IEC TS 60076-23:2018

IEC TS 60076-23 ®
Edition 1.0 2018-01
TECHNICAL
SPECIFICATION
colour
inside
Power transformers –
Part 23: DC magnetic bias suppression devices
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IEC TS 60076-23 ®
Edition 1.0 2018-01
TECHNICAL
SPECIFICATION
colour
inside
Power transformers –
Part 23: DC magnetic bias suppression devices

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-5228-4

– 2 – IEC TS 60076-23:2018 © IEC 2018
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Service conditions . 9
4.1 General . 9
4.2 Seismic conditions . 10
4.3 Unusual conditions. 10
5 Selection principle . 10
5.1 Classification and features of the devices . 10
5.2 Selection principle for DC current-limiting devices . 10
5.3 Selection principle for the DC current- blocking device . 11
5.4 Calculation and verification . 11
6 DC current-limiting device. 11
6.1 Functional requirements . 11
6.1.1 General . 11
6.1.2 Resistance . 11
6.1.3 Overvoltage protection . 12
6.1.4 Structure . 12
6.2 Ability to withstand effects of short-circuit current . 12
6.2.1 Ability to withstand thermal effects of short-circuit current . 12
6.2.2 Ability to withstand dynamic effects of short-circuit current . 12
6.3 Temperature rise . 13
6.3.1 Metal chip resistors . 13
6.3.2 Dry-type non-inductive epoxy-resin insulated resistors . 13
6.3.3 Other types of resistors . 13
6.4 Insulation level . 13
7 DC current-blocking device . 13
7.1 Functional requirements . 13
7.1.1 General . 13
7.1.2 Capacitance . 13
7.1.3 Bypass switches . 14
7.1.4 Function of device . 14
7.1.5 Structure . 14
7.2 Ability to withstand effects of short-circuit current . 15
7.2.1 Ability to withstand thermal effects of short-circuit current . 15
7.2.2 Ability to withstand dynamic effects of short-circuit current . 15
7.3 Temperature rise . 15
7.4 Insulation level . 15
8 Tests . 15
8.1 Test classification . 15
8.2 Routine tests. 16
8.2.1 General . 16
8.2.2 Visual inspection . 16
8.2.3 DC resistance measurement . 16

8.2.4 Capacitance measurement . 16
8.2.5 Insulation resistance measurement . 16
8.2.6 Withstand voltage test . 16
8.2.7 Gap discharge test . 17
8.2.8 Function check of DC current- blocking devices . 17
8.3 Type tests . 17
8.3.1 General . 17
8.3.2 Temperature rise test of DC current-limiting device . 17
8.3.3 Thermal stability test . 18
8.3.4 Dynamic stability test . 18
8.3.5 Lightning impulse test . 18
8.3.6 Ingress protection test . 18
9 Packing, transportation and storage requirements . 18
10 Nameplate specification . 18
11 Technical documentation requirements . 19
Annex A (informative) Generation mechanism of DC bias current of power
transformers caused by HVDC system . 20
Annex B (informative) Examples of harmful effects of DC bias current . 21
Annex C (informative) DC current-limiting device . 24
Annex D (informative) DC current-blocking device . 25
Annex E (informative) Information needed to calculate the DC bias current of
transformers . 26
E.1 General . 26
E.2 Information of grounding electrode of HVDC system . 26
E.3 Parameters of equipment in substations and converter stations . 26
E.4 Parameters of power transmission lines . 26
Annex F (informative) Methods of calculation of DC bias current . 27
F.1 Method based on modelling of underground electric field . 27
F.2 Method based on calculation model of resistor network with equivalent
voltage sources. 28
Annex G (informative) Application examples . 30

Figure A.1 – Schematic diagram of DC flowing path in the monopole ground return
mode . 20
Figure A.2 – Resistance network and ground electric field distribution . 20
Figure B.1 –Mechanism of DC bias . 21
Figure B.2 – Damage to transformer . 23
Figure C.1 – Electrical schematic diagram of DC current-limiting device . 24
Figure D.1 – Electrical schematic diagram of DC current-blocking device . 25
Figure F.1 – Schematic diagram of modelling for DC bias current calculation . 28
Figure F.2 – Ground potential around the grounding electrode of HVDC system . 28
Figure F.3 – Schematic diagram for calculation of DC bias current based on the
equivalent voltage source . 29

Table 1 – Test items . 15
Table 2 – Rated insulation level (kV) . 16
Table B.1 – Test results of DC bias influence on DC system . 22

– 4 – IEC TS 60076-23:2018 © IEC 2018
Table B.2 – Vibration data of transformer (mm/s) . 23
Table F.1 – The resistivity and thickness of layered soil . 27
Table G.1 – Test data of DC current, noise and vibration . 30
Table G.2 – Test data . 31

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 23: DC magnetic bias suppression devices

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. In
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• the required support cannot be obtained for the publication of an International Standard,
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• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 60076-23, which is a technical specification, has been prepared by IEC technical
committee 14: Power transformers.

– 6 – IEC TS 60076-23:2018 © IEC 2018
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
14/924/DTS 14/943/RVDTS
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60076, published under the general title Power transformers, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

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 us
understanding of its contents. Users should therefore print this document using a
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INTRODUCTION
In some cases, abnormal direct current (DC) is introduced into the AC power network and has
adverse effects upon neutral grounded power apparatuses such as power transformers.
• Case 1
Direct current flows into the AC power network through grounded neutral points of
transformers when an HVDC transmission system operates in monopole ground return
mode or in bipolar unbalanced mode.
• Case 2
Quasi-DC is induced in the AC power network by geo-magnetically induced current (GIC)
during the period of a solar magnetic storm.
• Case 3
Electric traction locomotives and some large capacity power electronic equipment may
cause DC current in AC power network.
DC current flowing through transformer windings may cause DC magnetic bias of the
transformers, presenting a safety risk for both the transformers and the power system. The
mechanism and harmful effects of DC bias are shown in Annex A and Annex B.
Two techniques for suppressing the transformer DC bias current are presented in this
document, respectively to limit or block the transformer bias current produced by the HVDC
transmission system.
The two techniques can also be used to suppress transformer DC bias caused by GIC,
electric traction locomotives and some large capacity power electronic equipment. However,
these issues are not included in this document due to their complexity.
This document defines the technical requirements for the two types of DC current suppression
devices that are connected to neutral points of power transformers and converter transformers.

– 8 – IEC TS 60076-23:2018 © IEC 2018
POWER TRANSFORMERS –
Part 23: DC magnetic bias suppression devices

1 Scope
This document specifies requirements for devices for the suppression of DC magnetic bias of
power transformers and convertor transformers. It includes requirements for service
conditions, structures, testing, packing, transport and storage.
The devices are connected to neutral points of power transformers and converter
transformers to suppress DC bias current in the case an HVDC system is operated in
monopole ground return mode or bipolar unbalanced mode. In the case of dedicated metallic
return HVDC system, the devices are useful to mitigate DC stray current flowing through
power transformers and converter transformers during transient conditions such as DC line
fault.
This document applies to DC magnetic bias suppression devices for operation at frequencies
of 50 Hz and 60 Hz on power systems having voltages above 110 kV.
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 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60068-3-3, Environmental testing – Part 3-3: Guidance – Seismic test methods for
equipments
IEC 60076-3, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-5, Power transformers – Part 5: Ability to withstand short circuit
IEC 60137, Insulated bushings for alternating voltages above 1000 V
IEC 60168, Tests on indoor and outdoor post insulators of ceramic material of glass for
systems with nominal voltages greater than 1000V
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 61071, Capacitors for power electronics
IEC 62271-1, High-voltage switchgear and controlgear – Part 1: Common specifications for
alternating current switchgear and controlgear
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
DC magnetic bias suppression device
electric device connected between the transformer neutral point and the earth to limit or to
block the DC bias current flowing through the transformer windings
3.2
DC current-limiting device
electric device connected between the transformer neutral point and the earth to limit the DC
bias current flowing through the transformer windings
Note 1 to entry: It normally consists of a resistor and a protection gap. For more information, see Annex C.
3.3
DC current- blocking device
electric device connected between the transformer neutral point and the earth to block the DC
bias current flowing through the transformer windings
Note 1 to entry: It normally consists of a capacitor, a mechanical bypass switch, a high speed bypass switch, AC
and DC sensors, and control devices. For more information, see Annex D.
3.4
mechanical bypass switch
mechanical switch connected in parallel to the capacitor in DC current-blocking device for the
purpose of bypassing the capacitor persistently
3.5
high speed bypass switch
high speed switch connected in parallel to the capacitor in DC current-blocking device for the
purpose of bypassing the capacitor quickly
3.6
DC bias current
DC current flowing through transformer windings which causes drift of the excitation
characteristic curve of transformer
4 Service conditions
4.1 General
This document gives detailed requirements for the DC current-limiting or blocking devices
under the following conditions.
a) Altitude
Height above sea-level not exceeding 1 000 m (3 300 ft).
b) Climate conditions
• Maximum ambient temperature: +40 °C.
• Minimum ambient temperature: −25 °C.
• Maximum daily temperature difference: 25 °C.
• Maximum relative outdoor humidity: 90 % at 40 °C.

– 10 – IEC TS 60076-23:2018 © IEC 2018
• Maximum wind speed: 35 m/s.
• Ice thickness: 10 mm.
• Sunshine intensity: ≤1 000 W/m (wind speed of 0,5 m/s).
4.2 Seismic conditions
Devices for operation under seismic conditions shall be qualified in accordance with
IEC 60068-3-3, subject to agreement between the manufacturer and the purchaser.
4.3 Unusual conditions
Any unusual service conditions, which can lead to special consideration in the design of the
device, shall be stated in the inquiry and order. These can be factors such as high altitude,
extreme high or low temperatures, tropical humidity, severe contamination. They can also
concern conditions for shipment, storage and installation, such as weight or space limitation.
5 Selection principle
5.1 Classification and features of the devices
DC magnetic bias suppression devices can be installed at the neutral points of the
transformers to suppress the DC bias current. These devices are divided into two categories:
DC current-limiting devices and DC current-blocking devices.
Resistor-type DC current-limiting devices limit the DC current flowing through transformer
windings by increasing the resistance between neutral points of transformers and earth,
without completely blocking the DC current. The installation of such a device at one
substation has little effect on the DC current flowing through transformer windings in other
substations.
Capacitor-type DC current-blocking devices completely block the DC current from flowing
through the transformer windings when connected between neutral points of transformers and
earth. The installation of such a device changes the distribution of DC current flowing in the
earth and through transformer windings in other substations.
5.2 Selection principle for DC current-limiting devices
To determine the resistance, capacity and other electrical properties of the DC current limiting
devices, several factors shall be taken into consideration. These factors include the tolerance
of the transformers to magnetic bias current, the short-circuit current level of the grid, the
insulation level of the neutral points, and the simulation result of the effect of installing
current-limiting devices.
In addition to selecting the appropriate resistance, the protection configuration of transformers
shall be assessed, to verify their compatibility with the resistor type DC current-limiting
devices.
Normally, no relay protection is required for DC current-limiting devices. However, an
overvoltage protection unit shall be included.
In the case where both short-circuit current and DC bias current need to be suppressed, the
serially connected reactor and resistor shall be used as the current-limiting component of the
device.
5.3 Selection principle for the DC current- blocking device
Normally, a low-voltage capacitor together with a high-speed bypass switch and a mechanical
bypass switch is adopted for the current-blocking devices to ensure that the capacitor is
bypassed during unsymmetrical faults.
The capacitance shall not introduce an overvoltage condition. Installation of the capacitor
shall not affect the function of the protection relays of the transformers.
In the case when both the short-circuit current and the DC bias current need to be suppressed,
a current-blocking capacitor in series with a reactor shall be installed. Verification shall be
made concerning the suppression effect of the short-circuit current, the configuration of the
relay protection and resonance condition.
5.4 Calculation and verification
Before the design and installation of the DC magnetic bias suppression devices, DC current
distribution in the earth shall be calculated by setting up calculation models. The type
selection of DC magnetic bias suppression devices shall be based on the calculation results.
The effect of installing the devices and the influence on transformers in other substations
shall be evaluated.
An electric field simulation method is recommended to set up the calculation model of the DC
magnetic bias current caused by monopole ground return operation of HVDC systems. A
resistor network method is recommended for quick assessment for engineering purposes. For
more information on calculation models, see Annex E and Annex F.
The calculation results of DC current distribution in transformer windings shall be compared
with the data of DC bias current of transformers collected in field measurements in all
substations within the distance of 50 km from the grounding electrode of the HVDC system
before the installation of the devices. Special attention should be paid to the transformers
within the distance of 10 km from the electrode in the measurements. The calculation model
should be used for engineering purposes only after verification and modification.
The examples of application of two types of suppression devices are shown in Annex G.
6 DC current-limiting device
6.1 Functional requirements
6.1.1 General
DC current-limiting device consists of a resistor and an overvoltage protection unit to protect
the resistor when a fault occurs in a power grid. A graphite ball protection gap is
recommended as the overvoltage protection.
The device normally works in the current-limiting mode, with the resistor connected directly to
the neutral of the transformer to limit the DC bias current.
6.1.2 Resistance
The resistance shall be determined by field technical conditions of the installation site through
comprehensive technical evaluations. Electrical ratings and characteristics of the DC
current-limiting devices should be evaluated to coordinate with the system conditions of the
power grid.
Normally, resistors made of stainless steel or cast iron should be used.

– 12 – IEC TS 60076-23:2018 © IEC 2018
Long-term normal alternating currents should not be less than 50 A.
The recommended range of resistance is 1,5 Ω to 5 Ω.
The tolerance of resistance for DC current-limiting devices is ±5 % at 25 °C.
If tapped resistors are utilized, 0,5 Ω per tap is recommended.
6.1.3 Overvoltage protection
Normally, a graphite ball discharge gap should be used as the overvoltage protection, and
should meet the following requirements:
• current carrying capacity of 10 kA/1 s, or determined by short-circuit current analysis of
the grid;
• the discharging voltage of switching impulse shall not be higher than 10
...