Yes, You can. After the PPF account has been in operation for 15 years, it can be extended for a duration of 5 years at a time. This extension can be taken any number of times and there is no limit on how many times a person takes this extension.
Answered above2- Or do I need to close my existing PPF account and open another PPF account for 15 years?
I think you can, but I am not sure. That you can you open another PPF account after closer of first one.3- Am I eligible to open another PPF account (after closer of first one)?