Power Transformers

Quote "What is permissible primary switching frequency "

Do you mean de-energizing the transformer? If so, why would you do this?

It is switching ( Energising / De-energysing)of transformer primary for Load transfer or load changeover between two transformers.

Only Manufacturer can answer this. usually distribution transformer are designed to handle the rush current at energizing, but the frequency of energizing / deenergizing is depending of the loss class of the transformer and its short circuit voltage, better to refer to the manufacturer regards

In distribution systems the transformer tripping and switching is normally carried out only rarely and when it is a must for operational or safety requirement.The switching of transformers cause over voltage switching surges which is harmful to the connected equipments and may cause failure. Switching Power transformers case transient high voltage and harmonics which will damage electronic equipments

My concern is about Power Transformer itself.

Considering power conditions in India,power transformer switching once or twice a day is common practice in many areas.

Considering Inrush current surge/ Effect on core Magnetisation properties , Mechnical stress and Insulation stress at the time of every switcing, is there any information available which states permissible switcings per day for Power Transformers with different % impidance and MVA capacity ?

As you stated the frequent tripping of power by the supplier (in your case the concerned state or private Electricity distribution company) is common practice in India. The existing design of the transformers in India can withstand such tripping.So for a maintenance engineer there is no solution for such a problem.Only point he can do is to take care of the utility equipments with surge suppressors. Can think about some surge suppression for the complete substation if it is economical . Only transformer manufacturers can carry out certain improvement in design of transformers to reduce the harmonics in switchings. Please read the article given in the link below which will give some ideas on the subject.

http://www.pacw.org/fileadmin/doc/WhitePapers/Protection/Transmission/X-erCharProt.pdf