IEC TS 62898-3-1:2020

IEC TS 62898-3-1 ®
Edition 1.0 2020-09
TECHNICAL
SPECIFICATION
colour
inside
Microgrids –
Part 3-1: Technical requirements – Protection and dynamic control

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IEC TS 62898-3-1 ®
Edition 1.0 2020-09
TECHNICAL
SPECIFICATION
colour
inside
Microgrids –
Part 3-1: Technical requirements – Protection and dynamic control

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.01 ISBN 978-2-8322-8764-4

– 2 – IEC TS 62898-3-1:2020 © IEC:2020
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Microgrid protection requirements . 15
4.1 General . 15
4.2 Main requirements specific to microgrids . 16
4.2.1 General . 16
4.2.2 Phase fault protection . 16
4.2.3 Earth fault protection . 17
4.3 General protection requirements . 17
4.3.1 General . 17
4.3.2 Dependability of protection . 17
4.3.3 Security of protection . 18
4.3.4 Availability and selectivity of protection . 18
4.3.5 Operating time (speed) of protection . 19
4.4 Particular requirements for non-isolated microgrids . 19
4.5 Particular requirements for isolated microgrids. 20
5 Protection systems for microgrids . 20
5.1 General . 20
5.2 Short-circuit protection . 21
5.2.1 Overcurrent protection . 21
5.2.2 Directional overcurrent protection . 23
5.2.3 Distance protection . 24
5.2.4 Directional power protection . 24
5.2.5 Differential protection . 24
5.3 System protection . 25
5.3.1 Under/over voltage protection . 25
5.3.2 Frequency protection . 26
5.4 Centralized protection systems . 26
6 Dynamic stability and control . 27
6.1 General . 27
6.2 Dynamic stability in microgrids . 27
6.2.1 General . 27
6.2.2 Disturbances in microgrids. 28
6.2.3 Voltage and frequency stability . 28
6.3 Dynamic control in microgrids . 29
6.3.1 General requirements . 29
6.3.2 Dynamic control functions . 29
6.3.3 Control elements in microgrids. 30
6.3.4 Control systems of microgrids . 31
6.3.5 Control of microgrids during grid-connected mode . 35
6.3.6 Control of microgrids during island mode . 35
Annex A (informative) Use cases for dynamic control of microgrids . 36

Bibliography . 40

Figure 1 – Ratio between maximum load current/minimum short-circuit current in the
microgrid . 22
Figure 2 – Control elements in microgrids . 30
Figure 3 – Hierarchical control levels of a microgrid . 32
Figure 4 – Centralized multilevel control of microgrids . 32
Figure A.1 – Simple microgrid platform for testing transient disturbance during motor
start-up . 36
Figure A.2 – Transient control strategy based on reactive current compensation
control . 36
Figure A.3 – Voltage profile during field testing of transient disturbance with and
without transient control device . 37
Figure A.4 – Current profile during field testing of transient disturbance with and
without transient control device . 37
Figure A.5 – Microgrid platform with high proportion of RES for testing dynamic
disturbance control . 38
Figure A.6 – Dynamic control strategy . 38
Figure A.7 – Voltage profile of field testing with and without dynamic control device . 39

– 4 – IEC TS 62898-3-1:2020 © IEC:2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MICROGRIDS –
Part 3-1: Technical requirements –
Protection and dynamic control

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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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
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Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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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 TS 62898-3-1, which is a Technical Specification, has been prepared by IEC subcommittee
8B: Decentralized Electrical Energy Systems of IEC technical committee 8: System aspects of
electrical energy supply.
The text of this Technical Specification is based on the following documents:

Draft TS Report on voting
8B/53/DTS 8B/59/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.
A list of all parts in the IEC 62898 series, published under the general title Microgrids, can be
found on the IEC website.
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.
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.

– 6 – IEC TS 62898-3-1:2020 © IEC:2020
INTRODUCTION
Microgrids can serve different purposes depending on the primary objectives of their
applications. They are usually seen as a means to manage reliability of supply in a grid
contingency and to facilitate local optimization of energy supply by controlling distributed energy
resources (DER). Microgrids also present a way to provide electricity supply in remote areas,
to use renewable energy as a systematic approach for rural electrification and to increase
resiliency and security of supply to end users.
Deployment of DER can cause a microgrid or distribution system of a grid to face several
challenges, including fault protection and dynamic control issues. There are, however, some
issues commonly faced in the protection and control of microgrids which are less prevalent in
large grids. These issues include: bidirectional flow of power resulting in voltage excursions
outside acceptable limits, fault current being supplied from multiple sources, loss of
synchronism between multiple sources when a fault occurs, potentially limited fault current
magnitude, lower inertia or lower primary time constant, regular changes in operational
configuration due to economic optimization, and intermittency of source-dependent renewable
distributed generators. These issues worsen when the microgrid contains several converter-
based generators (CBGs) and operates in island mode. As such, conventional protection and
control strategies may not be suitable or sufficient for microgrids. Protection systems different
from the conventional ones may be required. In some instances, protection systems may need
to be adjusted dynamically based on the operating state of the microgrid.
Conventional power systems have predominantly consisted of power sources, such as fossil
fuel-fired thermal power plants, hydro power plants and nuclear power plants, which are
relatively stable and easy to control. On the other hand, microgrids often contain many different
types of sources, many of which are intermittent. Hence, protection and dynamic control in
microgrids need to be more sophisticated than in conventional power systems. However, the
main grid contributes to the fault currents in the grid-connected mode of operation and hence
the fault currents are large enough to actuate conventional protection devices. Though it is
possible to employ conventional protection principles and existing standards for the protection
of microgrids operating in grid-connected mode, the existing protection settings should be
systematically assessed as the existence of DER may compromise the coordination of the
protection system.
Due to the specific characteristics of microgrids and their frequent use of converter-based
generators, disturbances in microgrids require special consideration. The disturbance problems
in microgrids can be addressed by dynamic control. Dynamic control can be classified as
transient disturbance control and dynamic disturbance control. Transient disturbance control
damps disturbances in microgrids caused by forced or unintended sudden and severe voltage
and current changes due to switching of large sources or loads, mode transfer or fault clearance,
and characterized by large magnitude and phase change and with a time duration of
milliseconds. Dynamic disturbance control regulates disturbances in microgrids caused by
forced or unintended voltage and current changes due to generator and load variation, and
characterized by magnitude and phase changes beyond the normal operating limits, and
continuing for milliseconds to seconds.
The initial characteristics of faults are very similar to initial characteristics of transient and
dynamic disturbances. Distinguishing the two types of incidents from each other is critical for
the proper operation of microgrids. Thus, protection and dynamic control of microgrids are
closely related and need to be coordinated with each other.
This part of IEC 62898 specifies requirements to address the above-mentioned protection and
dynamic control issues in microgrids.

IEC TS 62898 (all parts) intends to provide general guidelines and technical requirements for
microgrids.
a) IEC TS 62898-1 mainly covers the following issues:
• determination of microgrid purposes and application;
• preliminary study necessary for microgrid planning, including resource analysis, load
forecast, DER planning and power system planning;
• principles of microgrid technical requirements that should be specified during planning
stage;
• microgrid evaluation to select an optimal microgrid planning scheme.
b) IEC TS 62898-2 mainly covers the following issues:
• operation requirements and control targets of microgrids under different operation
modes;
• basic control strategies and methods under different operation modes;
• requirements of energy storage, monitoring and communication under different
operation modes;
• power quality.
c) IEC TS 62898-3-1 mainly covers the following issues:
• requirements for microgrid protection;
• protection systems for microgrids;
• dynamic control for transient and dynamic disturbances in microgrids;
Microgrids can be stand-alone or a sub-system of an interconnected grid. The technical
requirements in this Technical Specification are intended to be consistent with:
1) IEC 60364-7 (all parts and amendments related to low-voltage electrical installations);
2) IEC TS 62786, requirements for connection of generators intended to be operated in parallel
with the grid;
3) IEC TS 62257 (all parts) with respect to rural electrification;
4) IEC TS 62749 with respect to power quality;
5) IEC TS 62898-1;
6) IEC TS 62898-2;
7) IEC TS 63268;
– 8 – IEC TS 62898-3-1:2020 © IEC:2020
MICROGRIDS –
Part 3-1: Technical requirements –
Protection and dynamic control

1 Scope
The purpose of this part of IEC 62898 is to provide guidelines for the specification of fault
protection and dynamic control in microgrids. Protection and dynamic control in a microgrid are
intended to ensure safe and stable operation of the microgrid under fault and disturbance
conditions.
This document applies to AC microgrids comprising single or three-phase networks or both. It
includes both isolated microgrids and non-isolated microgrids with a single point of connection
(POC) to the upstream distribution network. It does not apply to microgrids with two or more
points of connection to the upstream distribution network, although such systems can follow the
guidelines given in this document. This document applies to microgrids operating at LV or MV
or both. DC and hybrid AC/DC microgrids are excluded from the scope, due to the particular
characteristics of DC systems (extremely large fault currents and the absence of naturally
occurring current zero crossings).
This document defines the principles of protection and dynamic control for microgrids, general
technical requirements, and specific technical requirements of fault protection and dynamic
control. It addresses new challenges in microgrid protection requirements, transient disturbance
control and dynamic disturbance control requirements for microgrids. It focuses on the
differences between conventional power system protection and new possible solutions for
microgrid protection functions.
Depending on specific situations, additional or stricter requirements can be defined by the
microgrid operator in coordination with the distribution system operator (DSO).
This document does not cover protection and dynamic control of active distribution systems.
This document does not cover product requirements for measuring relays and protection
equipment.
This document does not cover safety aspects in low voltage electrical installations, which are
covered by IEC 60364 (all parts and amendments related to low-voltage electrical installations).
Requirements relating to low voltage microgrids can be found in IEC 60364-8-2.
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 60364 (all parts), Low voltage electrical installations
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
code
collection of rules concerning rights and duties of the parties involved
in a certain part of the electric power system
EXAMPLE Grid code, distribution code.
[SOURCE: IEC 60050-617:2009, 617-03-03]
3.2
(electronic) (power) converter
(electronic) (power) convertor
operative unit for electronic power conversion, comprising one or more electronic valve devices,
transformers and filters if necessary and auxiliaries if any
Note 1 to entry: In English, the two spellings "convertor" and "converter" are in use, and both are correct. In this
document, the spelling "converter" is used in order to avoid duplications.
[SOURCE: IEC 60050-551:1998, 551-12-01, modified – The figure has been deleted.]
3.3
converter-based generator
CBG
generator of AC power that is naturally a DC source, or an AC source whose frequency is
different from the power frequency, and is connected to the electric power system through a
power converter
3.3.1
grid-forming CBG
generator which is connected to the network through a converter that can be controlled as a
voltage source capable of controlling voltage and frequency of the network
Note 1 to entry: There are also stiff grid-forming CBGs which are a special type of grid-forming CBGs delivering
power at constant frequency and voltage.
3.3.2
grid-supporting CBG
generator which is connected to the network through a converter with a power source capable
of actively assisting the regulation of voltage and frequency of the network
3.3.3
grid-following CBG
generator which is connected to the network through a converter with a power source that does
not have the capability to actively assist the regulation of voltage and frequency of the network
3.4
distributed energy resources
DER
generators (with their auxiliaries, protection and connection equipment), including loads having
a generating mode (such as electrical energy storage systems), connected to a low-voltage or
a medium-voltage network
[SOURCE: IEC 60050-617:2017, 617-04-20]

– 10 – IEC TS 62898-3-1:2020 © IEC:2020
3.5
distributed generation
embedded generation
dispersed generation
DG
generation of electric energy by multiple sources which are connected to the power distribution
system
[SOURCE: IEC 60050-617:2009, 617-04-09, modified – "distributed generation" has been listed
as a first preferred term and the abbreviated term "DG" has been added.]
3.6
distribution system operator
distribution network operator
distributor
DSO
party operating a distribution system
[SOURCE: IEC 60050-617:2009, 617-02-10, modified – The abbreviated term "DSO" has been
added.]
3.7
dynamic disturbance
series of voltage and current changes in a microgrid caused by output of renewable
energy sources reaching a sufficiently high proportion, non-linear loads, intentional islanding,
intermittency and output power fluctuation of renewable energy resources and grid side faults,
which continue for a period of 50 ms to 2 s
3.8
electrical energy storage
EES
installation able to absorb electrical energy, to store it for a certain amount of time and to
release electrical energy during which energy conversion processes may be included
EXAMPLE A device that absorbs AC electrical energy to produce hydrogen by electrolysis, stores the hydrogen,
and uses that gas to produce AC electrical energy is an electrical energy storage.
Note 1 to entry: The term “electrical energy storage” may also be used to indicate the activity that an apparatus,
described in the definition, carries out when performing its own functionality.
Note 2 to entry: The term “electrical energy storage” should not be used to designate a grid-connected installation,
"electrical energy storage system" is the appropriate term.
[SOURCE: IEC 62933-1:2018, 3.1]
3.8.1
energy intensive application
EES system application generally not very demanding in terms of step response performances
but with frequent and long charge and discharge phases at variable discharge powers
Note 1 to entry: Reactive power exchange with the electric power system is frequently present together with active
power exchange.
[SOURCE: IEC 62933-1:2018, 3.12, modified – "energy intensive application" has been listed
as a preferred term instead of an admitted term, "frequent" has been added in the definition,
"may be present" has been replaced with "is frequently present" in Note 1 to entry and Note 2
to entry has been deleted.]
3.8.2
power intensive application
EES system application generally demanding in terms of step response performances and with
frequent charge and discharge phase transition or with reactive power exchange with the
electric power system
[SOURCE: IEC 62933-1:2018, 3.13, modified – "power intensive application" has been listed
as a preferred term instead of an admitted term and the note has been deleted.]
3.9
fault ride-through
FRT
ability of a generating unit or power plant to stay connected during specified faults in the electric
power system
3.10
high voltage
HV
1) in a general sense, the set of voltage levels in excess of low voltage
2) in a restrictive sense, the set of upper voltage levels used in power systems for bulk
transmission of electricity
[SOURCE: IEC 60050-601:1985, 601-01-27]
3.11
island
portion of a power system, that is disconnected from the remainder of the
system, but remains energized
[SOURCE: IEC 60050-603:1986, 603-04-46]
3.11.1
intentional island
island resulting from planned action(s) of automatic protections, or from deliberate action by
the responsible network operator, or both, in order to keep supplying electrical energy to a
section of an electric power system
[SOURCE: IEC 60050-617:2017, 617-04-17]
3.11.2
unintentional island
island that is not anticipated by the relevant network operator
[SOURCE: IEC 60050-617:2017, 617-04-18]
3.12
low voltage
LV
set of voltage levels used for the distribution of electricity and whose upper limit is generally
accepted to be 1 000 V for alternating current
[SOURCE: IEC 60050-601:1985, 601-01-26]
3.13
medium voltage
MV
any set of voltage levels lying between low and high voltage

– 12 – IEC TS 62898-3-1:2020 © IEC:2020
Note 1 to entry: The boundaries between medium- and high-voltage levels overlap and depend on local circumstances
and history or common usage. Nevertheless the band 30 kV to 100 kV frequently contains the accepted boundary.
[SOURCE: IEC 60050-601:1985, 601-01-28]
3.14
microgrid
group of interconnected loads and distributed energy resources with
defined electrical boundaries forming a local electric power system at distribution voltage levels,
that acts as a single controllable entity and is able to operate in either grid-connected or island
mode
Note 1 to entry: This definition covers both (utility) distribution microgrids and (customer owned) facility microgrids.
[SOURCE: IEC 60050-617:2017, 617-04-22]
3.14.1
isolated microgrid
stand-alone microgrid
group of interconnected loads and distributed energy resources with defined electrical
boundaries forming a local electric power system at distribution voltage levels, that cannot be
connected to a wider electric power system
Note 1 to entry: Isolated microgrids are usually designed for geographical islands or for rural electrification.
[SOURCE: IEC 60050-617:2017, 617-04-23]
3.14.2
non-isolated microgrid
group of interconnected loads and distributed energy resources with defined electrical
boundaries forming a local electric power system at distribution voltage levels, that can be
connected to a wider electric power system
[SOURCE: IEC 60050-617:2017, 617-04-23, modified – The definition is a modification of that
of "isolated microgrid" (3.14.1).]
3.15
over-voltage ride-through
OVRT
ability of a generating unit or power plant to stay connected during a limited duration rise of
system voltage
Note 1 to entry: In some documents the expression "High Voltage Ride Through (HVRT)", is used for the same
capability.
3.16
phase locked loop
PLL
feedback circuit for synchronizing an oscillator with the phase of an input signal
[SOURCE: IEC 60050-713:1998, 713-10-48]
3.17
point of connection
POC
reference point on the electric power system where the user’s electrical facility is connected
[SOURCE: IEC 60050-617:2009, 617-04-01]

3.18
power quality
characteristics of the electric current, voltage and frequencies at a given point in an electric
power system, evaluated against a set of reference technical parameters
Note 1 to entry: These parameters might, in some cases, relate to the compatibility between electricity supplied in
an electric power system and the loads connected to that electric power system.
[SOURCE: IEC 60050-617:2009, 617-01-05]
3.19
power system stability
capability of a power system to regain a steady state, characterized by the synchronous
operation of the generators after a disturbance due, for example, to variation of power or
impedance
[SOURCE: IEC 60050-603:1986, 603-03-01]
3.20
protection system
arrangement of one or more protection equipments, and other devices intended to perform one
or more specified protection functions
Note 1 to entry: A protection system includes one or more protection equipments, instrument transformer(s), wiring,
tripping circuit(s), auxiliary supply(s) and, where provided, communication system(s). Depending upon the principle(s)
of the protection system, it may include one end or all ends of the protected section and, possibly, automatic reclosing
equipment.
Note 2 to entry: The circuit-breaker(s) are excluded.
[SOURCE: IEC 60050-448:1995, 448-11-04]
3.20.1
dependability of protection
probability for a protection of not having a failure to operate under given conditions for a given
time interval
[SOURCE: IEC 60050-448:1995, 448-12-07, modified – The figure has been deleted.]
3.20.2
reliability of protection
probability that a protection can perform a required function under given conditions for a given
time interval
Note 1 to entry: The required function for protection is to operate when required to do so and not to operate when
not required to do so.
[SOURCE: IEC 60050-448:1995, 448-12-05, modified – The figure has been deleted.]
3.20.3
security of protection
probability for a protection of not having an unwanted operation under given conditions for a
given time interval
[SOURCE: IEC 60050-448:1995, 448-12-06, modified – The figure has been deleted.]
3.20.4
selectivity of protection
ability of a protection to identify the faulty section and/or phase(s) of a power system

– 14 – IEC TS 62898-3-1:2020 © IEC:2020
[SOURCE: IEC 60050-448:1995, 448-11-06]
3.20.5
unwanted operation of protection
operation of a protection either without any power system fault or other power system
abnormality, or for a system fault or other power system abnormality for which that protection
should not have operated
[SOURCE: IEC 60050-448:1995, 448-12-03]
3.21
reliability
probability that an electric power system can perform a required
function under given conditions for a given time interval
Note 1 to entry: Reliability quantifies the ability of an electric power system to supply adequate electric service on
a nearly continuous basis with few interruptions over an extended period of time.
Note 2 to entry: Reliability is the overall objective in electric power system design and operation.
[SOURCE: IEC 60050-617:2009, 617-01-01]
3.22
stability
capability of a power system to regain or to retain a steady-state
condition, characterized by the synchronous operation of the generators and/or a steady
acceptable quality of the electricity supply, after a disturbance due, for example, to variation of
power or impedance
[SOURCE: IEC 60050-603:1986, 603-03-01, modified – "or to retain a steady state condition"
and " and/or a steady acceptable quality of the electricity supply" have been added.]
3.23
stability
capability of a microgrid to regain a steady state after being subjected to a
disturbance without involuntary load shedding
3.24
stability zone
operating area situated within the stability limits of the system state variables
[SOURCE: IEC 60050-603:1986, 603-03-12]
3.25
total harmonic ratio
total harmonic distortion
THD
ratio of the RMS value of the harmonic content to the RMS value of the fundamental component
or the reference fundamental component of an alternating quantity
Note 1 to entry: The total harmonic ratio depends on the choice of the fundamental component. If it is not clear
from the context which one is used an indication should be given.
Note 2 to entry: The total harmonic ratio may be restricted to a certain harmonic order. This is to be stated.
[SOURCE: IEC 60050-551:2001, 551-20-13]

3.26
transient disturbance
sudden and severe voltage and current changes in a microgrid caused by switching
of generation or load, unintentional islanding or faults, characterized by large magnitude and
phase changes and continuing for a period of 0 ms to 50 ms
3.27
transient stability of a power system
power system stability in which disturbances may have large rates of change and/or large
relative magnitudes
[SOURCE: IEC 60050-603:1986, 603-03-03]
3.28
under voltage ride-through
UVRT
ability of a generating unit or power plant to stay connected during a limited duration dip of
system voltage
Note 1 to entry: In some documents the expression “Low Voltage Ride Through (LVRT)”, is used for the same
capability.
4 Microgrid protection requirements
4.1 General
With conventional grids, the protection strategy depends on many factors and the complexity of
the protection systems can be very different in different applications. For example, protection
systems are different in residential LV grids from those used in meshed transmission grids. The
protection strategy in microgrids depends on many factors. The main factors are as follows.
a) Protection systems shall be adapted to the voltage level of the microgrid:
• microgrids based on LV networks: for example, residential house, building, rural
electrification (access to energy);
• microgrids with MV and LV distribution networks: for example, a campus with several
buildings and MV links between buildings, remote industrial plants, geographical islands
with an MV distribution network between substations.
b) Protection systems depend on the microgrid architecture. In traditional grids, protection
systems are closely linked to the grid architecture (meshed grid, closed or open ring, radial
structure, etc.). For microgrids, its electrical structure also has an impact on the protection
system complexity. For example, a microgrid where all the sources are connected to the
same busbar is easier to protect than a microgrid with distributed generators on several
busbars. The grid architecture and the locations of the power sources can significantly affect
the complexity and therefore the reliability and cost of the protection system. Thus, the
protection system design shall be considered at the same time as the grid architecture.
c) Protection systems are designed to meet specified reliability and availability requirements
of the power system. The protection strategy, in particular the fault selectivity, is a key factor
to reach the required availability level for a given microgrid, but these technical requirements
shall be balanced against the corresponding cost of the protection equipment.
d) Protection equipment commercially available on the market is not the same for MV and LV
applications:
– Multifunctional protection relays, which are commonly used in MV/HV grids, can be used
to implement complex protection systems. These devices can provide several protection
functions, an internal customized logic, and different setting groups, which can typically
be changed remotely via the relay's communication link. Complementary to the
protection relay, measuring sensors (current transformers, voltage transformers,
temperature sensors, etc.) and (in most cases) a reliable DC power supply system are
essential parts of the protection system.

– 16 – IEC TS 62898-3-1:2020 © IEC:2020
– At LV levels, simpler protection functions are often embedded in LV circuit-breakers.
However, protection systems shall also meet specific requirements linked to safety and
based on local regulations or international standards (e.g. IEC 60364 (all parts)). In
specific cases (e.g. point of connection to MV level, critical infrastructure), protection of
parts of the LV network can be based on MV protection relays to cover complex
protection systems. This option is already used for conventional grids.
e) Microgrid protection systems are particularly impacted by the short-circuit current levels.
Due to different operation modes (grid-connected and island) and the availability and control
of distributed generators, the short-circuit current level and characteristics could vary greatly
at any point within the microgrid. If synchronous generators are continuously operating, for
example in hydropower or biogas plants, the short-circuit current should be large enough to
make conventional protection effective. In the case of isolated microgrids, or non-isolated
microgrids operating in island mode, and which are served only by converter-based
generators, the short-circuit current is often low. In these cases, the generators might be
able to provide sufficient current to operate an overcurrent protection device with a lower
current setting but not those with a higher setting. The design of the microgrid protection
system needs to consider the short-circuit current from generators within the microgrid and
impact of the different operational modes.
4.2 Main requirements specific to microgrids
4.2.1 General
Microgrids can have a number of specific requirements in comparison with conventional grids.
Protection of microgrids with a high proportion of conventional synchronous machines (e.g.
geographical islands or industrial plants fed by conventional gensets) may resemble that of
conventional grids. However, microgrids with a higher proportion of converter-based-generators
have specific requirements that need to be considered.
4.2.2 Phase fault protection
In microgrids, phase fault detection is challenging due to the following issues:
a) Fault current contribution from multiple in-feeds
Conventional MV and LV power systems are typically operated in such a way that only
unidirectional short-circuit current flow occurs, i.e. they only have a single short-circuit
source (neglecting the contributions from motors). In microgrids, short-circuit sources can
be distrib
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