IEC TR 63401-3:2023

Dynamic characteristics of inverter-based resources in bulk power systems - Part 3: Fast frequency response and frequency ride-through from inverter-based resources during severe frequency disturbances

IEC TR 63401-3:2023, which is a Technical Report, provides an insight into the various forms of fast frequency response and frequency ride-through techniques that involve inverter-based generation sources (mainly wind and PV) in a bulk electrical system.
This document first focuses on extracting the clear definition of FFR from different references around the world, while studying the mechanism of FFR acting on system frequency and the unique features of FFR. It then compares various kinds of frequency response and demonstrates the relationship among synchronous inertia response, fast frequency response, and primary frequency response. Several system needs and conditions where FFR is suitable are identified. This document also focuses on the performance objectives, practicality and capabilities of various non-synchronous resources, and discusses the test methods for verifying FFR capability at different levels. Finally, it focuses on the ROCOF issues and on the robust performances of FFR. .

General Information

• Status: Published
• Publication Date: 13-Dec-2023
• ICS: 27.160 - Solar energy engineering
• ICS: 27.180 - Wind turbine energy systems
• ICS: 29.020 - Electrical engineering in general
• Technical Committee: SC 8A - Grid Integration of Renewable Energy Generation
• Drafting Committee: JWG 5 - TC 8/SC 8A/JWG 5
• Current Stage: PPUB - Publication issued
• Start Date: 14-Dec-2023
• Completion Date: 12-Jan-2024

Overview

IEC TR 63401-3:2023 - Dynamic characteristics of inverter-based resources in bulk power systems, Part 3 - is an IEC Technical Report that explains fast frequency response (FFR) and frequency ride‑through behaviors of inverter‑based resources (IBRs) such as wind, solar PV and battery systems during severe frequency disturbances. As a guidance document (TR), it consolidates global definitions, impact mechanisms, performance objectives, verification tests and modeling needs to support secure operation of bulk power systems with high non‑synchronous generation penetration.

Key topics and requirements

• Clear definition of FFR and mapping to existing regional usages (ENTSO‑E, NERC, ERCOT, etc.), including how FFR differs from synchronous inertia and primary frequency response (PFR).
• Impact mechanisms explaining how FFR affects system frequency, nadir and ROCOF (rate‑of‑change‑of‑frequency).
• Performance objectives for FFR: response time, response shape, maximum contribution and coordination with PFR.
• Technology and control options for delivering FFR - wind turbines, PV inverters, battery energy storage, HVDC; and control paradigms (grid‑following vs grid‑forming).
• Testing and verification methods at turbine/plant level: step, slope and simulation disturbance tests; measurement of start time, operate time delay and disengage time; ROCOF function testing and ride‑through criteria.
• ROCOF issues and withstand capability: recommended definitions, test specifications and operational limits for high‑ROCOF conditions.
• Modeling and simulation: improvements and dynamic model enhancements to capture FFR behavior and high‑ROCOF scenarios for bulk system studies.
• Operational context and case studies, including lessons from severe events (e.g., Great Britain 2019 blackout) and simulation studies showing system behavior with varying IBR penetrations.

Practical applications

• Developers and manufacturers can design inverter controls and reserve strategies to meet FFR performance objectives.
• Transmission system operators (TSOs) and planners use the report to set capability requirements, specify test regimes and assess system security under high non‑synchronous generation.
• Test laboratories and certifiers apply the described test methods to verify FFR and ROCOF functions at plant and device level.
• Modelers and academics use the recommended dynamic model improvements for stability studies and system planning with high renewable penetration.

Who should use this standard

• Transmission system operators, regulators and planners
• OEMs of wind, PV, battery and HVDC converters
• Test labs, certifiers and commissioning engineers
• Power system modelers, consultants and researchers

Related guidance

• Regional grid codes and guides (ENTSO‑E, NERC, ERCOT) and CIGRE working group outputs are discussed and cross‑referenced to help align FFR definitions and practices across jurisdictions.

Keywords: IEC TR 63401-3:2023, fast frequency response, FFR, inverter-based resources, frequency ride-through, ROCOF, bulk power systems, wind, PV, battery, grid-forming, grid-following, test methods, dynamic models.

Standards Content (Sample)

IEC TR 63401-3 ®
Edition 1.0 2023-12
TECHNICAL
REPORT
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Dynamic characteristics of inverter-based resources in bulk power systems –
Part 3: Fast frequency response and frequency ride-through from inverter-based
resources during severe frequency disturbances

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IEC TR 63401-3 ®
Edition 1.0 2023-12
TECHNICAL
REPORT
colour
inside
Dynamic characteristics of inverter-based resources in bulk power systems –

Part 3: Fast frequency response and frequency ride-through from inverter-

based resources during severe frequency disturbances

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160; 27.180; 29.020 ISBN 978-2-8322-8000-3

– 2 – IEC TR 63401-3:2023  IEC 2023
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms, definitions and abbreviated terms . 9
3.1 Terms and definitions . 9
3.2 Abbreviated terms . 9
4 Definition of fast frequency response (FFR) . 11
4.1 General . 11
4.2 Existing usage of term FFR . 11
4.2.1 FFR in Australia and Texas . 11
4.2.2 FFR and synthetic inertia in European Network of Transmission System
Operators for Electricity (ENTSO-E) . 15
4.2.3 FFR and synthetic inertia in EirGrid/SONI . 16
4.2.4 The enhanced frequency response and enhanced frequency control
capability in the UK . 18
4.2.5 FFR in North American Electric Reliability Council (NERC) and North
America . 18
4.3 Definition of FFR given by CIGRE JWG C2/C4.41 . 18
4.4 Recommended definition of fast frequency response (FFR) . 19
4.4.1 Clear definition . 19
4.4.2 Impact mechanism on system frequency . 19
4.5 Description of the relationship among synchronous inertia response, fast
frequency response, and primary frequency response . 20
4.5.1 Relationship between synchronous inertia response and fast frequency
response . 20
4.5.2 Relationship between fast frequency response and primary frequency
response . 21
4.5.3 Relationship between synchronous inertia response and primary
frequency response . 21
5 System needs and conditions where fast frequency response is warranted . 22
5.1 Higher ROCOF and lower nadir . 22
5.1.1 General . 22
5.1.2 Higher ROCOF . 23
5.1.3 Worse nadir . 24
5.1.4 Simulation study . 25
5.1.5 Blackout in Great Britain power grid on 9 August 2019 . 26
5.2 Large fluctuation of system frequency in power system operation . 29
5.2.1 General . 29
5.2.2 Frequency regulation scheme . 29
5.2.3 Relatively large load fluctuation . 30
5.2.4 Relatively weak and slow PFR . 30
6 Performance objectives for fast frequency response from inverter-based resources . 31
6.1 The response time of FFR . 31
6.2 The response characteristics and maximum response capacity of FFR . 32
6.3 Test performance for renewable generator equipped with fast frequency
response in China . 34
6.3.1 General . 34

6.3.2 Engineering construction . 34
6.3.3 Test practice and performance . 35
7 Available technologies, controls, and tuning considerations for fast frequency
response and primary frequency response. 35
7.1 Available technologies for fast frequency response . 35
7.1.1 Technology capabilities for FFR service. 35
7.1.2 Wind turbines . 36
7.1.3 Solar PV . 37
7.1.4 Battery energy storage . 38
7.1.5 HVDC . 40
7.2 Available controls for fast frequency response . 41
7.2.1 General . 41
7.2.2 Additional FFR control for grid-following converter . 41
7.2.3 Grid-forming converter control . 42
7.3 Tuning considerations for fast frequency response and primary frequency

response . 44
8 Test methods for verifying turbine-level or plant-level fast frequency response
capability . 45
8.1 General . 45
8.2 Selection of test equipment . 45
8.3 Test wiring method. 45
8.4 Selection of measuring conditions . 46
8.5 Step frequency disturbance test . 47
8.6 Slope frequency disturbance test . 47
8.7 Actual frequency disturbance simulation test . 48
8.8 Actual frequency disturbance simulation test . 48
9 Rate-of-change-of-frequency (ROCOF) definition and withstand capability for high
ROCOF conditions . 49
9.1 Definition of rate of change of frequency (ROCOF) . 49
9.2 Ride-through (withstand) capability for high ROCOF conditions . 51
10 Test specifications for high ROCOF conditions . 53
10.1 Performance specification . 53
10.1.1 Effective and operating ranges . 53
10.1.2 Accuracy related to the characteristic quantity . 53
10.1.3 Start time for rate of change of frequency (ROCOF) function . 54
10.1.4 Accuracy related to the operate time delay setting . 54
10.1.5 Voltage input . 54
10.2 Functional test methodology . 55
10.2.1 General . 55
10.2.2 Determination of steady-state errors related to the characteristic
quantity . 55
10.2.3 Determination of the start time . 63
10.2.4 Determination of the accuracy of the operate time delay . 65
10.2.5 Determination of disengaging time . 66
11 Modelling capabilities and improvements to dynamic models for fast frequency

response and related high ROCOF conditions . 67
11.1 General . 67
11.2 Dynamic models for fast frequency response and related high ROCOF
conditions . 68
11.2.1 Dynamic models of whole power systems . 68

– 4 – IEC TR 63401-3:2023  IEC 2023
11.2.2 Simplification of dynamic models . 73
11.3 Modelling improvements . 75
Bibliography . 77

Figure 1 – Proposed response times by ERCOT as of 2014 . 12
Figure 2 – Time elements of FFR . 14
Figure 3 – Impact mechanism on system frequency by FFR . 20
Figure 4 – System frequency in response to a large generation trip . 22
Figure 5 – Frequency characteristics under the same disturbance with various inverter-
based resources penetration . 26
Figure 6 – Frequency response in blackout in Great Britain power grid on 9 August 2019 . 27
Figure 7 – System frequency fluctuation under secondary frequency regulation due to
load fluctuation in a grid . 29
Figure 8 – Assignment of different modulations for quasi-steady-state frequency
fluctuation . 30
Figure 9 – Controlled contribution of electrical power provided by ROCOF-based FFR . 33
Figure 10 – The controlled contribution of electrical power provided by deviation-based
FFR . 34
Figure 11 – Scheme of the transfer function of ROCOF-based FFR for grid-following
converters . 41
Figure 12 – Scheme of the transfer function of deviation-based FFR for grid-following

converters . 42
Figure 13 – Schematic of the droop control of deviation-based FFR for grid-forming
converters . 43
Figure 14 – Time elements of FFR . 44
Figure 15 – Test wiring diagram . 46
Figure 16 – Test slope curve for ROCOF-based FFR . 48
Figure 17 – Schematic of increased ROCOF with increased renewable generation . 50
Figure 18 – The response of IBRs for frequency slope change (change from 45 Hz to
55 Hz in 1 s) . 51
Figure 19 – The response of IBRs for frequency step change of 1 Hz . 52
Figure 20 – Operate time and operate time delay setting . 54
Figure 21 – Example of test method for positive ROCOF function . 56
Figure 22 – Test method for measurement of reset value for ROCOF functions:
example for positive ROCOF function . 59
Figure 23 – Start time measurement of positive ROCOF function . 63
Figure 24 – Operate time delay measurement of positive ROCOF . 65
Figure 25 – Disengaging time measurement of ROCOF . 66
Figure 26 – Second generation BPS renewable energy system (RES) modules . 69
Figure 27 – Load modelling practices . 70
Figure 28 – WECC CLM . 72
Figure 29 – Electronically interfaced load model . 72
Figure 30 – Distributed energy resource model . 73
Figure 31 – The traditional SFR model . 73
Figure 32 – Improved model in light of ROCOF-based FFR and deviation-based FFR . 75

Figure 33 – Electrical power from wind turbines for different combinations of wind
power control strategies under 20 % wind power penetration in system . 76

Table 1 – Frequency response times of FFR . 13
Table 2 – Frequency response in Great Britain power grid on 9 August 2019 . 29
Table 3 – Summary of response times in different countries and regions . 31
Table 4 – Summary of response times for inverter-based resources . 31
Table 5 – Typical ranges of control parameters of FFR . 34
Table 6 – Inertia response and fast frequency regulation performance . 35
Table 7 – Input and output of a data collection point . 46
Table 8 – Test conditions for fast frequency response of renewable energy power plant . 46
Table 9 – Stepped frequency disturbance test . 47
Table 10 – Test conditions for actual frequency disturbance simulation . 48
Table 11 – Example of effective and operating ranges for over- and under-frequency
protection . 53
Table 12 – Example of effective and operating ranges for ROCOF protection . 53
Table 13 – Test points for ROCOF function . 57
Table 14 – Reporting of ROCOF accuracy . 58
Table 15 – Test points of reset value for ROCOF function . 62
Table 16 – Reporting of the reset value for ROCOF function . 63
Table 17 – Test points for minimum frequency protection function start time . 64
Table 18 – Test points to measure operate time delay . 65
Table 19 – Test points for accuracy of the operate time delay . 66
Table 20 – Test points of disengaging time for ROCOF function . 67

– 6 – IEC TR 63401-3:2023  IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 3: Fast frequency response and frequency ride-through from
inverter-based resources during severe frequency disturbances

FOREWORD
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IEC TR 63401-3 has been prepared by subcommittee 8A: Grid Integration of Renewable
Energy Generation, of IEC technical committee 8: System aspects of electrical energy supply.
It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
8A/130/DTR 8A/150/RVDTR
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 Report is English.
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/standardsdev/publications.
A list of all parts in the IEC 63401 series, published under the general title Dynamic
characteristics of inverter-based resources in bulk power systems, 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, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
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– 8 – IEC TR 63401-3:2023  IEC 2023
INTRODUCTION
Primary frequency response (PFR) denotes the autonomous reaction of system resources to
change in frequency. In most power systems, the main contributor to PFR is the governor
response of synchronous generation. In the systems with less synchronous generators, the
system inertia is relatively low and PFR capability is relatively weak and slow, so the system
frequency tends to change dramatically in severe power imbalance disturbances, which will
trigger under-frequency load shedding (UFLS) or OPC (over speed protection control) of
synchronous generators possibly. Therefore, it is an effective coping method to introduce
some new frequency responses in the systems with high penetration of inverter-based
resources.
This document studies fast frequency response (FFR) as a potential mitigation option in
maintaining grid security during severe frequency disturbances. Broadly, FFR is some kind of
rapid injection of electrical power from inverter-based resources or relief of loads that helps
arrest the decline of system frequency during severe disturbances.

DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 3: Fast frequency response and frequency ride-through from
inverter-based resources during severe frequency disturbances

1 Scope
This part of IEC 63401, which is a Technical Report, provides an insight into the various forms
of fast frequency response and frequency ride-through techniques that involve inverter-based
generation sources (mainly wind and PV) in a bulk electrical system.
This document first focuses on extracting the clear definition of FFR from different references
around the world, while studying the mechanism of FFR acting on system frequency and the
unique features of FFR. It then compares various kinds of frequency response and
demonstrates the relationship among synchronous inertia response, fast frequency response,
and primary frequency response. Several system needs and conditions where FFR is suitable
are identified. This document also focuses on the performance objectives, practicality and
capabilities of various non-synchronous resources, and discusses the test methods for
verifying FFR capability at different levels. Finally, it focuses on the ROCOF issues and on the
robust performances of FFR.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
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
3.2 Abbreviated terms
Abbreviated term Description
AEMO Australian Energy Market Operator
AGC automatic generation control
BESS battery energy storage systems
BMS battery management system
BPS bulk power system
– 10 – IEC TR 63401-3:2023  IEC 2023
Abbreviated term Description
CLM composite load model
CLOD complex load model
DFIG doubly fed induction generator
EFCC enhanced frequency control capability
EFR enhanced frequency response
European Network of Transmission System Operators for
ENTSO-E
Electricity
ERCOT Electric Reliability Council of Texas (US)
FCR frequency containment response
FFR fast frequency response
FLC frequency limit control
IBFFR inertia-based FFR
IBR inverter-based resources
MPPT the maximum power point tracking
NC RfG Network code on Requirements for Generators
NERC North American Electric Reliability Council
OPC over speed protection control
PCS power conversion system
PFR primary frequency response
PMSG permanent magnet synchronous generator
PMU phasor measurement units
PSSE Power System Simulator for Engineering
PV photovoltaic
RES renewable energy system
ROCOF rate of change of frequency
SFR system frequency response
SIR synchronous inertial response
SNSP system non-synchronous penetration
SONI System Operator for Northern Ireland
UFLS under-frequency load shedding
WECC Western Electricity Coordinating Council (US)

Abbreviated term Description
WSCC Western Systems Coordinating Council (US)
WTG wind turbine generators
4 Definition of fast frequency response (FFR)
4.1 General
In existing literature, there is no unified definition of fast frequency response, which seems
sometimes to have different meanings depending on the context.
The typical definitions from different organizations or authors are reviewed here. Some
existing usage does not give a clear definition in certain cases; thus the meaning of FFR is
speculated from the context. In this case the recommended definition of FFR from inverter-
based resources is given based on its impact mechanism on the system frequency.
4.2 Existing usage of term FFR
4.2.1 FFR in Australia and Texas
4.2.1.1 General
GE has prepared a report about FFR technology capabilities for the Australian Energy Market
Operator (AEMO), in which a description of FFR is given [1] . It is summarized as follows:
• Broadly, FFR is the rapid injection of power or relief of loading that helps arrest the
decline of system frequency during disturbances.
• FFR is similar to PFR but acts much faster, providing power during the arresting phase,
with the specific objective of providing arresting power before the frequency nadir.
• FFR and PFR both help to arrest frequency and interact with inertia to determine the
frequency nadir. FFR will also contribute to establishing the settling frequency if the FFR
is sustained past the time of the nadir into the rebound period.
• Both FFR and PFR are autonomous controls that act based on local conditions, that is,
they respond to quantities like local frequency (or machine speed) that can be measured
at, or very close to, the equipment providing the service.
The definition of FFR that was approved in the Electric Reliability Council of Texas (ERCOT)
NPPR 863 [2] as a new reserve service is shown below.
The automatic self-deployment and provision by a resource of their obligated response within
15 cycles after frequency meets or drops below a pre-set threshold, or a deployment in
response to an ERCOT Verbal Dispatch Instruction (VDI) within 10 minutes.
In general, this version of FFR is similar to PFR in function. The only difference between FFR
and PFR is the response time. Figure 1 shows an example relationship between the three
responses that was discussed in ERCOT.
___________
Numbers in square brackets refer to the Bibliography.

– 12 – IEC TR 63401-3:2023  IEC 2023

a) SIR
b) FFR
c) PFR
Figure 1 – Proposed response times by ERCOT as of 2014
It can be recognized that the three responses can contribute to mitigate the frequency nadir
when the frequency drop event occurs although the contribution levels are different. On the
other hand, the secondary frequency response is highly unlikely to contribute to mitigate the
frequency nadir because the delivered secondary frequency control signal is regularly
updated or renewed every few seconds, e.g. 5 s.
As seen from Figure 1, it is obvious that the initiating time of FFR is not zero. The response
time of the fast frequency response consists of five elements and is summarized as Table 1.
1) Measure – Measure and identify frequency deviation and fast frequency decrease.
2) Identify – Identify the occurrence of severe event that involves FFR.
3) Signal – Communicate action to be taken.
4) Activate – Actuate the resource.
5) Activate fully – Full response from the resource.

Table 1 – Frequency response times of FFR
Measure and
FFR option Signal Activate Activate fully
identify
≤ 40 ms to 60 ms
∼ 20 ms
Direct detection
(approximately 2 to 3 (approximately
cycles) 1 cycle)
∼ 40 ms to 60 ms ∼ 20 ms
Detection with PMU
(approximately 2 to 3 (approximately
cycles) 1 cycle)
≥ 100 ms
Local frequency detection nil
(approximately 5 to 6
cycles)
40 ms ∼ 500 ms
Wind turbine with IBFFR
(approximately (approximately
2 cycles)
30 cycles)
10 ms to 20 ms
Lithium batteries, flow
(approximately
batteries, supercapacitor
0,5 to 1 cycle)
40 ms
Lead-acid batteries
(approximately
2 cycles)
≤ 4 ms
Flywheels (non-inverter)
(approximately
instantaneously)
100 ms to 200 ms
Solar PV
(approximately
5 to 12 cycles)
50 ms to 500 ms
HVDC
(approximately
2,5 to 30 cycles)
There are several FFR options which cannot do without external detection and signalling:
wind turbines; lithium, flow and lead-acid batteries; flywheel energy storage systems (inverter-
interfaced); supercapacitor energy storage systems; solar photovoltaic (PV); and high-voltage
DC (HVDC) transmission. Inertia-based FFR (IBFFR, also known as "synthetic inertia") from
wind turbines can make a valuable contribution.
There are several FFR options that detect, signal and actuate by themselves such as flywheel
energy storage systems (non-inverter interfaced).
There are several FFR options that signal and actuate such as load.
In Figure 2, a simple illustration of the relationship between a frequency event and an FFR
response is shown.
– 14 – IEC TR 63401-3:2023  IEC 2023

Figure 2 – Time elements of FFR
At the beginning of the event (at 1 s), system frequency begins to drop at a ROCOF (as noted
above) that is proportional to the size of the event, and inversely proportional to the system
inertia. As the frequency drops until the event has been detected, actions requested from FFR
resources respond by providing arresting energy to slow and stop the frequency decline. A
primary objective is to ensure ROCOF is sufficiently reduced such that UFLS can operate
successfully, so energy delivered before hitting UFLS (green shading) is most valuable. Even
if UFLS occurs, FFR continues to arrest until the frequency nadir. This is the energy shaded
in orange. After the frequency nadir, the FFR energy complements the restorative energy
coming from the primary frequency response. This is shown in blue. As the figure suggests,
speed is important to avoid UFLS. However, the power industry has not asked for response
times that are as fast as the AEMO values in Table 1. Because of that, technologies will not
have been optimized for speeds less than several hundreds of milliseconds in some cases.
However, there will be modifications or trade-offs that could reduce response times
significantly in some cases. For example, an aggressive response from lithium batteries will
be at the expense of increased thermal stress on the battery cells, which could be mitigated
by increased parasitic losses from increased cooling capacity, or will reduce lithium battery
life expectancy in some cases. Rotating devices like flywheels and wind turbines will see
higher torques, etc.
4.2.1.2 The main challenge to achieve FFR
The main challenge to achieve FFR is to quickly and accurately identify a severe event where
the FFR is inseparable. Complicating this step is the fact that directly after an event,
frequency varies spatially. So, while one part of the grid could perceive a severe event,
another part of the grid will not. Additionally, triggering too much FFR will have adverse
consequences in some cases. Triggered by local ROCOF measurement and triggered by
direct event detection become two options that can be selected according to different
conditions.
Another difficulty is that very fast measurements will misinterpret transients to a certain extent,
switching operations or other actions that are not severe events as reasons to trigger. Risk of
false triggering is mitigated by longer periods for measurement and identification, but this
comes at the expense of FFR activation time. So, FFR response time is critical; however, a
balance between making high fidelity decisions to act and speed is needed. Fortunately, it
turns out that FFR needs to be fast but not incredibly fast. FFR needs to be started well
before UFLS or the occurrence of the frequency nadir. Analysis presented suggests that total
response times on the order of one quarter to one half second are sufficiently fast. It is non-
intuitive, but extremely fast FFR is less effective. If it is too fast, then it interferes with and
stifles full PFR response. Part of the planning process can include fine-tuning the response
time of FFR, thereby improving the efficacy of the FFR for critical conditions.
4.2.1.3 Response trigger options
There are two ways to detect the need to deploy FFR.
1) Direct event detection – Detect the specific condition of the disruption (e.g. the relay
action that results in losing AC link to main grid) and having a direct transfer trip scheme
to inform the resource(s). This can be done quickly (on the order of a few cycles) but
dedicated, fast communications are inseparable from it and it only addresses the specific
contingencies within its design criteria (i.e. if something else causes a frequency event,
the FFR won't trigger).
2) Frequency and ROCOF detection – Detect the frequency deviation and high ROCOF.
There are promising new technologies that claim to be able to do this very quickly.
However, accuracy and, especially, false triggering will still be an issue when attempting
to measure frequency and ROCOF very quickly after a major system fault in some cases.
Further, this approach has the limitation of only being applicable to frequency events (and
not, for example, excess interconnector loading that can cause an island to form or other
problems to evolve).
4.2.2 FFR and synthetic inertia in European Network of Transmission System
Operators for Electricity (ENTSO-E)
The ENTSO-E has published several documents related to FFR. It is summarized as follows.
• The word "frequency response" is widely used in the published grid code on Requirements
for Generators (RfG) [3], but RfG focuses on the performance requirement of the
frequency response from a functional perspective rather than details on technical
implementation to achieve the objectives. Based on the definitions in Article 2 in NC RfG,
"frequency response insensitivity" means the inherent feature of the control system
specified as the minimum magnitude of change in the frequency or input signal that results
in a change of output power or output signal. It can be speculated that frequency response
is more relevant to magnitude of change in the frequency than ROCOF.
• RfG defines synthetic inertia as the facility provided by a power park module or HVDC
system to replace the effect of inertia of a synchronous power generating module to a
prescribed level of performance. It responds to ROCOF like true inertia inherently provided
by synchronous generators [4].
• FFR is not defined as a specific term. It is described as the frequency response that
delivers energy in the very first seconds after disturbance. Although FFR qualifies "replace
the effect of inertia of a synchronous power generating module to a prescribed level of
performance", it is still controversial to be termed as synthetic inertia because it
emphasizes an active power block infeed rather than a power increase proportional to
ROCOF. FFR will be an alternative or supplement of synthetic inertia as reaching the nadir
is likely to take several seconds in some cases [5].

– 16 – IEC TR 63401-3:2023  IEC 2023
• In the Nordic synchronous area, fast frequency reserve is intended to be a fast, active
power support, responding to a frequency deviation [5]. There are three different
combinations for frequency activation level and maximum full activation time, that are
equally efficient for FFR provision, and the FFR provider can freely choose the most
suitable combination for each specific providing entity: 0,7 s maximum full activation time
for the activation level 49,5 Hz; 1,0 s maximum full activation time for the activation level
49,6 Hz; 1,3 s maximum full activation time for the activation level 49,7 Hz.
In summary, fast frequency response generally refers to the fast active power support
responding to frequency deviation, especially for the controlled contribution of electrical power
from a unit which responds quickly to changes in frequency in order to counteract the effect of
reduced inertial response. It can react proportionally to th
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