Medium Voltage Instrument Transformers

Medium Voltage Instrument Transformers

The transformers are designed for standard ambient temperature between –5 °C and +40 °C and can be installed at an altitude of no more than 1000 m above the sea level.

The primary winding and the core(s) with the secondary winding are completely resin-embedded in one casting process. In the case of the outdoor transformers, for an increase in the creep age distance, the insulation body is equipped with double sheds, the shape of which has been determined after many years of research. Their terminal screws are stainless steel grade.

Current transformers are produced up to 3000A, of 120 kA dynamic current and up to 5 measuring or protection cores. They can have single ratio or primary reconnection at a ratio of 1:2. Alternatively, secondary tapings can be provided. The ends of the primary winding are brought out by bolt-type terminals on the top of the insulating body, where the high-voltage connection and the primary reconnection are arranged. They can be single-bolt type or flat-pad type. The transformers can be supplied with integrated coupling electrode, in case capacitive voltage indicator is requested according to IEC 61234-5.

Both indoor and outdoor transformers are mounted on a steel hot-dip galvanized base according to ASTM A 123. The transformers can be earthed effectively with a M8 grounding screw at the base plate.

The outdoor secondary box is made of resin, embedded to the transformer’s body, assuring an IP 54 protection, and carry an additional grounding terminal. On the other hand, the Indoor secondary terminal box is sealed by a cover made of flame self-extinguishing MAKROLON and the windings’ terminals can be grounded directly with a screw that passes through the terminal and mounds to the transformer’s base. The connection of the transformer on the secondary side can be carried out over 3 PG16 cable inlets. The removable terminal box cover can be sealed.

High-quality raw materials from reputable suppliers ensure our excellent products quality. The fact that we use Swiss made outdoor cycloaliphatic resin in all our transformers, even for indoor applications, ensures that our products require considerably less maintenance, successfully combining high levels of quality and safety and have a long service life. Due to the fact that our resin is cycloaliphatic, it retains much stronger molecular bonds than the simple epoxy resin and is superior to any PU compound. It is environmentally friendly, in compliance with REACH regulations.

In order to achieve a partial discharge free insulation of high electrical and mechanical qualities resistant to ageing, the resin is mixed and poured under high vacuum, and cured under a long controlled heat and vacuum cycle. Furthermore, in order to minimize the stress that the insulation is subjected to due to high electrical fields, both the high voltage and low voltage windings together with the magnetic core, are wrapped in a semi conductive shield and extra conductive meshes are installed peripherally in key positions.

The magnetic cores are produced of high grade magnetic lamination, PERMAX or Mu metal in most current transformers, and annealed in a controlled atmosphere. Afterwards, they are tested and encapsulated in a sufficient amount of a special high temperature resistant elastic foam, in order for the insulation to be protected, due to the cores’ heat expansion.