Three-phase and three-phase-two-phase winding transformers and autotransformers are increasingly used in devices for suppressing higher current harmonics (HHH). It is known that over the last decade the shape of the current in electrical networks has significantly deteriorated. The reason for this was the higher harmonics of the current, which are generated by equipment with nonlinear loads: rectifiers, frequency-controlled electric drives, inverters, computers, arc units, etc.

     This autotransformer transmits the high voltage of 330 kV AC to 110/10 kV AC and has three windings in any of three phases: one primary and two secondary windings. They are wrapped on the magnetic core. The 330 kV wires carry 220 A nominal value AC. The secondary winding of 110 kV voltage has electrical connection to the primary winding of 330 kV voltage and is a part of the primary coils. However, only the part of electric current is transmitted by electric connection. All other current of 110 kV and 10 kV secondary windings is transformed by magnetic field.

    Autotransformer magnetic core is manufactured from the electrical steel sheets. In any phase the primary and secondary windings are placed one above the other on the three cores. In order to occupy a smaller volume of the coils the cores have polygonal cross section. The cooling channels are arranged among the magnetic core plates. Magnetic core with the wrapped windings is placed in a tank with the transformer oil. The oil circulated in the tank, it refrigerates the magnetic core and windings through convection. The magnetic flux of autotransformer is closed through magnetic core, oil, metal constructions and autotransformer tank walls.

    The especially strong electric field is created around the wires and around the autotransformer inlets of 330 kV voltage.

    Recently, all industrialized countries have begun to pay increased attention to this problem.

Along with economical and reliable “passive” filters for suppressing higher current harmonics, UPVGTs of transformer and autotransformer types have recently become widespread. These devices are very simple, reliable, economical and are winding transformers and autotransformers with a combined connection of phase windings to compensate for higher current harmonics. Such UPVGTs are successfully used when working with 12-, 18- and 24-pulse rectifiers, as well as in networks of large administrative buildings, industrial facilities, ships, etc., where the loads in the phases are distributed approximately equally. In addition, autotransformer-type UPVGTs with a neutral wire make it possible to compensate for zero-sequence currents. Therefore, in connection with the increase in the number of nonlinear loads and the use of UPVGT to suppress them, the issue of optimizing the latter becomes relevant.

    Higher current harmonics damage electrical and electronic equipment: rotors overheat and bearings of electric motors and generators quickly wear out; accidents occur in electric drive control systems; electrolytic capacitors explode; microprocessor equipment fails; false shutdowns of electrical equipment protection occur; The insulation quickly ages and electric lighting fixtures burn out. In addition, a problem arose everywhere with overloading the neutrals of three-phase networks with higher current harmonics, as a result of which the currents in the neutrals began to exceed the level of phase currents, and asymmetrical noise became unacceptably high. In North America, there is a standard that requires electricity consumers to take measures to suppress higher harmonic currents.

    The actuality of the problem of industrial electric and magnetic fields’ action to the workers health and to the electronic equipment arises in recent years.

    There are European (EU) standards limiting the maximum electric and magnetic fields values, which are dangerous for the human health and can damage the electronic equipment. Recent studies show that the small electric and magnetic fields can have influence to the health of human who works at the existing powerful electrical equipment, too. By EU Directive 2004/40/EC the magnetic field strength must not exceed 400 A/m and the magnetic flux density – 500 μT in the workplaces. Electric field strength should not exceed 10 kV/m. Implementation of this Directive is delayed in Lithuania. The Lithuanian main document governing the values of the electromagnetic field is the hygiene norm HN110: 2001 “The electromagnetic field of industrial frequency in the workplace.” Magnetic field strength should not exceed 900 A/m, and the electrical field strength should not exceed 5 kV/m during the working day (8 hours). Now the trend is considered and discussed to reduce the maximum allowable amount of magnetic flux density value to the values equal to 0.2 or 0.3 mT. These values are equivalent to the magnetic field strength values that are proportionally reduced to 160 – 240 A/m.

    High voltage transformer is one of the most powerful electrical equipment in the power system, which creates a strong electric and magnetic fields of 50 Hz frequency. Therefore, the magnetic and electric fields must be investigated in surroundings of the transformer with due attention. We investigate the 125 MVA autotransformer.

       In this article, the comparison of two topologies of autotransformer-based 30-pulse AC-DC converter has been presented. Topology A included polygon , hexagon, star , fork , and T-connected autotransformer-based 30-pulse AC-DC converters and topology B Included polygon , and T -connected autotransformer-based 30-pulse AC-DC converters. This study, based on technical and economic factors, compares different autotransformer-based 30 pulse AC-DC converters. Simulation results of six-pulse and different 30-pulse AC-DC converters feeding a DTCIMD load are scheduled and various quality criteria such as THD of ac mains current, power factor, displacement factor, distortion factor, and THD of the supply voltage at PCC are compared. Economic comparison of different 30 pulse AC-DC converters is based on the apparent power (kVA) ratings of the 30 pulse AC-DC converters.

    The purpose of the work is to present the main approaches to the development of an optimization model of dry winding transformers and autotransformers UPVGT.

    The main approaches to the development of an optimization model of a winding three-phase and three-phase-two-phase transformer and autotransformer of devices for suppressing higher current harmonics, the use of which is constantly increasing in industrialized countries, are presented. The developed simplified model makes it possible to optimize the transformer and autotransformer to minimize the mass, volume or cost of their active materials. The presented material will be useful to specialists in the field of electromechanics, electrical networks and converter technology.