Employer denying to give long term compensation

Sir, I was working in a Thermal Power Plant as an Mechanical Maintenance Engineer and joined on 04.09.2014 as a GET (Graduate Engineer Trainee) and resigned on 15.09.2017 giving notice period of 20 days instead of 90 days and relieved on 05.10.2017. At the time of leaving the organisation the HR said your salary will be adjusted with your leaves and salary according to the rules of company. During the Joining and then confirmation, I've been provided with Performance Grade Category B (LTCP - Long Term Compensation Plan) , according to which i've been paid Rs. 20000 on confirmation as first instalment, Rs. 40000 after one year of confirmation as second instalment. But it's next instalment of Rs. 90000, which is to be paid after 2 years of confirmation i.e. by 03.09.2017, has not been paid yet even after leaving the organisation on 05.10.2017 i.e. after completing 3 years in the company. Now i've mailed this to all the higher official and HR but they gave no reply. Indirectly they are excusing one or the other way and denying me my compensation. On calling the Head HR, he said you have been paid 17K approx as your full and final settlement and the LTCP Amount is being discussed with management and whatever would be decided would be given to you, if not then nothing will be given to you. I've all proofs regarding compensation like, Confirmation letter, Grade Certificate, Resignation acceptance letter, Experience letter, Relieving certificate but they did not gave me the Full and final settlement certificate and just transferred my salary of one month (17 k approx.) which was holded due to my resignation. Please tell me sir, what to do and whom to approach ?