eliminating leakage current in single phase transformerless z source inverter for photovoltaic...

University of Turkish Aeronautical AssociationFaculty of

Engineering

EEE department

Eliminating Leakage Currents in Single-PhaseTransformerless Z-Source Inverter for Photovoltaic

SystemsIbrahim Mahariq , and Hussein Mishbak

Department of Electrical and Electronics Engineering, University of Turkish Aeronautical Association, Ankara 06790, Turkey

Ibrahim Mahariq: • B.Sc, Palesitine Polytechnic University ( Electrical and Electronics Engineering) • M.Sc, Middle East Technical University (Design of Electrical Machines by FEM)• PhD1, Middle East Technical University (Computational Electromagnetics, Power sys)• PhD2, TOBB University of Economics and Technology (Photonics)• Assist. Prof. at the Department of Electrical and Electronics Engineering, University of

Turkish Aeronautical Association since Sep. 2014.

April 06,2016

[email protected]

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OUTLINE:1. Introduction .

2 .Operation principles of the modified topology : A- Structure of the Modified ZSI-TL .

C-Operation Mode Analysis .B- Modified PWM Strategy.

3. Leakage current analysis of the ZSI-TL .4. Simulation and results .5. Conclusion .

1. Introduction .

1- dc/ac inverter with a line-frequency transformer

2 -dc/ac inverter with a high frequency dc-dc converter

For PV grid-connected systems, two types of inverters are usually used.

Transformer can boost the voltage . Transformer guarantee galvanic isolation

between the grid and the PV system . Transformer is big, heavy and expensive

A high-frequency transformer is used to boost the voltage

The high-frequency transformer and switches in the dc/dc converter will cause additional power loss.

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Single-phase transformerless Z-source grid-connected PV inverter

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1. Introduction . Single-phase transformerless Z-source grid-connected PV inverter.

It has some advantages: A simple structure. Little output waveform

distortion.

OUTLINE:

2 .Operation principles of the modified topology :

A- Structure of the modified ZSI-TL .B- Modified PWM strategy.C- Operation mode analysis .

3. Leakage current analysis of the ZSI-TL .

4. Simulation and results .

5. Conclusion .

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1. Introduction .

A- Structure of the modified ZSI-TL .

The ZSI-TL adds one additional switch (S5) and one fast-recovery diode (D2).

Single-phase transformerless Z-source grid-connected PV inverter.

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A- Structure of the modified ZSI-TL .B- Modified PWM strategy.C-Operation mode analysis .



5. Conclusion .

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The switching patterns of the ZSI-TL. Here, S1 is ON and S2 is OFF during the positive half cycle.

Sinusoidal reference signals are used to modulate S3 . S4 and S5, with additional shoot-through time intervals added, commutate complementarily to S3.

S1 S2 S3 S4 S5 StateON OFF ON OFF OFF Freewheeling

mode

ON OFF ON ON ONShoot-throrgh mode

ON OFF OFF ON ON Active mode

The switching patterns During the positive half cycle

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B- Modified PWM strategy.

B- Modified PWM strategy. Similarly, during the negative half cycle, S3 is ON and

S4 is OFF.

S1 commutates at the switching frequency.

S2 and S5, with additional shoot-through time intervals added, commutate complementarily to S1.

S1 S2 S3 S4 S5 State

ON OFF ON OFF OFF

Freewheeling mode

ON ON ON OFF ONShoot-throrgh mode

OFF ON ON OFF ON Active mode

The switching patterns During the negative half cycle

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The inductors LZ1 = LZ2 = Land The capacitors CZ1 = CZ2 = C The Z-source network becomes symmetrical.

As a result, the following formula can be obtained as:

= = and = = (1)

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The shoot-through duty ratio ( ) should be limited to -M (Where M is the modulation index) .

The output peak voltage of the inverter :

= M = M = = B M ( 2 )

Equation (2) shows that the output voltage can be stepped up or down by choosing an appropriate boost factor, G . 𝑮=𝑩𝑴 (3)

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5. Conclusion .

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C-Operation Mode Analysis .The operation analysis of the ZVI-TL has six

modes :

Active mode: The output current increases through the switches S5 , S1 and S4.

Freewheeling mode

Shoot-through mode12

Active mode

Three modes during the positive half cycle.

Freewheeling mode : VAB =0V Shoot-through mode : The sum

of the capacitors voltage is greater than the dc source voltage ( + ˃ )

C-Operation Mode Analysis .

Freewheeling mode

Shoot-through modeActive mode

Three modes during the negative half cycle.

Freewheeling mode : VAB =0V Shoot-through mode : The sum

of the capacitors voltage is greater than the dc source voltage ( + ˃ ) Active mode: The output current increases through the switches S5 , S3 and S2.

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5. Conclusion .

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3. Leakage current analysis of the ZSI-TL .Model of the CM Voltage. - The CM model for the ZSI-TL including the most significant stray elements :

Common-mode model for ZSI-TL ( Full model )

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CPVg =The capacitance between the PV array and the ground .

Zpg =The series impedance between the ground connection points of the inverter and the grid.

The leakage current icm flows through the closed-loop path consisting of CPVg .

Model of the CM Voltage.

Common-mode model for ZSI-TL. (a) Full model. (b)Simplified model.

Without Zpg considered, the total CM voltage is :

= + 𝑳𝒂𝒄𝟐−𝑳𝒂𝒄𝟏

𝟐(𝑳𝒂𝒄𝟐+𝑳𝒂𝒄 𝟏)(𝑽 𝑨𝑵−𝑽 𝑩𝑵 ) (.…4)

From (4), if Lac1 = Lac2, the total CM voltage is only relevant to VAN and VBN. The CM voltage Vcm can be given by the following:

= (.….……5)

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Common-mode model for ZSI-TL. (a) Full model. (b)Simplified model.

and are determined by the PWM strategy of the ZSI-TL.

SW1 and SW2 represent the diodes of the ZSI-TL.

SW1 and SW2 are ON during the non-shoot-through states, and they are OFF during the shoot-through states.

L12 is obtained by the following:

𝑳𝒂𝒄 𝟏𝑳𝒂𝒄𝟐

𝟐(𝑳𝒂𝒄𝟏+𝑳𝒂𝒄 𝟐)(.….……6)=

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Model of the CM Voltage. In the active modes the CM voltage can

be expressed as:

Active mode

= = = (.….……7)

Where VAN =VC and VBN =VPV -VC (Taking Mode 3 as an example).

In the freewheeling modes the CM voltage can be obtained as:

Freewheeling mode

= = 𝑽 𝑷𝑽

𝟐 +𝑽 𝑷𝑽

𝟐𝟐

= (.….……8)

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Shoot-through mode

Simplified model

In the shoot-through modes SW1 and SW2 are OFF so that the path for the leakage current is blocked.

According to the above analysis, because the CM voltage is kept constant during the non-shoot-through states and the discharge path of the CM voltage is blocked during the shoot-through states, the leakage current is avoided .

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5. Conclusion .

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The circuit has been tested to verify the performance of the proposed ZVS-TL topology.

The detailed components and parameters are as follows:

VPV=100V. CZ1=CZ2=1000μF. LZ1=LZ2=4mH. Lac1=Lac2=4.5mH. Vg =220Vac. fg=50Hz. fS=4kHz. CPVg=0.1μF.

The voltages (VAN and VBN ) of the ZVI-TL

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4. Simulation and results . The leakage current waveform of the ZVI-TL with a paralleled capacitor in the grid-cycle

and in the PWM cycle is shown.

The leakage current icm is successfully limited to a very small value that is less than 50mA for the peak value this complies with the DIN VDE 0126-1-1 standard.

The leakage current waveform of the ZVI-TL with a paralleled capacitor in the grid-cycle and in the PWM cycle.

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It is clear that iL increases in the shoot-through mode and decreases in the non-shoot-through mode.

The dc-link voltage VFF¯ of the Z–source inductor and current iL waveforms.

The dc-link voltage and the Z–source inductor current iL waveforms.

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The dc-link voltage VFF¯ the Z–source inductor current iL waveforms

The grid current and voltage waveforms with filerThe grid current and voltage waveforms without filter

The grid current and voltage waveforms with filter and without filter.

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5. Conclusion .

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5. Conclusion .

A low ac output current distortion can be achieved because there is no dead time.

A modified transformerless Z-source PV grid-connected inverter in this paper has the following characteristics:

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A decoupling switch and two reversed-biased diodes are used to eliminate the leakage current.

A modified PWM strategy is implemented, which ensures a single power device switching per state transition.

No shoot-through issue leads to a greatly enhanced reliability.

These factors make the modified Z-source inverter suitable for high efficiency and low leakage current transformerless PV grid-connected applications.

Thanks for

Listening

Assist. Prof. Ibrahim Mahariq [email protected]

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