IEC TS 62257-9-2:2006

IEC/TS 62257-9-2 ®
Edition 1.0 2006-10
TECHNICAL
SPECIFICATION
colour
inside
Recommendations for small renewable energy and hybrid systems for rural
electrification –
Part 9-2: Microgrids
IEC/TS 62257-9-2:2006(E)
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IEC/TS 62257-9-2 ®
Edition 1.0 2006-10
TECHNICAL
SPECIFICATION
colour
inside
Recommendations for small renewable energy and hybrid systems for rural
electrification –
Part 9-2: Microgrids
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XA
ICS 27.160; 27.180 ISBN 2-8318-8863-8

– 2 – TS 62257-9-2 © IEC:2006(E)
CONTENTS
FOREWORD.4
INTRODUCTION.6

1 Scope.7
2 Normative reference .7
3 Terms and definitions .7
4 General .9
4.1 Limits of a microgrid .9
4.2 Voltage drops.9
4.3 Composition of a microgrid.9
5 Protection against electric shocks.11
6 Protection against overcurrents .11
7 Selection and erection of equipment.11
7.1 Equipment installation .11
7.2 Operational conditions and external influences.11
7.3 Characteristics of lines .13
7.4 Cables.14
7.5 Poles.14
7.6 Cable anchorage .17
7.7 Connections and accessories .17
7.8 Where poles are used for other purposes .19
7.9 Isolation and switching .19
7.10 Earthing arrangement, protective conductors and protective bonding
conductors .20
8 Verification and acceptance.22
8.1 General .22
8.2 Supervision of works .22
8.3 Verification before commissioning (on site acceptance).22
8.4 Operation tests.23

Annex A (informative) Characteristics of cables .24
Annex B (informative) Maximum circuit length.26

Figure 1 – Microgrid limits.9
Figure 2 – Microgrid consisting of a single-phase feeder.10
Figure 3 – Three phase system output, single phase distribution or three phase
service provided where needed.10
Figure 4 – Three phase system output, single phase distribution .11
Figure 5 – Diagram showing installation of twinned wooden poles forming an angle .16
Figure 6 – Examples of different pole arrangements .16
Figure 7 – Example of an overhead line .16
Figure 8 – Connection mode diagram.19
Figure 9 – Microgrid earthing scheme .21

TS 62257-9-2 © IEC:2006(E) – 3 –

Table 1 – Maximum values of voltage drops.9
Table 2 – Fuse ratings for protection from short-circuiting in 230 V a.c. microgrids
(overhead lines).20
Table 3 – Fuse ratings for protection from short-circuiting in 120 V a.c. microgrids
(overhead lines).20
Table 4 – Circuit breaker ratings for protection from short-circuiting in microgrids
(overhead lines).20
Table 5 – Characteristics of earthing components.22
Table A.1 – Example of characteristics of grid conductors for overhead lines (insulated
twisted conductors without carrier neutral) .24

– 4 – TS 62257-9-2 © IEC:2006(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RECOMMENDATIONS FOR SMALL RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION

Part 9-2: Microgrids
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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agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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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. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• The subject is still under technical development or where, for any other reason, there is
the future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 62257-9-2, which is a technical specification, has been prepared by IEC technical
committee 82: Solar photovoltaic energy systems.
This part of IEC 62257-9 is based on IEC/PAS 62111 (1999); it cancels and replaces the
relevant parts of IEC/PAS 62111.

TS 62257-9-2 © IEC:2006(E) – 5 –
This part of IEC 62257-9 is to be used in conjunction with the IEC 62257 series.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
82/412/DTS 82/443/RVC
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 publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 62257 series, under the general title Recommendations for small
renewable energy and hybrid systems for rural electrification, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication 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.

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 publication using a colour printer.

– 6 – TS 62257-9-2 © IEC:2006(E)
INTRODUCTION
The IEC 62257 series intends to provide to different players involved in rural electrification
projects (such as project implementers, project contractors, project supervisors, installers,
etc.) documents for the setting up of renewable energy and hybrid systems with a.c. nominal
voltage below 500 V, d.c. nominal voltage below 750 V and nominal power below 100 kVA.
These documents are recommendations:
• to choose the right system for the right place,
• to design the system,
• to operate and maintain the system.
These documents are focused only on rural electrification concentrating on but not specific to
developing countries. They should not be considered as all inclusive to rural electrification.
The documents try to promote the use of renewable energies in rural electrification; they do
not deal with clean mechanisms developments at this time (CO emission, carbon credit, etc.).
Further developments in this field could be introduced in future steps.
This consistent set of documents is best considered as a whole with different parts
corresponding to items for safety, sustainability of systems and at the lowest life cycle cost as
possible. One of the main objectives is to provide the minimum sufficient requirements,
relevant to the field of application that is: small renewable energy and hybrid off-grid systems.
Decentralized Rural Electrification Systems (DRES) are designed to supply electric power for
sites which are not connected to a large interconnected system, or a national grid, in order to
meet basic needs.
The majority of these sites are:
• isolated dwellings,
• village houses,
• community services (public lighting, pumping, health centres, places of worship or cultural
activities, administrative buildings, etc.),
• economic activities (workshops, microindustry, etc.).
The DRE systems fall into three categories:
• process electrification systems (for example for pumping),
• individual electrification systems (IES) for single users,
• collective electrification systems (CES) for multiple users.
Process or individual electrification systems exclusively consist of two subsystems:
• an electric energy generation subsystem,
• the user's electrical installation.
Collective electrification systems, however, consist of three subsystems:
• an electric energy generation subsystem,
• a distribution subsystem, also called microgrid,
• user’s electrical installations including interface equipment between the installations and
the microgrid.
TS 62257-9-2 © IEC:2006(E) – 7 –
RECOMMENDATIONS FOR SMALL RENEWABLE ENERGY
AND HYBRID SYSTEMS FOR RURAL ELECTRIFICATION

Part 9-2: Microgrids
1 Scope
The purpose this part of IEC 62257-9 is to specify the general requirements for the design
and the implementation of microgrids used in decentralized rural electrification to ensure the
safety of persons and property and their satisfactory operation according to the scheduled use.
This part of IEC 62257-9 applies to microgrids for decentralized rural electrification purposes.
The microgrids covered by this part of IEC 62257-9 are low voltage a.c., three-phase or
single-phase, with rated capacity less than or equal to 100 kVA. They are powered by a single
micropowerplant and do not include voltage transformation.
The low-voltage levels covered under this part of IEC 62257-9 are the 230 V 1-Ø/400 V 3-Ø
systems, 220 V 1-Ø/380 V 3-Ø, the 120 V 1-Ø/208 V 3-Ø, 60 Hz or 50 Hz systems.
This part of IEC 62257-9 specifies microgrids made of overhead lines because of technical
and economical reasons in the context of decentralized rural electrification. In particular cases,
underground cables can be used.
The requirements cover microgrids with radial architecture.
2 Normative reference
The following referenced documents are indispensable for the application 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 62257 (all parts), Recommendations for small renewable energy and hybrid systems for
rural electrification
IEC 60439 (all parts), Low-voltage switchgear and controlgear assemblies
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
carrier (messenger)
wire or a rope, the primary function of which is to support the cable in aerial installations,
which may be separate from or integral with the cable it supports
3.2
block
part of a line between two consecutive stoppage poles

– 8 – TS 62257-9-2 © IEC:2006(E)
3.3
earth
conductive mass of the earth, whose electric potential at any point is conventionally taken as
equal to zero
3.4
microgrid
subsystem of a DRES intended for power distribution of which the capacity does not exceed
100 kVA
NOTE The prefix «micro» is intended to express the low level of transmitting capacity.
3.5
micropowerplant
subsystem of a DRES for power generation up to 100 kVA
NOTE The prefix «micro» is intended to express the low power level generated (from a few kVA to a few tens of
kVA).
3.6
protective conductor
identification: PE
conductor provided for purposes of safety, for example protection against electric shock
[IEV 195-02-09]
3.7
PEN conductor
conductor combining the functions of both a protective earthing conductor and a neutral
conductor
[IEV 195-02-12]
3.8
power line
overhead or underground line installed to convey electrical energy for any purpose other than
communication
3.9
section of an overhead line
part of a line between two tension poles
NOTE A section generally includes several spans.
3.10
selectivity (or protection coordination)
ability of a protection to identify the faulty section and/or phase(s) of a power system
[IEV 448-11-06]
3.11
service connection line
conductors between the supplier’s mains and the customer’s installation
NOTE In the case of an overhead service connection, this means the conductor between a supply-line pole and the
customer’s installation.
3.12
span
part of a line between two consecutive poles

TS 62257-9-2 © IEC:2006(E) – 9 –
3.13
stay
steel wire, rope or rod, working under tension, that connects a point of a support to a separate
anchor
4 General
4.1 Limits of a microgrid
The microgrid is defined between the output terminals of the isolating device of the
micropowerplant and the input terminals of the user’s interface as illustrated in Figure 1.
Production Distribution User
sub-system sub-system sub-system
Micropowerplant Microgrid Installation
Other
User interface
Generator
equipment
IEC  1909/06
Figure 1 – Microgrid limits
4.2 Voltage drops
The maximum values of the voltage drops in the microgrid shall not exceed the values
indicated in Table 1.
Table 1 – Maximum values of voltage drops
Microgrid Voltage drop
Main line 6 %
Individual service connection line 1 %

4.3 Composition of a microgrid
Three microgrid schemes are specified in this part of IEC 62257-9 depending on the maximum
active power value required and the topography of the areas to be served.
• single-phase power system output: one single-phase feeder with multiple single phase
distribution (see Figure 2).
– 10 – TS 62257-9-2 © IEC:2006(E)

G
PEN
~
PEN 1 PEN 1 PEN 1
IEC  1910/06
Figure 2 – Microgrid consisting of a single-phase feeder
NOTE A community could be served by multiple single phase distribution driven by different single phase
generators.
• three phase system output: depending on the power needs of the customers, the layout
of the area to be served and the cost, two different distribution architectures can be used,
as shown in Figure 3 and Figure 4.
1) Case 1: Three-phase power system output; one three-phase feeder with three phase or
single phase distribution.
G
~
PEN
PEN 2 PEN PEN 1 PEN 3
IEC  1911/06
Figure 3 – Three phase system output, single phase distribution
or three phase service provided where needed

TS 62257-9-2 © IEC:2006(E) – 11 –
2) Case 2: Three-phase power system output; single phase distribution is used
throughout the community.
G
~
PEN
PEN 3 PEN 2 PEN 1
IEC  1912/06
Figure 4 – Three phase system output, single phase distribution
5 Protection against electric shocks
The microgrid shall be designed as a TN-C system (refer to IEC 62257-5).
6 Protection against overcurrents
The microgrid shall be provided with a device to protect against overcurrent. It shall be placed
at the interface with the micropowerplant
The characteristics of the device shall ensure that, at any point, negligible impedance faults
between phase conductor and protection conductor or earth will cause automatic
disconnection within a period of time of 0,4 s.
Special attention shall be paid to the selectivity with respect to the overcurrent protective
device of the user’s installation. The selectivity between protective devices in series should
ensure that any faulty section is interrupted.
7 Selection and erection of equipment
7.1 Equipment installation
All switch gear and control equipment shall be installed in cabinets or cases which allow
access only to authorized personnel. The cabinets shall comply with the IEC 60439 series.
7.2 Operational conditions and external influences
7.2.1 Ambient temperature
The conductors shall be chosen and installed so as to suit the highest local ambient
temperature.
– 12 – TS 62257-9-2 © IEC:2006(E)
The microgrid sections, including conductors and accessories, shall be installed within the
temperature limits specified by the product manufacturers and according to the manufacturers
instructions (see cable characteristics in Annex A).
7.2.2 Sources of heat
To avoid the effects of heat emitted by outside sources, the following methods or equally
efficient methods may be used to protect the cables:
• sufficient distance from sources of heat,
• protection screen,
• choice of line to allow for detrimental effects that may occur, local strengthening or change
of insulating material.
NOTE The heat given off by outside sources may be transmitted by convection, conduction or radiation.
7.2.3 Presence of water
The microgrid conductors and equipment shall be selected and installed to avoid damage by
water. Special precautions may be necessary for microgrid sections frequently exposed to
water or are liable to be immersed.
7.2.4 Risk of penetration of solid bodies
The microgrid conductors and equipment shall be selected and installed to minimize hazards
caused by the penetration of solid bodies. The cables and equipment shall ensure that the IP
protection degree is appropriate to the chosen location.
For locations where large quantities of dust appear, additional precautions shall be taken to
prevent the buildup of substances in quantities that are liable to affect the dissipation of heat
from the conductors.
7.2.5 Corrosive or polluting substance presence
When there is a possibility of corrosive substances occurring, including water, which are liable
to cause degradation or corrosion, all the parts of the line shall be suitably protected or
manufactured from material that resists such substances.
Different materials that may form electrolytic couples shall not be brought into contact with the
conductors unless special steps are taken to avoid the consequences of such contacts.
The materials that may cause mutual individual degradation or hazardous degradation shall
not be allowed to come into contact with other materials.
7.2.6 Mechanical requirements
For fixed installations in which medium, high or very high impact may occur, protection shall
be performed by any of the following arrangements:
• mechanical characteristics of the cables,
• chosen location,
• provision of complementary local or general mechanical protection,
or by any combination thereof.
The requirements provided in this part of IEC 62257-9 allow the project implementer to erect
microgrids matching the needs of consumers in rural areas and also matching normal climatic
conditions. If harsh conditions are expected, specific design studies shall be performed.

TS 62257-9-2 © IEC:2006(E) – 13 –
7.2.7 Equipment and supporting structures
Equipment and supporting structure, including their foundations, shall withstand the
anticipated mechanical stresses.
7.2.8 Vibration
The conductors and/or equipment supported by or attached to structures affected by medium
or high vibration conditions shall be appropriate to such conditions.
7.2.9 Other mechanical constraints for underground microgrid sections
The interior sizes of conduits and connecting accessories shall permit easy pulling or removal
of conductors or cables.
The curve radius shall be such that conductors or cables are undamaged (see cable
characteristics in Annex A).
The lines through which conductors or cables have to be pulled shall include suitable means
of access for pulling.
7.2.10 Presence of flora, mold or fauna
When any known or foreseen conditions represent a hazard because of the presence of flora,
mold or fauna, the microgrid equipment shall be selected and installed to include mitigation
measures against inherent damage.
Such protection measures include:
• choice of materials with appropriate mechanical properties,
• appropriate choice of location,
• prevention of access to animals.
7.2.11 Solar radiation
Insulated conductors and cables for overhead lines shall be rated to withstand U.V. exposure.
7.3 Characteristics of lines
7.3.1 General
The microgrid is in general designed with overhead lines made of insulated twisted
conductors.
7.3.2 Installation modes
There are two possibl
...