Electrical substation

On what factors does the design and size of a power transformer depend? What is the role played by control cables in a substation? Explain radiators,conservator,tap changers,breather(silica gel) used in a substation? What is the difference between post insulator,disc insulator and bushings used in a substation? How does the busbar frequency remain almost constant inspite of load fluctuations? Explain ferromagnetic core of a transformer?