In August 2011, the State Grid Corporation compiled and issued the First Catalogue of Key Promotion New Technologies. The distribution transformers of S13 and above (hereinafter referred to as distribution transformers) were listed in the catalogue, indicating that these distribution transformers will gradually become the leading products to replace the old high-loss transformers. According to "JB/T 3837-2010 Transformer Product Model Compilation Method", the no-load loss of S13 transformer is reduced by 20%-30% and the load loss is unchanged compared with that of S11 transformer with the same capacity; the no-load loss of amorphous alloy transformer is reduced by 60%-67% and the load loss is unchanged compared with that of S11 transformer. In this paper, energy-saving principle analysis, technical performance analysis and economic comparison are carried out for S13 and other types of distribution transformers, which can provide reference for selecting this type of distribution transformer.

Energy Saving Principle of 1 S13 Distribution Transformer

Traditional transformer cores adopt planar structure and lamination technology. There are some defects such as unbalance of three-phase magnetic circuit, inconsistency of local magnetic flux direction and magnetic conduction direction of silicon steel sheet, multiple air joints and so on, which restrict the improvement of energy efficiency level of transformer. At present, the energy-saving distribution transformer is mainly developing towards changing the core structure and improving the traditional lamination process.

Distribution of 1.1 S13 Stereo Coil Core

At present, many domestic transformer manufacturers produce S13 three-dimensional coil core distribution by changing the structure of transformer core (Fig. 1). The main features of the distribution transformer are as follows: (1) three identical single frames are assembled into equilateral triangle structure to make the three-phase core magnetic circuit completely balanced and the magnetic circuit shortest so as to reduce losses; (2) changing the traditional lamination mode, three single frames are made of continuous winding of silicon steel sheets, making full use of orientation of silicon steel sheets, seamless iron core, greatly reducing magnetoresistance and loss; (3) adopting retrogression; Fire technology can reduce process coefficient and eliminate internal stress, so that no-load loss and no-load current can be greatly reduced. In theory, the distribution of S13 three-dimensional coil core is easy to produce, but there are difficulties in actual production: first, the annealing process is difficult to master; secondly, the design technology of three-dimensional coil core is in the hands of a few transformer manufacturers, and there are technical barriers; thirdly, the winding process of three-dimensional coil core is more complex than that of overlapping core, which needs to be winded on the core column, and the winding efficiency is not as good as that of overlapping core.

In order to reduce the loss of core distribution (Fig. 2), manufacturers often choose high-quality grain-oriented cold-rolled silicon steel sheets, increase the thickness of Core Laminates and the amount of copper wire, or use high-price imported materials. At the same time, the energy consumption level is raised, which leads to the increase of production cost, and becomes a difficult problem for the popularization and application of S13 type stacked iron core distribution.

At present, the main energy-saving distribution transformers used in China are S11, S13 three-dimensional coil core, S13 stacked core and SH15 with amorphous alloy core. Among them, S11 core and amorphous alloy SH15 core are widely used.

2.1-year loss comparison

According to "Guidelines for Economic Operation of Power Transformers GB 13462-2008", the annual loss values of various types of distribution transformers are calculated by comparing the distribution transformers of S11, S13 and SH15 (Amorphous Alloys). Among them, the S13 type can be divided into two types: stacked core and coiled core. The calculation is based on the standard as shown in Table 1.

NL = P + KqQ0, Q0 I0% SnLL = f beta 2Pk + Kq f beta 2Qk, Qk Uk% Sn means annual power loss L = 8 760 * (NL + LL) / 10 000 P - rated no-load loss, kW;

Pk - rated load loss, kW; Sn - rated transformer capacity, kVA;

I 0% - No-load current percentage of transformer;

Uk%-impedance voltage percentage; Q0-no-load reactive power loss, kvar;

Qk - rated load leakage power, kvar; beta - average load coefficient;

F - Load fluctuation loss coefficient, 1.05;

Kq - Reactive power economic equivalent, take 0.1 kW/kvar.

The no-load loss of S13 type distribution transformer is 30% lower than that of S11 type, and the load loss is the same. Taking 315 kVA distribution transformer as an example, the loss parameters of each type of distribution transformer are shown in Table 2, the annual loss calculation results are shown in Table 3, and the annual loss reduction ratio relative to S11 distribution transformer is shown in Table 4, and the annual loss curve is shown in Figure 2.

2.2 Economic comparison

The electricity price is calculated at 0.65 yuan/kW.h. The annual operation cost of the distribution transformer with an average load rate of 30% is compared with Table 5.

Economic comparison adopts the method of equal annual value, that is to say, according to the predetermined rate of return on investment, the cash flow of related investment projects within their useful life is converted into the average annual equivalent amount. When converting cash flow, we can calculate the equal net cash flow each year according to the investment project, and analyze and evaluate the investment plan through the equal annual value. If the present value of an asset is represented by A, the present value of an asset by C1, the annual operating cost by C2, the annual cash flow by R, the residual value of fixed assets by S, the period by N and the interest rate by i, the equivalent annual value can be represented by A =-[C11-(1+i) -n i+C2]+R+S((1+i) n-1i. The following cases are compared by 315 kVA, 1+i, n-1i of allotment capacity. The average load rate is 30%, the operation life of the equipment is 20 years, the interest rate is 6%, and the residual value of the transformer is 1,000 yuan.

(1) The new 315kVA distribution project compares the S13 with the S11, assuming that the S11 and S13 distribution transformers produced by the winning bidder are produced according to the national standard energy consumption value. The price of model S11 is 32,000 yuan, and that of model S13 core is 50,000 yuan. The calculation results are shown in table 6.

(2) In the reconstruction of 315 kVA distribution project, S13 type transformer is used to replace S11 type transformer. Assuming that both S11 type and S13 type transformers are produced according to the national standard energy consumption value, the S11 type transformer has been used for five years, the residual value is 20,000 yuan, and the distribution price of S13 type stacked iron core is 59,000 yuan. The calculation results are as shown in table 7. As can be seen from Table 7, it is not economical to replace the S11 type distribution with the S13 type stacked core distribution.

3.1 Noise Size

Transformer noise not only pollutes the environment, endangers human health, but also affects the normal operation of equipment. At present, there is no standard stipulation on the sound level limit of amorphous alloy core distribution, which is determined only by the manufacturer and the user through consultation. According to the standard, the sound power level of 10 kV oil-immersed power transformer should not exceed 50 dB. "Technical parameters and requirements of GB/T 25438-2010 three-phase oil-immersed three-phase three-dimensional coil iron core distribution transformer" limits the sound level of three-phase coil iron core distribution transformer as shown in Table 8.

Compared with the shape size of S11 and SH15, the shape size of S13 stacked core or coiled core has no obvious change.

3.3 Transformer Weight

The distribution and transformation of three factories are investigated respectively. The distribution and transformation of S11, S13 and SH15 in factory A, S11 and S13 in three-dimensional iron core in factory B and SBH15 in factory C are compared as shown in table 9. Compared with amorphous alloys, the weight of S13 type and S11 type is at the same level, and the weight of S13 type is better than that of SH15 type.

3.4 Overload Capability

According to "GB/T 1094.7-2008 Power Transformer Part 7: Load Guidelines for Oil-immersed Power Transformers", transformer life is usually calculated by continuous operation under design ambient temperature and rated operating conditions. When load exceeds rated value and design ambient temperature, transformer aging accelerates. The over-load capacity of transformer refers to the ratio of allowable load current to rated current and its duration when the transformer operates at over-rated current. There is no data to show that the improvement of energy consumption level is related to the increase of overload capacity of distribution transformer. Compared with S13 coil core and laminated core, because coil core transformer has inner coil skeleton (fig. 4), the skeleton is thicker and coil heat can not be transmitted through the skeleton, the inner and outer coil heat is equal in unit area, but the inner coil has only one side heat dissipation, while the outer coil has two sides heat dissipation, which makes the temperature rise of inner coil much higher than the average temperature. The paper cylinder between inner coil and core column of laminated core transformer is thin, with good heat dissipation effect and small temperature difference between inner and outer layers. Therefore, even though the temperature rise test data of the two transformers are identical, the actual overload capacity of the coiled core transformer is weaker than that of the stacked core transformer because the hottest temperature rise of the coiled core transformer is higher than that of the stacked core transformer. Amorphous alloy core transformer, like other traditional silicon steel sheet core transformers, generally does not allow long-term overload operation. Overload operation of any distribution transformer shall comply with the relevant provisions of "GB/T 1094.7-2008 Power Transformer Part 7: Load Guidelines for Oil-immersed Power Transformers". The actual overload capacity of distribution transformer mainly depends on loss, insulation material heat resistance grade and environmental temperature. Transformer with low loss, fewer heat and low temperature rise has strong overload capacity. The characteristic of transformer core material is not the key factor to determine the transformer's overload capacity.

The no-load loss of amorphous alloy distribution transformer is lower than that of silicon steel core distribution transformer under the same load, and the insulation temperature rise is lower. The actual overload capacity of amorphous alloy distribution transformer may be stronger in theory.

3.5 Short Circuit Resistance

The ability of distribution transformer to withstand short circuit mainly depends on its heat resistance and dynamic stability. Under the action of electromagnetic force produced by strong short-circuit current, the distribution transformer in operation will appear displacement, damage and high temperature rise, which will increase the probability of damage. Short-circuit resistance of distribution transformer is related to manufacturing process. The manufacturer should improve the technological level of winding gasket processing, winding production, winding pre-assembly, inner winding bracing and assembly fastening, and strengthen product quality inspection and acceptance. There is no data to show that the ability of distribution transformer to withstand short circuit is related to the material of iron core. In theory, the coil core transformer body structure is more symmetrical and balanced, the stability of the three-dimensional triangle is better, and its short-circuit resistance ability can be stronger. In fact, the ability of distribution transformer to withstand short circuit is determined by its production technology level. Therefore, the ability of any distribution transformer to withstand short circuit does not increase with the improvement of energy consumption level, nor does it take the core structure and material as the basis of judgment, and should be based on product acceptance and experiment.

4 Conclusion

Based on the above analysis, the economy of type S13 is better than that of type S11. Technically, the same manufacturer's comprehensive performance level control of S13 type distribution transformer is not weaker than that of S11 type distribution transformer. Therefore, the choice of type S13 or type S11 is mainly determined by the price of the product. The smaller the absolute value obtained by the method of equal annual value, the less the annual investment, that is to say, the selection of this model to change the economy. SH15 amorphous alloy core is more economical than S13 core, but it has some obvious disadvantages, such as high noise and heavy weight. In use, the power supply department reflects that the overload capacity and short-circuit endurance of amorphous alloy distribution transformer are poor, but the research shows that the difference of iron core is not the decisive factor of overload capacity and short-circuit endurance. The thickness of amorphous alloy strip of SH15 amorphous alloy transformer core is very thin, only 0.025 mm, less than 1/10 of commonly used silicon steel sheet (2.3 mm). Influenced by manufacturing technology and long-distance transportation, under the premise of cost control, it is more difficult to allocate amorphous alloy core to improve the overload capacity than to withstand short-circuit capacity, which is the reason why some properties of amorphous alloy core are questioned. On the premise of ensuring economy, the pilot application of S13 type distribution transformer is advocated. Due to the uneven quality of manufacturer, it is necessary to strictly inspect the distribution transformer in order to ensure that the temperature rise, insulation performance, overload capacity, short-circuit withstanding capacity of the transformer in the network meet the relevant standards and the requirements of power supply enterprises. SH15 amorphous alloys are suitable for those areas which are insensitive to noise, weight and other factors, and should be applied in those areas where the load ratio is low and the no-load loss ratio is large. It can not only give full play to the advantages of low no-load loss of amorphous alloy distribution transformer, but also avoid burning due to long-term overload operation of amorphous alloy core distribution transformer.