how to improve the power factor to unity at 33 kv line?

You will have to get your VARS from either generation or capacitor banks. Plenty of references on the web.

Dear Gagan, the Capacitors you have to connect in Parallel and to set the Capacitors Bank in Star to Ground.
Regard
William

Dear Gagan, assuming that existing power factor of line is lagging, basic solution is to install various size capacitor banks with Automatic Power factor corrector relay (APFC Relay). Size and nos. of capacitors banks depends upon the existing Power Factor of 33KV line.Just contact any reputed capacitor vendor he will do these calculation for you.

a) at which end of the 33 kV line?
b) at which loading of the line?

c) why is unity power factor an "improvement"?

Dear Colleagues:
The capacitor bank must be Installed at the entrance to the load with PF lagging, do not improve up to 100% because in a moment can become Capacitive them and upload them to the voltage at the load above the nominal.
Automatic Drivers Using the PF advised only in cases where it is justified is a matter of $.
regards
William

Gagan you need to specify the load is on which voltage i.e. 33KV or any other voltage and the type of industry. Providing automatic power factor correction on 33KV bus is not economical.

Gagan!
1. Calculate vars needed for power factor correction. Try it with daily load curve (instant).
2. Look for the manufactures and ratings of the capacitor banks.
3. Apply them (instantaneous switching) in accordance with the instant power factor correction needed.
Good Luck