IEC TS 62257-9-7:2019

IEC TS 62257-9-7 ®
Edition 1.0 2019-10
TECHNICAL
SPECIFICATION
Renewable energy and hybrid systems for rural electrification –
Part 9-7: Recommendations for selection of inverters
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IEC TS 62257-9-7 ®
Edition 1.0 2019-10
TECHNICAL
SPECIFICATION
Renewable energy and hybrid systems for rural electrification –

Part 9-7: Recommendations for selection of inverters

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-7397-5

– 2 – IEC TS 62257-9-7:2019 © IEC 2019
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Overview . 7
5 System architecture and inverter selection . 8
6 General considerations . 8
6.1 Overview. 8
6.2 IP rating . 8
6.3 Ambient temperature. 8
6.4 Altitude . 9
6.5 Direct sunlight . 9
6.6 Efficiency . 9
6.7 No-load and stand-by power requirements . 9
6.8 Ventilation requirements . 9
6.9 Earthing arrangements . 9
6.10 Waveform quality . 9
6.11 Compatibility with loads that produce significant harmonics . 9
6.12 Electromagnetic interference. 10
6.13 Load growth . 10
6.14 Protection . 10
7 Variable speed drives and T1I-c systems – REN systems operating with no
storage, DC source to AC application . 10
7.1 General . 10
7.2 Characteristics of the inverter . 11
7.2.1 Frequency control . 11
7.2.2 MPPT . 11
7.3 Sizing of the inverter . 11
7.3.1 General . 11
7.3.2 AC voltage . 12
7.3.3 AC current . 12
7.3.4 DC voltage . 12
7.3.5 DC current . 12
7.4 VFD standards . 12
7.5 Variations on the architecture . 12
8 Uni-directional grid creating inverters that work with batteries – Type T2I systems
– REN production with energy storage . 12
8.1 General . 12
8.2 Characteristics of the inverter . 13
8.2.1 Grid creation . 13
8.3 Sizing of the inverter . 14
8.3.1 Input DC voltage . 14
8.3.2 Input DC current . 14
8.3.3 AC current and power . 14
8.3.4 AC voltage . 14
8.3.5 Power factor handling range . 14
8.4 Variations on the architecture . 14

9 Uni-directional inverters that synchronize to the grid – Type T3I systems – REN
and diesel production without energy storage . 15
9.1 General . 15
9.2 Characteristics of the inverter . 16
9.2.1 General . 16
9.2.2 Grid synchronizing . 16
9.2.3 MPPT tracking . 16
9.2.4 Active power control . 16
9.2.5 Grid support . 16
9.3 Sizing of the inverter . 16
9.3.1 Input DC voltage . 16
9.3.2 Input DC current . 17
9.3.3 Output AC current and power . 17
9.4 Variations on the architecture . 17
10 Bi-directional grid creating inverters that work with batteries – Type T4I systems,
RE and diesel with energy storage . 17
10.1 General . 17
10.2 Characteristics of the inverter . 18
10.2.1 Grid creation . 18
10.2.2 Battery management . 18
10.2.3 Frequency modulation . 18
10.2.4 Master-slave . 19
10.2.5 Reactive power control . 19
10.3 Sizing the inverter . 19
10.3.1 General . 19
10.3.2 Input DC voltage . 19
10.3.3 Input DC current . 19
10.3.4 Output AC voltage . 19
10.3.5 Output AC current . 19
10.4 Variations on the architecture . 20

Figure 1 – Type T1I-c system – DC to AC with no batteries . 11
Figure 2 – Type T2I system – DC to AC with batteries . 13
Figure 3 – Type T3I system – DC to AC with diesel but no batteries . 15
Figure 4 – Type T4I system – DC to AC with diesel and batteries . 18

Table 1 – Types of inverter and system architecture it is applicable to . 8

– 4 – IEC TS 62257-9-7:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RENEWABLE ENERGY AND HYBRID SYSTEMS
FOR RURAL ELECTRIFICATION –
Part 9-7: Recommendations for selection of inverters

FOREWORD
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Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62257-9-7, which is a Technical Specification, has been prepared by IEC technical
committee 82: Solar photovoltaic energy systems.

The text of this Technical Specification is based on the following documents:
Enquiry draft Report on voting
82/1473/DTS 82/1546A/RVDTS
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
This part of IEC 62257 is to be used in conjunction with IEC 62257 (all parts).
A list of all parts in the IEC 62257 series, published under the general title Renewable energy
and hybrid systems for rural electrification, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
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.
A bilingual version of this publication may be issued at a later date.

– 6 – IEC TS 62257-9-7:2019 © IEC 2019
RENEWABLE ENERGY AND HYBRID SYSTEMS FOR RURAL
ELECTRIFICATION –
Part 9–7: Recommendations for selection of inverters

1 Scope
This part of IEC 62257, which is a technical specification, specifies the criteria for selecting
and sizing inverters suitable for different off-grid applications integrating solar as an energy
source.
As well as off-grid system, this document can also apply to inverters where a utility grid
connection is available as a backup for charging batteries, but it is not intended to cover
applications in which inverters synchronize and inject energy back into a utility grid, even
though this capability may incidentally be a part of the functionality of the inverters.
Single and multi-phase applications are included.
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 60529, Degrees of protection provided by enclosures (IP Code)
IEC 61683, Photovoltaic systems – Power conditioners – Procedure for measuring efficiency
IEC 61800, (all parts), Adjustable speed electrical power drive systems
IEC 61800-3, Adjustable speed electrical power drive systems – Part 3: EMC requirements
and specific test methods
IEC 61800-5-1, Adjustable speed electrical power drive systems – Part 5-1: Safety
requirements – Electrical, thermal and energy
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC 62109, (all parts), Safety of power converters for use in photovoltaic power systems
IEC 62109-1, Safety of power converters for use in photovoltaic power systems – Part 1:
General requirements
IEC TS 62257-2, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 2: From requirements to a range of electrification systems
IEC TS 62257-7-1:2010, Recommendations for small renewable energy and hybrid systems
for rural electrification – Part 7-1: Generators – Photovoltaic generators

IEC TS 62257-7-4: Recommendations for renewable energy and hybrid systems for rural
electrification – Part 7-4: Generators – Integration of solar with other forms of power
generation within hybrid power systems
IEC 62548, Photovoltaic (PV) arrays – Design requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836 and the
following 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
3.1
renewable energy
REN
energy from a source that is not depleted when used
3.2
hybrid power plants
multi-sources system with at least two kinds of energy generation technology
3.3
storage
storage of electrical energy produced by one of the generators of the system in a battery
3.4
DC bus
node of the electrical system to which the DC input of the battery inverter is connected to
3.5
AC bus
node of the electrical system to which the AC output of the battery inverter is connected to
3.6
black start
process of starting an electrical power supply without relying on any other external generating
source
4 Overview
Inverters are used to convert DC power into AC power. For off-grid renewable applications,
there are different types of inverters that may be used depending upon the architecture of the
system.
Some systems work with batteries and use the firm energy they provide to form the conditions
of the micro-grid (grid-forming inverters) and some inverters synchronize to an existing grid.
In addition, some inverters are bi-directional inverter/chargers, which not only convert DC
battery current into AC, but also reverse the process and transform the AC current into a DC
battery charging current to permit battery charging from the AC side of the inverter.

– 8 – IEC TS 62257-9-7:2019 © IEC 2019
Off-grid inverters need to be correctly selected for the features which they possess, and sized
accordingly depending upon the loads they shall feed or the energy sources they are fed by.
5 System architecture and inverter selection
Different sorts of system architectures of isolated electrification systems are given in
IEC TS 62257-2. Not all of them require inverters, but several do. Depending on the situation
encountered, the inverters will need to possess certain features.
Table 1 shows the system architectures from IEC TS 62257-2 that need inverters, and gives
the general correspondence of which inverter is suitable for which application, with some
exceptions noted.
Table 1 – Types of inverter and system architecture it is applicable to
Type of inverter Principal system Description Example applications Variations on
architecture architecture
Variable Speed Drives T1I-c REN systems Solar water pumping Some drives may also
operating with no use diesel or grid
storage and no diesel
generators or grid
backup
Grid forming inverters T2I REN systems with Solar home systems Some systems may
that work with storage and no diesel allow generators or
batteries generators or grid grid to connect via an
backup additional battery
charger
REN and battery are
coupled on the DC bus
Grid-Tie Inverters that T3I REN systems Reduction of diesel Some systems may
synchronize to another operating with diesel consumption in off- use some battery
...