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Distribution Transformers
Distribution transformers can be grouped into 2 main types: the immersed type and the encapsulated dry type.
The distribution transformers within our range are grouped as follows according to their intended use:
- With off-load tap-changer or on-load tap-changer
- ONAN (naturally cooled) / ONAF (fan cooled)
- Hermetic or preservative tank
- Ceramic or plug-in insulator
- Gas-sealed hermetic transformers
- Rectifiers, phase shifters and grounding transformers
- Series/shunt reactors and panel mounted transformers
The “submerged” technology is the most used and represents approximately 80% of the world market for distribution transformers. Basic energy distribution uses this technology, which benefits from long experience, particularly in public distribution and industry. These transformers can be equipped with protection relays against internal faults and overloads to ensure the protection of goods and people as well as a retention tank to ensure the protection of the environment.
In Europe, the dielectric liquids used for the insulation and cooling of immersed transformers are: mineral oil, silicone oil, esters.
Mineral oil, derived from petroleum refining, is mainly used. Silicone oil and esters are synthetic liquids which are used to improve the fire behavior of immersed transformers. Standard NFC 27-300 and IEC 1100 (EN 61100) define the fire classification of these liquids, according to their characteristics.
The oil-immersed transformer consists of a magnetic circuit in oriented crystal laminations, copper or aluminum windings, a metal tank, a cover and MV and LV connection terminals.
Tank/Cover
The tank must be closed at its upper part by a bolted cover with a tightening torque limiter preventing the seal from creeping.
The tank must be designed in such a way as to avoid any risk of water stagnation.
The bottom of the tank is reinforced to allow handling on the ground with levers without risk of damage to the device.
The upper part of the cover must be equipped with lifting parts, drilled with a hole with a minimum diameter of 28 mm, allowing the draining and lifting of the transformer filled with oil.
Electrical continuity must be ensured between the tank, its cover and the magnetic circuit. On the cover there must be a filling opening, with a minimum diameter of 21 mm, raised by at least 30 mm in relation to the plane of the cover and at the bottom of the tank there must be a drain opening closed by a a metal cap ensuring the seal.
Connection
For medium voltage connection needs, the pole-top type transformer is fitted with porcelain bushings and the cabin-type transformer is fitted, depending on the case, either with porcelain bushings or with socket outlets.
Porcelain bushings must have a specific creepage distance of at least:
– 16mm/kV for the normal insulation transformer;
– 25mm/kV for the over-insulated transformer;
– 31mm/kV for the special transformer
For LV connection needs, cabin-type or pole-top transformers with 50 to 160 kVA power are equipped with 250 A porcelain bushings and cabin-type transformers with 250 to 1250 kVA power are equipped, depending on the case, either with porcelain bushings , or busbar type bushings according to specification HN 52 S62 with a rated current equal to:
– 1250 A for transformers from 250 to 800kVA;
– 2000 A for 1000 and 1250 KVA transformers.
Gaskets
All seals used must be made of synthetic rubber or any other material recognized as resistant to the action of dielectric oil used at operating temperature. Seals made from non-polymerizable pasty material or asbestos are excluded.
Tap Changer
The MV winding must be provided, in addition to the main tap, with taps corresponding to a transformation ratio range of ±5% (5 positions). These sockets are connected to a switch.
The tap changer is controlled by an operating device accessible on the lid or on the tank. This operation is carried out with the transformer de-energized. The operating device must be of appropriate design guaranteeing correct operation while ensuring the tightness of the device and the absence of corrosion by electrochemical couple (use of appropriate materials).Prise de terre des masses
A device must be provided for earthing the masses in accordance with the provisions of article 8-1 of IEC 76-1.
For the particular case of the Top of post transformer, an 8.5mm hole must be drilled on the end on the LV side of the bottom of the tank.
The earth symbol must be engraved or stamped in close proximity to the adopted device.
Bearing rollers (NF C 20 -210)
The cabin type transformers must be fitted with rollers that can be oriented in two perpendicular directions corresponding to the two axes of the transformer (NF C 51-151)
The dry-type transformer, which is intended for indoor installation only, must be of thermal class F. It can be with or without a protective metal casing.
When it is without protective casing, it must be installed in cabinets or cabins. The protective enclosure must satisfy degrees of protection IP53 and IK09.
Adjustment sockets must be made of easily accessible movable connections.
GENERAL PROVISIONS
Provisions common to cabin and pole-top transformers
Distances to respect for Bushings
A minimum distance in the air of 225 mm must be respected:
- between conductive parts of MV bushings;
- between LV bushings and conductive parts of MV bushings;
- between earth and conductive parts of MV bushings.
A minimum distance of 70mm between axis of LV bushings must be respected.
For “seaside” type transformers, the porcelain terminals are positioned with increased positioning as well as the phase spacing in order to have a higher creepage distance. at 31 mm/kV (C EI 815) and a spacing of 350 mm between MV phase connection axes. All the connection parts are tinned (copper parts)
Identification of Bushings
Terminals must be identified by means of symbols engraved on the cover or on metal plates fixed by the terminal fixing bolts or at the end of the bushings.
Degree of hermeticity
The oil-immersed transformer is:
- or with complete filling, in which case the balance of pressures (internal pressure and atmospheric pressure) must be achieved in the vicinity of normal conditions (20° C , 1,013 mbar). The words “warning: integrally filled transformer” must be affixed in the immediate vicinity of the filling hole;
- either breathable, in which case the cover must include a level indicator with a float, equipped with an orifice ensuring the breathing of the transformer.
Filling oil
The filling mineral oil must comply with the provisions of standard IEC 296 and/or standard NF C 37-101.
Cooling method
The oil-immersed transformer must be of ONAN type cooling mode in accordance with the provisions of article 3 of IEC 76-2.
The dry-type transformer shall be of AN or NAN type cooling mode, in accordance with the provisions of clause 9 of IEC 726.
Corrosion protection
The external surfaces of the tank and the lid must undergo a sandblasting or shot-blasting operation.
The wave panels need not undergo this operation if during their manufacture they are protected against the appearance of traces of rust, by oiling or by applying a layer of anti-rust paint.
Sandblasting or shot blasting must be followed by the application of anti-rust paint, the two operations having to take place on the same day.
Storage after sandblasting of transformer elements or mounted tanks is not tolerated. The external surfaces of the tank and the cover must then undergo the application, in at least two coats, of a gray paint.
The application of rustproofing and a minimum of two coats of paint is mandatory for conventional paint application processes. There is no requirement for intermediate protection (anti-rust) or a minimum number of coats of paint, for special processes that have received the approval of the Project Owner or a body mandated by him. The transformers must undergo an individual and systematic control of the thickness of the paint, at least in two points of the tank. The paint deposit must be at least 70 µm.
All hardware must be galvanized.
The transformer must be fitted with a weatherproof nameplate, fixed in a visible location and giving the indications listed below, marked indelibly:
– type of transformer;
– the serial number;
– Date of manufacture;
– name of the manufacturer;
– rated frequency (in Hz);
– Rated power (in KVA);
– rated MV voltage (in kV);
– rated LV voltage (in V);
– primary and secondary currents (A)
– insulation level (in kV)
– cooling mode;
Technical Specification of MV/LV Power Transformers
– coupling symbol;
– short-circuit voltage in %;
– number of phases;
– total mass;
– mass of the insulating oil (in the case of immersed transformers); – Isolation (normal, overinsulated or special).
SUBMERGED TYPE TRANSFORMERS
N0 | TESTS | IEC 76 REF STANDARD |
| INDIVIDUAL TESTS |
|
1 | Winding resistance measurement | Part 1 section 10-2 |
2 | Measurement of the transformation ratio and phase shift control | Part 1 section 10-3 |
3 | Measurement of short-circuit impedance and load losses | Part 1 section 10-4 |
4 | No-load current and loss measurement | Part 1 section 10-5 |
5 | Zero-sequence impedance measurement | Part 1 section 10-7 |
6 | No-load current harmonics measurement | Part 1 section 10-10 |
| Type Tests | Part 1 article 10 |
7 | Temperature rise test | Part 2 article 5 |
8 | Dielectric tests | Part 3 section 10-4 |
| Special Test |
|
9 | Short-circuit withstand test | Part 5 section 10-4 |
DRY TYPE TRANSFORMERS
N0 | TESTS | IEC 76 REF STANDARD |
| INDIVIDUAL TESTS |
|
1 | Winding resistance measurement | section 13 |
2 | Measurement of the transformation ratio and phase shift control | section 14 |
3 | Measurement of short-circuit impedance and load losses | section 15 |
4 | No-load current and loss measurement | section 16 |
5 | Dielectric test by applied voltage | section 17 |
6 | Dielectric test by induced voltage | section 18 |
| Type Tests |
|
7 | Lightning impulse test | Section 19 |
8 | Temperature rise test | section 21 |
| Special Test |
|
9 | Short-circuit withstand test | section 23 |
For large production series of transformers, all or part of the tests described above will be carried out by the manufacturer either in the factory or in an approved laboratory.
The test procedure and sanction are defined in the C EI 76 and 726 standards. The qualification tests will be carried out by an official or accredited laboratory, possibly in the presence of representatives of the Project Owner or a body mandated by him. . Said tests must be sanctioned by:
– A test report giving the detailed results of the tests with, in addition to the sanction, the mention “conclusive test” or “inconclusive test”;
– A certificate of conformity established if all the tests are conclusive.
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