Customs exemption for power-plant modernization allows reduced duty above a fixed basic rate subject to certification and undertaking. Goods of Chapters 84, 85 and 90 imported for renovation or modernisation of a power plant are exempt to the extent that customs duty exceeds an amount calculated at a basic ad valorem rate of 20% and the entire additional duty under section 3 is waived, subject to (i) certification by a Director level officer in the Central Electricity Authority that the scheme has techno economic clearance, (ii) recommendation by a Deputy Secretary level officer in the Department of Power, and (iii) an importer undertaking to use the goods as specified and to pay duty if they are not so used.