Are you still there in NTPC or left. If you are still continuing with this contract you have to keep on urging the contractor. If you have been issued with an apptt.letter use it. Probably you might write to the UPL failing which directly to NTPC itself being the Principal Employer. Take help of well wishers in NTPC with whom you worked with to influence the UPL. Don't irritate them, but keep requesting them. When all failed you have to approach state Labour Dept. for their help.
18th December 2018 From India, Bangalore
If you have left the services of the Contractor ( your immediate Employer And not Principal Employer), under the Contract labour Act/State rules you are legally entitled to get service certificate .
You write a strong letter asserting your right demanding the same to be delivered to you within seven days with a copy to the Labour Commissioner.
This perhaps will work.
If it does not , lodge a complaint with the Labor Commissioner.
18th December 2018 From India, Mumbai