WP6 Deliverable 6...accurately analyze the PV performance, detect issues and, on this base, permit the ... two research institutes experiences combined with the measurements performed

Checked by

WP6 Deliverable

Demonstration and validation report for tools and field testing

This project has received funding from the European Union’s Seventh

development and demonstration under grant agreement No

SUPSI – AIT

30/10/2015 – Final Version

Checked by Caroline Tjengdrawira – 3E

WP6 Deliverable 6.3


G. Corbellini (SUPSI)

Jan Slamberger (AIT)

This project has received funding from the European Union’s Seventh Framework Programme for research, technological

ation under grant agreement No 308991.

6.3


Programme for research, technological

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DEMONSTRATION AND VALIDATION REPORT

FOR TOOLS FOR FIELD TESTING Demonstration and Validation of Overall Tools – D6.3

CONTENTS

SUMMARY. ......................................................................................................... 3

1 DESCRIPTION OF TEST SITES ........................................................................ 4

1.1 TISO 10KW .......................................................................................... 4

1.2 CPT .................................................................................................... 5

1.3 AIT ..................................................................................................... 6

1.3.1 PART 1 ................................................................................................. 6

1.3.2 PART 2 ................................................................................................. 7

1.3.3 PART 3 ................................................................................................. 7

2 BEST PRACTICE FOR ON-SITE TESTING ............................................................. 8

2.1 PV MODULE STRINGS ................................................................................ 8

2.1.1 INSTRUMENTATION ................................................................................... 8

2.1.2 PROCEDURE ........................................................................................... 8

2.2 INVERTER ............................................................................................ 14

2.2.1 INSTRUMENTATION ................................................................................. 14

2.2.2 PROCEDURES ........................................................................................ 15

2.2.3 SAFETY ............................................................................................... 17

3 MEASUREMENT RESULTS ........................................................................... 18

3.1 TISO 10KW ........................................................................................ 18

3.1.1 SUMMARY ............................................................................................ 18

3.1.2 STRINGS ............................................................................................. 21

3.2 CPT .................................................................................................. 23

3.2.1 INVERTER ............................................................................................ 23

3.2.2 STRINGS ............................................................................................. 28

3.3 AIT ................................................................................................... 30

3.3.1 SUMMARY ............................................................................................ 30

3.3.2 INVERTERS ........................................................................................... 30

3.3.3 STRINGS ............................................................................................. 38

4 CONCLUSIONS....................................................................................... 46

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015 SUMMARY.

To validate the tool for on-site testing of PV plant, SUPSI and AIT have chosen to the

test them on 3 PV plants with different characteristics, all equipped with crystalline

silicon modules as required by the Performance Plus project.

The purpose of the tool is to provide innovative testing procedures and techniques to

accurately analyze the PV performance, detect issues and, on this base, permit the

operators to react any issue in underperformance in timeline manner, resolving the

problems where possible.

The 3 test sites are introduced in chapter 1, with all technical information and historical

main data. In chapter 2 we summarize the useful results regarding the operational

aspects of on-site measurement, summarizing best practices and lessons learnt from the

two research institutes experiences combined with the measurements performed during

this project.

In chapter 3 we present the electrical measures and the results of the processing using

the developed tool, these are split between the system level (inverter and strings) and

the strings level.

The knowledge base of the tool is described in deliverables D5.1, D5.2, D5.3 and D5.4 of

the Performance Plus project, while recommended procedures and tool description are

presented into the deliverable D5.5. Further possible developments of the tool are

depicted in the deliverable D7.2 where an integrated device for fault detection is

described together with its own implementation in the market and possible stakeholders.

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015 1 DESCRIPTION OF TEST SITES

The 3 test sites chosen represent different modules technologies (mono or poly

crystalline silicon), ages (from one to thirty years old), and inverter sizes (from 1.5 to 15

kW).

1.1 TISO 10KW

The first site chosen for the validation of the tool developed is a very old plant,

considered the first one connected to the grid in Europe, built in 1982; the plant is

located in Lugano (Switzerland) in the campus Trevano, where the PV group of SUPSI is

based.

The main characteristics of the PV plant are summarized in the following table.

LOCATION

Latitude 46.00 N Longitude 8.57 E Mounting Fixed

Tilt 30° Azimuth 8° E Installation 1982

PV MODULES

Number of modules 288 Manufacturer Arco Solar

Nominal Power 37W Number of cells 35

VOC 20.3V ISC 2.5A VMPP 16.1V IMPP 2.3A

α 0.1% β -0.41% γ -0.48%

Technology mono-Si

INVERTER

Number of inverters 6 Model SMA Sunnyboy

Number of strings 4 Modules per string 12

The modules of the TISO 10kW plant show many defects. Using the initial performance

as a reference for the comparison against the actual performance, we expect to see that

every module degradation modes is strongly present. For this reason, this analysis is not

so interesting.

Instead, we compared the measure performances in 2014 and 2015. This approach will

allow us to study which degradation modes are still active in the most recent years of

the plant operation and if there are any new optical or electrical losses affecting the

strings.

We do not expect to see Potential Induced Degradation (PID) failure mode because the

voltage on the module strings is limited to 250V, commonly considered too low to

generate Potential Induced Degradation.

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015 1.2 CPT

On the rooftop of a school in Lugano, we measured a PV plant of 24kWp DC that was

suspected of under-performance (in particular for one inverter). In the plant there are 96

PV modules connected to two inverters; one inverter is connected to the east-facing

modules, and the other to the west-facing modules.

These inverters can switch the output from one to two and three phases depending on

the power output; this feature improves significantly the efficiency, in particular at low

irradiance. This has to be considered when analyzing the efficiency curves of the

inverters.

LOCATION

Latitude 46.03 Longitude 8.96 Mounting Fixed

Tilt 6° Azimuth East-West Installation 2014

PV MODULES

Number of modules 96 Manufacturer Trina Solar


VOC 38.2V ISC 9A VMPP 30.6V IMPP 8.5A

α 0.05%/°C β -0.32%/°C γ -0.41%/°C

Technology Poly-Si

INVERTER

Number of inverters 2 Model Fronius


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015 1.3 AIT

AIT has chosen the building integrated photovoltaic plant (BIPV) for the test case. This

plant was built in the year 2008 as a part of shading system for offices building. Due to

installation into façade, there is a high risk of soiling and shading. The advantage of this

installation is the reflection of irradiance of office windows. This reflection increases the

total irradiance on the PV array plane for up to 30%. The BIPV plant is built from three

different technologies from modules. These are mono-Si, poly-Si and poly-Si with back

contacts technology. The technical data from PV plant are presented in the followed

subsections.

FIGURE 1: BIPV TEST CASE AT AIT

1.3.1 PART 1

LOCATION

Latitude 48.27 N Longitude 16.43 E Mounting BIPV

Tilt 30° Azimuth 5° W Installation 2008

PV MODULES

Number of modules 40 Manufacturer Solarwatt


VOC 33,7 ISC 5,28 VMPP 27,4 IMPP 4,81

α +0,03 %/K β -0,37 %/K γ -0,50 %/K

Technology mono-Si

INVERTER

Number of inverters 1 Model Sunways NT6000


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015 1.3.2 PART 2

LOCATION



PV MODULES



VOC 33,5 ISC 4,87 VMPP 27,3 IMPP 4,55

α +0,05 %/K β -0,31 %/K γ -0,41 %/K

Technology Poly-Si

INVERTER

Number of inverters 1 Model Sunways NT6000


1.3.3 PART 3

LOCATION



PV MODULES



VOC 32,8 ISC 5,34 VMPP 25,3 IMPP 4,79

α +0,05 %/K β -0,31 %/K γ -0,41 %/K

Technology Poly-Si

back-contact

INVERTER

Number of inverters 2 Model SMA SB3300


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2 BEST PRACTICE FOR ON-SITE TESTING

2.1 PV MODULE STRINGS

2.1.1 INSTRUMENTATION

I-V tracer

The main device for these studies is an I-V tracer to accurately measure the I-V

characteristics of the module string under study. The I-V tracer has a synchronized

measurement for module temperature and irradiation on the plane of the array.

The irradiation can be measured with a pyranometer or a reference cell. While a

pyranometer is more accurate in measuring the global irradiation on the plane of the

array, we suggest using a reference cell of the same technology (poly-crystalline or

mono-crystalline silicon) of the installed modules to avoid the spectral mismatch

between the sensors and the modules as much as possible.

The ISC is strongly dependent on irradiation (and almost not dependent on temperature)

so if the measured ISC deviates significantly from the expectation, it is recommended to

double check the calibration of the irradiation sensor. On the other hand the VOC is

strongly dependent on temperature (and typically have a very limited degradation

respect to original value as explained in D5.4) and therefore it is important to first verify

the accuracy temperature sensor (TBOM) before measuring the VOC.

Thermal camera

When a module string measurement is performed, it is very important to measure the

temperature at the back of the module properly as the temperature information will be

used to correct the module measured electrical parameters to the Standard Test

Condition (STC). In particular, we want to avoid the situation where the temperature

sensor (PT100, PT1000 or thermocouple) is installed in a non-representative location on

the module, for e.g. behind a cell that is affected by an hot spot, to avoid erratic

temperature reading which could falsely affect the STC correction calculations. In

addition, it is important to check the uniformity of the module temperature with a

thermal camera.

Dark I-V on-site

For the dark I-V measurement we prepared a new device using a traditional power

feeder and an I-V tracer developed internally in SUPSI, the MPTT3000, that can reach

very high accuracy on current and voltage measurements. With this setup we have been

able to measure the dark I-V characteristic of strings of modules and then use them in

the software for degradation modes analysis.

2.1.2 PROCEDURE

As explained in D5.5 and in the manual of the D5.6, SUPSI has built a synthetic

procedure to perform on field testing at module string level.

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performed on single modules or

parameters, most of them available from modules datasheet

FIGURE 2: PROCEDURE FOR STRI

As the flowchart illustrate, the string analysis is done by based on the behavior of the

parameters obtained from the I

In case of low measured short circuit current

irradiation transformed into electricity

1. Uniform soiling happens typically where small particles

affected modules (where “uniform” means both at module and at cell level

this case, we usually

effective irradiation)

cells and the modules are not limiting each other.

2. Uniform shading is very similar to uniform soiling

particulates coverage. T

irradiation yield.

3. Optical factors that limit the current in the cells: glass corrosion, degradation of

anti-reflective coating, and/or discoloration of the encapsulant. These factors

could be determined by performing visual inspection.

In both uniform soiling and uniform shading,

sensor is measuring the proper value

a reference cell made with same technology of the modules (mono

silicon). It is also important to

respect to the modules (position, no shading from surrounding objects, tilt, location etc.)

When the short circuit current is not affected, the next parameter to che

circuit voltage behavior. Most publications report a low or no degradation

This procedure, depicted in Figure 2, is useful for a qualitative analysis of the I

performed on single modules or strings and has an advantage of needi

parameters, most of them available from modules datasheet.

: PROCEDURE FOR STRING ANALYSIS


parameters obtained from the I-V tracer.

short circuit current, we can search for the reason of lower

irradiation transformed into electricity:

Uniform soiling happens typically where small particles uniformly

where “uniform” means both at module and at cell level

usually see a lower-than-expected ISC (that is only dependent on

) but no step in the I-V curve is observed, meaning that

modules are not limiting each other.

Uniform shading is very similar to uniform soiling but with the absence of

particulates coverage. The end effect is however the same, a

Optical factors that limit the current in the cells: glass corrosion, degradation of

reflective coating, and/or discoloration of the encapsulant. These factors

could be determined by performing visual inspection.

iform soiling and uniform shading, we have to be sure that our irradiation

sensor is measuring the proper value. It is therefore important to verify the value using

a reference cell made with same technology of the modules (mono-

It is also important to check that the sensor has been installed correctly with

(position, no shading from surrounding objects, tilt, location etc.)

When the short circuit current is not affected, the next parameter to che

Most publications report a low or no degradation

, is useful for a qualitative analysis of the I-V curves

needing only simple


we can search for the reason of lower

uniformly cover the

where “uniform” means both at module and at cell level). In

(that is only dependent on

, meaning that the

but with the absence of

however the same, a reduction in the

Optical factors that limit the current in the cells: glass corrosion, degradation of

reflective coating, and/or discoloration of the encapsulant. These factors

we have to be sure that our irradiation

. It is therefore important to verify the value using

or poly-crystalline

that the sensor has been installed correctly with

(position, no shading from surrounding objects, tilt, location etc.).

When the short circuit current is not affected, the next parameter to check is the open

Most publications report a low or no degradation (<10%) of the

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015 open circuit voltage VOC. When low VOC is observed, the first step is to check that the

module temperature is measured in a proper way. Best practice recommends collecting

the module temperature data at different points of a module (in the center or near the

frame). Unfortunately the cell temperature has big uncertainty and thus the

measurement needs to be performed very accurately. Best practice recommends placing

the sensors on the rear of the module to avoid the glass mass affecting the temperature

readout. The overall temperature uniformity should also be checked using a

thermographic camera. The effect of cables is usually seen as a “voltage drop” not

related to VOC; in open circuit there is no current and thus the energy losses in cables

are zero. Once we are sure that the voltage drop is not due to wrong module

temperature, the voltage drop could be explained only in two ways:

4. One or more sub-modules are bypassed, meaning that a failure has caused that

some bypass diodes to be shorted. If an entire module is shorted, the difference

between the expected and the actual VOC is a multiple of module VOC; this can be

detected by measuring module by module or using the shading method. In the

shading method, one module is shaded one at a time to understand which ones

cause losses; in this case we have to repeat the I-V measurement several times

to identify the problematic module.

5. If the VOC is accompanied by drop in fill factor, some modules could be affected by

Potential Induced Degradation. This phenomenon affect modules at the end of

string that are at higher voltages and can be partially recovered applying reverse

voltage; new devices are already available in the market to solve this problem.

When we measure a low fill factor, with a smooth curve and normal ISC and VOC, it means

that we have low shunt resistance or high series resistance (or both).

6. A typical signature of degradation is in series resistance. The RS of a PV module is

the sum of the single ohmic resistances in the different parts of the electrical

circuit in the PV module: cell solder bonds, emitter and base regions, cell

metallization, cell-interconnect bus-bars, and resistances in junction-box

terminations. An increasing value of RS could be attributed to various failures

inside modules, as depicted in D5.2 and in Figure 3.

FIGURE 3: SOURCES FOR THE SERIES RESISTANCE IN A PV MODULE

7. The shunt resistance RSH represents any parallel high-conductivity paths (shunts)

through the solar cell or on the cell edges as shown in Figure 4. Degradation in

RSH (decreasing) is due to crystal damage and impurities in and near the p-n

junction, and gives rise to the shunt current. These shunt paths divert currents

away from the intended load and their effects are detrimental to the module

performance especially at low irradiance level.

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FIGURE 4: SOURCES OF THE SHUNT RESISTANCE IN A PV MODULE

A more detailed analysis of the performance at string level can be performed with the

tool developed in the deliverable D5.6 of this project. With this tool the measured I-V

curve on-site are processed in order to take into account the losses on the cables and

the statistical variation of the parameters of the modules. It is also possible to simulate

the expected degradation of the modules after a defined period.

The comparison between the measured (and corrected to standard condition) and the

expected I-V curves can lead to insights on the actual degradation mode affecting the

string. The first table in Table 1 summarizes the ranking of the expected impacts of each

degradation modes on the I-V curve parameters, for example, the PID strongly reduces

the fill factor while there is not a significant effect on the short circuit current.

The second table includes value from dark measurement (without irradiation) that can

be performed by covering the modules properly or during night time; these measures

can be easily implemented in climates where it is not easy to find sunny condition

(irradiation higher than 600 W/m2) for long periods.

For detail explanation of how all the rankings are derived, please refer to the deliverable

D5.5 of the Performance Plus project.

TABLE 1: SUMMARY OF DEGRADTION EFFECTS ON I-V CURVES

Deg. modes FF FF low light ISC VOC ∆RS

OPTICAL ? ? - - ?

ELECTRICAL -- - - 0 ++

CELL - - - - ++

PID -- --- 0 - 0

Deg. modes �� VD_MAX VP J0

OPTICAL 0 0 0 0

ELECTRICAL -- + + 0

CELL - + - +++

PID -- -- - ++++

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015 2.1.2.1 CORRECTION OF I-V MEASUREMENT TO STC CONDITION

As explained in the deliverable D5.5, a new method has been

developed to obtain a single I-V curve from different

measurement on site. The goal is to obtain a more accurate

result avoiding the noise in the measurements and the

uncertainties coming from unstable measurement and estimation

of temperature coefficients.

The procedure is explained in details in D5.5 and considers an

averaging of each I-V curve based on its deviation from STC

(25°C and 1000 W/m2).

Moreover the tool offers the possibility to choose between five procedures for STC

correction, as documented in D5.5.

FIGURE 6: EXAMPLE OF CORRECTION TO STANDARD CONDITION, A NEW METHOD IS IMPLEMENTED

TO OBTAIN A SINGLE I-V CURVE FROM MORE MEASUREMENT IN DIFFERENT CONDITION

Figure 5:

selection of

method for STC

correction

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015 2.1.2.2 SIMULATION OF CABLES VOLTAGE DROP AND DEGRADATION

An optional mini tool has been incorporated in the overall

performance assessment tool to allow for the simulation of the

degradation of performance and the effect of the voltage drop

due to cables and energy dissipation for the Joule effect.

Typically the voltage drop could be neglected but in some cases,

especially for new installations with string inverters, this could

be relevant.

The left chart in Figure 8 shows the result of the simulation of

the degradation modes affecting the TISO plant including the

voltage drop due to cables. Here we can see that the voltage

drop due to cables is no longer significant for plants which have

been operational for 30 years while the degradation effect is

clearly dominant. In the picture at right we point out an

example we faced in a multi MW installation on an industrial

rooftop with inverters installed in the building quite far from the

PV modules.

FIGURE 8: LEFT: EXAMPLE OF DEGRADATION SIMULATION FOR A TISO STRING OF 12 MODULES

AFTER 30 YEARS OF OPERATION, THE EFFECT OF THE VOLTAGE DROP DUE TO CABLES IS

NEGLIGIBLE WHILE THE SHAPE OF THE CURVE IS – RIGHT: THE EFFECT OF 200M OF CABLES OF

SECTION 8MM2

Figure 7: settings

for the simulation

of the degradation

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015 2.2 INVERTER

2.2.1 INSTRUMENTATION

The main device for analyzing

efficiency of the PV systems. This instrument measures voltage and current on

side, and voltage, current and cos

power, the AC power and the

FIGURE 9: MODEL OF THE SYSTE

where:

• �� is an efficiency factor

• �� is the DC current

• �� is the VC voltage

• �� is the AC current

• �� is the AC voltage

• �� is the power factor

The performance ratio of PV system could be calculated with synchronous measurements

of the inverter, the irradiance on the PV plane and

FIGURE 10: MODEL OF THE SYSTE

��

where:

• �� is the performance ratio of

analyzing PV systems and inverters is the instrument for testing

PV systems. This instrument measures voltage and current on

and voltage, current and cos� on the AC-side. From the measured data the DC

the inverter efficiency are calculated.

: MODEL OF THE SYSTEM – INVERTER ON-SITE TESTING

�� · �� · �� · ��

efficiency factor of the inverter

is the power factor


irradiance on the PV plane and the PV module temperature.

: MODEL OF THE SYSTEM – SYSTEM ON-SITE TESTING

� ∑ �� · �� · �� ∑ �! · "#$%�&�

"'() · *1 , - · ./01�� 2 /34�56��

performance ratio of the system

PV systems and inverters is the instrument for testing the

PV systems. This instrument measures voltage and current on the DC-

measured data the DC


module temperature.

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015 • 7 is the sample number

• 8 is the number of samples

• �� is the nominal power*

• - is the temperature coefficient for power at Maximum Power Point

• /01� is the temperature of the cell of the PV-module

• /34� is the temperature at STC = 25°C

• 934� is the global irradiation at STC = 1000 W/m2

• 9:1� is the global irradiation on the plane of the array

*The nominal power from the module name plates can be used. In this case the

guaranteed values could be verified.

2.2.2 PROCEDURES

Whenever the monitoring system is showing significant energy losses with respect to

expected energy yield, the string analysis should be perform to possibly identify the root

causes. If the string analysis leads to no particular results, the operator should perform

an investigation at the inverter level.

Similar to the modules and strings analysis procedure developed by SUPSI for the DC

side, AIT (Austrian Institute of Technology) has developed a procedure for

troubleshooting at inverter level. The procedure is summarized in Figure 11.

First, the visual inspection of inverter has to be done to identify the mechanical and

degradation failures of inverter. Additionally the installation has to be checked if they are

following the guidelines in the manual of the inverter manufacturer. Installation and

operation according the manual are the basis for well performing inverters.

If no deviations are found, the procedure will be followed with the check of inverter error

log files. If defined errors are found, the failure indicator has to be followed; otherwise

the procedure has to be followed with VDC und IDC measurement to check short term

stability of the Maximum Power Point Tracker (MPPT). This check requires stable ambient

conditions. The possible failures of instability are failure of DC-Controller or Cin defect

(switching ripple).

If the MPPT is stable, the level V/I measurement has to be done next; the check will be

performed on string level. A single string of modules will be connected to the inverter

and the measured power will be stored, after a few second the same single string will be

measured with I-V curve tracer; these two measurements have to be performed during

stable ambient conditions. The possible failures are multiple maxima, algorithm runaway

and the failure of PV-modules.

If no incorrectness of the MPPT are found, the procedure has to be followed with VAC, IAC,

cos ϕ measurement to check grid disturbances. The possible causes of failure are failure

of AC-Controller, failure of output filter or low grid quality.

If no grid disturbances are found, the procedure has to be followed with PAC and PDC

comparison to show conversion efficiency. The possible causes of low conversion

efficiency are stable and variable energy losses.

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015 If the conversion efficiency is normal, the communication check has to be done to check

the correctness of provided status by the inverter. The possible causes of this failure are

failure on the physical connections and the failures on interfaces.

If no failure is found, the change of set points (P/Q, V/Q, Pmax) has to be done (if

applicable). After that, the communication check has to be repeated.

If there still no failure was found, the laboratory characterization has to be done to

precisely identify the failure causes.

FIGURE 11: INVERTER CHECK PROCEDURE

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015 2.2.3 SAFETY

The on-site inverter and system testing is performed on PV systems under operation.

The PV arrays have up to 1000V voltage and some 10A current in the case of parallel

coupling. The main risk is the risk of electric shock. Before any connection is made, the

PV array and inverter must be switched off. The place where the measurement is taken

has to be dry and without dust. The measurement cables have to be insulated and

certified for high voltage. The second risk is a risk of height. Special safety protection

and procedure are required to avoid incidents such as falling from height.

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015 3 MEASUREMENT RESULTS

3.1 TISO 10KW

For the TISO installation, the main test goal is to evaluate the progression of the

degradation in recent years. From previous tests it is quite clear that every degradation

mode (except for PID) has been occurring for several years causing the strong under-

performance (>20% respect to PMPP) we see today. Overall the system is performing well

albeit the plant has been operational for 33 years; these could be attributed to the fact

that the modules were hand-made and use thicker silicon solar cells (400 µm).

3.1.1 SUMMARY

The test measurements were carried out in the summer of 2014 and the summer of

2015. Several repetitions were conducted to achieve a condition close enough to STC in

order to reduce the uncertainties coming from the temperature coefficients and the low

irradiance efficiency (quite difficult in this case where the modules present high series

resistances and many shunt patterns for current leakage).

TABLE 2: TOTAL PLANT POWER REFERRED TO STC CONDITION IN DIFFERENT PERIODS

1982 2014 2015 Power corrected to STC [W] 10241 8056 7726 Degradation rate reference 79% 75%

In Figure 12 we present a summary of the degradation of performances between the

2014 and 2015 measurements. As it can be seen from the graph, performance

degradation in average of 4% is observed over the period of 12 months.

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FIGURE 12: DEGRADATION OF PERFORMANCE AT STRING LEVEL BETWEEN 2014 AND 2015, MOST

STRINGS SHOW VALUES ABOUT 4%

Recalling the deliverables D5.2 and D5.5, we categorized the degradation modes into

four categories:

• Optical

• Electrical

• Cells cracking

• Potential Induced Degradation (PID)

The following figures present the observations from the visual inspection of the PV

modules. From Figure 13 and 15 it is clear that the modules are suffering from optical

losses; this phenomenon seems to be stable from some years. With the on-site testing

we confirmed this intuition and understood that additional losses of the last year should

be addressed to electrical losses in particular to new shunt paths inside the module or

increasing of series resistance due to hot spots (Figure 14) or other electrical defects.

-2%

0%

2%

4%

6%

8%

10%

12%

14%

Power degradation @STC [W]

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FIGURE 13: VISUAL APPEARANCE OF SOME MODULES OF THE TISO PLANT, MOST OF THEM ARE

SUFFERING FROM YELLOWING CAUSING CLEAR OPTICAL LOSSES

FIGURE 14: IMAGES FROM THE THERMAL CAMERA SHOWING HOT SPOTS CORRESPONDING TO THE

JUNCTION BOXES ON THE BACKSIDE OF THE MODULES

Other important defects are shown in Figure 15, in particular regarding the defects

associated with the degradation of the bus bars and the cell degradation subsequent to

strong hot spots (in summer we measured temperature of about 105°C).

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FIGURE 15: BURNT CELLS, AND BUS BARS DEFECTS ARE SHOWN FROM THE TISO PLANT

3.1.2 STRINGS

We present one very representative case of the results of the I-V measurements of

strings and the application of the developed software (D5.6) presented in D5.5.

The other strings measured showed a very similar behaviour in the extracted parameters

to the string analyzed in detail here.

The result of the first string analysis is reported in Figure 16; in this case we used all the

features of the tool, including cables voltage drop, expected degradation, light and dark

I-V measurement.

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FIGURE 16: ANAYSIS OF THE REPRESENTATIVE STRING

FIGURE 17: SUMMARY OF DEVIATIONS BETWEEN REFERENCE AND MEASURED VALUES

TABLE 3: REFERENCE TABLE FOR LIGHT I-V DEVIATION

Deg.

modes FF FF low light ISC VOC ∆RS

OPTICAL ? ? - - ?

ELECTRICAL -- - - 0 ++

CELL - - - - ++

PID -- --- 0 - 0

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015 TABLE 4: REFERENCE TABLE FOR DARK I-V DEVIATION

Deg. modes �� VD_MAX VP J0

OPTICAL 0 0 0 0

ELECTRICAL -- + + 0

CELL - + - +++

PID -- -- - ++++

In the Figure 16 and Tables 3-4 we summarized the results of the deviation analysis

between the measured and the reference I-V curves between 2014 and 2015.

Comparing the last row of the Figure 17 and the rows of the Table 3 and 4, it is pretty

clear that the main degradation mode affecting the string is the electrical one.

To understand which degradation mode is addressable we recommend comparing the

reference table (Table 3) with the output of the tool (Table 4) and check which line has

more similar rankings (plus and minus). In addition, combining the light and dark I-V

measurements we have more chances to figure out the main degradation mode affecting

the string.

We have also found that magnitudes of the variations of the parameters are high,

implying that the electrical degradation is severe. In this case, both the light and dark I-

V measurements agree on the root cause.

The measurement on other strings, not reported here, confirmed that electrical

degradation is the main one causing the strong degradation occurring in the plant (from

79% to 75% in only one year).

3.2 CPT

For this test plant, we started by measuring the whole system, then focused on one

inverter, and then on the strings connected to this inverter to detect which could be the

root cause of the energy losses.

3.2.1 INVERTER

To evaluate the performance of the plant as a whole we start our measurements from

the inverters, and then focused on strings when we understood that the problem was at

that level.

We repeated these measures in different occasions and then summarized the results in

the figures below. The repeated measurements were carried out under different ambient

conditions and consequently different behaviours of the PV plant due to thermal,

irradiation or soiling effects were observed.

3.2.1.1 INVERTER 1

In the Figure 18-21 we present the results of the tool for the inverter analysis. In this

case, the results do not indicate significant under-performance on the plant, considering

the Performance Ratio (Figure 19) falls regularly between 85% and 90% and the

conversion efficiency rarely falls below 95% (Figure 18).

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015 In the Figure 21 the DC losses is about 8%, indicating that the DC part of the plant is

not affected by significant under-performance. This value, on the long term, is in

agreement with values calculated with PVSyst or similar software.

FIGURE 18: ANALYIS OF THE INVERTER 1


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FIGURE 21: SANKEY DIAGRAM OF INVERTER 1

3.2.1.2 INVERTER 2

On the second inverter we repeated the same measurements to evaluate the overall

performances. From Figure 22 we see that the conversion efficiency is almost always

above 95% as claimed in the inverter datasheet.

In Figure 23 we see that in several occasions the PR is very low, in particular in sunny

conditions. From Figure 24 the efficiency is always good and does not depend on the

voltages so we considered the DC part (strings of modules) as the main possible cause

of under-performances.

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FIGURE 25: SANKEY DIAGRAM FOR INVERTER 2

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015 3.2.2 STRINGS

3.2.2.1 TESTING OF SOILING EFFECT

From Figure 26 we understand that there was a significant under performance in one

inverter (array PV losses 16% are clearly too much, and the PR was sometimes very low)

and therefore we put our focus on the strings connected to this inverter.

We tested all the strings connected to this inverter and selected one to show here as an

example for the use of the tool.

FIGURE 26: STRING SUSPECTED OF CAUSING UNDER PERFORMANCE

TABLE 5: REFERENCE AND RESULTING TABLES FOR THE STRING

Comparing the reference table (Table 5, left) and the tables with measured values (Table

5, right) the most reasonable degradation mode indicated is the optical one, meaning

there could be something that reduces the irradiation available at the cell of the PV

Deg. modes FF FF low light ISC VOC

OPTICAL +- +- - -

ELECTRICAL -- - - 0

CELL - - - -

PID -- --- 0 -

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015 modules with respect to the reference cell used to measure the irradiation on the plane

of modules.

A visual inspection carried out on the string found a high level of soiling, we then

repeated the measurement and the results are shown Figure 27.

FIGURE 27: THE SAME STRING AFTER CLEANING IT

After cleaning the modules we repeated the measurements and the results are also

shown in Figure 27; in this case the differences between the reference parameters and

the calculated ones after the cleaning are negligible (all of them are lower than 1%), i.e.

within the uncertainty threshold of the measurement equipment. We therefore concluded

that the whole electrical losses are attributable to the soiling effect on the modules,

confirming the assumption from the tool.

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015 3.3 AIT

As mentioned in section 1.3, AIT has chosen a building integrated photovoltaic plant

(BIPV) for the test case. This plant was built in the year 2008 as a part of shading

system for offices building. Due to installation into façade, there is a high risk of soiling

and shading. The advantage of this installation is the reflection of irradiance of office

windows. This reflection increases the total irradiance on the PV array plane for up to

30%. The BIPV plant is built from three different module technologies. These are mono-

Si, poly-Si and poly-Si with back contacts technology.

3.3.1 SUMMARY

On the monitoring the under-performance of the PV system has been found. First, the

test of the system and inverter has been made (subsection 3.3.2). The analyses of this

test (see Figures 28-43) show that the main losses are on the PV array side (up to 22%);

meanwhile, the inverter efficiencies are in the range of the datasheets values.

Second, the measurements of PV module strings have been done. Significant steps in

the I-V curve are observed in all cases (see Figures 44-51), indicating, according to the

procedure summarized in the Figure 2, that a mismatch of module parameters within the

same string is present and influencing the electrical output. The main reasons for those

mismatches could be partly shading, non-homogenous soiling or cracks/burns of the

cells.

After analyses the large deviations of the shunt resistance RSH in all PV strings have been

found (see Tables 6-9). The shunt resistance RSH is defined as the derivation of the

voltage (slope of the IV curve) near the short-circuit current point. In the case where the

I-V curve is not smooth, the calculation of RSH is not accurate and should not been taken

into account.

The test with IR-camera should be made to define exact reason of the losses of this

power plant. Unfortunately, the PV modules from this power plant are integrated into

façade and therefore difficult to make reliable IR-tests.

3.3.2 INVERTERS

During the testing of the inverter and the system performance, the DC current, the DC

voltage, the AC current, the AC voltage, cos �, ambient temperature, module

temperature and irradiance on the PV plane have been measured. The tool for

system/inverter analysis has calculated the theoretical DC power, actual DC power and

actual AC power from the captured data. Figure 28 presents the inverter efficiency from

the inverter in the part 1 of the installation.

The mean efficiency is 96.6%; this is in the range of the datasheet value (maximum

efficiency 97%, Euro efficiency 96.5%). Figure 29 presents the Performance Ratio from

the inverter in part 1. The mean value is 69.4%; this is lower than expected. Normally

the value of the Performance Ratio is expected to be above 80%. Figure 30 presents the

ratio between the AC voltage and the DC power at the inverter; additionally the values of

the inverter efficiency at measurement points are presented by colour. The inverter

efficiency is higher at lower power and higher voltage. Figure31 presents the Sankey

diagram for the power losses. As in can be seen, the main losses are PV array losses

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015 (22.1%), the second losses are degradation losses (estimated 5.6%), the third losses

are inverter losses (2.5%), and the last losses are cable and contact losses (0.5%). The

analysis from part two and three show the similar issues (see Figures 32-43).

Regarding to the results of the inverter/system analysis (the main losses are on PV array

side), the PV module string analysis have to be done to define reasons of higher PV array

losses (see subsection 3.3.3).

FIGURE 28: PART 1 INVERTER 1: CONVERSION EFFICIENCY

FIGURE 29: PART 1 INVERTER 1: PERFORMACE RATIO

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FIGURE 30: PART 1 INVERTER 1: INVERTER EFFICIENCY

FIGURE 31: PART 1 INVERTER 1: LOSSES

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3.3.3 STRINGS

Eight strings of the AIT plants have been tested using the tool for the degradation modes

detection. The modules have been in operation for seven years; this was taken into

account when evaluating the parameters.

In all cases the strings show significant steps in the I-V curve, indicating, according to

the procedure summarized in the Figure 2, that a mismatch of modules parameters

inside the same string is present and influencing the electrical output.

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015 3.3.3.1 PART 1

The results from the first part of the installation are shown in Figures 44-45 and then

summarized in Table 6-7.

Analyzing the parameters extracted by the tool from the light I-V measurements, it

seems reasonable to assume that a combination of electrical and cell cracking

degradation occurs. The very low fill factor can lead often to electrical problems in the

module. PID mode should be excluded because it would not affect the photocurrent and

also, the module structure is glass/glass so PID, if present, is active only near the edges

of the module.

FIGURE 44: PART 1 INVERTER 1 STRING 1: PV STRING ANALYSIS

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TABLE 6: PART 1: COMPARISON BETWEEN DATASHEET AND MEASURED DATA

Datasheet Light @ STC Δ [%] Light @ STC Δ [%]

IPH 5.28 4.64 -12.20 4.70 -11.02

I0 6.86 5.95 -13.17 3.83 -44.21

n 1.17 1.30 11.10 1.14 -2.87

RSH 1.01 663.06 65264.61 511.71 50345.05

RS 10.74 10.26 -4.48 9.29 -13.55

ISC 5.28 4.57 -13.40 4.61 -12.71

VOC 674.00 656.55 -2.59 662.96 -1.64

FF 0.74 0.65 -11.93 0.65 -12.31

String 1

Part 1

Inverter 1

String 2

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015 TABLE 7: SUMMARY OF PARAMETERS AND REFERENCE TABLE

3.3.3.2 PART 2

The same analysis has been applied on the part 2 of the installation. The results are very

similar, so we can also attribute the performance issue to the electrical and cell levels.

The results of the testing are summarized in Figures 46-47 and Table 8.


Deg. modes FF FF low light ISC VOC

OPTICAL +- +- - -

ELECTRICAL -- - - 0

CELL - - - -

PID -- --- 0 -

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Datasheet Light @ STC Δ [%] Light @ STC Δ [%]

IPH 4.87 4.98 2.28 4.98 2.21

I0 4.48 7.54 68.09 7.50 67.15

n 1.16 1.16 0.03 1.17 1.26

RSH 1.71 563.06 32763.96 538.03 31303.02

RS 11.50 12.12 5.37 13.41 16.59

ISC 4.87 4.90 0.61 4.89 0.48

VOC 670.00 648.66 -3.19 654.59 -2.30

FF 0.79 0.65 -18.39 0.64 -18.60

String 2String 1

Part 2

Inverter 1

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015 3.3.3.3 PART 3

The same analysis has been applied the part 3 of the installation. The results are very

similar, so we can also attribute the performance issue to the electrical and cell levels.

The results of the testing are summarized in Figures 48-51 and Table 9.


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Datasheet Light @ STC Δ [%] Light @ STC Δ [%] Light @ STC Δ [%] Light @ STC Δ [%]

IPH 5.34 5.14 -3.79 5.08 -4.93 5.21 -2.50 5.10 -4.44

I0 2.24 1.41 -37.32 6.87 206.49 7.64 240.61 1.01 -55.10

n 1.20 1.16 -3.41 1.12 -7.20 1.14 -5.28 1.15 -4.25

RSH 4.98 333.77 6597.42 372.50 7374.50 292.97 5778.66 325.81 6437.64

RS 7.59 6.71 -11.55 6.84 -9.85 7.78 2.57 7.14 -5.86

ISC 5.34 5.03 -5.76 4.98 -6.73 5.09 -4.73 5.02 -5.99

VOC 328.00 320.18 -2.38 318.92 -2.77 322.03 -1.82 321.64 -1.94

FF 0.69 0.63 -9.19 0.64 -7.80 0.62 -10.10 0.63 -8.89

String 2String 1String 2String 1

Inverter 1 Inverter 2

Part 3

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015 4 CONCLUSIONS

The overall objective the Work Package (WP) 5 of the Performance Plus was to define

new procedures and software for field testing that can improve the quality of

measurements and the resulting information to operate and optimize the energy output

(and so the LCOE) of PV plants. Moreover the tools aim to simplify the job of PV plant

operators by improving the quality of on-site measurement and thus reducing the

associated uncertainties.

The tools and procedure developed and reported in the deliverables D5.5 and D5.6 of the

Work Package 5 have been applied to three real PV installations, all equipped with silicon

crystalline modules and small size inverters. This report presented the results of these

demonstration works.

In the first test case, the tools have been applied to a very old PV installation (>30year

old) in Lugano (Switzerland) to understand which module degradations are still affecting

the plant performance in the recent years. From this test case, we have attributed that

the recent losses are due to electrical issues in the modules while the optical

phenomenon (such as the yellowing) has stabilized. These results have been obtained by

combining light and dark I-V measurements.

In the second test case, the tools were tested on a rooftop installation, also in Lugano

(Switzerland). By testing both the DC and AC parts of the installation we found that the

inverter showed regular performances. Analysis showed the main root cause of the

losses is the soiling on the PV modules.

In the third test case, the tools were applied on a BIPV installation at Vienna (Austria).

The analysis indicates the issues to be at electrical and cells level.

By testing the new procedures and software for field testing we developed in WP5, we

have demonstrated that it has been possible to analyze PV installations of different ages

and types and investigate the root causes that cause the under-performance and energy

losses of the studied PV plants. These tools offer promising results for further

development into a fixed device which can be physically installed in PV installation as

explained in the deliverable D7.2 of the Performance Plus.

Documents

WP6 Deliverable 6...accurately analyze the PV performance, detect issues and, on this base, permit the ... two research institutes experiences combined with the measurements performed