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Transformer winding temperature is a combination of temperature rise and ambient temperature. The temperature of the hottest part that has the most serious effect on the insulation aging is the most attention, but it cannot be obtained by adding the average temperature rise to the ambient temperature. It is necessary to explore the difference between the average and the hot spot. The hot spot temperature of the winding varies with both the temperature rise of the winding and the ambient temperature. For the difference between the average winding temperature (in liter) and the hot spot temperature (in liter), strictly speaking, transformers with different designs, especially transformers with different cooling modes, are different. When the load multiple is different, the difference also changes. China usually soaks oil into 13K. Because it cannot be determined that the average temperature plus 13K is the temperature at the hottest point, it is better not to say “the hottest point†when the hot spot temperature rises. The average temperature rise of the winding specified by the transformer standard is not more than 65K, so the hot spot temperature rise is 78K. Calculated at an equivalent maximum annual temperature of 20 degrees Celsius, 98 degrees Celsius becomes the reference value for the thermal aging effect of the oil-immersed transformer insulation.
2. Oil-immersed transformer load type normal periodic load: refers to the transformer in the rated or exceeded (within the limit) rated current and alternately running below the rated value of the cycle, as long as the longer period of operation, the average Relative aging rate is not greater than 1 operating mode. Different capacity sizes, transformer hot spot temperature limit is 120 to 140 degrees Celsius.
Long-term emergency first aid cyclical load: Due to related transformer maintenance, operating mode changes, and user load increase, in order to reduce the loss of power outage, some transformers will periodically exceed (within the limit) the rated current for a long time . The relative aging rate during the entire run may be greater than 1 or much greater than 1. Different transformer hot spot temperature limit is 130 ~ 150 degrees Celsius. It is necessary to consider the social and economic benefits and decide to adopt this mode of operation. The risk is small.
Short-term first-aid load: Equivalent to the accident overload in the original procedure, but now it needs to be more scientific in terms of temperature control. When a transformer in a station is faulty or the system is partially faulty, the operation mode in which part of the unremovable load needs to be transferred to the transformer is urgently needed. In addition to the current limit, the hot spot temperature limit may reach 160 degrees Celsius, and the power distribution may only be affected by the current. Multiple 2 limit. Considering that insulation has certain risks, only half an hour is allowed.
In general, when the ambient temperature is high, the transformer output is limited by the hot spot temperature limit, and at a lower ambient temperature, the load current multiple will be a constraint condition. In the case of electricity stress, according to the limit of the emergency load, it is advantageous to ensure continuous power supply.
3. 98 degrees Celsius should not be used as a "limit value"
This summer, long-term high temperature and heat in many areas, quasi-fixed at 95 degrees Celsius (<98 degrees Celsius) of the winding thermometer alarm frequently, in particular, also installed a shunt reactor of this type of thermometer. Because the shunt reactor is fully loaded (ambient temperature is 40 degrees Celsius for normal operating conditions, such as the average temperature of 65K design, hot spot temperature of up to 118 degrees Celsius), the general use of self-cooling and no spare cooler can vote, so some plus Fans, some water or not, is very troublesome.
The reason why the alarm temperature is set so low is to confuse the design conditions of the equipment and the concept of operating conditions. There are misunderstandings and misunderstandings. The contradiction in the use of 98 degrees Celsius limit for some transformers is not because of the special design of these transformers, but the low operating load rate and low temperature rise. This is the case of most step-down transformers; There are few small and medium-sized transformer mounted winding thermometers, which are not subject to this restriction (in fact, the relationship between the top oil temperature and the winding temperature has a corresponding law to follow).
4. The relationship between the life of the transformer and the temperature, if it does not take into account the internal quality of the transformer fiber insulation and moisture, oxygen and other insulation aging, according to the Monsinger rule of heat aging 6 degrees can be expressed as: "In the temperature range of 80 to 140 degrees Celsius Inside, the coefficient of aging of oil-paper insulation with temperature changes is taken as a constant, that is, for every 6K increase in temperature, the aging rate is increased by a factor of 2. "The load guidelines set the aging rate at 98 degrees Centigrade to "1", so the temperature is reduced to The aging rate is "0.5" at 92 degrees Celsius, and "2" when the temperature rises to 104 degrees Celsius. With this rate, the aging rate is 4,8,16 for every 6K increase. It should be noted that the multiple of the aging rate cannot be directly equivalent to The value of reduced life expectancy is affected by many factors. The “20-year design life span†is a common practice for decades. Apart from the fact that it has died due to a defect, the actual life span is much longer. However, the promise of a longer life expectancy should not be used as a basis for directly evaluating the product. Its performance index and manufacturing level should be primarily considered. Although the remaining life of the transformer insulation can not be calculated, according to the load guide can be calculated in a certain load period of the "relative aging value." Aging or segmentation of different aging rates and times can be used to calculate the relative loss of life over a longer period of time (by days, months, or years). In short, the effect of different aging rates on the life span is related to the duration of time, and the principle that low temperature compensation can be used for high temperature loss. For example, within 24 hours a day, the average aging rate of 2 hours, the average aging rate of 10 hours, the average aging rate of 12 hours is 0.8, then 2 × 2 + 10 × 1 + 12 × 0.8 = 23.6, the calculation is slightly lower At 24, it can be considered that the normal life loss is 24 hours.
Transformer winding hot spot temperature troubles operation
1. Winding Temperature Rise and Hot Spot Temperature The temperature rise of a transformer winding is the temperature difference between the temperature at which the winding conductor heats up due to the current and the temperature of the external (oil immersed into the tank) cooling medium, which can be expressed in “Kâ€. It is an indicator for transformer design and evaluation of thermal characteristics. The temperature rise test can obtain the average temperature rise of the winding under rated load. The temperature rise varies with the magnitude of the load current, and has nothing to do with changes in the ambient temperature.