Photovoltaic devices - Part 14: Guidelines for production line measurements of single-junction PV module maximum power output and reporting at standard test conditions

IEC TR 60904-14:2020 provides guidelines for measurements of the maximum power (Pmax) output of single-junction photovoltaic (PV) modules and for reporting at standard test conditions (STC) in industrial production line settings. As it is desirable to have consistent measurement practices across the industry, this document describes the following features of such measurements:
- Essential elements, in order to provide common understanding;
- Common issues or complications;
- Sources of error and uncertainty, including recommendations to minimize them.

General Information

Status
Published
Publication Date
24-Nov-2020
Current Stage
PPUB - Publication issued
Completion Date
25-Nov-2020
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IEC TR 60904-14
Edition 1.0 2020-11
TECHNICAL
REPORT
colour
inside
Photovoltaic devices –
Part 14: Guidelines for production line measurements of single-junction PV
module maximum power output and reporting at standard test conditions
IEC TR 60904-14:2020-11(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 60904-14
Edition 1.0 2020-11
TECHNICAL
REPORT
colour
inside
Photovoltaic devices –
Part 14: Guidelines for production line measurements of single-junction PV
module maximum power output and reporting at standard test conditions
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-9082-8

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 60904-14:2020 © IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms and definitions ...................................................................................................... 7

4 Measurement principles ................................................................................................... 7

4.1 Measurement of I-V curves ..................................................................................... 7

4.2 Standard test conditions ......................................................................................... 8

4.2.1 General ........................................................................................................... 8

4.2.2 Additional practical considerations ................................................................... 9

4.2.3 Deviations from STC, errors, and uncertainties ................................................ 9

4.3 Solar simulators .................................................................................................... 12

4.3.1 General ......................................................................................................... 12

4.3.2 Solar simulator requirements in relevant IEC documents ............................... 13

4.3.3 Spectral distribution of irradiance .................................................................. 13

4.3.4 Spatial non-uniformity of irradiance................................................................ 14

4.3.5 Temporal instability of irradiance ................................................................... 15

4.3.6 Angular distribution of irradiance ................................................................... 16

4.4 Reference modules ............................................................................................... 16

4.4.1 General ......................................................................................................... 16

4.4.2 Irradiance monitoring ..................................................................................... 17

4.4.3 Reference device requirements in relevant IEC documents ............................ 17

4.4.4 Reference module characteristics .................................................................. 18

4.5 Calibration and operation procedures of measurement equipment ........................ 19

4.5.1 General ......................................................................................................... 19

4.5.2 Calibration and operation procedure requirements in relevant IEC

documents ..................................................................................................... 19

4.5.3 Calibration frequency ..................................................................................... 20

4.5.4 Factors affecting calibration of measurement equipment ................................ 20

5 Measurement systems analysis ..................................................................................... 25

5.1 General ................................................................................................................. 25

5.2 Evaluation of measurement repeatability and reproducibility ................................. 25

Bibliography .......................................................................................................................... 28

Figure 1 – Application of IEC documents to report STC P values, adapted from
max

published work [1]. .................................................................................................................. 8

Figure 2 – Graphical summary of STC (cross-sectional view of module) ................................. 9

Figure 3 – Example traceability chain for production PV modules, adapted from

published work [10] ............................................................................................................... 18

Dashed lines represent optional validation checks. ............................................................... 22

Figure 4 – Flowchart describing solar simulator irradiance adjustment based on

reference module P ; adapted from published work [7] ..................................................... 22

max

Figure 5 – Illustration of four-terminal electrical connection to a PV module .......................... 24

---------------------- Page: 4 ----------------------
IEC TR 60904-14:2020 © IEC 2020 – 3 –

Table 1 – Factors contributing to P uncertainty ............................................................... 11

max

Table 2 – Summary of effects due to deviations from STC and factors of 4.2.2 ..................... 12

Table 3 – Types of production module P errors that can be partially cancelled by
max

setting solar simulator irradiance using a matched reference module .................................... 21

Table 4 – Small sampling analysis (n = 4, p = 4) over a single day, including variation

in module placement and module electrical connection [10] (solar simulator irradiance

was set for each trial using the reference module’s P ) .................................................... 26

max
---------------------- Page: 5 ----------------------
– 4 – IEC TR 60904-14:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC DEVICES –
Part 14: Guidelines for production line measurements
of single-junction PV module maximum power output
and reporting at standard test conditions
FOREWORD

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a Technical Report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 60904-14, which is a Technical Report, has been prepared by IEC technical committee

82: Solar photovoltaic energy systems.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
82/1748/DTR 82/1785A/RVDTR

Full information on the voting for the approval of this Technical Report can be found in the

report on voting indicated in the above table.
---------------------- Page: 6 ----------------------
IEC TR 60904-14:2020 © IEC 2020 – 5 –

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 60904 series, published under the general title Photovoltaic devices,

can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct understanding

of its contents. Users should therefore print this document using a colour printer.

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– 6 – IEC TR 60904-14:2020 © IEC 2020
PHOTOVOLTAIC DEVICES –
Part 14: Guidelines for production line measurements
of single-junction PV module maximum power output
and reporting at standard test conditions
1 Scope

This document provides guidelines for measurements of the maximum power (P ) output of

max

single-junction photovoltaic (PV) modules and for reporting at standard test conditions (STC) in

industrial production line settings. Such measurements typically:

• Record current-voltage (I-V) data while illuminating the module with a solar simulator;

• Are performed on 100 % of manufactured modules, in order to determine whether they meet

nameplate requirements for various bins spanning different power output levels.
This type of measurement is widespread and performed in high volume by PV module

manufacturers worldwide. As it is desirable to have consistent measurement practices across

the industry, this document describes the following features of such measurements:

• Essential elements, in order to provide common understanding;
• Common issues or complications;
• Sources of error and uncertainty, including recommendations to minimize them.

Understanding of P measurement uncertainties is expected to be useful in application of

max
other IEC documents, such as IEC 61215-1 and IEC 62941, where P tolerances and
max

uncertainties must be determined. Whenever possible, this document references specific IEC

documents covering topics in more detail. Where no such documents exist, this document

provides guidance and recommendations based on other publications relevant to the PV

industry.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document (including any

amendments) applies.

IEC 60512-2-1, Connectors for electronic equipment – Tests and measurements – Part 2-1:

Electrical continuity and contact resistance tests – Test 2a: Contact resistance – Millivolt level

method

IEC 60891, Photovoltaic devices – Procedures for temperature and irradiance corrections to

measured I-V characteristics

IEC 60904-1:2020, Photovoltaic devices – Part 1: Measurement of photovoltaic current-voltage

characteristics

IEC 60904-2:2015, Photovoltaic devices – Part 2: Requirements for photovoltaic reference

devices

IEC 60904-3:2019, Photovoltaic devices – Part 3: Measurement principles for terrestrial

photovoltaic (PV) solar devices with reference spectral irradiance data
---------------------- Page: 8 ----------------------
IEC TR 60904-14:2020 © IEC 2020 – 7 –

IEC 60904-4, Photovoltaic devices – Part 4: Reference solar devices – Procedures for

establishing calibration traceability

IEC 60904-7, Photovoltaic devices – Part 7: Computation of the spectral mismatch correction

for measurements of photovoltaic devices

IEC 60904-9:2020, Photovoltaic devices – Part 9: Classification of solar simulator

characteristics

IEC 60904-10, Photovoltaic devices – Part 10: Methods of linear dependence and linearity

measurements

IEC 61215-1, Terrestrial photovoltaic (PV) modules – Design qualification and type approval -

Part 1: Test requirements

IEC 61215-2:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type

approval – Part 2: Test procedures
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

IEC 61853-2:2016, Photovoltaic (PV) module performance testing and energy rating – Part 2:

Spectral responsivity, incidence angle and module operating temperature measurements

IEC 62941:2019, Terrestrial photovoltaic (PV) modules – Quality system for PV module

manufacturing

ISO/IEC Guide 98-3:2008, Uncertainty of measurement – Part 3: Guide to the uncertainty of

measurement (GUM:1995)

JCGM 200, International vocabulary of metrology – Basic and general concepts and associated

terms (VIM)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61836,

IEC 60904‑1, IEC 60904-9, ISO/IEC Guide 98-3, and JCGM 200 apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO online browsing platform: available at http://www.iso.org/obp
4 Measurement principles
4.1 Measurement of I-V curves

Figure 1 summarizes the application of IEC documents in order to measure PV module I-V

curves and report STC P values in production line settings. The following symbols refer to

max
common parameters measured directly from a module I-V curve:
• Short-circuit current and open-circuit voltage: I and V , respectively
SC OC
• Current and voltage at the maximum power point: I and V , respectively
Pmax Pmax
• Maximum power: P , which is equal to the product I × V
max Pmax Pmax
---------------------- Page: 9 ----------------------
– 8 – IEC TR 60904-14:2020 © IEC 2020

Documents referenced by the correction procedures of IEC 60891 and IEC 60904‑7 are omitted for simplicity.

Figure 1 – Application of IEC documents to report STC P values,
max
adapted from published work [1]
4.2 Standard test conditions
4.2.1 General

STC give the essential properties of the light to which the PV module is exposed and the thermal

state at which the PV module is kept during measurement of its I-V characteristic. STC are

intended to give a common and standardized reference point for a practical assessment of PV

module performance in both laboratory and industrial settings; they are not necessarily intended

to describe all conditions under which a PV module operates when installed outdoors. STC are

defined in IEC TS 61836 as follows:

a) A spectral irradiance equal to the global spectral irradiance defined in IEC 60904-3

(essential light property);
b) Total irradiance equal to 1 000 W m (essential light property);

c) Cell junction temperature within the module equal to 25 °C (thermal state of the PV module).

IEC 60904-3:2019, Clause 5 further specifies that the angular distribution of irradiance is

defined such that "the complete radiation hits the solar device perpendicularly under normal

incidence." STC are summarized graphically in Figure 2.
___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 10 ----------------------
IEC TR 60904-14:2020 © IEC 2020 – 9 –
Key
1 PV module under test
E(λ) global spectral irradiance as defined in IEC 60904-3:2019
G total irradiance equal to 1 000 W m
T cell junction temperature within the module equal to 25 °C

ϕ normal (perpendicular) incidence angle of irradiance as defined in IEC 60904-3:2019

Figure 2 – Graphical summary of STC (cross-sectional view of module)

It is commonly understood that STC describe spatially uniform conditions across the module

area, and temporally uniform conditions during measurement of the module I-V curve. For

example:

• Spectral irradiance and total irradiance should be uniform across the module area, and for

the duration required to measure the module I-V curve;

• Cell junction temperature should be uniform for all cells in the module, and for the duration

required to measure the module I-V curve.
4.2.2 Additional practical considerations

The measured electrical characteristics of a PV module can vary depending on additional

factors not included in STC, such as:

• The prior illumination history of the module (light soaking or metastability effects);

• The instantaneous rate of change of voltage or current versus time during measurement of

the I-V curve (capacitive or hysteresis effects);

• The electrical connection to the module (contact resistance and cable resistance).

Stabilization techniques for metastabilities specific to different PV technologies are described

in the IEC 61215 series (e.g. IEC 61215-2:2016, MQT 19). Methods for minimizing capacitive

effects are described in IEC 60904-1:2020, Annex B. Effects of contact resistance and cable

resistance are described in 4.5.4.3.
4.2.3 Deviations from STC, errors, and uncertainties

In practice, actual measurement conditions always deviate from STC. This can cause errors in

measured I-V curve parameters relative to their STC values, as summarized in Table 2. Errors

in various I-V curve parameters related to total irradiance and temperature can be corrected by

the methods of IEC 60891 if they fall within the relevant ranges allowed by that standard. Errors

in I due to spectral irradiance can be corrected by the methods of IEC 60904-7.
---------------------- Page: 11 ----------------------
– 10 – IEC TR 60904-14:2020 © IEC 2020

Even after all significant errors have been identified and corrected for, reported I-V curve

parameters, including P , will still have some (non-zero) uncertainty. For guidance on

max

evaluating and reporting all measurement aspects contributing to the overall uncertainty, refer

to ISO/IEC Guide 98-3. If the uncertainty of a correction is comparable in magnitude to the error

that it is intended to correct, such correction may be omitted, provided the error is still

considered in the uncertainty evaluation. Unless otherwise specified, uncertainty values stated

in this document are standard uncertainties (coverage factor k = 1), consistent with

ISO/IEC Guide 98-3:2008, 6.1. In addition to the corrections described above, reduction of P

max

uncertainty is generally accomplished by one or more of the following strategies:

• Minimizing deviations from STC as in 4.3;

• Setting solar simulator total irradiance using a reference module that is matched to the

production module under test. “Matched” refers to similarity in spectral responsivity, as well

as several other factors described in 4.4.4.

Often, several interacting factors contribute to P uncertainty, as described in the remainder

max

of this subclause. In general, a single factor (for example, the solar simulator’s characteristics)

cannot be used in isolation to reliably evaluate P uncertainties. However, these factors can

max
still be grouped together for convenience:

• Characteristics of the irradiance produced by the solar simulator as described in 4.3;

• Dimensional, optical, and electrical characteristics of the reference device used to set

irradiance described in 4.4, and in particular, the device’s matching (or lack of matching) to

the production module under test as described in 4.4.4;

• Dimensional, optical, and electrical characteristics of the production module under test;

• Operation procedures of the solar simulator and other measurement equipment as
described in 4.5.

Table 1 (adapted from a previous publication [1]) lists these factors approximately according to

their expected contribution to P uncertainty reported from various publications [3][5][7][10],

max

with largest uncertainty contributions listed first. Table 1 can be considered a checklist for PV

uncertainty evaluation according to
module manufacturers to be used in developing a P
max
ISO/IEC Guide 98-3.

Not all factors in Table 1 will affect all PV module manufacturers equally; therefore, each PV

manufacturer should implement a detailed evaluation of the elements that can contribute to the

P uncertainty within their specific production processes and production lines. Factors having

max

significant uncertainty contributions should be systematically monitored to keep P

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uncertainties within acceptable limits. The procedures described in Clause 0 can be used as a

starting point for quantifying some uncertainty factors using gauge study methods.

Table 2 lists these factors according to their effect on the module I-V curve parameters. Each

factor affects P indirectly, since P = I × V .
max max Pmax Pmax
---------------------- Page: 12 ----------------------
IEC TR 60904-14:2020 © IEC 2020 – 11 –
Table 1 – Factors contributing to P uncertainty
max
Source of uncertainty and Factors to be considered
reference to this document
Reference modules Stated uncertainty of I or P from calibration certificate
SC max
Subclause 4.4
Use of I or P for setting solar simulator irradiance as in 4.5.4.1
SC max
Drift of I or P since last calibration of reference module
SC max
Matching of dimensional, optical, and electrical characteristics between
reference module and production module under test as in 4.4.4
Drift of electrical connector resistance since last calibration
Optical Spectral distribution of irradiance
Subclause 4.3 Spatial distribution of irradiance
Angular distribution of irradiance
Placement of modules in test plane, possibly including translational
misalignments and/or rotational misalignments (x, y, z, and θ)
Electrical Four-terminal connection (Kelvin connection) to module

Subclause 4.5.4.3 Drift of electrical connector resistance of I-V measurement equipment

Subclause 4.3.5 Uncertainty of data acquisition channels (from manufacturer datasheets and

calibration certificates), including possible drift of channel properties over time

(e.g. gain and offset)
Uncertainty of shunt resistor used for current measurement, including possible
drift of its electrical resistance over time
Capacitive (hysteresis) effects during I-V measurement
Thermal Uncertainty of temperature sensor(s)
Subclause 4.5.4.2 Temperature non-uniformity across PV module
Deviation of reference module temperature from its calibration temperature
Deviation of production module under test from target temperature
Deviation of cell junction temperature from measured module temperature
Drift of module temperature during exposure to simulated sunlight

Measurement capability Measurement of reference modules under intermediate precision condition

Clause 0 Measurement of production modules under intermediate precision condition

Measurement corrections Corrections related to total irradiance and temperature (IEC 60891)

Corrections related to spectral irradiance (IEC 60904-7)
Uncertainty of correction factors
---------------------- Page: 13 ----------------------
– 12 – IEC TR 60904-14:2020 © IEC 2020
Table 2 – Summary of effects due to deviations from STC and factors of 4.2.2
Type of deviation and Primary effect Secondary Critical metrics
reference to this document effect

Spectral distribution of Module current None Spectral irradiance of solar simulator

irradiance
(I , I ) Mismatch in spectral responsivity of
SC Pmax
Subclause 4.3.3 reference module and production module
under test
Spectral mismatch correction factor as
per IEC 60904-7 (if used)
Spatial non-uniformity of Module current
Module voltage Spatial distribution of irradiance across
irradiance module area
(I , I ) (V )
SC Pmax Pmax
Subclause 4.3.4 Variation of cell electrical and optical
characteristics within module
Mismatch in module and cell dimension
between reference module and
production module under test
Mismatch in cell electrical interconnection
between reference module and
production module under test

Module temperature Module voltage Module current Deviation of cell junction temperature

from measured module temperature
Subclause 4.5.4.2 (V , V ) (I , I )
OC Pmax SC Pmax
Temperature non-uniformity across
module
Drift of module temperature during
exposure to simulated sunlight
Temperature correction factors as per
IEC 60891 (if used)

Capacitive effects Module current Module voltage Rate of change of voltage, current, and

irradiance during I-V measurement
Subclause 4.3.5
...

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