I am working on 10KV surge arrester. I am basically working on the temperature effect on ageing of surge arrester

Good that you are working on a nice topic. Probably the rating of the arrester block will be 3kV, 4.5 kV and sometimes 5.45kV . IF they assemble three blocks in series the rating of the arrester will be 9kV if the block is 3kV rated. So, What is the rating of your ZnO block? How do you age the surge arrester? How do you measure the temperature inside the arrester on blocks?

I was working on 22 kV and 24 kV surge arrester with porcelain housing. I used thermal, electrical, combined (thermal+electrical) and grouting of conductive solution with defined conductivity according to the soil under controlled thermal and electrical stress. At present I am working on 24 kV surge arrester with silicone insulation; new and old after some years of operation.

@ Bystrik Dolnik : What is the rating of  the ZnO surge arrester blocks used in 22kV and 24kV arrester? 

Also, what study you are carrying out on the arrester? I mean to say the end result?

The result is, after 1000 hours of stress, the parameters (VA characteristic and third harmonic of the leakage current) has improved. Then the parameters have been gradually worsened (during next 3000 hours), except for the sample with injected conductive solution. On the surge arrester with silicone insulation I study, in addition to the previous, the change in the leakage current (FFT, resistive and capacitive component) and active power losses.

How do you measure the Resistive leakage current? Do you use any software for it ?

First, I measure the total leakage current and the supply voltage with digital oscilloscope. Then I use software (script in Octave) to determine the resistive leakage current. I use a voltage signal as reference.

Thanks for all the answers. In your previous answers you had mentioned you have used thermal, electrical, combined (thermal+electrical) studies. What does it exactly mean?

During the thermal stress of the samples, I used increased temperature, 1000 hours 135 °C and 140 °C for 3000 hours. During the electric stress and the thermal stress of the samples (combined), I used increased temperature, 1000 hours 135 ° C and 140 ° C for 3000 hours, the voltage applied to the samples was increased by 18% (1.18 * ULG, line-to-ground). During the electric stress I applied line-to-ground voltage, line-to-line voltage and rated voltage of the surge arrester for specific time period.

Can you please inform me the significance of doing ageing at 140oC for 3000hrs which is equivalent to > 350 years of stress . wherein the arrester's average life is less than 30 years. I think thats why the standard IEC 60099-4 is recommending ageing  at 115oC for 1000hrs (equivalent for 110years at 40oC ambient). Even if your ambient temperature is high, stressing it at 140oC for 3000hrs may not be required.

The aim was to further accelerate the ageing process and continuously monitor changes in electrical parameters.