IEC TS 62257-301:2025

IEC TS 62257-301 ®
Edition 1.0 2025-08
TECHNICAL
SPECIFICATION
Renewable energy off-grid systems -
Part 301: Generators - Integration of solar with other forms of power generation
within hybrid power systems
ICS 27.160  ISBN 978-2-8327-0622-0

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Overview . 7
5 Types of hybrid systems . 7
5.1 General . 7
5.2 Multi-master rotating machine dominated mini-grid . 8
5.2.1 General. 8
5.2.2 Design . 8
5.2.3 Design and selection of fuel powered generators. 9
5.2.4 Design and selection of renewable energy component . 9
5.2.5 Design and selection of batteries . 10
5.2.6 Control system . 10
5.3 Single switched master mini-grid . 11
5.3.1 General. 11
5.3.2 Design . 12
5.3.3 Design and selection of fuel powered generators. 12
5.3.4 Design and selection of renewable energy component . 12
5.3.5 Design and selection of batteries . 13
5.3.6 System control . 13
5.4 Multi-master inverter dominated mini-grid . 15
5.4.1 General. 15
5.4.2 Design . 15
5.4.3 Design and selection of fuel powered generators. 15
5.4.4 Design and selection of renewable energy component . 15
5.4.5 Design and selection of batteries . 16
5.4.6 Control. 16
Bibliography . 18

Figure 1 – Typical multi-master rotating machine dominated mini-grid architecture . 8
Figure 2 – Single switched master mini-grid architecture . 11
Figure 3 – Multi-master inverter dominated mini-grid architecture . 15

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Renewable energy off-grid systems -
Part 301: Generators - Integration of solar with other forms
of power generation within hybrid power systems

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 this end and
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC TS 62257-301 has been prepared by IEC technical committee 82: Solar photovoltaic energy
systems. It is a Technical Specification.
This first edition cancels and replaces IEC TS 62257-7-4, published in 2019.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/2408/DTS 82/2463/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
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
off-grid systems, 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 webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
INTRODUCTION
The IEC 62257 series provides technical standardization to different stakeholders (including
but not limited to project developers, financing agencies, testing agencies, installers, etc.)
involved in electrification projects for access to electricity for those not solely connected to the
regional grid, through the setting up of off-grid renewable energy and hybrid systems (including
micro-grids) with a voltage less than or equal to 1 000 V for AC (alternating current) or a voltage
less than or equal to 1 500 V for DC (direct current).
Access to electricity is one of the predominant policy actions designed to increase the well-
being of populations, together with access to clean water, improved healthcare, education,
personal advancement, and economic development. Increasing access to electricity through
utilization of renewable off-grid electricity also directly or indirectly supports various United
Nations Sustainable Development Goals (https://sdgs.un.org/goals), depending on the
application.
The IEC 62257 technical specifications focus on enabling access to electricity by concentrating
on, but not being specific to, developing countries. This series should not be considered as all-
inclusive for access to electricity. That means that the technical specifications could be used
for rural electrification, also for electrification of remote sites in developed countries, or any
requirement for electricity access that cannot be met by attaching solely to the national utility
grid.
One of the main objectives of this series is to provide the minimum sufficient recommendations,
including items for safety, sustainability of systems, and at the lowest life cycle cost, relevant
to the renewable energy and hybrid off-grid systems field of application.
Where connection to a national or regional electricity grid is not possible or reliable enough to
provide the desired level of electrification, renewable energy off grid and hybrid systems are
typically used to provide it. These autonomous renewable energy and hybrid systems can
provide electricity without connection to the larger utility grid.
Using renewable energy off-grid and hybrid systems to create “access to electricity” applies to
cases where the regional grid is too far away and/or too costly for the user(s) to connect to the
regional grid, and where autonomous power systems can be used to supply these services.

1 Scope
This part of IEC 62257, which is a technical specification, specifies the design and
implementation of hybrid off-grid solar systems, where solar energy provides energy to a load
in conjunction with other sources of energy. Such systems can either include or not include an
energy storage system. There are a variety of different system architectures and applications,
and many ways in which these energy sources can be combined. This document distinguishes
between different sorts of hybrid system applications and gives guidance on the design and
integration of these systems.
It applies to single-phase and three-phase applications, and it covers situations where grid is
available as an additional source of power for charging batteries and maintaining system
reliability, but this document does not cover situations in which energy is fed back into a utility
grid, although such systems can incidentally possess this function.
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 TS 61836, Solar photovoltaic energy systems - Terms, definitions and symbols
IEC TS 62257-7-2, Recommendations for renewable energy and hybrid systems for rural
electrification - Part 7-2: Generator set - Off-grid wind turbines
IEC TS 62257-7-3, Recommendations for renewable energy and hybrid systems for rural
electrification - Part 7-3: Generator set - Selection of generator sets for rural electrification
systems
IEC TS 62257-350 , Renewable energy off-grid systems - Part 350: Recommendations for
selection of inverters
IEC 62509, Battery charge controllers for photovoltaic systems - Performance and functioning
IEC 62548-1, Photovoltaic (PV) arrays - Part 1: Design requirements
IEC TS 62738, Ground-mounted photovoltaic power plants - Design guidelines and
recommendations
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 terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
___________
Under preparation. Stage at the time of publication: IEC DTS 62257-350:2025.
3.1
renewable energy
REN
primary energy the source of which is constantly replenished and will not become depleted
Note 1 to entry: Examples of renewable energy are: wind, solar, geothermal, hydropower.
Note 2 to entry: Fossil fuels are non renewable.
[SOURCE: IEC 60050-617:2009, 617-04-11]
3.2
hybrid system
multi-sources system with at least two kinds of energy generation technology
3.3
electrical energy storage system
EES system
EESS
installation with defined electrical boundaries, which can include civil engineering works, energy
conversion equipment and related ancillary equipment, comprising at least one electrical energy
storage, which extracts electrical energy from an electric power system, stores this energy
internally in some manner and injects electrical energy into an electric power system
[SOURCE: IEC 60050-631:2023, 631-01-02, modified – “grid-connected installation” has been
replaced by “installation” and the notes have been omitted]
3.4
DC bus
node of the electrical system to which the DC ports of the PCE are connected
3.5
AC bus
node of the electrical system to which the AC ports of the PCE and gensets are connected
3.6
black start
process of starting an electrical power system without relying on any other external generating
source
3.7
renewable fraction
fraction of energy delivered to a load that originates from renewable power sources
3.8
battery inverter
inverter used to control the charge in a dedicated battery
3.9
microgrid
group of interconnected loads and distributed energy resources
with defined electrical boundaries forming a local electric power system at voltage levels of
distribution of electricity, that acts as a single controllable entity and is able to operate in island
mode
Note 1 to entry: This definition covers both (utility) distribution microgrids and (customer owned) facility microgrids.
Note 2 to entry: The terms “mini grid” and “micro grid” are synonymous.
[SOURCE: IEC 60050-617:2009, 617-04-22, modified – the admitted term “embedded
microgrid” has been omitted, Note 2 to entry has been added]
3.10
diversion load
resistive load whose sole purpose is to ensure that the genset is working at its minimum
operating capacity
3.11
PCE
power conversion equipment
device(s) that perform electronic conversion from alternating current to direct current or vice
versa, or direct current to direct current at a different voltage
3.12
hybrid inverter
inverter which can work with different power sources
Note 1 to entry: Examples of power sources are PV, battery, gensets, etc.
4 Overview
Off-grid solar hybrid systems, where a solar energy source is combined with other sources of
energy such as diesel or wind, have two primary differences in functionality compared with
solar-only off-grid systems. Firstly, the intermittent nature of solar can be compensated for by
making use of an additional energy source. In systems with batteries, this can help to maintain
the batteries charged and can change the way the battery bank is sized with respect to the solar
resource: for example, a fuel powered generator can be relied upon to charge the batteries if
they would normally have discharged after an overcast period. Secondly, the renewable aspect
of the hybrid system can reduce the fuel consumption of a system which would normally be
relying solely on fuel by injecting a certain amount of renewable energy into a system which is
normally sustained only by fuel powered generators.
5 Types of hybrid systems
5.1 General
This document makes three principal distinctions according to the primary regulator of the
energy output:
a) Multi-master rotating machine dominated grids (similarly in b) and c)) where a diesel
generator creates the grid, and to which other energy sources synchronize.
b) Single switched master mini-grid systems with battery banks where either the battery
inverter creates the grid and the other energy sources synchronize to the battery inverter
output, or the diesel generator creates the grid.
c) Multi-master inverter mini-grid where certain inverters participate in the creation of the grid
alongside the diesel generator.
This list is not the comprehensive range of architectures available, and there can be subtle
variations upon the architectures, for example making use of other technologies such as DC
coupled gensets.
These different architectures are used for different renewable fractions and for systems with
power generators spread over a wide area. Each separate system is defined based on the
application requirements.
5.2 Multi-master rotating machine dominated mini-grid
5.2.1 General
A multi-master machine dominated grid is a system where the characteristics of the grid such
as voltage and frequency are created by a rotating generator, for example a diesel generator,
and to which other renewable sources synchronize.
A typical application of this is for sites with a high availability requirement, such as mining. The
renewable energy resource is used to offset the fuel consumption of the fuel powered
generators somewhat, but never to completely replace them. See Figure 1.

Figure 1 – Typical multi-master rotating machine dominated mini-grid architecture
5.2.2 Design
A typical arrangement is to have one rotating generator or a bank of them connected to an
appropriately sized grid, taking present and future loads into account. Connected in parallel to
the generation sets are grid following inverters connected to a renewable energy resource,
typically solar. Everything is integrated via a control mechanism, the purpose of which is to
ensure the correct and optimal performance of the whole system, which it does by utilizing the
output of the inverters, and by switching fuel powered generators on and off according to the
load being supplied.
Of particular importance is the protection of the generator sets by ensuring that under most load
conditions, the generator sets are not being over-loaded or under-loaded: under-loading of a
generation set can cause incomplete combustion of fuel, creating a ‘wet-stacking’ condition
which can damage the engine. For efficient combustion the generator should typically be run at
least at 30 % of its rated power but manufacturers' instructions can allow for a wider variation
in power. Backfeed protection shall be provided on the genset.
The other significant issue is how to maintain stability of the mini-grid in the face of rapid
changes of the renewable contribution due to clouds, etc. This can be maintained by ensuring
there is enough unused capacity in the generator sets (spinning reserve) that can be quickly
activated to compensate for the drop in renewable contribution, or to integrate a small amount
of battery storage into the system for PV smoothing. The spinning reserve should be sufficient
to support the load if the largest power source goes off-line for the time required to start reserve
generators. Load forecast and production forecast should be considered to minimize the
spinning reserve and therefore the fuel consumption.
All generating equipment shall be capable of operating at the same nominal AC voltage and
frequency.
5.2.3 Design and selection of fuel powered generators
Detailed characteristics and design requirements for fuel powered generators for rural
electrification, including this application, are described in IEC TS 62257-7-3.
The system shall be designed in such a way that for the typical expected loads the generator
sets will avoid under-loading, while providing some spare capacity for spinning reserve, and s
...