IEC TS 62786-1:2023

Distributed energy resources connection with the grid - Part 1: General requirements

IEC TS 62786-1:2023, which is a Technical Specification, provides principles and general technical requirements for distributed energy resources (DER) connected to an electric power network (in the following: the "network"). It applies to the planning, design, operation and connection of DER to networks. It includes general requirements, connection scheme, choice of switchgear, normal operating range, immunity to disturbances, active power response to frequency deviations, reactive power response to voltage changes, EMC and power quality, interface protection, connection and start to generate electrical power, active power management, monitoring, control and communication, and conformance tests.
It is supplemented by additional parts of IEC 62786 series, covering specific aspects.
This document specifies interface and interoperability requirements for connection of DER to a network operating at a nominal frequency of 50 Hz or 60 Hz. These requirements are intended for application at the point of connection (POC) of the DER to the grid. In some situations, the requirements can be the applied at the AC terminals of the generator. Additional parts of IEC 62786 provide more specific requirements.
DER include distributed generation and electrical energy storage in the form of synchronous generators, asynchronous generators, power converters, etc., connected to the medium voltage (MV) or low voltage (LV) network.

Raccordement au réseau des ressources énergétiques décentralisées – Exigences générales

General Information

• Status: Published
• Publication Date: 27-Nov-2023
• ICS: 29.240.01 - Power transmission and distribution networks in general
• Technical Committee: TC 8 - System aspects of electrical energy supply
• Drafting Committee: JWG 10 - TC 8/JWG 10
• Current Stage: PPUB - Publication issued
• Start Date: 28-Nov-2023
• Completion Date: 20-Oct-2023

Relations

• Replaces: IEC TS 62786:2017 - Distributed energy resources connection with the grid
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC TS 62786-1 ®
Edition 1.0 2023-11
TECHNICAL
SPECIFICATION
colour
inside
Distributed energy resources connection with the grid –
Part 1: General requirements
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IEC TS 62786-1 ®
Edition 1.0 2023-11
TECHNICAL
SPECIFICATION
colour
inside
Distributed energy resources connection with the grid –

Part 1: General requirements
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.01  ISBN 978-2-8322-7927-4

– 2 – IEC TS 62786-1:2023 © IEC 2023
CONTENTS
FOREWORD . 5
1 Scope and object . 7
2 Normative references . 7
3 Terms, definitions and abbreviated terms . 8
3.1 Terms and definitions . 8
4 Requirements for generating plants . 14
4.1 General . 14
4.2 Connection scheme . 14
4.3 Choice of switchgear . 14
4.3.1 General . 14
4.3.2 Interface switch . 15
4.4 Normal operating range . 15
4.4.1 General . 15
4.4.2 Operating frequency range . 15
4.4.3 Operating voltage range . 15
4.5 Immunity to disturbances . 16
4.5.1 General . 16
4.5.2 Rate of change of frequency (ROCOF) immunity . 16
4.5.3 Undervoltage ride through (UVRT) requirements . 18
4.5.4 Overvoltage ride through (OVRT) requirements . 19
4.5.5 Rapid phase angle change immunity . 20
4.6 Active power response to frequency deviation . 21
4.7 Power response to voltage changes . 21
4.7.1 General . 21
4.7.2 Voltage support by reactive power . 21
4.7.3 Reactive power control modes . 22
4.7.4 Voltage related active power control . 22
4.7.5 Voltage related reactive power response . 23
4.7.6 Additional reactive current requirements on generating plants . 24
4.8 EMC and power quality . 29
4.8.1 General . 29
4.8.2 Direct current (DC) injection . 29
4.9 Interface protection . 29
4.9.1 General . 29
4.9.2 Requirements on voltage and frequency protection . 31
4.9.3 Means to detect islanding situation . 32
4.9.4 Digital input to the interface protection . 33
4.10 Connection and starting to generate electrical power . 33
4.10.1 General . 33
4.10.2 Connection of synchronous-type generators . 33
4.10.3 Autoreclose of distribution lines . 33
4.11 Ceasing and reduction of active power on set point. 33
4.12 Remote information exchange . 34
4.12.1 General . 34
4.12.2 Monitoring and control . 34
4.12.3 Communication . 34

4.13 Requirements regarding single fault tolerance of interface protection system
and interface switch . 35
5 Conformance tests . 35
Annex A (normative) Operating frequency range . 36
Annex B (normative) Operating voltage range . 37
Annex C (normative) Undervoltage ride through capability of DER . 38
Annex D (normative) Overvoltage ride through parameters . 39
Annex E (informative) Reactive power control behaviour of DC to AC power
conversion equipment used in DER in conjunction with synchronous generators . 40
E.1 Background. 40
E.2 Power system with DER configuration . 40
E.3 Reactive power directions with leading/lagging of current phase angle to
voltage phase . 40
E.4 Reactive power control of synchronous generators . 41
E.5 Reactive power control of power conversion equipment . 42
E.6 Reactive power effect as reactive and capacitive load . 42
E.7 Conclusion . 43
Annex F (normative) Summary of power thresholds to be defined by individual
countries . 44
Bibliography . 45

Figure 1 – Example of electricity generating plant connected to a network (schematic
view of switches) . 12
Figure 2 – Under frequency ride through capability requirements of DER . 17
Figure 3 – Over frequency ride through capability requirements of DER . 17
Figure 4 – Instantaneous frequency change ride through requirement of DER . 18
Figure 5 – Under voltage ride through capability requirements of DER . 18
Figure 6 – Power recovering requirements for UVRT operation of DER (example) . 19
Figure 7 – Over voltage ride through capability . 20
Figure 8 – Example of instantaneous voltage phase angle change during UVRT . 20
Figure 9 – Typical power-frequency response curve. 21
Figure 10 – Typical volt-watt response curve . 23
Figure 11 – Typical volt-var response curve . 24
Figure 12 – Principle of voltage support during faults and voltage steps (DBL: lower
deadband limit; DBH: higher deadband limit) . 27
Figure 13 – Accuracy requirement for additional reactive current in positive and
negative sequence (LTR: limit of tolerance range) . 28
Figure 14 – Operation of protection functions . 32
Figure E.1 – Configuration schematic of a power system . 40
Figure E.2 – Reactive and active power control image of syncronous generators and DER . 42

Table 1 – Operating frequency requirements of DER . 15
Table 2 – Operating voltage requirements of DER . 16
Table 3 – Volt watt mode parameters (example) . 22
Table 4 – volt-var response set-point values for reference voltages (example) . 23
Table 5 – Interface protection functions . 30

– 4 – IEC TS 62786-1:2023 © IEC 2023
Table A.1 – Continuous operating frequency range . 36
Table A.2 – Limited operating frequency range . 36
Table B.1 – Continuous operating POC voltage range . 37
Table B.2 – Limited operating voltage range . 37
Table C.1 – UVRT capability of DER with an interface to the grid based on a
synchronous generator . 38
Table C.2 – UVRT capability of DER with an interface to the grid based on non-

synchronous generators (eg. converters, DFIG, etc.) . 38
Table D.1 – OVRT capability of DER . 39
Table E.1 – Operation and behaviours of synchronous generators, dispersed loads and
DER 43
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DISTRIBUTED ENERGY RESOURCES CONNECTION WITH THE GRID –

Part 1: General requirements
FOREWORD
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IEC TS 62786-1 has been prepared by IEC Technical Committee 8: System aspects of electrical
energy supply. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
8/1656/DTS 8/1677/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.

– 6 – IEC TS 62786-1:2023 © IEC 2023
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62786 series, published under the general title Distributed energy
resources connection with the grid, 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 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 document indicates
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of its contents. Users should therefore print this document using a colour printer.

DISTRIBUTED ENERGY RESOURCES CONNECTION WITH THE GRID –

Part 1: General requirements
1 Scope and object
This part of IEC 62786, which is a Technical Specification, provides principles and general
technical requirements for distributed energy resources (DER) connected to an electric power
network (in the following: the "network"). It applies to the planning, design, operation and
connection of DER to networks. It includes general requirements, connection scheme, choice
of switchgear, normal operating range, immunity to disturbances, active power response to
frequency deviations, reactive power response to voltage changes, EMC and power quality,
interface protection, connection and start to generate electrical power, active power
management, monitoring, control and communication, and conformance tests.
It is supplemented by additional parts of IEC 62786 series, covering specific aspects.
This document specifies interface and interoperability requirements for connection of DER to a
network operating at a nominal frequency of 50 Hz or 60 Hz. These requirements are intended
for application at the point of connection (POC) of the DER to the grid. In some situations, the
requirements can be the applied at the AC terminals of the generator. Additional parts of
IEC 62786 provide more specific requirements.
DER include distributed generation and electrical energy storage in the form of synchronous
generators, asynchronous generators, power converters, etc., connected to the medium voltage
(MV) or low voltage (LV) network.
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 60255-12, Electrical relays – Part 12: Directional relays and power relays with two input
energizing quantities
IEC 60255-127, Measuring relays and protection equipment − Part 127: Functional
requirements for over/under voltage protection
IEC 60255-151, Measuring relays and protection equipment − Part 151: Functional
requirements for over/under current protection
IEC 60255-181, Measuring relays and protection equipment − Part 181: Functional
requirements for frequency protection
IEC 61000 (all parts), Electromagnetic compatibility (EMC)
IEC 61850 (all parts), Communication networks and systems for power utility automation
IEC 62116, Utility-interconnected photovoltaic inverters – Test procedure of islanding
prevention measures
– 8 – IEC TS 62786-1:2023 © IEC 2023
IEC TS 62749, Assessment of power quality – Characteristics of electricity supplied by public
networks
3 Terms, definitions and abbreviated terms
3.1 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
3.1.1
black start
startup of an electric power system from a blackout through internal energy resources
3.1.2
converter-type generator
generator that produces electrical power and is connected to the network via a converter,
including doubly fed induction machines
3.1.3
declared supply voltage
U
C
supply voltage agreed by the power system operator and the network user
Note 1 to entry: Generally declared supply voltage U is the nominal voltage U but it may be different according
C n
to the agreement between the DSO and the network user.
3.1.4
distributed energy resource
DER
generators (with their unit 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, modified – unit has been added.]
3.1.5
distribution network
electric power network for the distribution of electric power from and to network users for which
a distribution system operator (DSO) is responsible
3.1.6
distribution system operator
DSO
party operating a distribution network
Note 1 to entry: In some countries, a DSO is also referred to as DNO (distribution network operator).
[SOURCE: IEC 60050-617:2009, 617-02-10, modified – Note 1 to entry has been added.]

3.1.7
electrical proximity
state of two or more pieces of equipment linked to one another by connections the impedances
of which are negligible as compared to other impedances involved
3.1.8
flicker
impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or
spectral distribution fluctuates with time
Note 1 to entry: Flicker can be calculated by short-term flicker indicator P and long-term flicker indicator P .
st lt
[SOURCE: IEC 60050-161:1990, 161-08-13, modified – Note 1 to entry has been added.]
3.1.9
fundamental frequency
frequency of the fundamental component of a periodic quantity
Note 1 to entry: For the purpose of this document, the fundamental frequency is the same as the power supply
frequency, e.g. 50 Hz or 60 Hz.
[SOURCE: IEC 60050-103:2009, 103-07-21, modified – Note 1 to entry has been added.]
3.1.10
fundamental component
sinusoidal component of the Fourier series of a periodic quantity having the frequency of the
quantity itself
[SOURCE: IEC 60050-103:2009, 103-07-19]
3.1.11
generating unit
set of equipment connected together whose primary purpose is to generate electrical power.
3.1.12
generating plant
group of generating units including auxiliaries connected to one POC
3.1.13
harmonic component
sinusoidal component of the Fourier series of a periodic quantity, the harmonic order of which
is an integer number greater than one
th
Note 1 to entry: A component of harmonic n (with n>1) is generally designated n harmonic. the designation of the
st
fundamental component as the "1 harmonic" is not recommended.
[SOURCE: IEC 60050-103:2009, 103-07-25]
3.1.14
interface protection
combination of protection relay functions which open the interface switch of a generating unit
and prevents its closure, whichever is appropriate, in the case of:
• a fault in the electric power network;
• an unintentional islanding situation;
• voltage and/or frequency being outside the tolerance of their operating ranges for
continuous operation
– 10 – IEC TS 62786-1:2023 © IEC 2023
3.1.15
interharmonic frequency
frequency which is a non-integer multiple of the reference fundamental frequency
Note 1 to entry: By extension from harmonic order, the interharmonic order is the ratio of an interharmonic frequency
to the fundamental frequency. This ratio is not an integer (Recommended notation: "m").
Note 2 to entry: In the case where m<1 the term subharmonic frequency can be used.
[SOURCE: IEC 60050-551:2001, 551-20-06, modified – Note 1 to entry and Note 2 to entry
have been added.]
3.1.16
interharmonic component
sinusoidal component of a periodic quantity having an interharmonic frequency
Note 1 to entry: Its value is normally expressed as an RMS value.
[SOURCE: IEC 60050-551:2001, 551-20-08, modified – The existing note has been deleted and
a new Note 1 to entry has been added.]
3.1.17
interoperability
property permitting diverse systems or components to work together for a specified purpose
[SOURCE: IEC 60050-871:2018, 871-05-06]
3.1.18
long-term flicker indicator
measure of flicker evaluated over a specified time interval of a relatively long duration, using
successive values of the short-term flicker indicator
Note 1 to entry: The duration is typically 2 hours, using 12 successive values of P , in accordance with
st
IEC 61000-4-15.
[SOURCE: IEC 60050-161:1990, 161-08-19]
3.1.19
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]
3.1.20
medium voltage
MV
any set of voltage levels lying between low and high voltage
Note 1 to entry: The boundaries between medium and high voltage levels that overlap and depend on local
circumstances as well as history or common usage. Nevertheless, the band 1 kV to 35 kV is considered as the
accepted medium voltage boundary.
Note 2 to entry: Because of existing network structures, boundary between MV and HV can be different from country
to country.
[SOURCE: IEC 60050-601:1985, 601-01-28, modified – The existing note has been modified
and Note 2 to entry has been added]

3.1.21
point of connection
POC
reference point on the electric power system where the user’s electrical facility is connected
Note 1 to entry: See Figure 1.
[SOURCE: IEC 60050-617:2009, 617-04-01]
3.1.22
power converter
electronic equipment that converts
• AC to DC (rectifier)
• DC to AC (inverter)
• DC to DC (DC-to-DC converter)
• AC to AC (AC-to-AC converter)
[SOURCE: IEC TR 61850-90-7:2013, 3.1.16]
3.1.23
port
location giving access to a device or network where electromagnetic energy or signals may be
supplied or received or where the device or network variables may be observed or measured
[SOURCE: IEC TR 62109.1:2010, 3.64]
3.1.24
power factor
under periodic conditions, ratio of the absolute value of the active power P to the apparent
power S:
IIP
λ =
S
Note 1 to entry: Under sinusoidal conditions, the power factor is the absolute value of the active factor.
[SOURCE: IEC 60050-131:2002, 131-11-46]
3.1.25
rate of change of frequency
ROCOF
amount of frequency change per unit of time
3.1.26
short-term flicker indicator
a measure of flicker evaluated over a specified time interval of a relatively short duration
Note 1 to entry: The duration is typically 10 minutes, in accordance with IEC 61000-4-15.
[SOURCE: IEC 60050-161:1990, 161-08-18]
3.1.27
short-time withstand current
the current that a circuit or a switching device in the closed position can carry during a specified
short time under prescribed conditions of use and behaviour

– 12 – IEC TS 62786-1:2023 © IEC 2023
[SOURCE: IEC 60050-441:1984, 441-17-17]
3.1.28
single fault tolerance
built-in capability of a system to provide continued correct execution of its function in the
presence of a single fault
3.1.29
system operator
party responsible for safe and reliable operation of a part of the electric power system in a
certain area and for connection to other parts of the electric power system
[SOURCE: IEC 60050-617:2009, 617-02-09]
3.1.30
switch
device for changing the electric connections among its terminals
Note 1 to entry: See Figure 1.
[SOURCE: IEC 60050-151:2001, 151-12-22]

Figure 1 – Example of electricity generating plant connected to a network
(schematic view of switches)
3.1.31
main switch
switch installed as close as possible to the point of connection, for protection against internal
faults and disconnection of the whole plant from the network
Note 1 to entry: See also Figure 1.

3.1.32
unit auxiliaries
any auxiliary equipment specific to the unit and indispensable for its operation
Examples: mills, circulating pumps, induced draught fans.
[SOURCE: IEC 60050-602:1983, 602-02-29]
3.1.33
interface switch
switch (circuit breaker, switch or contactor) installed in the producer’s network, for separating
the part(s) of the producer’s network that contain at least one generation unit from the network
Note 1 to entry: See also Figure 1.
Note 2 to entry: In some situations, the interface switch can be used to enable island operation of part of the
producer’s network, if technically feasible.
3.1.34
generating unit switch
switch installed electrically close to the terminals of each generating unit of the generating plant,
for protection and disconnection of that generating unit
Note 1 to entry: See also Figure 1.
3.1.35
voltage deviation
difference between the supply voltage at a given instant and the declared supply voltage
[SOURCE: IEC 60050-614:2016, 614-01-4]
3.1.36
voltage dip
sudden voltage reduction at a point in an electric power system, followed by voltage recovery
after a short time interval, from a few periods of the sinusoidal wave of the voltage to a few
seconds
[SOURCE: IEC 60050-614:2016, 614-01-08]
3.1.37
voltage fluctuation
series of voltage changes or a continuous variation of the RMS or peak value of the voltage
Note 1 to entry: Whether the RMS or peak value is chosen depends upon the application.
[SOURCE: IEC 60050-161:1990, 161-08-05, modified – Note to entry 1 has been modified.]
3.1.38
voltage swell
sudden increase of voltage at a point in an electrical system followed by voltage reduction after
a short period of time from a few cycles to a few seconds
3.1.39
voltage unbalance
condition in a polyphase system in which the RMS values of the phase element voltages
(fundamental component), or the phase angles between consecutive phase element voltages,
are not all equal
Note 1 to entry: The degree of the inequality is usually expressed as the ratios of the negative- and zero-sequence
component to the positive-sequence component.

– 14 – IEC TS 62786-1:2023 © IEC 2023
Note 2 to entry: In this standard, voltage unbalance is considered in relation to 3-phase systems.
[SOURCE: IEC 60050-614:2016, 614-01-32, modified – Note 1 to entry and Note 2 to entry
have been added.]
3.2 Abbreviated terms
DER distributed energy resource
DFIG doubly fed induction generator
DSO distribution system operator
EHV extra high voltage
EMC electromagnetic compatibility
HV high voltage
OVRT overvoltage ride through
LV low voltage
UVRT undervoltage ride through
MV medium voltage
POC point of connection
PV photovoltaic
ROCOF rate of change of frequency
4 Requirements for generating plants
4.1 General
When connecting a DER to the network, consideration should be given to all generating units
and loads connected or to be connected in the same electrical proximity. The combined effect
of DER should be considered when selecting an appropriate POC for the DER concerned.
4.2 Connection scheme
DER shall follow the requirements of the system operator in POC. Different requirements can
be subject to agreement between the operator of the DER and the system operator depending
on the electric power system needs.
4.3 Choice of switchgear
4.3.1 General
Switches shall be chosen based on the characteristics of the electric power system in which
they are intended to be installed. For this purpose, the short circuit current at the installation
point shall be assessed taking into account the contribution of the generating plant.
Switches and their protections shall be suitable for the type of generating units installed.
Related applicable standard shall be used.
Depending on local system requirements, generating plant should be equipped at the
connection point with isolating devices and visible signs that are easily operated and lockable.

The method for isolating the generating plant connected to a MV network shall be accessible to
the system operator at all times, unless the system operator requires or permits using an
alternative method.
NOTE In some countries, accessibility is also required for DER connected to a LV network.
4.3.2 Interface switch
Interface switches shall have a breaking and making capacity corresponding to the rated current
of the generating plant and corresponding to the short circuit contribution of the generating
plant.
The short-time withstand current of the switching devices shall be coordinated with maximum
short circuit current at the POC.
In case of loss of auxiliary power supply to the switchgear, the interface switch shall disconnect
immediately. An uninterruptible power supply or an equivalent device may be required by the
system operator.
The interface switch can be combined with either the main switch or with the generating unit
switch. In case of a combination, the single switch should be compliant with the requirements
of the two separate switches. As a consequence, at least two switches in series should be
present between any generating unit and the POC. One of the two switches can include
electronic switch (gate blocking) in case of converter connection.
4.4 Normal operating range
4.4.1 General
A DER when generating power shall have the capability to operate in the operating ranges
specified below regardless of the topology and the settings of the interface protection.
4.4.2 Operating frequency range
DER with rated power exceeding the power threshold for frequency deviation P , (see
th-fd
Annex F), as defined by individual countries, shall withstand frequency deviations in
accordance with those specified in Table 1.
Table 1 – Operating frequency requirements of DER
Frequency DER actions required
f < f
min2
Instantaneous disconnection permitted
f > f
max2
f ≤ f < f
min2 min1
Operate for a minimum time T
f1
f < f ≤ f
max1 max2
f ≤ f ≤ f
Operate continuously
min1 max1
The recommended range values of f , f , f , f and T are specified in Annex A.
min1 min2 max1 max2 f1
4.4.3 Operating voltage range
A DER with a rated power exceeding the power threshold for voltage deviation P (see
th-vd
Annex F) as defined by individual countries, should provide capability of withstanding voltage
deviations at POC, in accordance with those specified in Table 2.

– 16 – IEC TS 62786-1:2023 © IEC 2023
Table 2 – Operating voltage requirements of DER
Voltage at POC DER actions required
U < U
min2
Disconnection allowed after time T
u2
U > U
max2
U ≤ U < U
min2 min1
Disconnection allowed after time T
u1
U < U ≤ U
max1 max2
U ≤ U ≤ U
Operate continuously
min1 max1
If the voltage drops below U , undervoltage ride through limits as specified in 4.5.3 shall
min2
apply.
If voltage exceeds U , overvoltage ride through limits as specified in 4.5.4 shall apply.
max2
The recommended range values of U , U , U , U , T and T are specified in
min1 min2 max1 max2 u1 u2
Annex B.
4.5 Immunity to disturbances
4.5.1 General
The following withstand capabilities shall be fulfilled regardless of the topology and the settings
of the interface protection.
NOTE An event on the HV and EHV transmission network can affect numerous small-scale units on MV and LV
level. Depending on the penetration of distributed generation, a significant loss of active power can occur.
4.5.2 Rate of change of frequency (ROCOF) immunity
Regarding the ROCOF withstand capability, the generating unit shall be able to operate with a
ROCOF as specified by the system operator in individual countries and by subparts of this
series of standards. Immunity level of ROCOF may be configured via communication interfaces.
Three examples follow.
Example 1
After a frequency event on the grid and whilst frequency remains within required frequency
operation limits, a DER should maintain continuous operation whilst the absolute value of
ROCOF does not exceed:
a) a higher threshold ROCOF for the first t seconds, or
hi ROCOFhi
b) a lower threshold ROCOF for the first t seconds,
lo ROCOFlo
where ROCOF > ROCOF and t < t
hi lo ROCOFhi ROCOFlo.
ROCOF and ROCOF are the higher and the lower ROCOF threshold values, respectively.
hi lo
They are expressed in Hz/s and specified by individual countries.
t is the minimum time during which a DER should withstand violation of ROCOF .
ROCOFhi hi
t is the minimum time during which a DER should withstand violation of ROCOF
ROCOFlo lo
is also exceeded, in which case the above requirement applies).
(unless ROCOF
hi
t and t are expressed in seconds and specified by individual countries.
ROCOFhi ROCOFlo
Example 2
Figure 2 and/or Figure 3 may be defined by the system operator.
If frequency is within "stay connected" area, a DER shall maintain the connection with the
network. If frequency moves outside of “stay connected” area, a DER may disconnect. The
system operator should choose appropriate values for f , f , f , t , t , t , t . The chosen values
0 1 2 0 1 2 3
shall be coordinated with the operating frequency range (see 4.4.2).

Figure 2 – Underfrequency ride through capability requirements of DER

Figure 3 – Overfrequency ride through capability requirements of DER
Example 3
Depending on local system requirements, DER shall be capable of staying connected to the
grid during frequency step change caused by autoreclose of transmission line. General
requirement of immunity of DER to frequency step change is illustrated in Figure 4.

– 18 – IEC TS 62786-1:2023 © IEC 2023

Figure 4 – Instantaneous frequency change ride through requirement of DER
4.5.3 Undervoltage ride through (UVRT) requirements
Depending on local system requirements, DER shall stay connected with the grid for a minimum
time during a voltage dip at the POC. General requirements of immunity of DER to voltage dips
are illustrated in Figure 5.
Figure 5 – Undervoltage ride through capability
requirements of DER
If the voltage drop at the POC is within "stay connected" area, the DER shall maintain the
connection with the network. If the voltage drop at the POC moves out of "stay connected" area,
the DER may disconnect.
After the fault is cleared, the DER should recover a specified proportion of its pre-fault output
power, S , within a specified time, T , after stable network conditions are detected as shown
rec rec
in Figure 6.
These requirements apply to all types of short circuit faults. For three phase DER, the lowest
phase-to-phase voltage shall be used. Depending on local system requirements, the lowest
phase-to-neutral voltage may be used instead.

The recommended value ranges for parameters t , t , t , t , t , t , U , U , U , U , U , U in
1 2 3 4 5 6 1 2 3 4 5 6
Figure 5 are listed in Annex C, Table C.1. The chosen values shall be coordinated with the
operating voltage range (see 4.4.3).
If required, a DER connected to MV networks shall participate in dynamic network support (such
as fast reactive current injection for inverter based DER), see also 4.7.2 and 4.7.3.

Figure 6 – Power recovering requirements for UVRT operation of DER (example)
4.5.4 Overvoltage ride through (OVRT) requirements
Depending on local system requirements, DER shall stay connected with the grid for a minimum
time during a voltage swell at the POC. General requirements of immunity of DER to voltage
swell are illustrated in Figure 7.
If the voltage at the POC is within "stay connected" area, DER shall maintain the connection
with the network. If the voltage at the POC moves out of "stay connected" area, DER may
disconnect.
For three phase DER, the highest phase-to-phase voltage shall be used. Depending on local
system requirements, the highest phase-to-neutral voltage may be used instead.
The recommended range values for parameters in Figure 7 are listed in Annex D, Table D.1.
The chosen values shall be coordinated with the operating voltage range (see 4.4.3).

– 20 – IEC TS 62786-1:2023 © IEC 2023

Figure 7 – Overvoltage ride through capability
4.5.5 Rapid phase angle change immunity
Depending on local system requirements, generating units shall be capable of staying
conn
...