IEC TR 60919-3:2009
(Main)Performance of high-voltage direct current (HVDC) systems with line-commutated converters - Part 3: Dynamic conditions
Performance of high-voltage direct current (HVDC) systems with line-commutated converters - Part 3: Dynamic conditions
IEC/TR 60919-3:2009 provides general guidance on the dynamic performance of high-voltage direct current (HVDC) systems. Dynamic performance is meant to include those events and phenomena whose characteristic frequencies or time domain cover the range between transient conditions and steady state. Diode valves are not considered in this specification. This report (IEC 60919-3) which covers dynamic performance, is accompanied by publications for steady-state (IEC 60919-1) and transient (IEC 60919-2) performance. All three aspects should be considered when preparing two-terminal HVDC system specifications. This report should not be used directly as a specification for a specific project, but rather to provide the basis for an appropriate specification tailored to fit actual system requirements for a particular electric power transmission scheme. This report does not intend to discriminate between the responsibility of users and manufacturers for the work specified. This second edition cancels and replaces the first edition, which was issued as a technical specification in 1999. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) this report concerns only line-commutated converters;
b) significant changes have been made to the control system technology;
c) some environmental constraints, for example audible noise limits, have been added;
d) the capacitor coupled converters (CCC) and controlled series capacitor converters (CSCC) have been included.
Fonctionnement des systèmes à courant continu haute tension (CCHT) munis de convertisseurs commutés par le réseau - Partie 3: Conditions dynamiques
La CEI/TR 60919-3:2009 contient des indications générales sur le fonctionnement dynamique des réseaux à courant continu haute tension (CCHT). On utilise le terme fonctionnement dynamique pour désigner les événements et les phénomènes dont les fréquences caractéristiques ou la plage temporelle correspondent à un état entre des conditions transitoires et celles du régime établi. Les valves à diodes ne sont pas étudiées dans cette spécification. Le présent rapport (CEI 60919-3) qui porte sur le fonctionnement dynamique est accompagné de publications concernant le fonctionnement en régime établi (CEI 60919-1) et le fonctionnement dans des conditions transitoires (CEI 60919-2). Il convient de tenir compte des trois aspects lors de la préparation des spécifications d'un réseau CCHT à deux extrémités. Il convient de ne pas utiliser ce rapport comme la spécification d'un projet particulier, mais plutôt en tant que base dans la préparation d'une spécification plus appropriée, permettant de répondre aux exigences réelles d'un réseau pour un schéma particulier de transport d'énergie électrique. Ce rapport n'a pas pour objet de distinguer la responsabilité de l'utilisateur de celle du fabricant en ce qui concerne le projet spécifié. Cette deuxième édition annule et remplace la première édition parue comme spécification technique en 1999. Elle constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) ce rapport technique ne concerne que les convertisseurs commutés par le réseau;
b) des changements significatifs ont été apportés à la technologie du système de contrôle;
c) certaines contraintes environnementales ont été ajoutées, par exemple les niveaux de bruit audible maximaux;
d) les convertisseurs connectés au travers de condensateurs (CCC) et les convertisseurs à condensateurs en série contrôlés (CCSC) ont été ajoutés.
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IEC TR 60919-3 ®
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Performance of high-voltage direct current (HVDC) systems with line-
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Part 3: Dynamic conditions
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IEC TR 60919-3 ®
Edition 2.2 2021-10
CONSOLIDATED VERSION
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STANDARD
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Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.200; 29.240.99 ISBN 978-2-8322-4509-5
IEC TR 60919-3 ®
Edition 2.2 2021-10
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REDLINE VERSION
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Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
– 2 – IEC TR 60919-3:2009+AMD1:2016
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CONTENTS
1 Scope . 7
2 Normative references . 7
3 Outline of HVDC dynamic performance specifications. 8
3.1 Dynamic performance specification . 8
3.2 General comments . 9
4 AC system power flow and frequency control . 9
4.1 General . 9
4.2 Power flow control . 9
4.2.1 Steady-state power control requirements . 9
4.2.2 Step change power requirement . 10
4.3 Frequency control . 12
5 AC dynamic voltage control and interaction with reactive power sources . 13
5.1 General . 13
5.2 Voltage and reactive power characteristics of an HVDC substation and other
reactive power sources . 13
5.2.1 General . 13
5.2.2 Converter as active/reactive power source . 14
5.2.3 Voltage characteristics of a.c. networks depending on the power
loading at the busbar of the HVDC substation . 16
5.2.4 Voltage characteristics of a.c. filters, capacitor banks and shunt
reactors for power compensation at the HVDC substation. 18
5.2.5 Voltage characteristics of static var compensator (SVC) . 18
5.2.6 Voltage characteristics of synchronous compensator (SC) . 19
5.2.7 Voltage characteristics of static synchronous compensator
(STATCOM) . 19
5.3 Voltage deviations on the busbar of an HVDC substation . 19
5.4 Voltage and reactive power interaction of the substation and other reactive
power sources . 20
5.4.1 HVDC converters, switchable a.c. filters, capacitor banks and shunt
reactors . 20
5.4.2 HVDC converters, switchable reactive power sources, SVC . 21
5.4.3 HVDC converters, switchable reactive power sources and
synchronous compensators . 21
5.4.4 HVDC converters, switchable reactive power sources, STATCOM . 22
6 AC system transient and steady-state stability . 23
6.1 General . 23
6.2 Characteristics of active and reactive power modulation . 23
6.2.1 General . 23
6.2.2 Large signal modulation. 24
6.2.3 Small signal modulation . 25
6.2.4 Reactive power modulation . 25
6.3 Classification of network situations . 26
6.4 AC network in parallel with the HVDC link . 26
6.5 Improvement of the stability within one of the connected a.c. networks . 30
6.6 Determination of the damping control characteristics . 30
6.7 Implementation of the damping controller and telecommunication
requirements . 31
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7 Dynamics of the HVDC system at higher frequencies . 31
7.1 General . 31
7.2 Types of instability . 32
7.2.1 Loop instability (harmonic instability) . 32
7.2.2 Current loop instability . 32
7.2.3 Core saturation instability . 32
7.2.4 Harmonic interactions . 32
7.3 Information required for design purposes . 33
7.4 Means available for preventing instabilities . 34
7.5 Damping of low order harmonics by control action . 34
7.6 Demonstration of satisfactory performance at higher frequencies . 34
8 Subsynchronous oscillations . 35
8.1 General . 35
8.2 Criteria for subsynchronous torsional interaction with an HVDC system . 36
8.3 Screening criteria for identifying generator units susceptible to torsional
interactions . 37
8.4 Performance considerations for utilizing subsynchronous damping controls
controllers (SSDCs) . 38
8.5 Performance testing . 38
8.6 Turbine generator protection . 38
9 Power plant interaction . 39
9.1 General . 39
9.2 Specific interactions . 39
9.2.1 General . 39
9.2.2 Frequency variation effects . 39
9.2.3 Frequency controls interactions . 40
9.2.4 Overvoltage effects . 40
9.2.5 Harmonics . 40
9.2.6 Subsynchronous and shaft impact effects . 40
9.2.7 Resonance . 41
9.2.8 Overvoltages . 41
9.2.9 Stresses in a.c. switching equipment . 41
9.2.10 Under-frequency . 41
9.2.11 Starting procedure for an HVDC converter . 41
9.3 Special considerations for a nuclear plant . 41
Figure 1 – Elements for reactive power compensation at an HVDC substation . 14
Figure 2 – P/Q diagram of a converter . 15
Figure 3 – Reactive power requirements of a weak a.c. system depending on the active
power loading for various constant voltage characteristics at the a.c. bus of an HVDC
substation . 17
Figure 4 – Representation of the a.c. network . 17
Figure 5 – Example of voltage – current characteristic showing possible current
modulation range in the absence of telecommunication between rectifier and inverter . 25
Figure 6 – Reactive power modulation in an HVDC transmission operating at minimum
extinction angle γ . 27
min
Figure 7 – Reactive power modulation in an HVDC transmission operating at extinction
angle γ > γ . 28
min
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Figure 8 – Stability improvement of an a.c. link or network . 29
Figure 9 – Principle arrangements of a damping controller . 29
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INTERNATIONAL ELECTROTECHNICAL COMMISSION
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PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
FOREWORD
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This consolidated version of the official IEC Standard and its amendments has been
prepared for user convenience.
IEC TR 60919-3 edition 2.2 contains the second edition (2009-10) [documents
22F/183/DTR and 22F/192/RVC], its amendment 1 (2016-03) [documents 22F/376/DTR and
22F/382A/RVC] and its amendment 2 (2021-10) [documents 22F/634/DTR and
22F/654/RVDTR].
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendments 1 and 2. Additions are in green text, deletions are in
strikethrough red text. A separate Final version with all changes accepted is available
in this publication.
– 6 – IEC TR 60919-3:2009+AMD1:2016
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60919-3, which is a technical report, has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
This edition includes the following significant technical changes with respect to the previous
edition:
a) this report concerns only line-commutated converters;
b) significant changes have been made to the control system technology;
c) some environmental constraints, for example audible noise limits, have been added;
d) the capacitor coupled converters (CCC) and controlled series capacitor converters (CSCC)
have been included.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60919 series, under the general title: Performance of high-voltage
direct current (HVDC) systems with line-commutated converters, can be found on the IEC
website.
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the stability date indicated on the IEC web site under webstore.iec.ch
in the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
A bilingual version of this publication may be issued at a later date.
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PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
1 Scope
This Technical Report provides general guidance on the dynamic performance of high-voltage
direct current (HVDC) systems. Dynamic performance, as used in this specification, is meant
to include those events and phenomena whose characteristic frequencies or time domain
cover the range between transient conditions and steady state. It is concerned with the
dynamic performance due to interactions between two-terminal HVDC systems and related
a.c. systems or their elements such as power plants, a.c. lines and buses, reactive power
sources, etc. at steady-state or transient conditions. The two-terminal HVDC systems are
assumed to utilize 12-pulse converter units comprised of three-phase bridge (double way)
connections. The converters are assumed to use thyristor valves as bridge arms, with gapless
metal oxide arresters for insulation coordination and to have power flow capability in both
directions. Diode valves are not considered in this specification. While multi-terminal HVDC
transmission systems are not expressly considered, much of the information in this
specification is equally applicable to such systems.
Only line-commutated converters are covered in this report, which includes capacitor
commutated converter circuit configurations. General requirements for semiconductor line-
commutated converters are given in IEC 60146-1-1, IEC 60146-1-2 and IEC 60146-1-3.
Voltage-sourced converters are not considered.
This report (IEC 60919-3) which covers dynamic performance, is accompanied by publications
for steady-state (IEC 60919-1) and transient (IEC 60919-2) performance. All three aspects
should be considered when preparing two-terminal HVDC system specifications.
A difference exists between system performance specifications and equipment design
specifications for individual components of a system. While equipment specifications and
testing requirements are not defined herein, attention is drawn to those which would affect
performance specifications for a system. There are many possible variations between
different HVDC systems, therefore these are not considered in detail. This report should not
be used directly as a specification for a specific project, but rather to provide the basis for an
appropriate specification tailored to fit actual system requirements for a particular electric
power transmission scheme. This report does not intend to discriminate between the
responsibility of users and manufacturers for the work specified.
2 Normative references
The following referenced documents are indispensable for the application 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 60146-1-1, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specification of basic requirements
IEC/TR 60146-1-2, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-2: Application guide
IEC 60146-1-3, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-3: Transformers and reactors
– 8 – IEC TR 60919-3:2009+AMD1:2016
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IEC TR 60919-1:20052020, Performance of high-voltage direct current (HVDC) systems with
line-commutated converters – Part 1: Steady-state conditions
IEC TR 60919-2:2008, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 2: Faults and switching
IEC TR 60919-2:2008/AMD1:2015
IEC TR 60919-2:2008AMD2:2020
3 Outline of HVDC dynamic performance specifications
3.1 Dynamic performance specification
A complete dynamic performance specification for an HVDC system should consider the
following clauses:
– a.c. system power flow and frequency control (see Clause 4);
– a.c. dynamic voltage control and interaction with reactive power sources (see Clause 5);
– a.c. system transient and steady-state stability (see Clause 6);
– dynamics of the HVDC system at higher frequencies (see Clause 7);
– subsynchronous oscillations (see Clause 8);
– power plant interaction (see Clause 9).
Clause 4 deals with using active power control of the HVDC system to affect power flow
and/or frequency of related a.c. systems in order to improve the performance of such a.c.
systems. The following aspects should be considered at the design of HVDC active power
control modes:
a) to minimize the a.c. power system losses under steady-state operation;
b) to prevent a.c. line overload under steady-state operation and under a disturbance;
c) to coordinate with the a.c. generator governor control;
d) to suppress a.c. system frequency deviations under steady-state operation and under a
disturbance.
In Clause 5, the voltage and reactive power characteristics of the HVDC substation and other
reactive power sources (a.c. filters, capacitor banks, shunt reactors, SVC (static var
compensator), synchronous compensators) as well as interaction between them during control
of the a.c. bus voltage are considered.
In Clause 6, a discussion is provided concerning methods of controlling active and reactive
power of an HVDC link to improve the steady-state and/or transient stability of the
interconnected a.c. system by counteracting electromechanical oscillations.
Clause 7 deals with dynamic performance of an HVDC system in the range of half
fundamental frequency and above due to both characteristic and non-characteristic harmonics
generated by converters. Means for preventing instabilities are also discussed.
In Clause 8, the phenomenon of amplification of torsional, mechanical oscillations in turbine-
generators of a thermal power plant at their natural frequencies, due to interaction with an
HVDC control system (constant power and current regulation modes), is considered.
Specifications for subsynchronous damping control are defined.
The interaction between a power plant and an HVDC system located electrically near to it is
considered in Clause 9, taking into account some special features of the nuclear power plant
and requirements for the reliability of the HVDC system.
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3.2 General comments
Any design requirements for future HVDC systems being specified should fall within the
design limits covered in publications on steady-state (IEC 60919-1) and transient
(IEC 60919-2) performance. It is recommended that during preparation of the dynamic HVDC
system performance specification, the proper HVDC system control strategy should be
identified based on detailed power system studies. The priorities of control signal inputs and
the way they are processed should be specified.
4 AC system power flow and frequency control
4.1 General
Active power control of an HVDC system can be used to control the power flow and/or
frequency in related a.c. systems in order to improve the performance of a.c. systems in
steady-state operation and under disturbance.
In this clause, the HVDC active power operation modes, which are used to improve the a.c.
system performance for the following purposes, will be covered:
– HVDC power control to minimize the total power system losses under steady-state
operation;
– HVDC power control for prevention of a.c. line overload under a disturbance as well as
steady state;
– coordinated HVDC power control with an a.c. system generator governor control;
– HVDC power control for suppression of an a.c. system frequency deviation under a
disturbance as well as steady state.
HVDC active and/or reactive power modes used to improve a.c. system dynamic and transient
stability or improve a.c. voltage control is discussed in Clauses 5 and 6.
4.2 Power flow control
4.2.1 Steady-state power control requirements
The power of an HVDC system is sometimes controlled to minimize overall power system
losses, to prevent a.c. line overloading, and to coordinate with the governor control of a.c.
system generators. Such power control requirements differ from time to time according to the
role of HVDC systems in the overall power system.
When an HVDC system is used to transmit power from remote generating stations, the HVDC
transmission power control is coordinated with the governor control of the power station
generators. In this case, the generator voltage, frequency or the rotor speed may be used as
a reference to the HVDC power control system.
When two a.c. power systems are connected by an HVDC link, the HVDC power is controlled
to a pre-determined pattern under normal circumstances, but an additional function can be
incorporated to this HVDC power control so that the frequency of either or both a.c. power
systems is controlled. When one of the a.c. systems is an isolated system, such as one
supplying a separate island, frequency control of this isolated a.c. system may have to be
realized by the HVDC system.
The a.c. system frequency control by an HVDC system is discussed in 4.3.
When two a.c. systems are interconnected by more than one d.c. link or d.c. and a.c. links, or
when a d.c. system exists within an a.c. system, HVDC power may be controlled in order to
minimize the total transmission losses of the interconnected systems.
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In some cases of a.c./d.c. system configurations described above, the HVDC power change
control can be used to prevent overloading of one or more transmission lines in the power
system.
In certain special HVDC control schemes, such as the one designed to improve a.c. system
performance by increasing the d.c. power during and after a disturbance, the steady-state d.c.
transmission power may have to be set at a restricted value so that the d.c. power does not
exceed the d.c. rated power, including overload capability, when the control is initiated. It is
important to consider also the additional reactive power supply required both by the HVDC
converters and the a.c. systems in such a situation.
The following items a) to g) need to be considered in the specification of steady-state control
requirements. Note that at the time of preparing the specification, the complete steady-state
control requirements may not have been determined or designed, but allowance for possible
future inputs is necessary.
a) When a power flow control system is designed to have more than one function, including
the a.c. system frequency control, the HVDC control system should be so designed that
priorities are set between each control function.
b) Under steady-state conditions, the control for prevention of a.c. line overloading is usually
given higher priority over other power flow controls. The control for minimization of power
system losses is implemented either by setting the d.c. power to a pattern which has been
pre-determined by the power system data, or in response to an on-line computation which
is conducted in the central load dispatching office. Usually, its control response is
relatively slow, being several seconds or several minutes, even in the latter case.
c) In isolated systems or systems with a relatively large d.c. infeed, frequency is often
maintained by the HVDC power. In such a case, HVDC frequency control could have a
priority over system loss minimization, but may be limited by overload protection.
d) The change in reactive power demand accompanying the power changes may result in
frequent switching of reactive power equipment. In such a case, it is necessary to figure
out particular a.c. voltage control measures such as reactive power control by converter
units, or to set limits of the magnitude of HVDC power change.
e) The need for special power order adjustment signals unique to the power system should
be identified, studied, and specified. The signals cannot be permitted to cause d.c. current
or power, or a.c. voltage to deviate beyond equipment and system ratings and limits. The
priority of two or more input signals having simultaneous demand on d.c. link power
should be established and coordinated.
f) Bipolar d.c. links normally require that d.c. power and current be effectively shared
between poles. For loss of one pole, an overload strategy for the remaining pole could be
developed to minimize disruption to a.c. system power flow, voltage and frequency.
g) Disruption of the telecommunication link between the sending and receiving system of the
d.c. link should not cause disruption to the a.c. power system. A minimum specification
requirement is that power transmission is maintained at the same power level which
existed before the telecommunication failure. If additional functions such as frequency
control are required during temporary outage of the telecommunication link, these should
be specified.
4.2.2 Step change power requirement
Under certain power system conditions, it may be required to change the HVDC power in
steps in order to improve the performance of a.c. systems during and after power system
disturbances. Under certain circumstances, the step change may involve d.c. power reversal.
A step change of d.c. power is realized by changing the set value of d.c. power order or by
changing the power range in response to an input signal. The rate of change of power and
limit to the magnitude of the d.c. power change demanded by the step change should be
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adjustable within specified limits according to a.c. system requirements. For example,
different ramp rates may be required for different events. Special considerations may be
required when the step change would include power reversal.
Power system disturbances to be considered in specifying d.c. power step changes may
include: a.c. line trip, loss of large power supply source or large drop in a.c. system frequency
and sudden increase or decrease of power system load with its corresponding large frequency
deviation.
In some of the above cases of power system disturbances, the a.c. systems will also be
supported by the a.c. frequency control provided by the d.c. system.
In specifying and designing HVDC control functions, the effects of the step change power
functions should be surveyed in detail for various power system conditions. It is best to
specify limits and ranges for power changes and ramp rates rather than specific settings.
Setting adjustment can be made with the d.c. system in operation.
The signals for initiation of HVDC step power changes include overload relay signals or trip
signals of particular transmission lines which are transmitted to the HVDC substation, or a.c.
system frequency which is detected at the HVDC substation or at some point in a.c. systems.
The time delay involved in a telecommunication system which transmits these initiation
signals may affect the a.c. or d.c. system performance. Therefore, in some cases, a high
speed telecommunication system may be required. When the transmission delay time is large,
this effect should be taken into account.
There are some cases in which signals are sent to both HVDC substations, or more than one
signal is received by an HVDC substation. In these cases, it is necessary to set priorities of
control functions.
The magnitude of d.c. power step change may be limited by a.c. and d.c. system conditions,
and it may be required under certain circumstances to detect the changes in system
conditions to update the values of such limits.
In particular, when there is a large step change in d.c. power, the a.c. voltage may change
substantially. For this reason, it may be required to study the allowable range of a.c. voltage
fluctuation to determine the limits on step power changes, or introduce special a.c. voltage
control measures.
The allowable limits of a.c. voltage deviation can be different for steady-state operation and
transient conditions and should be specified.
When an HVDC system is connected to a high impedance and/or low inertia a.c. system, the
step change in d.c. power may have adverse effects on the voltage stability, transient
stability, and frequency of the a.c. system. In such cases, the magnitude and rate of change
of power may have to be limited, or other special measures may have to be provided, to
prevent deterioration of the a.c. system dynamic performance. When an HVDC system
interconnects two a.c. systems, the effect of d.c. power step change must be evaluated in
detail not only for the a.c. system in which a disturbance occurs, but also for the other a.c.
system in which a fault does not occur.
When the d.c. step change of power causes the d.c. current to fall below the minimum
allowable operational current of the HVDC system, which is usually 5 % to 10 % of the rated
current, the converter operation should be set to the positive minimum current. Otherwise the
converter should be blocked after the allowable period of low current operation, or be
specified to operate down to zero current. One possible measure to overcome minimum
allowable operational current is to set the power flows of two poles in opposite direction and
let the power flow of two poles cancel each other when the HVDC system configuration is
– 12 – IEC TR 60919-3:2009+AMD1:2016
+AMD2:2021 CSV IEC 2021
bipolar. The difference in the power flows of each pole is the actual operating power flow of
the overall HVDC system.
Because of inverter control limitations and possible risks to a.c. system operation, it is not
advisable to request a current order step change larger than the current margin unless special
control actions are taken upon loss of telecommunications.
Certain considerations may be required when an HVDC system is to be started up from a no
load stand-by state in response to a step change power order (see Clause 7 of IEC 60919-1).
4.3 Frequency control
The a.c. system frequency control by the HVDC system can be applied for the following
purposes:
a) frequency control of the receiving and/or sending end a.c. system for a d.c. transmission
from remote power sources;
b) frequency control of an a.c. system in an isolated island or a small a.c. system when it is
interconnected to a large a.c. system through a d.c. system;
c) frequency control of either of the a.c. systems interconnected by an HVDC system, also
taking the frequency of the other system into account.
The a.c. system frequency control is executed either as a continuous function of frequency
under steady-state conditions, or when the frequency deviation of the a.c. system exceeds
certain limits. It may only be activated under certain circumstances such as when the local
a.c. system connected to the HVDC substation is disconnected (islanded) from the main a.c.
system. Accordingly, the specification should state the duties and performance requirements
of the frequency control function.
If the frequency at the receiving end is controlled by varying or modulating the power
transmitted by the d.c. link, there must be coordination of the d.c. link frequency control with
any governor control on associated a.c. generators. It may be possible to use transient
frequency deviation capability of an asynchronous sending end system for support of the
receiving end, provided the a.c. generating equipment is designed accordingly.
When an HVDC substation is electrically far from the centre of the a.c. system, the phase
angle of a.c. voltage at the HVDC substation changes substantially with power changes. In
such circumstances, the speed of response of the frequency signal can be reduced. To avoid
this lower speed of response, the frequency signal can be detected at the centre of the a.c.
system and transmitted to the HVDC substation.
In frequency control it may be required to provide limits of power change and rate of power
change which maintains the a.c. system voltage fluctuation within an allowable range, or
utilize special voltage regulation measures such as reactive power control by converters or
SVC. The allowable limits of voltage fluctuation during steady state frequency control should
be specified.
When the d.c. contribution to a.c. system frequency control is implemented, it is possible that
generator frequency control is degraded unless the controls are properly coordinated. When
two different power systems are interconnected, it may be required to provide appropriate
dead band or to select suitable gain in the frequency control by the HVDC system so that only
large or fast frequency fluctuations are compensated by the d.c. power control, and small or
slow frequency fluctuations are controlled by the power stations belonging to the individual
a.c. systems.
The frequency control designed to correct for severe disturbances, such as those caused by
the tripping of large generation units, may be realized more effectively if the generator unit
trip signal is transmitted to the HVDC substation to initiate the control action.
+AMD2:2021 CSV IEC 2021
Fast and large magnitude of d.c. power change for frequency control may produce
overvoltage or voltage dip in the a.c. systems. Such a situation may be relieved by limiting the
rate of power change or by fast reactive power compensations. The allowable overvoltage or
voltage dip, and the allowable duration time should be specified.
One possible measure for continuous and smooth operation of frequency control is to set the
power flows of two poles in opposite direction and let the power flow of two poles cancel each
other when the HVDC system configuration is bipolar. This special operation mode is called
“Frequency control with zero power setting.” However, note that there is additional system
loss and accompanying polarity reversals, which happen when crossing the border of
minimum current.
It is sometimes difficult to set optimal parameters of frequency control since the power system
configuration often changes due to outages of transmission lines and/or substations for
maintenance. This could be accounted for by adopting multi variable frequency control.
When d.c. power control is performed for the purpose of frequency control, it is usually
necessary to provide high speed telecommunication channels, such as a microwave channels
or fiber optic channels, between two HVDC substations. In case of loss of telecommunication
between the two substations, frequency control is usually limited to the network connected to
the current controlling substation.
When the frequency detection point is located far from the HVDC substation control terminal,
or when it is intended to initiate the frequency control action by special signals issued from
the a.c. system, telecommunication channels are required.
In any case, the effect of telecommunication time delay should be taken into account.
For a discussion of telecommunication channels, refer to Clause 13 of IEC 60919-1.
5 AC dynamic voltage control and interaction with reactive power sources
5.1 General
Change in reactive power flow due to load change, switching operations or faults produce
voltage fluctuations in the a.c. network. In high impedance a.c. systems, i.e. in systems with
low short-circuit capacity, larger voltage fluctuation can be expected and the need for voltage
control is more pronounced.
Sudden voltage changes in the network should be limited e.g. to less than 3 % if occurring
frequently and to less than 10 % if happening seldom. Appropriate values should be specified.
High temporary overvoltages in excess of the normal operating range due to large load
changes and load r
...
IEC TR 60919-3 ®
Edition 2.1 2016-03
CONSOLIDATED VERSION
TECHNICAL
REPORT
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Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
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IEC TR 60919-3 ®
Edition 2.1 2016-03
CONSOLIDATED VERSION
TECHNICAL
REPORT
colour
inside
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.200; 29.240.99 ISBN 978-2-8322-3241-5
IEC TR 60919-3 ®
Edition 2.1 2016-03
CONSOLIDATED VERSION
REDLINE VERSION
colour
inside
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
– 2 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
CONTENTS
FOREWORD. 5
1 Scope . 7
2 Normative references . 7
3 Outline of HVDC dynamic performance specifications . 8
3.1 Dynamic performance specification . 8
3.2 General comments . 9
4 AC system power flow and frequency control . 9
4.1 General . 9
4.2 Power flow control . 9
4.2.1 Steady-state power control requirements . 9
4.2.2 Step change power requirement . 10
4.3 Frequency control . 12
5 AC dynamic voltage control and interaction with reactive power sources . 13
5.1 General . 13
5.2 Voltage and reactive power characteristics of an HVDC substation and other
reactive power sources . 13
5.2.1 General . 13
5.2.2 Converter as active/reactive power source . 14
5.2.3 Voltage characteristics of a.c. networks depending on the power
loading at the busbar of the HVDC substation . 16
5.2.4 Voltage characteristics of a.c. filters, capacitor banks and shunt
reactors for power compensation at the HVDC substation . 18
5.2.5 Voltage characteristics of static var compensator (SVC) . 18
5.2.6 Voltage characteristics of synchronous compensator (SC) . 19
5.2.7 Voltage characteristics of static synchronous compensator
(STATCOM) . 19
5.3 Voltage deviations on the busbar of an HVDC substation . 19
5.4 Voltage and reactive power interaction of the substation and other reactive
power sources . 20
5.4.1 HVDC converters, switchable a.c. filters, capacitor banks and shunt
reactors . 20
5.4.2 HVDC converters, switchable reactive power sources, SVC . 21
5.4.3 HVDC converters, switchable reactive power sources and
synchronous compensators . 21
5.4.4 HVDC converters, switchable reactive power sources, STATCOM . 22
6 AC system transient and steady-state stability . 23
6.1 General . 23
6.2 Characteristics of active and reactive power modulation . 23
6.2.1 General . 23
6.2.2 Large signal modulation . 24
6.2.3 Small signal modulation . 25
6.2.4 Reactive power modulation . 25
6.3 Classification of network situations . 26
6.4 AC network in parallel with the HVDC link . 26
6.5 Improvement of the stability within one of the connected a.c. networks . 30
6.6 Determination of the damping control characteristics . 30
IEC 2016
6.7 Implementation of the damping controller and telecommunication
requirements . 31
7 Dynamics of the HVDC system at higher frequencies . 31
7.1 General . 31
7.2 Types of instability . 32
7.2.1 Loop instability (harmonic instability) . 32
7.2.2 Current loop instability . 32
7.2.3 Core saturation instability . 32
7.2.4 Harmonic interactions . 32
7.3 Information required for design purposes . 33
7.4 Means available for preventing instabilities . 34
7.5 Damping of low order harmonics by control action . 34
7.6 Demonstration of satisfactory performance at higher frequencies . 34
8 Subsynchronous oscillations . 35
8.1 General . 35
8.2 Criteria for subsynchronous torsional interaction with an HVDC system . 36
8.3 Screening criteria for identifying generator units susceptible to torsional
interactions . 37
8.4 Performance considerations for utilizing subsynchronous damping controls
controllers (SSDCs) . 38
8.5 Performance testing . 38
8.6 Turbine generator protection . 38
9 Power plant interaction . 39
9.1 General . 39
9.2 Specific interactions . 39
9.2.1 General . 39
9.2.2 Frequency variation effects . 39
9.2.3 Frequency controls interactions . 40
9.2.4 Overvoltage effects . 40
9.2.5 Harmonics . 40
9.2.6 Subsynchronous and shaft impact effects . 40
9.2.7 Resonance . 41
9.2.8 Overvoltages . 41
9.2.9 Stresses in a.c. switching equipment . 41
9.2.10 Under-frequency . 41
9.2.11 Starting procedure for an HVDC converter . 41
9.3 Special considerations for a nuclear plant . 41
Bibliography . 42
Figure 1 – Elements for reactive power compensation at an HVDC substation . 14
Figure 2 – P/Q diagram of a converter . 15
Figure 3 – Reactive power requirements of a weak a.c. system depending on the active
power loading for various constant voltage characteristics at the a.c. bus of an HVDC
substation . 17
Figure 4 – Representation of the a.c. network . 17
Figure 5 – Example of voltage – current characteristic showing possible current
modulation range in the absence of telecommunication between rectifier and inverter . 25
Figure 6 – Reactive power modulation in an HVDC transmission operating at minimum
extinction angle γ . 27
min
– 4 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
Figure 7 – Reactive power modulation in an HVDC transmission operating at extinction
angle γ > γ . 28
min
Figure 8 – Stability improvement of an a.c. link or network . 29
Figure 9 – Principle arrangements of a damping controller . 29
IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This consolidated version of the official IEC Standard and its amendment has been prepared
for user convenience.
IEC TR 60919-3 edition 2.1 contains the second edition (2009-10) [documents 22F/183/DTR and
22F/192/RVC] and its amendment 1 (2016-03) [documents 22F/376/DTR and 22F/382A/RVC].
In this Redline version, a vertical line in the margin shows where the technical
content is modified by amendment 1. Additions are in green text, deletions are in
strikethrough red text. A separate Final version with all changes accepted is
available in this publication.
– 6 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60919-3, which is a technical report, has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
This edition includes the following significant technical changes with respect to the previous
edition:
a) this report concerns only line-commutated converters;
b) significant changes have been made to the control system technology;
c) some environmental constraints, for example audible noise limits, have been added;
d) the capacitor coupled converters (CCC) and controlled series capacitor converters (CSCC)
have been included.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60919 series, under the general title: Performance of high-voltage
direct current (HVDC) systems with line-commutated converters, can be found on the IEC
website.
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
A bilingual version of this publication may be issued at a later date.
IEC 2016
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
1 Scope
This Technical Report provides general guidance on the dynamic performance of high-voltage
direct current (HVDC) systems. Dynamic performance, as used in this specification, is meant
to include those events and phenomena whose characteristic frequencies or time domain
cover the range between transient conditions and steady state. It is concerned with the
dynamic performance due to interactions between two-terminal HVDC systems and related
a.c. systems or their elements such as power plants, a.c. lines and buses, reactive power
sources, etc. at steady-state or transient conditions. The two-terminal HVDC systems are
assumed to utilize 12-pulse converter units comprised of three-phase bridge (double way)
connections. The converters are assumed to use thyristor valves as bridge arms, with gapless
metal oxide arresters for insulation coordination and to have power flow capability in both
directions. Diode valves are not considered in this specification. While multi-terminal HVDC
transmission systems are not expressly considered, much of the information in this
specification is equally applicable to such systems.
Only line-commutated converters are covered in this report, which includes capacitor
commutated converter circuit configurations. General requirements for semiconductor line-
commutated converters are given in IEC 60146-1-1, IEC 60146-1-2 and IEC 60146-1-3.
Voltage-sourced converters are not considered.
This report (IEC 60919-3) which covers dynamic performance, is accompanied by publications
for steady-state (IEC 60919-1) and transient (IEC 60919-2) performance. All three aspects
should be considered when preparing two-terminal HVDC system specifications.
A difference exists between system performance specifications and equipment design
specifications for individual components of a system. While equipment specifications and
testing requirements are not defined herein, attention is drawn to those which would affect
performance specifications for a system. There are many possible variations between
different HVDC systems, therefore these are not considered in detail. This report should not
be used directly as a specification for a specific project, but rather to provide the basis for an
appropriate specification tailored to fit actual system requirements for a particular electric
power transmission scheme. This report does not intend to discriminate between the
responsibility of users and manufacturers for the work specified.
2 Normative references
The following referenced documents are indispensable for the application 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 60146-1-1, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specification of basic requirements
IEC/TR 60146-1-2, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-2: Application guide
IEC 60146-1-3, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-3: Transformers and reactors
– 8 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
IEC TR 60919-1:2005 2010, Performance of high-voltage direct current (HVDC) systems with
line-commutated converters – Part 1: Steady-state conditions
IEC TR 60919-1:2010/AMD1:2013
IEC TR 60919-2:2008, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 2: Faults and switching
IEC TR 60919-2:2008/AMD1:2015
3 Outline of HVDC dynamic performance specifications
3.1 Dynamic performance specification
A complete dynamic performance specification for an HVDC system should consider the
following clauses:
– a.c. system power flow and frequency control (see Clause 4);
– a.c. dynamic voltage control and interaction with reactive power sources (see Clause 5);
– a.c. system transient and steady-state stability (see Clause 6);
– dynamics of the HVDC system at higher frequencies (see Clause 7);
– subsynchronous oscillations (see Clause 8);
– power plant interaction (see Clause 9).
Clause 4 deals with using active power control of the HVDC system to affect power flow
and/or frequency of related a.c. systems in order to improve the performance of such a.c.
systems. The following aspects should be considered at the design of HVDC active power
control modes:
a) to minimize the a.c. power system losses under steady-state operation;
b) to prevent a.c. line overload under steady-state operation and under a disturbance;
c) to coordinate with the a.c. generator governor control;
d) to suppress a.c. system frequency deviations under steady-state operation and under a
disturbance.
In Clause 5, the voltage and reactive power characteristics of the HVDC substation and other
reactive power sources (a.c. filters, capacitor banks, shunt reactors, SVC (static var
compensator), synchronous compensators) as well as interaction between them during control
of the a.c. bus voltage are considered.
In Clause 6, a discussion is provided concerning methods of controlling active and reactive
power of an HVDC link to improve the steady-state and/or transient stability of the
interconnected a.c. system by counteracting electromechanical oscillations.
Clause 7 deals with dynamic performance of an HVDC system in the range of half
fundamental frequency and above due to both characteristic and non-characteristic harmonics
generated by converters. Means for preventing instabilities are also discussed.
In Clause 8, the phenomenon of amplification of torsional, mechanical oscillations in turbine-
generators of a thermal power plant at their natural frequencies, due to interaction with an
HVDC control system (constant power and current regulation modes), is considered.
Specifications for subsynchronous damping control are defined.
The interaction between a power plant and an HVDC system located electrically near to it is
considered in Clause 9, taking into account some special features of the nuclear power plant
and requirements for the reliability of the HVDC system.
IEC 2016
3.2 General comments
Any design requirements for future HVDC systems being specified should fall within the
design limits covered in publications on steady-state (IEC 60919-1) and transient
(IEC 60919-2) performance. It is recommended that during preparation of the dynamic HVDC
system performance specification, the proper HVDC system control strategy should be
identified based on detailed power system studies. The priorities of control signal inputs and
the way they are processed should be specified.
4 AC system power flow and frequency control
4.1 General
Active power control of an HVDC system can be used to control the power flow and/or
frequency in related a.c. systems in order to improve the performance of a.c. systems in
steady-state operation and under disturbance.
In this clause, the HVDC active power operation modes, which are used to improve the a.c.
system performance for the following purposes, will be covered:
– HVDC power control to minimize the total power system losses under steady-state
operation;
– HVDC power control for prevention of a.c. line overload under a disturbance as well as
steady state;
– coordinated HVDC power control with an a.c. system generator governor control;
– HVDC power control for suppression of an a.c. system frequency deviation under a
disturbance as well as steady state.
HVDC active and/or reactive power modes used to improve a.c. system dynamic and transient
stability or improve a.c. voltage control is discussed in Clauses 5 and 6.
4.2 Power flow control
4.2.1 Steady-state power control requirements
The power of an HVDC system is sometimes controlled to minimize overall power system
losses, to prevent a.c. line overloading, and to coordinate with the governor control of a.c.
system generators. Such power control requirements differ from time to time according to the
role of HVDC systems in the overall power system.
When an HVDC system is used to transmit power from remote generating stations, the HVDC
transmission power control is coordinated with the governor control of the power station
generators. In this case, the generator voltage, frequency or the rotor speed may be used as
a reference to the HVDC power control system.
When two a.c. power systems are connected by an HVDC link, the HVDC power is controlled
to a pre-determined pattern under normal circumstances, but an additional function can be
incorporated to this HVDC power control so that the frequency of either or both a.c. power
systems is controlled. When one of the a.c. systems is an isolated system, such as one
supplying a separate island, frequency control of this isolated a.c. system may have to be
realized by the HVDC system.
The a.c. system frequency control by an HVDC system is discussed in 4.3.
When two a.c. systems are interconnected by more than one d.c. link or d.c. and a.c. links, or
when a d.c. system exists within an a.c. system, HVDC power may be controlled in order to
minimize the total transmission losses of the interconnected systems.
– 10 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
In some cases of a.c./d.c. system configurations described above, the HVDC power change
control can be used to prevent overloading of one or more transmission lines in the power
system.
In certain special HVDC control schemes, such as the one designed to improve a.c. system
performance by increasing the d.c. power during and after a disturbance, the steady-state d.c.
transmission power may have to be set at a restricted value so that the d.c. power does not
exceed the d.c. rated power, including overload capability, when the control is initiated. It is
important to consider also the additional reactive power supply required both by the HVDC
converters and the a.c. systems in such a situation.
The following items a) to g) need to be considered in the specification of steady-state control
requirements. Note that at the time of preparing the specification, the complete steady-state
control requirements may not have been determined or designed, but allowance for possible
future inputs is necessary.
a) When a power flow control system is designed to have more than one function, including
the a.c. system frequency control, the HVDC control system should be so designed that
priorities are set between each control function.
b) Under steady-state conditions, the control for prevention of a.c. line overloading is usually
given higher priority over other power flow controls. The control for minimization of power
system losses is implemented either by setting the d.c. power to a pattern which has been
pre-determined by the power system data, or in response to an on-line computation which
is conducted in the central load dispatching office. Usually, its control response is
relatively slow, being several seconds or several minutes, even in the latter case.
c) In isolated systems or systems with a relatively large d.c. infeed, frequency is often
maintained by the HVDC power. In such a case, HVDC frequency control could have a
priority over system loss minimization, but may be limited by overload protection.
d) The change in reactive power demand accompanying the power changes may result in
frequent switching of reactive power equipment. In such a case, it is necessary to figure
out particular a.c. voltage control measures such as reactive power control by converter
units, or to set limits of the magnitude of HVDC power change.
e) The need for special power order adjustment signals unique to the power system should
be identified, studied, and specified. The signals cannot be permitted to cause d.c. current
or power, or a.c. voltage to deviate beyond equipment and system ratings and limits. The
priority of two or more input signals having simultaneous demand on d.c. link power
should be established and coordinated.
f) Bipolar d.c. links normally require that d.c. power and current be effectively shared
between poles. For loss of one pole, an overload strategy for the remaining pole could be
developed to minimize disruption to a.c. system power flow, voltage and frequency.
g) Disruption of the telecommunication link between the sending and receiving system of the
d.c. link should not cause disruption to the a.c. power system. A minimum specification
requirement is that power transmission is maintained at the same power level which
existed before the telecommunication failure. If additional functions such as frequency
control are required during temporary outage of the telecommunication link, these should
be specified.
4.2.2 Step change power requirement
Under certain power system conditions, it may be required to change the HVDC power in
steps in order to improve the performance of a.c. systems during and after power system
disturbances. Under certain circumstances, the step change may involve d.c. power reversal.
A step change of d.c. power is realized by changing the set value of d.c. power order or by
changing the power range in response to an input signal. The rate of change of power and
limit to the magnitude of the d.c. power change demanded by the step change should be
IEC 2016
adjustable within specified limits according to a.c. system requirements. For example,
different ramp rates may be required for different events. Special considerations may be
required when the step change would include power reversal.
Power system disturbances to be considered in specifying d.c. power step changes may
include: a.c. line trip, loss of large power supply source or large drop in a.c. system frequency
and sudden increase or decrease of power system load with its corresponding large frequency
deviation.
In some of the above cases of power system disturbances, the a.c. systems will also be
supported by the a.c. frequency control provided by the d.c. system.
In specifying and designing HVDC control functions, the effects of the step change power
functions should be surveyed in detail for various power system conditions. It is best to
specify limits and ranges for power changes and ramp rates rather than specific settings.
Setting adjustment can be made with the d.c. system in operation.
The signals for initiation of HVDC step power changes include overload relay signals or trip
signals of particular transmission lines which are transmitted to the HVDC substation, or a.c.
system frequency which is detected at the HVDC substation or at some point in a.c. systems.
The time delay involved in a telecommunication system which transmits these initiation
signals may affect the a.c. or d.c. system performance. Therefore, in some cases, a high
speed telecommunication system may be required. When the transmission delay time is large,
this effect should be taken into account.
There are some cases in which signals are sent to both HVDC substations, or more than one
signal is received by an HVDC substation. In these cases, it is necessary to set priorities of
control functions.
The magnitude of d.c. power step change may be limited by a.c. and d.c. system conditions,
and it may be required under certain circumstances to detect the changes in system
conditions to update the values of such limits.
In particular, when there is a large step change in d.c. power, the a.c. voltage may change
substantially. For this reason, it may be required to study the allowable range of a.c. voltage
fluctuation to determine the limits on step power changes, or introduce special a.c. voltage
control measures.
The allowable limits of a.c. voltage deviation can be different for steady-state operation and
transient conditions and should be specified.
When an HVDC system is connected to a high impedance and/or low inertia a.c. system, the
step change in d.c. power may have adverse effects on the voltage stability, transient
stability, and frequency of the a.c. system. In such cases, the magnitude and rate of change
of power may have to be limited, or other special measures may have to be provided, to
prevent deterioration of the a.c. system dynamic performance. When an HVDC system
interconnects two a.c. systems, the effect of d.c. power step change must be evaluated in
detail not only for the a.c. system in which a disturbance occurs, but also for the other a.c.
system in which a fault does not occur.
When the d.c. step change of power causes the d.c. current to fall below the minimum
allowable operational current of the HVDC system, which is usually 5 % to 10 % of the rated
current, the converter operation should be set to the positive minimum current. Otherwise the
converter should be blocked after the allowable period of low current operation, or be
specified to operate down to zero current. One possible measure to overcome minimum
allowable operational current is to set the power flows of two poles in opposite direction and
let the power flow of two poles cancel each other when the HVDC system configuration is
– 12 – IEC TR 60919-3:2009+AMD1:2016 CSV
IEC 2016
bipolar. The difference in the power flows of each pole is the actual operating power flow of
the overall HVDC system.
Because of inverter control limitations and possible risks to a.c. system operation, it is not
advisable to request a current order step change larger than the current margin unless special
control actions are taken upon loss of telecommunications.
Certain considerations may be required when an HVDC system is to be started up from a no
load stand-by state in response to a step change power order (see Clause 7 of IEC 60919-1).
4.3 Frequency control
The a.c. system frequency control by the HVDC system can be applied for the following
purposes:
a) frequency control of the receiving and/or sending end a.c. system for a d.c. transmission
from remote power sources;
b) frequency control of an a.c. system in an isolated island or a small a.c. system when it is
interconnected to a large a.c. system through a d.c. system;
c) frequency control of either of the a.c. systems interconnected by an HVDC system, also
taking the frequency of the other system into account.
The a.c. system frequency control is executed either as a continuous function of frequency
under steady-state conditions, or when the frequency deviation of the a.c. system exceeds
certain limits. It may only be activated under certain circumstances such as when the local
a.c. system connected to the HVDC substation is disconnected (islanded) from the main a.c.
system. Accordingly, the specification should state the duties and performance requirements
of the frequency control function.
If the frequency at the receiving end is controlled by varying or modulating the power
transmitted by the d.c. link, there must be coordination of the d.c. link frequency control with
any governor control on associated a.c. generators. It may be possible to use transient
frequency deviation capability of an asynchronous sending end system for support of the
receiving end, provided the a.c. generating equipment is designed accordingly.
When an HVDC substation is electrically far from the centre of the a.c. system, the phase
angle of a.c. voltage at the HVDC substation changes substantially with power changes. In
such circumstances, the speed of response of the frequency signal can be reduced. To avoid
this lower speed of response, the frequency signal can be detected at the centre of the a.c.
system and transmitted to the HVDC substation.
In frequency control it may be required to provide limits of power change and rate of power
change which maintains the a.c. system voltage fluctuation within an allowable range, or
utilize special voltage regulation measures such as reactive power control by converters or
SVC. The allowable limits of voltage fluctuation during steady state frequency control should
be specified.
When the d.c. contribution to a.c. system frequency control is implemented, it is possible that
generator frequency control is degraded unless the controls are properly coordinated. When
two different power systems are interconnected, it may be required to provide appropriate
dead band or to select suitable gain in the frequency control by the HVDC system so that only
large or fast frequency fluctuations are compensated by the d.c. power control, and small or
slow frequency fluctuations are controlled by the power stations belonging to the individual
a.c. systems.
The frequency control designed to correct for severe disturbances, such as those caused by
the tripping of large generation units, may be realized more effectively if the generator unit
trip signal is transmitted to the HVDC substation to initiate the control action.
IEC 2016
Fast and large magnitude of d.c. power change for frequency control may produce
overvoltage or voltage dip in the a.c. systems. Such a situation may be relieved by limiting the
rate of power change or by fast reactive power compensations. The allowable overvoltage or
voltage dip, and the allowable duration time should be specified.
One possible measure for continuous and smooth operation of frequency control is to set the
power flows of two poles in opposite direction and let the power flow of two poles cancel each
other when the HVDC system configuration is bipolar. This special operation mode is called
“Frequency control with zero power setting.” However, note that there is additional system
loss and accompanying polarity reversals, which happen when crossing the border of
minimum current.
It is sometimes difficult to set optimal parameters of frequency control since the power system
configuration often changes due to outages of transmission lines and/or substations for
maintenance. This could be accounted for by adopting multi variable frequency control.
When d.c. power control is performed for the purpose of frequency control, it is usually
necessary to provide high speed telecommunication channels, such as a microwave channels
or fiber optic channels, between two HVDC substations. In case of loss of telecommunication
between the two substations, frequency control is usually limited to the network connected to
the current controlling substation.
When the frequency detection point is located far from the HVDC substation control terminal,
or when it is intended to initiate the frequency control action by special signals issued from
the a.c. system, telecommunication channels are required.
In any case, the effect of telecommunication time delay should be taken into account.
For a discussion of telecommunication channels, refer to Clause 13 of IEC 60919-1.
5 AC dynamic voltage control and interaction with reactive power sources
5.1 General
Change in reactive power flow due to load change, switching operations or faults produce
voltage fluctuations in the a.c. network. In high impedance a.c. systems, i.e. in systems with
low sh
...
IEC/TR 60919-3 ®
Edition 2.0 2009-10
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
Fonctionnement des systèmes à courant continu haute tension (CCHT) munis
de convertisseurs commutés par le réseau –
Partie 3: Conditions dynamiques
IEC/TR 60919-3:2009
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IEC/TR 60919-3 ®
Edition 2.0 2009-10
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 3: Dynamic conditions
Fonctionnement des systèmes à courant continu haute tension (CCHT) munis
de convertisseurs commutés par le réseau –
Partie 3: Conditions dynamiques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
X
CODE PRIX
ICS 29.200; 29.240.99 ISBN 978-2-88910-332-4
– 2 – TR 60919-3 © IEC:2009
CONTENTS
FOREWORD.4
1 Scope.6
2 Normative references .6
3 Outline of HVDC dynamic performance specifications.7
3.1 Dynamic performance specification .7
3.2 General comments .8
4 AC system power flow and frequency control.8
4.1 General .8
4.2 Power flow control.8
4.2.1 Steady-state power control requirements.8
4.2.2 Step change power requirement .9
4.3 Frequency control .11
5 AC dynamic voltage control and interaction with reactive power sources .12
5.1 General .12
5.2 Voltage and reactive power characteristics of an HVDC substation and other
reactive power sources.12
5.2.1 General .12
5.2.2 Converter as active/reactive power source .13
5.2.3 Voltage characteristics of a.c. networks depending on the power
loading at the busbar of the HVDC substation.15
5.2.4 Voltage characteristics of a.c. filters, capacitor banks and shunt
reactors for power compensation at the HVDC substation.17
5.2.5 Voltage characteristics of static var compensator (SVC) .17
5.2.6 Voltage characteristics of synchronous compensator (SC).18
5.3 Voltage deviations on the busbar of an HVDC substation .18
5.4 Voltage and reactive power interaction of the substation and other reactive
power sources.19
5.4.1 HVDC converters, switchable a.c. filters, capacitor banks and shunt
reactors .19
5.4.2 HVDC converters, switchable reactive power sources, SVC.20
5.4.3 HVDC converters, switchable reactive power sources and
synchronous compensators .20
6 AC system transient and steady-state stability.21
6.1 General .21
6.2 Characteristics of active and reactive power modulation.21
6.2.1 General .21
6.2.2 Large signal modulation.22
6.2.3 Small signal modulation.23
6.2.4 Reactive power modulation.23
6.3 Classification of network situations.24
6.4 AC network in parallel with the HVDC link .24
6.5 Improvement of the stability within one of the connected a.c. networks .28
6.6 Determination of the damping control characteristics.28
6.7 Implementation of the damping controller and telecommunication
requirements .29
7 Dynamics of the HVDC system at higher frequencies .29
7.1 General .29
7.2 Types of instability .30
TR 60919-3 © IEC:2009 – 3 –
7.2.1 Loop instability (harmonic instability) .30
7.2.2 Current loop instability.30
7.2.3 Core saturation instability .30
7.2.4 Harmonic interactions.30
7.3 Information required for design purposes .31
7.4 Means available for preventing instabilities .32
7.5 Damping of low order harmonics by control action.32
7.6 Demonstration of satisfactory performance at higher frequencies .32
8 Subsynchronous oscillations.33
8.1 General .33
8.2 Criteria for subsynchronous torsional interaction with an HVDC system.34
8.3 Screening criteria for identifying generator units susceptible to torsional
interactions .35
8.4 Performance considerations for utilizing subsynchronous damping controls .36
8.5 Performance testing .36
8.6 Turbine generator protection .36
9 Power plant interaction .37
9.1 General .37
9.2 Specific interactions .37
9.2.1 General .37
9.2.2 Frequency variation effects.37
9.2.3 Frequency controls interactions .37
9.2.4 Overvoltage effects .38
9.2.5 Harmonics .38
9.2.6 Subsynchronous and shaft impact effects .38
9.2.7 Resonance .39
9.2.8 Overvoltages .39
9.2.9 Stresses in a.c. switching equipment .39
9.2.10 Under-frequency.39
9.2.11 Starting procedure for an HVDC converter.39
9.3 Special considerations for a nuclear plant .39
Bibliography.40
Figure 1 – Elements for reactive power compensation at an HVDC substation .13
Figure 2 – P/Q diagram of a converter .14
Figure 3 – Reactive power requirements of a weak a.c. system depending on the active
power loading for various constant voltage characteristics at the a.c. bus of an HVDC
substation .16
Figure 4 – Representation of the a.c. network .16
Figure 5 – An example of voltage – current characteristic showing possible current
modulation range in the absence of telecommunication between rectifier and inverter .23
Figure 6 – Reactive power modulation in an HVDC transmission operating at minimum
extinction angle γ .25
min
Figure 7 – Reactive power modulation in an HVDC transmission operating at extinction
angle γ > γ .26
min
Figure 8 – Stability improvement of an a.c. link or network.27
Figure 9 – Principle arrangements of a damping controller.27
– 4 – TR 60919-3 © IEC:2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60919-3, which is a technical report, has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
This second edition cancels and replaces the first edition, which was issued as a technical
specification in 1999. It constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
TR 60919-3 © IEC:2009 – 5 –
a) this report concerns only line-commutated converters;
b) significant changes have been made to the control system technology;
c) some environmental constraints, for example audible noise limits, have been added;
d) the capacitor coupled converters (CCC) and controlled series capacitor converters (CSCC)
have been included.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
22F/183/DTR 22F/192/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60919 series, under the general title: Performance of high-voltage
direct current (HVDC) systems with line-commutated converters, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – TR 60919-3 © IEC:2009
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)
SYSTEMS WITH LINE-COMMUTATED CONVERTERS –
Part 3: Dynamic conditions
1 Scope
This Technical Report provides general guidance on the dynamic performance of high-voltage
direct current (HVDC) systems. Dynamic performance, as used in this specification, is meant
to include those events and phenomena whose characteristic frequencies or time domain
cover the range between transient conditions and steady state. It is concerned with the
dynamic performance due to interactions between two-terminal HVDC systems and related
a.c. systems or their elements such as power plants, a.c. lines and buses, reactive power
sources, etc. at steady-state or transient conditions. The two-terminal HVDC systems are
assumed to utilize 12-pulse converter units comprised of three-phase bridge (double way)
connections. The converters are assumed to use thyristor valves as bridge arms, with gapless
metal oxide arresters for insulation coordination and to have power flow capability in both
directions. Diode valves are not considered in this specification. While multi-terminal HVDC
transmission systems are not expressly considered, much of the information in this
specification is equally applicable to such systems.
Only line-commutated converters are covered in this report, which includes capacitor
commutated converter circuit configurations. General requirements for semiconductor line-
commutated converters are given in IEC 60146-1-1, IEC 60146-1-2 and IEC 60146-1-3.
Voltage-sourced converters are not considered.
This report (IEC 60919-3) which covers dynamic performance, is accompanied by publications
for steady-state (IEC 60919-1) and transient (IEC 60919-2) performance. All three aspects
should be considered when preparing two-terminal HVDC system specifications.
A difference exists between system performance specifications and equipment design
specifications for individual components of a system. While equipment specifications and
testing requirements are not defined herein, attention is drawn to those which would affect
performance specifications for a system. There are many possible variations between
different HVDC systems, therefore these are not considered in detail. This report should not
be used directly as a specification for a specific project, but rather to provide the basis for an
appropriate specification tailored to fit actual system requirements for a particular electric
power transmission scheme. This report does not intend to discriminate between the
responsibility of users and manufacturers for the work specified.
2 Normative references
The following referenced documents are indispensable for the application 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 60146-1-1, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specification of basic requirements
IEC/TR 60146-1-2, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-2: Application guide
IEC 60146-1-3, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-3: Transformers and reactors
TR 60919-3 © IEC:2009 – 7 –
IEC/TR 60919-1:2005, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 1: Steady-state conditions
IEC/TR 60919-2:2008, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 2: Faults and switching
3 Outline of HVDC dynamic performance specifications
3.1 Dynamic performance specification
A complete dynamic performance specification for an HVDC system should consider the
following clauses:
– a.c. system power flow and frequency control (see Clause 4);
– a.c. dynamic voltage control and interaction with reactive power sources (see Clause 5);
– a.c. system transient and steady-state stability (see Clause 6);
– dynamics of the HVDC system at higher frequencies (see Clause 7);
– subsynchronous oscillations (see Clause 8);
– power plant interaction (see Clause 9).
Clause 4 deals with using active power control of the HVDC system to affect power flow
and/or frequency of related a.c. systems in order to improve the performance of such a.c.
systems. The following aspects should be considered at the design of HVDC active power
control modes:
a) to minimize the a.c. power system losses under steady-state operation;
b) to prevent a.c. line overload under steady-state operation and under a disturbance;
c) to coordinate with the a.c. generator governor control;
d) to suppress a.c. system frequency deviations under steady-state operation and under a
disturbance.
In Clause 5, the voltage and reactive power characteristics of the HVDC substation and other
reactive power sources (a.c. filters, capacitor banks, shunt reactors, SVC (static var
compensator), synchronous compensators) as well as interaction between them during control
of the a.c. bus voltage are considered.
In Clause 6, a discussion is provided concerning methods of controlling active and reactive
power of an HVDC link to improve the steady-state and/or transient stability of the
interconnected a.c. system by counteracting electromechanical oscillations.
Clause 7 deals with dynamic performance of an HVDC system in the range of half
fundamental frequency and above due to both characteristic and non-characteristic harmonics
generated by converters. Means for preventing instabilities are also discussed.
In Clause 8, the phenomenon of amplification of torsional, mechanical oscillations in turbine-
generators of a thermal power plant at their natural frequencies, due to interaction with an
HVDC control system (constant power and current regulation modes), is considered.
Specifications for subsynchronous damping control are defined.
The interaction between a power plant and an HVDC system located electrically near to it is
considered in Clause 9, taking into account some special features of the nuclear power plant
and requirements for the reliability of the HVDC system.
– 8 – TR 60919-3 © IEC:2009
3.2 General comments
Any design requirements for future HVDC systems being specified should fall within the
design limits covered in publications on steady-state (IEC 60919-1) and transient
(IEC 60919-2) performance. It is recommended that during preparation of the dynamic HVDC
system performance specification, the proper HVDC system control strategy should be
identified based on detailed power system studies. The priorities of control signal inputs and
the way they are processed should be specified.
4 AC system power flow and frequency control
4.1 General
Active power control of an HVDC system can be used to control the power flow and/or
frequency in related a.c. systems in order to improve the performance of a.c. systems in
steady-state operation and under disturbance.
In this clause, the HVDC active power operation modes, which are used to improve the a.c.
system performance for the following purposes, will be covered:
– HVDC power control to minimize the total power system losses under steady-state
operation;
– HVDC power control for prevention of a.c. line overload under a disturbance as well as
steady state;
– coordinated HVDC power control with an a.c. system generator governor control;
– HVDC power control for suppression of an a.c. system frequency deviation under a
disturbance as well as steady state.
HVDC active and/or reactive power modes used to improve a.c. system dynamic and transient
stability or improve a.c. voltage control is discussed in Clauses 5 and 6.
4.2 Power flow control
4.2.1 Steady-state power control requirements
The power of an HVDC system is sometimes controlled to minimize overall power system
losses, to prevent a.c. line overloading, and to coordinate with the governor control of a.c.
system generators. Such power control requirements differ from time to time according to the
role of HVDC systems in the overall power system.
When an HVDC system is used to transmit power from remote generating stations, the HVDC
transmission power control is coordinated with the governor control of the power station
generators. In this case, the generator voltage, frequency or the rotor speed may be used as
a reference to the HVDC power control system.
When two a.c. power systems are connected by an HVDC link, the HVDC power is controlled
to a pre-determined pattern under normal circumstances, but an additional function can be
incorporated to this HVDC power control so that the frequency of either or both a.c. power
systems is controlled. When one of the a.c. systems is an isolated system, such as one
supplying a separate island, frequency control of this isolated a.c. system may have to be
realized by the HVDC system.
The a.c. system frequency control by an HVDC system is discussed in 4.3.
When two a.c. systems are interconnected by more than one d.c. link or d.c. and a.c. links, or
when a d.c. system exists within an a.c. system, HVDC power may be controlled in order to
minimize the total transmission losses of the interconnected systems.
TR 60919-3 © IEC:2009 – 9 –
In some cases of a.c./d.c. system configurations described above, the HVDC power change
control can be used to prevent overloading of one or more transmission lines in the power
system.
In certain special HVDC control schemes, such as the one designed to improve a.c. system
performance by increasing the d.c. power during and after a disturbance, the steady-state d.c.
transmission power may have to be set at a restricted value so that the d.c. power does not
exceed the d.c. rated power, including overload capability, when the control is initiated. It is
important to consider also the additional reactive power supply required both by the HVDC
converters and the a.c. systems in such a situation.
The following items a) to g) need to be considered in the specification of steady-state control
requirements. Note that at the time of preparing the specification, the complete steady-state
control requirements may not have been determined or designed, but allowance for possible
future inputs is necessary.
a) When a power flow control system is designed to have more than one function, including
the a.c. system frequency control, the HVDC control system should be so designed that
priorities are set between each control function.
b) Under steady-state conditions, the control for prevention of a.c. line overloading is usually
given higher priority over other power flow controls. The control for minimization of power
system losses is implemented either by setting the d.c. power to a pattern which has been
pre-determined by the power system data, or in response to an on-line computation which
is conducted in the central load dispatching office. Usually, its control response is
relatively slow, being several seconds or several minutes, even in the latter case.
c) In isolated systems or systems with a relatively large d.c. infeed, frequency is often
maintained by the HVDC power. In such a case, HVDC frequency control could have a
priority over system loss minimization, but may be limited by overload protection.
d) The change in reactive power demand accompanying the power changes may result in
frequent switching of reactive power equipment. In such a case, it is necessary to figure
out particular a.c. voltage control measures such as reactive power control by converter
units, or to set limits of the magnitude of HVDC power change.
e) The need for special power order adjustment signals unique to the power system should
be identified, studied, and specified. The signals cannot be permitted to cause d.c. current
or power, or a.c. voltage to deviate beyond equipment and system ratings and limits. The
priority of two or more input signals having simultaneous demand on d.c. link power
should be established and coordinated.
f) Bipolar d.c. links normally require that d.c. power and current be effectively shared
between poles. For loss of one pole, an overload strategy for the remaining pole could be
developed to minimize disruption to a.c. system power flow, voltage and frequency.
g) Disruption of the telecommunication link between the sending and receiving system of the
d.c. link should not cause disruption to the a.c. power system. A minimum specification
requirement is that power transmission is maintained at the same power level which
existed before the telecommunication failure. If additional functions such as frequency
control are required during temporary outage of the telecommunication link, these should
be specified.
4.2.2 Step change power requirement
Under certain power system conditions, it may be required to change the HVDC power in
steps in order to improve the performance of a.c. systems during and after power system
disturbances. Under certain circumstances, the step change may involve d.c. power reversal.
A step change of d.c. power is realized by changing the set value of d.c. power order or by
changing the power range in response to an input signal. The rate of change of power and
limit to the magnitude of the d.c. power change demanded by the step change should be
– 10 – TR 60919-3 © IEC:2009
adjustable within specified limits according to a.c. system requirements. For example,
different ramp rates may be required for different events. Special considerations may be
required when the step change would include power reversal.
Power system disturbances to be considered in specifying d.c. power step changes may
include: a.c. line trip, loss of large power supply source or large drop in a.c. system frequency
and sudden increase or decrease of power system load with its corresponding large frequency
deviation.
In some of the above cases of power system disturbances, the a.c. systems will also be
supported by the a.c. frequency control provided by the d.c. system.
In specifying and designing HVDC control functions, the effects of the step change power
functions should be surveyed in detail for various power system conditions. It is best to
specify limits and ranges for power changes and ramp rates rather than specific settings.
Setting adjustment can be made with the d.c. system in operation.
The signals for initiation of HVDC step power changes include overload relay signals or trip
signals of particular transmission lines which are transmitted to the HVDC substation, or a.c.
system frequency which is detected at the HVDC substation or at some point in a.c. systems.
The time delay involved in a telecommunication system which transmits these initiation
signals may affect the a.c. or d.c. system performance. Therefore, in some cases, a high
speed telecommunication system may be required. When the transmission delay time is large,
this effect should be taken into account.
There are some cases in which signals are sent to both HVDC substations, or more than one
signal is received by an HVDC substation. In these cases, it is necessary to set priorities of
control functions.
The magnitude of d.c. power step change may be limited by a.c. and d.c. system conditions,
and it may be required under certain circumstances to detect the changes in system
conditions to update the values of such limits.
In particular, when there is a large step change in d.c. power, the a.c. voltage may change
substantially. For this reason, it may be required to study the allowable range of a.c. voltage
fluctuation to determine the limits on step power changes, or introduce special a.c. voltage
control measures.
The allowable limits of a.c. voltage deviation can be different for steady-state operation and
transient conditions and should be specified.
When an HVDC system is connected to a high impedance and/or low inertia a.c. system, the
step change in d.c. power may have adverse effects on the voltage stability, transient
stability, and frequency of the a.c. system. In such cases, the magnitude and rate of change
of power may have to be limited, or other special measures may have to be provided, to
prevent deterioration of the a.c. system dynamic performance. When an HVDC system
interconnects two a.c. systems, the effect of d.c. power step change must be evaluated in
detail not only for the a.c. system in which a disturbance occurs, but also for the other a.c.
system in which a fault does not occur.
When the d.c. step change of power causes the d.c. current to fall below the minimum
allowable operational current of the HVDC system, which is usually 5 % to 10 % of the rated
current, the converter operation should be set to the positive minimum current. Otherwise the
converter should be blocked after the allowable period of low current operation, or be
specified to operate down to zero current. One possible measure to overcome minimum
allowable operational current is to set the power flows of two poles in opposite direction and
let the power flow of two poles cancel each other when the HVDC system configuration is
TR 60919-3 © IEC:2009 – 11 –
bipolar. The difference in the power flows of each pole is the actual operating power flow of
the overall HVDC system.
Because of inverter control limitations and possible risks to a.c. system operation, it is not
advisable to request a current order step change larger than the current margin unless special
control actions are taken upon loss of telecommunications.
Certain considerations may be required when an HVDC system is to be started up from a no
load stand-by state in response to a step change power order (see Clause 7 of IEC 60919-1).
4.3 Frequency control
The a.c. system frequency control by the HVDC system can be applied for the following
purposes:
a) frequency control of the receiving and/or sending end a.c. system for a d.c. transmission
from remote power sources;
b) frequency control of an a.c. system in an isolated island or a small a.c. system when it is
interconnected to a large a.c. system through a d.c. system;
c) frequency control of either of the a.c. systems interconnected by an HVDC system, also
taking the frequency of the other system into account.
The a.c. system frequency control is executed either as a continuous function of frequency
under steady-state conditions, or when the frequency deviation of the a.c. system exceeds
certain limits. It may only be activated under certain circumstances such as when the local
a.c. system connected to the HVDC substation is disconnected (islanded) from the main a.c.
system. Accordingly, the specification should state the duties and performance requirements
of the frequency control function.
If the frequency at the receiving end is controlled by varying or modulating the power
transmitted by the d.c. link, there must be coordination of the d.c. link frequency control with
any governor control on associated a.c. generators. It may be possible to use transient
frequency deviation capability of an asynchronous sending end system for support of the
receiving end, provided the a.c. generating equipment is designed accordingly.
When an HVDC substation is electrically far from the centre of the a.c. system, the phase
angle of a.c. voltage at the HVDC substation changes substantially with power changes. In
such circumstances, the speed of response of the frequency signal can be reduced. To avoid
this lower speed of response, the frequency signal can be detected at the centre of the a.c.
system and transmitted to the HVDC substation.
In frequency control it may be required to provide limits of power change and rate of power
change which maintains the a.c. system voltage fluctuation within an allowable range, or
utilize special voltage regulation measures such as reactive power control by converters or
SVC. The allowable limits of voltage fluctuation during steady state frequency control should
be specified.
When the d.c. contribution to a.c. system frequency control is implemented, it is possible that
generator frequency control is degraded unless the controls are properly coordinated. When
two different power systems are interconnected, it may be required to provide appropriate
dead band or to select suitable gain in the frequency control by the HVDC system so that only
large or fast frequency fluctuations are compensated by the d.c. power control, and small or
slow frequency fluctuations are controlled by the power stations belonging to the individual
a.c. systems.
The frequency control designed to correct for severe disturbances, such as those caused by
the tripping of large generation units, may be realized more effectively if the generator unit
trip signal is transmitted to the HVDC substation to initiate the control action.
– 12 – TR 60919-3 © IEC:2009
Fast and large magnitude of d.c. power change for frequency control may produce
overvoltage or voltage dip in the a.c. systems. Such a situation may be relieved by limiting the
rate of power change or by fast reactive power compensations. The allowable overvoltage or
voltage dip, and the allowable duration time should be specified.
One possible measure for continuous and smooth operation of frequency control is to set the
power flows of two poles in opposite direction and let the power flow of two poles cancel each
other when the HVDC system configuration is bipolar. This special operation mode is called
“Frequency control with zero power setting.” However, note that there is additional system
loss and accompanying polarity reversals, which happen when crossing the border of
minimum current.
It is sometimes difficult to set optimal parameters of frequency control since the power system
configuration often changes due to outages of transmission lines and/or substations for
maintenance. This could be accounted for by adopting multi variable frequency control.
When d.c. power control is performed for the purpose of frequency control, it is usually
necessary to provide high speed telecommunication channels, such as a microwave channels
or fiber optic channels, between two HVDC substations. In case of loss of telecommunication
between the two substations, frequency control is usually limited to the network connected to
the current controlling substation.
When the frequency detection point is located far from the HVDC substation control terminal,
or when it is intended to initiate the frequency control action by special signals issued from
the a.c. system, telecommunication channels are required.
In any case, the effect of telecommunication time delay should be taken into account.
For a discussion of telecommunication channels, refer to Clause 13 of IEC 60919
...
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