IEC TS 62257-7-4:2019

IEC TS 62257-7-4 ®
Edition 1.0 2019-05
TECHNICAL
SPECIFICATION
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
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IEC TS 62257-7-4 ®
Edition 1.0 2019-05
TECHNICAL
SPECIFICATION
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
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-6823-0

– 2 – IEC TS 62257-7-4:2019 © IEC 2019
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 Overview . 6
5 Types of hybrid systems. 7
5.1 General . 7
5.2 Multi-master rotating machine dominated mini-grid . 7
5.2.1 General . 7
5.2.2 Design. 8
5.2.3 Design and selection of diesel generators . 8
5.2.4 Design and selection of renewable energy component. 9
5.2.5 Design and selection of batteries . 9
5.2.6 Control system . 9
5.3 Single switched master mini-grid . 10
5.3.1 General . 10
5.3.2 Design. 11
5.3.3 Design and selection of diesel generators . 12
5.3.4 Design and selection of renewable energy component. 12
5.3.5 5.2.4 Design and selection of batteries . 12
5.3.6 System control . 13
5.4 Multi-master inverter dominated mini-grid . 14
5.4.1 General . 14
5.4.2 Design. 15
5.4.3 Design and selection of diesel generators . 15
5.4.4 Design and selection of renewable energy component. 15
5.4.5 Design and selection of batteries . 15
5.4.6 Control . 15

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 . 14

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RECOMMENDATIONS FOR RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION –

Part 7-4: Generators – Integration of solar w ith other forms
of pow er generation w ithin hybrid power systems

FOREWORD
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Technical Specification are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62257-7-4, which is a Technical Specification, has been prepar ed by IE C t ec hni c al
committee 82: Solar photovoltaic energy systems.

– 4 – IEC TS 62257-7-4:2019 © IEC 2019
The text of this Technical Specification is based on the following documents:
Draf t TS Report on voting
82/1477/DTS 82/1545A/RVDTS
Full information on the voting for the approval of this Technical Specification can be found i n
the report on voting indicated in the above table.
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 Recommendations
for 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.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
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 t o
the specific document. At this date, the document will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or amended.

A bilingual version of this publication may be issued at a later date.

RECOMMENDATIONS FOR RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION –

Part 7-4: Generators – Integration of solar w ith other forms
of pow er generation w ithin hybrid power systems

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 l o a d
in conjunction with other sources of energy. Such sy stems may or may not include an energy
storage system. There are a variety of different system architectures and appl i c at ions , and
many ways in which these energy sources can be combined. Th i s doc um ent di s t inguis hes
between different sorts of hybrid system applications and gives guidance on t he des i gn and
integration of these systems.
It applies to single-phase and three-phase applications, and it covers situations where gri d i s
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 may incidentally possess this function.
2 Normative references
The following documents are referred to in the text in such a way the 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 symb ols
IEC TS 62257-7-2, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 7-2: Generator set – Off-grid wind turb ines
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 e l e c t r ifi ca ti o n
systems
IEC TS 62257-9-7, Recommendations for renewable energy and hybrid systems for rural
electrification – Part 9-7: Selection of inverters
IEC 62509, Battery charge controllers for photovoltaic systems – Performance and functioning
IEC 62548, Photovoltaic (PV) arrays – Design requirements
IEC TS 62738, Ground-mounted photovoltaic power plants – Design guidelines and
recommendations
____________
To be published.
– 6 – IEC TS 62257-7-4:2019 © IEC 2019
3 T e rms 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 syste m
multi-sources system with at least two kinds of energy generation technology
3.3
storage
storage of 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
3.7
renewable fraction
fraction of energy delivered to a load that originates from renewable power sources
4 Overview
Off-grid solar hybrid systems, where a solar energy source is combined with other sour c e s o f
energy such as diesel or wind, have two primary differences in functi o n a l ity c om pared wi t h
solar-only off-grid sys tems. Firstly, the intermittent nature of solar can be compensated fo r b y
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 diesel 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 consumpt i o n o f a s y s t e m whi c h
would normally be relying solely on fuel by injecting a certain a m o u n t o f renewabl e energy
into a system which is normally sustained only by diesel generators.

5 T ype s of hybrid systems
5.1 General
This document makes three principal distinctions according to t he pri m ary regul at or of t he
energy output:
a) Multi-master rotating machine dominated mini-grid systems where a diesel generator
creates the grid, and to which other energy sources need to 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 i n ve r t e r
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 the r e m a y b e s u b t l e
variations upon the architectures, for example making use of other technologi e s s u ch a s D C
coupled gensets.
These different architectures represent different renewable fractions and spat ial distrib u t io n s
of the power generators. A rotating machine dominated mini-grid is more suitable for
applications with a low renewable fraction, where renewable energy is used to offset fuel
consumption in order to increase the renewable fraction and maintain constant service,
5.2 Multi-maste r rotating machine dominated mini-grid
5.2.1 General
A multi-master machine dominated mini-grid is a system whereby the c h a r a c t e r ist ics o f t h e
grid such as voltage and frequency are created by a fuel powered rotating machine such a s a
diesel generator, and to which other renewable sources synchronize.
A typical application of this is for sites with a high diese l generat i on s et dem and, s uc h as
mining. The renewable energy resource is used to offset the fuel consum p t i o n o f t h e d i e s e l
generators somewhat, but never to completely replace them. See Figure 1.

– 8 – IEC TS 62257-7-4:2019 © IEC 2019

Figure 1 – Typica l multi-maste r rota ting ma chine dominated mini-grid a rchitecture
5.2.2 Design
A typical arrangement is to have a diesel generator or a bank of diesel generators conn e ct e d
to the mini-grid distribution network, suitable sized according to the current or future predicted
on-site loads. Connected in parallel to the generation sets are grid s y n c h r o n i zin g i n ve r t e r s
connected to a renewable energy resource, typically solar for this applicatio n . E ve r y t h i n g i s
integrated together via a control mechanism, the purpose of which is ensure th e c o r r e c t a n d
optimal performance of the whole system, which it does by controlling the maximum outpu t o f
the inverters, and by switching diesel generators in and out according to the load being
demanded.
Of particular importance is the protection of the generator sets by ensurin g t hat under m os t
load conditions, the generator sets are not being over-loaded or under-loaded: under-l oadi ng
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 40 % of its rated power but manufacturers' instructio n s m a y a l l o w
for a wider variation in power.
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 ensurin g
there is enough unused capacity in the generator sets (spinning reserve) that can b e q u i c k l y
activated in order to compensate for the drop in renewable contribution, or to integrate a small
of amount of battery storage into the system.
All generating equipment shall be capable of operating at the same nomina l A C vo l t a g e a n d
frequency.
5.2.3 De sign a nd se le ction of diesel generators
Detailed characteristics and design requirements for diesel generators for rural electrification ,
including this application, are described in IEC TS 62257-7-3.
In general, the system shall be designed in such a way that for the typical expected loads t he
generator sets will avoid wet-stacking, while providing some spare capacity for spinning
reserve, and so typically they need to be configured so they will operate at between 40 % t o

80 % of their capacity, or values that comply with manufacturer’s recommendations. W hen the
loads on site are particularly variable, then this can be ensured by having several smaller
generator sets instead of one large generator set.
It should be noted that even if there are minimum recommended loadi ngs for t he generat or
sets, it may be acceptable to have them running under low loadings under some
circumstances if it is the only way to avoid a black-out.
5.2.4 De sign a nd se le ction of renewable energy component
The design of the photovoltaic array and the installation requirements are the same as
described in IEC 62548. For large scale ground mounted systems, the design requirements o f
IEC TS 62738 may also apply.
The design and installation requirements of the wind component are to be des c ri bed i n IE C
TS 62257-7-2.
Because the generator sets dictate the grid conditions, it is necessary to make use of
inverters that can synchronize to the grid. The characteristics o f s u i t a b l e i n ve r t e r s for t hi s
application can be found in IEC TS 62257-9-7.
The percentage contribution of renewable generation at any one time should be done s o t hat
the generator sets will continue to work within the recommended minimum and maximum
loading conditions.
When there are multiple gensets, it is possible to have a greater percentage of solar
penetration into the system. The higher the solar penetration, the more important it is t o h a ve
effective control over their energy output. It is not recommended to have a penetration
percentage of greater than 60 % renewables into this type of multi-master rot ati ng m ac hine
dominated system.
The harmonics tolerance of the diesel generators shall be checked against the h a r m o n i cs o f
the solar and battery inverters.
5.2.5 De sign a nd se le ction of batt
...