360 MVA Main Transformer in a 220 kV Grid Substation

The use of high-capacity main transformers has been explored in the construction of substations in areas with rapid load growth and strong concentration, and work has been carried out on the selection of high-capacity transformer parameters. In this paper, a technical and economic comparison of the main transformer capacity of 360 MVA or 240 MVA in the substation is carried out. It is concluded that by using 220 kV 360 mva main transformer, the substation can save 5% of the equipment investment and about 14% of the floor space, while reducing the operation and maintenance workload, etc.

The actual operation results show that the equipment is operating well and that all performance parameters are within the design range.

220 kV 360 MVA transformer

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Why choose daelim?

There are currently no technical barriers to Daelim’s high-capacity three-phase 220 kV 360 MVA generator step-down transformer, which can be manufactured according to the relevant EHV standards and IEC standards.

From a manufacturing point of view, Daelim transformers do not have any problems with lifting and testing of the entire unit, and large parts can be transported by water, land or rail.

The 220 kV 360 MVA transformer made by Daelim is currently in operation in areas with high load growth and concentration.

The results show that the equipment operates well and that all parameters are within the design range. The design and construction experience can be used as a reference for the application of 360 MVA and larger capacity 220 k V transformers in the grid.

Table of Contents

Current Status of 220 kV 360 MVA Main Transformer Applications

According to the main transformer selection content in the general design and equipment specifications of the relevant power grid companies, the maximum capacity of the main transformer for 220 k V substations can be selected as 240 MVA.

Certain power generation enterprises and industrial users of steel and aluminium have a small number of transformers with a capacity of 300 MVA in operation, but these are limited to double-volume transformers, which are different from the three-volume generator step-down transformers commonly used in power grids.

The 360 mva main transformers in 220 kV substations are still mainly of 180 MVA and 240 MVA capacity. However, 360 MV/220 kV transformers can better reduce the area used in substations and reduce construction and operating costs.

Advantages of the 360 MVA Main Transformer

Saving floor space

In order to analyse the advantages and disadvantages of 360 MVA main transformers, this paper has designed two preliminary schemes for substations, scheme 1 main transformer construction scale is 4 x 240 MVA; scheme 2 main transformer construction scale is 3 x 360 MVA.

Both options use outdoor GIS equipment for 220 kV and 110 kV distribution, with the same number of outlet circuits and the same scale of 35 kV distribution and other facilities in the station.

General electrical plan of scheme 1 capacity of single main transformer is 240 MVA
General electrical plan of scheme 2 capacity of single main transformer is 360 MVA

(General electrical plan of scheme 2 capacity of single main transformer is 360 MVA)

Comparing the two options, the substation’s footprint is saved by about 15% compared to option 2 because option 1 reduces the number of main transformers and the number of main transformer triple inlet intervals, sectional intervals and busbar equipment intervals.

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Investment savings

The total capacity of the substation main transformers in the above two options is 1080 MVA and 960 MVA respectively, with Option 2 increasing the power supply capacity by 120 MVA compared to Option 1. Both options solve the problem of landing points for the 110 kV substation in the region, and the functions and positioning of the substation are basically the same and comparable in terms of technical and economic aspects.

220 kV 360 MVA power transformer

In terms of basic comparative project investment, the total cost of Option 2 is 5% less than that of Option 1.

Land acquisition costs for Option 2 are approximately 14% less than Option 1.

In terms of major electrical primary equipment costs and corresponding equipment foundation investments, the number of main transformer inlet intervals for each voltage level in Option 2 is less than that in Option 1.

Therefore, with the exception of the main transformer costs, all other equipment costs are lower under Option 2 than under Option 1.

In addition, Option 2 has one less main transformer than Option 1, and the station-wide main transformer fire protection facilities (e.g. transformer water spray system or foam fire extinguishing system) have been reduced accordingly.

Similarly, the main transformer protection system, the oil chromatography online monitoring device and the corresponding secondary cable usage are all reduced accordingly, the amount of auxiliary facilities related to the main transformer is reduced, and the maintenance workload of the equipment after commissioning is also reduced accordingly.

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Selection of Main Transformer Capacities and Wiring Groups

In accordance with regional grid requirements and planning, the transformer voltages for high, medium and low voltage are set at 220 kV, 110 kV and 35 kV, with full capacity (i.e. 360 MVA) used for high and medium voltage.

generator step-up transformer

Considering the role and function of high-capacity transformers in the grid, split transformers are not recommended for the low-voltage side of the 220 kV 360 MVA transformer.

The use of split transformers on the LV side, although effective in reducing the duty cycle of individual windings, would increase the overall size of the transformer and pose a risk to manufacture and transport.

At the same time, when 1 of the branches is short-circuited out of operation, the short-circuit resistance of the other branch will be greatly reduced and the split transformer is not conducive to safe and stable operation.

Therefore, the low-voltage side capacity is chosen to be configured according to 1/3 of the main transformer capacity, i.e. 120 MVA, which allows the low-voltage side switching output capacity to be unrestricted without affecting the short-circuit resistance of the main transformer.

The 220 kV 360 MVA transformer studied in this paper is restricted by the regional grid plan and the coupling group is of the YN/yn0/yn0 type, with no triangular windings in the transformer.

generator step-down transformer

In this case, it is common practice to add an additional 10 kV balancing winding to the transformer. The balance winding is triangularly wired and generally does not output external power, but mainly provides a path for the third harmonic current, eliminating the third harmonic flux and thus the third harmonic component of the voltage.

The configuration of the balancing winding capacity is based on the experience of the balancing winding in transformers, the mode of operation and the structural form of the balancing winding.

At the same time, the opinions of the various manufacturers are combined to ensure that the transformer can meet the requirements of use, when it acts as a balancing winding only, and is generally designed for a 10 kV class with a capacity of approximately 1/3 of the rated capacity of the transformer.

Therefore, the 220 kV 360 MVA transformer balance winding capacity studied in this paper is set at 120 MVA.

In summary, a reasonable transformer capacity ratio is 360 MVA/360 MVA/120 MVA+120 MVA (10 kV balancing winding capacity) when the three sides of the transformer are 220 kV, 110 kV and 35 k V respectively; the final choice of the coupling group is YN/yn0/yn0+d type.

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Selection of Cooling Methods and Noise Analysis for 220 kV 360 mva Main Transformer

At present, the common cooling methods for 220 kV 360 MVA transformers are oil-immersed self-cooling, oil-immersed air-cooling, forced oil circulation air-cooling and forced oil circulation water-cooling.

The following are specific solutions for the cooling method suitable for 220 kV 360 MVA transformer

generator step-down transformer

(1) Natural oil circulation self-cooling method: As the capacity of the main transformer used in this case is too large, the volume of the self-cooling transformer external radiator will exceed the sides of the body by a lot and the structure is unreasonable, therefore the manufacturers do not recommend the natural oil circulation self-cooling method.

(2) Natural oil circulation air-cooling method: has the characteristics of mature technology and stable operation. When using this method transformer noise level can be controlled below 70 dB, the minimum can be controlled at 65 d B (but the cost will increase).

(3) forced oil circulation air-cooled way: oil tank body structure is more compact and reasonable, shape size is smaller, strong short-circuit resistance; but the failure rate and operation and maintenance workload increases, should use better quality oil pump; transformer noise level can only be controlled at 75 dB.

Oil-immersed transformers forced oil circulation air-cooling system is the cooling system emerged in the 1990s, with high reliability requirements of the working power supply, cooling system self-cooling performance is low, maintenance and repair, energy consumption and noise high weaknesses, if the air-cooling system failure, will cause transformer overload shutdown, affecting the safe operation of the grid and reliable power supply.

The oil-immersed transformer natural cooling/natural oil circulation air-cooled cooling system has the advantages of good self-cooling performance, low maintenance, low energy consumption and noise, etc., and is therefore widely used.

Based on the above analysis, it is recommended that the 220 kV 360 mva main transformer should be naturally oil circulating air-cooled.

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Other Main Parameters of the 220 kV 360 MVA Transformer

generator step-up transformer

For parameters such as losses, noise and temperature rise of the transformer, the recommended values for the main technical parameters of the 360 MVA/220 kV transformer were obtained by combining the opinions of all parties, taking into account economy and practicality, and by referring to the parameters of the 360 MVA/330 k V main transformer that has already been maturely applied.

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