Three-phase Transformerless Grid Connected Quasi Z-Source Inverter for Solar Photovoltaic Systems with Minimal Leakage Current

1
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

2
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

3
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
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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

Galvanic Isolation
Without Galvanic Isolation
Fig Classification of GCI
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Introduction 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
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Introduction 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
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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

Fig Common mode voltage of 3-ph System
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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 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
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Space 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
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Transformerless 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.
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Common 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
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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.
• 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
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Vcmv for different space vectors
Space Vector VCMV
Odd (V1,V3,V5)
Even (V2,V4,V6)
Zero (V0) 0
Zero (V7)
Vst (all) 0

1. VCMV for odd vector is 50 less than for even
   vector.
2. VCMV0 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
   system.
5. Careful selection of the switching pattern and
   voltage vector for inverter switching reduce the
   VCMV and corresponding leakage current.

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Odd 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
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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,
• 
  , i ?1,3,5

Fig. Matlab Simulink model of Odd SVPWM Generation
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Switching Pattern and Vcmv
Fig Sector-I switching pattern and vCMV of q-ZSI
for odd SVPWM
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Results and Discussion
Vrms242V
Irms0.94A
Fig Output voltage and load current of
Transformerless q-ZSI
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Results 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
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Results and Discussion
Vcmv, peak 172V Vcmv, RMS 64V
Fig Common mode voltage of transformerless q-ZSI
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Results and Discussion

• Fig Leakage current of transformerless q-ZSI

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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.

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THANK YOU!
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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
  

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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.solaraccre
  ditation.com.au/approvedproducts/inverters.html