Design Considerations
Mercedes Pereyra Boue
International Applications Engineer
Mercedes.Pereyra@solectria.com
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Summary
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Components
Inverter selection
Over sizing
Example Pv Syst
Example single line
Career recommendations
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Components of a Typical Grid-Tied PV
System
PV Module
= PV Panel
Combiner Box
= String Combiner Box
= Array Combiner Box
Inverter
String
Subcombiner Box
= Subarray Combiner Box
= Recombiner Box
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Components of a Typical Grid-Tied PV
System – SLD View
PV Module String
Fuse
DC Disconnect
Subcombiner = Recombiner
Inverter
“Home Runs”
= PV Output Circuits
[NEC]
Combiner Box
= String Combiner Box
= Array Combiner Box
PV Source Circuit
[NEC]
= String Circuit Solectria – A Yaskawa Company / Company Confidential © 2015
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Component list (typical)
•Modules Racking
•Can you
list all the components of a
•Combiner
boxes
photovoltaic
•Circuit
protection system?
(fuses and CBs)
•Inverters (string vs. central)
•Disconnects
•Meters
•Load centers
•MV Transformer / Switchgear
•Monitoring systems
•Weather Stations
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Inverters
Options.. Single phase or micro?
Central vs. string?
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3-P String vs. Central Inverters
There are many considerations…
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DC Wiring
DC Wiring Type
Combiner Boxes
AC Combining Panel
AC Collection System
Mounting (wall vs. pad)
Communications Wiring
Monitoring Granularity
Ground Fault
Arc Fault
Full Power Temp
Wire Coloring
Wiring Isolation?
Central Point of Utility Control
Weight
Shipping Cost
Efficiency
Oversizability
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MPPT Range
String Balancing
NEMA Rating
Design Flexibility
Failure Points/Part Count
Noise
Permitting
AC Voltage
1000/1500Vdc Module?
DC Disconnects
Revenue Grade Metering
Utility Acceptance
Protective Relaying
Remote Shutdown
Mismatch Losses
MPPT Range
Commissioning
Service
O&M
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PVI 36 TL
PVI 50-100KW
Performance
3ɸ-String Inverters
 Efficiency 0% to 1% CEC Efficiency Gain
 *Minor Benefit* Lower Mismatch Losses
 *Minor Benefit* Lower Start Voltage
 Array may be susceptible to PID
 Downtime: More failures/shorter duration/duration of
downtime may vary (higher percentage of the array that down,
the short the downtime)
Central Inverters
 Array not susceptible to PID
 Downtime: Less failures/shorter duration/greater percentage of
the system down
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Design Considerations
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3ɸ-String Inverters have an 8 string input
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Racking designed around 8 strings per row allows for
scalable “cookie cutter” designs
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Arc-Fault Detection
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3ɸ-String Inverters have integrated arc-fault detection.
Fault is isolated to one inverter
Central inverters require arc-fault combiners
Fault is isolated to the arc-fault string combiner
Source- Implementing Arc Detection in Solar Applications
Brett Novak, Texas Instruments
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Service and Warranty
3ɸ-String Inverters
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Service Frequency: More points of failures
=> More visits (may not be more frequent)
Smaller percentage of system effected
Duration of Downtime: Shorter
Service: “Rip and Replace,” Replacing on
roof may be a challenge.
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Service and Warranty
Central Inverters
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Service Frequency: Fewer points of failure => Fewer visits
Larger percentage of system effected
Duration of Downtime: Longer
Service: Manufacturer Trained Rep
Warranty: 5 year
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What Architecture is Best?
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The “best” architecture is the one that optimizes the financial
performance of the project.
Financial performance is determined by:
 Engineering Cost
 Installation Cost (Equipment and Labor)
 Performance/Uptime
Answer: It depends. Best
architecture will be project
specific.
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Modules
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Microinverter vs DC optimizer
• Both: MPPT and real-time monitoring.
• Less power loss due to shading,
module mismatch and other problems
•Greater flexibility
•Optimizers require no trunk
•Low Part Count
•Low Heat
•Enhanced safety for installers due to
•High Efficiency
the absence of high voltage DC wires
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Oversizing
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Oversizing
What is the Array-to-Inverter Ratio?
• The ratio of Array’s STC Nameplate Power to Inverter’s Rated Power
• Also called: DC/AC Ratio, Oversizing Ratio, Overloading Ratio
Example:
Array: 120kW
Inverter: PVI 100KW
120kW = 1.2
100kW
…the Array-to-Inverter
Ratio is 1.2 (120%)
120kW PV Array
PVI 100KW
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Oversizing
Historically:
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Due to high module prices, optimal oversizing: 1.10 to 1.20
Design Goal:
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Optimize Annual Energy Harvest per Panel = Optimize Specific Yield (Annual
kWh/kWdc)
Little to No clipping
Minimal shading
Today:
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Cheaper Module Prices
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Larger array feeding a fixed size inverter results in greater production
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Greater production across the same fixed AC costs
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Time-of-Use Utility Rates
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Generate most energy when it has the most value
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Daily Output Profile: Clipping Day
• Oversized systems produce more energy
• Oversized systems match TOU rate structures
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Daily Output Profile: Non-Clipping Day
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How to determine “Optimal” Array
to Inverter Ratio
Perform Oversizing Analysis
using simulation program
(PVsyst, PV*SOL, SAM) =>
Feeds Financial Model
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Higher or Lower A-I Ratio?
The further away your system is from “Optimal” the Higher the “Best” Array-to-Inverter Ratio
HIGHER Array-to-Inverter Ratios:
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Large Commercial/Utility Scale Projects
Cheaper Modules
Low/No Tilt Angle
High Soiling/Don’t want to clean
Off-optimal orientation
Shading (inter-row, obstructions)
Fixed AC kW interconnection
LOWER Array-to-Inverter Ratios:
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High Altitude
Constant “Cooler” temps year round
Residential Systems, Light Commercial Systems
Clipping
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Oversizing Reverences
SolarPower World: “Supersize It,” J. Fiorelli, M. Zuercher, June 2013
SolarPro Magazine: “Designing for Value in Large-Scale PV Systems,” G.
Everts and M. LeDucq, June/July 2013
SolarPro Magazine: “Optimal PV-to-Inverter Sizing Ratio,” A. Gregg, C.
Adcock, B. Brooks, April/May 2010
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Example PVSyst
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Software
•PVWatts
http://pvwatts.nrel.gov/
•SAM
https://sam.nrel.gov/
•Helioscope
https://helioscope.folsomlabs.com/
•PVSyst – 30 days trial! http://www.pvsyst.com/en/
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Intro
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Preliminary design
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Databases
PAN file
NASA
database /
Meteonorm
OND file
• Photon Magazine Database
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Project design – Grid Connected
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Main screen
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Site and Meteo
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Orientation
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System design
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Losses
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Shading
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Shading
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Report – What to look for?
Location? Los Mochis PVSyst REV3.pdf
Meteo Data?
Modules?
Inverter ?
DC:AC ratio
Shading
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Production
PR = E_Grid / (GlobInc * Pnom)
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Recommendations
•Internships!
•Massachusetts
Clean Energy Internship Program
•http://www.masscec.com/programs/massachusetts-clean-energy-internship-program
•Career
fair (UMass, UB, Northeastern, Tufs)
•Target
the company you like! and check for alumni
•Attend conference, and trade
•Keep
in touch / network
•Learn
Autocad / PvSyst now! 
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shows (SPI, Intersolar, IEEE)
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Questions?
Thanks!!
mechiboue@yahoo.com.ar
Mercedes.pereyra@solectria.com
https://www.linkedin.com/in/mpereyraboue
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