Dry-type transformers

KPM, LLC has been manufacturing dry-type transformers on the basis of the genuine technology for a long time. Since 2008 a significant number of transformers has been manufactured and supplied on industrial and power facilities of the Russian Federation and the States of the former Soviet Union. Best practices have been gained in operating transformers in different conditions including adverse conditions.

Transformers are made for mains with the voltage from 0.4 kV up to 20 kV and the power of up to 12500 kVA.

Purpose

Power transformers are the electrical machines intended for transformation of current and voltage from one mains with particular rating to the other mains. It is conventional to divide power transformers into general-purpose and special-purpose transformers.

Due to the power and voltage class dry-type general-purpose transformers are intended for distribution mains, production facilities mains and tracking substations of electrified railway transport. This includes in particular reducing transformers, which convert voltage of higher rating into the lower rating, and coupling transformers, which provide for power exchange between the distribution mains of different voltage class.

Dry-type special-purpose transformers have modified characteristics and design, which ensures operation in special-purpose conditions. These include welding transformers, converter transformers for rectifying units and inverter installations, isolating transformers and other transformers of special-purpose design, including those specifically manufactured on the basis of the Customer’s requirements and specifications.

Design

KPM, LLC transformers design is genuine. The design is developed following the production and operation history of dry-type reactors with different voltage classes. The main design feature is the windings manufacturing process.

General transformer arrangement is typical for transformers with the power of up to 12 MVA. Magnetic core has frame structure, the core is imbricated. Pressurization is implemented by bands, and the tightening in the upper and lower part is implemented by yoke with studs. Additional vertical studs increase general structure strength to provide its resistance to loads during transportation and operation. Concentric windings — low-voltage winding is located closer to the bar, high-voltage winding is located outside.

Magnetic core is imbricated and made of high-quality electrotechnical steel. Cold-rolled high-quality steel. Bands used for bars pressurizing are implemented by glass tape. This is an elastic, durable and long-term material with high resistance to external effects, the material is non-electrically-conductive and non-magnetic.

The transformer windings are cylindrical, multilayer, made of rectangular wire and have vertical air ducts. Windings are divided into sections by horizontal gaps. Wire transposition is implemented between the sections in order to reduce losses in the winding and equalise potentials in parallel paths. All the electrical connections are implemented by argon-arc welding.

Air ducts provide for effective air circulation inside the windings and good cooling thereof. Thus, we succeeded in increasing the transformer overload capacity in all the modes.

The windings are self-supported:  they do not require bearing structural elements to provide strength thereto as each winding is the monolithic structure in general. Such windings structure is called air-barrier monolith.

The windings monolithic character is achieved by manufacturing process thereof. In general, the above manufacturing process is similar to the manufacturing process of windings for KPM, LLC dry-type reactors.

A special rectangular single wire with composite insulation consisting of polyimide-fluoroplastic (PMF) film and two layers of glass-fiber fabric saturated with heat-proof silicone varnish of heat-resistance class 200 (200 °С) as per GOST 8865-93 is used for windings fabrication.

PMF film is the base wire insulation which ensures the required level of electric insulation. Glass-fiber fabric saturated with varnish acts as structural and protective material. It forms protective casing around the conductor in PMF film and ensures solid, monolithic clutching of conductors with each other.

Glass-fiber fabric is durable, unexposed to corrosion effect and putrefactive process and keeps its mechanical properties throughout the article service life. The varnish used for saturation has excellent adhesive and saturating properties, as well as bonding characteristics. The varnish is characterized by high heat conductivity, which ensures good heat abstraction from the conductors. The varnish has high moisture and water resistance, it is water-repellent and hardly combustible and has high heat resistance.

In general, the insulation stands high and low temperature for a long period of time without changing its properties, it is highly stable and resistant to chemical substances and radiation effect. The windings insulation is hardly combustible in accordance with GOST 12.1.044-89, it stands long-term (up to 8 minutes) exposure to open flame without ignition. The ignition probability of the transformer windings insulation is practically equal to zero even in case of heavy and long-term short-circuits.

As compared to well-known air-barrier insulation KPM, LLC technology has a number of advantages:

  • The winding has great mechanical strength, which ensures high resistivity to mechanical stresses during transportation and installation, as well as resistivity to dynamic forces during short-circuit currents passing.
  • High insulation strength is ensured.
  • The windings insulation is proof against the action of moisture, temperature, corrosive chemical substances.
  • The windings insulation materials are durable and they practically do not change their properties throughout the whole service life.
  • The transformer windings are not the source of ignition and fire.

As compared to cast insulation the air-barrier monolith ensures higher load-voltage characteristics of the transformer.

Winding operation is implemented on the machines with smooth adjustment of rotation speed and rotating torque adjustment on the engine shaft, which ensures high repeatability of the windings parameters.

After winding and welding operations the windings are saturated. The saturation is implemented by means of full immersion into the tank with silicone compound. The compound does not act as interlayer insulation, it is designed to add density to the windings. After saturation the winding is placed into the heating furnace, where it is cured during 12 hours at the temperature of 160 — 180 °C. Saturation-curing cycle is repeated many times.

After saturation and curing the winding is ready for installation on the bar. The transformer is painted by fire-resistant enamel for aesthetic purposes and for protection against ultraviolet rays. Silicone insulating enamel is used for painting, this enamel is a pigment suspension in the varnish. The enamel has high level of dielectric strength and high mechanical-and-physical properties.

After the transformer assembly the production final stage presumes full cycle of commissioning tests in proprietary electric laboratory.

KPM, LLC transformers special features and characteristics

The transformers genuine design contributes to their excellent operational characteristics. It concerns in particular load-carrying capacity and heat resistance. These characteristics are essential for dry-type transformers, as dry-type transformers traditionally underperform in comparison to oil transformers particularly due to their load-carrying capacity.

As GOST R 54827-2011 "Dry-Type Transformers.  General Technical Specifications" and GOST 11677-85 "Power Transformers. General Technical Specifications" does not contain clearly defined requirements for load-carrying capacity of dry-type transformers, many manufacturers of dry-type transformers have their own interpretations.

Transformer overcurrent may take place due to various reasons. In all cases overheating of the transformer windings takes place as a result of excessive current flowing through them. It happens in the following cases:

  • At short-circuit currents flowing (SC). In this case flowing of heavy currents takes place, these currents may exceed the current ratings by ten folds during very short period of time (from tens of milliseconds up to 3-4 seconds). In this case winding is heated rapidly and then may cool down after short-circuit (SC) clearance. In this case it is reasonable to mention not the transformer load-carrying capacity, but the heat-resistance class of the windings insulation.

The insulation of wire used for fabrication of KPM, LLC transformer windings has excellent heat-resistance indicator — class 200 (200 ºС) as per GOST 8865-93.

  • At load fluctuations. Temporary (from several dozens of seconds up to several dozens of minutes) build-ups of the consumed power are typical for a number of load types. For example, such modes are the engines start-ups and self-startings. Intermittent service mode may be such a typical mode for controlled drives. In such modes and other similar modes the step-up of currents, flowing through the transformer windings, from several dozens of percent up to several folds may take place.
  • At the mains mode emergency change. In this case the temporary (during the period from several dozens of minutes up to several hours) increase of load (by one and half - two times) on the transformer may occur.

Item 2.1.21 of "Regulations for Operation of Consumer Electrical Installations” specifies that short-time overload of the transformers in excess of current rating is allowed with all the cooling systems operating regardless of duration and value of the preceding load and coolant temperature. It means that the transformer shall carry overload (current above the rating) even in case if prior to overload it has been operating for a long time at full load (100% from the rating) and at the maximum operating ambient temperature.

Load-carrying capacity indicators of KPM, LLC transformers are specified below in comparison to the indicators required by the Regulations for Operation of Consumer Electrical Installations (ROCEI). All the stated values are given for KPM, LLC transformers with natural air cooling, which have been operating at full load (100% of current rating) before the overload specified in the Table. In case the load is lower than the rating and at the ambient temperature below the one specified in the Table, transformers are able to carry higher overloads.

Transformer Allowable overcurrent in excess of the rating, % relative to the current rating
Duration: 5 min. 10 min. 18 min. 20 min. 32 min. 45 min. 60 min. 80 min. 2 hours 3 hours 24 hours
Oil transformer, in accordance with ROCEI item 2.1.21 - 100% - 75% - 60% - 45% 30% - -
Dry-type transformer, in accordance with ROCEI item 2.1.21 60% - 50% - 40% 30% 20% - - - -
KPM, LLC transformer at the ambient temperature of +20 ºС 100% 100% 90% 70% 55% 45% 40% 38% 32% 25% 25%
KPM, LLC transformer at the ambient temperature of +40 ºС 100% 100% 60% 57% 38% 30% 27% 23% 20% 20% 20%

Thus KPM, LLC dry-type transformers are vastly superior to the requirements, specified for dry-type transformers by the Regulations for Operation of Consumer Electrical Installations, and are comparable to oil transformers by their load-carrying capacity.