The factory was established in year 1968 under the name “Elprom Trafo” with its main activity being manufacturing of MV/LV Distribution Transformers. From 1968 till 1996 the factory functioned as a completely state owned manufacturing entity. In 1996 it was transformed into a Public Liability Company, with the Ministry of Economy being its sole Principal. Following the government strategy for privatization of the industrial sector, in the middle of 1999 two private companies and individual share-holders acquired the public shares.
In November, 2004 “MEGA EL” LTD. Bulgaria – a private electrical contracting company – acquired the control share of the factory. Nowadays “MEGA EL” LTD is the majority shareholder being in control of 80% of the factory’s shares.

A brief detail of the Company as of 30.12.2013 is given below
Year of establishment: 1968
Total factory area: 56,204 sq. m,
Area underlying buildings and structures: 13544 sq. m,
Export sales (2013): 4 042 Thousand Euro
Man power strength: 142
Majority Owner: MEGA EL ltd
Annual Capacity: 10000 pieces

Currently the factory manufactures oil filled transformers with rated power up to 8000 kVA Different series has been developed, depending on no-load losses and load losses, in accordance to the international standard EN 50464-1:2007. The manufactured standard series of oil filled transformers operate at frequency of 50Hz (60Hz), ambient temperature up to 500C and altitude up to 2500m. Special modification according to work conditions, number of phases, vector-group symbol as well single phase transformers can be made upon customer request.
The transformers are conservator type or hermetically sealed oil-immersed with corrugated tank walls or with radiators. The rated power order corresponds to the order according to IEC, continuous rated operation and natural cooling (ONAN). Upon customer request special transformers can be designed and manufactured including earthing and auxiliary transformers.
- Indoor and outdoor operation;
- Frequency 50 Hz (60 Hz);
- Altitude up to 2500m
- Climatic conditions with maximum ambient air temperature 50 ̊C
- Moderately polluted atmosphere with medium corrosion aggression and without conducting explosion – or fire – hazardous gases, vapors or dust.
The transformers of the series are manufactured and tested according to the requirements of IEC 60076, the prescription of CENELEC – HD 428.181, DIN 42500 and BDS 3067 and can be also manufactured according to the requirements of other national standards.
The no-load losses, load losses and impedance voltage correspond to EN 50464-1: 2007 (HD 428.1S1:1992 List A of CENELEC). Their tolerances and these for the other rated data are according to IEC 60076, DIN 42500 and BDS 3067. However the factory can meet most stringent specifications. The design staff would fulfill any customer requirements.
At the ambient temperature 400C the maximum temperature rise of windings measured by resistance is 65 ̊C for continuous operation and the temperature rise for oil measured by thermometer under the tank cover is 60 ̊C. In case of higher ambient temperature, the temperature rise is re-calculated.
The transformers can be overloaded depending on the daily load cycle and the ambient temperature according to the requirements of IEC 354 without their lifetime being affected.
The nose level corresponds to EN 50464-1:2007, IEC 551, NEMA TR1 and BDS 3067.
- Each transformer is subjected to routine tests.
- Type tests are performed on each transformer type. These include temperature rise test and lightning impulse test.
- Special tests are performed on each transformer type including short-circuit test and noise level measurement. These are performed upon special request of the customer.
The core is three limb plane type made out of high quality cold rolled electrical steel with grain oriented structure with low specific losses. Contemporary automated lines cut the steel sheets delivered in rolls longitudinally and transversally. The stacking of the core sheets is done by the „step-lap” method, which along with the overlap joints at 45° C of the lamination sheets provides a lower level of no-load losses and a higher mechanical strength of the overlapping zones.
The windings are made out of high quality copper E Cu 99.90-99.97 conductors with round, rectangular cross-section and copper sheet. They have an insulation of cable paper or enamel resistant to hot transformer oil. The windings are of cylindrical type situated concentrically on the core limbs. The HV winding is a multi-layer one for the lower ratings and two- or four-section multi-layer, ‘block coils’ or continuously wound disk winding for the higher ratings. The LV winding is situated at the core limb and is foil cylindrical type made out of profiled section copper for the ratings including 250 kVA and out of copper foil for ratings 400 kVA and above. The combinations of LV foil winding and multi layer HV winding results in a significant reduction of the axial electrodynamics’ forces at short circuit for the high ratings. The optimal design of the windings and employment of most contemporary technology, the optimal selected cooling ducts, properly dimensioned insulation, effective tightening in radial and axial direction result in a high electrical, thermal and dynamic withstand capability according to the requirements of IEC.
The upper and lower yoke are tightened by steel beams and bolts situated outside the yokes. Axial bolts do the axial tightening of the windings. The active part is fixed to the tank cover and is supported by the tank bottom. The off-load tap-changer is situated vertically between two adjacent phases. The leads from the HV winding are insulated with thick insulation paper layer and insulation tubes and press board.
The standard type of the transformer tanks is rectangular with corrugated walls made out of cold rolled steel sheets with low carbon content. Contemporary lines for automatic corrugation and plasma welding produce the tank walls. In the upper part the walls are welded to a flange frame to which the tank cover is bolted. At the bottom the walls are welded to the tank bottom. The transport carriage with wheels is fitted to the tank bottom allowing bi-directional movement of the transformer. Welding is done in protective atmosphere Ar-CO2. High quality of the welding seams is guaranteed by tests with penetrating agents in a special test field and by testing with over pressure of 40 kPa.
- Porcelain bushings according to DIN 42531 and EN 50180 for the HV terminals and DIN 42530 and EN 50386 for the LV terminals — standard version.
- Special epoxy resin bushing and plug-in cable connector at the HV side (Elastimold system) — optional.
- Air cable box — optional
The transformer oil is inhibited, without PCB according to IEC 296, IEC 60296.
“Elprom Trafo CH” Plc is managed by a Board of Directors and headed by Executive Director. The present Executive Director is Miss Kristina Mihailova. The company is privately owned.
“Elprom Trafo CH” Plc has its own design office and the factory almost entirely depends upon its own developed technology and design. The engineering staff is well experienced. The design is carry out on software. The team of young designers continues its hard work to find new ways to optimize the production cycle. Increasing of production range is being developed, as well as manufacturing of transformers from the next power class. The work on the project for manufacturing of transformers with fire resistance equal to the one of dry transformers also continues.
The company holds a quality system certificate EN ISO-9001-2008, BS OHSAS 18001:2007, ISO 14001:2004 issued by GCS Global Certification Services in the year 2010. The ISO-9001-2008 quality procedures are being widely applied from the incoming materials control through design and production to the outgoing testing and dispatch.
The annual production rate of their production is indicated below.
• Distribution transformers
2008 2009 2010 2011 2012
Thousand Euro 6926
Thousand Euro 8567
Thousand Euro 6446
Thousand Euro 5535
Thousand Euro

“ELPROM TRAFO CH” supplies most of the transformers to customers of Africa, Asia, Balkan region and Middle East as well as to Bulgarian market and recently to EU. Our long term business partners are in many countries from these regions. At the moment 70% of the total production of “Elprom Trafo CH” is shipped abroad and 30% goes to the local market.
All components of the transformers (core, windings LV and HV, tank) are manufactured in “Elprom Trafo CH” Plc. Most of accessories are imported. Tap changers, thermometers and oil-level indicators are outsourced mainly from Italy and Turkey. The core steel is mainly of American, Russian origin or EU countries. Copper wires are of Bulgarian and Turkish made, etc.
The testing lab Trafo Test, where all of the specified routine, type and special tests of the produced transformers are performed as per IEC 60076, is an independent one, located on the territory of “Elprom Trafo CH” Plc. The entire testing equipment is regularly calibrated and records are well maintained.
This laboratory has certification on ISO 17025 by the Bulgarian Accreditation Authority.
Export sales of distribution transformers and the import of raw materials and equipment are held by Elprom Trafo’s commercial office enlisted here below. The commercial office is entitled to perform the trading activity of Elprom Trafo CH Plc with all domestic and international partners.