Title: Three-phase Transformerless Grid Connected Quasi Z-Source Inverter for Solar Photovoltaic Systems with Minimal Leakage Current
1Three-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
2Overview
- Introduction of Grid Connected Inverter
- Transformerless System-Pros Cons
- Modulation Technique
- Transformerless Quasi Z-Source Inverter
- Analysis of Common Mode Voltage
- Result and Discussion
3Introduction 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
4Introduction Classification of GCI
- Grid Connected Inverters are classified into two
catagories based on the electrical isolation
between the PV panels and the utility grid
Galvanic Isolation
Without Galvanic Isolation
Fig Classification of GCI
5Introduction Pros Cons of Transformerless system
Advantages Increase Efficiency (2)
Advantages Reduced Size
Advantages Reduced Weight
Advantages Reduce Cost
Disadvantages The galvanic isolation between the PV generator and the grid is lost.
Disadvantages 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.
Disadvantages Corrosion in the underground equipment
Disadvantages Malfunction of CT and PT due to saturation
6Introduction Parasitic Capacitance and Leakage
Current
- 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
- 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.
Fig Leakage current path
7Leakage 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
Fig Common mode voltage of 3-ph System
8Leakage 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 currents should never be greater than 300
mA. - 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.
Leakage Current lt300mA for fire safety
lt30mA for human safety Table An
instantaneous current and disconnection time
German Standard VDE0126-1-1 German Standard VDE0126-1-1
RMS Leakage Current Jump Value mA Disconnection Time s
30 0.3
60 0.15
100 0.04
9Space Vector Pulse Width Modulation Technique
- SVPWM is a modulation technique for 3-?
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.
V1 application
SVPWM
10Transformerless Quasi Z-Source Inverter
- Quasi Z-Source Inverter is suitable for grid
connected distributed generation, specially
Solar PV - 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
-
Fig 3-? transformerless Quasi Z-Source Inverter
for grid connected PV System. Potential leakage
current paths are shown as dotted lines.
11Common Mode Voltage Analysis
- Common Mode Voltage Analysis during Active and
Zero State - For Odd Vector (V1,V3,V5)
- e.g. for V1(100)
- For Even Vector (V2,V4,V6)
- e.g. for V2(110)
- For Zero Vector V7(111)
- For Zero Vector V0(000)
- where,
Fig Equivalent circuits of the q-ZSI during
active and zero state
12Common 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. - Diode D1 is open circuited in this mode and the
DC link voltage is zero. - R,Y,B,RY,YB,BR,RYB
- During Shoot-through
Fig Equivalent circuits of the q-ZSI during
Shoot-through state
13Vcmv for different space vectors
Space Vector VCMV
Odd (V1,V3,V5)
Even (V2,V4,V6)
Zero (V0) 0
Zero (V7)
Vst (all) 0
- VCMV for odd vector is 50 less than for even
vector. - VCMV0 during Tst .
- No need to have extra circuitry to block/isolate
the leakage current during Tst - Increase the efficiency and reliability of the
system. - Careful selection of the switching pattern and
voltage vector for inverter switching reduce the
VCMV and corresponding leakage current.
14Odd SVPWM Modulation Technique
- Odd Space Vector Pulse Width Modulation technique
- is used to reduce the leakage current of
q-ZSI - A Single leg shoot-through vector
R,Y,B - is adopted here to reduce the number of
switching - states and corresponding switching loss.
- The time duration of each vector (T1,T3,T5) for
six switches are calculated in terms of a
reference voltage angle (a), switching time
period (Tz), input voltage (Uin) and shoot
through time period (Tst).
Fig Odd Voltage Space Vector
15Odd 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,
-
, i ?1,3,5
Fig. Matlab Simulink model of Odd SVPWM Generation
16Switching Pattern and Vcmv
Fig Sector-I switching pattern and vCMV of q-ZSI
for odd SVPWM
17Results and Discussion
Vrms242V
Irms0.94A
Fig Output voltage and load current of
Transformerless q-ZSI
18Results and Discussion
Fundamental 341.8 Vpeak
(241.7 Vrms) Total Harmonic Distortion
(THD) 0.62 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
Output Voltage Harmonics
Fundamental 1.315 Ipeak
(0.93 Irms) Total Harmonic Distortion
(THD) 0.67 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
Load Current Harmonics
Fig Harmonic analysis of transformerless q-ZSI
19Results and Discussion
Vcmv, peak 172V Vcmv, RMS 64V
Fig Common mode voltage of transformerless q-ZSI
20Results and Discussion
- Fig Leakage current of transformerless q-ZSI
21Conclusions
- 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.
22THANK YOU!
23Backup 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
24Backup 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.solaraccre
ditation.com.au/approvedproducts/inverters.html