IEC TR 63401-2:2022
(Main)Dynamic characteristics of inverter-based resources in bulk power systems - Part 2: Sub- and Super-synchronous control Interactions
Dynamic characteristics of inverter-based resources in bulk power systems - Part 2: Sub- and Super-synchronous control Interactions
IEC TR 63401-2:2022, which is a technical report, covers the "control interactions" in converter interfaced generators e.g, wind and PV with the frequency of the resulting oscillation below twice the system frequency. SSCI can be categorized into:
1) SSCI in DFIG is caused by the interaction between DFIG wind turbine converter controls and the series compensated network.
2) SSCI involving FSC (both type-4 wind turbine or PV generators) is caused by the interaction between wind turbine or solar PV's FSC controls and weak AC grid.
This technical report is organized into nine clauses. Clause 1 gives a brief introduction and highlights the scope of this document. Clause 4 presents the historical background of various types of subsynchronous oscillation (SSO) and revisits the terminologies, definitions, and classification in the context of classical SSR and emerging SSCI issues to better understand and classify the emerging interaction phenomena. Clause 5 provides the description, mechanism, and characteristics of the SSCI phenomenon in the framework of real-world incidents, including the SSCI events in the ERCOT, Guyuan, and Hami wind power systems. Clause 6 proposes two benchmark models to study the SSCI DFIG and FSC-based wind turbines or PV generators. Clause 7 gives an overview of existing and emerging modeling and stability analysis approaches to investigate the SSCI phenomenon. Clause 8 outlines various techniques to mitigate the SSCI. It discusses various SSCI mitigation schemes, such as bypassing the series capacitor, selective tripping of WTGs, generator, and plant-level damping control schemes. Clause 9 highlights the need for future works towards standardization of terms, definitions, classification, analysis methods, benchmark models, and mitigation methods.
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IEC TR 63401-2 ®
Edition 1.0 2022-06
TECHNICAL
REPORT
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Dynamic characteristics of inverter-based resources in bulk power systems –
Part 2: Sub- and super-synchronous control interactions
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IEC TR 63401-2 ®
Edition 1.0 2022-06
TECHNICAL
REPORT
colour
inside
Dynamic characteristics of inverter-based resources in bulk power systems –
Part 2: Sub- and super-synchronous control interactions
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160, 27.180, 29.020 ISBN 978-2-8322-3929-2
– 2 – IEC TR 63401-2:2022 IEC 2022
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 10
2 Normative references . 10
3 Terms and definitions . 10
4 Terms, definitions and classification . 11
4.1 Existing terms, definitions and historical background . 11
4.1.1 General . 11
4.1.2 Subsynchronous resonance (SSR) . 12
4.1.3 Device dependent SSO (DDSSO) . 13
4.2 Necessity to revisit the terms and classification . 13
4.3 Revisiting the terms and classification . 13
4.3.1 General . 13
4.3.2 Torsional interaction . 14
4.3.3 Network resonance . 15
4.3.4 Control interaction . 15
4.4 Clause summary . 16
5 SSCI incidents in real-world wind power systems . 16
5.1 General . 16
5.2 SSCI in DFIGs connected to series-compensated networks . 17
5.2.1 ERCOT SSCI incident in 2009 . 17
5.2.2 ERCOT SSCI events in 2017 . 18
5.2.3 SSCI events in Guyuan wind power system . 20
5.3 SSCI in FSC-based generators connected to weak AC network . 24
5.3.1 SSCI event in Hami wind power system . 24
5.4 Clause summary . 27
6 Modeling and analysis approaches . 28
6.1 Preview . 28
6.2 Time-domain modeling and analysis approaches . 28
6.2.1 General . 28
6.2.2 Nonlinear time-domain EMT simulation . 28
6.2.3 Controller hardware-in-the-loop simulation . 28
6.2.4 Linearized state-space modeling and modal analysis . 29
6.2.5 Discussions on time-domain approaches for SSCI studies . 30
6.3 Frequency-domain modeling and analysis approaches . 30
6.3.1 Frequency scanning . 30
6.3.2 Complex torque coefficient method . 31
6.3.3 Impedance-based modeling and analysis . 33
6.4 Guidelines on the approaches to SSCI studies . 39
6.5 Clause summary . 40
7 Proposed benchmark models . 40
7.1 Overview. 40
7.2 Benchmark model based on Guyuan wind power system . 40
7.2.1 General . 40
7.2.2 Configuration and parameters of the WTGs and Guyuan substation . 41
7.2.3 Parameters of the DFIG's converter control . 41
7.2.4 Series-compensated electrical network . 41
7.2.5 Case study . 41
7.3 Benchmark model based on Hami wind power system . 42
7.3.1 General . 42
7.3.2 Configuration and parameters of FSCs . 43
7.3.3 Configuration and parameters of LCC-HVDC . 43
7.3.4 Synchronous generators . 45
7.3.5 Electrical network . 45
7.3.6 Case studies . 45
7.4 Clause summary . 46
8 Mitigation methods . 46
8.1 General . 46
8.2 Bypassing the series capacitor . 47
8.3 Selective tripping of WTGs . 47
8.4 Network/Grid-side subsynchronous damping controller (GSDC) . 48
8.5 Generation-side subsynchronous mitigation schemes . 50
8.5.1 Adjusting the wind turbine converter control parameters . 50
8.5.2 Adding an SSDC in the RSC control loop . 51
8.5.3 Adding an SSDC in the GSC control loop . 53
8.6 Protection schemes . 54
8.7 Clause summary . 54
9 Future work . 54
Annex A (Informative) . 56
Bibliography . 60
Figure 1 – Multi-frequency oscillations in the modern power system with high-share of
renewables and power electronic converters . 10
Figure 2 – Timeline of the historical developments of SSO terms, definitions and
classification [12] . 11
Figure 3 – Terms and classification of SSR by IEEE [13] . 12
Figure 4 – Classification of subsynchronous interaction based on the origin [12]. 14
Figure 5 – Reclassification of subsynchronous interactions based on the interaction
mechanism . 14
Figure 6 – Timeline of SSCI events reported around the world . 16
Figure 7 – Structure of the ERCOT wind power system in 2009 [16]. 17
Figure 8 – Oscilloscope record of the 2009 SSCI event in the ERCOT system [19] . 18
Figure 9 – Structure of the ERCOT wind power system in 2017 [24]. 18
Figure 10 – Event#1 August 24, 2017: current, voltage and frequency spectrum of the
current during the SSCI event and after bypassing the series capacitor [24] . 19
Figure 11 – Event#2 September 27, 2017: current, voltage and frequency spectrum of
the current during the SSCI event [24] . 20
Figure 12 – Event#3 October 27, 2017: current, voltage and frequency spectrum of the
current during the SSCI event [24] . 20
Figure 13 – Geographical layout of the Guyuan wind power system, Hebei Province,
China . 21
Figure 14 – Power flow measured at the 200 kV side of the Guyuan step-up
transformer . 22
Figure 15 – Field recorded line current and frequency spectrum . 22
– 4 – IEC TR 63401-2:2022 IEC 2022
Figure 16 – Field recorded voltage and frequency spectrum . 23
Figure 17 – Hami wind power system, Xinjiang, China [27] . 24
Figure 18 – Current (upper plot) and active power (lower plot) . 25
Figure 19 – Frequency spectrum of the current (upper plot) and active power (lower
plot) . 25
Figure 20 – Field measured active power of a wind farm (a) From 09:46 to 09:47 (b)
From 11:52 to 11:53. 26
Figure 21 – Torsional modes and frequency variation of the unstable oscillation . 26
Figure 22 – Torsional speed of modes 1 to 3 of unit #2 in Plant M . 27
Figure 23 – Configuration of CHIL simulation . 29
Figure 24 – Three-phase subsystem represented in the dq domain using equivalent
small-signal impedance. 34
Figure 25 – Three-phase subsystem represented in the sequence domain using
equivalent small-signal impedance . 34
Figure 26 – Impedance measurement in a simple system . 36
Figure 27 – A simple system in the impedance model, consisting of two separable
components: source and load . 38
Figure 28 – Impedance model with voltage and current as input and output of the
source and load sides; system stability is determined by the two transfer function
matrices, Z (s) and Z (s) . 38
s l
Figure 29 – The unified dq-frame INM of a typical power system . 38
Figure 30 – Recommended guidelines for the SSCI stability analysis of a real-world
wind power system . 40
Figure 31 – One-line diagram of the proposed benchmark model adopted from the
Guyuan wind power system . 41
Figure 32 – Simulation results of benchmark model (a) phase A current (b) frequency
spectrum of the current (c) subsynchronous current component . 42
Figure 33 – One-line diagram of the proposed benchmark model adopted from the
Hami wind power system . 42
Figure 34 – The structure of the LCC HVDC system . 43
Figure 35 – AC filters and reactive power compensations . 44
Figure 36 – Three tuned DC filtersTT12/24/45 . 44
Figure 37 – The common electrical network . 45
Figure 38 – SSO in the second benchmark model (a) the SG rotor speed (b)
subsynchronous frequency component in the speed (c) time-frequency analysis of the
rotor speed . 46
Figure 39 – A system-wide SSCI mitigation scheme based on selective tripping of
WTGs . 48
Figure 40 – (a) A series-compensated wind power system with GSDC (b) design and
configuration of GSDC including SSDC and SCG . 49
Figure 41 – CHIL test results of GSDC (a) active power (b) subsynchronous current . 50
Figure 42 – SSCI mitigation by increasing the K of the inner controllers of the GSC
p
(a) voltage at PCC (b) current phase-A (c) active and reactive power . 51
Figure 43 – SSCI mitigation by reducing the PLL bandwidth (a) voltage at PCC (b)
current phase-A (c) active and reactive power . 51
Figure 44 – Control diagram of the virtual resistor for DFIG's RSC controllers . 52
Figure 45 – The SSCI damped out when the virtual resistor is enabled at 2 seconds in
simulation (a) voltage at PCC (b) current phase-A (c) active and reactive power . 52
Figure 46 – Control diagram of GSC of a typical FSC wind turbine . 53
Figure 47 – The SSCI mitigated after the virtual resistor is switched-on (a) voltage at
PCC (b) current phase-A (c) active and reactive power . 53
Table 1 – Comparison of the characteristics of real-world SSCI events . 27
Table 2 – Main Features of time-domain approaches for SSCI studies . 30
Table A.1 – Number of DFIGs in the wind farms of Guyuan system . 56
Table A.2 – DFIG and step-up transformer parameters (Base capacity = 1,5 MW) . 56
Table A.3 – GSC control parameters . 56
Table A.4 – RSC control parameters . 57
Table A.5 – Transmission lines and their parameters in Guyuan wind power system . 57
Table A.6 – Electrical parameters of the VSC . 57
Table A.7 – Specific parameters of the converter transformer . 57
Table A.8 – Parameters of AC filters on the rectifier side (800 MW) . 58
Table A.9 – Parameters of AC filters on the inverter side (800 MW) . 58
Table A.10 – The control parameters of the LCC-HVDC system . 58
Table A.11 – The rated parameters and electrical parameters of the synchronous
generator . 59
Table A.12 – 660 MW steam turbine shafting equivalent lumped parameters . 59
Table A.13 – The common electrical network parameters (500 kV transmission line) . 59
– 6 – IEC TR 63401-2:2022 IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –
Part 2: Sub- and super-synchronous control interactions
FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 63401-2 has been prepared by subcommittee SC 8A: Grid Integration of renewable
energy generation, of IEC technical committee TC 8: Systems aspects of electrical energy
supply. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
8A/99/DTR 8A/103/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,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
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– 8 – IEC TR 63401-2:2022 IEC 2022
INTRODUCTION
Advancements in power electronic converters have led to an increased proportion of converter
based renewable power generators in modern electric power systems. Power electronic
converters use multi-time scale converter control structures to achieve smooth grid connection.
Such control interactions cause oscillation with the frequency ranging from a few hertz to
several kilohertz, which can interact with other converter-based devices or system components
such as static compensators (STATCOM), series capacitors and weak AC grids. The
interactions of converter control with series-compensated or weak AC grid cause oscillation in
the subsynchronous and its complementary supper synchronous frequency ranges, named as
sub- and super-synchronous control interaction or simply sub-synchronous control interaction
(SSCI).
In the past decade, several incidents have been reported where wind turbine and photovoltaic
(PV) converter controls interacted with series-compensated or weak AC grids at
subsynchronous and/or supersynchronous frequencies. Post-event investigations have shown
that the converter controls actively participate in these interactions. Unlike classical sub-
synchronous resonance (SSR), SSCI is a system-wide phenomenon rather than a localized
converter control issue. The mechanism and characteristics of SSCI are greatly influenced by
converter control structures and parameters, generation resource intermittency, network
topology change, grid strength, etc. Such factors distinguish the converter control participated
interactions in converter-based generators from the classic SSR phenomenon associated with
the conventional power generators. The oscillation caused by SSCI seriously threatens the
stable and reliable operation of wind power systems.
Power systems with high-penetration of power electronic converters face a variety of oscillatory
stability issues. Power electronic converter-based components such as converter-based wind
turbine generators (WTGs), photovoltaic (PV), flexible AC transmission system (FACTS) and
high voltage DC (HVDC) can interact with each other and/or with the series-compensated or
weak AC networks. As a result of such interactions, oscillation from a few hertz to tens or
hundreds of hertz could be triggered, as illustrated in Figure 1.
The interaction between doubly-fed induction generators (DFIGs) and series compensated
transmission lines was first reported in the electric reliability council of Texas (ERCOT) wind
power system in 2009. The frequency of triggered oscillation was 20 Hz to 30 Hz. Later on, from
2010 to 2016, frequent oscillation events were reported between DFIG and series-compensated
network in the Guyuan system located in Hebei, China. In 2015, a new type of interaction was
reported in the Hami wind power system in Western China. Post-event investigations showed
that the full-scale converter (FSC) interacted with the weak AC grid causing strong sub- and
super-synchronous oscillation. The frequency of oscillation originating from the FSC wind
turbines matched with the shafts' natural frequencies of the nearby steam turbine generators,
which resulted in intense torsional vibrations. In 2019, a power outage event in the UK's
National Grid was also found to have been worsened by a 9 Hz oscillation. The converter
controls of the FSCs in the Hornsea offshore wind farm participated in the event and amplified
the negative resistance effect, which led to the sudden shutdown of the wind farm.
The frequency of oscillation triggered by the interactions between converter generators (e.g.
wind or PV) and series-compensated or weak AC grid falls in the range of sub- and/or super-
synchronous frequency. Due to the active participation of converter controls, the interaction is
widely known as the subsynchronous 'control' interaction (SSCI). Note that although the
frequency of the 2019 event in the UK's National Grid is below the system's synchronous
frequency, careful consideration must be given before characterizing this event as an SSCI
event.
Besides SSCI, several high-frequency resonance events have also been reported around the
world. For example, the harmonic instability with frequency ranging from 100 Hz to 1 000 Hz in
the Borwin1 offshore wind power project in the North Sea of Europe. In 2017, a high-frequency
resonance was reported in the Yunnan grid after the Luxi project was put in operation. The high-
frequency resonance occurred between the modular multilevel (MMC)-HVDC and the AC grid,
triggering the 1 272 Hz and its complementary frequency oscillation. Similar events involving
interactions between converter-based devices and the grid have occurred around the world.
The interaction phenomenon causing such high-frequency oscillation is widely known as high-
frequency resonance or harmonic resonance.
This technical report aims at revisiting the existing terms and definitions, proposing benchmark
models, modeling and analysis methods and mitigation schemes to better understand, analyze
and control SSCI.
– 10 – IEC TR 63401-2:2022 IEC 2022
DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –
Part 2: Sub- and super-synchronous control interactions
1 Scope
Based on the interaction phenomenon and frequency range, this part of IEC 63401, which is a
technical report, covers the "control interactions" in converter interfaced generators e.g, wind
and PV with the frequency of the resulting oscillation below twice the system frequency. SSCI
can be categorized into:
1) SSCI in DFIG is caused by the interaction between DFIG wind turbine converter controls
and the series compensated network.
2) SSCI involving FSC (both type-4 wind turbine or PV generators) is caused by the interaction
between wind turbine or solar PV's FSC controls and weak AC grid.
Figure 1 – Multi-frequency oscillations in the modern power system
with high-share of renewables and power electronic converters
This technical report is organized into nine clauses. Clause 1 gives a brief introduction and
highlights the scope of this document. Clause 4 presents the historical background of various
types of subsynchronous oscillation (SSO) and revisits the terminologies, definitions, and
classification in the context of classical SSR and emerging SSCI issues to better understand
and classify the emerging interaction phenomena. Clause 5 provides the description,
mechanism, and characteristics of the SSCI phenomenon in the framework of real-world
incidents, including the SSCI events in the ERCOT, Guyuan, and Hami wind power systems.
Clause 6 proposes two benchmark models to study the SSCI DFIG and FSC-based wind
turbines or PV generators. Clause 7 gives an overview of existing and emerging modeling and
stability analysis approaches to investigate the SSCI phenomenon. Clause 8 outlines various
techniques to mitigate the SSCI. It discusses various SSCI mitigation schemes, such as
bypassing the series capacitor, selective tripping of WTGs, generator, and plant-level damping
control schemes. Clause 9 highlights the need for future works towards standardization of terms,
definitions, classification, analysis methods, benchmark models, and mitigation methods.
2 Normative references
There are no normative references in this document.
3 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
4 Terms, definitions and classification
4.1 Existing terms, definitions and historical background
4.1.1 General
This subclause gives a brief overview of the historical developments to define and classify the
subsynchronous oscillation issues in traditional power systems. Figure 2 shows a timeline of
the classical SSR events and the historical development of terms and definitions related to SSR.
The formation of a series resonance circuit in induction generators in the presence of a series
capacitor is not a new phenomenon. This series resonance phenomenon was first observed
and named as the induction generator effect (IGE) in 1937 [1] . The resonance phenomenon
related to the turbine's shaft emerged after the two consecutive resonance incidents that
occurred in Mohave generating stations in 1970 and 1971 [2]. The Mohave incidents happened
due to the excitation of the shaft's torsional modes, resulting in severe damage to the shafts of
the turbo-generators. Post-incident studies on the Mohave incidents led to defining several
terminologies for the first time, which have been reported in [3]. The concept of SSR, IGE,
torsional interaction (TI) and torque amplification (TA) in a series compensated induction
generator was presented. Until this time, SSR was thought to be triggered by the series
capacitors in the transmission line. To harmonize the SSR research community, in 1976, a
bibliographic report was published by the IEEE in which the work was classified into induction
machine effect (IME) and torsional torque oscillation (TTOs) [4]. However, a few years later,
another shaft failure event in the Navajo power station revealed that SSR can also be triggered
by HVDC converters [5], [6]. Such a phenomenon was later named as the subsynchronous
torsional interaction (SSTI) [5]. In an attempt to standardize and redefine the terms, in 1980,
the IEEE SSR working group proposed standard definitions of the terms, such as SSR, IGE, TI,
and TA to streamline the research community [7]. Subsequently, a second bibliographic
supplement was published, which apart from the existing terms/definitions included a new term
called device-dependent subsynchronous oscillation (DDSSO). The DDSSO was defined as the
oscillation caused by power system devices, such as the power system stabilizers (PSSs) and
static var compensators (SVCs) [8]. The SSR field tests at Square Butte showed that the HVDC
system was involved in adverse interaction with the shaft of an adjacent turbine generator [9].
Subsequently, the second, third, and fourth supplements to the bibliographic report introduced
a new classification in 1985, that is the DDSSO [4], [10], [11].
Figure 2 – Timeline of the historical developments of SSO terms,
definitions and classification [12]
___________
Numbers in square brackets refer to the Bibliography.
– 12 – IEC TR 63401-2:2022 IEC 2022
In 1992, the IEEE's working group put forward standard terms to define and classify the SSR
issues in conventional turbo-generators [13]. The SSO was divided into SSR and DDSSO. The
SSR was further divided into self-excitation (SE), IGE, TI, and shaft TA, as depicted in Figure
3.
Figure 3 – Terms and classification of SSR by IEEE [13]
4.1.2 Subsynchronous resonance (SSR)
4.1.2.1 General
According to the IEEE definition of the term SSR [13], "it is an electric power system condition
where the electric network exchanges energy with a turbine generator at one or more of the
natural frequencies of the combined system below the synchronous frequency of the system".
The SSR is further divided into self-excitation (SE), also called steady-state SSR, and transient
SSR to include TA. The steady-state SSR or SE covers the SSR caused by IGE and TI [13].
4.1.2.2 Induction generator effect (IGE)
SE of a series compensated induction generator is caused by the IGE, that is when the rotor
circuits turn faster than the rotating magnetic field produced by the subsynchronous armature
currents. Under this condition, the rotor resistance to subsynchronous current as viewed from
the armature terminals becomes negative. The IGE occurs when this negative resistance is
more than the sum of the armature and network resistance at a certain subsynchronous
frequency [13].
4.1.2.3 Torsional interaction (TI)
TI is the interplay between the mechanical systems (turbine-generator) and a series
compensated electrical network. The shaft of the turbine-generator responds to system
disturbances at its natural frequencies and produces corresponding subsynchronous voltages
at the generator terminals. If this subsynchronous frequency matches with the electrical
resonance frequency of the network, the corresponding stator current induces a torque, which
excites the torsional oscillations. Each time, the magnitude of the torque increases, resulting in
growing oscillations [13].
4.1.2.4 Torque amplification (TA)
TA occurs following a large disturbance in a series capacitor compensated system. The system
disturbance causes electromagnetic torque oscillation at the complement of the electrical
network's natural frequency. If, somehow, this frequency aligns with one of the natural
frequencies of the shaft, a resonance between the network's electrical and shaft's mechanical
frequencies occurs [13].
4.1.3 Device dependent SSO (DDSSO)
DDSSO is defined as the oscillation caused by the interaction between turbine generators and
a wide range of fast-acting controllers of the power system components, such as HVDC
converters, static VAR compensators, and high-speed governor controls.
4.2 Necessity to revisit the terms and classification
The mechanism and characteristics of the SSO associated with the WTGs are quite different
from the previously reported conventional SSR events involving turbine generators. For
example, the mechanism and characteristics of the interaction phenomenon depend on the
structure and parameters of the wind turbine or PV's converter control, which actively
participates in the interaction. Furthermore, the SSCI in converter-based generators is not just
related to the converter controls; it is rather a system-level issue that is also influenced by other
system-wide parameters. The parameters influencing the mechanism and features of the
oscillation include wind speed, number of online WTGs, wind turbine converter controls, and
their parameters, degree of series compensation, network topology, and stiffness of the AC grid.
Another key difference is that the frequency coupling is sometimes very strong, which leads to
a very large supersynchronous oscillation component in addition to the subsynchronous
component. The frequency coupling effect is obvious in full-converter WTGs, in which
sometimes, the magnitude of the supersynchronous oscillation is even larger than the
subsynchronous oscillation. These characteristics are very different from the characteristics of
SSR in turbo-generators. Thus, the terms, definitions, and classification should be redefined to
better understand the mechanism of SSCI in WTGs.
4.3 Revisiting the terms and classification
4.3.1 General
It is recommended that the term "subsynchronous oscillation or SSO" should be used as a
general term for an "oscillation" caused by any phenomenon that results in the "oscillation" with
its frequency being within the sub-/super-synchronous range. Thus, the SSR, SSCI, IGE, TI,
TA and DDSSO should be considered as "phenomena" whereas the "subsynchronous
oscillation" as the cause of this phenomenon.
In an electric power system, the subsynchronous interaction phenomena can be better
understood by categorizing the contributive system components into the "base" and
"interactive" components [12], where
• Base components are the system components that are prone to be interacted by other
system components present in the power system;
• Interactive components are the system components that have the potential to initiate or
trigger the interaction.
Figure 4 shows a bunch of base and interactive components in a typical power system, that
could potentially interact with each other and trigger oscillation in the range of sub-/ super-
synchronous frequency. Based on the origin, the subsynchronous interactions can be classified
into: 1) torsional interaction, 2) network resonance, and 3) control interaction, as illustrated in
Figure 5. The revisited terms and classification are valid for the subsynchronous interaction in
both conventional and renewable generations.
– 14 – IEC TR 63401-2:2022 IEC 2022
Figure 4 – Classification of subsynchronous interaction based on the origin [12]
Figure 5 – Reclassification of subsynchronous interactions
based on the interaction mechanism
4.3.2 Torsional interaction
The torsional interaction is defined as the interaction in which the mechanical dynamics of the
generator shafts (either turbine generator or wind generators) interact with the converter
controls of power electronic devices, such as the HVDC, and FACTS.
Referring to Figure 4, the torsional interaction can occur due to the interaction between the
mechanical dynamics of any of the base components (such as traditional steam turbines, hydro
turbines with low generator turbine ratio, type 1 to 3 wind turbines, and large motors) interact
with any of the interactive components (such as fixed series compensation (SC), HVDC
converter, FACTS converter, power system stabilizers (PSSs), governor controls, and breaker
switching). This type of interaction covers all types of subsynchronous oscillations involving the
shaft dynamics of conventional turbine generators as well as WTGs. It covers the
shaft/torsional-related terms proposed in the IEEE's reader guide. The shaft or torsional
dynamics terms by the IEEE are TI, TA, and SSTI.
The frequency of oscillation caused by the torsional interaction is in the range of
subsynchronous frequenc
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