The Ultimate Guide to Underground transformer

In this paper, we compare and analyze the different setting options of underground substation in super high-rise buildings, and its technical economy, and consider the power loss on the distribution trunk line during the whole life cycle of the project while fully considering the length of the distribution trunk line. The capacity of single underground transformer and underground electrical transformer transportation in super high-rise buildings are discussed, and it is suggested that the capacity of single residential underground transformer should not exceed 1 000 kVA.

underground transformer

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Table of Contents

How to choose the installation location of Underground substation?

The number of high-voltage distribution levels of the same voltage level should not be more than two, so the high-voltage power supply and distribution system of super high-rise buildings generally adopts the structure of main underground substation – substation underground substation, according to two levels. set.

The usual practice is to set up the central underground substation in the basement, set up the sub-underground substation according to the business layout, and lead the high-voltage cable from the central distribution station to the sub-underground substation.

In addition to the basic requirements of safety and reliability, the design of underground substation in super high-rise buildings should also take into account the positioning and characteristics of the building itself, the flexibility of its operation, and the economy of the owner’s investment and operation. The difference in the number and location of the substation will have a great impact on the installation and operation cost, which requires the designer to pay full attention to this point.

In the design stage, we should fully communicate with the owner and determine the location of each substation and its power supply range after rigorous technical and economic comparison, instead of simply using a single indicator such as low-voltage power supply radius as the basis for setting the substation.

underground power transformer

The following is an example to illustrate that the podium of this project is a commercial building and the tower is an office building with a simple business and a small podium area, with a total construction area of 228,000 m2, of which the basement has 4 floors and a total of 48,000 m2 and the upper floor has 75 floors, with a building height of 350 m and a standard floor area of about 2,300 m2. Three options were considered for the location of the underground substation in the office area of the tower.

The 1#underground substation in the basement is the central underground substation, and 2#~4# are sub-underground substations (1#underground substation is designed for electricity supply to the basement and podium).

When comparing the schemes, we mainly compare the investment of main line, voltage loss, equipment investment of underground substation, and the floor space of underground substation.

Since the power supply and distribution system has not yet been designed in the scheme stage, the calculation model must be appropriately simplified in order to obtain the comparison results.

a. Only the subunderground substation supplying power to the tower part is economically compared, and the main underground substation supplying power to the podium and underground garage is not involved in the comparison.

b. Assuming that all the underground substations are close to the core, the water level section distance is not considered in the calculation of the radius of power supply and the length of power supply trunk line.

c. The standard floor area of the tower is 2,300 m2 , the core area is 440 m2 , and the office area after deducting the core is 1,860 m2 , and the underground transformer load estimation is calculated based on the standard floor area of 2,300 m2 and 80 VA/m2 .

d. Only the vertical dense busbar supplying power to the office area is calculated, and the other power supply trunks are not compared for the time being.

e. Dense busbar is considered for the power supply trunk in the office area, and the power supply is staggered according to double busbars for odd and even floors (e.g. two busbars are allocated for floors 6 to 21, one of which supplies power to odd floors and the other supplies power to even floors.) After estimation, one circuit is allocated for each 8 floors, 1 250 A circuit breaker is installed, and 1 600 A busbar is matched.

For the short supply area, one circuit is assigned for every 4 floors, with 630 A circuit breaker and 800 A busbar.

f. The capacity of a single transformer is temporarily considered at a maximum of 1 600 kVA.

Option Ⅲunderground substation is more dispersed, so the total installed capacity of the underground transformer is larger than the other two options. In this section of the economic comparison, the capacity of a single underground transformer is temporarily considered as no more than 1 600 kVA, and the number of underground transformers used in scheme Ⅰ, Ⅱ and Ⅲ are all 10, only the distribution location is different.

According to the analysis in Chapter 2 later in this paper, the actual design will control the single unit capacity of the underground transformer in the above-ground part, i.e., if the capacity of the underground transformer is limited to 1 000 kVA, the number of underground transformers in the above-ground part of Scheme Ⅱ and Ⅲ will reach 14, and the number of underground substations will be 14. If the capacity of underground transformer is limited to 1,000 kVA, the number of underground transformers in the above-ground part of Scheme Ⅱ and Ⅲ will reach 14, the area of underground substation will be increased by about 40%, and the investment in primary equipment of underground substation will be increased, but the vertical trunk line will not be affected. As the power supply in the middle and low area is provided by the underground substation in the basement, it is expected that 2 underground transformers will be added.

Each of the three options has its own advantages and disadvantages, through a comprehensive comparison, the final choice of option Ⅲ. Although Option III does not occupy the least amount of floor space, it has the lowest overall cost considering the one-time investment and 50-year operation cost. Therefore, the layout of the underground substation requires special consideration of the one-time investment in the distribution trunk line and the losses in the later operation.

In addition, there is another advantage of Option Ⅲ, because the underground substation is more decentralized, there are only two vertical dense busbars in each power supply area (other vertical cable trunks are also less than the other two options), and the area required for electrical pipe wells is smaller, which can save a certain area of pipe wells compared with the other two options, which can play a certain role in saving the owner’s investment.

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How to transport Underground transformer?

underground electrical transformer

One of the characteristics of super high-rise buildings is that most of them have an underground substation above ground level, usually on the refuge or electromechanical floor. Since the service life of an underground transformer is unlikely to be greater than the building life, and there is no guarantee that the electrical product will not be damaged during its normal life, this raises the question of how to repair or replace an underground electrical transformer.

The mass of residential underground transformer is generally much larger than the load capacity of the elevator, but the size is generally smaller than the elevator car size. At present, the following ways are generally adopted in super high-rise buildings to solve the vertical transportation problem of underground transformer after completion.

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Adopting freight elevator to transport the underground power transformer directly

residential underground transformer

In this way, a freight elevator is required to be set up and stop at the floor where each underground substation is located, and a separate service elevator is usually set up in the core of super high-rise buildings. Under this premise, it can be divided into two approaches

a. Underground distribution transformer with less than 400 kVA is installed in the underground substation, which can be directly transported by the freight elevator with a capacity of 1 800 kg due to its small size and light weight. From the perspective of existing super high-rise buildings, the installation of a freight elevator with a capacity of 1,800 kg is perfectly acceptable to most developers. In the project of Changsha Kaifu Wanda Plaza in which I participated, Wanda headquarters explicitly requested that the transformer on the upper floor should not be larger than 400 kVA and the freight elevator should be used for direct transportation.

The advantage of this solution is that the transformer underground electrical can be directly transported by ordinary elevator, which is easy to replace. The disadvantage is that the capacity of the underground power transformer is too small, and the distribution trunk line cannot be enlarged, so that the peak time difference between loads can not be fully utilized, resulting in an increase in the overall installation capacity of the underground transformer; in addition, if there is a larger load capacity of equipment in the upper part of the building, it will also be very difficult to install the underground electrical transformer due to its small capacity. electrical transformer capacity is too small and difficult to design.

This type of solution is generally only applicable to simple super high-rise buildings under 200 m in height, such as office buildings.

b. Adopt the freight elevator with large capacity, such as 3 t / 4 t freight elevator. The larger the load capacity, the larger the size of the car, and therefore the larger the area of the core cylinder, which will have a greater impact on the economic efficiency of the developer.

On the other hand, our purpose is to transport residential underground transformer, and the plane size of underground transformer is not big, so the general car size can meet it, but the weight per unit area is bigger, so what we really need is a kind of elevator with regular car size but bigger load capacity. Therefore, what we really need is an elevator with regular car size but larger load capacity, which generally needs to be customized by the elevator manufacturer. And because the vertical lifting distance of freight elevator is large, the speed requirement is also high.

With large load capacity and high speed, the choice of elevator will be narrower, and the developer usually does not conduct equipment bidding at the planning stage, so it is impossible to deepen the requirements of elevator, so in many cases, it is impossible to determine the adoption of this solution at the early stage. However, if the owner accepts the plan of installing a large capacity freight elevator, he can consider relaxing the capacity of the underground distribution transformer on the upper floor to 1,000 kVA, which will be much more flexible in the design of the power supply and distribution system and avoid the disadvantages of plan a.

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Underground power transformer transported by elevator shaft

underground distribution transformer

Before replacing the transformer underground electrical, the elevator car is lowered to the bottom, the original elevator traction ropes are removed, and a set of lifting equipment and pulley sets are installed in the shaft to lift the underground transformer through the shaft. This solution has complicated construction process and long construction period, which will have a big impact on the normal operation of the building.

This solution requires that the size of the underground transformer not be larger than the internal dimensions of the elevator shaft, but the potential impact on future operations needs to be explained to the future operator during the project planning stage.

Some operators (e.g., luxury hotels, etc.) may not be able to accept the long-term impact on their normal business activities.

In the case discussed in Chapter 1 of this paper, transportation through the elevator shaft was used.

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The complex power supply and distribution system of super high-rise building is difficult to maintain, so it should be repeatedly discussed and compared with several solutions to make it safe, economical and reliable when designing. Through the above analysis, the following conclusions can be drawn.

a. single transformer underground electrical capacity, can effectively reduce the total installed capacity of underground distribution transformer, but the transportation of underground transformer is a big problem, the designer must be fully considered at the project proposal stage The designer must fully consider this problem in the project proposal stage, and should not just increase the capacity of the underground transformer.

b. The location of the underground transformer should take into account the length of the distribution trunk line, and the voltage drop should not be the only consideration for the layout of the underground substation. Due to the large number of underground substation in super high-rise building, the length of distribution trunk line is very considerable, and the power loss consumed in the distribution trunk line during the whole life cycle of the project should be fully considered.

c. Combining the transportation of the underground transformer and the length of the distribution trunk line, it is recommended that the single unit capacity of the above-ground transformer should not exceed 1,000 kVA; the underground transformer is relatively centralized, and it is considered that the underground substation to the upper and lower sections respectively (it is appropriate not to cross another refuge level).

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