Microgrids - Part 3-1: Technical requirements - Protection and dynamic control

IEC TS 62898-3-1:2020(E) provides 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.

General Information

Status
Published
Publication Date
20-Sep-2020
Current Stage
PPUB - Publication issued
Completion Date
21-Sep-2020
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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
IEC TS 62898-3-1:2020-09(en)
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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

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® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 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

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IEC TS 62898-3-1:2020 © IEC:2020 – 3 –

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

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– 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

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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:
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IEC TS 62898-3-1:2020 © IEC:2020 – 5 –
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.

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– 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.
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IEC TS 62898-3-1:2020 © IEC:2020 – 7 –

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;
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– 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.
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IEC TS 62898-3-1:2020 © IEC:2020 – 9 –

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]
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– 10 – IEC TS 62898-3-1:2020 © IEC:2020
3.5
distributed generation
embedded generation
dispersed generation

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 fr
...

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