IEC 60076-24:2020

IEC 60076-24 ®
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 24: Specification of voltage regulating distribution transformers (VRDT)

Transformateurs de puissance –
Partie 24: Spécification des transformateurs de distribution régulateurs de
tension (VRDT)
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IEC 60076-24 ®
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 24: Specification of voltage regulating distribution transformers (VRDT)

Transformateurs de puissance –

Partie 24: Spécification des transformateurs de distribution régulateurs de

tension (VRDT)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.180 ISBN 978-2-8322-8564-0

– 2 – IEC 60076-24:2020 © IEC 2020
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 Environmental conditions . 7
5 Electrical characteristics and general requirements . 7
5.1 General . 7
5.2 Regulation steps . 7
5.3 Voltage steps . 8
5.4 Regulation range . 8
6 Service life . 8
7 Parallel operation of the VRDT . 8
8 Loss level . 8
9 Sound power level . 8
10 Rating plate . 8
11 Tests . 9
11.1 Routine tests. 9
11.2 Functional tests . 9
11.3 Type tests . 9
11.4 Special tests . 9
11.5 Tests for the control unit (electronic or data transfer part) . 10
11.5.1 Functionality test during erection and commissioning. 10
11.5.2 Type tests . 10
Bibliography . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 24: Specification of voltage regulating
distribution transformers (VRDT)

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) 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 60076-24 has been prepared by IEC technical committee 14: Power
transformers.
The text of this International Standard is based on the following documents:
FDIS Report on voting
14/1050/FDIS 14/1055/RVD
Full information on the voting for the approval of this International Standard 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 series, published under the general title Power transformers,
can be found on the IEC website.

– 4 – IEC 60076-24:2020 © IEC 2020
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.
POWER TRANSFORMERS –
Part 24: Specification of voltage regulating
distribution transformers (VRDT)

1 Scope
This document applies to medium power transformers from 25 kVA up to 3 150 kVA with highest
voltage for equipment up to 36 kV, or in low voltage (LV) networks with highest voltage for
equipment of up to 1,1 kV equipped with voltage regulating devices.
Voltage regulating distribution transformers are transformers equipped with components to
control primary or secondary voltage for on-load voltage regulation purposes.
The main objective of the installation of a VRDT is to regulate the LV network voltage level (i.e.
400 V), to avoid violation of the limits defined by relevant standards or regulations. The VRDT
must operate properly as a step down and step up transformer.
Transformers covered by this document comply with the relevant requirements set out in
IEC 60076 (all parts) and, unless otherwise stated in this document, they also comply with
European Standards EN 50160 and EN 50588-1.
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 60068-2-1, Environmental testing – Part 2-1: Tests – Test A: Cold
IEC 60068-2-2, Environmental testing – Part 2-2: Tests – Test B: Dry heat
IEC 60068-2-11, Basic environmental testing procedures – Part 2-11: Tests – Test Ka: Salt mist
IEC 60068-2-30, Environmental testing – Part 2-30: Tests – Test Db: Damp heat, cyclic
(12 h + 12 h cycle)
IEC 60068-2-78, Environmental testing – Part 2-78: Tests – Test Cab: Damp heat, steady state
IEC 60076-1, Power transformers – Part 1: General
IEC 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-5, Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-7, Power transformers – Part 7: Loading guide for mineral-oil-immersed power
transformers
IEC 60076-10, Power transformers – Part 10: Determination of sound levels
IEC 60076-11, Power transformers – Part 11: Dry-type transformers

– 6 – IEC 60076-24:2020 © IEC 2020
IEC 60076-12, Power transformers – Part 12: Loading guide for dry-type power transformers
IEC 60255-21-1, Electrical relays – Part 21: Vibration, shock, bump and seismic test on
measuring relays and protection equipment – Section 1: Vibration tests (sinusoidal)
IEC 60255-21-2, Electrical relays – Part 21: Vibration, shock, bump and seismic tests on
measuring relays and protection equipment – Section 2: Shock and bump tests
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 61000-6-2, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards – Immunity
standard for industrial environments
IEC 61000-6-4, Electromagnetic compatibility (EMC) – Part 6-4: Generic standards – Emission
standard for industrial environments
IEC 61000-6-5, Electromagnetic compatibility (EMC) – Part 6-5: Generic standards – Immunity
for equipment used in power station and substation environment
IEC 61010-1, Safety requirements for electrical equipment for measurement, control, and
laboratory use – Part 1: General requirements
IEC 61010-2-201, Safety requirements for electrical equipment for measurement, control, and
laboratory use – Part 2-201: Particular requirements for control equipment
EN 50708, Power transformers – Additional European requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60076-1, EN 50708,
and the following 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
voltage regulating distribution transformer
VRDT
transformer equipped with components to control primary or secondary voltage for on-load
voltage regulation purposes within the scope of this document
3.2
regulation voltage
ratio Ud/UN or 100 Ud/UN (percentage) where: UN is the rated voltage of the transformer; Ud is
the voltage which would be developed at no-load at the terminals of the transformer, connected
on the tapping concerned, by applying rated voltage to an untapped winding related to
IEC 60214
3.3
regulation range
variation range of the regulation voltage expressed as a percentage, compared with value "100"
Note 1 to entry: If this factor ranges from 100 + a to 100 − b, the tapping range is said to be: + a % − b % or ± a %
if a = b.
3.4
regulation steps
ratio between regulation range divided by number of steps
3.5
regulating unit
system or device that physically facilitates the change in voltage of the transformer, for
example, electromagnetic or electronic devices
3.6
control unit
unit which operates the regulating unit
Note 1 to entry: It can have various modes of operation, for example, automatic, manual or remote control. Manual
mode is mandatory for erection and commissioning. It can also have a human-machine interface.
4 Environmental conditions
Service conditions for VRDTs are the same as those specified for power transformers in
IEC 60076-1 and IEC 60076-11.
The purchaser shall identify in the enquiry to the manufacturer any service conditions not
covered by the normal service conditions as specified in IEC 60076-1 and IEC 60076-11 and,
for the control unit, the normal service conditions as specified in IEC 60068-2 (all parts). Any
different applications shall be agreed between manufacturer and purchaser.
NOTE 1 Transformer with regulating unit:
Application liquid immersed transformer: indoor or outdoor use and storage.
Application dry transformer: indoor use and storage.
NOTE 2 Control unit:
Application is indoor use and storage.
5 Electrical characteristics and general requirements
5.1 General
As per IEC 60076-1:2011, 5.3 for transformers connected to generators, the value of the rated
voltage is as given in IEC 60076-1:2011, 5.4.3, the 110 % of overvoltage is at no load and
limited at 5 % at rated power.
For specifying the rated low voltage, the purchaser shall take into account the possible over-
excitation which can increase no-load losses, depending on the technology of the VRDT.
5.2 Regulation steps
Unless otherwise specified, there are 3 to 11 on-load steps.
The regulation can be symmetric or asymmetric.
Unless otherwise agreed, step sizes are to be uniform to give constant flux voltage variation
(CFVV).
– 8 – IEC 60076-24:2020 © IEC 2020
5.3 Voltage steps
Unless otherwise specified, voltage steps are 1,0 % up to 3,0 % and shall be agreed between
manufacturer and purchaser.
5.4 Regulation range
Unless otherwise specified, the on-load regulation range is ±5 % up to ±10 % of the rated
voltage ratio for symmetrical regulation.
6 Service life
Loading guides (IEC 60076-7 and IEC 60076-12) are applicable.
Deviations shall be agreed between manufacturer and purchaser.
The manufacturer shall indicate the number of operations for which the VRDT is designed.
The manufacturer shall indicate a future need for firmware updates of the control unit.
7 Parallel operation of the VRDT
For parallel operation of the VRDT, special precautions should be taken to avoid high circulating
currents.
NOTE Depending on the impedance of the connections between the VRDTs, significant circulating currents are
possible (IEC 60076-8). A typical solution can be a master/follower arrangement or a single grid controller.
8 Loss level
Measurement of the loss level shall be in accordance with IEC 60076-1.
The maximum loss level for the VRDT (transformer and regulating unit) shall be compliant with
local standards, for example, EN 50708.
No-load and load losses shall include regulating unit losses.
9 Sound power level
Measurement of the sound power shall be in accordance with IEC 60076-10.
The maximum sound power level shall be compliant with local standards, for example,
EN 50708.
Operating noise during the switching process of voltage regulation should not be taken into
account.
This noise and its values shall be agreed between manufacturer and purchaser.
10 Rating plate
Rating plate requirements are in accordance with IEC 60076-1, IEC 60076-11, and with
applicable local standards, for example, EN 50708.

For voltage regulating distribution transformers, a visible marking of "VRDT" shall be placed on
the rating plate.
11 Tests
11.1 Routine tests
Apply IEC 60076-1 and IEC 60076-11, but with different test sequences, in accordance with
this document.
11.2 Functional tests
With the regulating unit fully assembled on the transformer, the following sequence of
operations shall be performed as a routine test, without any failure:
a) with the transformer de-energized, three complete cycles of operation (a cycle of operation
goes from one end of the tapping range to the other, and back again);
b) with the transformer de-energized, and with the auxiliary voltage reduced to 85 % of its rated
value, one complete cycle of operation;
c) with the transformer energized at rated voltage and frequency at no load, one complete
cycle of operation.
11.3 Type tests
If the regulating unit contains electronic components, the test conditions shall be agreed
between manufacturer and purchaser.
If the operation of the regulating unit occurs in conditions that differ from those of IEC 60076-1,
a heat run test according to IEC 60076-2 shall be agreed between purchaser and manufacturer.
With the regulating unit fully assembled on the transformer, the following sequence of
operations shall be performed, without any failure:
With one winding short-circuited and, as far as practicable, rated current in the tapped winding,
10 cycles of tap-change operations across the complete range.
NOTE In order to check the correct coordination of over-current protection and to have all information regarding the
operation of the VRDT, the purchaser can request the measurement of the impedance voltage and load losses on all
the taps, related to the high voltage (HV) rated voltage and rated power of the VRDT.
The functional tests shall be repeated.
11.4 Special tests
The transformer, including the regulating unit, shall pass the short-circuit withstand test in
accordance with IEC 60076-5.
If the regulating unit contains electronic components, the test conditions shall be agreed
between manufacturer and purchaser.
Partial discharge measurement shall be agreed between manufacturer and purchaser
(e.g. IEC 60076-13).
The functionality test (11.5.1) shall be repeated.

– 10 – IEC 60076-24:2020 © IEC 2020
11.5 Tests for the control unit (electronic or data transfer part)
11.5.1 Functionality test during erection and commissioning
The transformer is connected on either the HV or LV side and the other side is open for testing.
Voltage shall be measured at the open side.
The control unit is in manual operation mode and is then switched through the complete
regulation range.
For each switching operation, the relative change in the measured voltage shall be equivalent
to the regulation steps.
If the VRDT has an automatic mode then with rated voltage applied to the HV, manually set the
regulator to one of the extreme positions, then switch to automatic mode and check that the
system regulates the LV voltage to the rated value.
11.5.2 Type tests
The VRDT shall satisfy the following criteria:
• safety requirements according to IEC 61010-1 and IEC 61010-2-201;
• electromagnetic compatibility according to IEC 61000-6-2, IEC 61000
...