The concept of "carbon peak" and "carbon neutrality" shows China's firm determination to save energy and reduce emissions. Energy saving and emission reduction cannot be separated from the utilization of new energy. Photovoltaic power generation is a well-known power generation method in new energy. , its load characteristics are also different from the most extensive thermal power generation in China, and different load characteristics also make photovoltaic power stations have different requirements for transformers. Generally speaking, in order to integrate photovoltaic power plants with a power of more than 10MW into the grid, a transformer is usually required to play a role in it, but the role is changed from step-down to step-up. Transformers can be classified into: power transformers, rectifier transformers, furnace transformers, reactors, instrument transformers, voltage regulating transformers, transformers, etc. According to the capacity classification, transformers with a capacity of 10~6300kVA can be called small and medium-sized transformers, and those with a capacity of 6300kVA~63000kVA are called large transformers. According to the number of windings, they can be divided into two-winding transformers, three-winding transformers and auto-transformers.
In fact, the charging head of the mobile phone, the power adapter of the notebook computer... all have functions similar to transformers, but these "small" gadgets cannot be used in photovoltaic power plants. For photovoltaic power plants of 10MW and above, it is necessary to use The power transformer is used to boost the voltage, and the power generated by itself is connected to the grid and supplied to the users on the grid. Power transformers can be divided into dry-type transformers and oil-immersed transformers by insulating medium, and oil-immersed transformers can be further divided into mineral oil transformers and vegetable oil transformers. Transformers used in photovoltaic power plants need to be selected from these three transformers, and the selection of transformers requires discussion of the load characteristics of photovoltaic power plants. Photovoltaic power generation is inseparable from light, and the load characteristics of photovoltaic power plants are naturally closely related to light.
So do we have to choose the transformer according to the time when the sunshine is the strongest in summer? the answer is negative. For photovoltaic power generation, (even if it is overloaded), the overload operation state of the transformer only occurs at noon. As long as the temperature rise of the transformer does not exceed the specified limit when the overload is the most serious on that day, the operating life of the transformer will not be longer Affected, which provides room for the design of the capacity ratio. (From Liu Fengjue's "Calculation of Temperature Rise for Transformer Overload Operation in Photovoltaic Power Stations") We do not need to select transformers according to the load when the sun is the strongest in summer, as long as the temperature rise does not exceed GB/T 1094.2 "Liquid Immersion Type" Transformer temperature rise" and other relevant standards, the service life of the transformer will hardly be affected. The transformer required for photovoltaic power generation is used for boosting grid-connected power supply, and the maximum load of its operation is predictable. Because the number and power of the connected solar cells are designed by us. This means that we can accurately select the transformer according to the design load, and due to the characteristics of photovoltaic power generation mentioned above, within the temperature rise limit, short-term overload operation will not affect the life of the transformer.
Therefore, we found that due to the characteristics of photovoltaic power generation requiring light, the design load of the transformer of the photovoltaic power station does not need to be higher than the daily peak of power generation (when the light is the strongest around noon), it only needs to ensure that the temperature of the transformer during overload operation is guaranteed. The liter does not exceed the limit value in the standard. Since the cost recovery period of photovoltaic power plants is also long, the cost naturally needs to be taken into consideration. Due to the high price of dry-type transformers, unless there are special considerations, they are generally not selected. For mineral oil and natural ester (vegetable oil) transformers, there is a difference between them. Another difference. ——Under the same conditions, the liquid-immersed transformer using natural ester (vegetable oil) has a higher heat resistance level than the mineral oil transformer. According to Appendix C of IEC60076-14/GB1094.14, without replacing the insulating paper, Vegetable oil can improve the heat resistance level of the insulation system. For example, the original 105℃ heat resistance level insulation system can be increased to 120℃ level, which is enough to cope with the higher temperature rise caused by short-time overload.
Thanks to the above, transformers using vegetable oil (natural esters) can withstand higher temperature rise under overload conditions, i.e. transformers using vegetable oil (natural esters) can operate at higher loads than mineral oil transformers, To put it bluntly, in a photovoltaic power station with the same peak power, a natural ester (vegetable oil) transformer with a smaller capacity can be purchased and used, compared with a mineral oil transformer, thereby saving costs. In addition to the cost advantage, vegetable oil (natural ester) also has good environmental friendly properties, with a degradation rate of close to 100%, no environmental burden, and no toxicity, and is harmless to animals, plants and humans. At the same time, it perfectly meets the carbon emission requirements related to the national strategy, that is, the carbon emission in the entire life cycle is only 1/64 of that of traditional mineral oil. These attributes are very consistent with the environmental protection characteristics of photovoltaic power generation itself. In addition, what is more important is the excellent fire performance of vegetable oil (natural ester) transformers. As of September 2019, there have been more than 2 million vegetable oil (natural ester) transformers in operation worldwide, and none of them use vegetable oil (natural ester). The transformer has a record of fire. Photovoltaic projects are usually located in remote areas. Once a fire accident occurs, it is difficult to get fire rescue immediately. Therefore, it is a perfect solution to enhance the fire protection performance of the facilities in the photovoltaic power station as much as possible, and replace the traditional mineral oil transformer with vegetable oil transformer.
