IEC 60214-2:2004

INTERNATIONAL IEC
STANDARD 60214-2
First edition
2004-10
Tap-changers –
Part 2:
Application guide
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INTERNATIONAL IEC
STANDARD 60214-2
First edition
2004-10
Tap-changers –
Part 2:
Application guide
 IEC 2004  Copyright - all rights reserved
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mechanical, including photocopying and microfilm, without permission in writing from the publisher.
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– 2 – 60214-2  IEC:2004(E)
CONTENTS
FOREWORD.4

INTRODUCTION.6

1 Scope.7

2 Normative references .7

3 Terms and definitions .7

4 Symbols and abbreviations.7

5 Types of tap-changer.8
5.1 General .8
5.2 On-load tap-changers.8
5.3 Off-circuit tap-changers .15
5.4 Liquid-immersed tap-changers .15
5.5 Dry-type tap-changers .16
5.6 Other types .17
5.7 Protective devices .18
6 Selection of tap-changers .21
6.1 General .21
6.2 On-load tap-changers.22
6.3 Off-circuit tap-changers .27
7 Location of liquid-immersed tap-changers.29
7.1 Tap selectors .29
7.2 Diverter and selector switches.29
8 Fittings .29
8.1 Valves, air-release vents and liquid sampling devices .29
8.2 Liquid level gauges .29
8.3 Low liquid level alarms .30
8.4 Nameplate and other plates.30
8.5 Devices to aid maintenance.30
8.6 Dehydrating breathers .30
9 Field service (operation, maintenance and monitoring) .30
9.1 Operation .30

9.2 Maintenance.32
9.3 Monitoring in service .32
10 Information to be provided by the transformer manufacturer .34
10.1 Information required at the enquiry or order stage for an on-load tap-changer .34
10.2 Information required with enquiry or order for off-circuit tap-changers .36
10.3 Documentation .36
11 Protection and safety.37
11.1 Protection.37
11.2 Safety aspects.37
11.3 Immersing medium .37

60214-2  IEC:2004(E) – 3 –
Figure 1 – External separate selector and diverter compartments (for mounting on the

end or side of the transformer).9

Figure 2 – External mounted selector switch tap-changer (for mounting on the end or

side of the transformer).10

Figure 3 – External mounted in-tank with separate barrier board.10

Figure 4 – In-tank separate selector and diverter switch .12

Figure 5 – In-tank selector switch tap-changer .13

– 4 – 60214-2  IEC:2004(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
TAP-CHANGERS –
Part 2: Application guide
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
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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
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interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
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6) All users should ensure that they have the latest edition of this publication.
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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.
International Standard IEC 60214-2 has been prepared by IEC technical committee 14: Power
transformers.
This standard cancels and replaces IEC 60542 (1976) and its amendment 1 (1988). This first
edition constitutes a technical revision of that standard.
The text of this standard is based on the following documents:
FDIS Report on voting
14/490/FDIS 14/492/RVD
Full information on the voting for the approval of this standard 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.

60214-2  IEC:2004(E) – 5 –
IEC 60214 consists of the following parts, under the general title Tap-changers:

Part 1: Performance requirements and test methods

Part 2: Application guide
The committee has decided that the contents of this publication will remain unchanged until

the maintenance result 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.

– 6 – 60214-2  IEC:2004(E)
INTRODUCTION
The recommendations in this application guide represent advice to the tap-changer

manufacturer and purchaser.
It is stressed that the responsibility for the correct application of the fully assembled tap-

changers in connection with the transformer lies with the manufacturer of the transformer.

60214-2  IEC:2004(E) – 7 –
TAP-CHANGERS –
Part 2: Application guide
1 Scope
This part of IEC 60214 is intended to assist in the selection of tap-changers designed in

accordance with IEC 60214-1 for use in conjunction with the tapped windings of transformers
or reactors. It is also intended to assist in understanding the various types of tap-changers
and their associated equipment available. The application guide covers on-load tap-changers
(resistor and reactor types) and off-circuit tap-changers.
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 60076-1:1993, Power transformers – Part 1: General
Amendment 1(1999)
IEC 60076-3:2000, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-5:2000, Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-11:2004, Power transformers – Dry-type transformers
IEC 60214-1:2003, Tap-changers – Part 1: Performance requirements and test methods
IEC 60296:2003, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
IEC 60354:1991, Loading guide for oil-immersed power transformers
IEC 60599:1999, Mineral oil-impregnated electrical equipment in service – Guide to the

interpretation of dissolved and free gases analysis
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60214-1 apply.
4 Symbols and abbreviations
DGA Dissolved gas analysis
HVDC High-voltage direct current
PST Phase-shifting transformer
___________
There exists a consolidated edition 2.1 (2000) that includes edition 2.0 and its amendment.

– 8 – 60214-2  IEC:2004(E)
5 Types of tap-changer
5.1 General
Tap-changers are devices that will vary the turns ratio of a transformer and hence regulate

the voltages of that transformer. Tap-changers that can perform this operation can broadly be

divided into two fundamental types as follows:

– on-load tap-changers;
– off-circuit tap-changers.
5.2 On-load tap-changers
5.2.1 General
The on-load tap-changer is designed to change tap position and hence vary the turns ratio of
the transformer whilst it is both energized and on load. It performs this function without any
interruption of the supply. This is achieved with mechanically operated devices that will select
the various tap positions and switch the load currents and step voltages.
On-load tap-changing can be employed by using various switching principles.
The two most common switching principles are:
– high-speed transition resistor type switching; and
– transition reactor (preventive autotransformer) type switching.
5.2.2 Resistor-type on-load tap-changers
5.2.2.1 General
Resistor-type on-load tap-changers can be divided into two distinctive types:
– external out-of-tank tap-changers (air environment), as described in 5.2.2.2;
– in-tank tap-changers (liquid environment), as described in 5.2.2.3.
The operating sequences of the different resistor type tap-changers are shown in
IEC 60214-1, Table A.1.
5.2.2.2 External resistor-type on-load tap-changers
5.2.2.2.1 General
These tap-changers are self-contained in their own tanks (except dry-type tap-changers) and
mounted on the side or end of the transformer. Four types of tap-changer arrangements are
considered, all employing the high-speed transition resistor switching principle.
5.2.2.2.2 External separate selector and diverter compartments
This type of tap-changer has two separate compartments, one for the pre-selection of the
transformer taps called the tap selector compartment and the other for the on-load switching
called the diverter switch compartment. The two compartments have separate liquid chambers
and both are isolated from the liquid in the main transformer tank, although the tap selector
compartment and the main tank may share the same conservator. The taps from the
transformer are taken to the selector contacts through a liquid-tight barrier board. The tap
selector compartment contains clean liquid enabling it to withstand the required higher
voltages across the contacts. The diverter switch compartment isolates the carbonized liquid
and gases. As can be seen from Figure 1, the tap-changer bolts on the side or end of the
transformer. This arrangement is generally used for the larger MVA transformers.

60214-2  IEC:2004(E) – 9 –
IEC  1390/04
Key
1 Transformer windings 3 Liquid and gas tight barrier 5 Drive mechanism
2 Tap leads 4 Diverter switch compartment 6 Tap selector compartment

Figure 1 – External separate selector and diverter compartments
(for mounting on the end or side of the transformer)
The switching can be by arc extinction in the liquid or by vacuum interrupters/power
electronics.
5.2.2.2.3 External mounted separate selector and diverter in single compartment
This type of tap-changer uses separate selector and diverter contact systems in a similar way
to the double compartment arrangement in 5.2.2.2.2 but combines them in a single
compartment.
The switching can be by arc extinction in the liquid or by vacuum interrupters/power
electronics.
5.2.2.2.4 External mounted selector switch tap-changers
Selector switch tap-changers are contained in a single compartment, normally bolted on the

side or the end of the transformer (see Figure 2). Again, the transformer taps are taken to the
tap-changer contacts through a liquid-tight barrier board. The selection and switching are
carried out using common contacts in the same liquid and chamber. These tap-changers tend
to be used on the smaller MVA and voltage class transformers.

– 10 – 60214-2  IEC:2004(E)
IEC  1391/04
Key
1 Transformer windings 3 Liquid and gas tight barrier 5 Drive mechanism
2 Tap leads 4 Selector switch compartment
Figure 2 – External mounted selector switch tap-changer
(for mounting on the end or side of the transformer)
5.2.2.2.5 External mounted in-tank with separate barrier board
By using an in-tank tap-changer in a separate pocket with a liquid-tight barrier board between
the tap-changer and the transformer, it effectively becomes a separate bolt-on type of tap-
changer. The liquid from the selector is totally isolated from the transformer although the
selector compartment and the main tank may share the same conservator.
Figure 3 shows how this arrangement works and gives all the advantages of the separate tank
tap-changer for the higher voltage classes.

4 5
3 6
1 2
IEC  1392/04
Key
1 Transformer windings 3 Liquid and gas tight barrier 5 Diverter switch
2 Tap leads 4 Tap selectors 6 Drive mechanism
Figure 3 – External mounted in-tank with separate barrier board

60214-2  IEC:2004(E) – 11 –
5.2.2.2.6 Advantages and disadvantages of external mounted on-load tap-changers

External mounted on-load tap-changers generally have the advantage of being easier for the

user to maintain. Access to the complete tap-changer and all contacts is obtained by removal

of inspection covers. Because the selectors are always in a separate chamber, the DGA of

the transformer is not affected by capacitive sparking of the selector and change-over of
selector contacts. Monitoring of the separate selector compartments can be carried out in

isolation allowing early diagnosis of selector problems and the ability to differentiate between

selector and main transformer defects. Due to voltage clearance considerations, external

mounted out-of-tank on-load tap-changers have the disadvantage of not being practical for

line-end applications above 145 kV.

5.2.2.3 In-tank resistor type on-load tap-changers
5.2.2.3.1 General
As their name implies, these tap-changers are fitted inside the transformer. They are normally
suspended from the lid of the transformer either at one end in the case of a single three-
phase pole or along the side when three single-phase poles are used. A pole can consist of
one, two or three phases. Two poles can also be used for some Delta applications. Three
types are considered, all of them having high speed resistor switching.
5.2.2.3.2 In-tank separate selector and diverter switch
With this type of tap-changer, separate selectors are mounted underneath the diverter switch
and operate in the same liquid as the transformer. There are two selector rings per phase,
one containing the odd-numbered taps and the other the even-numbered taps, and moving
contact arms radially pre-select a tap position without current flowing prior to the diverter
switching to that position.
The diverter switch is mounted in a gas- and liquid-tight insulated compartment which
separates the arcing gases and carbonized liquid from the transformer liquid. Usually the
diverter compartment is equipped with a separate conservator breathing to atmosphere.
This type of tap-changer is used for the higher MVA ratings and voltage classes. Figure 4
shows how this arrangement works.

– 12 – 60214-2  IEC:2004(E)
2 4
3 5
IEC  1393/04
Key
1 Transformer windings 3 Tap selectors 5 Drive mechanism
2 Tap leads 4 Diverter switch
Figure 4 – In-tank separate selector and diverter switch
The switching can be by arc extinction in the liquid or by vacuum interrupters/power
electronics.
5.2.2.3.3 In-tank selector switch tap-changers
In-tank selector switch tap-changers carry out their selection and switching in the one
compartment using the same contacts and liquid. The fixed contacts are radially mounted
around a vertical insulated compartment with the three phases mounted one above the other.
The carbonized liquid and gases are separated from the main transformer liquid by the liquid-
tight compartment. The moving contacts are fixed to a central insulated drive shaft.
This type of tap-changer tends to be used for the lower MVA and voltage class transformers
(see Figure 5).
60214-2  IEC:2004(E) – 13 –
1 2 4 5
IEC  1394/04
Key
1 Transformer windings 3 Selector switch terminals 5 Drive mechanism
2 Tap leads 4 Change-over selector terminals
Figure 5 – In-tank selector switch tap-changer
5.2.2.3.4 In-tank separate selector and diverter switch in single compartment
This type of tap-changer uses separate contacts for the selection of the taps and different
contacts for the switching. Normally, all moving selector contacts would pre-select a tap
before the diverter contacts switch to that tap. The sequence of operation would be similar to
a tap-changer described in 5.2.2.3.2 but both sets of contacts are contained in the one
compartment. The switching can be by arc extinction in the liquid or by vacuum
interrupters/power electronics.
5.2.2.3.5 Advantages and disadvantages of in-tank on-load tap-changers
In-tank on-load tap-changers have the advantage of being more suitable for the higher
voltage class line-end applications. They can also benefit the transformer manufacturer by
allowing the tap-changer to be connected to the transformer prior to processing. For a given
power rating, an in-tank tap-changer will generally have a smaller oil volume. This has the

advantage of smaller oil volumes to handle during maintenance but a disadvantage that more
frequent maintenance may be required. Where in-tank tap-changer types have their selectors
and change-over selectors in particular, operating in the same liquid as the transformer, the
DGA of the transformer can be influenced by capacitive arcing from the contacts.
5.2.3 Reactor-type on-load tap-changers
5.2.3.1 General
Reactor-type on-load tap-changers are normally designed to be applied to the low voltage
winding of the transformer. They can be divided into two distinctive types:
– external out-of-tank tap-changers (air environment), as described in 5.2.3.2;
– in-tank tap-changers (liquid environment), as described in 5.2.3.3.

– 14 – 60214-2  IEC:2004(E)
5.2.3.2 Reactor-type external on-load tap-changers

5.2.3.2.1 General
These tap-changers are self-contained in their own tanks and mounted on the side or end of
the transformer. The taps from the transformer regulating winding are taken to the tap

selector contacts through a liquid tight barrier board (component of the on-load tap-changer).

The liquid from the tap-changer is totally isolated from the transformer main tank and,

therefore, can be checked separately by liquid sampling. Three types of tap-changer

arrangements are considered.
5.2.3.2.2 Diverter switch and tap selector

This type of tap-changer consist of change-over selectors and tap selectors, designed to
select tap connections, and transfer switches (diverter switches or vacuum interrupters),
designed to break and make current and, therefore, perform the arcing duty of the tap-
changing operations. The separate devices are normally located in a common liquid
compartment similar to that shown in Figure 1. This design of a tap-changer tends to be used
on the larger MVA transformers.
The operating sequence of a reactor type tap-changer with diverter switch and tap selector is
shown in IEC 60214-1, Figure B.5.
5.2.3.2.3 Selector switch (arcing tap switch) tap-changers
This type of tap-changer will incorporate a selector switch (arcing tap switch) which performs
the functions of making/breaking current and selection of tap connections, combining the
duties of a tap selector and a diverter switch. The selector switch and the change-over
selector, if they exist, are contained in one single compartment. This design of a tap-changer
tends to be used on the smaller MVA transformers.
The operating sequence of reactor type tap-changers with selector switch is shown in
IEC 60214-1, Figure B.1.
5.2.3.2.4 Vacuum interrupter (with by-pass switches) and tap selector
This tap-changer design also consists of a single liquid compartment containing change-over
and tap selectors to select taps and one vacuum interrupter per phase to break and make
current. In addition, they may be equipped with by-pass switches, mainly designed to by-pass
the vacuum interrupter when the tap-changer is not performing a tap-change operation. By
using vacuum interrupters to perform the breaking duty of the tap-changing operations, the
carbonization of the insulating liquid is minimized which enables the device to withstand
higher voltages. This arrangement is generally used for the larger MVA transformers.

The operating sequence of a reactor-type tap-changer with vacuum interrupter and tap
selector is shown in IEC 60214-1, Figure B.7.
5.2.3.3 Reactor-type internal on-load tap-changers
These designs are exclusively used in voltage regulators. The tap-changer is fitted inside the
transformer tank and is normally located on top of the transformer adjacent to the preventive
autotransformer (reactor).
This type of tap-changer will incorporate a load transferring selector switch (arcing tap switch)
which performs the functions of switching current and tap selection. Since tap changing is
performed in the same tank as the main transformer windings the insulating liquid should be
checked frequently to insure the dielectric integrity of the transformer. The transformer taps
are taken to the tap-changer contacts that are located on an insulated board. The selection
and switching are carried out using common contacts.

60214-2  IEC:2004(E) – 15 –
The transformer manufacturer needs to take into account the potentially heavy carbonization

of the liquid in the main transformer tank.

5.3 Off-circuit tap-changers
The off-circuit tap-changer is designed to change tap position and hence vary the turns ratio

of the transformer whilst it is de-energized.

This is achieved with mechanically operated devices that will select the various taps. The

fixed contacts may be arranged in a circular configuration (for rotary types) or in a straight

line (for rack and slide types). Normally, the drive mechanism is manual, but motor drive units

are also available.
This type of tap-changer is usually mounted inside the transformer tank with the drive
mechanism mounted on the transformer lid or on the wall of the transformer tank.
5.4 Liquid-immersed tap-changers
5.4.1 General
Liquid-immersed tap-changers cover all the types described in 5.2 and 5.3 that require a
liquid for use as either insulation or arc quenching during the switching operation. A typical,
and the most common, liquid is mineral oil (transformer oil) according to IEC 60296. Other
types of liquids may be employed for insulation and switching purposes but care has to be
exercised to ensure its compatibility with the tap-changer under consideration.
5.4.2 Liquid immersed on-load tap-changers
The liquid used for on-load tap-changers, as well as having electrical insulation and switching
functions, also acts as a lubricant and coolant. The most commonly used liquid for tap-
changers is mineral oil to IEC 60296. Although this oil has relatively poor lubricating
properties, it is nevertheless essential for the mechanical operation of the tap-changer. It is
therefore recommended that the tap-changer manufacturer be consulted before operating
mechanically off circuit in a non-immersed (unfilled) condition.
Other liquids that are sometimes used in transformers for fire-retardant and environmental
purposes may not be suitable for on-load tap-changers. Silicone fluids have very poor
lubricating properties and no arc-quenching ability and cannot be used for on-load tap-
changers. Synthetic esters and High Molecular Weight (HMW) paraffins have good lubrication
and arc extinction properties and may be suitable for some on-load tap-changers. The
temperature operating range may be restricted due to higher viscosities than transformer oil
at lower temperatures.
Where a liquid other than mineral oil according to IEC 60296 is being considered, the tap-
changer manufacturer should be consulted to establish its suitability.
Liquid immersed on-load tap-changers tested to IEC 60214-1 are suitable for operation down
to –25 °C in transformer oil according to IEC 60296. For temperatures below –25 °C, the tap-
changer manufacturer may recommend a lower viscosity oil, the installation of heaters in the
switching and mechanism compartments or other precautions to prohibit tap-changing whilst
below a given temperature limit.
Where temperatures below –25 °C are envisaged, the tap-changer manufacturer should be
consulted.
– 16 – 60214-2  IEC:2004(E)
5.4.3 Liquid-immersed off-circuit tap-changers

Liquid-immersed off-circuit tap-changers are tested to operate in mineral oil to IEC 60296;

however, in service they may be required to operate on one position for long periods of time,

and, if operating in high liquid temperatures, pyrolytic carbon may eventually form on the

contacts. For this reason, a lower temperature rise value is stipulated in 7.2.2 of IEC 60214-1.

The type of material used for the contacts should be suitable for the intended application.

Prolonged operation on one position may influence the pyrolitic carbon formation; hence,

silver plating/silver plating, silver plating/copper, copper/copper and copper/brass contact

materials may be preferable.
During transformer maintenance, it is recommended that the off-circuit tap-changer is
operated to clean the contacts (see 9.1.3).
Unlike liquid-immersed on-load tap-changers, off-circuit tap-changers do not require arc
quenching or good lubricating properties. Also, cold temperature viscosity is not so important
to their operation. For these reasons, the use of many different types of fire-retardant fluids
are possible.
Where a liquid other than mineral oil according to IEC 60296 is being considered, the tap-
changer manufacturer should be consulted to establish its suitability.
5.5 Dry-type tap-changers
Dry-type tap-changers are usually used in conjunction with dry-type transformers. This type of
tap-changer has several advantages compared to the conventional tap-changer in mineral oil
such as reduced fire hazards and prevention of water pollution.
In contrast to conventional tap-changers, which are located in separate housings filled with
insulating liquid or inside the main transformer tank, dry-type tap-changers can be subject to
different service conditions due to environmental influences. They can be used with or without
a housing in indoor or outdoor installations.
In liquid-filled on-load tap-changers the insulating liquid is used as insulating, switching and
cooling medium and as lubricant. Dry-type on-load tap-changers usually use vacuum
interrupters as switching elements and gas (SF or air) as the insulating medium as well as
the cooling medium. Lubrication is achieved by the application of grease on the movable
mechanical parts. Usually, the lubrication measures have to be repeated during maintenance
work. In order to reduce the need of frequent lubrication measures, especially for dry-type
tap-changers in an SF -gas environment, contacts, bearings and gears are specially designed
to significantly reduce mechanical fatigue and the necessary mechanical torque.

The following list of applications of dry-type tap-changers can be deduced from the different
types of dry-type transformers.
a) Dry-type tap-changers for gas-filled dry-type transformers
The tap-changer and the transformer are incorporated in a pressurized and gas-filled
(mainly SF ) enclosure.
b) Dry-type tap-changers for totally enclosed dry-type transformers
The tap-changer and the transformer are incorporated in an unpressurized enclosure,
cooled by the circulation of internal air.
c) Dry-type tap-changers for enclosed dry-type transformers
The tap-changer and the transformer are incorporated in a ventilated enclosure, cooled by
the circulation of external air.

60214-2  IEC:2004(E) – 17 –
d) Dry-type tap-changers for non-enclosed dry-type transformers

The tap-changer is used in conjunction with a transformer which is installed without a

protective enclosure (mainly indoor applications). The dry-type tap-changer may have its

own enclosure (usually a ventilated enclosure).

For dry-type tap-changers the normal service conditions such as altitude, temperature of

cooling air and humidity will be in accordance with IEC 60076-11 if applicable.

Furthermore, the purchaser should check, when selecting an appropriate dry-type tap-changer

for a certain application, whether the dry-type transformer will still meet the stated climatic,

environmental and fire behaviour classes according to IEC 60076-11 once the selected tap-
changer has been incorporated in the transformer. In the case of dry-type on-load tap-
changers, it has to be considered that despite the use of vacuum interrupters as switching
elements in common designs, arcing and hot spots can occur at, for example,
– change-over selectors (if applicable);
– commutation sparks at non-enclosed mechanical switching elements (if applicable);
– temperature rise of the transition resistors.
Dry-type on-load tap-changers that are not totally enclosed are not suitable for use in
explosion hazardous areas.
When using dry-type on-load tap-changers in SF gas atmosphere, the effects of the above-
mentioned sparks or hot spots should be considered, as SF gas may decompose.
It can be assumed that no decomposition of the SF gas takes place below 150 °C. At
temperatures higher than 200 °C some metals may have a decomposing effect on the SF
gas. At temperatures of 500 °C and higher, SF gas starts to decompose into its constituent
elements, with the decomposition process being directly proportional to the quantity of energy
converted.
The SF gas decomposes into gaseous and solid by-products, some of which may have toxic
effects. Therefore, careful handling of the SF gas in use is required, for example, during
maintenance work. Adequate precautions should be taken to ensure the safety of personnel.
5.6 Other types
5.6.1 General
There are also other less common types of tap-changers not fully covered by the above types.
The standards, type and routine tests to be applied are those relevant for the design. Other

tests to fulfil the intention of the standards and to support the tap-changer manufacturer’s
technical data of the products may also be made on the tap-changer.
The following are some other types of tap-changers described.
5.6.2 Electronic tap-changers
In an electronic on-load tap-changer, the transferring of load from one tap to another is
performed by power electronics such as thyristors and thereby no arcing will take place.
Electronic tap-changers are usually designed to replace only the diverter switch function but
can also take care of the tap selector function. The electronic tap-changer can be of totally
dry type or have the electronic immersed in liquid. It can also be of dry type but cooled with
some other type of media.
– 18 – 60214-2  IEC:2004(E)
5.7 Protective devices
5.7.1 General
According to 5.1.4 of IEC 60214-1, the use of protective devices is required for on-load tap-
changers to minimize the risk of fire or explosion resulting from an internal failure within the

diverter or selector switch compartment.

Protective devices for diverter or selector switches are designed to counter the action of the

following stresses:
– inadmissible increase of pressure within the diverter or selector switch liquid compartment;

– on-load tap-changer operation with excessive transformer overcurrents;
– on-load tap-changer operation at liquid temperatures below the limit of –25 °C given in
IEC 60214-1 or, in some cases, above an agreed maximum limit.
If the tap selector is assembled in its own and separate liquid compartment, then protective
devices designed to counter the action of an inadmissible increase of pressure within the tap
selector liquid compartment may be employed.
In some applications, it may be necessary to supervise the simultaneous operation of different
poles of an on-load tap-changer or of different phases to avoid excessive circulating currents
caused by a possible out-of-step position of the on-load tap-changers.
5.7.2 Increase of pressure within diverter or selector switch liquid compartments
5.7.2.1 General
Faults occurring in on-load tap-changers usually have the effect of converting electrical
energy into heat due to arcing. The heat vaporizes the insulating liquid, which will result in an
increase of pressure within the liquid compartment. The amount of energy released during a
fault depends upon various factors such as the rated capacity of the transformer, the
operating voltage, the on-load tap-changer through-current, the short-circuit power of the grid,
the connection of the star point, the length of the fault arc, etc.
Protective devices for supervision of the increase of pressure within diverter or selector switch
liquid compartments have to respond to every form of abnormal energy release, from long-
term low-energy phenomena to an explosive energy release. However, the energy release
during normal operation should not operate the protective devices. Such supervision can be
achieved by direct pressure sensing or by monitoring the surge speed of the liquid flow,
resulting from the pressure increase to the separate conservator. Each on-load tap-changer
should have such a protective device. In the case of on-load tap-changers consisting of more
than one pole, each pole should be equipped with a separate protective device.

5.7.2.2 Liquid-flow controlled relay
Liquid-flow controlled relays inserted into the pipes between the on-load tap-changer switching
compartment and the conservator are most frequently used. Such relays are actuated by an
increased liquid flow from the diverter and selector switch liquid compartment to the
conservator. They respond to relatively low- up to high-power short-duration disturbances within
the diverter switch compartment by tripping the circuit breaker of the transformer, thus avoiding
or limiting damage to the on-load tap-changers and the transformer.
Liquid-flow controlled relays have been used for many years in transformer engineering
applications and have the advantage of proven reliability and little or no evidence of spurious
operations. The disadvantage is that the response time of the relay, being essentially
hydraulic, is relatively long compared to some other relay types. Dual element relays,
incorporating a contact that is operated by accumulation of gas, are not applicable to diverter
or selector switches as they inherently generate gas during their normal operation.

60214-2  IEC:2004(E) – 19 –
The liquid-flow controlled relay should be installed in the pipe leading from the on-load tap-

changer switching compartment to the conservator, located as near as possible to the on-load

tap-changer switching compartment. The pipe to the liquid conservator should be installed

with a rising inclination sufficient to ensure the free escape of switching gas. For further

information, see the installation instructions of the manufacturer.

Liquid-flow controlled relays would normally be set for tripping the transformer to reduce

danger to personnel, and to limit consequential damage. Alarm-only systems are not

recommended.
5.7.2.3 Overpressure relay
Pressure-sensing devices are often used, either alone or in addition to liquid-flow controlled
relays. Usually, they are mounted to the outside of the on-load tap-changer switching
compartment and respond to internal static and dynamic pressures. However, such
overpressure relays will not be actuated by weak disturbances, as they will not reach the
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