IEC 62271-211:2024

IEC 62271-211 ®
Edition 2.0 2024-09
INTERNATIONAL
STANDARD
High-voltage switchgear and controlgear –
Part 211: Direct connection between power transformers and gas-insulated
metal-enclosed switchgear for rated voltages above 52 kV
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IEC 62271-211 ®
Edition 2.0 2024-09
INTERNATIONAL
STANDARD
High-voltage switchgear and controlgear –

Part 211: Direct connection between power transformers and gas-insulated

metal-enclosed switchgear for rated voltages above 52 kV

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.130.10 ISBN 978-2-8322-9578-6

– 2 – IEC 62271-211:2024 © IEC 2024
CONTENTS
FOREWORD . 4
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Normal and special service conditions . 10
4.1 Normal service conditions . 10
4.2 Special service conditions . 10
5 Ratings . 10
5.1 General . 10
5.2 Rated voltage (U ) . 10
r
5.3 Rated insulation level (U , U , U ) . 10
d p s
5.4 Rated frequency (f ) . 10
r
5.5 Rated continuous current (I ) . 11
r
5.6 Rated short-time withstand current (I ) and rated thermal short-time
k
current (I ) . 11
th
5.7 Rated peak withstand current (I ) and rated dynamic current (I ) . 11
p d
5.8 Rated duration of short-circuit (t ) and of thermal short-time current (t ) . 11
k th
6 Design and construction . 11
6.1 Gas and vacuum tightness . 11
6.2 Gas pressure for insulation of the bushing inside the gas-insulated
switchgear (GIS) enclosure . 12
6.3 Pressure withstands requirements . 12
6.4 Standard dimensions and tolerances. 12
6.4.1 General . 12
6.4.2 Single-phase direct connection between oil-filled transformer and
switchgear . 13
6.4.3 Three-phase direct connection between oil-filled transformer and
switchgear . 13
6.4.4 Connection between gas-insulated transformer and switchgear . 13
6.4.5 Transformer tolerances . 13
6.4.6 Mounting of the transformer on its foundation . 13
6.5 Limits of supply . 13
6.6 Mechanical forces applied on the connection interface . 15
6.7 Mechanical forces applied on the bushing flange . 15
6.8 Horizontal and vertical displacement . 17
6.9 Vibrations . 17
7 Type tests . 17
7.1 General . 17
7.2 Dielectric tests . 17
7.2.1 Dielectric tests of bushing . 17
7.2.2 Dielectric tests of transformer connection in a single-phase enclosure . 18
7.2.3 Dielectric tests of transformer connection in a three-phase enclosure . 18
7.3 Cantilever loads withstand tests . 18
7.4 Gas tightness tests . 18
8 Routine tests . 18

8.1 General . 18
8.2 External pressure test of the bushing . 18
8.3 Tightness tests . 19
9 Guide to the selection of switchgear and controlgear (informative) . 19
10 Information to be given with enquiries, tenders and orders (informative) . 19
11 Transport, storage, installation, operating instructions and maintenance. 19
12 Safety . 19
13 Environmental aspects . 20
Bibliography . 25

Figure 1 – Typical direct connection between power transformer and gas-insulated
metal-enclosed switchgear . 21
Figure 2 – Standard dimensions for typical direct connection between power

transformer and gas-insulated metal-enclosed switchgear . 22
Figure 3 – Transformer tolerances for a typical direct connection shown on the
example of a three-phase power transformer connected to a single-phase
gas-insulated metal-enclosed switchgear . 23
Figure 4 – Transformer tolerances for a typical direct connection shown on the
example of a three-phase power transformer connected to a three-phase gas-insulated
metal-enclosed switchgear up to 170 kV . 24

Table 1 – Moments and forces applied on the bushing flange and transformer . 16

– 4 – IEC 62271-211:2024 © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE SWITCHGEAR AND CONTROLGEAR –

Part 211: Direct connection between power transformers and
gas-insulated metal-enclosed switchgear for rated voltages above 52 kV

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
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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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
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the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 62271-211 has been prepared by subcommittee 17C: Assemblies, of IEC technical
committee 17: High-voltage switchgear and controlgear. It is an International Standard.
This second edition cancels and replaces the first edition of IEC 62271-211:2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) re-numbering of clauses according to IEC 62271-1:2017,
b) Clause 3: updating definition about bushing (3.1), updating some pressure definitions (3.6,
3.7, 3.8, 3.9), rewording definition about proctor density (3.11), new term very-fast-front
overvoltage (3.12),
c) Clause 5 (former clause 4): add a subclause 5.1 General, according to IEC 62271-1:2017
and IEC 62271-203:2022,
1) subclause 5.5: new first paragraph, rewording second paragraph,
2) subclause 5.8: modify the term "Rated duration of thermal short-time current" of the
bushing,
d) Clause 6 (former Clause 5): restructure and rewording of subclauses:
1) 6.1 (former 5.3): requirements about gas and vacuum tightness of the transformer
bushing
2) 6.3 (former 5.2): harmonization with IEC 62271-203:2022 about typical maximum
pressure in service for SF , other gases and gas mixtures,
3) 6.4 (former 5.8), rewording
4) 6.5 (former 5.1), some rewording and modification
5) 6.6 (former 5.4), some rewording, updated references
6) 6.7 (former 5.5), some rewording
7) 6.8 (former 5.6), some rewording
8) 6.9 (former 5.7), slight rewording,
e) Clause 7 (former clause 6) type tests: some rewording and clarifications about references,
f) Clause 8 (former clause 7) routine tests:
-mixtures and other gases than SF ,
1) 8.2 (former 7.2): add a paragraph about SF
6 6
2) 8.3 (former 7.3): update reference to relevant on-site test according to
IEC 62271-203:2022,
g) Clause 9 Guide to the selection of switchgear and controlgear (new): informative, to have a
reference to IEC 62271-203:2022,
h) Clause 11 (former 10): updated headline and updated reference according to
IEC 62271-1:2017,
i) new Clauses 12 Safety and 13 Environmental aspects: Adding of references to safety and
environmental aspects,
j) correction of errors in Corrigendum 2 of IEC 62271-211:2017,
k) modified orientation of Figure 1 to Figure 4 for easier reading of the tables,
The text of this International Standard is based on the following documents:
Draft Report on voting
17C/935/FDIS 17C/945/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62271 series, published under the general title High-voltage
switchgear and controlgear, can be found on the IEC website.

– 6 – IEC 62271-211:2024 © IEC 2024
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
HIGH-VOLTAGE SWITCHGEAR AND CONTROLGEAR –

Part 211: Direct connection between power transformers and
gas-insulated metal-enclosed switchgear for rated voltages above 52 kV

1 Scope
This part of IEC 62271 is applicable to single- and three-phase direct connections between gas-
insulated metal-enclosed switchgear (GIS) for rated voltages above 52 kV and transformer
arrangements to establish electrical and mechanical interchange ability and to determine the
limits of supply for the transformer connection.
Direct connections are immersed on one end in the transformer oil or insulating gas and on the
other end in the insulating gas of the switchgear.
Transformer arrangements are single-phase transformers with single-phase enclosed
arrangement, three-phase transformers with three single-phase enclosed arrangements or
three-phase transformers with a three-phase enclosed arrangement with three transformer
bushings.
The connection satisfies the requirements of IEC 62271-203 for gas-insulated metal-enclosed
switchgear, IEC 60076 for power transformer and IEC 60137 for completely immersed
bushings.
For the purpose of this document the term “switchgear” is used for “gas-insulated metal-
enclosed switchgear and the term “transformer” is used for “power transformer”.
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 60076 (all parts), Power transformers
IEC 60076-1:2011, Power transformers – Part 1: General
IEC 60137:2017, Insulated bushings for alternating voltages above 1 000 V
IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – Part 1: AC
IEC 62271-1:2017, High-voltage switchgear and controlgear – Part 1: Common specifications
for alternating current switchgear and controlgear
IEC 62271-1:2017/AMD1:2021
IEC 62271-203:2022, High-voltage switchgear and controlgear – Part 203: Gas-insulated metal-
enclosed switchgear for rated voltages above 52 kV
IEC 62271-207:2023, High-voltage switchgear and controlgear – Part 207: Seismic qualification
for gas-insulated switchgear assemblies, metal enclosed and solid-insulation enclosed
switchgear for rated voltages above 1 kV

– 8 – IEC 62271-211:2024 © IEC 2024
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62271-1, as well as
the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
bushing
device that enables one or several conductors to pass through an enclosure and insulate the
conductors from it
[SOURCE: IEC 60050-471:2007 [1] , 471-02-01, modified – "an enclosure" inserted after "pass
through" and "a partition such as a wall or a tank" deleted. Notes 1 and 2 were deleted.]
3.2
completely immersed bushing
bushing, both ends of which are intended to be immersed in an insulating medium other than
ambient air (e.g. oil or gas)
[SOURCE: IEC 60050-471:2007 [1], 471-02-04]
3.3
gas-insulated switchgear (GIS) enclosure
part of gas-insulated metal-enclosed switchgear retaining the insulating gas under the
prescribed conditions necessary to maintain safely the highest insulation level, protecting the
equipment against external influences and providing a high degree of protection to personnel
Note 1 to entry: The enclosure can be single-phase or three-phase.
[SOURCE: IEC 62271-203:2022, 3.103, modified – the acronym GIS has been added]
3.4
main circuit end terminal
part of the main circuit of a gas-insulated metal-enclosed switchgear forming part of the
connection interface
[SOURCE: IEC 62271-209:2019 [2], 3.2]
3.5
transformer connection enclosure
part of the gas-insulated metal-enclosed switchgear which houses one end of a completely
immersed bushing fitted on a power transformer and a main circuit end terminal
___________
Numbers in square brackets refer to the Bibliography.

3.6
maximum external operating gas pressure
maximum pressure of the gaseous insulating medium in which the bushing is partially or
completely immersed when in operation
Note 1 to entry: It is at least equal to the maximum pressure in the transformer connection enclosure of the GIS at
the highest temperature that the gas used for insulation can reach under specified maximum service conditions.
Note 2 to entry: In case of gas-insulated transformers it is also the maximum insulating pressure of the gaseous
insulating medium in which the end of the bushing is immersed into the power transformer, when in operation, the
bushing-power transformer connection assembly carrying its rating continuous current at the maximum ambient
temperature.
[SOURCE: IEC 60137:2017, 3.32 – modified: Note 1 to entry and Note 2 to entry were added]
3.7
enclosure design pressure
relative pressure used to determine the design of the enclosure
Note 1 to entry: It is at least equal to the maximum pressure in the enclosure at the highest temperature that the
gas used for insulation can reach under specified maximum service conditions.
Note 2 to entry: The transient pressure occurring during and after a breaking operation (e.g. circuit-breaker) is not
considered in the determination of the design pressure.
[SOURCE: IEC 62271-203:2022, 3.114]
3.8
filling pressure p for insulation
re
filling density ρ for insulation
re
pressure (in Pa), for insulation, referred to the standard atmospheric air conditions of +20 °C
and 101,3 kPa (or density), which may be expressed in relative or absolute terms, to which the
assembly is filled before being put into service
[SOURCE: IEC 62271-1:2017, 3.6.5.1, modified – deleted the terms "and/or switching", deleted
"or automatically replenished"]
3.9
minimum functional pressure p for insulation
me
minimum functional density ρ for insulation
me
pressure (in Pa), for insulation, referred to the standard atmospheric air conditions of +20 °C
and 101,3 kPa (or density), which may be expressed in relative or absolute terms, at which and
above which the characteristics of the switchgear-power-transformer connection are maintained
[SOURCE: IEC 62271-1:2017, 3.6.5.5, modified – deleted the terms "and/or switching", added
"the characteristics of the switchgear-power-transformer connection" deleted "rated
characteristics of switchgear and controlgear"]
3.10
insulated junction
all parts which are needed to insulate between the transformer and the switchgear, including
but not limited to the insulating flange
3.11
proctor density
highest dry density of a soil for a given compaction effort depending on the amount of water the
soil contains during soil compaction of controlled magnitude according Proctor Standard test
Note 1 to entry: Proctor Standard test is defined in ASTM D-698 [3]. However, other tests methods exist providing
similar information, but not always equivalent, like for example ISO 17892-2 [4], BS 1377-1 [5], UNE 103500 [6], NF
P 94-093 [7] and DIN 18127 [8].

– 10 – IEC 62271-211:2024 © IEC 2024
3.12
very-fast-front overvoltage
VFFO
transient overvoltage, usually unidirectional with time to peak T ≤ 0,1 µs, and with or without
f
superimposed oscillations at frequency 30 kHz < f < 100 MHz
Note 1 to entry: Transient overvoltage generated by switching operations in GIS have a time to peak in a range of
nanoseconds. They are usually named VFTO (very fast transients overvoltage) or related to the grounded
encapsulation named as very-fast-front transient ground potential rises.
[SOURCE: IEC 60071-1:2019 [9], 3.17.2.3, modified – Note 1 to entry added]
4 Normal and special service conditions
4.1 Normal service conditions
Subclause 4.1 of IEC 62271-203:2022 is applicable.
4.2 Special service conditions
Subclause 4.2 of IEC 62271-203:2022 is applicable.
5 Ratings
5.1 General
When dimensioning a direct connection between a switchgear and a power-transformer the
following rated values apply.
5.2 Rated voltage (U )
r
The rated voltage shall be the rated voltage of the switchgear, selected from the following
standard values:
72,5 kV – 100 kV – 123 kV – 145 kV – 170 kV – 245 kV – 300 kV – 362 kV – 420 kV – 550 kV
NOTE Values 800 kV and higher are not considered because there is little experience at this time.
5.3 Rated insulation level (U , U , U )
d p s
The rated insulation level for the GIS part in the direct connection enclosure shall comply with
the related rated voltage values given in the product standard IEC 62271-203. The rated
insulation level for the transformer bushing shall be selected from the values given in the
product standard IEC 60137.
NOTE Transformers can be tested at other insulation level values, according to their relevant standard.
The rated insulation level for a direct connection shall fulfil at least the requirements of
IEC 62271-203.
5.4 Rated frequency (f )
r
Subclause 5.4 of IEC 62271-1:2017 is applicable.

5.5 Rated continuous current (I )
r
This rating defines the RMS value of the current the direct connection can carry continuously
for its service conditions (see Clause 4).
The connection interface is shown in Figure 1 as parts 3 and 4. The dimensions of the
connection interfaces shown in Figure 2 allow a maximum value of 3 150 A for the rated
continuous current at normal service conditions i.e. maximum ambient temperature of 40 °C
(see 4.1).
The contact surfaces of the connection interface shall be silver-coated, copper-coated or bare
copper.
For the rated continuous current, the connection between switchgear and power transformer
shall be designed so that the temperature of the transformer connection enclosure and the
temperature of the connection interface do not exceed the values given in 7.5.6 of
IEC 62271-1:2017.
5.6 Rated short-time withstand current (I ) and rated thermal short-time current (I )
k th
For the rated short-time withstand current of the GIS part of the direct connection, 5.6 of
IEC 62271-1:2017 applies; for the rated thermal short-time current of the bushing 4.3 of IEC
60137:2017 applies.
It is recommended to assign the same value for both short-time rated currents for GIS and
bushing.
5.7 Rated peak withstand current (I ) and rated dynamic current (I )
p d
For the rated peak withstand current of the GIS part of the direct connection, 5.7 of
IEC 62271-1:2017 applies; for the rated dynamic current of the bushing 4.4 of IEC 60137:2017
applies.
It is recommended to assign the same value for the rated peak withstand current for the GIS
and the rated dynamic current for the bushing.
5.8 Rated duration of short-circuit (t ) and of thermal short-time current (t )
k th
For the rated duration of the short-circuit current of the GIS part of the direct connection, 5.8 of
IEC 62271-1:2017 applies; for the rated duration of the thermal short-time current of the bushing
4.3 of IEC 60137:2017 applies.
It is recommended to assign the same value for the rated duration of short-circuit for the GIS
and the rated duration of the thermal short-time current for the bushing.
6 Design and construction
6.1 Gas and vacuum tightness
Subclause 6.16 of IEC 62271-203:2022 is applicable with the following addition:
For conditions up to the maximum external operating gas pressure inside the transformer
connection enclosure of the GIS, the bushing shall minimize the gas insulated media diffusing
into the transformer. The leak rate shall be equal or less than 10 Pa∙cm /s, see 9.10.3 of
IEC 60137:2017.
– 12 – IEC 62271-211:2024 © IEC 2024
The bushing shall prevent liquid insulating media entering from the transformer or the bushing
into the GIS. Visual inspection is sufficient evidence, see 9.10.3. of IEC 60137:2017.
The bushing shall be capable of withstanding the vacuum conditions when the transformer
connection enclosure is evacuated, as part of the gas filling process of the GIS and shall be
capable of withstanding the vacuum conditions when the transformer is evacuated as part of its
gas or oil filling process.
The relevant requirements and acceptance criteria for the bushing are described in 8.11 to 8.13
and 9.7 to 9.10 of IEC 60137:2017.
In the case of a gas-insulated transformer the gas compartment of the transformer shall be
completely separated and managed independently from the switchgear.
6.2 Gas pressure for insulation of the bushing inside the gas-insulated switchgear
(GIS) enclosure
The filling pressure p (or density) of any gas for insulating is assigned by the switchgear
re
manufacturer.
The minimum functional pressure for insulation p , used to determine the design of the
me
bushing insulation, shall be agreed between the switchgear manufacturer and the bushing
manufacturer.
However, the minimum functional pressure for insulation p of the bushing inside the GIS
me
enclosure shall be below or equal to the minimum functional pressure of GIS.
NOTE If SF is used as the insulating gas, a minimum functional gas pressure for insulation of not more than
0,35 MPa (absolute) at 20 °C is commonly used to determine the bushing insulation.
6.3 Pressure withstands requirements
The bushing connected to the GIS shall be capable of withstanding the maximum pressure in
service of the GIS. Typical maximum pressures in service are up to 1,1 MPa (absolute) for SF
and up to 1,5 MPa (absolute) for other gases and gas mixtures.
NOTE The former given value for SF of "at least 0,85 MPa (absolute) at 20 °C" is approximately in correlation with
a "typical maximum pressure in service of 1,1 MPa (absolute) for SF " at a maximum value of 80 °C at the outer
surface of a metallic encapsulation, considering rated continuous current of 3 150 A and maximum ambient
temperature of 40 °C. The above mentioned typical maximum pressure values were introduced in
IEC 62271-203:2022.
The transformer connection enclosure and all pressurized connected parts of the GIS shall
satisfy the requirements provided in 6.104 of IEC 62271-203:2022 for the design pressure
determined by the switchgear manufacturer as specified in 6.104.2 of IEC 62271-203:2022.
The enclosure design pressure of the transformer connection enclosure may be lower than the
gas pressure which is used to determine the mechanical strength of the bushing in a type test.
6.4 Standard dimensions and tolerances
6.4.1 General
Standard dimensions are specified to ensure compatibility between switchgear and transformer
connections conforming to this document and agreement is required between the switchgear
and transformer manufacturers.

6.4.2 Single-phase direct connection between oil-filled transformer and switchgear
Standard dimensions for single-phase transformer connection enclosures, main circuit end
terminals, bushing end terminals and bushing flanges are shown in Figure 2.
6.4.3 Three-phase direct connection between oil-filled transformer and switchgear
The minimum dimensions of the three-phase direct connection between oil-filled transformer
and switchgear are defined by the minimum phase-to-phase distance arising out of d and the
minimum phase to ground distance arising out of d /2 taking into account three single-phase
bushings as specified in Figure 2.
Transformer tolerances for a typical direct connection of a three-phase power transformer
connected to a single-phase gas-insulated metal enclosed switchgear are shown as an example
in Figure 3, see also 6.4.5.
Three-phase direct connections are considered up to 170 kV with reference to Figure 4.
6.4.4 Connection between gas-insulated transformer and switchgear
In case of a gas insulated transformer, the standard dimensions of Figure 2 could be
inappropriate. Therefore, the detailed dimensions of the direct connection between the gas
insulated power transformer and the gas-insulated metal-enclosed switchgear will deviate. It is
the responsibility of the switchgear manufacturer and the transformer manufacturer to define it.
This connection can take place by using a partition rather than a bushing.
NOTE For direct connection between gas-insulated transformer and switchgear there is little experience currently.
6.4.5 Transformer tolerances
Tolerances shown in Figure 3 and Figure 4 are given for guidance as reference tolerances
permitted after installation of the transformer ready for operation including the installed
bushings.
6.4.6 Mounting of the transformer on its foundation
Transformers with direct connections shall be supplied with appropriate jacking pads for lifting
and appropriate angles for the fixing of the transformer on its foundation; both facilities allowing
a horizontal and vertical movement to adapt to the needs for the connection to the switchgear.
Care shall be taken for mounting the transformer on its foundation in order not to restrict the
allowable operation tolerances in Figure 3 and Figure 4 when directly connected to a
switchgear.
Transformers installed on rails usually do not comply with this requirement.
To comply with the requirement of a horizontal movement of the transformer during installation
and not interfering with the installation process, either in indoor or outdoor, it is best practice in
civil engineering for such purpose to apply anchor bolts in conjunction with the usage of
corrugated permanent formwork.
6.5 Limits of supply
A typical direct connection is shown in Figure 1.
The limits of supply of the switchgear manufacturer and transformer manufacturer shall be
according to Figure 1.
– 14 – IEC 62271-211:2024 © IEC 2024
To minimize circulating currents between the switchgear and the transformer, bonding
conductors according to IEC 61936-1 shall be provided by the switchgear manufacturer
between the different single-phase switchgear enclosures; capable of carrying continuously the
rated current of the transformer in normal operation. If circulating currents higher than 250 A
are expected via the single-phase transformer connection enclosure, an insulated junction
between the GIS and the transformer is necessary, see Figure 1 Part 9. This applies to the
direct connection of three-phase and single-phase power transformers according to IEC 60076
series.
NOTE 1 The tank of a transformer is not designed to carry any considerable undefined continuous circulating
currents. Practical experiences have shown that a load current of up to 1 250 A can be carried without additional
precautions in the enclosure of the interface between the GIS and the transformer. This is due to the fact that
approximately 80 % of the load current is carried by the bonding conductor of the switchgear, to be installed preferably
at the end of the switchgear enclosure at the transformer. Thus, approximately 20 % of the load current, therefore
up to 250 A, could generate continuous circulating currents in the transformer enclosure. The confirmation of the
current value in the bonding conductor and through the transformer enclosure can be made by calculation.
NOTE 2 In case of higher circulating currents the user will clarify with the transformer manufacturer to confirm the
maximum acceptable current via the transformer encapsulation to avoid the insulated junction, if possible.
An insulated junction may be also needed to achieve isolation between the transformer tank
and the neighbouring earthed switchgear enclosures, and to achieve correct operation of the
user’s protection schemes for GIS and transformer faults.
The insulation level across the insulated junction shall be designed to withstand a
power-frequency test voltage of 5 kV RMS, for 1 min.
To limit the very-fast-front transient ground potential rises which may occur when a switching
device operates, non-linear resistors may be connected in parallel with the insulated junction.
The number and the characteristics of the non-linear resistors shall be determined by the
switchgear manufacturer. See [10].
According to this document two different locations (shown in Figure 1) are acceptable for the
insulated junction
a) location "A" between the flange of the transformer connection enclosure, part 6 in Figure 1,
and the flange of the bushing, part 10 in Figure 1, or
b) location "B" between the transformer connection enclosure, part 6 in Figure 1, and the next
switchgear housing, part 14 in Figure 1.
If the insulated junction is needed, the standard dimensions in accordance with Clause 8 shall
be kept.
Independent of the existence of an insulated junction, bonding conductors between the single-
phase enclosures of the switchgear need to be provided by the switchgear manufacturer in
accordance with IEC 61936-1 and IEC 62271-203.
Bonding conductors always need to be provided between single-phase enclosed direct
connection arrangements of single-phase, two-phase or three-phase transformers.
Bonding conductors need not be provided for a three-phase transformer with a three-phase
enclosed direct connection arrangement.
In the case of not having an insulated junction, bonding conductors shall be designed to limit
the circulating current via the transformer tank to the permitted values as specified in this
clause.
Both with and without an insulated junction the appropriate bonding connection shall be made
at the end of the transformer connection enclosure towards the bushing, i.e. location marked
"A" in Figure 1.
In case of location marked "B" in Figure 1 the bonding conductor shall be made at the end of
the switchgear housing towards the bushing.
The transformer, including the bushing, shall be designed according to the relevant product
standards to withstand very-fast-front overvoltage (VFFO) generated by the switchgear.
If an insulated junction is applied, its design shall be also able to withstand the same value of
the very fast transient overvoltages generated by the switchgear.
NOTE 3 Alternatively to the insulating junction an insulating spacer can be used.
6.6 Mechanical forces applied on the connection interface
Normal loads, whose occurrence and level can be planned or predicted, applicable to bushings
at the connection interface (parts 3 and 4 in Figure 1) are given in IEC 60137:2017, Table 1,
level I (see cantilever operating loads); the minimum normal load however shall not be less than
2 kN, acting either transversely or axially to the connection interface. Table 1 of IEC 60137:2017
gives values for bushing installed up to 30° from the vertical.
If higher normal loads are to be expected at the connection interface, then the higher loads
shall be selected from IEC 60137:2017, Table 1, Level II (heavy load).
For exceptional circumstances, whose probability of occurrence during lifetime of products is
very small or accidental (e.g. short-circuit current or seismic loads), the cantilever test load
level II of Table 1 in IEC 60137:2017 shall be used.
In case of seismic requirements as special service conditions according to subclause 4.2 of
IEC 62271-203:2022, a seismic calculation shall be carried out to identify the location of
mechanical reinforcements, see IEC 62271-207.
NOTE IEC 62271-207 covers seismic qualification requirements, test procedures for qualifications, qualification by
combined tests and numerical analysis and evaluation of the seismic qualification.
It is the responsibility of the switchgear manufacturer to ensure that the seismic loads are not
exceeding the specified load level II of Table 1 in IEC 60137:2017 or to agree with the bushing
manufacturer that the bushing shall withstand the higher forces.
The selection of the appropriate bushing is under the responsibility of the transformer
manufacturer considering the maximum occurring short circuit forces between phase and
ground, respectively also between phases in the case of a three-phase enclosure at both ends
of the bushing.
In the case of different rated withstand currents of the GIS in respect to those of the transformer,
the higher rated withstand currents apply to calculate the mechanical forces to be supported by
the connection interface.
6.7 Mechanical forces applied on the bushing flange
The flange of the bushing attached to the transformer connection enclosure is subjected in
service, in addition to the maximum external operating gas pressure inside the transformer
connection enclosure of the GIS, to the following loads:
– part of the weight of the switchgear not supported by the switchgear's own supporting
structures;
– part of the wind load, if applicable, not supported by the switchgear's own supporting
structures;
– expansion or contraction stresses due to the temperature variations of the transformer tank
and the gas-insulated switchgear (GIS) enclosure.

– 16 – IEC 62271-211:2024 © IEC 2024
These loads result in the simultaneous application, at the centre of the bushing flange, of:
– a bending moment M ;
– a shearing force F ;
t
– a tensile or compressive force F .
a
The bushing flange and its connection to the transformer shall be capable of withstanding, in
service, the values of M , F and F specified in Table 1. The responsibility of an adequate

0 t a
design to cover these requirements is related to the transformer and bushing manufactur
...