IEC TR 62544:2011

IEC/TR 62544 ®
Edition 1.0 2011-08
TECHNICAL
REPORT
High-voltage direct current (HVDC) systems – Application of active filters

IEC/TR 62544:2011(E)
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IEC/TR 62544 ®
Edition 1.0 2011-08
TECHNICAL
REPORT
High-voltage direct current (HVDC) systems – Application of active filters

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 29.240.99 ISBN 978-2-88912-627-9

– 2 – TR 62544 © IEC:2011(E)
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references. 7
3 Terms and definitions . 7
3.1 Active and passive filters . 8
3.2 Active filter topologies . 8
shunt active filter . 8
3.3 Power semiconductor terms . 9
3.4 Converter topologies . 9
4 Active filters in HVDC applications . 9
4.1 General . 9
4.2 Semiconductor devices available for active filters . 11
5 Active d.c. filters . 11
5.1 Harmonic disturbances on the d.c. side . 11
5.2 Description of active d.c. filters . 12
5.2.1 General . 12
5.2.2 Types of converters available . 12
5.2.3 Connections of the active d.c. filter . 13
5.2.4 Characteristics of installed active d.c. filters . 15
5.3 Main components in an d.c. active filter . 16
5.3.1 General . 16
5.3.2 Passive part . 16
5.3.3 Current transducer . 18
5.3.4 Control system . 18
5.3.5 Amplifier . 19
5.3.6 Transformer . 19
5.3.7 Protection circuit and arrester . 19
5.3.8 Bypass switch and disconnectors . 19
5.4 Active d.c. filter control . 19
5.4.1 General . 19
5.4.2 Active d.c. filter control methods . 20
5.5 Example – Performance of the Skagerrak 3 HVDC Intertie active d.c. filter . 23
5.6 Conclusions on active d.c. filters . 24
6 Active a.c. filters in HVDC applications . 25
6.1 General . 25
6.2 Harmonic disturbances on the a.c. side of a HVDC system . 25
6.3 Passive filters . 26
6.3.1 Conventional passive filters. 26
6.3.2 Continuously tuned passive filters . 26
6.4 Reasons for using active filters in HVDC systems . 27
6.5 Operation principles of active filters . 28
6.5.1 Shunt connected active filter . 28
6.5.2 Series connected active filter . 29
6.6 Parallel and series configuration . 29
6.6.1 General . 29
6.6.2 Hybrid filter schemes . 29
6.7 Converter configurations . 30

TR 62544 © IEC:2011(E) – 3 –
6.7.1 Converters . 30
6.8 Active a.c. filter configurations . 32
6.8.1 Active a.c. filters for low voltage application . 32
6.8.2 Active a.c. filters for medium voltage application . 33
6.8.3 Active a.c. filters for HVDC applications . 33
6.9 Series connected active filters . 34
6.10 Control system . 34
6.10.1 General . 34
6.10.2 Description of a generic active power filter controller . 35
6.10.3 Calculation of reference current . 36
6.10.4 Synchronous reference frame (SRF) . 37
6.10.5 Other control approaches . 37
6.10.6 HVDC a.c. active filter control approach . 38
6.11 Existing active a.c. filter applications . 38
6.11.1 Low and medium voltage. 38
6.11.2 High voltage applications . 38
6.12 Overview on filter solutions for HVDC systems . 39
6.12.1 Solution with conventional passive filters . 39
6.12.2 Solution with continuously tuned passive filters . 40
6.12.3 Solution with active filters . 40
6.12.4 Solution with continuously tuned passive filters and active filters . 41
6.12.5 Study cases with the CIGRÉ HVDC model . 41
6.13 ACfilters for HVDC installations using VSC . 43
6.14 Conclusions on active a.c. filters . 43
Bibliography . 45

Figure 1 – Shunt connection . 8
Figure 2 – Series connection . 8
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost . 10
Figure 4 – Simple current source converter . 13
Figure 5 – Simple voltage sourced converter . 13
Figure 6 – Possible connections of active d.c. filters . 14
Figure 7 – Filter components in the active filter . 17
Figure 8 – Impedance characteristics of different passive filters . 17
Figure 9 – Basic control loop of an active d.c. filter . 21
Figure 10 – Measured transfer function of external system, Baltic Cable HVDC link . 22
Figure 11 – Feedforward control for the active d.c. filter . 22
Figure 12 – Measured line current spectra, pole 3 operated as monopole . 24
Figure 13 – Continuously tuned filter . 26
Figure 14 – Example of current waves . 28
Figure 15 – Series and parallel connection . 29
Figure 16 – Hybrid configuration . 30
Figure 17 – Three phase current-source converter . 31
Figure 18 – Three phase 2 level voltage-sourced converter (three-wire type) . 31
Figure 19 – Three phase 3 level voltage-sourced converter (three-wire type) . 32
Figure 20 – Single-phase voltage sourced converter . 32

– 4 – TR 62544 © IEC:2011(E)
Figure 21 – Active filter connected to the HV system through a single-tuned passive
filter . 33
Figure 22 – Active filter connected to the HV system through a double-tuned passive
filter . 34
Figure 23 – Using an LC circuit to divert the fundamental current component . 34
Figure 24 – Per-phase schematic diagram of active filter and controller . 35
Figure 25 – Block diagram of IRPT . 36
Figure 26 – Block diagram of SRF . 38
Figure 27 – Plots from site measurements . 39
Figure 28 – Filter configuration and a.c. system harmonic impedance data . 42

Table 1 – The psophometric weighting factor at selected frequencies . 12
Table 2 – Voltage to be supplied by the active part with different selections of passive
parts . 18
Table 3 – Major harmonic line currents, pole 3 operated as monopole . 24
Table 4 – Preferred topologies for common LV and MV applications . 30
Table 5 – Performance Requirements . 41
Table 6 – Parameters of filters at a.c. substation A (375 kV) . 42
Table 7 – Parameters of filters at a.c. substation B (230 kV) . 43
Table 8 – Performance results of filters . 43

TR 62544 © IEC:2011(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
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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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".
This Technical Report cancels and replaces IEC/PAS 62544 published in 2011. This first
edition constitutes a technical revision.
IEC/TR 62544, 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 electronics.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
22F/242/DTR 22F/250/RVC
– 6 – TR 62544 © IEC:2011(E)
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.
The committee has decided that the contents of this publication 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.
A bilingual version of this publication may be issued at a later date.

TR 62544 © IEC:2011(E) – 7 –
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
1 Scope
This technical report gives general guidance on the subject of active filters for use in high-
voltage direct current (HVDC) power transmission. It describes systems where active devices
are used primarily to achieve a reduction in harmonics in the d.c. or a.c. systems. This
excludes the use of automatically retuned components.
The various types of circuit that can be used for active filters are described in the report, along
with their principal operational characteristics and typical applications. The overall aim is to
provide guidance for purchasers to assist with the task of specifying active filters as part of
HVDC converters.
Passive filters are specifically excluded from this report.
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/TS 60071-5, Insulation co-ordination – Part 5: Procedures for high-voltage direct current
(HVDC) converter stations
IEC 60633, Terminology for high-voltage direct-current (HVDC) transmission
IEC 61000 ( all parts), Electromagnetic compatibility (EMC)
IEC 61975, High-voltage direct current (HVDC) installations – System tests
IEC/TR 62001:2009, High-voltage direct current (HVDC) systems – Guidebook to the
specification and design evaluation of A.C. filters
IEC/TR 62543, High-voltage direct current (HVDC) power transmission using voltage sourced
converters (VSC)
IEEE 519, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical
Power Systems
3 Terms and definitions
For the purposes of this technical report, the terms and definitions given in IEC 60633 and
IEC 62001:2009 for passive a.c. filters, as well as the following apply.
NOTE Only terms which are specific to active filters for HVDC are defined in this clause. Those terms that are
either identical to or obvious extensions of IEC 60633 or IEC 62001:2009 terminology have not been defined.

– 8 – TR 62544 © IEC:2011(E)
I I
s l
Voltage Harmonic
source generator
I
f
Active
filter
IEC  1820/11
Figure 1 – Shunt connection
U U
s l
U
Net f
Load
source
Harmonic
Active
producing
filter
load
IEC  1821/11
Figure 2 – Series connection
3.1 Active and passive filters
3.1.1
active filter
a filter whose response to harmonics is either wholly or partially governed by a controlled
converter
3.1.2
passive filter
a filter whose response to harmonics is governed by the impedance of its components
3.2 Active filter topologies
3.2.1
shunt active filter
an active filter connected high-voltage (HV) to low-voltage (LV) or HV to ground such that it
experiences the full a.c. or d.c. voltage of the HVDC system or its a.c. connection (see
Figure 1)
3.2.2
series active filter
an active filter connected between the HVDC converter and the a.c. or d.c. supplies such that it
must withstand the full HVDC system current, either a.c. or d.c. (see Figure 2)
3.2.3
shunt and series active filter
an active filter containing both series and shunt elements as defined above

TR 62544 © IEC:2011(E) – 9 –
3.3 Power semiconductor terms
NOTE There are several types of power semiconductor devices which can be used in active filters for HVDC and
currently the IGBT is the major device used in such converters. The term IGBT is used throughout this report to
refer to the switched valve device. However, the report is equally applicable to other types of devices with turn-off
capability in most of the parts.
3.3.1
insulated gate bipolar transistor
IGBT
a controllable switch with the capability to turn-on and turn-off a load current
NOTE 1 An IGBT has three terminals: a gate terminal (G) and two load terminals - emitter (E) and collector (C).
NOTE 2 By applying appropriate gate to emitter voltages, current in one direction can be controlled, i.e. turned on
and turned off.
3.3.2
free-wheeling diode
FWD
power semiconductor device with diode characteristic.
NOTE 1 A FWD has two terminals: an anode (A) and a cathode (K). The current through the FWDs is in opposite
direction to the IGBT current.
NOTE 2 FWDs are characterized by the capability to cope with high rates of decrease of current caused by the
switching behaviour of the IGBT.
3.3.3
IGBT-diode pair
arrangement of IGBT and FWD connected in inverse parallel
3.4 Converter topologies
3.4.1
pulse width modulation
PWM
a converter operation technique using high frequency switching with modulation to produce a
particular waveform when smoothed
3.4.2
two-level converter
a converter in which the voltage at the a.c. terminals of the VSC unit is switched between two
discrete d.c. voltage levels
3.4.3
three-level converter
a converter in which the voltage at the a.c. terminals of the VSC unit is switched between three
discrete d.c. voltage levels
3.4.4
multi-level converter
a converter in which the voltage at the a.c. terminals of the VSC unit is switched between more
than three discrete d.c. voltage level.
4 Active filters in HVDC applications
4.1 General
The conversion process in an HVDC transmission system introduces harmonic currents into
the d.c. transmission lines and the a.c. grid connected to the HVDC converters. These

– 10 – TR 62544 © IEC:2011(E)
harmonic currents may cause interference in the adjacent systems, like telecommunication
equipment. The conventional solution to reduce the harmonics has been to install passive
filters in HVDC converter stations [1] . When the power line consists of cables, this filtering is
normally not necessary. The development of power electronics devices and digital computers
has made it possible to achieve a new powerful way for a further reduction of harmonic levels,
namely, active filters.
The active filters can be divided into two groups, active a.c. and d.c. filters. Active d.c. filter
installations are in operation in several HVDC links and have been economically competitive
due to more onerous requirements for telephone interference levels on the d.c. overhead lines
(Figure 3). An active a.c. filter is already in operation as well. In addition to the active d.c. filter
function of mitigating the harmonic currents on the d.c. overhead lines, the active a.c. filters
may be part of several solutions in the HVDC scheme to improve reactive power exchange with
the a.c. grid and to improve dynamic stability.
Filter
Passive d.c. filter
cost
Active d.c. filter
Allowable interference level
IEC  1822/11
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost
The features of active filters are the following:
• Active a.c. and d.c. filters consist of two parts, a passive part and a corresponding active part
which are loaded with the same currents. Due to the fact that the passive a.c. filter is used
to supply the HVDC converter demand of reactive power and thereby loaded with the
fundamental current, the required rating of the d.c. filter active part is lower than the one of
the a.c. filter active part.
• The control philosophy for the active d.c. filter is less complex than for the a.c. one.
• The present HVDC applications where active a.c. filters are feasible will be limited, due to
the fact that a.c. filters are also required to supply the HVDC converter demand of reactive
power. The filter size is therefore often well above the filtering demand.
Many recent and future HVDC projects use new converter technologies which allow the
reactive compensation to be separated from the a.c. filters and thereby make the active a.c.
filter more feasible. For line-commutated converters, capacitor commutated converters (CCC)
and the controlled series capacitor converter (CSCC) allow reduced reactive power absorption.
Moreover, self-commutated converters (which include most voltage sourced converters) are
able to control active and reactive power independently, avoiding the need for separate
reactive power compensation altogether.
___________
Figures in square brackets refer to the Bibliography.

TR 62544 © IEC:2011(E) – 11 –
4.2 Semiconductor devices available for active filters
Three types of power semiconductor devices, suitable for use in an active filter, are available at
present:
• metal-oxide-semiconductor field-effect transistor (MOSFET);
• insulated gate bipolar transistor (IGBT);
• gate turn-off thyristor (GTO) and other thyristor-derived devices such as the gate
commutated thyristor (GCT) and integrated gate commutated thyristor (IGCT).
The MOSFET is an excellent switching device capable of switching at very high frequencies
with relatively low losses, but with limited power handling capability.
The IGBT has a switching frequency capability which, although very good and sufficient to
handle the frequencies within the active d.c. filter range, is inferior to the MOSFET. However
the IGBT power handling is significantly higher than the MOSFET.
The GTO-type devices has the highest power handling capacity, but with a relatively limited
switching speed far below the required frequency range for active d.c. filter. The use of GTO-
type devices will probably be limited to handle frequencies below a few hundred of hertz.
The relatively high frequency band for active d.c. filtering excludes the use of thyristors and
GTO. Even though the MOSFET and IGBT are suited as switching elements in a power stage,
the limited power handling capacity on MOSFET and the installed cost evaluations tend to point
on the use of IGBT in future power stages.
5 Active d.c. filters
5.1 Harmonic disturbances on the d.c. side
The main reason for specifying demands on the d.c. circuit is to keep disturbances in nearby
telephone lines within an acceptable limit, which will vary depending on whether the telephone
system consists of overhead lines or underground cables which are generally shielded and
therefore have a better immunity [2]. A summary is given below to illustrate the demands which
made it feasible to install the active filters. As described, the demand on disturbances can
appear as an harmonic current on the d.c. line or as an induced voltage U in a fictive
ind
telephone line. It should be kept in mind that the harmonic demand, the specific HVDC system
and surroundings (earth resistivity, telephone system, etc.) all together define the d.c. filter
solution.
The specified requirements:
• The induced voltage U in a theoretically 1 km telephone line situated 1 km from the d.c.
ind
overhead line shall be below 10 mV for monopolar operation.
• A one minute mean value of the equivalent psophometric current I fed into the d.c. pole
pe
overhead line shall be below 400 mA.
The mentioned induced voltage and the equivalent psophometric current are defined as:
U = (2π ⋅ f ⋅MI⋅⋅ p ) (1)
∑ n nn
ind
n=1
I = ()k ⋅⋅pI (2)
pe ∑ n n n
p
n=1
– 12 – TR 62544 © IEC:2011(E)
where
th
f is the frequency of the n harmonic,
n
M is the mutual inductance between the telephone line and the power line,
k = f n/800,
n 1 ×
th
I is the vectorial sum of the n harmonic current flowing in the line conductors (common
n
mode/earth mode current),
th
p is the n psophometric weighting factor defined by CCITT Directives 1963 [3] (see also
n
Table 1),
th
p is the 16 psophometric weighting factor.
The characteristic harmonics n = 12, 24, 36, 48 as well as the non-characteristic harmonics up
to n = 50 shall be considered.
Table 1 – The psophometric weighting factor at selected frequencies
Frequency, H
50 100 300 600 800 1 000 1 200 1 800 2 400 3 000
z
n
1 2 6 12 16 20 24 36 48 60
p factor
0,0007 0,009 0,295 0,794 1,0 1,122 1,0 0,76 0,634 0,525
n
P k
0,00004 0,001 0,111 0,595 1,0 1,403 1,5 1,71 1,902 1,969
n × n
5.2 Description of active d.c. filters
5.2.1 General
Active d.c. filters use a controllable converter to introduce currents in the network, presenting a
waveform which counteracts the harmonics. This subclause describes types of power stages,
converters to be used in active filters and the possible connections in HVDC schemes.
5.2.2 Types of converters available
5.2.2.1 General
Two basic types of switching converters are possible in an active d.c. filter; the current-source
converter using inductive energy storage and the voltage-sourced converter (VSC) using
capacitive energy storage.
5.2.2.2 Current source converters
In a current-source converter, the d.c. element is a current source, which normally consists of a
d.c. voltage source power supply in series with an inductor. For correct operation, the current
should flow continuously in the inductor. Hence if a.c. current is not required current must be
by-passed within the converter. This fact restricts the switching actions. A simple current-
source converter is shown in Figure 4.
5.2.2.3 Voltage sourced converters (VSC)
In the VSC, the d.c. element is a voltage source. This may be a d.c. power supply or, in the
case of an active d.c. filter application, an energy storage unit. In practice, the voltage source
for an active d.c. filter power stage is usually a capacitor with a small power supply to offset the
power stage losses. A VSC also has the property that its a.c. output appears as a voltage
source.
A circuit of simple VSC is shown in Figure 5.

TR 62544 © IEC:2011(E) – 13 –
L I
IEC  1823/11
Key
1 AC current
Figure 4 – Simple current source converter
C
+
V
–
IEC  1824/11
Key
1 AC voltage
Figure 5 – Simple voltage sourced converter
5.2.2.4 Comparison between current and voltage sourced converters
The current-source converter has a high internal impedance for currents through the converter,
while the VSC has a low impedance. The VSC has no constraints on the switching pattern
which can be employed, while the current-source converter is restricted as described above.
The necessity for continuous current in the current-source converter, combined with the fact
that (neglecting superconductivity) an inductor has higher losses than a capacitor, ensures that
the losses in the current-source converter are higher than in the VSC. Another parameter
influencing losses is that a current-source converter needs switching devices which can block
reverse voltage. Most of the available semiconductors do not fulfil this requirement. In this case
an extra diode in series with each device is necessary and this again increases the losses.
Some GTOs are able to support reverse voltage, but these are less common than the GTOs
which do not support reverse voltage. The former have higher losses than the more common
devices.
Conclusion: Considering the above properties of current-source converter and VSC, the type
most suited for power stage applications, particularly high power, is the VSC. The VSC has
been preferred in all HVDC projects applicable today.
5.2.3 Connections of the active d.c. filter
5.2.3.1 General
Advantages and disadvantage of connecting the active filters at locations shown in Figure 6
have been discussed in several papers [4], [5], [6]. The active filters can either be connected
as shunt active filters or as series active filters.

– 14 – TR 62544 © IEC:2011(E)
Smoothing
reactor
Pole
Active
filter 4
Passive
d.c. filter
HVDC
converter
Active Active
filter 3
filter 2
Active Active
filter 5a filter 5b
Active
filter 1
Electrode
IEC  1825/11
Figure 6 – Possible connections of active d.c. filters
5.2.3.2 "Active filter 1" connection
The active d.c. filter realised in HVDC schemes today is connected as the shunt “Active filter 1”
in Figure 6. By connecting the active filter in series with the passive d.c. filter, usually a 12/24th
double tuned filter, the active filter rating can be reduced. A VSC is chosen in order to make
the smallest influence on the original function of the passive filter, especially on frequencies
where the control algorithm is not active.
5.2.3.3 "Active filter 2" connection
The “Active filter 2” in Figure 6 is similar to the shunt “Active filter 1” solution. The power
consumption of the tuning circuit in the passive filter will probably reduce the efficiency to inject
harmonic currents to counteract the disturbance current and thereby increase the rating of the
converter. There may be additional inductance inserted in series with the active part.
5.2.3.4 "Active filter 3" connection
The “Active filter 3” in Figure 6 is a series active filter described in [11], but there is a lack of
knowledge of such a system. The active filter converter must be connected to the HVDC
. To prevent saturation of the coupling transformer T by the
system by a coupling transformer T
c c
direct load current of the HVDC converter I , the core must have an air gap.
dconv
In this way, the coupling transformer T is a d.c. reactor with a galvanic insulated auxiliary
c
winding to connect the active filter (converter). To achieve no ripple voltage at the point of
connection of the passive d.c. filter and therefore no ripple current in the d.c. pole line, the
active filter must generate across the main winding T a voltage which compensate the ripple
c
voltage U of the d.c. side of the HVDC converter.
r
The a.c. load current I of the main winding of T is determined by U and its inductance value
r c r
L , the converter transformer inductance and the smoothing reactor inductance. The rating of
r
2 ×
T is determined by (I + I ) L . The rating of the active filter (converter) is determined by
c dconv r r
U /L . Hence the economical optimisation between the active and passive part of the active
dr r
TR 62544 © IEC:2011(E) – 15 –
filter can be adjusted by increasing L . The rating of T will be increased and the rating of the
r c
active filter part will be decreased or vice versa.
The smoothing reactor (which is already designed for U ) is eventually an alternative for T ,
dr c
although is must be relocated to the neutral side of the HVDC converter valve and provided
with an auxiliary winding.
The advantages of this connection are:
• There are no harmonics in the HVDC converter direct current.
• The control algorithm of a series filter will probably be simplified compared to the shunt
filter control.
The disadvantages are:
• Even by an optimal design, the rating of T and the active filter part will be considerable.
c
• The T side of the HVDC converter has no earth potential, which should be considered in the
c
design of the HVDC converter and the transformer T .
c
5.2.3.5 "Active filter 4" connection
The “Active filter 4” in Figure 6 is a series active filter fundamentally with the same
configuration and problems as the “Active filter 3”. The filter is connected at the pole bus on the
line side of the d.c. filter capacitor. The major advantage of this arrangement is that the active
filter rating (due to the fact that the HVDC converter output ripple voltage is attenuated already
by the passive filter) will be considerably less than the “Active filter 3” connection. The
disadvantage of this arrangement is that the filter is situated at line potential and that the filter
must conduct the whole direct current.
5.2.3.6 "Active filter 5" connection
There has not been any information describing “Active filter 5a and 5b” in Figure 6. The
application of such a filter is expected to be limited to either higher frequencies or lower
frequencies and not the whole frequency range as the “Active filter 1 and 2”.
5.2.3.7 Conclusion on active filter connections
The advantages and disadvantages of the most possible connections of the active part of the
d.c. filter have been described above. The main conclusion is that series connections of active
filters on the d.c. side are possible but with the facts available today not recommendable.
The injected power for active filtering can be reduced by choosing the optimum line injection
point on the passive circuit or the d.c. line. All active d.c. filter applications implemented today
and in the near future will use the “Active filter 1” solution in Figure 6. The remaining of this
paper therefore discusses the “Active filter 1” solution.
5.2.4 Characteristics of installed active d.c. filters
The active d.c. filters (Figure 7) are connected in feedback control loop. The line current is
measured by a current transducer. The current signal is passed through a light guide into a
computer. The computer calculates a signal to feed a VSC, so that the current injected at the
pole line is in opposition to the measured line current.
Characteristics of the active d.c. filters:
• frequency range 300 Hz to 3 000 Hz;
• the achieved harmonic current attenuation is high, at least 10 times attenuation in addition
to achievements with the passive part alone, at all chosen frequencies in the whole
frequency range (see Figure 29);

– 16 – TR 62544 © IEC:2011(E)
• adaptable to variations of network frequency;
• compensate detuning effects of the passive d.c. filter;
• comparatively small size. The active part of the active d.c. filter can be fully assembled and
tested at the factory and then transported to site;
• significant changes in characteristics of the active d.c. filter can be achieved any time after
commissioning within the active filter ratings by software changes without hardware
modification.
5.3 Main components in a d.c. active filter
5.3.1 General
The active d.c. filter is a hybrid filter consisting of a passive and an active part. The passive
part can usually be defined as a double tuned passive filter which connects the active part with
the d.c. line. The active part in the d.c. active filter is shown in Figure 7. All the components in
the active part shall ensure proper function of the active filter in steady state conditions and
during faults. Testing of active filters during system tests for high-voltage direct-current
installations should be carried out in accordance with IEC 61975.
5.3.2 Passive part
The main function of the passive part is to connect the active part with the high voltage d.c.
line. The reasons for choosing a double tuned filter are both an optimisation of the VSC cost
compared with the double tuned circuit and to ensure a reasonable performance if the active
part is not in operation.
The choice of the characteristics for the passive part, together with the size of the smoothing
reactor, will influence the rating of the active part. The following example illustrates the rating
requirements of the active part with a fixed size smoothing reactor when
• only a capacitor is used;
th
• a single tuned 12 harmonic filter is used;
th
• a double tuned 12/24 harmonic filter is used.
Table 2 shows a scheme calculated from some typical measured current val
...

IEC TR 62544 ®
Edition 1.2 2020-02
CONSOLIDATED VERSION
TECHNICAL
REPORT
High-voltage direct current (HVDC) systems – Application of active filters

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IEC TR 62544 ®
Edition 1.2 2020-02
CONSOLIDATED VERSION
TECHNICAL
REPORT
High-voltage direct current (HVDC) systems – Application of active filters

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.99 ISBN 978-2-8322-7860-4

IEC TR 62544 ®
Edition 1.2 2020-02
CONSOLIDATED VERSION
REDLINE VERSION
High-voltage direct current (HVDC) systems – Application of active filters

– 2 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
3.1 Active and passive filters . 8
3.2 Active filter topologies . 8
shunt active filter . 8
3.3 Power semiconductor terms . 9
3.4 Converter topologies . 9
4 Active filters in HVDC applications . 10
4.1 General . 10
4.2 Semiconductor devices available for active filters . 11
5 Active d.c. filters . 11
5.1 Harmonic disturbances on the d.c. side . 11
5.2 Description of active d.c. filters. 12
5.2.1 General . 12
5.2.2 Types of converters available . 12
5.2.3 Connections of the active d.c. filter . 14
5.2.4 Characteristics of installed active d.c. filters . 16
5.3 Main components in a d.c. active filter . 17
5.3.1 General . 17
5.3.2 Passive part . 17
5.3.3 Current transducer . 19
5.3.4 Control system . 19
5.3.5 Amplifier . 20
5.3.6 Transformer . 20
5.3.7 Protection circuit and arrester . 20
5.3.8 Bypass switch and disconnectors . 20
5.4 Active d.c. filter control . 20
5.4.1 General . 20
5.4.2 Active d.c. filter control methods . 21
5.5 Example – Performance of the Skagerrak 3 HVDC Intertie active d.c. filter . 24
5.6 Conclusions on active d.c. filters . 25
6 Active a.c. filters in HVDC applications . 26
6.1 General . 26
6.2 Harmonic disturbances on the a.c. side of a HVDC system . 26
6.3 Passive filters . 27
6.3.1 Conventional passive filters . 27
6.3.2 Continuously tuned passive filters . 27
6.4 Reasons for using active filters in HVDC systems. 28
6.5 Operation principles of active filters . 29
6.5.1 Shunt connected active filter. 29
6.5.2 Series connected active filter . 30
6.6 Parallel and series configuration . 30
6.6.1 General . 30
6.6.2 Hybrid filter schemes . 30

+AMD2:2020 CSV © IEC 2020
6.7 Converter configurations . 31
6.7.1 Converters . 31
6.8 Active a.c. filter configurations. 34
6.8.1 Active a.c. filters for low voltage application . 34
6.8.2 Active a.c. filters for medium voltage application . 34
6.8.3 Active a.c. filters for HVDC applications. 34
6.9 Series connected active filters . 36
6.10 Control system . 36
6.10.1 General . 36
6.10.2 Description of a generic active power filter controller . 36
6.10.3 Calculation of reference current . 37
6.10.4 Synchronous reference frame (SRF) . 38
6.10.5 Other control approaches . 39
6.10.6 HVDC a.c. active filter control approach . 39
6.11 Existing active a.c. filter applications . 39
6.11.1 Low and medium voltage . 39
6.11.2 High voltage applications . 39
6.12 Overview on filter solutions for HVDC systems . 40
6.12.1 Solution with conventional passive filters . 40
6.12.2 Solution with continuously tuned passive filters . 41
6.12.3 Solution with active filters . 42
6.12.4 Solution with continuously tuned passive filters and active filters . 42
6.12.5 Study cases with the CIGRÉ HVDC model . 42
6.13 ACfilters for HVDC installations using VSC . 44
6.14 Conclusions on active a.c. filters . 45
Bibliography . 46

Figure 1 – Shunt connection . 8
Figure 2 – Series connection . 8
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost . 10
Figure 4 – Simple current source converter . 13
Figure 5 – Simple voltage sourced converter . 14
Figure 6 – Possible connections of active d.c. filters . 15
Figure 7 – Filter components in the active filter . 18
Figure 8 – Impedance characteristics of different passive filters. 18
Figure 9 – Basic control loop of an active d.c. filter . 22
Figure 10 – Measured transfer function of external system, Baltic Cable HVDC link . 23
Figure 11 – Feedforward control for the active d.c. filter . 23
Figure 12 – Measured line current spectra, pole 3 operated as monopole . 25
Figure 13 – Continuously tuned filter. 27
Figure 14 – Example of current waves . 30
Figure 15 – Series and parallel connection . 31
Figure 16 – Hybrid configuration . 31
Figure 17 – Three phase current-source converter . 32
Figure 18 – Three phase 2 level voltage-sourced converter (three-wire type) . 33
Figure 19 – Three phase 3 level voltage-sourced converter (three-wire type) . 33

– 4 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
Figure 20 – Single-phase voltage sourced converter . 34
Figure 21 – Active filter connected to the HV system through a single-tuned passive
filter . 35
Figure 22 – Active filter connected to the HV system through a double-tuned passive
filter . 35
Figure 23 – Using an LC circuit to divert the fundamental current component . 36
Figure 24 – Per-phase schematic diagram of active filter and controller . 37
Figure 25 – Block diagram of IRPT . 37
Figure 26 – Block diagram of SRF . 39
Figure 27 – Plots from site measurements . 41
Figure 28 – Filter configuration and a.c. system harmonic impedance data . 43

Table 1 – The psophometric weighting factor at selected frequencies . 12
Table 2 – Voltage to be supplied by the active part with different selections of passive
parts . 19
Table 3 – Major harmonic line currents, pole 3 operated as monopole . 25
Table 4 – Preferred topologies for common LV and MV applications . 31
Table 5 – Performance Requirements . 43
Table 6 – Parameters of filters at a.c. substation A (375 kV) . 44
Table 7 – Parameters of filters at a.c. substation B (230 kV) . 44
Table 8 – Performance results of filters . 44

+AMD2:2020 CSV © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
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
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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 62544 edition 1.2 contains the first edition (2011-08) [documents 22F/242/DTR
and 22F/250/RVC], its amendment 1 (2016-04) [documents 22F/377/DTR and 22F/381A/
RVC] and its amendment 2 (2020-02) [documents 22F/519/DTR and 22F/525/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 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
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 62544, 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 electronics.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
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
"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.
+AMD2:2020 CSV © IEC 2020
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
1 Scope
This technical report gives general guidance on the subject of active filters for use in high-
voltage direct current (HVDC) power transmission. It describes systems where active devices
are used primarily to achieve a reduction in harmonics in the d.c. or a.c. systems. This
excludes the use of automatically retuned components.
The various types of circuit that can be used for active filters are described in the report,
along with their principal operational characteristics and typical applications. The overall aim
is to provide guidance for purchasers to assist with the task of specifying active filters as part
of HVDC converters.
Passive filters are specifically excluded from this report.
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/TS 60071-5, Insulation co-ordination – Part 5: Procedures for high-voltage direct current
(HVDC) converter stations
IEC 60633, Terminology for high-voltage direct-current (HVDC) transmission
IEC 61000 ( all parts), Electromagnetic compatibility (EMC)
IEC 61975, High-voltage direct current (HVDC) installations – System tests
IEC/TR 62001:2009, High-voltage direct current (HVDC) systems – Guidebook to the
specification and design evaluation of A.C. filters
IEC TR 62001-1:2016, High-voltage direct current (HVDC) systems – Guidance to the
specification and design evaluation of AC filters – Part 1: Overview
IEC/TR 62543, High-voltage direct current (HVDC) power transmission using voltage sourced
converters (VSC)
IEEE 519, IEEE Recommended Practices and Requirements for Harmonic Control in
Electrical Power Systems
3 Terms and definitions
For the purposes of this technical report, the terms and definitions given in IEC 60633 and
IEC TR 62001-1:20092016 for passive a.c. filters, as well as the following apply.
NOTE Only terms which are specific to active filters for HVDC are defined in this clause. Those terms that are
either identical to or obvious extensions of IEC 60633, IEC TR 62001-1:2009 and 62747 terminology have not been
defined.
– 8 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
I I
s l
Voltage Harmonic
source generator
I
f
Active
filter
IEC  1820/11
Figure 1 – Shunt connection
U U
s l
U
Net f
Load
source
Harmonic
Active
producing
filter
load
IEC  1821/11
Figure 2 – Series connection
3.1 Active and passive filters
3.1.1
active filter
a filter whose response to harmonics is either wholly or partially governed by a controlled
converter
3.1.2
passive filter
a filter whose response to harmonics is governed by the impedance of its components
3.2 Active filter topologies
3.2.1
shunt active filter
an active filter connected high-voltage (HV) to low-voltage (LV) or HV to ground such that it
experiences the full a.c. or d.c. voltage of the HVDC system or its a.c. connection (see
Figure 1)
3.2.2
series active filter
an active filter connected between the HVDC converter and the a.c. or d.c. supplies such that
it must withstand the full HVDC system current, either a.c. or d.c. (see Figure 2)
3.2.3
shunt and series active filter
an active filter containing both series and shunt elements as defined above

+AMD2:2020 CSV © IEC 2020
3.3 Power semiconductor terms
NOTE There are several types of power semiconductor devices which can be used in active filters for HVDC and
currently the IGBT is the major device used in such converters. The term IGBT is used throughout this report to
refer to the switched valve device. However, the report is equally applicable to other types of devices with turn-off
capability in most of the parts.
3.3.1
insulated gate bipolar transistor
IGBT
a controllable switch with the capability to turn-on and turn-off a load current
turn-off semiconductor device with a gate terminal (G) and two load terminals emitter (E) and
collector (C)
NOTE 1 An IGBT has three terminals: a gate terminal (G) and two load terminals - emitter (E) and collector (C).
NOTE 2 By applying appropriate gate to emitter voltages, current in one direction can be controlled, i.e. turned on
and turned off.
3.3.2
free-wheeling diode
FWD
power semiconductor device with diode characteristic.
NOTE 1 A FWD has two terminals: an anode (A) and a cathode (K). The current through the FWDs is in opposite
direction to the IGBT current.
NOTE 2 FWDs are characterized by the capability to cope with high rates of decrease of current caused by the
switching behaviour of the IGBT.
3.3.3
IGBT-diode pair
arrangement of IGBT and FWD connected in inverse parallel
3.3.4
turn-off semiconductor device
controllable semiconductor device which may be turned on and off by a control signal
EXAMPLE Insulated gate bipolar transistor (IGBT).
NOTE There are several types of turn-off semiconductor devices which can be used in active filters for HVDC.
Currently, the IGBT is the major device used in such converters. The term IGBT is used throughout this Technical
Report to refer to the turn-off semiconductor device. However, this Technical Report is equally applicable to other
types of devices with turn-off capability in most of the parts.
3.4 Converter topologies
3.4.1
pulse width modulation
PWM
a converter operation technique using high frequency switching with modulation to produce a
particular waveform when smoothed
3.4.2
two-level converter
a converter in which the voltage at between the a.c. terminals of the voltage sourced
converter (VSC) unit and the VSC unit midpoint is switched between two discrete d.c. voltage
levels
3.4.3
three-level converter
a converter in which the voltage at between the a.c. terminals of the voltage sourced
converter (VSC) unit and the VSC unit midpoint is switched between three discrete d.c.
voltage levels
– 10 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
3.4.4
multi-level converter
a converter in which the voltage at the a.c. terminals of the VSC unit is switched between
more than three discrete d.c. voltage level.
4 Active filters in HVDC applications
4.1 General
The conversion process in an HVDC transmission system introduces harmonic currents into
the d.c. transmission lines and the a.c. grid connected to the HVDC converters. These
harmonic currents may cause interference in the adjacent systems, like telecommunication
equipment. The conventional solution to reduce the harmonics has been to install passive
filters in HVDC converter stations [1] . When the power line consists of cables, this filtering is
normally not necessary. The development of power electronics devices and digital computers
has made it possible to achieve a new powerful way for a further reduction of harmonic levels,
namely, active filters.
The active filters can be divided into two groups, active a.c. and d.c. filters. Active d.c. filter
installations are in operation in several HVDC links and have been economically competitive
due to more onerous requirements for telephone interference levels on the d.c. overhead lines
(Figure 3). An active a.c. filter is already in operation as well. In addition to the active d.c.
filter function of mitigating the harmonic currents on the d.c. overhead lines, the active a.c.
filters may be part of several solutions in the HVDC scheme to improve reactive power
exchange with the a.c. grid and to improve dynamic stability.
Filter
Passive d.c. filter
cost
Active d.c. filter
Allowable interference level
IEC  1822/11
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost
The features of active filters are the following:
• Active a.c. and d.c. filters consist of two parts, a passive part and a corresponding active part
which are loaded with the same currents. Due to the fact that the passive a.c. filter is used
to supply the HVDC converter demand of reactive power and thereby loaded with the
fundamental current, the required rating of the d.c. filter active part is lower than the one
of the a.c. filter active part.
• The control philosophy for the active d.c. filter is less complex than for the a.c. one.
• The present HVDC applications where active a.c. filters are feasible will be limited, due to
the fact that a.c. filters are also required to supply the HVDC converter demand of reactive
power. The filter size is therefore often well above the filtering demand.
___________
Figures in square brackets refer to the Bibliography.

+AMD2:2020 CSV © IEC 2020
Many recent and future HVDC projects use new converter technologies which allow the
reactive compensation to be separated from the a.c. filters and thereby make the active a.c.
filter more feasible. For line-commutated converters, capacitor commutated converters (CCC)
and the controlled series capacitor converter (CSCC) allow reduced reactive power
absorption. Moreover, self-commutated converters (which include most voltage sourced
converters) are able to control active and reactive power independently, avoiding the need for
separate reactive power compensation altogether.
4.2 Semiconductor devices available for active filters
Three types of power semiconductor devices, suitable for use in an active filter, are available
at present:
• metal-oxide-semiconductor field-effect transistor (MOSFET);
• insulated gate bipolar transistor (IGBT);
• gate turn-off thyristor (GTO) and other thyristor-derived devices such as the gate
commutated thyristor (GCT) and integrated gate commutated thyristor (IGCT).
The MOSFET is an excellent switching device capable of switching at very high frequencies
with relatively low losses, but with limited power handling capability.
The IGBT has a switching frequency capability which, although very good and sufficient to
handle the frequencies within the active d.c. filter range, is inferior to the MOSFET. However
the IGBT power handling is significantly higher than the MOSFET.
The GTO-type devices has the highest power handling capacity, but with a relatively limited
switching speed far below the required frequency range for active d.c. filter. The use of GTO-
type devices will probably be limited to handle frequencies below a few hundred of hertz.
The relatively high frequency band for active d.c. filtering excludes the use of thyristors and
GTO. Even though the MOSFET and IGBT are suited as switching elements in a power stage,
the limited power handling capacity on MOSFET and the installed cost evaluations tend to
point on the use of IGBT in future power stages.
5 Active d.c. filters
5.1 Harmonic disturbances on the d.c. side
The main reason for specifying demands on the d.c. circuit is to keep disturbances in nearby
telephone lines within an acceptable limit, which will vary depending on whether the telephone
system consists of overhead lines or underground cables which are generally shielded and
therefore have a better immunity [2]. A summary is given below to illustrate the demands
which made it feasible to install the active filters. As described, the demand on disturbances
can appear as an harmonic current on the d.c. line or as an induced voltage U in a fictive
ind
telephone line. It should be kept in mind that the harmonic demand, the specific HVDC system
and surroundings (earth resistivity, telephone system, etc.) all together define the d.c. filter
solution.
The specified requirements:
• The induced voltage U in a theoretically 1 km telephone line situated 1 km from the d.c.
ind
overhead line shall should be below 10 mV for monopolar operation limited to a set value.
• A one minute mean value of the equivalent psophometric current I fed into current I at any
pe pc
point on the d.c. pole overhead line shall should be below 400 mA limited to a set value.
The mentioned induced voltage and the equivalent psophometric current are defined as:

– 12 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
U (2π⋅ f⋅MI⋅⋅ p ) (1)
∑ n nn
ind
n=1
(2)
I ()k⋅⋅pI
pe ∑ n n n
p
n=1
where
th
f is the frequency of the n harmonic,
n
M is the mutual inductance between the telephone line and the power line,
k = f n/800,
n 1 ×
th
I is the vectorial sum of the n harmonic current flowing in the line conductors (common
n
mode/earth mode current),
th
p is the n psophometric weighting factor defined by CCITT Directives 1963 [3] (see also
n
Table 1),
th
p is the 16 psophometric weighting factor.
The characteristic harmonics n = 12, 24, 36, 48 as well as the non-characteristic harmonics
up to n = 50 shall be considered.
Table 1 – The psophometric weighting factor at selected frequencies
Frequency, H
50 100 300 600 800 1 000 1 200 1 800 2 400 3 000
z
n
1 2 6 12 16 20 24 36 48 60
p factor
0,0007 0,009 0,295 0,794 1,0 1,122 1,0 0,76 0,634 0,525
n
P k
0,00004 0,001 0,111 0,595 1,0 1,403 1,5 1,71 1,902 1,969
n × n
5.2 Description of active d.c. filters
5.2.1 General
Active d.c. filters use a controllable converter to introduce currents in the network, presenting
a waveform which counteracts the harmonics. This subclause describes types of power
stages, converters to be used in active filters and the possible connections in HVDC schemes.
5.2.2 Types of converters available
5.2.2.1 General
Two basic types of switching converters are possible in an active d.c. filter; the current-source
converter using inductive energy storage and the voltage-sourced converter (VSC) using
capacitive energy storage.
5.2.2.2 Current source converters
In a current-source converter, the d.c. element is a current source, which normally consists of
a d.c. voltage source power supply in series with an inductor. For correct operation, the
current should flow continuously in the inductor. Hence if a.c. current is not required current
must be by-passed within the converter. This fact restricts the switching actions. A simple
current-source converter is shown in Figure 4.
=
=
+AMD2:2020 CSV © IEC 2020
5.2.2.3 Voltage sourced converters (VSC)
In the VSC, the d.c. element is a voltage source. This may be a d.c. power supply or, in the
case of an active d.c. filter application, an energy storage unit. In practice, the voltage source
for an active d.c. filter power stage is usually a capacitor with a small power supply to offset
the power stage losses. A VSC also has the property that its a.c. output appears as a voltage
source.
A circuit of simple VSC is shown in Figure 5.
L I
IEC  1823/11
L I
IEC
Key
1 AC current
Figure 4 – Simple current source converter

– 14 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
C
+
V
–
Key IEC  1824/11
1 AC voltage
Figure 5 – Simple voltage sourced converter
5.2.2.4 Comparison between current and voltage sourced converters
The current-source converter has a high internal impedance for currents through the
converter, while the VSC has a low impedance. The VSC has no constraints on the switching
pattern which can be employed, while the current-source converter is restricted as described
above. The necessity for continuous current in the current-source converter, combined with
the fact that (neglecting superconductivity) an inductor has higher losses than a capacitor,
ensures that the losses in the current-source converter are higher than in the VSC. Another
parameter influencing losses is that a current-source converter needs switching devices which
can block reverse voltage. Most of the available semiconductors do not fulfil this requirement.
In this case an extra diode in series with each device is necessary and this again increases
the losses. Some GTOs are able to support reverse voltage, but these are less common than
the GTOs which do not support reverse voltage. The former have higher losses than the more
common devices.
Conclusion: Considering the above properties of current-source converter and VSC, the type
most suited for power stage applications, particularly high power, is the VSC. The VSC has
been preferred in all HVDC projects applicable today.
5.2.3 Connections of the active d.c. filter
5.2.3.1 General
Advantages and disadvantage of connecting the active filters at locations shown in Figure 6
have been discussed in several papers [4], [5], [6]. The active filters can either be connected
as shunt active filters or as series active filters.

+AMD2:2020 CSV © IEC 2020
Smoothing
reactor
Pole
Active
filter 4
Passive
d.c. filter
HVDC
converter
Active Active
filter 3
filter 2
Active Active
filter 5a filter 5b
Active
filter 1
Electrode
IEC  1825/11
Figure 6 – Possible connections of active d.c. filters
5.2.3.2 "Active filter 1" connection
The active d.c. filter realised in HVDC schemes today is connected as the shunt “Active filter
1” in Figure 6. By connecting the active filter in series with the passive d.c. filter, usually a
12/24th double tuned filter, the active filter rating can be reduced. A VSC is chosen in order to
make the smallest influence on the original function of the passive filter, especially on
frequencies where the control algorithm is not active.
5.2.3.3 "Active filter 2" connection
The “Active filter 2” in Figure 6 is similar to the shunt “Active filter 1” solution. The power
consumption of the tuning circuit in the passive filter will probably reduce the efficiency to
inject harmonic currents to counteract the disturbance current and thereby increase the rating
of the converter. There may be additional inductance inserted in series with the active part.
5.2.3.4 "Active filter 3" connection
The “Active filter 3” in Figure 6 is a series active filter described in [11], but there is a lack of
knowledge of such a system. The active filter converter must be connected to the HVDC
system by a coupling transformer T . To prevent saturation of the coupling transformer T by the
c c
direct load current of the HVDC converter I , the core must have an air gap.
dconv
In this way, the coupling transformer T is a d.c. reactor with a galvanic insulated auxiliary
c
winding to connect the active filter (converter). To achieve no ripple voltage at the point of
connection of the passive d.c. filter and therefore no ripple current in the d.c. pole line, the
a voltage which compensate the ripple
active filter must generate across the main winding T
c
voltage U of the d.c. side of the HVDC converter.
r
The a.c. load current I of the main winding of T is determined by U and its inductance value
r c r
L , the converter transformer inductance and the smoothing reactor inductance. The rating of
r
2 ×
T is determined by (I + I ) L . The rating of the active filter (converter) is determined by
c dconv r r
U /L . Hence the economical optimisation between the active and passive part of the active
dr r
– 16 – IEC TR 62544:2011+AMD1:2016
+AMD2:2020 CSV © IEC 2020
filter can be adjusted by increasing L . The rating of T will be increased and the rating of the
r c
active filter part will be decreased or vice versa.
The smoothing reactor (which is already designed for U ) is eventually an alternative for T ,
dr c
although is must be relocated to the neutral side of the HVDC converter valve and provided
with an auxiliary winding.
The advantages of this connection are:
• There are no harmonics in the HVDC converter direct current.
• The control algorithm of a series filter will probably be simplified compared to the shunt
filter control.
The disadvantages are:
• Even by an optimal design, the rating of T and the active filter part will be considerable.
c
• The T side of the HVDC converter has no earth potential, which should be considered in the
c
design of the HVDC converter and the transformer T .
c
5.2.3.5 "Active filter 4" connection
The “Active filter 4” in Figure 6 is a series active filter fundamentally with the same
configuration and problems as the “Active filter 3”. The filter is connected at the pole bus on
the line side of the d.c. filter capacitor. The major advantage of this arrangement is that the
active filter rating (due to the fact that the HVDC converter output ripple voltage is attenuated
already by the passive filter) will be considerably less than the “Active filter 3” connection.
The disadvantage of this arrangement is that the filter is situated at line potential and that the
filter must conduct the whole direct current.
5.2.3.6 "Active filter 5" connection
There has not been any information describing “Active filter 5a and 5b” in Figure 6. The
application of such a filter is expected to be limited to either higher frequencies or lower
frequencies and not the whole frequency range as the “Active filter 1 and 2”.
5.2.3.7 Conclusion on active filter connections
The advantages and disadvantages of the most possible connections of the active part of the
d.c. filter have been described above. The main conclusion is that series connections of
active filters on the d.c. side are possible but with the facts available today not
recommendable.
The injected power for active filtering can be reduced by choosing the optimum line injection
point on the passive circuit or the d.c. line. All active d.c. filter applications implemented today
and in the near future will use the “Active filter 1” solution in Figure 6. The remaining of this
paper therefore discusses the “Active filter 1” solution.
5.2.4 Characteristics of installed active d.c. filters
The active d.c. filters (Figure 7) are connected in feedback control loop. The line current is
measured by a current transducer. The current signal is passed
...

IEC TR 62544 ®
Edition 1.1 2016-04
CONSOLIDATED VERSION
TECHNICAL
REPORT
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inside
High-voltage direct current (HVDC) systems – Application of active filters

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IEC TR 62544 ®
Edition 1.1 2016-04
CONSOLIDATED VERSION
TECHNICAL
REPORT
colour
inside
High-voltage direct current (HVDC) systems – Application of active filters

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.99 ISBN 978-2-8322-3332-0

IEC TR 62544 ®
Edition 1.1 2016-04
CONSOLIDATED VERSION
REDLINE VERSION
colour
inside
High-voltage direct current (HVDC) systems – Application of active filters

– 2 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
3.1 Active and passive filters . 8
3.2 Active filter topologies . 8
shunt active filter . 8
3.3 Power semiconductor terms . 9
3.4 Converter topologies . 9
4 Active filters in HVDC applications . 10
4.1 General . 10
4.2 Semiconductor devices available for active filters. 11
5 Active d.c. filters . 11
5.1 Harmonic disturbances on the d.c. side . 11
5.2 Description of active d.c. filters . 12
5.2.1 General . 12
5.2.2 Types of converters available . 12
5.2.3 Connections of the active d.c. filter . 14
5.2.4 Characteristics of installed active d.c. filters . 16
5.3 Main components in a d.c. active filter . 17
5.3.1 General . 17
5.3.2 Passive part . 17
5.3.3 Current transducer . 19
5.3.4 Control system . 19
5.3.5 Amplifier . 20
5.3.6 Transformer . 20
5.3.7 Protection circuit and arrester . 20
5.3.8 Bypass switch and disconnectors . 20
5.4 Active d.c. filter control . 20
5.4.1 General . 20
5.4.2 Active d.c. filter control methods . 21
5.5 Example – Performance of the Skagerrak 3 HVDC Intertie active d.c. filter . 24
5.6 Conclusions on active d.c. filters . 25
6 Active a.c. filters in HVDC applications . 26
6.1 General . 26
6.2 Harmonic disturbances on the a.c. side of a HVDC system. 26
6.3 Passive filters . 27
6.3.1 Conventional passive filters . 27
6.3.2 Continuously tuned passive filters . 27
6.4 Reasons for using active filters in HVDC systems . 28
6.5 Operation principles of active filters . 29
6.5.1 Shunt connected active filter . 29
6.5.2 Series connected active filter . 30
6.6 Parallel and series configuration . 30
6.6.1 General . 30
6.6.2 Hybrid filter schemes . 30

© IEC 2016
6.7 Converter configurations . 31
6.7.1 Converters . 31
6.8 Active a.c. filter configurations . 34
6.8.1 Active a.c. filters for low voltage application . 34
6.8.2 Active a.c. filters for medium voltage application . 34
6.8.3 Active a.c. filters for HVDC applications . 34
6.9 Series connected active filters . 36
6.10 Control system . 36
6.10.1 General . 36
6.10.2 Description of a generic active power filter controller . 36
6.10.3 Calculation of reference current . 37
6.10.4 Synchronous reference frame (SRF) . 38
6.10.5 Other control approaches . 39
6.10.6 HVDC a.c. active filter control approach . 39
6.11 Existing active a.c. filter applications . 39
6.11.1 Low and medium voltage . 39
6.11.2 High voltage applications . 39
6.12 Overview on filter solutions for HVDC systems . 40
6.12.1 Solution with conventional passive filters . 40
6.12.2 Solution with continuously tuned passive filters . 41
6.12.3 Solution with active filters . 42
6.12.4 Solution with continuously tuned passive filters and active filters . 42
6.12.5 Study cases with the CIGRÉ HVDC model . 42
6.13 ACfilters for HVDC installations using VSC . 44
6.14 Conclusions on active a.c. filters . 45
Bibliography . 46

Figure 1 – Shunt connection . 8
Figure 2 – Series connection . 8
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost . 10
Figure 4 – Simple current source converter . 13
Figure 5 – Simple voltage sourced converter . 14
Figure 6 – Possible connections of active d.c. filters . 15
Figure 7 – Filter components in the active filter . 18
Figure 8 – Impedance characteristics of different passive filters . 18
Figure 9 – Basic control loop of an active d.c. filter . 22
Figure 10 – Measured transfer function of external system, Baltic Cable HVDC link . 23
Figure 11 – Feedforward control for the active d.c. filter . 23
Figure 12 – Measured line current spectra, pole 3 operated as monopole . 25
Figure 13 – Continuously tuned filter . 27
Figure 14 – Example of current waves . 30
Figure 15 – Series and parallel connection . 31
Figure 16 – Hybrid configuration . 31
Figure 17 – Three phase current-source converter . 32
Figure 18 – Three phase 2 level voltage-sourced converter (three-wire type) . 33
Figure 19 – Three phase 3 level voltage-sourced converter (three-wire type) . 33

– 4 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
Figure 20 – Single-phase voltage sourced converter . 34
Figure 21 – Active filter connected to the HV system through a single-tuned passive
filter . 35
Figure 22 – Active filter connected to the HV system through a double-tuned passive
filter . 35
Figure 23 – Using an LC circuit to divert the fundamental current component . 36
Figure 24 – Per-phase schematic diagram of active filter and controller . 37
Figure 25 – Block diagram of IRPT . 37
Figure 26 – Block diagram of SRF . 39
Figure 27 – Plots from site measurements . 41
Figure 28 – Filter configuration and a.c. system harmonic impedance data . 43

Table 1 – The psophometric weighting factor at selected frequencies . 12
Table 2 – Voltage to be supplied by the active part with different selections of passive
parts . 19
Table 3 – Major harmonic line currents, pole 3 operated as monopole . 25
Table 4 – Preferred topologies for common LV and MV applications . 31
Table 5 – Performance Requirements . 43
Table 6 – Parameters of filters at a.c. substation A (375 kV) . 44
Table 7 – Parameters of filters at a.c. substation B (230 kV) . 44
Table 8 – Performance results of filters . 44

© IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
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
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
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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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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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services carried out by independent certification bodies.
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
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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 62544 edition 1.1 contains the first edition (2011-08) [documents 22F/242/DTR and
22F/250/RVC] and its amendment 1 (2016-04) [documents 22F/377/DTR and 22F/381A/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 62544:2011+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/TR 62544, 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 electronics.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
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.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
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that it contains colours which are considered to be usefu
understanding of its contents. Users should therefore print this document using a
colour printer.
© IEC 2016
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
APPLICATION OF ACTIVE FILTERS
1 Scope
This technical report gives general guidance on the subject of active filters for use in high-
voltage direct current (HVDC) power transmission. It describes systems where active devices
are used primarily to achieve a reduction in harmonics in the d.c. or a.c. systems. This
excludes the use of automatically retuned components.
The various types of circuit that can be used for active filters are described in the report,
along with their principal operational characteristics and typical applications. The overall aim
is to provide guidance for purchasers to assist with the task of specifying active filters as part
of HVDC converters.
Passive filters are specifically excluded from this report.
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/TS 60071-5, Insulation co-ordination – Part 5: Procedures for high-voltage direct current
(HVDC) converter stations
IEC 60633, Terminology for high-voltage direct-current (HVDC) transmission
IEC 61000 ( all parts), Electromagnetic compatibility (EMC)
IEC 61975, High-voltage direct current (HVDC) installations – System tests
IEC/TR 62001:2009, High-voltage direct current (HVDC) systems – Guidebook to the
specification and design evaluation of A.C. filters
IEC/TR 62543, High-voltage direct current (HVDC) power transmission using voltage sourced
converters (VSC)
IEEE 519, IEEE Recommended Practices and Requirements for Harmonic Control in
Electrical Power Systems
3 Terms and definitions
For the purposes of this technical report, the terms and definitions given in IEC 60633 and
IEC 62001:2009 for passive a.c. filters, as well as the following apply.
NOTE Only terms which are specific to active filters for HVDC are defined in this clause. Those terms that are
either identical to or obvious extensions of IEC 60633, IEC 62001:2009 and 62747 terminology have not been
defined.
– 8 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
I I
s l
Voltage Harmonic
source generator
I
f
Active
filter
IEC  1820/11
Figure 1 – Shunt connection
U U
s l
U
Net f
Load
source
Harmonic
Active
producing
filter
load
IEC  1821/11
Figure 2 – Series connection
3.1 Active and passive filters
3.1.1
active filter
a filter whose response to harmonics is either wholly or partially governed by a controlled
converter
3.1.2
passive filter
a filter whose response to harmonics is governed by the impedance of its components
3.2 Active filter topologies
3.2.1
shunt active filter
an active filter connected high-voltage (HV) to low-voltage (LV) or HV to ground such that it
experiences the full a.c. or d.c. voltage of the HVDC system or its a.c. connection (see
Figure 1)
3.2.2
series active filter
an active filter connected between the HVDC converter and the a.c. or d.c. supplies such that
it must withstand the full HVDC system current, either a.c. or d.c. (see Figure 2)

© IEC 2016
3.2.3
shunt and series active filter
an active filter containing both series and shunt elements as defined above
3.3 Power semiconductor terms
NOTE There are several types of power semiconductor devices which can be used in active filters for HVDC and
currently the IGBT is the major device used in such converters. The term IGBT is used throughout this report to
refer to the switched valve device. However, the report is equally applicable to other types of devices with turn-off
capability in most of the parts.
3.3.1
insulated gate bipolar transistor
IGBT
a controllable switch with the capability to turn-on and turn-off a load current
turn-off semiconductor device with a gate terminal (G) and two load terminals emitter (E) and
collector (C)
NOTE 1 An IGBT has three terminals: a gate terminal (G) and two load terminals - emitter (E) and collector (C).
NOTE 2 By applying appropriate gate to emitter voltages, current in one direction can be controlled, i.e. turned
on and turned off.
3.3.2
free-wheeling diode
FWD
power semiconductor device with diode characteristic.
NOTE 1 A FWD has two terminals: an anode (A) and a cathode (K). The current through the FWDs is in opposite
direction to the IGBT current.
NOTE 2 FWDs are characterized by the capability to cope with high rates of decrease of current caused by the
switching behaviour of the IGBT.
3.3.3
IGBT-diode pair
arrangement of IGBT and FWD connected in inverse parallel
3.3.4
turn-off semiconductor device
controllable semiconductor device which may be turned on and off by a control signal
EXAMPLE Insulated gate bipolar transistor (IGBT).
NOTE There are several types of turn-off semiconductor devices which can be used in active filters for HVDC.
Currently, the IGBT is the major device used in such converters. The term IGBT is used throughout this Technical
Report to refer to the turn-off semiconductor device. However, this Technical Report is equally applicable to other
types of devices with turn-off capability in most of the parts.
3.4 Converter topologies
3.4.1
pulse width modulation
PWM
a converter operation technique using high frequency switching with modulation to produce a
particular waveform when smoothed
3.4.2
two-level converter
a converter in which the voltage at between the a.c. terminals of the voltage sourced
converter (VSC) unit and the VSC unit midpoint is switched between two discrete d.c. voltage
levels
– 10 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
3.4.3
three-level converter
a converter in which the voltage at between the a.c. terminals of the voltage sourced
converter (VSC) unit and the VSC unit midpoint is switched between three discrete d.c.
voltage levels
3.4.4
multi-level converter
a converter in which the voltage at the a.c. terminals of the VSC unit is switched between
more than three discrete d.c. voltage level.
4 Active filters in HVDC applications
4.1 General
The conversion process in an HVDC transmission system introduces harmonic currents into
the d.c. transmission lines and the a.c. grid connected to the HVDC converters. These
harmonic currents may cause interference in the adjacent systems, like telecommunication
equipment. The conventional solution to reduce the harmonics has been to install passive
filters in HVDC converter stations [1] . When the power line consists of cables, this filtering is
normally not necessary. The development of power electronics devices and digital computers
has made it possible to achieve a new powerful way for a further reduction of harmonic
levels, namely, active filters.
The active filters can be divided into two groups, active a.c. and d.c. filters. Active d.c. filter
installations are in operation in several HVDC links and have been economically competitive
due to more onerous requirements for telephone interference levels on the d.c. overhead
lines (Figure 3). An active a.c. filter is already in operation as well. In addition to the active
d.c. filter function of mitigating the harmonic currents on the d.c. overhead lines, the active
a.c. filters may be part of several solutions in the HVDC scheme to improve reactive power
exchange with the a.c. grid and to improve dynamic stability.
Filter
Passive d.c. filter
cost
Active d.c. filter
Allowable interference level
IEC  1822/11
Figure 3 – Conceptual diagram of allowable interference level and d.c. filter cost
The features of active filters are the following:
• Active a.c. and d.c. filters consist of two parts, a passive part and a corresponding active part
which are loaded with the same currents. Due to the fact that the passive a.c. filter is used
to supply the HVDC converter demand of reactive power and thereby loaded with the
___________
Figures in square brackets refer to the Bibliography.

© IEC 2016
fundamental current, the required rating of the d.c. filter active part is lower than the one
of the a.c. filter active part.
• The control philosophy for the active d.c. filter is less complex than for the a.c. one.
• The present HVDC applications where active a.c. filters are feasible will be limited, due to
the fact that a.c. filters are also required to supply the HVDC converter demand of
reactive power. The filter size is therefore often well above the filtering demand.
Many recent and future HVDC projects use new converter technologies which allow the
reactive compensation to be separated from the a.c. filters and thereby make the active a.c.
filter more feasible. For line-commutated converters, capacitor commutated converters (CCC)
and the controlled series capacitor converter (CSCC) allow reduced reactive power
absorption. Moreover, self-commutated converters (which include most voltage sourced
converters) are able to control active and reactive power independently, avoiding the need for
separate reactive power compensation altogether.
4.2 Semiconductor devices available for active filters
Three types of power semiconductor devices, suitable for use in an active filter, are available
at present:
• metal-oxide-semiconductor field-effect transistor (MOSFET);
• insulated gate bipolar transistor (IGBT);
• gate turn-off thyristor (GTO) and other thyristor-derived devices such as the gate
commutated thyristor (GCT) and integrated gate commutated thyristor (IGCT).
The MOSFET is an excellent switching device capable of switching at very high frequencies
with relatively low losses, but with limited power handling capability.
The IGBT has a switching frequency capability which, although very good and sufficient to
handle the frequencies within the active d.c. filter range, is inferior to the MOSFET. However
the IGBT power handling is significantly higher than the MOSFET.
The GTO-type devices has the highest power handling capacity, but with a relatively limited
switching speed far below the required frequency range for active d.c. filter. The use of GTO-
type devices will probably be limited to handle frequencies below a few hundred of hertz.
The relatively high frequency band for active d.c. filtering excludes the use of thyristors and
GTO. Even though the MOSFET and IGBT are suited as switching elements in a power stage,
the limited power handling capacity on MOSFET and the installed cost evaluations tend to
point on the use of IGBT in future power stages.
5 Active d.c. filters
5.1 Harmonic disturbances on the d.c. side
The main reason for specifying demands on the d.c. circuit is to keep disturbances in nearby
telephone lines within an acceptable limit, which will vary depending on whether the
telephone system consists of overhead lines or underground cables which are generally
shielded and therefore have a better immunity [2]. A summary is given below to illustrate the
demands which made it feasible to install the active filters. As described, the demand on
disturbances can appear as an harmonic current on the d.c. line or as an induced voltage
U in a fictive telephone line. It should be kept in mind that the harmonic demand, the
ind
specific HVDC system and surroundings (earth resistivity, telephone system, etc.) all together
define the d.c. filter solution.
The specified requirements:
– 12 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
• The induced voltage U in a theoretically 1 km telephone line situated 1 km from the d.c.
ind
overhead line shall be below 10 mV for monopolar operation.
• A one minute mean value of the equivalent psophometric current I fed into the d.c. pole overhead
pe
line shall be below 400 mA.
The mentioned induced voltage and the equivalent psophometric current are defined as:
U (2p⋅ f⋅MI⋅⋅ p ) (1)
∑ n nn
ind
n=1
I ()k⋅⋅pI (2)
pe ∑ n n n
p
n=1
where
th
f is the frequency of the n harmonic,
n
M is the mutual inductance between the telephone line and the power line,
k = f n/800,
n 1 ×
th
I is the vectorial sum of the n harmonic current flowing in the line conductors (common
n
mode/earth mode current),
th
p is the n psophometric weighting factor defined by CCITT Directives 1963 [3] (see also
n
Table 1),
th
p is the 16 psophometric weighting factor.
The characteristic harmonics n = 12, 24, 36, 48 as well as the non-characteristic harmonics
up to n = 50 shall be considered.
Table 1 – The psophometric weighting factor at selected frequencies
Frequency, H
50 100 300 600 800 1 000 1 200 1 800 2 400 3 000
z
n 1 2 6 12 16 20 24 36 48 60
p factor
0,0007 0,009 0,295 0,794 1,0 1,122 1,0 0,76 0,634 0,525
n
P k
0,00004 0,001 0,111 0,595 1,0 1,403 1,5 1,71 1,902 1,969
n × n
5.2 Description of active d.c. filters
5.2.1 General
Active d.c. filters use a controllable converter to introduce currents in the network, presenting
a waveform which counteracts the harmonics. This subclause describes types of power
stages, converters to be used in active filters and the possible connections in HVDC schemes.
5.2.2 Types of converters available
5.2.2.1 General
Two basic types of switching converters are possible in an active d.c. filter; the current-
source converter using inductive energy storage and the voltage-sourced converter (VSC)
using capacitive energy storage.
=
=
© IEC 2016
5.2.2.2 Current source converters
In a current-source converter, the d.c. element is a current source, which normally consists of a
d.c. voltage source power supply in series with an inductor. For correct operation, the current
should flow continuously in the inductor. Hence if a.c. current is not required current must be
by-passed within the converter. This fact restricts the switching actions. A simple current-
source converter is shown in Figure 4.
5.2.2.3 Voltage sourced converters (VSC)
In the VSC, the d.c. element is a voltage source. This may be a d.c. power supply or, in the
case of an active d.c. filter application, an energy storage unit. In practice, the voltage source
for an active d.c. filter power stage is usually a capacitor with a small power supply to offset the
power stage losses. A VSC also has the property that its a.c. output appears as a voltage
source.
A circuit of simple VSC is shown in Figure 5.

IEC  1823/11
L I
IEC
Key
1 AC current
Figure 4 – Simple current source converter

– 14 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
C
+
V
–
Key IEC  1824/11
1 AC voltage
Figure 5 – Simple voltage sourced converter
5.2.2.4 Comparison between current and voltage sourced converters
The current-source converter has a high internal impedance for currents through the
converter, while the VSC has a low impedance. The VSC has no constraints on the switching
pattern which can be employed, while the current-source converter is restricted as described
above. The necessity for continuous current in the current-source converter, combined with
the fact that (neglecting superconductivity) an inductor has higher losses than a capacitor,
ensures that the losses in the current-source converter are higher than in the VSC. Another
parameter influencing losses is that a current-source converter needs switching devices
available semiconductors do not fulfil this
which can block reverse voltage. Most of the
requirement. In this case an extra diode in series with each device is necessary and this
again increases the losses. Some GTOs are able to support reverse voltage, but these are
less common than the GTOs which do not support reverse voltage. The former have higher
losses than the more common devices.
Conclusion: Considering the above properties of current-source converter and VSC, the type
most suited for power stage applications, particularly high power, is the VSC. The VSC has
been preferred in all HVDC projects applicable today.
5.2.3 Connections of the active d.c. filter
5.2.3.1 General
Advantages and disadvantage of connecting the active filters at locations shown in Figure 6
have been discussed in several papers [4], [5], [6]. The active filters can either be connected
as shunt active filters or as series active filters.

© IEC 2016
Smoothing
reactor
Pole
Active
filter 4
Passive
d.c. filter
HVDC
converter
Active Active
filter 3
filter 2
Active Active
filter 5a filter 5b
Active
filter 1
Electrode
IEC  1825/11
Figure 6 – Possible connections of active d.c. filters
5.2.3.2 "Active filter 1" connection
The active d.c. filter realised in HVDC schemes today is connected as the shunt “Active filter
1” in Figure 6. By connecting the active filter in series with the passive d.c. filter, usually a
12/24th double tuned filter, the active filter rating can be reduced. A VSC is chosen in order
to make the smallest influence on the original function of the passive filter, especially on
frequencies where the control algorithm is not active.
5.2.3.3 "Active filter 2" connection
The “Active filter 2” in Figure 6 is similar to the shunt “Active filter 1” solution. The power
consumption of the tuning circuit in the passive filter will probably reduce the efficiency to
inject harmonic currents to counteract the disturbance current and thereby increase the rating
of the converter. There may be additional inductance inserted in series with the active part.
5.2.3.4 "Active filter 3" connection
The “Active filter 3” in Figure 6 is a series active filter described in [11], but there is a lack of
knowledge of such a system. The active filter converter must be connected to the HVDC
system by a coupling transformer T . To prevent saturation of the coupling transformer T by the
c c
direct load current of the HVDC converter I , the core must have an air gap.
dconv
In this way, the coupling transformer T is a d.c. reactor with a galvanic insulated auxiliary
c
winding to connect the active filter (converter). To achieve no ripple voltage at the point of
connection of the passive d.c. filter and therefore no ripple current in the d.c. pole line, the
active filter must generate across the main winding T a voltage which compensate the ripple
c
voltage U of the d.c. side of the HVDC converter.
r
The a.c. load current I of the main winding of T is determined by U and its inductance value
r c r
L , the converter transformer inductance and the smoothing reactor inductance. The rating of
r
2 ×
T is determined by (I + I ) L . The rating of the active filter (converter) is determined by
c dconv r r
U /L . Hence the economical optimisation between the active and passive part of the active
dr r
– 16 – IEC TR 62544:2011+AMD1:2016 CSV
© IEC 2016
filter can be adjusted by increasing L . The rating of T will be increased and the rating of the
r c
active filter part will be decreased or vice versa.
The smoothing reactor (which is already designed for U ) is eventually an alternative for T ,
dr c
although is must be relocated to the neutral side of the HVDC converter valve and provided
with an auxiliary winding.
The advantages of this connection are:
• There are no harmonics in the HVDC converter direct current.
• The control algorithm of a series filter will probably be simplified compared to the shunt
filter control.
The disadvantages are:
• Even by an optimal design, the rating of T and the active filter part will be considerable.
c
• The T side of the HVDC converter has no earth potential, which should be considered in the
c
design of the HVDC converter and the transformer T .
c
5.2.3.5 "Active filter 4" connection
The “Active filter 4” in Figure 6 is a series active filter fundamentally with the same
configuration and problems as the “Active filter 3”. The filter is connected at the pole bus on
the line side of the d.c. filter capacitor. The major advantage of this arrangement is that the
active filter rating (due to the fact that the HVDC converter output ripple voltage is attenuated
already by the passive filter) will be considerably less than the “Active filter 3” connection.
The disadvantage of this arrangement is that the filter is situated at line potential and that the
filter must conduct the whole direct current.
5.2.3.6 "Active filter 5" connection
There has not been any information describing “Active filter 5a and 5b” in Figure 6. The
application of such a filter is expected to be limited to either higher frequencies or lower
frequencies and not the whole frequency range as the “Active filter 1 and 2”.
5.2.3.7 Conclusion on active filter connections
The advantages and disadvantages of the most possible connections of the active part of the
d.c. filter have been described above. The main conclusion is that series connections of
active filters on the d.c. side are possible but with the facts available today not
recommendable.
The injected power for active filtering can be reduced by choosing the optimum line injection
point on the passive circuit or the d.c. line. All active d.c. filter applications implemented
today and in the near future will use the “Active filter 1” solution in Figure 6. The remaining of
this paper therefore discusses the “Active filter 1” solution.
5.2.4 Characteristics of installed active d.c. filters
The active d.c. filters (Figure 7) are connected in feedback control loop. The line current is
measured by a current transducer. The current signal is passed through a light guide into a
computer. The computer calculates a signal to feed a VSC, so that the current injected at the
pole line is in opposition to the measured line current.
Characteristics of the active d.c. filters:
• frequency range 300 Hz to 3 000 Hz;
...