Maximum power point tracking efficiency of grid connected photovoltaic inverters

IEC 62891:2020 provides a procedure for the measurement of the efficiency of the maximum power point tracking (MPPT) of inverters used in grid-connected photovoltaic (PV) systems. Both the static and dynamic MPPT efficiency are considered. Based on the static MPPT efficiency calculated in this document and steady state conversion efficiency determined in IEC 61683 the overall efficiency can be calculated. The dynamic MPPT efficiency is indicated separately.

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Status
Published
Publication Date
14-Jul-2020
Current Stage
PPUB - Publication issued
Completion Date
15-Jul-2020
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IEC 62891
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
colour
inside
Maximum power point tracking efficiency of grid connected photovoltaic
inverters
IEC 62891:2020-07(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62891
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
colour
inside
Maximum power point tracking efficiency of grid connected photovoltaic
inverters
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-8470-4

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

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 62891:2020 © IEC 2020
CONTENTS

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

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

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

3 Terms and definitions ...................................................................................................... 6

3.1 Inverter input (PV generator) ................................................................................... 6

3.2 Inverter output (grid) ............................................................................................... 7

3.3 Measured quantities ................................................................................................ 7

3.4 Calculated quantities .............................................................................................. 8

4 MPPT efficiencies ............................................................................................................ 9

4.1 General description ................................................................................................. 9

4.2 Test set-up ........................................................................................................... 10

4.3 Static MPPT efficiency .......................................................................................... 11

4.3.1 Test conditions .............................................................................................. 11

4.3.2 Measurement procedure ................................................................................ 12

4.3.3 Evaluation – Calculation of static MPPT efficiency ......................................... 13

4.4 Test conditions for dynamic MPPT efficiency ........................................................ 13

4.4.1 Dynamic MPPT efficiency .............................................................................. 13

4.4.2 Measurement procedure ................................................................................ 14

4.4.3 Evaluation – Calculation of the dynamic MPPT efficiency .............................. 14

5 Calculation of the overall efficiency ............................................................................... 15

Annex A (normative) Requirements on the measuring apparatus ......................................... 16

A.1 PV generator simulator ......................................................................................... 16

A.1.1 General ......................................................................................................... 16

A.1.2 Requirements on the static characteristic ...................................................... 16

A.1.3 Requirement on the transient stability ............................................................ 17

A.1.4 Requirements on the dynamic characteristic .................................................. 17

A.1.5 Requirements on electrical characteristic ....................................................... 17

A.1.6 Calibration – Uncertainty ............................................................................... 17

A.2 AC power supply ................................................................................................... 17

Annex B (normative) Test conditions for dynamic MPPT efficiency ....................................... 18

B.1 Test profiles .......................................................................................................... 18

B.2 Test sequence with ramps 10 % – 50 % G (See Table B.1) ............................ 20

STC

B.3 Test sequence with ramps 30 % – 100 % G (See Table B.2) .......................... 21

STC

B.4 Start-up and shut-down test with slow ramps (See Table B.3 and Figure B.3) ....... 21

B.5 Total test duration ................................................................................................. 22

Annex C (normative) Models of current/voltage characteristic of PV generator ..................... 23

C.1 PV generator model for MPPT performance tests .................................................. 23

C.2 Alternative PV generator model for MPPT performance tests ................................ 27

Annex D (normative) Efficiency weighting factors ................................................................. 29

D.1 European efficiency .............................................................................................. 29

D.2 CEC efficiency ...................................................................................................... 29

Annex E (normative) Specification of the static MPPT and conversion efficiency in

terms of normalised rated AC power ..................................................................................... 30

E.1 General ................................................................................................................. 30

E.2 Re-normalisation of output power P to the rated output power P .................. 30

AC AC,r
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IEC 62891:2020 © IEC 2020 – 3 –
E.3 Representation of the conversion efficiency in terms of normalised rated

output power ......................................................................................................... 30

E.4 Interpolation on normative nodes .......................................................................... 31

E.5 Result ................................................................................................................... 33

Bibliography .......................................................................................................................... 34

Figure 1 – Example test set-up for MPPT efficiency measurements ...................................... 11

Figure B.1 – Test sequence for fluctuations between small and medium irradiation

intensities ............................................................................................................................. 18

Figure B.2 – Test sequence for fluctuations between medium and high irradiation

intensities ............................................................................................................................. 19

Figure B.3 – Test sequence for the start-up and shut-down test of grid connected

inverters ............................................................................................................................... 22

Figure C.1 – Irradiation-dependent V-I- and V-P characteristic of a c-Si PV generator .......... 25

Figure C.2 – Irradiation-dependent V-I- and V-P characteristic of a thin-film PV

generator .............................................................................................................................. 26

Table 1 – Test specifications for static MPPT efficiency ........................................................ 12

Table A.1 – General requirements on the simulated I/V characteristic of the PV

generator .............................................................................................................................. 16

Table B.1 – Dynamic MPPT-Test 10 %  50 % G (valid for the evaluation of
STC

η ) .............................................................................................................................. 20

MPPTdyn
Table B.2 – Dynamic MPPT-Test 30 %  100 % G (valid for the evaluation of
STC

) .............................................................................................................................. 21

MPPTdyn
Table B.3 – Dynamic MPPT- Slow Ramp 1 %  10 % G (valid for the evaluation of
STC

η ) .............................................................................................................................. 21

MPPTdyn

Table C.1 – Technology-dependent parameters .................................................................... 24

Table C.2 – MPP-values obtained with the cSi PV model ...................................................... 25

Table C.3 – MPP-values obtained with the TF-PV mode ....................................................... 27

Table D.1 – Weighting factors and partial MPP power levels for the calculation of the

European efficiency .............................................................................................................. 29

Table D.2 – Weighting factors and partial MPP power levels for the calculation of the

CEC efficiency (California Energy Commission) .................................................................... 29

Table E.1 – Measured quantities at the conversion efficiency test ......................................... 30

Table E.2 – Conversion efficiency in term of rated AC power ................................................ 31

Table E.3 – Allowed limits for the nodes of the normalised AC power ................................... 31

Table E.4 – Sought values by means of interpolation ............................................................ 32

Table E.5 – Interpolated conversion efficiencies ................................................................... 33

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– 4 – IEC 62891:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MAXIMUM POWER POINT TRACKING EFFICIENCY
OF GRID CONNECTED PHOTOVOLTAIC INVERTERS
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To

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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.

International Standard IEC 62891 has been prepared by IEC technical committee 82: Solar

photovoltaic energy systems.
The text of this standard is based on the following documents:
FDIS Report on voting
82/1723/FDIS 82/1736/RVD

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.

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

---------------------- Page: 6 ----------------------
IEC 62891:2020 © IEC 2020 – 5 –

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

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

related to the specific publication. At this date, the publication 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.
---------------------- Page: 7 ----------------------
– 6 – IEC 62891:2020 © IEC 2020
MAXIMUM POWER POINT TRACKING EFFICIENCY
OF GRID CONNECTED PHOTOVOLTAIC INVERTERS
1 Scope

This document provides a procedure for the measurement of the efficiency of the maximum

power point tracking (MPPT) of inverters used in grid-connected photovoltaic (PV) systems.

Both the static and dynamic MPPT efficiency are considered. Based on the static MPPT

efficiency calculated in this document and steady state conversion efficiency determined in

IEC 61683 the overall efficiency can be calculated.
The dynamic MPPT efficiency is indicated separately.

NOTE This document addresses PV inverters connected to an AC grid. However, this procedure may also be

used for other power conversion devices with MPPT functionality used in PV systems, such as charge controllers

or optimizers.
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 61683, Photovoltaic systems – Power conditioners – Procedure for measuring efficiency

IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

EN 50160, Voltage characteristics of electricity supplied by public distribution networks

3 Terms and definitions

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

well as the following:

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
3.1 Inverter input (PV generator)
3.1.1
maximum input voltage
DCmax
allowed maximum voltage at the inverter input
Note 1 to entry: Exceeding of V may destroy the equipment under test.
DCmax
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IEC 62891:2020 © IEC 2020 – 7 –
3.1.2
minimum input voltage
DCmin

minimum input voltage for the inverter to energize the utility grid, independent of mode of

operation
3.1.3
rated input voltage
DC,r

input voltage specified by the manufacturer, to which other data sheet information refers

3.1.4
maximum MPP voltage
MPPmax

maximum voltage at which the inverter can convert its rated power under MPPT conditions

3.1.5
minimum MPP voltage
MPPmin

minimum voltage at which the inverter can convert its rated power under MPPT conditions

Note 1 to entry: The actual minimum MPP voltage may depend on the grid voltage level.

3.1.6
rated input power
DC,r

rated input power of the inverter, which can be converted under continuous operating

conditions
3.1.7
maximum input current
DC,max
maximum input current of the inverter under continuous operating conditions

Note 1 to entry: At inverters with several independent inputs, this value may depend on the chosen input

configuration.
3.2 Inverter output (grid)
3.2.1
rated grid voltage
AC,r
utility grid voltage to which other data sheet information refers
3.2.2
rated power
AC,r
active power the inverter can deliver in continuous operation
3.3 Measured quantities
3.3.1
PV simulator MPP-Power
MPP, PVS
MPP power provided by the PV simulator
3.3.2
input power
measured input power of the device under test
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– 8 – IEC 62891:2020 © IEC 2020
3.3.3
PV simulator MPP voltage
MPP, PVS
MPP voltage provided by the PV simulator
3.3.4
input voltage
measured input voltage of the device under test
3.3.5
PV simulator MPP current
MPP, PVS
MPP current provided by the PV simulator
3.3.6
input current
measured input current of the device under test
3.3.7
output power
measured AC output power of the device under test
3.3.8
output voltage
measured AC voltage
3.3.9
output current
measured AC output current of the device under test
3.4 Calculated quantities
3.4.1
MPPT efficiency, energetic
MPPT

ratio of the energy drawn by the device under test within a defined measuring period T to the

energy provided theoretically by the PV simulator at the maximum power point (MPP):

p ( t )⋅dt
η = (1)
MPPT
p ( t )⋅ dt
MPP
where
P (t) is the instantaneous value of the power drawn by the device under test;

P (t) is the instantaneous value of the MPP power provided theoretically by the PV

MPP
simulator.
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IEC 62891:2020 © IEC 2020 – 9 –
3.4.2
conversion efficiency, energetic
conv

.ratio of the energy delivered by the device under test at the AC terminal within a defined

measuring period T to the energy accepted by the device under test at the DC terminal:

p ()t ⋅dt
η = (2)
conv
p ()t ⋅dt
where

P (t) is the instantaneous value of the delivered power at the AC terminal of the device

under test;

P (t) is the instantaneous value of the accepted power at the DC terminal of the device

under test.
3.4.3
overall efficiency, energetic

ratio of the energy delivered by the device under test at the AC terminals within a defined

measuring period T to the energy provided theoretically by the PV simulator:
pt()⋅dt
η = respectively ηη ⋅η (3)
t t conv MPPT
pt()⋅dt
MPP
3.5
photovoltaic array simulator

current source emulating the static and dynamic behaviour of a PV array, in particular the

current-voltage characteristic (see IEC TS 61836).
Note 1 to entry: The requirements are outlined in Clause A.1.
4 MPPT efficiencies
4.1 General description

The MPPT efficiency describes the accuracy of an inverter to set its operating conditions to

match the maximum power point on the characteristic curve of a PV generator. The overall

MPPT efficiency is divided into static and dynamic efficiency components.

Because inverters with poor MPPT performance operate at a DC input voltage that is different

from MPP voltage, and static power conversion efficiency depends on DC input voltage, the

measurements of static MPPT efficiency and static power conversion efficiency according to

4.3 shall be performed simultaneously.
a) Static MPPT efficiency
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– 10 – IEC 62891:2020 © IEC 2020
The static MPPT efficiency is determined by means of measurement as follows:
η VI⋅⋅ ΔT (4)
MPPTstat ∑ DC,iiDC,
PT⋅
MPP,PVS M
where
V is the sampled value of the inverter’s input voltage;
DC,i
I is the sampled value of the inverter’s input current;
DC,i
is the overall measuring period;
ΔT is the period between two subsequent sample values.

The static MPPT efficiency describes the accuracy of an inverter to regulate on the maximum

power point on a given static characteristic curve of a PV generator.
V and I shall be sampled at the same time.
DC,i DC,i
b) Dynamic MPPT efficiency

Variations of the irradiation intensity and the resulting transition of the inverter to the new

operation point are not considered with the static MPPT efficiency. For the evaluation of this

transient characteristic the dynamic MPPT efficiency is specified. The dynamic MPPT

efficiency is defined as:
η VI⋅⋅ ΔT (5)
MPPTdyn ∑ DC,i DC,i i
PT⋅ Δ
∑ MPP,PVS,j j
where
ΔT is the period in which the power P is provided;
j MPP,PVS,j
ΔT is the period in which the power V and I are sampled.
i DC,i DC,i
4.2 Test set-up

The generic test set-up for single phase grid connected inverters is depicted in Figure 1. The

diagram can also be considered as a single-phase representation of a test-circuit for multi

phase inverters.
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IEC 62891:2020 © IEC 2020 – 11 –
Key
EUT Equipment under test (inverter);
I DC current meter;
V DC voltage meter;
P DC power meter;
V AC voltage meter;
P AC power meter.
Figure 1 – Example test set-up for MPPT efficiency measurements

The DC source connected to the PV input of the inverter shall be a PV simulator in

accordance to the specifications in Clause A.1.

The AC supply of the inverter shall be in accordance to the specifications in Clause A.2.

For the conversion efficiency, the DC and AC voltages shall be measured as close as

possible to the inverter terminals. For MPPT efficiency, the DC voltage shall be measured as

close as possible to the PV simulator. For combined conversion and MPPT efficiency

measurements, two voltage measurements will be required at the output of the PVS and the

DC input of the EUT, in order to avoid measurement errors resulting from the voltage drop

between the PVS and the EUT.
4.3 Static MPPT efficiency
4.3.1 Test conditions
The measurement of the conversion and static MPPT efficiency shall be performed
simultaneously with test specifications as defined in Table 1.

For test devices with several independent MPPT input terminals, the measurements shall be

performed for all input configurations as intended by the manufacturer. Unless otherwise

provided by the manufacturer, the total power shall be split equally on the individual input

terminals.
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– 12 – IEC 62891:2020 © IEC 2020
Table 1 – Test specifications for static MPPT efficiency

MPP voltage of the Simulated I/V MPP power of the simulated I/V characteristic normalised

d f
simulated I/V characteristic to rated DC power , P /P
MPP,PVS DC,r
characteristic (see Annex C)
0,05 0,10 0,20 0,25 0,30 0,50 0,75 1,00
of the PV generator
V or c-Si
MPPmax
a,c
(0,8 · V )
DCmax
V c-Si
DC,r
V c-Si
MPPmin
V or TF
MPPmax
a,c
(0,7 · V )
DCmax
V TF
DC,r
V TF
MPPmin

The MPP voltages at the different test conditions (V , V , V ) shall be kept constant during the test

MPPmax DC,r MPPmin
for each power level.

The lower of the two values shall be used. The specified MPP voltages ensure that the correct MPPT

operation is not affected by reaching voltage limits.

For devices under test that are not intended for the operation with thin-film technologies, these measuring

points can be omitted.
For other cell technologies the value V = n·V shall be set accordingly.
MPPmax DCmax

In order to specify the static MPPT efficiency in terms of normalised rated AC power, the procedure in

Annex E shall be used.

If this value is not specified by the manufacturer, V = (V + V )/2 shall be used.

DC,r MPPmax MPPmin

If this value is not specified by the manufacturer, it can be defined as P = P / η , in which η is

DC,r AC,r conv,r conv,r

the conversion efficiency at rated DC voltage. If the rated conversion efficiency is not specified, it shall be

measured.

The measurement shall be performed at nominal grid voltage V in order to avoid any

AC,r

impact of the grid voltage level on the measurement results. Deviations shall be documented

in the measurement report.

The measurement should be performed at an ambient temperature of 25 °C ± 5 °C. Other

ambient temperatures can be mutually agreed. The actual ambient temperature shall be

specified in the test report.
4.3.2 Measurement procedure

For each of the above specified test conditions a corresponding I/V characteristic has to be

defined which shall be emulated by means of the PV simulator.

NOTE The requirements on the accuracy of the defined characteristic are outlined in Annex C.

After commissioning the device under test the stabilization of the MPP tracking shall be

awaited firstly.

Given the multitude of various MPPT methods and their parameters, a specific waiting period

is not defined in this standard. The stabilization time depends on the characteristics of the

device under test and shall be set accordingly in each case. The stabilization time shall be

documented in the test report. If a stabilisation of the MPPT can’t be observed due to the

behaviour of the device under test, a latency of at least 5 min is defined.

The measuring time for each test condition as specified in Table 1 amounts to 10 min. For the

first power level of each MPP voltage setting, the stabilisation of the MPPT-tracker has to be

awaited. If a stabilisation cannot be observed a stabilisation time of at least 5 min is defined.

---------------------- Page: 14 ----------------------
...

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