waste to heat power plant

If u connect all the loads one SWBD the reliability is less and controling the motors is difficult and design swbd is difficult generally it will design more than 7.2 time of current rating in order to with stand the motor starting current.

I assume you mean:

by 'feeders for steam turbine generators' - the auxiliary supply for boiler, steam turbine and generator;

by 'transformer feeders for plant consumption load' – the transformers that feed the plant loads.

The plant loads are important and so the plant load transformers are connected to the most reliable section of switchboard or bus system. The steam turbine generators auxiliary supply is generally connected to the same bus or switchboard so that the auxiliary supply is available for the start up of boiler, turbine and generator. It is not necessary to connect it from the same switchboard but it should be connected to some source where power supply is available (either from utility supply or other emergency generators) for the start up of the boiler, steam turbine and generator.

- MS

Your assumption is correct,

Can we conclude that it is desirable to connect on the same switchboards or is there any demerits of the same.

Yes, it is desirable. The bottom line is - it should be connected to the switchboards where power is available even if the STG (Steam turbine generator) is off so that the start up auxiliary power is available for the Boiler (for feed water pump, draft fan etc.) and Turbine (lube oil filter pump, condensate pump etc.).

Demerit:

If, in case, the plant switchboard goes down and the STG set needs to start, you can't do it. Remember the start up of a STG is a process of couple of hours depending on how big the capacity is.

Solution:

It can be avoided to have a quick start emergency generator (DG set) with a transfer switch connected to the STG auxiliary supply bus.

- MS