The document presents a detailed study on a modified transformerless z-source inverter topology for photovoltaic grid-connected systems. It discusses the operational principles, modified PWM strategy, and leakage current analysis, demonstrating improvements in efficiency and reduced leakage current. Simulation results validate the performance of the proposed inverter design, highlighting its advantages in terms of reliability and reduced distortion.

1. University of Turkish Aeronautical
Association
Faculty of Engineering
EEE department
Ibrahim 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
imahariq@thk.edu.tr

2. 1
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 .

3. 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.
2

4. Single-phase transformerless Z-source grid-connected
PV inverter
3
1. Introduction .
 Single-phase transformerless Z-source grid-connected PV inverter.
 It has some advantages:
 A simple structure.
 Little output waveform
distortion.

5. 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 .
4
1. Introduction .

6. 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.
5

7. OUTLINE:
1. Introduction .
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 .
6

8.  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 State
ON OFF ON OFF OFF Freewheeling
mode
ON OFF ON ON ON Shoot-throrgh
mode
ON OFF OFF ON ON Active mode
The switching patterns During the positive half cycle
7
B- Modified PWM strategy.

9. 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 ON Shoot-throrgh
mode
OFF ON ON OFF ON Active mode
The switching patterns During the negative half cycle
8

10.  The inductors LZ1 = LZ2 = L
and
 The capacitors CZ1 = CZ2 = C
The Z-source network becomes symmetrical.
 As a result, the
following formula can
be obtained as:
𝑽 𝑪𝟏 = 𝑽 𝑪𝟐 = 𝑽 𝑪 and 𝑽 𝑳𝟏 = 𝑽 𝑳𝟐 = 𝑽 𝑳 ( 1 )
9
B- Modified PWM strategy.

11.  The shoot-through duty ratio ( 𝒅 𝒐 ) should be limited to 𝟏-M
(Where M is the modulation index) .
 The output peak voltage of the inverter :
𝒗
−
𝒂𝒃= M 𝑽 𝑭𝑭 ¯ =
𝑻 𝒔
𝑻 𝟏−𝑻 𝟎
𝑽 𝑷𝑽 M =
𝑴
𝟏−𝟐𝒅 𝟎
𝑽 𝑷𝑽 = B M 𝑽 𝑷𝑽 ( 2 )
 Equation (2) shows
that the output
voltage can be
stepped up or
down by choosing
an appropriate
boost factor, G .
𝑮 = 𝑩𝑴 ( 3 )
10
B- Modified PWM strategy.

12. OUTLINE:
1. Introduction .
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 .
11

13. 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 mode
12
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 (𝑉𝐶1 + 𝑉𝐶2˃ 𝑽 𝑷𝑽 )

14. 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 (𝑉𝐶1 + 𝑉𝐶2˃ 𝑽 𝑷𝑽 )
 Active mode: The output current
increases through the switches S5 , S3
and S2.
13

15. OUTLINE:
1. Introduction .
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 .
14

16. 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 )
15
 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 .

17. Model of the CM Voltage.
Common-mode model for ZSI-TL. (a) Full model.
(b)Simplified model.
 Without Zpg considered, the total CM
voltage 𝒗 𝒕𝒄𝒎 is :
𝑽 𝑡𝑐𝑚= 𝑽 𝑨𝑵+𝑽 𝑩𝑵
𝟐
+
𝑳 𝒂𝒄𝟐 − 𝑳 𝒂𝒄𝟏
𝟐(𝑳 𝒂𝒄𝟐 + 𝑳 𝒂𝒄𝟏)
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:
𝑽 𝑐𝑚= 𝑽 𝑨𝑵+𝑽 𝑩𝑵
𝟐
…….….(5)
16
3. Leakage current analysis of the ZSI-TL .

18. Model of the CM Voltage.
Common-mode model for ZSI-TL. (a) Full model.
(b)Simplified model.
 𝑽 𝑨𝑵 and 𝑽 𝑩𝑵 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:
𝑳 𝒂𝒄𝟏 𝑳 𝒂𝒄𝟐
𝟐(𝑳 𝒂𝒄𝟏 + 𝑳 𝒂𝒄𝟐)
…….….(6)𝑳 𝟏𝟐=
17
3. Leakage current analysis of the ZSI-TL .

19. Model of the CM Voltage.
 In the active modes the CM voltage can
be expressed as:
Active mode
𝑽 𝑐𝑚= 𝑽 𝑨𝑵+𝑽 𝑩𝑵
𝟐
=
𝑽 𝑪−𝑽 𝑷𝑽−𝑽 𝑪
𝟐
=
𝑽 𝑷𝑽
𝟐
…….….(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 𝒄𝒎= 𝑽 𝑨𝑵+𝑽 𝑩𝑵
𝟐
=
𝑽 𝑷𝑽
𝟐
+
𝑽 𝑷𝑽
𝟐
𝟐
=
𝑉 𝑃𝑉
2
…….….(8)
18
3. Leakage current analysis of the ZSI-TL .

20. 3. Leakage current analysis of the ZSI-TL .
Model of the CM Voltage.
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 .
19

21. OUTLINE:
1. Introduction .
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 .
20

22. 4. Simulation and results .
 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
21

23. 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.
22

24. 4. Simulation and results .
 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 𝑽𝑭𝑭−
and the Z–source inductor
current iL waveforms.
23
The dc-link voltage VFF¯ the Z–source inductor current iL waveforms

25. 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.
24
4. Simulation and results .

26. OUTLINE:
1. Introduction .
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 .
25

27. 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:
26
 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.

28.  Assist. Prof. Ibrahim Mahariq
 imahariq@thk.edu.tr
Contact information: