IEC TR 63065:2017

IEC TR 63065 ®
Edition 1.0 2017-09
TECHNICAL
REPORT
Guidelines for operation and maintenance of line commutated converter (LCC)
HVDC converter station
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IEC TR 63065 ®
Edition 1.0 2017-09
TECHNICAL
REPORT
Guidelines for operation and maintenance of line commutated converter (LCC)

HVDC converter station
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.200; 29.240.01 ISBN 978-2-8322-4802-7

– 2 – IEC TR 63065:2017 © IEC 2017
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms, definitions, and abbreviated terms . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 7
4 Operation . 8
4.1 Operation policy . 8
4.1.1 Target reliability and availability . 8
4.1.2 Operation cost . 9
4.1.3 Manned or unmanned . 9
4.2 Operation condition and limits . 9
4.3 Operations of an HVDC system . 10
4.3.1 General . 10
4.3.2 Typical operation configuration . 10
4.3.3 Set up the control mode . 12
4.3.4 Operation procedure . 13
4.4 Operations of HVDC equipment . 16
4.4.1 Converter valves. 16
4.4.2 Converter transformers and oil immersed smoothing reactors . 19
4.4.3 AC/DC breakers and switchgear . 22
4.4.4 AC/DC filters . 24
4.4.5 Control and protections . 27
4.4.6 DC measurement instruments . 29
4.4.7 Valve cooling system . 30
4.4.8 Auxiliary power system . 33
5 Maintenance . 34
5.1 Maintenance policy . 34
5.1.1 General . 34
5.1.2 Corrective maintenance . 34
5.1.3 Time-based maintenance (TBM) . 34
5.1.4 Condition-based maintenance (CBM) . 35
5.1.5 Reliability-centred maintenance (RCM) . 35
5.1.6 Maintenance programme . 35
5.2 Maintenance during operation . 36
5.2.1 Routine maintenance for converter transformers . 36
5.2.2 Maintenance for control and protections . 36
5.2.3 Maintenance for DC measurements . 37
5.2.4 Routine maintenance for valve cooling system . 37
5.3 Maintenance under outage . 37
5.3.1 Converter valves. 37
5.3.2 Converter transformers . 38
5.3.3 AC/DC breakers and switchgear . 39
5.3.4 AC/DC filters . 40
5.3.5 DC measurements . 40
5.3.6 Valve cooling system . 41
5.3.7 AC/DC arresters . 41

6 Fault analysis and troubleshooting . 42
6.1 General . 42
6.2 System disturbances . 42
6.3 Station faults . 43
6.4 General information for fault analysis . 43
6.4.1 General . 43
6.4.2 Interpreting events and the TFR . 43
6.4.3 Checking plant circuit diagram and application software logics . 43
6.4.4 Analyzing the equipment status . 43
6.4.5 Simulation . 44
6.4.6 Site test . 44
6.4.7 Involving the HVDC supplier . 44
7 Training . 44
7.1 General . 44
7.2 Operator training program . 44
7.2.1 General . 44
7.2.2 Training courses . 44
7.2.3 Participation during installation and commissioning . 45
7.3 Maintenance training program . 45
7.3.1 General . 45
7.3.2 Training courses . 45
7.3.3 Training during equipment installation and testing . 46
7.3.4 Continuous transfer of knowledge . 46
8 Spare parts . 46
9 Tools . 47
10 Documentation . 48
10.1 General . 48
10.2 Documents to be provided by the supplier . 48
10.3 Documents to be prepared by the operators . 48
10.4 Statistics and analysis . 48
Bibliography . 50

Table 1 – Basic tools needed for operation and maintenance of an HVDC converter
station . 47

– 4 – IEC TR 63065:2017 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GUIDELINES FOR OPERATION AND MAINTENANCE
OF LINE COMMUTATED CONVERTER (LCC)
HVDC CONVERTER STATION
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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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.
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 TR 63065, which is a technical report, has been prepared by IEC technical committee 115:
High Voltage Direct Current (HVDC) transmission for DC voltages above 100 kV.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
115/153/DTR 115/163/RVDTR
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 document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 6 – IEC TR 63065:2017 © IEC 2017
GUIDELINES FOR OPERATION AND MAINTENANCE
OF LINE COMMUTATED CONVERTER (LCC)
HVDC CONVERTER STATION
1 Scope
This Technical Report provides general guidance on basic principles and general proposals
for the safe and economic operation and maintenance of an LCC converter station.
These guidelines are based on the operation and maintenance practices that have been used
successfully during the last decades at HVDC converter stations all over the world, and can
be referred to by new HVDC users to optimize operation and maintenance policy and assist in
performing the operation and maintenance work.
This document focuses only on the operation and maintenance of the equipment inside an
LCC converter station, including back-to-back HVDC systems. The operation and
maintenance of HVDC overhead transmission lines, HVDC cables and voltage sourced
converter (VSC) are not covered by this document.
NOTE Usually the agreement between the purchaser and the suppliers of the HVDC converter station includes
specific requirements regarding contractual requirements of particular systems. Such specific requirements will
supersede the general/typical description mentioned in this document and all functions mentioned in this document
are not necessarily applicable for all systems.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60633, Terminology for high-voltage direct current (HVDC) transmission
IEC 60919 (all parts), Performance of high-voltage direct current (HVDC) systems with line-
commutated converters
IEC 61975, System tests for High-voltage direct current (HVDC) installations
IEC TS 62672-1, Reliability and availability evaluation of HVDC systems – Part 1: HVDC
systems with line commutated converters
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60633 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

3.1.1
manned station
MS
HVDC converter station that is operated locally by several operators, 24 h a day
Note 1 to entry: In some systems the manned station controls and observes not only its own converter station but
also the opposite unmanned station.
3.1.2
unmanned station
US
HVDC converter station that is operated remotely with no operator on site
3.1.3
time based maintenance
TBM
maintenance carried out in accordance with a specified time schedule, in most cases annually
3.1.4
condition-based maintenance
CBM
necessary maintenance performed based on the equipment's condition, sometimes combined
with TBM
3.1.5
reliability centred maintenance
RCM
necessary maintenance performed after analyzing the performance of the equipment
3.1.6
operator
person operating the converter station
3.1.7
asset manager
person that manages the plant/asset for the overall planning, operation, maintenance and
performance in accordance with a set of criteria as assigned by the asset owner
3.2 Abbreviated terms
BOD break over diode
BPS by-pass switch
CBM condition-based maintenance
CT current transformer
DGA dissolved gas analysis
GIS gas insulated switchgear
GRTS ground return transfer switch
HMI human-machine interface
LCC line commuted converter
MCB micro circuit breaker
MRTB metallic return transfer breaker
MV medium voltage
NBS neutral bus switch
OEM original equipment manufacturer

– 8 – IEC TR 63065:2017 © IEC 2017
OLT open line test
OLTC on line tap changer
OTDR optical time domain reflectometer
RCM reliability-centred maintenance
RPC reactive power control
SCADA supervisory control and data acquisition
SER sequence of event records
TBM time-based maintenance
TFR transient fault record
4 Operation
4.1 Operation policy
4.1.1 Target reliability and availability
Generally, an availability of 97 % is requested as the minimum value in the design
specification of a new HVDC project. This is based on a forced unavailability of less than 1 %
and a scheduled unavailability of about 2 %. With good quality control applied in the design,
manufacture, installation and testing stages, as well as appropriate operating and careful
maintenance during operation, this requirement can be met in most modern HVDC systems.
According to the survey conducted by CIGRE B4 AG4 ([2] to [6] ) for the period 2003 to 2014,
typical target performance indicators for an HVDC system can be:
a) forced unavailability < 1 %;
b) availability > 97 %.
Scheduled unavailability varies from system to system depending on which operation and
maintenance policy is taken. This can be seen from the performance reports of CIGRE B4
AG4. For example, some systems perform maintenance using 24 h shifts, whereas others
work only 8 h a day and leave the system out of service for 16 h each day and on weekends.
In other systems, maintenance is performed when the generator is not available (thermal unit
out for maintenance or low water conditions, etc.). That is why the typical scheduled
unavailability and overall unavailability is not given above.
The number of forced outages, including sudden interruption of power transmission both by
protections and by manual emergency shutdown, is an important parameter for reliability.
Usually, for a bipolar LCC HVDC scheme, the number of monopole trips is designed to be
four times or fewer a year and the number of bipolar trips below once every ten years.
According to the operation practice of modern HVDC systems, the number of monopole trips
can be even limited to once a year. In systems with more than one converter in series per
pole, the converters' trips can be limited to two trips per converter per station.
Outage hours, a parameter that indicates not only the health of the equipment but also the
ability of maintenance, also affect the reliability and availability considerably. The outage
hours would be longer due to severe damage of the equipment, complex fault analyzing and
troubleshooting, or long waiting times for tools or spare parts. Every effort should be made to
put the HVDC system back into full operation as soon as possible.
Refer to IEC TS 62672-1 for the main performance indicators.
__________
Numbers in square brackets refer to the Bibliography.

4.1.2 Operation cost
The operation cost covers the following aspects:
a) human resources (salary for the operation and maintenance staff),
b) consumables,
c) maintenance and tests,
d) auxiliary power and cooling waters,
e) spare parts,
f) improvements and upgrade, and
g) other costs.
There are a number of variables that will impact the operational cost of an HVDC station. The
human resources cost for an unmanned station is much less than for a manned station. The
maintenance cost for an RCM station is much less than for a TBM station. The spare parts
cost for an old station is usually higher than for a new station.
Due to the above reasons, it is hard to give an average cost for running an HVDC converter
station for one year. The costs can be reduced if optimal operation, maintenance and spare
part policies are considered.
4.1.3 Manned or unmanned
Traditionally, an LCC HVDC converter station is a manned station, where several operators
monitor the status of the HVDC link and the equipment 24 h a day. Even for a manned station,
the power flow on the HVDC system may be dispatched remotely. As development in
automation increases, as well as the improvement of HVDC equipment, unmanned stations or
less-manned stations (e.g. manned only during office hours) are more common. Many stations
in Europe (like FennoSkan, SwePol) are unmanned. An unmanned converter station is
operated remotely from an operation centre or even a dispatch centre, and manned only when
something has failed.
Some important HVDC converter stations, such as Itaipu (Brazil), Nelson River (Canada) and
Fulong (China),which are responsible for transmitting 70 % of the power generated by nearby
hydro plants, are still manned stations. Equipment failure can be fixed faster at a manned
station. This in turn contributes to the reliability and availability of the HVDC link.
Whether a station is manned or unmanned can be evaluated technically and economically. If it
takes more than two hours to drive from a nearby city to the station, if the equipment failure
rate of the station is still high, or if the converter station is critical to the grid, it may be better
to man the station.
NOTE Some parts of the text in this document are applicable for manned stations only and are not applicable for
unmanned stations, as there is no operator in the station.
4.2 Operation condition and limits
Before entering commercial operation of an HVDC converter station, the following conditions
are normally required to be fulfilled:
a) the system test (IEC 61975) and the trial operation have been completed successfully;
b) the operating and maintenance staff is available and has been trained;
c) the communication between the dispatch centre and the converter station has been set up;
d) the standard operating procedure for operation and maintenance of the HVDC station has
been established; and
e) spare parts are available.
– 10 – IEC TR 63065:2017 © IEC 2017
Keeping and preserving the life of an HVDC converter station/link is the most important
aspect to manage. Generally, HVDC systems are planned, designed and expected to have a
lifetime of 30 years or more. To meet this requirement, operation limits should be clearly
defined and faithfully followed by the operating and maintenance staff. They at least include:
1) the current, voltage and temperature of the main equipment are within the limits defined
by HVDC suppliers;
2) the temperature, humidity and cleanliness of the valve hall, valve cooling system, relay
buildings and control rooms can meet the requirements of normal operation of related
equipment or devices;
3) the maximum acceptable current for electrodes and station grounds is such that it will not
affect nearby industry pipes or the environment;
4) environment (oil, water, PCB, etc.) management systems and procedures are in place and
the staff regularly reviews these procedures.
4.3 Operations of an HVDC system
4.3.1 General
Subclause 4.3 gives a generic guidance as to how to operate an HVDC system. Note that the
suppliers’ documentation for the given HVDC system shall always be consulted for the
particular HVDC system.
4.3.2 Typical operation configuration
4.3.2.1 General
Availability, reliability, and flexibility for operation and maintenance are closely related to the
operational configuration of HVDC systems. The general configuration and performance of
high voltage direct current (HVDC) system are given in IEC 60919.
4.3.2.2 Point-to-point HVDC system
A point-to-point HVDC system is mainly for bulk transmission. It usually consists of two
separate poles, which can be operated individually or together in a bipolar arrangement. This
kind of system can be operated in three operating configurations.
a) Bipolar ground return with bipolar power control
This is the normal operation mode for a bipolar HVDC system. Active power order is
shared between each pole and the current of both poles is balanced so that the earth
current is kept to a low value, typically less than 10 A. Furthermore, if one pole is tripped,
part of its power can be taken up by the healthy pole so that less power will be lost. The
above benefits make this the most commonly used operation modes in a bipolar HVDC
system.
During bipolar operation, the bipole neutral bus can be connected to either the electrode
or the station ground. This allows the electrode or electrode lines maintenance job to be
done without interruption of normal power transmission. However, it may be noted that
when the station ground is used, in the event of a trip of a pole, another pole will also be
tripped as a consequence of increased station ground current. Attention should also be
paid to a possible station ground potential change at pole ground fault.
Under contaminated conditions, which are often a combination of rain or fog in an HVDC
station or along the corridor of its DC lines, the affected pole can continue the power
transmission by running at reduced voltage, for instance 70 % or 80 % of rated DC
voltage.The current of each pole is balanced by the control system and the earth current is
still limited to a low value. The HVDC system can resume the rated voltage when the
weather conditions improve.
In the case where the DC equipment of one pole develops a fault such that the power or
current needs to be limited to a certain level, the defective pole can be set to pole power
control or pole current control.

b) Monopole metallic return
This is an optional operation mode for a bipolar system. If one pole for a bipolar HVDC
system is not available, and if long-term flow of high earth current is not allowable and the
DC line of the other pole is still available, the remaining pole can be connected to both DC
lines and earthed at one predefined station. The equipment belonging to the outage pole
can be checked or repaired and put into operation again. Compared to monopole ground
return, the power loss on the DC line will be doubled, so once the faulty pole is available
again, the operating pole can be transferred back first from metallic return to ground return
and then to bipolar ground return.
c) Monopole ground return
This is also an optional operation mode for a bipolar HVDC system. If one pole is not
available, for example when it is under construction, maintenance, or it is tripped by
protections, and if long-term flow of earth current is acceptable, the remaining pole is
connected to the electrode via the MRTB and can go on operating. The equipment
belonging to the faulty pole, as well as its DC line, can be checked or repaired and put into
operation again.
4.3.2.3 Back-to-back HVDC system
A back-to-back HVDC system is mainly used for asynchronous connections. In this
arrangement, there are no DC transmission lines or DC filters, and both converters are
located at one station. The valves for both the rectifier and the inverter are typically located in
one valve hall.
There is only one operating configuration for a back-to-back system. The 12-pulse converters
of both the rectifier and the inverter are connected directly through busbars. Some back-to-
back systems are comprised of several units so that the loss of a converter will not lead to a
total loss of power transmission or network islanding.
4.3.2.4 Two 12-pulse converters per pole
If the DC voltage or DC current of the converter reach their limits, two 12-pulse converters are
connected in series or in parallel for higher voltage or higher current. This configuration is
applied to Nelson River, Itaipu, and the UHVDC systems in China and India.
The operational configuration of 12-pulse converters per pole is the same as for bipolar
systems except that the system can still be operated in bipole mode when one converter is
out of service (forced or scheduled). In this case, the pole will operate at half the normal
voltage or half the DC current capability.
4.3.2.5 Set up the operation configuration
Operators should evaluate the state of main circuit equipment including:
a) AC configuration and minimum availability of AC filter;
b) availability of HVDC equipment of each pole;
c) allowed ground return current;
d) connectivity of DC lines;
e) ensuring all major or critical alarms have already been acknowledged and reset.
The operational configuration can then be set up by connecting both poles or a single pole, in
ground return or metallic return, and the DC lines of each pole or of both poles.

– 12 – IEC TR 63065:2017 © IEC 2017
4.3.3 Set up the control mode
4.3.3.1 General
A combination of different DC voltage settings, power direction, power control methods, power
control modes, reactive power control methods and reactive power control modes gives a
large combination of options.
Bipolar power control ensures that the total power of the DC bipolar transmission remains at
the ordered value and that the current is equally distributed between the two poles, thus
minimizing the earth current. This is the main mode of operation, and to fully synchronize the
two stations, telecommunication has to be in service.
Pole power control keeps the transmitted DC power equal to the power order given by the
operator. To keep the power constant, the DC voltage variation is compensated by adjusting
the DC current accordingly. The current order I is obtained by dividing the total power order
o
by the DC voltage of the converter.
Pole current control keeps DC current equal to the current order given by the operator.
4.3.3.2 Basic control mode
Before starting the power transmission of an HVDC system, the operators should set up the
following control modes:
a) reference DC voltage of each pole;
b) power transmission direction of each pole;
c) select control method and control mode of active power control;
d) enable joint control as long as telecommunication is available;
e) select control method and control mode of reactive power control;
f) enable or disable the supplementary control function such as frequency control, damping
control.
4.3.3.3 Additional control functions
The inherent high-speed power control capability of the HVDC transmission system may be
used for different objectives such as frequency control, power modulation, and power
oscillation damping.
Frequency control modifies the DC power transfer to assist the connected AC systems in
recovering from severe contingencies by limiting AC system frequency deviation above and
below the nominal frequency. The characteristics and dead bands of the frequency controls
are determined during the design studies.
Contingencies involving loss of generation in the inverter AC system and loss of load in the
rectifier AC system may require that the power on the DC system be rapidly increased to
improve the performance of the AC systems. Contingencies involving a loss of generation in
the rectifier AC system or a loss of load in the inverter AC system may require an automatic
reduction in DC power transfer. Power modulation functions are available both in bipolar and
monopolar operation.
Some HVDC systems of which both ends are connected to the same synchronized AC system
apply power modulation that can dampen the power oscillations that may be caused either by
a large disturbance or specific system conditions. The power flow of the HVDC system is
quickly controlled to repress power oscillation.
Therefore, before starting the power transmission of an HVDC system, operators should also
set up the following additional controls.

a) Enable or disable automatic frequency control and set up its parameters. Automatic
frequency control should be enabled if the HVDC link is connected to a weak AC system
or an islanded system.
b) Enable or disable power modulation and set up its parameters. Power modulation is
commonly used for network stability control. After loss of AC lines, DC transmission power
can be ramped down if needed. After tripping of DC link, the generators connected to the
rectifier and the loads connected to the inverter can be turned off.
c) Enable or disable oscillation damping and set up its parameters. Oscillation damping is
mainly used for the rectifier that is fed from thermal power generators and is weakly
connected to a power grid. To avoid subsynchronous oscillation, the power at special
frequencies is measured and modulated.
4.3.4 Operation procedure
4.3.4.1 Control position and control authority
Control orders to the HVDC system, either in digital or analogue, can be given from the
dispatch centre, SCADA, or from backup or local control locations. By default orders from
SCADA are accepted by the control system, unless the dispatch centre takes over the control
rights or when backup control is enabled.
Only the master station can control the power transmission of an HVDC link. If the slave
station needs to control the power, it needs to first take master control from the other station
first.
To increase the security of the HVDC system, only authorized operators should control the
equipment in corresponding areas such as the DC yard, AC yard, AC filters, and auxiliary
power system.
Operators shall log onto the human-machine interface (HMI) before performing any operation.
4.3.4.2 OLT
The open line test (OLT) is a test that is used by the operators to energize the pole DC side
with direct voltage for the purpose of testing the insulation on the DC side, as well as the
converter. The OLT can be performed either in manual or automatic mode.
The OLT is performed in one station at a time because the DC line will be energized up to the
pole disconnector of the other station. When one pole is in open line test the other pole may,
depending on the system design and operation permission, be operated independently.
The OLT is usually part of the commissioning of the HVDC scheme, in some cases it is also
carried out after annual maintenance or a DC line fault. But it is not necessary to carry out an
OLT every time before deblocking a pole. Some HVDC systems have never been subjected to
an OLT in the last thirty years, even after pole maintenance.
4.3.4.3 Deblock
Once the main circuit and the control mode are chosen, i.e. an operating mode is set up, the
HVDC system can be deblocked for power transmission. Before deblock, the status of the
main circuit equipment, control and protections and auxiliary systems should be confirmed to
ensure that the HVDC system is ready for operation. If any of the systems indicates a severe
alarm, maintenance staff should investigate its cause and should take steps to restore the
system to normal condition.
The inverter is always deblocked first to build up DC voltage, and then the rectifier is
deblocked to build up the DC current. The control of the two stations is synchronised via
telecommunication. In the event of a telecommunication failure, operators of the two stations
shall cooperate by telephone to manually deblock the HVDC link.

– 14 – IEC TR 63065:2017 © IEC 2017
4.3.4.4 Power ramp
During the change of power generation and load consumption, the power reference of an
HVDC link should also be adjusted dynamically. Operators receive the power orders or a daily
power curve from the dispatch centre and execute power ramps accordingly.
To ramp up or down the transmission power, the operator should:
a) set up the power ramp speed;
b) set up the power reference at the give time;
c) hold the existing ramp rate or set a new reference if needed.
Corresponding events will be generated after starting the power ramp and when the ramp
completes.
To simplify the above operations, an automatic bipolar power curve is used for some systems.
In this option, the bipolar power order is controlled automatically in response to a pre-
programmed power transfer curve, which will define the power transfer over the daily, weekly

or monthly load cycles.
4.3.4.5 Changing the operation mode
Even though it is safer and more economical to have a DC line running in bipolar ground
return mode with rated DC voltage, normal power direction and bipolar power control,
sometimes it is necessary to change to another operation mode due to the unavailability of
HVDC equipment, HVDC line or other reasons.
a) Change to monopole ground return or metallic return mode.
If one pole of a bipolar HVDC system is not available, for example when it is tripped by
protections, and if long-term flow of high earth current is acceptable, the remaining pole
can go into operation with the ground return until the other pole is available again and the
HVDC system can revert back to bipolar ground return mode. However, if long-term flow of
high earth current is undesirable while the DC line of the defective pole is still available,
the remaining pole can be transferred from ground return to metallic return by starting the
corresponding sequence. Once the other pole is available again, the operating pole can be
transferred back first from metallic return to ground return and then to bipolar ground
return mode.
b) Change to reduced voltage mode
If continuous electro-discharging is observed under contaminated conditions, often caused
by a combination with rain or fog in the converter station or along the corridor of DC lines,
the defective pole can continue the power transmission by running at reduced voltage, for
instance 80 % of rated DC voltage.
When operating in the reduced voltage mode, the taps of converter transformers are
lowered to the position resulting in minimum voltage, and the thyristor valve might operate
at a relatively higher firing angle so that the snubber circuits are subjected to additional
stresses. Additionally, more reactive power is consumed in this mode.
The HVDC system can be returned to rated voltage when the weather conditions improve.
c) Change to pole power/current control mode
When the DC equipment of one pole develops a problem so that its power or current
needs to be limited, this defective pole can be set to pole power control or pole current
control. In both cases, the pole in bipolar control mode will try to balance the current of
each pole so that the earth current can be still limited to a low value.
d) Overloaded
After one pole is blocked by protections, the remaining pole might be ov
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