IEC TR 62001-3:2016

High-voltage direct current (HVDC) systems - Guidance to the specification and design evaluation of AC filters - Part 3: Modelling

IEC TR 62001-3:2016(E), which is a Technical Report, provides guidance on the harmonic interaction across converters, pre-existing harmonics, AC network impedance modelling and simulation of AC filter performance. The scope of this document covers AC side filtering for the frequency range of interest in terms of harmonic distortion and audible frequency disturbances. It excludes filters designed to be effective in the PLC and radio interference spectra. This document concerns the "conventional" AC filter technology and line-commutated high-voltage direct current (HVDC) converters. This first edition of IEC TR 62001-3, together with IEC TR 62001-1, IEC TR 62001-2 and IEC TR 62001-4, cancels and replaces IEC TR 62001 published in 2009. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to IEC TR 62001: a) expanded and supplemented Clause 6; b) new Clause 4; c) new Clause 5; d) new annexes on the location of worst case network impedance; e) accuracy of network component modelling at harmonic frequencies; f) further guidance for the measurement of harmonic voltage distortion; g) project experience of pre-existing harmonic issues; h) worked examples showing impact of pre-existing distortion; i) comparison of calculation methods.

General Information

• Status: Published
• Publication Date: 21-Sep-2016
• ICS: 29.200 - Rectifiers. Convertors. Stabilized power supply
• Technical Committee: SC 22F - Power electronics for electrical transmission and distribution systems
• Drafting Committee: MT 21 - TC 22/SC 22F/MT 21
• Current Stage: PPUB - Publication issued
• Start Date: 22-Sep-2016
• Completion Date: 31-Dec-2016

Relations

• Replaces: IEC TR 62001:2009 - High-voltage direct current (HVDC) systems - Guidebook to the specification and design evaluation of A.C. filters
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC TR 62001-3 ®
Edition 1.0 2016-09
TECHNICAL
REPORT
colour
inside
High-voltage direct current (HVDC) systems – Guidance to the specification and
design evaluation of AC filters –
Part 3: Modelling
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IEC TR 62001-3 ®
Edition 1.0 2016-09
TECHNICAL
REPORT
colour
inside
High-voltage direct current (HVDC) systems – Guidance to the specification and

design evaluation of AC filters –

Part 3: Modelling
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.200 ISBN 978-2-8322-3655-0

– 2 – IEC TR 62001-3:2016 © IEC 2016
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
2 Normative references. 10
3 Harmonic interaction across converters . 10
3.1 General . 10
3.2 Practical experience of problems . 11
3.3 Indicators of where harmonic interaction is significant . 13
3.4 Interaction phenomena . 14
3.5 Impact on AC filter design . 15
3.5.1 General . 15
3.5.2 AC side third harmonic . 15
3.5.3 Direct current on the AC side . 16
3.5.4 Characteristic harmonics . 16
3.6 General overview of modelling techniques . 16
3.6.1 General . 16
3.6.2 Time domain AC-DC-AC interaction model . 18
3.6.3 Frequency domain AC-DC-AC interaction model . 19
3.6.4 Frequency domain AC-DC interaction model . 19
3.6.5 Frequency domain current source model . 19
3.7 Interaction modelling . 20
3.7.1 General . 20
3.7.2 Coupling between networks . 20
3.7.3 Driving forces. 21
3.7.4 System harmonic impedances . 22
3.8 Study methods . 22
3.8.1 Frequency domain . 22
3.8.2 Time domain . 22
3.9 Composite resonance . 23
3.10 Core saturation instability . 23
3.11 Particular considerations for back-to-back converters . 23
3.12 Issues to be considered in the design process . 24
3.12.1 General . 24
3.12.2 Fundamental frequency and load issues . 24
3.12.3 Negative phase sequence . 25
3.12.4 Pre-existing harmonic distortion . 26
3.12.5 AC network impedance . 27
3.12.6 Converter control system . 28
3.12.7 Combination with "classic" harmonic generation . 29
3.12.8 Relative magnitude of pairs of low-order harmonics . 29
3.12.9 Superposition of contributions . 30
3.13 Parallel AC lines and converter transformer saturation . 30
3.14 Possible countermeasures . 32
3.14.1 AC (and/or DC) filters . 32
3.14.2 DC control design . 32
3.14.3 Operating restrictions and design protections . 33

3.15 Recommendations for technical specifications . 33
3.15.1 General . 33
3.15.2 Specified design data . 33
3.15.3 Requirements regarding calculation techniques . 34
4 AC network impedance modelling . 35
4.1 General . 35
4.2 Implications of inaccurate definition of network impedance . 36
4.3 Considerations for network modelling . 37
4.3.1 General . 37
4.3.2 Project life expectancy and robustness of data . 37
4.3.3 Network operating conditions . 37
4.3.4 Network impedances for performance and rating calculations . 38
4.3.5 Modelling of network components . 39
4.3.6 Representation of loads at harmonic frequencies . 40
4.4 Network harmonic impedance envelopes . 40
4.5 Methods of determining envelope characteristics . 43
4.5.1 General . 43
4.5.2 Low order harmonics . 43
4.5.3 Mid-range and higher order harmonics . 44
4.5.4 Balancing of risk and benefit . 45
4.5.5 Consideration of tolerances on harmonic bands . 46
4.5.6 Two separate envelopes for one harmonic band . 48
4.5.7 Critical envelope parameters . 49
4.5.8 Impedance envelopes for performance and rating conditions . 49
4.6 Examples of the impact of different network impedance representations . 50
4.6.1 Effect of network envelope parameters on resultant distortion . 50
4.6.2 Effect of network minimum resistance on filter rating . 53
4.7 Interharmonic impedance assessment . 54
4.8 Measurement of network harmonic impedance . 56
4.9 Conclusions . 57
5 Pre-existing harmonics . 57
5.1 General . 57
5.2 Modelling and measurement of pre-existing harmonic levels . 58
5.3 Harmonic performance evaluation, methods and discussion . 60
5.3.1 General . 60
5.3.2 "Incremental" harmonic performance evaluation . 60
5.3.3 "Aggregate" harmonic performance evaluation . 61
5.3.4 Both "incremental" and "aggregate" performance evaluation . 62
5.3.5 "Incremental" and "maximum magnification factor" harmonic
performance evaluation . 63
5.4 Calculation of total harmonic performance indices . 63
5.5 Harmonic rating evaluation . 64
5.6 Difficulties with the voltage source/worst network model for rating . 65
5.6.1 Background . 65
5.6.2 Illustration of the voltage source/worst network method . 66
5.7 Further possible calculation procedures for rating evaluation . 68
5.7.1 Using measured levels of pre-existing distortion . 68
5.7.2 Applying compatibility level voltage source at the filter busbar. 70

– 4 – IEC TR 62001-3:2016 © IEC 2016
5.7.3 Limiting the filter bus harmonic voltage to a maximum level for filter
rating (MLFR) . 72
5.7.4 Limiting total source distortion to the defined THD . 73
5.7.5 Limiting harmonic order of pre-existing distortion . 75
5.8 Conclusions . 75
Annex A (informative) Location of worst-case network impedance . 76
Annex B (informative) Accuracy of network component modelling at harmonic
frequencies . 79
B.1 General . 79
B.2 Loads . 79
B.3 Transformers . 82
B.3.1 Transformer reactance . 82
B.3.2 Transformer resistance . 82
B.4 Transmission lines . 85
B.5 Synchronous machines . 87
B.6 Modelling of resistance in harmonic analysis software . 88
Annex C (informative) Further guidance for the measurement of harmonic voltage
distortion . 91
Annex D (informative) Project experience of pre-existing harmonic issues . 93
D.1 General . 93
D.2 Third harmonic overload of filters in a back-to-back system . 93
D.3 Third and fifth harmonic overload of filters in a line transmission . 94
th
D.4 Overload of a DC side 6 harmonic filter . 94
Annex E (informative) Worked examples showing impact of pre-existing distortion . 96
E.1 General . 96
E.2 Pre-existing distortions . 97
E.2.1 Example 1 – Illustration of magnification . 97
E.2.2 Impact of network impedance parameters . 101
Annex F (informative) Comparison of calculation methods . 103
F.1 General . 103
F.2 Reference case – Converter generated harmonics only . 106
F.3 Method 1 – Source voltages behind impedance sector . 106
F.4 Method 2 – Source voltages at filter bus (see 5.7.2) . 106
F.5 Method 3 – Limiting the filter bus harmonic voltage to a maximum level for
filter rating (MLFR) (see 5.7.3) . 107
F.6 Method 4 – Limiting total source distortion to the THD level (see 5.7.4) . 107
th
F.7 Method 5 – Pre-existing harmonics considered only up to the 10 , with
10 % margin on converter generation for remainder (see 5.7.5) . 110
Bibliography . 111

Figure 1 – Key elements of a complete AC-DC-AC harmonic interaction model . 17
Figure 2 – Equivalent circuit for evaluation of harmonic interaction with DC side
interaction frequency greater than AC side fundamental frequency . 21
th th
Figure 3 – DC side 6 harmonic voltage due to AC side 5 harmonic (fixed angle) and
th
7 harmonic (varying angle) . 27
th th
Figure 4 – Simple circuit used to represent AC network impedance at 5 and 7
harmonics . 28
Figure 5 – Example of a single impedance locus for harmonic orders 2 to 49 . 41

Figure 6 – Example of simple circle envelope encompassing all scatter points for
harmonic orders 2 to 49 . 42
th th
Figure 7 – Example of an impedance envelope for 7 to 13 harmonic with
associated scatter plots . 44
th th
Figure 8 – Example of an impedance envelope for 13 to 19 harmonic with
associated scatter plots . 45
th th
Figure 9 – Example of an impedance envelope for 19 to 25 harmonic with
associated scatter plots . 45
Figure 10 – Example of the need to extend the band of harmonics to allow for
resonance effects . 47
Figure 11 – Application of tolerance range in percentage of the harmonic number . 48
Figure 12 – Application of tolerance range in percentage of the harmonic number,
th
zoomed to show 11th and 13 harmonics . 48
Figure 13 – Example showing two impedance envelopes for a particular band . 49
Figure 14 – Example of impedance envelopes under "performance" and "rating"
th th
conditions for harmonic orders 4 to 7 . 50
Figure 15 – Example of impedance envelopes "performance" and "rating" conditions
th st
for harmonic orders 25 to 31 . 50
Figure 16 – Discrete envelopes for different groups of harmonics . 51
Figure 17 – Example showing a distributed generation causing about 15 % attenuation
of ripple control signal at the PCC . 55
Figure 18 – Generic circuit model for calculation of harmonic performance or rating . 59
Figure 19 – Illustration of basic voltage quality concepts with time/location statistics
covering the whole system . 60
Figure 20 – Circuit model for calculation of incremental performance . 61
th
Figure 21 – Equivalent circuit of a network for the h harmonic . 66
Figure 22 – Typical voltage magnification factor . 67
Figure 23 – Pre-existing distortion set to measured levels (plus margin) . 68
Figure 24 – Pre-existing distortion applied directly at the filter bus . 70
Figure 25 – Harmonic voltage stress on a shunt capacitor with IEC planning levels
applied . 72
Figure A.1 – Equivalent circuit model for demonstration of worst-case resonance
between AC filters and the network . 76
Figure A.2 – Diagram indicating vectors Z , Z and Z . 77
F N H
Figure B.1 – Typical equivalent load network . 80
Figure B.2 – Relative error of equivalent load loss resistance R of using [28]
n
compared with Electra 167 [27] model . 83
Figure B.3 – Effect of temperature on transformer load loss . 84
Figure B.4 – Ratio between harmonic and fundamental frequency resistance as
calculated for balanced mode components and calculated from averages of reduced Z
matrix resistance values . 86
Figure B.5 – Ratio between harmonic and fundamental frequency resistance as
calculated for balanced mode components and calculated from averages of reduced Z

matrix resistance values, for varying earth resistivity . 87
Figure B.6 – Comparison of synchronous machine reactance between [4-1]
recommendation and test measurements for a salient pole hydro generator of 370 MVA . 87
Figure B.7 – Comparison of synchronous machine resistance between [17]
recommendation and test measurements for a salient pole hydro generator of 370 MVA . 88
Figure B.8 – Comparison of different approximations for resistance variations . 89

– 6 – IEC TR 62001-3:2016 © IEC 2016
Figure B.9 – Network impedance for Araraquara substation . 90
Figure E.1 – Harmonic models for converter and for pre-existing distortion . 97
Figure E.2 – Geometrical visualisation of selecting worst-case impedance for converter
harmonics . 97
Figure E.3 – Simple filter scheme to illustrate magnification . 98
Figure E.4 – Plots illustrating magnification of various pre-existing harmonics . 101
Figure F.1 – Network impedance sector used in example . 103
Figure F.2 – Assumed filter scheme for examples of different methods of calculation . 104
Figure F.3 – IEC planning levels used for source voltages in the study . 105

Table 1 – Dominant frequencies in AC–DC harmonic interaction . 15
Table 2 – Comparison of calculated harmonic voltage distortion between two methods
of representing network harmonic impedance . 52
Table 3 – Comparison of calculated harmonic voltage distortion considering the
variation of network impedance angle . 53
Table 4 – Comparison of calculated filter harmonic current considering the variation of
network minimum resistance and filter detuning . 54
Table 5 – Amplification factor tanΦ at different network impedance angles . 66
Table 6 – Variation of calculated filter harmonic current as a function of detuning . 71
Table B.1 – Constants for resistance adjustment – five parameter equations . 89
Table E.1 – Parameters of elements of a simplified filter scheme shown in Figure E.3 . 98
Table E.2 – Voltage and current distortion for Z = 1 Ω and varying Φ . 101
min
Table E.3 – Voltage and current distortion for Φ = ±85° and varying Z . 102
min
Table F.1 – Table F.1 – Parameters of components of filters shown in Figure F.2 . 104
Table F.2 – Component rating calculated using different calculation methods . 106
Table F.3 – Rating calculations using Method 3 – for BP1113 C1 . 107
Table F.4 – Rating calculations using Method 3 – for HP24 R1 . 109
Table F.5 – Rating calculations using Method 4 – for BP1113 C1 . 110

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
GUIDANCE TO THE SPECIFICATION AND
DESIGN EVALUATION OF AC FILTERS –

Part 3: Modelling
FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62001-3, which is a Technical Report, has been prepared by subcommittee 22F:
Power electronics for electrical transmission and distribution systems, of IEC technical
committee 22: Power electronic systems and equipment.
This first edition of IEC TR 62001-3, together with IEC TR 62001-1, IEC TR 62001-2 and
IEC TR 62001-4, cancels and replaces IEC TR 62001 published in 2009. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to IEC TR 62001:

– 8 – IEC TR 62001-3:2016 © IEC 2016
a) expanded and supplemented Clause 6;
b) new Clause 4;
c) new Clause 5;
d) new annexes on the location of worst case network impedance;
e) accuracy of network component modelling at harmonic frequencies;
f) further guidance for the measurement of harmonic voltage distortion;
g) project experience of pre-existing harmonic issues;
h) worked examples showing impact of pre-existing distortion;
i) comparison of calculation methods.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
22F/411/DTR 22F/415/RVC
Full information on the voting for the approval of this document can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 62001 series, published under the general title High-voltage direct
current (HVDC) systems – Guidance to the specification and design evaluation of AC filters,
can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability dateindicated 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
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
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INTRODUCTION
The IEC TR 62001 series is structured in four parts:
Part 1 – Overview
This part concerns specifications of AC filters for high-voltage direct current (HVDC)
systems with line-commutated converters, permissible distortion limits, harmonic
generation, filter arrangements, filter performance calculation, filter switching and reactive
power management and customer specified parameters and requirements.
Part 2 – Performance
This part deals with current-based interference criteria, design issues and special
applications, field measurements and verification.
Part 3 – Modelling
This part addresses the harmonic interaction across converters, pre-existing harmonics,
AC network impedance modelling, simulation of AC filter performance.
Part 4 – Equipment
This part concerns steady-state and transient ratings of AC filters and their components,
power losses, audible noise, design issues and special applications, filter protection,
seismic requirements, equipment design and test parameters.

– 10 – IEC TR 62001-3:2016 © IEC 2016
HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –
GUIDANCE TO THE SPECIFICATION AND
DESIGN EVALUATION OF AC FILTERS –

Part 3: Modelling
1 Scope
This part of IEC TR 62001, which is a Technical Report, provides guidance on the harmonic
interaction across converters, pre-existing harmonics, AC network impedance modelling and
simulation of AC filter performance.
The scope of this document covers AC side filtering for the frequency range of interest in
terms of harmonic distortion and audible frequency disturbances. It excludes filters designed
to be effective in the PLC and radio interference spectra.
This document concerns the "conventional" AC filter technology and line-commutated high-
voltage direct current (HVDC) converters.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC TR 61000-3-6:2008, Electromagnetic compatibility (EMC) – Part 3-6: Limits – Assessment
of emission limits for the connection of distorting installations to MV, HV and EHV power
systems
IEC 61000-4-30, Electromagnetic compatibility (EMC) – Part 4-30: Testing and measurement
techniques – Power quality measurement methods
3 Harmonic interaction across converters
3.1 General
In order to facilitate the analysis of harmonic generation by an HVDC converter, simplifying
assumptions are often made. Typically, the HVDC converter is regarded as a generator of
harmonic currents, with an infinite internal impedance. Such an assumption is reasonably
valid for practical purposes for most harmonics, and is the basis of the calculation methods
described in IEC TR 62001-1.
The customer should be aware, however, that such a simplified approach has limitations, and
can lead to incorrect analysis and design in some circumstances. In practice, the converter is
a link between the AC and DC side harmonic systems, and the AC side harmonic currents
may be strongly influenced by the harmonic impedance and harmonic current flows on the DC
side.
This is particularly true for low-order harmonics, and it is strongly recommended that the
analysis of third harmonic distortion and filtering requirements should take into account the
th th
AC/DC side harmonic interaction. At the 11 and 13 harmonics, the interaction effect can

also be significant. At higher frequencies, although interactions occur, their practical impact
on filter design and harmonic performance will normally be negligible.
Subclauses 3.2 to 3.15 give an overview of the interaction phenomena, focusing on practical
implications for AC filter design. The technical specification should make it clear that such
phenomena have to be taken into account, and the customer should be able to address the
subject in his evaluation of the bidders’ designs.
The terms "harmonic interaction" and "cross-modulation" are used synonymously in this
report. "Cross-modulation" is to be understood here as the process of harmonic transfer
across one converter, not, as it is sometimes used in a more specific sense, as the transfer of
harmonics from one AC system to another via the intervening HVDC link.
CIGRE Technical Brochure 143 [1] discusses in detail the technical aspects related to the
subject. This is a comprehensive review of the subject and included valuable references to
other publications. However, it concentrates on the theoretical aspects of calculation
procedures. CIGRE Technical Brochure 533 [2] contains more guidance on the practical
requirements for specifying and evaluating the treatment of cross-modulation during a tender
and subsequent design process and some aspects not included or only briefly covered in [1].
Some of the fundamental conclusions of [1] [2] and other referenced books and papers have
been summarised in this document.
Harmonic interaction across the converters can be a cause of problems, some examples of
which are illustrated in 3.2. Proper consideration of cross-modulation during the design
process can be of benefit, not only in avoiding such future problems in operation, but also in
rd
possibly simplifying designs. There are examples of where 3 harmonic filtering would have
been necessary when using a simplified classic calculation with a stiff current source, but
shown to be unnecessary when a full interaction model was applied, taking into account the
impedances on both sides of the converter. It should therefore not always be assumed that
consideration of cross-modulation will introduce problems or make the design more difficult –
it may actually resolve some difficult issues.
This document does not recommend prescribing calculation procedures and conditions in the
customer’s technical specification. In practice, issues involving harmonic interaction have
been treated in very different ways and using different study methods by various HVDC
contractors in the past. However, customers need comparable bids and want to be in control
of the risks associated with this phenomenon. Clause 3 will therefore pinpoint the important
assumptions that need to be defined in a technical specification and it will recommend that
contractors should justify their chosen calculation procedure and verify its accuracy.
3.2 Practical experience of problems
There has been considerable experience from operational HVDC schemes of adverse
harmonic interactions between AC and DC sides of the converter. Several experiences are
described in detail in [1]. A brief summary of some illustrative issues is given below.
One of the earliest incidents of reported interaction is related to the Kingsnorth HVDC link [3].
The particular combination of DC reactors and DC cable capacitance of the Willesden pole
resulted in a series resonance condition at the fundamental frequency in the DC circuit. A
nd
small 2 harmonic present on the AC side therefore resulted in high fundamental current on
the DC side, which in turn gave cause to unequal firing pulse spacing. This resulted in a
further contribution to the fundamental frequency voltage on the DC side and created small
direct currents in the converter transformers, which tended to saturate the transformer cores,
nd
generating further 2 harmonic distortions on the AC side. An additional flux control loop in
the HVDC control system solved the problem. This was one of the earliest examples of what
___________
Numbers in square brackets refer to the Bibliography.

– 12 – IEC TR 62001-3:2016 © IEC 2016
is known as "core saturation instability" which has subsequently been observed on several
HVDC schemes.
nd
At the Chateauguay back-to-back converter station, a similar phenomenon occurred. A 2
harmonic resonance condition was observed at the AC side of the converter [4]. The initially
nd
small pre-existing voltage distortion at the 2 harmonic was transferred to the DC side of the
converter. The resulting fundamental current on the DC side transferred back to the AC side
nd
as a 2 harmonic current. Due to a parallel resonance of the AC network impedance at the
nd
2 harmonic, this converter current gave rise to a corresponding voltage distortion, and the
loop was closed. The problem was solved by introducing an auxiliary direct current controller
which created an external damping for the fundamental frequency component of the direct
nd
current. Shunt filters tuned to the 2 harmonic have also been installed on the AC side.
During commissioning of the Gezhouba-Shanghai HVDC transmission, non-characteristic DC
harmonic currents of orders 2, 6 and 18 were observed which caused an unduly large
equivalent disturbing current. It was found that pre-existing voltage distortions of order 3, 5
and 7 on both AC sides caused a number of non-characteristic voltage distortions on the DC
side. These met with near-resonant conditions at these frequencies on the DC side resulting
in the observed current distortions. The implementation of additional resistive damping in the
DC filters, as well as changes to the neutral capacitor, solved the problem.
th
For the same scheme, it was also reported that the 11 harmonic AC side converter current
th th
was significantly higher, and the 13 harmonic significantly lower, when a 12 harmonic
current of not negligible value flows in the DC circuit. Possibly, this is the case for most HVDC
schemes, but is rarely mentioned.
In the design of the Quebec-New England multi-terminal scheme, a 60 Hz component in the
DC circuit caused by induction from nearby AC lines was anticipated due to the planned DC
line route. This induced fundamental frequency current would cross the converter and
generate direct current in the transformer winding, which could lead to core saturation. The
design of the scheme therefore included a series blocking filter tuned to the fundamental
frequency inserted in the DC neutral of each converter.
A second issue in the same project was related to the Radisson converter station [5]. The
station is located in an area of the far north of Canada where geomagnetic activity is strong
and the ground resistivity is high. This combination can create high direct currents in
th th
transformer neutrals during geomagnetic storms. During such an event, 5 and 7 harmonic
distortion on the AC side due to transformer saturation produced excessive 6th harmonic on
the DC side resulting in the failure of a DC filter arrester. It was found that the converter
th
impedance as seen from the AC side, which was heavily determined by 6 harmonic
impedance of the DC side, shifted the resonance frequency between the AC n
...