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Transformers with a nominal capacity of up to 500 MVA and a nominal voltage of up to 420 kV, filled with mineral oil or environmentally friendly, biodegradable and non-flammable insulating liquids.

  • The mineral oils used in oil transformers are free of PCBs and other harmful substances.
  • In fire-resistant and water conservation areas, transformers with synthetic oils, that is, environmentally friendly liquids, are used. These liquids have outstanding characteristics such as a higher flash point and biodegradability.

Using modern computing techniques, new materials and technologies, progress is made in the development of innovative products. Power transformers and autotransformers with a rated power of up to 500 MVA and a rated voltage of up to 420 kV are filled with mineral oil or an environmentally friendly, biodegradable and non-flammable insulating liquid.

The highlights of these high-power transformers can be summarized as follows:


The focus is to provide customers with reliable and environmentally friendly products. KOLEKTOR ETRA transformers have low loss and noise levels.

  • They contribute to environmental protection by using mineral oils in submerged transformers, free of harmful substances.
  • In areas where protection against fire and water conservation is required, Kolektor Etra applies synthetic oils with a higher flash point and biodegradability.
  • The commitment includes repairs, reconstructions and various modifications of transformers and their components.
  • Special attention is paid to reducing transformer noise, reducing the need for noise barriers at transformer stations.
  • Another significant contribution to the preservation of the environment consists of the environmentally friendly dismantling of transformers at the end of their life cycle.

Magnetic core

Three kinds of magnetic steel sheets are used: conventional oriented, high magnetic oriented HI-B and laser treated. The core is stacked using the staggered pitch system, improving efficiency and reducing noise. The heat-shrinkable polyglass bands secure the laminations, preventing movement during transportation and operation. The lower yoke and legs are coated to reduce vibrations caused by magnetostriction forces.


Made of high-purity electrolytic copper, the windings offer specific electrical conductivity and various insulation classes. Interleaved windings are used for transformers with lightning impulse voltages greater than 550 kV. Continuous disc, helical and double helical windings are used as per required specifications.

Active Part Assembly

After thermally and mechanically stabilizing the windings, they are assembled with laminated wood plates. Proper drying of the insulation and oil filling under vacuum guarantee high dielectric, thermal and dynamic strength of the transformer.

Isolation System

The insulation system comprises solid and liquid dielectric insulation. Calculations of dielectric insulation loads are performed using modern computer-aided design methods.


Designed to withstand full vacuum and overpressure, the tanks are constructed with reinforced flat walls. Two types of tanks are manufactured: the classic tank and the bell-shaped tank. Corrosion protection is adapted to climatic conditions or customer requirements.

Refrigeration system

The transformers are equipped with a cooling system that can efficiently dissipate the heat generated during operation. The system is designed and tested for permanent operation with declared capacity within the specified temperature rise limits. If there are no special overload requirements, the system is designed in accordance with IEC 60076-7. It includes radiators, various types of oil coolers or heat exchangers. They can be separated from the tank and divided into several batteries or chillers, allowing quick assembly or disassembly of installations. The cooling system can be separated from the tank and placed in separate facilities with better conditions for heat dissipation. In such cases, the system has pumps installed for faster circulation of the oil.

Testing and quality assurance

Each individual element, such as the core, winding, tank and other parts, undergoes a thorough mechanical and electrical inspection prior to assembly. During the assembly process, the following operations are carried out: measurement of the voltage relationship between phases, measurement of resistance, evaluation of insulation resistance, measurement of short circuit loss prior to the drying process, supervision of the drying process and testing dielectric strength of transformer oil. Once completed, the transformer undergoes routine testing and measurements in accordance with regulations, and each new type of transformer also undergoes specific testing. Measurements between phases are carried out in the workshops, while the final tests of the transformers are carried out in our own high voltage laboratory, which is fully equipped and designed with cutting-edge technology, located near the plant. production.

To minimize frequency disturbances, of great importance in partial discharge tests and corona measurement, a cage of metal plates is used on the walls of the laboratory, and the electrical supply is carried out through filters. We have a group of highly experienced and professionally trained employees, capable of performing routine, specific and special tests in accordance with international standards.

The main equipment of the high voltage laboratory includes:

  • 3500/1200/500 kVA motor-generator group, with specifications of 12/6/6 kV and frequencies of 50/200/300 Hz.
  • 500 kV test transformer, which has a regulating transformer, compensating reactor, capacity divider and control panel equipped with a digital measuring instrument.
  • Computer-controlled impulse voltage generator with a capacity of 2000 kV and 200 kJ, provided with an RC divider and short wave device.
  • Mobile equipment for resistance measurement and temperature rise testing with automatic temperature readings.
  • Digital partial discharge measurement system.
  • Equipment to measure tgδ and transformer capacitance.
  • Precision broadband power analyzer.
  • Frequency response analyzer (FRA).
  • Turns ratio of the three-phase transformer.
  • Noise source identification system with automatic microphone positioning system.
  • Sound intensity analyzer.
  • Sound level meter.
  • Automated test unit for transformer oil dielectric strength testing.
  • Test unit for dissolved gas analysis.