7.1 Regarding the grading issue of the casing box:
The issue of the casing of box transformers has become more complex due to the influence of standards. Originally, there was no mention of casing classification in the technical conditions for ordering box transformers formulated by the Ministry of Electric Power of China or the standards for modular substations formulated by the Ministry of Machinery. However, since the introduction of IEC-1330 (1995 edition) "High voltage/low voltage pre installed box substations" into China and the equivalent Chinese GB/T17467-1998 standard, there have been provisions on casing classification. The article on casing in IEC-130 (1995 edition) is now excerpted:
1.3.12 Housing level:
Under the normal operating conditions specified in section 2.1 of this standard, the difference between the temperature rise of a transformer inside the casing and the temperature rise of the same transformer outside the casing, the rated value (capacity and loss) of the transformer corresponds to the maximum rated value of the pre installed substation.
4.11 Rated housing level
The rated housing level is the housing level corresponding to the maximum rated capacity of the prefabricated substation
The rated shell level is used to determine the load factor of the transformer, so that the temperature of the transformer does not exceed the limits given in IEC76 or IEC726 and described in Appendix D.
There are three rated housing levels: levels 10, 20, and 30 correspond to the maximum temperature rise difference of 10K, 20K, and 30K, respectively
With the implementation of this standard, the box transformer must undergo a temperature rise test both inside and outside the casing during type testing.
The source of this regulation in the IEC-1330 standard is that the average temperature in Europe is lower than that in China, and most of its transformers are fully sealed. The standard also stipulates that natural cooling is used. For different levels of box transformers, the test results must be multiplied by a load factor of less than 1, so all transformers installed inside must be operated with reduced capacity.
However, according to the situation in our country, box transformer manufacturers will never allow large capacity transformers to be sold as reduced capacity box transformers. Therefore, since its inception, domestic box transformers have adopted temperature based blowing and cooling measures to ensure that the box transformers can reach the output capacity of the installed transformers.
The power industry standard DL/T "High Voltage/Low Voltage Prefabricated Box Substation", which was implemented on September 1, 2002, specifies a "Level 0" enclosure level to address this issue.
7.2 Regarding the material issue of the casing:
The material issue of the outer shell of the box transformer is also a very important issue, and the author has a gradual understanding of this. Initially, in leading the joint design work of the box transformer, anti rust aluminum plates were selected because of the size limitations of the plates and the need to prevent rain at the joints, which made the structure complex and time-consuming. Later, some foreign products were considered to use steel plates with double-layer inner lining and automotive paint on the outside in compact structures, but they were not resistant to wind and sand in northern regions. For non-metallic shells, the author also participated in the initial evaluation meeting, during which experts pointed out that their heat dissipation was insufficient. This was because during the type test to determine the temperature rise test of the box transformer, it was not easy to pass due to the insufficient heat dissipation of the outer shell, which led to the problem of the front shell grading.
Through recent years of practice, box transformers belong to outdoor equipment, and their weather resistance needs special attention. No matter what material is used for the metal casing, it is impossible to paint it as a whole, so the appearance is always poor soon. However, using "GRC" (Glass fiber reinforced cement) glass fiber reinforced cement casing has shown its advantages, because under outdoor travel conditions, the highest temperature in the surrounding environment in summer can sometimes reach+55 ℃. At the same time, the radiation heat from sunlight can reach up to 1000W/㎡. Due to its thermal conductivity being nearly 100 times smaller than that of metals, GRC can not only withstand high temperatures of+55 ℃, but also withstand the radiation heat of solar energy, especially during the hot summer season when electricity is in high demand. During the period when the transformer needs to operate under overload, the box transformer of the GRC shell fully demonstrates its superiority. The heat dissipation problem can be solved by using a fan that starts automatically with temperature rise for exhaust.
In addition, condensation cannot occur inside the GRC casing, so temperature sensors and heaters are not required in the high-pressure chamber to prevent condensation.