At our power plant, we want to install the subject protection relay(s) in our 132 KV switchyard. The purpose is to open the 132 KV line breakers in event of under/over voltage and frequency to keep the power plant running in island mode. We have a single busbar scheme and two lines in parallel going to same grid station at remote end.

Should we conenct our new relay using Busbar PT or two relays on transmission line using line PTs.

Please suggest the pros and cons of both.

It would seem to me that when you have a fault that will bring your voltage and frequencies down the bus and line would both see it, but I would tend to agree with stevenal in that you should look to the line PT's when you are going to restore the system.

I didn't understand what sabap means by 'bus overcurrent protection. Neverthless, to me it appears that this is problem of overcurrent coordination from line overcurrent relay to the generator overcurrent relay.
Generators are not normally provided with a simple overcurrent protection precisely to avoid maloperations such as above, rather voltage restrained or voltage controlled over current protection is provided. Sabap may like to check the type of protection existing for their generator. The setting ensures that the relay doesn't operate with any speed unless the overcurrent is coupled with drop in generator terminal voltage (signalling the fault is close to generator terminals).
Distance protection in lines should operate for faults in the line irrespective of the load carried by the line. The relay setting is in terms of line impedance and the relay operation is not affected by the magnitude of current in the line (generally speaking).

You might also want to think about a valve that can dump some of the live steam from the steam generator to to condenser as well as a high speed throttle that slams shut and then very quickly cracks open enough to supply the off grid needs of your main turbine. If you are a nuclear plant both devices would help avoiding having to scram the reactor.

You might also want to consider an auxiliary steam turbine to operate the plant auxiliaries off grid during demanding conditions such as peak periods or lightning storms. An alternate wise investment would be a gas turbine generator for the same purpose and for black starting the plant.

You might also want to think about extra hydraulic accumulators for the turbine governor and extra direct current battery supported hydraulic pumps for the governor. Also some extra battery supported pumps for lubrication with hydraulic accumulators to supply oil while the extra lubrication pumps are starting. The system disturbances on 14 August 2003 caused a lot of turbine governors to run out of hydraulic pressure which tripped off the boiler or scrammed the reactor. Fossil fuel and nuclear plants were equally affected by loss of hydraulic pressure as the governors tried to keep up with swings in frequency.