Tax liability on income tax refund. in which assessment year?

I am an NRI. I filed my tax return for AY 17 - 18 in June 2017. This was process in March 24 2019 and refund credited last week. I got INR 45000 as interest for delayed refund. TDS of 15000 was deducted from the interest at 30%. Will this interest be considered my income in AY 17 - 18 or AY 19 - 20?

The reason for asking is that if it's in AY 17 - 18, the tax is 30% as deducted. If it's my income in AY 19 - 20, then the tax liability is 10% due to my lower income for AY 19 - 20 where I can claim a refund this year.

Please guide to understand in which AY the interest income need to be accounted.