Temperature has a great influence on the operation reliability of the capacitor unit. Whether the hottest temperature of a capacitor unit under the most severe operating conditions will exceed the allowable value can be verified by thermal stability tests. Due to the lack of research basis on the capacitor hottest spot temperature measurement

the test results of thermal stability of capacitor unit are not accurate enough. Consequently

taking a capacitor unit UHV BAM6.56-556-1W used in the UHV（ultra high voltage） project as the test object

we adopted the finite element analysis software to calculate the thermal stability. At the same time

lots of FBG（fiber bragg grating） sensors were embedded in the capacitor unit to measure the temperature distribution under thermal equilibrium. The temperature value and the position of the hottest spot in the capacitor unit were found out through the research and the relationship between the hot spot temperature and the temperature of the shell was also revealed. Finally

the effects of placement mode

structure parameters and ambient temperature on the temperature distribution of capacitor were analyzed. Research results not only provide a relatively accurate and simple measurement method for the hottest temperature of UHV shunt capacitor unit

but also offer a reference for the design of capacitor’s heat dissipation.