Distribution transformers

K.Yu.Guda
The modern concept of power supply is based on engineering ideas formulated at the end of the 20th century, which include the principles of producing alternating current, its consumption by an electric drive, transformation using transformers, as well as high and low voltage networks with electrical consumers connected to them. These basic principles have allowed the creation of advanced energy supply systems both in Europe and around the world.
One of the central links in this system rightfully belongs to transformers that convert electricity in terms of voltage - first, increasing the voltage at the place of electricity production, and then lowering it at the places of its consumption.
In this article, only the lowest-power transformers found in power systems are considered - the so-called distribution transformers, which provide electricity to industrial and domestic consumers. Moreover, when considering these transformers, the main emphasis is on the indicator of their energy efficiency, as one of the main sources of large-scale energy savings.
General information about losses in distribution transformers. Distribution transformer losses account for a significant portion of the total losses in transmission and distribution systems. For example, an analysis of the transmission and distribution networks of the Pacific coast of the United States carried out at the end of the 20th century showed that losses in distribution transformers are more than 30%, while only 2% is lost in transformers of supply substations. A similar picture takes place in domestic distribution transformers. Considering the significant number of such transformers in the power system and their long service life, such transformers represent a significant energy saving reserve. Therefore, from the point of view of energy saving, an increase in the efficiency of distribution transformers by only 0.1 % is already justified, since such transformers are constantly energized and during their round-the-clock and year-round operation, savings from reducing no-load losses (h.x.) for 20 ... 30 years turns out to be quite significant. The magnitude of losses in the windings - short-circuit losses (short-circuit) - depends on the load of the transformer, which is why these losses are also called load losses. Although distribution transformer manufacturing plants have a design life of about 25 years for such transformers, many of them will last much longer without failure. So, the average post-war European distribution transformer served for about 30 ... 40 years. One of the reasons for this longevity is that companies, in the face of a growing demand, installed an excessive number of transformers, which is why many of them worked for a long time in low load modes. In principle, most of these transformers have acceptable technical characteristics, with the exception of energy efficiency indicators, which, unlike powerful transformers, did not receive due attention until the early 70s of the last century.
European and international practice for determining the energy efficiency of distribution transformers. In the EU countries, most of the requirements for distribution transformers are determined by national (BSI, NF, DIN, NEN, UNE OTEL), international (ISO, IEC), as well as the European (EN, HD) standards. The main task of these standards is to ensure acceptable requirements for the characteristics of such transformers, their safety, uninterrupted operation during the entire service life, and environmental protection. In connection with the need to achieve large-scale energy savings in all sectors of the national economy in technologically developed countries, over the past few decades, great attention has also been paid to solving the problem of a significant increase in the energy efficiency of such transformers, despite the fact that until now there is no clear formulation of this concept. distribution transformers is defined by the standard HD428 "Three-phase distribution transformers with an operating frequency of 50 Hz from 50 to 2500 kVA with oil cooling and a maximum voltage not exceeding 36 kV."
Similar standard, - standard HD538. defines the energy efficiency level of dry-cooled distribution transformers. нормы потерь короткого замыкания распределительных трансформаторовAccording to the standard HD428 for distribution transformers with oil cooling and a maximum voltage of up to 24 kV, the main efficiency parameters are given in table. 1 norms of losses of short circuit (short circuit) and no-load. As can be seen from the table for oil transformers, three levels of short-circuit losses are allowed. (A, B and C) and three levels of x.x loss. (A ', B' and C '), which are determined by a special method with a certain margin of error. If the transformer does not comply with the level of losses given in Table 1 during testing, the manufacturer either rejects it or agrees with the buyer the amount of monetary compensation. Conversely, if the actual losses of large transformers significantly exceed the requirements of the corresponding level, the manufacturer can receive additional remuneration from the buyer.уровнинагрузки распределительного трансформатораThus, the standard HD428 allows you to select three levels of load (short-circuit) and three levels of h.x. - from the least effective combination A-A 'to the most effective C-C, and of the theoretically possible nine combinations, this standard allows the selection of only five combinations shown in Fig. 1, where the combination A-A' is taken as the basis of comparison (highlighted in red bold the line, values (percentages) are calculated from this basis). The really achievable level of loss reduction in distribution transformers can be judged on the basis of such a typical example: for a transformer with a rated power of 630 kVA, the difference in total losses (short-circuit and c.h. losses) between the extreme values (combinations of loss levels A-A 'and C-C) is about 1.5 kW.
The values of short-circuit losses given in Table 1. and h.x., five combinations of permissible combinations of loss levels (Fig. 1), as well as the dependences of losses on the load of transformers considered below, are the methodological basis on the basis of which the energy efficiency of oil distribution transformers is determined. Note that the actual losses of distribution transformers change significantly with changing load: in the x.x. mode. there are only c.h. losses, and under load, short-circuit losses are added to them, as can be seen from the example shown in Fig. 2 of the dependence of the total (a) and relative, equal to 100% minus the efficiency (b), losses from the load for a transformer with a rated power of 400 kV * A and a voltage of 24 kV. Figure 2 shows the following combinations of combinations of loss levels (energy efficiency levels): 1 - A-A '; 2 - A-C '; 3 - B-B '; 4 - C-B '; 5 - C-C '.
The graph of the dependence of the relative losses on the load, shown in Fig. 2, b, clearly shows that the minimum values of losses fall on loads equal to approximately 50% rated power. диапазоны нагрузки распределительных трансформаторовMoreover, if transformers of levels А-А 'and В-В' have different load ranges optimal from the point of view of reducing losses, then transformers С-С 'in any case have a loss value 20 ... 30% less than transformers of levels А-А' and B — B '.
The dependence of the relative losses of the full load in the transformer on the rated power is shown in Fig. 3, where numbers 1-5 indicate the same combinations of combinations of loss levels (energy efficiency levels), as in Fig. 2. These dependences (with a few exceptions) show that the higher the rated power of the transformer, the lower the full load loss.зависимость потерьтрансформатора от мощностиSince the overall efficiency of a transformer directly depends on the load, it is possible to make a conclusion about the energy efficiency of a particular distribution transformer only when the total losses are calculated for a certain period of time (for a year or for the entire period of operation), which is a rather difficult task.
   Let us now consider the question of the really achievable values of loss reduction in oil-filled distribution transformers. Unfortunately, until now, a single internationally recognized criterion has not yet been developed, according to which one or another distribution transformer could be unequivocally considered energy efficient - even though oil-immersed transformers of C-C 'level have the lowest losses. Therefore, a number of experts refer to the following transformers as energy efficient:
  1.With oil-cooled level C-C 'as standard HD428.
  2. Dry transformers with voltage up to 24-36 kV, having a loss value by 20% less than according to the standard HD53B.
The basis for such benchmarks is the technical feasibility of manufacturing transformers with such levels of losses by almost all manufacturers. The third possible way to assess energy efficiency can serve as individual technical features of transformers, which, although not directly related to efficiency, are nevertheless associated with it. These features include: the use of special types of windings, advanced grades of transformer steels in mahfilament, fabrication of a magnetopod from amorphous iron (AMDT) and etc.
It is quite obvious that the resources for reducing losses in distribution transformers are far from being exhausted and can be reduced in the future, first of all, by using methods of reducing short-circuit losses at transformer plants. and x.x., given in Table 2.
 Thus, there are quite large potential reserves for reducing losses in distribution transformers and, consequently, increasing the level of their energy efficiency. However, the realization of these reserves requires the adoption of a number of difficult decisions that stimulate the purchase by consumers of more expensive (albeit quickly recouped) energy-efficient transformers, and manufacturers - the release of such transformers, the organization of which will require large additional capital investments for the modernization of production. Among the possible (albeit difficult to implement) solutions in the European Union, the following are considered:

  1. Voluntary agreement or EU Directive establishing permitted levels of use for distribution transformers.
  2. Inclusion of the requirements for mandatory minimum criteria for the energy efficiency of distribution transformers in the formation of national standards, which in the absence of clear uniform criteria for the concept of "energy efficient transformer" will be difficult to implement.
  3. Application of discounts (subsidies), tax incentives, etc. when purchasing energy efficient transformers, which is constrained by the lack of a clear definition of the term "energy efficient transformer".
  4. Application of a simple consumer labeling system, illustrating the degree of efficiency of the transformer under various loads, etc.

  потери в распределительных трансформаторахпотери в распределительных трансформаторахNote that in the EU countries for a long time at tenders for the purchase of transformers, including energy-saving distribution transformers, an approach has been used to calculate the price taking into account losses for the entire service life of a transformer (25 years), thus a transition from material-intensive to science-intensive production is carried out, allowing produce energy efficient electrical equipment. The energy saving potential of energy efficient distribution transformers in the European Union is shown in Fig. 4, where the following combinations of loss levels are indicated:
  1 —C-AMDT;
  2 - A-AMDT;
  3 - C-C '.
The last two combinations refer to transformers with a magnetic core made of amorphous iron.
   Minsk energy saving transformers TMG-12 series
Having analyzed the European and international approaches to determining the energy efficiency of distribution transformers, let us consider a typical example of creating energy-saving transformers at the Minsk Electrotechnical Plant. The Minsk Electrotechnical Plant has developed new TMG12 oil-immersed distribution transformers that meet the European standard CENELEC and having the lowest level of losses х.х. and k.z. in comparison with serially produced in the CIS. Analogues of this series of transformers are produced only by such leading world manufacturers as SIEMENS, ABB, AREVA... A general view of one of the energy-saving distribution transformers of the TMG12 series is shown in Fig. 5.трансформатор тмг Technical characteristics of transformers of the TMG12 series are given in Table 3. Let's pay attention to the main advantages of distribution transformers of the TMG12 series in comparison with the transformers of the earlier series.
Loss of h.x. and k.z. distribution transformers of TMG12 series are reduced by 30% in comparison with transformers of other series due to the fact that:

  1.    Magnetic cores of transformers are made of special grades of high quality silicon steels with the highest resistance and low hysteresis losses.
  2.  For the manufacture of transformers, a larger amount of material is used, which is optimally distributed between the mass of the magnetic circuit and the winding.
  3.  The magnetic core is manufactured using the most advanced technology Staplap and consists of plates with oblique joints, without holes in the active steel.
  4.  The thickness of the plates does not exceed 0.3 mm, and the plates themselves are varnished to isolate one from the other.
  5.  The assembly of the transformer is carried out by highly qualified personnel using equipment from the world's leading manufacturers, which excludes any possible mechanical damage to steel and minimizes losses.

The annual savings on losses in TMG12 transformers with a capacity of 630 kV * A is 6.7 thousand kWh, and in TMG12 transformers with a capacity of 1000 kV-A - 5.4 thousand kWh.

The difference in price between the TMG12 series transformer in comparison with the earlier series transformers is about 10%. The payback period for additional investments, taking into account this difference, for a TMG12 transformer with a capacity of 630 kV-A is less than 1 year, and for transformers of the same series with a capacity of 1000 kV-A - less than 2 years.

Replacement 100 pcs. conventional transformers with a capacity of 630 and 1000 kV-A transformers of the TMG12 series save money on the installation of three additional substations with a capacity of 630 kV-A.

In addition, transformers of the TMG12 series have improved noise characteristics, which is also their important advantage complementing high energy efficiency indicators.

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