Three-phase Transformerless Grid Connected
Quasi Z-Source Inverter for Solar Photovoltaic
Systems with Minimal Leakage Current
Yam P. Siwakoti
(PhD Student)
Supervisor:
Prof. Graham E. Town
Department of Electronic Engineering,
Macquarie University, NSW 2109
Australia
1
Overview
 Introduction of Grid Connected Inverter
 Transformerless System-Pros & Cons
 Modulation Technique
 Transformerless Quasi Z-Source Inverter
 Analysis of Common Mode Voltage
 Result and Discussion
2
Introduction: Grid Connected Inverter for PV
 Power Generated by the PV panel (1) is connected to the grid (4)
via the grid connected inverter (2) and an energy meter (3).
 Grid Connected PV system should comply with the specific
standards which are regulated by electrical utility.






Total Harmonic Distortion (THD) of current
Power Factor (PF)
Leakage current and DC current injection
Voltage/Phase/Frequency
Islanding Operation
Grounding
 Different International Standards:
IEEE1547, VDE-0126-1-1, IEC61727, AS4777.2
Fig: Grid Connected Solar PV System
3
Introduction: Classification of GCI
 Grid Connected Inverters are classified into two catagories based
on the electrical isolation between the PV panels and the utility
grid
Grid Connected
Inverter
Galvanic Isolation
Without Galvanic Isolation
With
Transformer
Transformerless
Low Frequency
Transformer
High Frequency
Transformer
Fig: Classification of GCI
4
Introduction: Pros &Cons of Transformerless system
Increase Efficiency (+2%)
Reduced Size
Advantages
Reduced Weight
Reduce Cost
The galvanic isolation between the PV generator and the grid is lost.
Leakage currents (common-mode currents) through the stray
capacitance (Cp) between the PV array and the ground.
Disadvantages
Inverter could inject direct current (DC) to the grid saturate
distribution transformer.
Corrosion in the underground equipment
Malfunction of CT and PT due to saturation
5
Introduction: Parasitic Capacitance and Leakage Current
Electrical Grid
 Parasitic Capacitance formed between
panels and metallic frame (50nF/kW-150nF/kW).
 Parasitic capacitance depends on:



Surface area of PV array and ground frame
Dust, humidity and salt cover
Atmospheric conditions
Transformerless
Inverter
Cpg
Leakage Circulating Current
Fig: Leakage current path
 Series resonant circuit consisting of
Cpg, the PV generator, filtering elements and ground resistance (Rg).
 If the switching frequency of the inverter is close to the resonant
frequency of the series circuit then large leakage current flows through
the ground.
 Leakage current cause severe EMI (conducted and radiated), grid current
distortion, and additional losses in the system and potential hazard to
humans.
 The amplitude of the leakage current must be limited to within safe limits
when connected to grid.
6
Leakage Current and Common Mode Voltage
 The value of leakage current depends on the amplitude and frequency
content of the voltage fluctuations and Cpg.
 The modulation technique used in the inverter is the most dominant
factor in determining the common mode voltage and leakage currents.
 The common mode voltage for three phase system is defined as:
VRO
P
vCMV
R
=
v  vYN  v BN
 RN
3
VYO
Y
3 ̴
VBO
O
B
N
Grid
Fig: Common mode voltage of 3-ph System
7
Leakage Current and Common Mode Voltage
 German national standard VDE
DIN 0126-1-1 is taken into
consideration because it is the
most comprehensive standard in
the field of solar electricity.
Leakage Current <300mA for fire safety
& <30mA for human safety
 Leakage currents should never be
greater than 300 mA.
German Standard VDE0126-1-1
 In case of leakage currents higher
than 300 mA, the system should
shut-off in 0.3 seconds.
 Residual Current Monitoring Unit
(RCMU) is used to detect the
abnormal current.
Table: An instantaneous current and
disconnection time
RMS Leakage Current
Jump Value
[mA]
Disconnection Time
[s]
30
0.3
60
0.15
100
0.04
8
Space Vector Pulse Width Modulation Technique
 SVPWM is a modulation technique for 3-Ф inverters.
 Total eight switching state: 6-Active States & 2-Zero states
 Duty-cycles of switch are computed from the selected switching state
vectors out of eight possible switching vectors.
 The main advantage of SVPWM is the flexibility to choose space vectors
and their placement in the switching cycle to achieve required
performance specifications for the inverter with minimum switching
transitions.
Vq
V3 (010)
V1 (100)
V2 (110)
Vref
V4 (011)
V0 (000)
V7 (111)
V5 (001)
α°
V1 (100)
R
Vd
Y
B
V6 (101)
SVPWM
V1 application
9
Transformerless Quasi Z-Source Inverter
 Quasi Z-Source Inverter is suitable for grid connected
generation, specially Solar PV
distributed
 Buck-Boost capability
(Changing the modulation index (m) and shoot-through time period Tst)
 Single stage power conversion  improve power conversion efficiency and reliability
 Continuous and constant current drawn from source
 Less stress on switching components
C2
P
+
Cp
L1
=
L2
Grid
D1
S1
Rg
PV
Array
S3
R
Uin
Y
C1
Rg
Cp
S5
S2
_
N
S4
B
S6
Filter
VRO
Lf
VYO
Lf
VBO
O
Lf
Ileakage
Fig: 3-Ф transformerless Quasi Z-Source Inverter for grid connected PV System.
Potential leakage current paths are shown as dotted lines.
10
Common Mode Voltage Analysis …
R
Common Mode Voltage Analysis during
(T1-Tst/3)/2
(T3-Tst/3)/2 (T5-Tst/3)/2
For Even Vector (V2,V4,V6):
e.g. for V2(110)
v  v  vBN BU in  BU in  0 2 BU in
 RN YN


3
3
3
iC2
C2
iL
For Zero Vector V7(111)
Cp
+
Rg
+
PV
Array Uin
For Zero Vector V0(000)
vCMV  vNo 
VL1
-
D1
iL2
+
VL2 -
+
+
C1
iC1
iIN
R/Y/B for odd &
RY/YB/BR for even
L2
iD1
_
Rg
N
BU in  BU in  BU in
 BU in
3
L1
P
Cp
vCMV  v No 
+
(T5-Tst/3)/2 (T3-Tst/3)/2 (T1-Tst/3)/2
Tz
v v v
BU in  0  0 BU in
 v No  RN YN BN 

3
3
3
vCMV  vNo
-V
Tst/2 C2
Tst/2
For Odd Vector (V1,V3,V5):
e.g. for V1(100)
vCMV
R
V1
V3
V5
Vst Vst
V5
V3
V1
(101) (101) (001) (010) (100)
Active (100)
and(010)
Zero(001)
State
VC1
B*Uin
_
_
YB/BR/RY for odd &
B/R/Y for even
Fig: Equivalent circuits of the q-ZSI during
active and zero state
vRN  vYN  vBN 0  0  0

0
3
3
where,
B  1 21 Tst
Tz
11
Common Mode Voltage Analysis …
Common Mode Voltage Analysis during Shoot-through State
In the shoot-through mode the upper and lower switch are turned on at the
same time to store energy in inductors (L1 and L2) and capacitors (C1 and
C2) for voltage boost.
(T6-Tst/3)/2
(T2-Tst/3)/2 (T4-Tst/3)/2
Tst/2
Tst/2
(T4-Tst/3)/2 (T2-Tst/3)/2 (T6-Tst/3)/2
Tz is zero.
Diode D1 is open circuited in this mode and the DC link voltage
- VC2

(Vst ), k  {R,Y,B,RY,YB,BR,RYB}
+
k
iL
vRN  vYN  vBN
000

0
3
3
L1
P
Cp
+
Rg
+
PV
Array Uin
During Shoot-through:
vCMV  v No 
iC2
C2
VL2
L2
-
- UD1 +
C1
iC1
Cp
+
R/Y/B
VL2 -
+
_
Rg
iL2
VC1
_
iIN
N
R/Y/B
Fig: Equivalent circuits of the q-ZSI during
Shoot-through state
12
Vcmv for different space vectors
Space Vector
Odd (V1,V3,V5)
Even (V2,V4,V6)
VCMV
BU in
3
2 BU in
3
1) VCMV for odd vector is 50 % less than for
even vector.
2) VCMV=0 during Tst .
3) No
need
to
have
extra
circuitry
to
block/isolate the leakage current during Tst
4) Increase the efficiency and reliability of the
Zero (V0)
0
system.
5) Careful selection of the switching pattern
Zero (V7)
BUin
and voltage vector for inverter switching
reduce the VCMV and corresponding leakage
current.
Vst (all)
0
13
Odd SVPWM Modulation Technique
Vq
 Odd Space Vector Pulse Width Modulation technique
V3 (010)
is used to reduce the leakage current of q-ZSI
I

 A Single leg shoot-through vector (Vst ), k {R,Y,B}
Vref
α°
II
k
III
V1 (100)
Vd
is adopted here to reduce the number of switching
V5 (001)
states and corresponding switching loss.
Fig: Odd Voltage Space Vector
 The time duration of each vector (T1,T3,T5) for six switches are calculated
in terms of a reference voltage angle (α), switching time period (Tz), input
voltage (Uin) and shoot through time period (Tst).
T1 
TZ TZ Vref

3
U in
cos(  )
T3 
3TZ Vref
TZ TZ Vref

cos(  ) 
3
2U in
2U in
sin( )
T5 
3TZ Vref
TZ TZ Vref

cos(  ) 
3
2U in
2U in
sin( )
14
Odd SVPWM Modulation Technique…
 Shoot-through state is introduced in each sector along with the active
vector dwell time for voltage gain at the output.
 The effective dwell time is then,
T
Ti eff  Ti  st , i ∈{1,3,5}
3
[s1]
VAref V/_0
Vd
VBref V/_120
Sa
[s2]
Sast
[s3]
Sb
[s4]
Sbst
[s5]
Sc
[s6]
Vref
f(u)
S3
S5
R
Y
B
f(u)
Alpha
Vq
VCref V/_240
S1
Cartesian to
Polar
Odd_SVPWM
S2
S4
S6
Vtr
Repeating
Sequence1
2
B
Scst
Boost Factor
Embedded
MATLAB Function
Fig. Matlab Simulink model of Odd SVPWM Generation
15
Switching Pattern and Vcmv
Shoot-through State
RUP
RDOWN
V1 (100)
YUP
YDOWN
BUP
R
BDOWN
Y
B
(B/3)Uin
Vcmv
R
R
V1
V3
V5
Vst Vst
V5
V3
V1
(100) (010) (001) (101) (101) (001) (010) (100)
(T1-Tst/3)/2
(T3-Tst/3)/2 (T5-Tst/3)/2
Tst/2
Tst/2
(T5-Tst/3)/2 (T3-Tst/3)/2 (T1-Tst/3)/2
Tz
Fig: Sector-I switching pattern and vCMV of q-ZSI for
odd SVPWM
16
Results and Discussion
400
Vrms=242V
300
Output Voltage (Vo) [V]
200
100
0
-100
-200
-300
-400
0.8
0.82
0.84
0.86
0.88
0.9
Simulation time [s]
0.92
0.94
0.96
0.98
1
1.5
Irms=0.94A
Load Current (Io) [A]
1
0.5
0
-0.5
-1
-1.5
0.8
0.82
0.84
0.86
0.88
0.9
Simulation time [s]
0.92
0.94
0.96
0.98
1
Fig: Output voltage and load current of Transformerless
q-ZSI
17
Fundamental = 341.8 Vpeak
(241.7 Vrms)
Total Harmonic Distortion
(THD) = 0.62%
Mag (% of Fundamental)
Fundamental (60Hz) = 341.8 , THD= 0.62%
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
Frequency (Hz)
120
140
160
180
200
0.6
0.4
0.2
0
0
20
40
60
80
100
Frequency (Hz)
120
140
0 Hz (DC): 0.06%
60 Hz (Fnd): 100.00%
120 Hz (h2): 0.45%
180 Hz (h3): 0.01%
240 Hz (h4): 0.27%
Fundamental = 1.315 Ipeak
(0.93 Irms)
Total Harmonic Distortion
(THD) = 0.67%
Fundamental (60Hz) = 1.315 , THD= 0.67%
0.8
Mag (% of Fundamental)
Load Current Harmonics
Output Voltage Harmonics
Results and Discussion…
160
180
200
0 Hz (DC): 0.04%
60 Hz (Fnd): 100.00%
120 Hz (h2): 0.52%
180 Hz (h3): 0.02%
240 Hz (h4): 0.27%
Fig: Harmonic analysis of transformerless q-ZSI
18
Results and Discussion…
Common Mode Voltage (Vcmv) [V]
400
350
Vcmv, peak = 172V
Vcmv, RMS = 64V
300
Common Mode Voltage (Vcmv) [V]
200
Peak Value of Vcmv
180
RMS value of Vcmv
160
140
120
100
80
60
40
20
0
0.998
0.9982
0.9984
0.9986
0.9988
0.999
0.9992
0.9994
0.9996
0.9998
1
Simulation time [s]
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
Simulation time [s]
0.6
0.7
0.8
0.9
1
Fig: Common mode voltage of transformerless q-ZSI
19
Results and Discussion…
Fig: Leakage current of transformerless q-ZSI
0.1
0.05
0
-0.05
-0.1
0
0.02
0.1
0.2
0.3
Leakage Current (Il) [A]
Leakage Current (Il) [A]
I leakage, peak = 10mA
I leakage, RMS = 5mA
0.01
0.4 0
0.5
0.6
Simulation time [s]
0.7
0.8
0.9
1
-0.01
-0.02
0.999
0.9991
0.9992
0.9993
0.9994
0.9995
0.9996
Simulation time [s]
0.9997
0.9998
0.9999
1
20
Conclusions
• Odd Space Vector Pulse Width Modulation
Technique is effective in reducing the common
mode voltage and leakage current of q-ZSI.
• Boost capability of q-ZSI is maintained by
applying single leg shoot-through state.
• The leakage current is 10mApeak /5mArms , way
below the German standard of 300mA.
• Transformerless Quasi Z-Source Inverter is
safe to connect to the grid.
21
22
Backup Slides
•
•
•
Chinese manufactures dominated the
global industry in 2010, with close to
11,000 megawatts of PV cell
production. This was the seventh
consecutive year in which China at
least doubled its PV output.
Taiwan was a distant second with
3,600 megawatts produced, followed
by Japan with 2,200 megawatts,
Germany with 2,000 megawatts, and
the United States with 1,100.
The top five countries thus accounted
for 82 percent of total world PV
production.
Source: http://www.freesolar.com.au/about-us/our-suppliers
23
Backup Slides
Grid-connect inverters - testing standards
 Standards Australia has released three standards which are pertinent
to grid connected inverter systems. These are:
 AS 4777.1 - 2005 Grid connection of energy systems via inverters Part
1: Installation requirements.
 AS 4777.2 - 2005 Grid connection of energy systems via inverters Part
2: Inverter requirements.
 AS 4777.3 - 2005 Grid connection of energy systems via inverters Part
1: Grid protection requirements.
 Inverters must be tested against AS 4777.2 and 3 - 2005 (or
equivalent) and AS3100 (or equivalent) by an appropriate testing
laboratory.
Source: http://www.solaraccreditation.com.au/approvedproducts/inverters.html
24