.
Fig.9 Ideal waveforms of the improved inverter with double frequency SPWM



SIMULATION RESULTS AND DISCUSSION
u
u

u
dc dc dc
1 1 U
U U
In the case of simulations the generated common mode
(13)
voltage of the inverter topology and modulation strategy can
cm 2 AN BN 2 2
2 2
be shown using a simple resistance as load since the utility
Mode 6: similarly, whenS2 andS5 are turned OFF, the voltage uAN rises and uBN falls until their values are equal, and the antiparallel diode of S1 conducts. Therefore uAB = 0V and the inductor current decreases through the switch S3 and the antiparallel diode of S1. The commonmode voltage ucm still is a constant Udc/2 referring to (10).
grid has no influence on the common mode behavior of the system. The simulations were done in MATLAB/SIMULINK with switching frequency fsw =8kHz.To simplify the simulation the PV array was simulated with PV panel voltage udc =380V.The parasitic capacitance Cpv =75 nF, load resistance R =2.5 k, filter inductances Lf=1.8 H and the filter capacitance Cf =2F.
u 1 u

u 1 U dc U dc U dc
(14)
cm 2 AN
BN
2 2 2 2

Conventional Unipolar SPWM Strategy
Fig10 Simulation of transformerless PV system with conventional Unipolar SPWM
(a)
Fig.11 Simulated results by employing conventional Unipolar SPWM strategy
(a) commonmode current. (b) commonmode voltage. (c) Load current. (d) Load Voltage. (e) THD of conventional Unipolar SPWM.

Conventional Bipolar SPWM Strategy
Fig.12 Simulation of transformerless PV system with conventional Bipolar SPWM
(a)
(b)
(c)
(d)
(e)
Fig.13 Simulated results by employing conventional Bipolar SPWM strategy
(a) commonmode current. (b) commonmode voltage. (c) Load current. (d) Load Voltage. (e) THD of conventional Unipolar SPWM.

Improved Unipolar SPWM Strategy
Fig.14 Simulation of transformerless PV system with improved Unipolar SPWM
(a)
(b)
(c)
(d)
(e)
Fig.15 Simulated results by employing improved Unipolar SPWM strategy

commonmode current. (b) commonmode voltage. (c) Load current. (d) Load Voltage. (e) THD of conventional Unipolar SPWM.


Improved Bipolar SPWM Strategy
Fig.16 Simulation of transformerless PV system with conventional Bipolar SPWM
(a)
(b)
(c)
(d)
(e)
Fig.17 Simulated results by employing improved Bipolar SPWM strategy

commonmode current. (b) commonmode voltage. (c) Load current. (d) Load Voltage. (e) THD of conventional Unipolar SPWM.
Table 1 Performance comparison of various methods
Systems
Parameters
Conventional System
Improved System
Unipolar SPWM
Bipolar SPWM
Unipolar SPWM
Bipolar SPWM
Common mode Current (amps)
0.0005
0.0027
0.0011
0.0007
Load Current (amps)
0.11
0.12
0.08
0.08
% THD
18.08
18.07
1.72
1.51
Case wise discussion is done below

When we use unipolar PWM technique in half bridge or full bridge inverter common mode current minimize to microampere but there is still DC injection in grid.

When we use bipolar PWM technique in half bridge or full bridge inverter common mode current minimize to milliampere but there is still DC injection in grid.

When we use unipolar PWM technique in improved transformerless inverter common mode current minimize to milliampere and there is no DC injection in grid.

When we use bipolar PWM technique in improved transformerless inverter common mode current minimize to milliampere and there is no DC injection in grid, and THD is also improved.



CONCLUSION
This paper presented an improved inverter topology for transformerless PV systems. The unipolar SPWM and double frequency SPWM control strategies are both implemented with threelevel output in the presented inverter, which can guarantee

Not to generate the commonmode leakage current because the condition of eliminating commonmode leakage current is met completely.

Furthermore, the switching voltages of all commutating switches are half of the input dc voltage and the switching losses are reduced greatly.

The high efficiency and convenient thermal design are achieved thanks to the decoupling of two additional switches S5 and S6 .Moreover, by adopting the doublefrequency SPWM, the higher frequency and lower current ripples are achieved

The higher quality and lower THD of the grid connected current are obtained, or the smaller filter inductors are employed and the copper losses and core losses are reduced accordingly.
REFERENCES

B. Yang, W. Li, Y. Gu, W. Cui, X. He, "Improved Transformerless Inverter with CommonMode Leakage Current Elimination for a Photovoltaic GridConnected Power System," Power Electronics, IEEE Transactions on, vol.27, no.2, pp.752762, Feb. 2012..

Yi Tang; Wenli Yao; Poh Chiang Loh; Frede Blaabjerg,"Highly Reliable Transformerless Photovoltaic Inverters With Leakage Current and Pulsating Power Elimination," IEEE Trans. Ind. Elec., vol. 63, no. 2, pp. 10161026, 2016.

Ramin Rahimi; Babak Farhangi; Shahrokh Farhangi,"New topology to reduce leakage current in threephase transformerless gridconnected photovoltaic inverters" 7th Power Electronics and Drive Systems Technologies Conference (PEDSTC), pp.421426, 2016

Freddy Tan Kheng Suan; Nasrudin Abd Rahim; Hew Wooi Ping,"An improved threephase transformerless photovoltaic inverter with reduced leakage currents" Clean Energy and Technology (CEAT), pp. 14, 2014