Trasformatori in Resina e Olio

General specification for three-phase

Cast Resin Transformers – HV/LV

Power 100 to 3150 kVA

 1   -    Scope

Three-phase transformers of cast resin type, class F insulation system with natural (AN) cooling for indoor installation, destined for use in three-phase HV/LV distribution systems.dry-type-193x300

If required forced cooling (AF) to increase the rated power up to 40%.

On request, the initial guarantee can be extended to 5 years, subject to validation of the first commissioning procedure, as advised by the supplier.

2   -    Standards

These transformers will be in compliance with the following standards:

              – IEC 60076-11

              - prEN 50541-1

Specific Standards according to country requests      

These transformers will be manufactured in accordance with:

a quality system in conformity with ISO 9001

- an environmental management system in conformity with ISO 14001, both certified by an official independent organisation.

 3   -    Description

          3.1  -    Magnetic core

 This will be made from laminations of insulated silicon steel, and will be protected against corrosion with a coat of varnish.

 In order to reduce the power consumption due to transformer no-load losses, the magnetic core is stacked using overlapping-interlocking technology.

 In order to reduce the noise produced by the magnetic core, it is equipped with noise-damping devices.

           3.2  -    LV windings

 The LV winding is produced using aluminium or copper foils (according to the manufacturer’s preference) in order to cancel out axial stress during short circuit ; this foil will be insulated between each layer using a heat-reactivated class F pre-impregnated epoxy resin film

The ends of the winding are protected and insulated using a class F insulating material.

The whole winding assembly will be polymerised throughout by being autoclaved for 2 hours at 130°C, which will ensure:

     – High level of resistance to industrial environments

     – Excellent dielectric withstand

     – Very good resistance to radial stress in the instance of a bolted short circuit.


          3.3  -    HV windings

They will be separated from the LV windings to give an air gap between the MV and LV circuits in order to avoid depositing of dust on the spacers placed in the radical electrical field and to make maintenance easier.

 These will be made of aluminium or copper wire or foil (according to the manufacturer’s preference) with class F insulation.

 The HV windings will be vacuum cast in a class F fireproof epoxy resin casting system composed of:

     – an epoxy resin

     – an anhydride hardener with a flexibility additive

     – a flame-retardant filler.


The flame-retardant filler will be thoroughly mixed with the resin and hardener. It will be composed of trihydrated alumina powder (or aluminium hydroxide) or other flame-retardant products to be specified, either mixed with silica or not.

The casting system will be of class F. The interior and exterior of the windings will be reinforced with a combination of glass fibre to provide thermal shock withstand

 3.4  -    MV winding support spacers

 These will provide sufficient support in transport, operation and during bolted short circuit conditions as well as in the case of an earthquake.

These spacers will be circular in shape for easy cleaning. They will give an extended tracking line to give better dielectric withstand under humid or high dust conditions.

These spacers will include an Elastomer cushion that will allow it to absorb expansion according to load conditions. This Elastomer cushion will be incorporated in the spacer to prevent it being deteriorated by air or UV.


          3.5  -    HV connections


The HV connections will be made from above on the top of the connection bars. Each bar will be drilled with a 13 mm hole ready for connection of cable lugs on terminal plates.

The HV connection bars will be in rigid copper bars protected by heat shrinkable tubing.

HV connections in cables are not allowed, in order to avoid all risk of contact, due to cables flapping.

The HV connections will be in copper.

           3.6  -    LV connections

 The LV connections will be made from above onto bars located at the top of the coils on the opposite side to the HV connections.

Connection of the LV neutral will be directly made to the LV terminals between the LV phase bars.

The LV connection bars will be in copper or in tinned aluminium (according to preference of the manufacturer).

The output from each LV winding will comprise a tin-plated aluminium or copper connection terminal, enabling all connections to be made without using a contact interface (grease, by-metallic strip: out of scope of supplying).

These will be assembled according to current practices, notably using spring washers under the fixings and nuts.

Devices in the 630 to 2500 kVA range will be easy to connect using factory-built electrical ducting through an optional interface. Stress withstand in the instance of a bolted short circuit on the connector will be guaranteed by the manufacturer.


          3.7  -    HV tapping


The tapping which act on the highest voltage adapting the transformer to the real supply voltage value, will be off-circuit bolted links.

Tapping with connection cables are not allowed.

These bolted links will be attached to the HV coils.

4   -    Accessories and standard equipment

           These transformers will be equipped with:

              – 4 flat bi-directional rollers

              – lifting lugs

              – haulage holes on the undercarriage

              – 1 earthling terminals

              – 1 rating plate

              – 1 “Danger Electricity” warning label (T 10 warning)

              – 1 routine tests certificate

              – 1 instruction manual for installation, commissioning and maintenance in English.


5   -    Thermal protection


These transformers will be equipped with a thermal protection device which will comprise:


- 3 thermal detection systems (1 by phase), installed in the active part of the transformer. The sensors will be placed in a tube to enable them to be replaced if ever necessary.

- An electronic converter with two independent monitoring circuits equipped with a changeover switch, one for “Alarm 1″ the other for “Alarm 2″. The position of the relays will be indicated by different coloured indicator lights. A third indicator light will indicate the presence of voltage.

These three indicator lights will be on the front of the converter. The electronic converter will be installed away from the transformer.

- A plug-in terminal block for connection of the sensors to the electronic converter.


The sensors will be supplied assembled and wired to the terminal block fixed on the upper part of the transformer. The converter will be supplied loose with the transformer, packaged complete with its wiring diagram.


 6   -    Metal enclosure


On request, these transformers will be equipped with a metal enclosure for indoor installation comprising an integral IP 31 (except the base which may be IP 21) metal enclosure, that can be dismantle on request, with:

On request specified in the annex, these transformers can be supplied mounted equipped with protective metal enclosure:


  • either for indoor installation, with degree of protection IP 31 (except the bottom which may be IP 21),
  • Ø or for outdoor installation with degree of protection IP 35 (except the bottom which may be IP 31). This outdoor installation will require some adjustments related to weather and environmental extremes of the place.

These enclosures will not be dismantled (if requested, it can be dismantled) with:

- an anti-corrosion protection in the manufacturer’s standard colour

- lifting-lugs enabling the transformer and enclosure assembly to be handled.

- a bolted access panel on the enclosure front to allow access to the HV connections and to the tapping. This will be fitted with handles; it will have one “Danger Electricity” warning label (“T10 “ warning), a rating plate and a visible braid for earthling.

- blanked off holes for fitting on the bolted access panel to enable it to be locked.

- 2 undrilled gland plates on the roof: one on the HV side, one on the LV side (drilling and cable gland not supplied).

- 1 plate at the right HV side on the bottom of the enclosure for the HV cables for connections from the bottom.

- as an option, a HV cables clamping system shall be provided when the cables are coming from the bottom

7   -    Electrical protection

           7.1     – Protection relay

 The installation must have a protection relay to protect the transformer from:

     – overload,

     – short-circuit (internal or external),

     – earth fault,

     – overflow.


          7.2     – MV surge arresters

It is advisable to check that the installation will not be subjected to overvoltage of any kind (atmospheric or switching overvoltage).

If there is a risk, the transformer should be protected by phase-earth surge arresters installed directly on the MV connection terminals (top or bottom). 

Phase-earth surge arresters are absolutely essential in the following cases:

- If the lightning impact level Nk is greater than 25. The risk of direct or induced atmospheric overvoltage is directly proportional to Nk.

- During the occasional switching (less than 10 operations a year) of a transformer with a weak load, or during a magnetisation period.

They are highly recommended in the following case:

- If the substation is supplied by a network including overhead parts, then a cable which is longer than 20 m (for example, an overhead-underground network).


7.3      – RC filters (repetitive switching operations)

If the installation is likely to be subjected to repetitive switching operations (e.g. connected with a process), it should be protected from the resulting surges, which are particularly harmful to the transformer. .

The ideal solution for protecting the installation completely from these surges (with high frequency oscillations), consists in fitting an RC damping filter between the phases and the earth.

This RC filter should be placed as close as possible to the transformer’s primary terminals.

This gets rid of the high frequency phenomenon, and limits voltages at the transformer terminals.

The filter should consist of 3 50 Ohm resistors (of the RWST type), and 3 0.25 µF capacitors, insulation level 24 kV.

It may be placed either in a separate metal enclosure or, preferably, inside the metal enclosure of the transformer.

8   -    Electrical tests

           8.1  -    Routine tests

 These tests will be carried out on all the transformers after the manufactu­ring, enabling an official test certificate to be produced for each one:

              – measurement of windings resistance

              – measurement of the transformation ratio and vector group

              – measurement of impedance voltage and load loss

              – measurement of no load loss and no load current

              – applied voltage dielectric test

              – induced voltage dielectric test

       – measurement of partial discharges. For this measurement, the acceptance criterion will be:

  • partial discharges less than or equal to 10 pC at 1.30 Un, or
  • partial discharges less than or equal to 5 pC at 1.30 Un (Special test)

 (All these tests are defined in the IEC 60076-11 and IEC 60076-1 to 60076-3 standards).

 When transformers are equipped with a protection enclosure, (see § 6), they will be tested in their enclosure.

 8.2  -    Type tests or special tests

 These tests can be requested as option, but are subject to prior agreement of the supplier:

- temperature rise test carried out in accordance with the simulated loading method as defined by the IEC 60076-11 standard

- lightning impulse test in accordance with IEC 60076-3

- short circuit test in accordance with IEC 60076-5

- noise level measurements in accordance with IEC 60076-10.

(All these tests are defined by the IEC  60076-11 and IEC 60076-1 to 60076-5 standards).


9   -    Climatic and Environmental classifications

These transformers will be of environmental class E3 and of climatic class C2 as defined in IEC 60076. E3 and C2 classes will be indicated on the rating plate.

The manufacturer must produce a test report from an official laboratory for a transformer of the same design as those produced.

The tests must have been performed in accordance with IEC 60076-11 for C2 climatic class and IEC 60076-16 for E3 environmental class.

On request, temperature behaviour at -50°C can be specified and will be applicable for transportation, storage and running of the transformer.


10     -    Fire behaviour classification

These transformers will be of class F1 as defined in IEC 60076-11. F1 class will be indicated on the rating plate.

The manufacturer must produce a test report from an official laboratory on a transformer of the same design as those produced and on the same transformer which have initially passed the here above Climatic and Environmental tests.

This test must have been performed in accordance with IEC 60076-11.