The MLREs at the daily and monthly scales show that the site ROCK1 with higher elevation and latitude has a higher CD value, which indicates that the temperature dynamics at the site with higher elevation and latitude are of much higher complexity. The results of the interpolating from cokriging method based on the variogram at seasonal and annual scales show that the higher CD values mainly distribute on complex landform such as mountain area, while the lower CD values mainly distribute on the comparative flat landform such as basin area. The results indicate that the complex temperature dynamics come from the complex landform.AcknowledgmentsThis work was supported by National Basic Research Program of China (973 Program; no. 2010CB951003).
The authors are grateful to the editor and referees whose comments helped them for improving the paper’s quality.
The life of a transformer is defined as the life of the paper insulation [1]. The paper insulation deteriorates with several factors, primarily including temperature, moisture content, and oxygen content. In all these factors, temperature has a major impact because the contribution of moisture or oxygen to the insulation deterioration can be minimized with modern oil preservation systems [2]. The temperature of a transformer, often considered as hot spot temperature (HST), is primarily controlled by ambient temperature and load, where ambient temperature is the dominated factor. On one hand, some types of load are directly affected by ambient temperature, such as the cooling load in summer: the higher the ambient temperature is, the greater the load will be.
On the other hand, the load capability of a transformer is generally governed by ambient temperature. Transformers have to operate under the prescribed limit of HST, and ambient temperature is an uncontrollable factor in the influential factors of HST (ambient temperature and load); therefore, if the ambient temperature is high, the load capability of a transformer is always low, and vice versa. From these two aspects, it can be seen that ambient temperature is an important factor in estimating the transformer life.There are some literatures that studied the impact of an increase of ambient temperature on transformer life. Reference [3] used a standardized engineering approach according to IEEE Standard C57.
91 [4] to preliminarily assess the impact of increased ambient temperature on transformer loss of life at five locations in the USA. For an assumed 4��C rise in ambient temperature during 1900�C2100, the predicted loss of life in the interval from 1990 to 2045 rises approximately 32% at Brefeldin_A a relatively warm location and 8% at a relatively cold location. Reference [5] applied an improved model of top oil temperature rise differing from the IEEE C57.