Basis for Choosing the Nominal Voltage of the Network

The rated medium voltages of electric networks are various and numerous in value; question arises why it is so. By trial and error, technicians have come to the cheapest solution in each case. The two objects of equal value can be expensive or cheap depending on the quantity of processed products, that is per unit cost can be diverse. Per unit power cost of electricity delivery depends primarily on the cost of power lines and transformer substations which for both of them increases with rated voltage. However in per unit cost of a network, the share of power lines increases but of transformers decreases with the power (with supply zone) increasing. So per unit cost is optimum (minimum) at a certain value of both voltage and the supply zone. These factors (and others) differed in different regions, which led to many rated voltages. If at a given rated voltage the per unit power cost decreases with the supply zone increase, then we have cost decreasing network, otherwise - cost increasing network, which leads to a different network planning approach.