Tutorial on
Photovoltaic power converters
Mahinda Vilathgamuwa, Geoff Walker
Queensland University of Technology, Brisbane
Gamini Jayasinghe
University of Tasmania
CRICOS No. 00213J
Queensland University of Technology
Tutorial content
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Introduction (MV)
PV inverters (MV)
PV dc-dc converters (GW)
PV-grid interaction (GW)
Grid-storage (SGJ)
Active power decoupling (SGJ)
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Introduction
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What are photovoltaics ?
• Photovoltaic (PV) systems convert light energy
directly into electricity.
• Commonly known as “solar cells.”
• Simpler systems power the small calculators
we use every day. More comprehensive
systems provide a significant portion of the
electricity in Australia and in other countries
• PV represents one of the most promising means of
maintaining our energy intensive standard of living
while not contributing to global warming and
pollution.
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Introduction
• The sun, which is the key source of energy in
photovoltaic systems can be considered as a
huge nuclear fusion reactor that produces
3.89×1020 MW of power having approximately
60000C surface temperature.
• Sun radiates its energy in a wide spectrum and
the earth’s atmosphere receives around 1370
W/m2 of sun’s energy.
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Introduction
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All such energy reached at the earth’s atmosphere cannot be
received by the earth’s surface as atmospheric gases and water
vapour attenuates solar radiation while around one third is reflected
back to the outer space and another significant portion is scattered.
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Principles of PV cell operation
(a)p-n junction with its current and voltage indicated,
(b) symbolic notation of p-n junction (diode), and
(c) V-I characteristics of the p-n junction.
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Principles of PV cell operation
Fig. 1.6. Principle of the operation of a photovoltaic cell.
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PV cell equivalent circuit
(a) PV cell equivalent circuit with current direction
indicating it as a passive device,
(b) PV cell equivalent circuit with current direction
indicating it as an active source,
(c) V-I and V-P characteristics for the circuit shown in (b).
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PV cell characteristics
Photovoltaic cell characteristics with varied insolation,
(a) V-I characteristic, (b) V-P characteristic
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PV cell equivalent circuit
PV cell equivalent circuit considering cell series and parallel resistances.
PV cell V-I characteristics, considering the effect of (a) Rs, and (b) Rp
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Modularization
(a) PV module, and (b) PV array
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Modularization
V-I characteristics when modules are connected in
(a) series, and (b) parallel
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Modular characteristics
(a) PV array with uneven insolation, and (b) its P-V characteristics
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Grid integration of PV modules
Grid integration configurations of PV modules using
(a) centralized inverters, (b) modularized system with dc-dc converters,
and (c) modularized system with DC-AC inverters
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Central inverter based PV power conversion systems
Central inverter based PV power conversion systems with
(a) isolated DC-DC converter, and
(b) non-isolated DC-DC converter
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Central inverter based PV power conversion systems
Central inverter based PV system with an impedance source
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String based PV power conversion systems
Multi-string inverters with (a) non-isolated and (b) isolated DC-DC converters
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Transformerless PV inverters
(a)Basic elements of the transformerless PV inverter and
(b) its implementation with a half-bridge converter
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Multi-level inverter
topologies
a) diode clamped
three-level inverter,
b) capacitor clamped
three-level inverter,
c) cascaded multi-level
inverter,
d) modular multi-level
inverter
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Microinverters
Micro inverter based PV power conversion systems
(a) basic configuration and (b) topology classification
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Transformerless microinverters
Transformerless micro
inverters with grid-connected
(a) full-bridge
(b) half-bridge and
(c) three level inverter
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Grid isolated micro inverters
Micro inverters with DC-link
based on
(a) half-bridge converter
(b) fly-back converter and
(c) push-pull converter
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DC-link less or high-frequency-link micro inverters
Q5
Q1
Q3
Q1
C2
Cin
C1
Q4
Q2
Q9
Q1 1
Q10
Q1 2
AC
Q4
Q8
(a)
Q8
C2
Q7
Q2
Q6
Q3
L1
AC
Q7
Filter
C1
Filter
Q6
L1
Cin
Q5
(b)
HFL micro inverters with
(a) half-wave (b) full-wave cycloconverters
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Microinverter control strategies
Controllers of the micro inverter with a (a) DC-link and (b) HFL
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Customer distributed generation sources
and their integration to the grid
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Queensland University of Technology
UQ St Lucia 1.22 MWp PV array
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Sir Samuel Griffith Centre, Nathan campus
6 storey, 6 star green star Off grid building
PV arrays
303 kW on roof, tilted 15° to North
84 kW on Northern façade shades
Aircon chiller and cooling towers
Two x 200kW DC-AC inverters
Two x 160kW Solar inverters
Chilled water storage
8 x 400V DC Lithium Battery strings,
900 kWh usable capacity , 3000 cycles
200 kW electrolyser
Hydride storage system for 100kg H2
60 kW fuel cell
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Energy flows
Electrolyser
Hydride
storage
PV array
PV inverter
Fuel
Cell
Battery
inverter
Batteries
Building
load
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Progress –
Now completed …
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Australia:
Falling demand, but rising electricity prices
http://grattan.edu.au/static/files/assets/965
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5afc0/804-shock-to-the-system.pdf
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Both consumption and demand are dropping …
What will EVs and batteries do?
http://www.climatespectator.com.au/commentary/
electricity-demands-speedy-descent
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http://www.wattclarity.com.au/2012/12/a-more-detailed-look-atR
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how-demand-has-trended-over-15-summers-in-the-nem
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Today Demand reduction = Distributed generation
Caloundra 2015
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PV capital cost less than
$2/Wp, even without subsidy
(NB this was 2013!)
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Expensive grid connected PV systems?
(2015)
http://www.solarchoice.net.au/blog/solar-pva university for the real world
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system-price-index-january-2015
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Payback usually under 7 years
Invest $5000 for 3 kWp system
Generation (Brisbane) approx
= 3 x 4.4 kWh/day x 365 days
= 4,818 kWh annually
If offsetting retail tariff at $0.279 / kWh
= $1345 / year. (inc GST)
Simple payback in under four years.
BUT with new Qld feed-in tariff at $0.00 / kWh
= $0 / year.
New mantra: self consume, don’t export.
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Today Demand reduction = Distributed generation
Caloundra 2015
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Currimundi 3A (as at Oct 14)
- 2642 customers
- predominantly residential
- 937 solar PV systems (37%)
- 2870 kW installed generation
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Renewables in South Australia, 2013-14
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Wind:
22% of capacity, 31% of delivered energy
Rooftop PV: 10% of capacity, 6% of delivered energy
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real world Advisory-Functions/South-Australian-Electricity-ReportCRICOS No. 00213J
Renewables in South Australia – future growth?
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Local context –
ENERGEX
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$7 billion in assets
1.3M+ retail customers
600,000 poles
36,000+ km of OH line,
13,000 km UG cables
250 + substations
Mix of CBD, residential,
C&I and rural network
3,800 + staff
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Local context – Ergon Energy
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$6 billion in assets
500,000+ retail customers
1 million poles
30,000+ km of line
Highly radial & sparse network
– 70% zone subs and
– 50% feeders are radial
– 4.2 connections per line km
(nat avg 24)
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35 power stations
Extensive use of SWER and
Stand-alone generation (sea
blue area)
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LV Distribution networks:
“suburban streets”
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Voltages are low –
– 415 V three phase, 240 V single
Distances must be short –
– Under 1 km from distribution
transformer
Powerflows are low –
– Up to rating of distribution
transformer (50 – 500 kVA)
Losses are moderate to high
Voltage drops are the largest
(proportionately), up to 6 - 7%
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Approx. 300m
Distribution
Transformer
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LV feeder – unverified modelling
Va 241.5 V
Vb 237.6 V
Vc 246.9 V
Va 235.6 V
Vb 239.3 V
Vc 246.1 V
Va 247.3 V
Vb 247.0 V
Vc 248.2 V
Va 230.1 V
Vb 225.1 V
Vc 245.2 V
Va 234.3 V
Vb 238.5 V
Vc 246.3 V
Va 238.7 V
Vb 235.3 V
Vc 247.3 V
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Va 0.959
Vb 0.938
Vc 1.022
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TransmissionApprox.
lineNormal cyclic rating:
Pluto > 700 A or 40 MVA @ 33 kV
Resistance
Moon > 400 A or 8 MVA @ 11kV
Aluminium cable used for overhead
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Mars > 300 A or 200 kVA @ LV
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Transmission line
Reactance
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Inductance and thus reactance
is a function of line spacing.
For a single phase system:
Lab = µ0/π loge dab/rm H/m
= 0.2 (loge d/GMR) mH/km
= 0.2 (0.25 + loge d/r) mH/km
d = conductor separation (mm)
r = conductor radius (mm)
rm = GMR = geometric mean
radius of conductors (mm)
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For a three phase system,
must use GMD instead of d
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GMD = 3√ (dab dac dbc)
= 3√ (900*1550*650) = 968 mm.
GMR = r e-1/4 = 0.7788 * r
= 5.2 mm for Moon
L = 0.2(loge GMD/GMR) mH/km
= 0.2(loge 968/5.2)
= 1.046 mH/km
X1 = 2 π f L = 329 mΩ/km
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Transmission line
Impedance
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Impedance is the combination of resistance and reactance
For the 11 kV Moon distribution line discussed:
Z = R + j X = 0.284 + j 0.329 Ω/km = 0.434 Ω/km @ 49°
Conductor name
Moon 7/4.75 AAC - LV
flat
Banana 6/1/3.75
ACSR/GZ
Calc.
Inductance
(ohm/km)
Z arg
(degrees
AC Resistance Z mag
(ohm/km)
(ohm/km) )
0.329
0.284
0.4343
49.2
0.331
0.582
0.6693
29.6
7/.080 HDC
0.383
0.986
0.9860
21.2
LVABC 95mm2
0.084
0.398
0.4074
11.9
UG 240mm2 Al XLPE
Tyree 200 kVA
transformer
0.078
0.162
0.1735
25.7
0.036
0.0096
0.0375
75.2
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P
Voltage drop
V
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For short transmission lines (true
for distribution lines),
– shunt admittance (capacitance)
can be neglected, and
– Series impedance lumped
For a simple radial AC circuit
VS = VR + I . Z
Voltage drop vs. load power is a
parabola
V
Note that for a DC circuit,
VS = VR + I . R and voltage drop vs
current (or load power) is a straight line.
Transfer
Capability
P
P
Pmax
0
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Voltage drop as per AS 3008
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∆V = IP.R + IQ.X
= I cos θ . R + I sin θ. X
= I cos θ . Z cos Φ + I sin θ . Z sin Φ
= I . Z cos (θ - Φ)
VS = VL + I . Z cos (θ - Φ)
I = I /θ = the load current
IP = I cos θ = real or in-phase component of load current
IQ = I sin θ = imaginary, quadrature or reactive component of load current
p.f. = cos θ = displacement power factor,
θ = phase delay of the load current relative to the load voltage.
Z = Z /Φ = transmission line impedance = R + j X
R = Z cos Φ, X = Z sin Φ, and thus tan Φ = X/R
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Effect of VARs on volt drop
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The voltage drop for a unity power factor load current is only seen
across the resistive portion of the transmission line and transformer
impedance.
Equally, a purely reactive load will only cause a voltage drop (if
inductive, that is, absorbing VARs from the grid) or rise (if capacitive,
exporting VARs) across the reactive portion of the transmission line and
transformer impedance.
The voltage drop seen at the load due to real power flow (in-phase
current) can be partially or even completely cancelled by a proportionate
or equal voltage rise due to VARs generated by a capacitive load
(leading current) in parallel with the load.
Complete cancellation occurs when the displacement power factor angle
θ (where p.f. = cos θ) is set equal to the transmission line impedance
angle Φ (where tan Φ = X/R).
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AS3008 fig.2
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Switched capacitor
TX line compensation
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VR
P
The addition of leading VARs
on a transmission line
– Improves power factor,
– Lifts the load voltage
– Increases maximum
possible power transfer
Switched capacitors can be
used to regulate bus voltage
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Solutions already waiting – SMA inverters as example
Tools for grid stabilization and support:
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Grid Stability Management –
power limiting on utility request
Reduction of active power in cases of
over-frequency
Ability to supply/absorb reactive power
during PV operation, and during night if
programmed
Ability to control cos phi / VAR
Stay connected during grid failures
(FRT limited)
Delivers reactive current in cases of
failure
And of course generate power from PV
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http://iea-pvps.org/fileadmin/
6_SMA_Grid_Management_APVA_UNSW.pdf
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Storage!
Solution #2
Currimundi 3A (as at Oct 14)
- 2642 customers
- predominantly residential
- 937 solar PV systems (37%)
- 2870 kW installed generation
= 3.06 kW average system size
New max of 2700 kVA
1900 kWh of storage
= 2 kWh per customer
New min of 420 kVA
1900 kWh of storage
= 2 kWh per customer
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BYD EPS-3000 UPS solution
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BYD LiFePO4 battery,
2.4, 4,8 or 7.2 kWh
3 kW inverter output power
190 kg, 475 x 795 x 655 mm WxHxD
Indoor installation
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Available now from Australia PV installers
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Sony, Samsung, Bosch, …
Sony Lithium module
• 52 Vdc, 1.2 kWh
Li-ion, 6000+ cycles
• 432 x 421 x 80mm
• 17kg
Samsung all-in-one
energy storage system
• 5 kVA PV/battery
inverter (240V 22A)
• 60 Vdc, 3.6 kWh
Li-ion, 6000 cycles
• 1000 x 267 x 680mm
• 95 kg
Bosch all-in-one energy
storage system
• 5 kVA PV/battery
inverter (240V 22A)
• 4.4 to 13.2 kWh, 96 V
Li-ion, 6000 cycles
• 597 x 706 x 1693 mm
• 220 kg
http://rfisolar.com.au/browse-products/energystorgae.html
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Playing the market with
3.6 kWh of storage?
Buy cheap SA
wind power
Battery empty,
Peak is over anyway
Feed into the
morning peak
Sell your stored SA
wind power to your
neighbours during peak
Send your PV to local
C&I rather than store
But not at lunch …
market tanks at lunch!
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Examples of technology adoption
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Note slow (dishwasher) vs. rapid (microwave)
Note complete (refrigerator) vs. partial (automobile, air-conditioning)
Which curve will PV or battery storage follow, and when?
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Many possible LV voltage regulation solutions?
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dSTATCOMs?
Move customers between phases to rebalance load & generation?
Re-tap distribution transformers?
Add switched capacitors at end of LV feeders?
Tie feeders together (mesh rather than radial?!!)
Set PV inverters to operate with a lagging power factor
Set inverter air-conditioners to operating with a leading power factor
Set PV or A/C units to operate as dSTATCOMs
Add storage to PV converters … or Air conditioners?
Add storage to dSTATCOMs?
Work out a better tariff?
Electric Vehicles?
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Converter topologies for grid connection of
PV
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Converter topologies for
grid connection of PV:
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Centralized inverters,
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String inverters,
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Module Integrated Converters
(MICs)
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DC
AC
DC
AC
DC
DC
AC
DC
AC
AC
Cascaded dc-dc MICs and
DC
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DC
AC
DC
DC
DC
DC
DC
DC
DC
DC
Bypass dc-dc MICs.
AC
DC
DC
Focus now on module integrated
converters (MICs): DC-DC and
micro-inverters.
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DC
DC
DC
DC
DC
AC
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Motivation for Per-Module PV converters
Advantages
• Per module MPPT
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Partial shading of array OK
Non planar array OK
Mismatched modules OK
Failed modules OK
Per module monitoring
Incremental expansion
Longer strings (upto 30%)
Safety advantages
Drawbacks
• Electronics in harsh
environment with PV
– Reliability penalty
– Maintenance penalty
• Cost penalty?
• Efficiency penalty?
Note that the cost of one additional PV module < $1/W
and Balance of System (BoS) now more significant.
Advantages and drawbacks must be viewed in this light.
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Baseline for comparison: String inverters
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One inverter, one grid connection,
one or two long PV strings
Typical specifications:
– 2 MPPT boost converters,
eg 125-440Vdc, 15A each
– 220-240Vac at 3-5kW
Full bridge is actually a more complex
arrangement such as “HERIC” or “H5”
to manage common mode voltages
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Boost Converter
DC Bus
DC-AC Inverter
AC filter
PV
Module
etc.
PV
Module
Z. Li, S. Kai, F. Lanlan, W. Hongfei, and X. Yan, "A
Family of Neutral Point Clamped Full-Bridge Topologies
for Transformerless Photovoltaic Grid-Tied Inverters,"
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Power Electronics, IEEE Transactions on, vol. 28, pp.
730-739, 2013.
String inverter with transformer isolation
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One inverter, one grid connection,
Shorter strings are possible
Simple full bridge, with no additional
MPPT boost stage required
Example specifications:
– Nominal 85V or 170V, 25A input
– 240Vac at 1.6 or 3.1 kW
Paralleling of PV strings permits
better resilience to shading, possibly
better safety, but
Higher currents, and transformer
isolation may compromise efficiency
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DC Bus
DC-AC Inverter
Transformer / AC filter
PV
Module
TX1
TRANS1
PV
Module
etc.
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Module Integrated Converters with AC output
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Many small inverters, with one or two PV
modules per inverter. Large step-up is
required, efficiency penalty.
Wiring to PV array is AC.
Short strings are possible
Example specifications:
– Nominal 30-60V input
– 220-240Vac at 250 or 500W
Larger examples (1kW) and
three phase examples are appearing
Step up Converter
DC Bus
DC-AC Inverter
PV
Module
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B. Sahan, Arau, x, S. V. jo, No, x, et al., "Comparative
Evaluation of Three-Phase Current Source Inverters for
Grid Interfacing of Distributed and Renewable Energy
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Systems," Power Electronics, IEEE Transactions on,
vol. 26, pp. 2304-2318, 2011.
AC filter
etc.
Module Integrated DC-DC “Optimisers”
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Many small DC-DC converters,
one per PV module, one single
DC-AC grid connect inverter.
Our initial work assumed best
solution was buck, or
alternatively boost (2003-2006)
Built and tested two phase buck
Erickson’s group (CoPEC) chose
non-inverting buck-boost (2008-)
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Module DC-DC Converter
DC Bus
DC-AC Inverter
AC filter
PV
Module
etc.
Series connected
Module DC-DC Converter
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Commercialisation of Optimisers, Maximisers
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Optimisers commercialised by
Solaredge (2006-), Tigo (2007-),
and many more
Example Optimiser specs:
– Nominal 30-60V input and
output, 10A max, 250W
Module DC-DC Converter
real world
DC-AC Inverter
AC filter
PV
Module
etc.
Series connected
Module DC-DC Converter
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DC Bus
The Inclusion of wireless or
powerline communications
an integral part of the offering.
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IC vendors support of Optimisers, Maximisers
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National Semiconductor (now TI)
first offered “SolarMagic” in 2008
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Other manufacturers offering PV
specific power or support products
Example: ST SPV2010
Interleaved four phase DC-DC
boost converter with integrated
MPPT algorithm
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Module integration of Optimisers, Maximisers
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Some optimisers integrated into
PV module junction box…
(Tigo + Trina, and many others)
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Module integration of Optimisers, Maximisers
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… and now into the module
laminate itself
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C. Deline, B. Sekulic, J. Stein, S. Barkaszi, J. Yang, and
S. Kahn, "Evaluation of Maxim module-Integrated
electronics at the DOE Regional Test Centers," in
CRICOS No. 00213J
Photovoltaic Specialist Conference (PVSC), 2014 IEEE
40th, 2014, pp. 0986-0991.
Bypass DC-DC MICs
•
•
•
•
If PV modules are still connected
in series, most power can flow in
series connection.
Only the power difference
between modules due to
shading or mismatch needs
processing by DC-DC converters
Advantages
– Much lower power ratings
– Lower efficiency acceptable
– Safety + monitoring
But
– DC-AC converter must again
perform overall MPPT
– Paralleling strings???
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DC
AC
DC
AC
DC
DC
DC
AC
DC
DC
DC
AC
DC
DC
DC
AC
DC
DC
AC
DC
DC
AC
DC
DC
DC
DC
DC
DC
DC
AC
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CRICOS No. 00213J
Processing only PV module power differences
•
•
•
The DC-DC converters can have
– Much lower power ratings
– Lower efficiency acceptable
Then smaller simpler converters
are OK, and inverting modules
are a natural fit –
But
PV
Module
PV
Module
Cuk Converter
PV
Module
Flyback Converter
PV
Module
Buck-boost Converter
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G. R. Walker and J. C. Pierce, "Photovoltaic
DC-DC module integrated converter for novel
cascaded and bypass grid connection topologiesCRICOS No. 00213J
- design and optimisation," PESC 2006
Module Integrated DC-DC “Optimisers”
•
Each module only needs to process the difference in current between
neighbouring modules.
0.39
3.6
0.66
2
3.6
0.66
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0.13
3.6
3.33
0.39
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CRICOS No. 00213J
Not a new idea (2001) – “Generation Control”
•
T. Shimizu, M. Hirakata, T. Kamezawa, and H. Watanabe,
"Generation control circuit for photovoltaic modules," Power
Electronics, IEEE Transactions on, vol. 16, pp. 293-300, 2001.
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CRICOS No. 00213J
Excellent research has continued recently
•
J. T. Stauth, M. D. Seeman, and K. Kesarwani, "Resonant SwitchedCapacitor Converters for Sub-module Distributed Photovoltaic Power
Management," Power Electronics, IEEE Transactions on, vol. 28, pp.
1189-1198, 2013.
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CRICOS No. 00213J
•
Switched capacitors operated
at their resonant frequency (or
a subharmonic) to achieve a
lower equivalent impedance.
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CRICOS No. 00213J
Energy Storage Interfacing and
Active Power Decoupling in PV
Systems
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CRICOS No. 00213J
Outline
• Energy storage
– Storage technologies
– Grid, PV and ES characteristics
– PE converter based interfacing technologies
• Active power decoupling (APD)
– Double frequency power ripple in single phase PV systems
– Large capacitor based solution
– PE converter based solutions for APD
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CRICOS No. 00213J
PV power in a day
•
•
Large power fluctuations present during 11am -4pm period
may be due to clouds
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CRICOS No. 00213J
Source: Instantaneous PV power, UQ Centre, St Lucia: 14 Mar 2015. available at http://solar.uq.edu.au/user/reportPower.php?pa=2-2&dtra=day&dts=2015-0314&etp=n
Intermittency - How it affects
• Isolated PV systems
–
–
–
–
Might be the only source
Demand-supply imbalance
Surplus/lack of power
Results in voltage fluctuations
• Grid connected residential PV systems
– Power fluctuations are passed into the grid
– Might require automatic tap change in
distribution transformers
• Large scale PV systems
– Power fluctuations may cause instabilities
– Need spinning reserve
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CRICOS No. 00213J
Images from http://www.bernardalvarez.com/a_few_things_to_consider_when_buying_rural_and_off_grid_property , https://www.choice.com.au/homeimprovement/energy-saving/solar/articles/solar-panel-payback-times , http://arena.gov.au/project/agl-solar-project/
Energy storage is the promising solution
• In isolated PV systems
– Possible to match the
demand and supply
– ESS absorbs power
fluctuations
– Regulated voltage
• In grid connected
residential PV systems
– Not essential if the grid is
strong
• In large scale PV systems
– Regulate output power
– Improve system stability
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Image at top right is from http://news.panasonic.com/press/news/official.data/data.dir/en120223-3/en120223-3.html
CRICOS No. 00213J
Other advantages of energy storage
• Load shifting
– Store energy during low demand and discharge during high
demand
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Image from http://www.codaenergy.com/solutions/microgridscampus/
CRICOS No. 00213J
Other advantages of energy storage -LVRT
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CRICOS No. 00213J
Other advantages of energy storage - GSS
• Reactive power support
– Similar to STATCOM
– Improve voltage regulation
|Vs| (kV)
3
2.5
2
0.2
• Harmonic suppression
MV/HV
TF
transmission
Energy
storage
element
Cdc
Id, Iq and Iq* (A)
Inverter
TF
Filter
Sensitive
loads
DC-DC
converter
Reactive power
exchange
real world
0.7
0.8
0.9
Time (s)
1
Vd with STATCOM
Vd without STATCOM
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Time (s)
1
Iq*
Iq
Id
0
-500
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Time (s)
1
500
0
-500
0.2
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0.6
500
0.2
ia,stat (A)
Cbuf
DC-DC
converter
0.5
2.5
2
0.2
Distribution
bus
PV
panel
0.4
3
Vd (kV)
Generator
0.3
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CRICOS No. 00213J
Time (s)
1
Energy storage technologies
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Emerging Energy Storage Technologies in Europe. Rapport Frost& Sullivan, 2003.
CRICOS No. 00213J
Cost ESSs
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Electricity Storage Association. www.electricitystorage.org.
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CRICOS No. 00213J
Battery technologies for small PV systems
• Lead-acid
– Not very expensive
– Well matured technology
– Low efficiency and cycle life
• Li-ion
– Highest efficiency and cycle life
– Expensive
– Popular in consumer electronics,
electric vehicles and low power
systems
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real world
Images from , http://www.lowtechmagazine.com/2015/05/sustainability-off-grid-solar-power.html
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http://commons.wikimedia.org/wiki/File:Lithium-Ionen-Accumulator.jpg
CRICOS No. 00213J
Battery technologies for large PV systems
• Flow batteries
– Zinc-Bromide , Vanadium- Redox, IronChromium
– moderate cost, efficiency and cycle life
– Suitable for large scale PV systems
• Molten-salt batteries
– Sodium-Sulfur, Sodium-Nickel
– moderate cost, efficiency and cycle life
– Suitable for large scale PV systems
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CRICOS No. 00213J
real world
Images from http://redflow.com and Renato M, Michael M, Giorgio C, “ZEBRA ELECTRIC ENERGY STORAGE SYSTEM: FROM R&D TO MARKET,”
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THE hi.tech.expo, pp. 1-20, Nov. 2008
Battery characteristics
Rp1
+
-
VSOC
R0
+
-
Cp1
Rp2
+
+
-
VB
Cp2
Voltage (V)
A simplified battery equivalent circuit
• Steady power for a long
period
• Voltage drop across the
internal resistance R0
• R0 limits the maximum
power
VI curves for charging and discharging
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An example VI chart for a RedFlow battery
R
VI chart for the Redflow battery is from the ZBM Installation and Operation Manual. redflow.com
CRICOS No. 00213J
Supercapaciors
• AKA EDLC
• Very high capacity (F)
• High cycle life
• High power rating
Maxwell Technologies
• Fast charging/discharging
Panasonic
Cap XX
• Suitable for absorbing fast
Saft
Epcos
Batscap
Nichicon
power fluctuations
Ness
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real world
http://commons.wikimedia.org/wiki/File%3AElectric_double-layer_capacitor_(2_models)_-1_NT.PNG
, www.Maxwell.com
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Skeleton
CRICOS No. 00213J
Supercapaciors model and characteristics
(b) Simplified SC equivalent circuit
1

(a) An example SC equivalent circuit
e
arg
Ch
rge
sha
Di
Only 50% voltage drop
is required for 75% of
energy discharge
1
2
Su p ercap acito r
V o ltag e (V )
400
A typical VI characteristic of a SC
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300
Voltage drop
200
100
0
0
20
40
60
Energy (kJ)
Energy
taken out
CRICOS No. 00213J
80
Battery supercapacitor combination
• Batteries got high energy
capacity (good for absorbing long
fluctuations)
• SCs got high power capacity
(good for absorbing short fluctuations)
• Combination can provide both
high power and high energy
capacity
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CRICOS No. 00213J
PV characteristic
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•
MPPT is required
for better utilization
•
Output voltage
should be allowed
to vary in a certain
range
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CRICOS No. 00213J
Grid characteristic and requirements
• Number of phases
– 1 or 3
• Voltage
– regulated (230V, 400V etc)
• Frequency
• Angle
– Synchronization is required
• Other requirements
Voltage (V)
– Regulated (50Hz, or 60Hz)
200
va
vb
vc
0
-200
0
5
10
15
– THD
– Islanding
– FRT
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CRICOS No. 00213J
20
Time (ms)
Let’s put things together
Voltage (V)
Power (W)
Power
flow
200
va
vb
vc
0
-200
0
Voltage (V)
I = constant
Battery
real world
10
15
20
Time (ms)
Grid
PV module
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5
( )
SC
Time (t)
At least an
inverter is
required for
grid
connection
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CRICOS No. 00213J
Simple direct connection
PV
panel
Q1
Q3
Q2
Q4
R0
CSC
Cdc
VSOC
Grid
Inverter
Maximum power
point curve
• Strongest element decides
the operating point
• MPPT cannot be guaranteed
• Underutilization of SC
Supercapacitor Battery
voltage range voltage
range
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Voltage (V)
Required PV
panel output
voltage
range
CRICOS No. 00213J
Interfacing converter for the PV module
L1
PV
panel
D1
Q3
Q2
Q4
R0
Q5
Cbuf
Q1
CSC
Cdc
VSOC
Inverter
MPPT is possible
•
But limited control over
battery power
•
SC is still underutilized
•
Required SC voltage is large
Power (W)
DC-DC converter
•
Grid
Maximum power
point curve
DC-link
voltage
range
Voltage (V)
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Required PV panel
output voltage range
Battery
Supercapacitor voltage
voltage range
range
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Interfacing converter to PV and SC
L1
PV
panel
D1
Cbuf
Cdc
Battery
Power (W)
DC-DC converter
Maximum power
point curve
Q2
Q4
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L3
ESR
CSC
Battery
voltage
range
real world
Supercapacitor
Q9
Voltage (V)
Required PV panel
output voltage range
Grid
Inverter
Q8
DC-link
voltage
range
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Q3
R0
Q5
VSOC
Supercapacitor
voltage range
Q1
DC-DC converter for
the supercapacitor
• Still the control over battery
power is limited
CRICOS No. 00213J
Separate converters for PV, SC and Battery
Power (W)
Filter
• Individual control
• Better dc-link
voltage regulation
• Better utilization
and low voltage for
battery and SC
Maximum power
point curve
• Better ESS interfacing
arrangement
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Supercapacitor
voltage range
Battery
voltage
range
DC-link
voltage
Voltage (V)
PV panel output
voltage range
CRICOS No. 00213J
Connection to the grid through inverters
Filter
• Suitable for large
scale PV systems
DC
Cbuf
Cdc
Q3
Q2
Q4
Grid
Filter
Filter
PV
panel
Q1
DC
INverter for the PV panel
Supercapacitor
L3
Q9
CSC
DC-DC converter for the
supercapacitor
real world
Q7
Q6
Q8
R0
Cdc1
Q10
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Q5
VSOC
Battery
Filter
Filter
ESR
Inverter For the battery
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CRICOS No. 00213J
Interfacing converter requirements
• Buck-boost operations
– Buck operation for charging
– Boost operation for discharging
• Bidirectional power flow
• Galvanic isolation
– Reduce leakage current
– Safely add/remove modules
• High voltage gain
– Multi-stage systems
• High efficiency
– Zero switching
• Cooling
– Liquid, forced air, natural air
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CRICOS No. 00213J
Fundamental bi-directional dc-dc converter
•
Simple operation and control
•
Low voltage gain
• Requires a large inductor to reduce current ripple
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CRICOS No. 00213J
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Combining two dc-dc converters
Q6
Q8
Q8
L3
R0
L3
Vdc
L2
R0
Vdc
VSOC
Q7
L2
Q6
Vdc1
Q9
Q9
Q7
VSOC
Interleaved dc-dc converter
Two stage dc-dc converter
•
Reduce voltage ripple
•
Voltage gain
•
Power sharing
•
No ripple reduction
•
No gain improvement
•
No power sharing
•
Reduce inductor size and
switch ratings
•
Controller is relatively complex
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CRICOS No. 00213J
Buck-boost converter
Q1
R0
Cdc1
VSOC
Buck-boost type bi-directional DC-DC
converter
If the battery voltage is close
to the dc-link voltage both
buck and boost operations
may be required
•
Negative output voltage
real world
Q3
Cdc2
Q4
cascade bi-directional buck-boost
converter.
•
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Q2
L1
•
Positive output voltage with
two additional switches
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CRICOS No. 00213J
Vdc
Cúkc converter and Luo converter
Bi-directional forms of the Cúkc converter
•
Two
‒ switches,
‒ capacitors
‒ inductors
‒ diodes
Luo DC-DC converter
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•
Reduce current ripples
•
Require large inductors and
capacitors
CRICOS No. 00213J
Isolated dc-dc converter
• Provide galvanic isolation
• ESS modules can be added, removed, replaced safely
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CRICOS No. 00213J
Zero switching converter topologies
(a) series resonance, (b) parallel resonance, (c) series parallel resonance, (d) LLC
resonance
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CRICOS No. 00213J
SQ1
SQ2
SQ3
SQ4
SQ5
SQ6
A simulation study with a BESS
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CRICOS No. 00213J
Controller block diagrams
VPV
VPV
IPV
X
PI
dP/dV based VPVref
+
MPP algorithm
-
PPV
IPVref
+
-
PI
mQ5
SQ5
PWM
IPV
MPPT controller
DC-DC converter controller
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Grid interfacing inverterCRICOS
controller.
No. 00213J
Boost operation
Simulation results
Pg
Pg
Pb
PPV
Buck operation
PPV
PPV
Pg
Pb
Pb
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CRICOS No. 00213J
Integrated energy storage interfacing
• ESS interfacing dc-dc converters add
– Additional cost
– Power losses
– Complexity
• If the ESS can be integrated into the grid
connecting inverter those can be reduce
– It is possible
– There are certain limitations as well
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CRICOS No. 00213J
Integrated ESS interfacing – Dual inverter
dc-link
Idc
SMaT
PV cell array
with MPPT
SMbT
1
SMcT
SMaB
1
m
SMbB
SMcB
2
3
+
0
Cb1
Idcx
SA1
SAbT
1
+
-
1
-
1
SAcT
SAaB
0
SA2
1
2m
Grid
2
1
1
2m
1
2m
1-k
Vdcx
Cb2
Lb2
-
SAaT
+
vas vbs vcs
SAbB
SAcB
Auxiliary inverter
PA
1  k  P

M
k
Auxiliary
inverter
1
2m
1
2m
1
1
2m
1
3
0
1

-2
1
0.5
-1
1
1
m
1.5
m
1
m
1
2m
80
Ppv
Pgrid
Pb
60
40
Power (kW)
Battery 2
Vb2
1
1
m
ias ibs ics
Main inverter
Battery 1
Vb1
k - power sharing
coefficient
Vdc
C1
Lb1
1
2m
20
0
-20
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-40
0
0.05
0.1
0.15
CRICOS No. 00213J
0.2
Time (S)
0.25
Integrated ESS interfacing – Three-level NPC
vqs
120
020
121
220
221
210
021
110
010
022
122
200
211
111 222
000
011
001
100
vds
101
012
112
201
212
002
202
102
Vsc>Vb
4
Pout
Pin
Psc
Pbl
2
Power (kW)
• DC-link voltage is
not constant
• Filters required for
SC and battery
• Unbalanced operation
• Vector pattern changes
0
-2
-4
0
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0.05
0.1
0.15
0.2
0.25
0.3
0.35
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Time (s)
0.4
Integrated ESS interfacing –Capacitor clamped
three-level inverter
• Unequal and imbalanced
capacitor voltages
• Vector pattern changes
• Dc-link voltage can be regulated
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CRICOS No. 00213J
Integrated ESS interfacing – HCMLI
Power (kW)
80
60
Pout
40
20
Pin
0
Psc
-20
-40
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (S)
Vdc (V) and Idc (A)
• Unequal and imbalanced
capacitor voltages
• Vector pattern changes
• Variable dc-link voltage
1500
1000
Vdc
500
Idc
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (S)
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CRICOS No. 00213J
Power ripple in single phase system
L1
PV
panel
D1
Q1
Q3
Q2
Q4
Q5
Cbuf
Cdc
DC-DC converter
Grid
Inverter
Power (W)
sin
sin
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CRICOS No. 00213J
Power decoupling with a large capacitor
1
2
2
2
• Increase the dc-link voltage to
reduce the required capacitance
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CRICOS No. 00213J
DC-link vs requiredcapacitance
2
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CRICOS No. 00213J
Active power decoupling (APD)
• Ripple port can be realised in number of ways
– A power converter in parallel with PV module in the DC-bus.
– A power converter in series with the power flow.
– A third port connected to the isolation transformer.
– A power converter in the AC side of the micro inverter.
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CRICOS No. 00213J
Different ripple port arrangements
(a) A power converter in parallel with PV module
in the DC-bus.
(c) A third port connected to the isolation
transformer.
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(b) A power converter in series with the
power flow.
(d) A power converter in the AC side of the
micro inverter.
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CRICOS No. 00213J
Parallel power port with PV module
(a) A dc-dc converter parallel to PV module, (b), (c) flyback converter
•
Store power
‒ APD circuit operates as a boost converter
•
Release power
‒ APD circuit operates as a buck converter
•
Low efficiency
•
Required capacitance is still large
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CRICOS No. 00213J
Power converter in series with power flow
•
•
•
•
Series power buffer can be placed at both sides
Placing at the secondary side is advantages
Phase shift modulation
Conduction loss increases
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CRICOS No. 00213J
Third port at the isolation transformer
• Possible to have a large voltage at the ripple port
• Required capacitance is low if the voltage is high
• Efficiency is low
‒ increased number of semiconductor devices
‒ imperfect coupling between magnetics
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CRICOS No. 00213J
Ripple port at the ac side
• Required capacitance is low due to
‒ high voltage
‒ higher allowable voltage ripple across the power decoupling
capacitor
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CRICOS No. 00213J
A simulation study with a third port
With APD
PV voltage
Input current
Output power
Without APD
Fluctuations are
absorbed by the third
port
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Ripple port voltage
•
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CRICOS No. 00213J
Summary and conclusions
• Energy storage plays an important role in isolated and
commercial PV systems
• Battery-supercapacitor combination is capable of absorbing
both short term and long term fluctuations
• Separate interfacing converters improve the utilization and
control flexibility
• Integrated ESS reduces power loss and cost
• Power decoupling is required to ensure MPPT
• Increase in the voltage reduces the required capacitance
• PE converter based APD topologies help reduce required
capacitance
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