1) The document discusses modeling power optimizers in PVsyst software to recover losses from mismatch between photovoltaic modules and strings. Mismatch losses occur from differences in current and voltage output from modules due to shading, temperature variations, and component differences. 2) Power optimizers can help compensate for these mismatch losses by boosting both the current and voltage from partially shaded strings and modules to maximize energy output. Different types of optimizers, including full buck-boost and buck-only optimizers, are evaluated in the document for their ability to recover losses. 3) According to the simulations and analysis in the document, while power optimizers are useful for monitoring and flexibility, their ability to recover electrical shading losses in row

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Modeling PV power optimizers with PVsyst
8th PVPMC Workshop
9-10.5.2017 Albuquerque, USA
André Mermoud, Bruno Wittmer
Bruno.Wittmer@pvsyst .com

2. Page 2Page 2
Overview
• Introduction to mismatch losses
• Electrical shading losses in row-based installations
• Recovery of electrical shading losses with power
optimizers

3. Page 3Page 3
Mismatch losses
Operate PV cells at same current.
The ‘weakest’ element limits the current.
Current mismatch comes from spread in cell
characteristics and from irradiance differences.
Need to operate strings at same voltage.
PV-curve determines best operating point.
Voltage mismatch comes from spread in cell
characteristics, different irradiance and different
temperature.
Serial connections (Cells and Modules)
suffer from Current mismatch
Parallel connections (Strings)
suffer from Voltage mismatch
Current
mismatch
Voltage
mismatch
If serial mismatch becomes large, the
bypass diodes get activated
Parallel mismatch will not trigger
bypass diodes
Power optimizers are designed to
compensate the mismatch

4. Page 4Page 4
Mismatch losses in PVsyst
Module mismatch losses
- Variation of module properties (Vmpp, Impp)
- Variation of temperature (Vmpp)
Electrical Shading & Orientation losses
- IV Mismatch due to shading
- Mismatch due to different orientations
Constant loss factor in simulation (default 1%).
A dedicated tool allows to make a detailed
analysis based on IV-curves, to estimate this
loss factor
Calculated in detail in the simulation.
Leads to current and voltage mismatch
Current mismatch is small
(will not activate bypass diodes)
Mainly current mismatch due to
shadings
When Current mismatch becomes large
enough to activate bypass diodes
=> Voltage Mismatch
Fully recoverable with optimizers Partially recoverable with optimizers
But additional optimizer efficiency loss!
Typically ≈1%

5. Page 5Page 5
Module and Sub-Modules
Sub-module (sub-string)
protected by bypass-diode Module in landscape orientation
Shadings in a row-based
installation will progress upwards.
Row-based installations:
Modules within a string have the same shadings
1/3 of submodules get shaded at once
Consider a module with 72 PV cells and 3 bypass diodes
=> 24 cells / sub-module

6. Page 6Page 6
Bypass Diodes Behavior
Significant
shift of VmppSignificant
shift of Vmpp
Single Cell shaded All Cells in one sub-module shaded
Large current mismatch activates bypass diode
Large mismatch is caused by shadings
Mismatch of a single cell leads already to bypass of entire sub-module
When a bypass diode becomes active, Vmpp decreases by 1/3 (1/2 for second bypass diode)
The energy dissipated in the bypass diode, accounts for an additional loss
: 1st sub-module (shaded)
: 1st + 2nd sub-module
: 1st + 2nd + 3rd sub-module
: unshaded module
: Shaded cells
: Unshaded cells
Bypass diode
active
Bypass diode
not active
Bypass diode
active
Bypass diode
not active

7. Page 7Page 7
Shading a single module
First row of cells shaded Four rows of cells shadedTwo rows of cells shaded
: shaded cell(s)
: unshaded cells
: unshaded module
The shading proceeds in steps of sub-modules:
Each shaded sub-module shifts the step in the
IV-Curve to lower V
: 1st sub-module
: 1st + 2nd sub-module
: 1st + 2nd + 3rd sub-module

8. Page 8Page 8
Shading losses with one or two rows of shaded
cells are the same
First row of cells shaded Two rows of cells shaded
: 1st sub-module
: 1st + 2nd sub-module
: 1st + 2nd + 3rd sub-module
Linear shading loss: first row of cells
Electrical shading loss: second row of cells
Linear shading loss: Two row of cells
Electrical shading loss: none
In a partially shaded sub-module the unshaded cells are also lost and not recoverable!
The active bypass diode also dissipates power, which is not recoverable.
: unshaded module
The losses are of equal magnitude
but are accounted for differently

9. Page 9Page 9
Shading a string of modules
A string of modules is equivalent to a string of sub-modules => the behavior is similar to the module behavior
: shaded sub-modules
: shaded + unshaded sub-modules
: resulting IV-curve
First row of cells shaded
One sub-module shaded
Three rows of cells shaded
Two sub-modules shaded
Two rows of cells shaded
One sub-module shaded
Optimizers on sub-module level
can recover diffuse part of shaded modules
Shadow growth
(8 sub-modules get shaded at a time)
: unshaded string

10. Page 10Page 10
Shading losses in a string of modules
: shaded sub-modules
: shaded + unshaded sub-modules
: resulting PV-curve
First row of cells shaded Four rows of cells shadedTwo rows of cells shaded
Optimizers on sub-module level
can recover diffuse part of shaded modules
: unshaded string (2322W)
Shadow growth
(8 sub-modules get shaded at a time)
Displaced MPP can reach
inverter voltage limits!

11. Page 11Page 11
Parallel connection of shaded strings
Identical IV-Curves add up in current
=> multiply current (power) with number of strings
IV curves are not identical
MPPs are at different voltages
=> Mismatch losses
Same shading on each string Shaded and unshaded strings in parallel
: shaded sub-modules
: 1st string
: 1st + 2nd string
: unshaded strings
: shaded + unshaded
1451 W
2990 W
1494 W
3142 W

12. Page 12Page 12
Parallel connection of shaded and unshaded strings
Single shaded string Shaded and unshaded strings in parallel Shaded and two unshaded strings in parallel
Pmpp Module:
Pmpp Strings:
8 x 187 W = 1494 W
1494 W 1451 W + 1691 W = 3142 W 481 W + 2 x 2319 W = 5119 W
8 x (187 W + 290 W) = 3810 W 8 x (187 W + 2 x 290 W) = 6126 W
shaded string
shaded string shaded string
unshaded strings
pull MPP to
higher Voltage
1494 W 1451 W
1691 W
481 W
2319 W
2319 W
2322 W
1494 W
1451 W
481 W

13. Page 13Page 13
Losses in a partially shaded string
Single shaded string
Unshaded strings
Relative loss =
The order in which the sub-modules
get shaded does not matter!
Loss in group of parallel strings
Power of single unshaded string
Irradiance loss
Electrical
shading loss
1/3 of sub-modules
3 or more strings in parallel:
When 1/3 of the submodules in one string are shaded,
the contribution of the shaded string is reduced to diffuse part

14. Page 14Page 14
Power Optimizer Working Principle
Full optimizer (Buck-Boost)
Current & Voltage boost
‘Buck-only’
Current boost
Optimizers expand MPP to an extended IV-range
=> Mismatch in serial and parallel connections can be compensated
MPP
constant
power
Current
boost
MPP
constant
power
Current
boost
‘Buck-only’ optimizers risk to reach
current limit (or inverter lower voltage limit)
MPP
constant
power
Current
boost
Voltage
boost
constant
power
Current
boost Voltage
boost
‘Buck-only’ optimizers can act either
on module or on sub-module level
: Optimizer Output
: Module IV curve

15. Page 15Page 15
Optimizers and Module Mismatch
Full optimizer‘Buck-only’
‘Buck-only’
per sub-module
Optimizers recover the Module mismatch
(small mismatch, not triggering bypass diodes)
315 W 295 W 275 W 255 W
105 W
98 W
92 W
S = 1140 W
S = 295 W
In PVsyst, systems with optimizers get as default 0% module mismatch

16. Page 16Page 16
Optimizers and Shaded Sub-modules
Full optimizer‘Buck-only’
‘Buck-only’
per sub-module
Optimizers per sub-module can recover
the diffuse part of the shaded sub-modules
Extended MPP plateau
Extended MPP plateau
Recovery of
sub-module mismatch
Optimizer Imax is reached
in this example!
MPP Extension to lower voltage risks to hit Inverter limitation!: Optimizer Output
: Shaded IV curves

17. Page 17Page 17
Optimizers and Shaded Strings
Full optimizer ‘Buck-only’
‘Buck-only’
per sub-module
Optimizers per sub-module can recover
the diffuse part of the shaded sub-modules
Sub-modules are
boosted independently
: Optimizer Output
: Shaded IV curves
Firs row of
sub-modules
shaded
Firs two rows of
sub-modules
shaded

18. Page 18Page 18
Strings in parallel
Optimizers effectively recover the Voltage mismatch between
strings
MPP Extension to lower voltage risks to hit Inverter limitation!
Shaded and unshaded strings in parallel
No optimizer
Pmpp = 3.73 kW
‘Buck-only’
per sub-module
Pmpp = 4.67 kW
‘Buck-only’
Pmpp = 3.76 kW
Full optimizer
Pmpp = 4.51 kW
Inverter Vmpp min : Resulting IV/PV curves

19. Page 19Page 19
Impact on yearly simulation
- Situations with optimizer benefits do not happen very
often
- Typically they happen with shadings => sun is low and
incidence angle high (small impact)
- The overall benefit in row-based installations is small.
- ‘Buck-only per submodule’ can recover the diffuse
irradiance on shaded sub-modules
- Buck-only per sub-module and ‘Full optimizers’ recover
voltage mismatch
on shaded/unshaded string combinations
Sun Height: 22°
: Shadow
: Shaded String
Pmpp = 3.76 kW
Electrical
shading lossess

20. Page 20Page 20
Conclusions
Using optimizers in row-based PV installations:
• Monitoring at the PV Module level
• Possibility to centrally disconnect Modules
• More flexibility in string design (some optimizers also allow especially long strings)
• The module mismatch is always recovered with optimizers (PVsyst will set it to 0% by default)
=> useful on the long run, when module degradation increases mismatch
• Additional losses due to optimizer efficiency
(similar order of magnitude as initial module mismatch)
• In row-based installations the recovery of electrical shading losses is rather small
Recovering electrical shading losses with power optimizers comes mainly into play with
irregular shading situations
Buck only optimizers are more likely to reach inverter voltage limitations