NHPC has signed power purchase agreements with Tata Power-Delhi Distribution for its seven power stations — Bairasiul, Salal, Tanakpur, Uri-I, Sewa-II, Chamera-I and Chamera-III. The agreements, signed for a period of 35 years, will become effective from the date of start of commercial operations of the respective power stations, the public sector power generator said in stock exchange filings. Investors will closely monitor the execution of the agreement.