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Specification of voltage regulating distribution transformers (VRDT)
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 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization forStandardization (ISO) in accordance with conditions determined by agreement between the two organizations.
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.
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
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
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
Transformer equipped with components to control primary or secondary voltage for on-load voltage regulation purposes within the scope of this document.
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.
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.
Ratio between regulation range divided by number of steps
System or device that physically facilitates the change in voltage of the transformer, for example, electromagnetic or electronic devices
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.
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
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 overexcitation which can increase no-load losses, depending on the technology of the VRDT.
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).
Unless otherwise specified, voltage steps are 1,0 % up to 3,0 % and shall be agreed between manufacturer and purchaser.
Unless otherwise specified, the on-load regulation range is ±5 % up to ±10 % of the rated voltage ratio for symmetrical regulation.
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.
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.
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.
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.
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.
Apply IEC 60076-1 and IEC 60076-11, but with different test sequences, in accordance with this document.
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.
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.
Apply IEC 60076-1 and IEC 60076-11, but with different test sequences, in accordance with this document.
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