higher number insulating oil distribution transformer core tank core forming magnetic field wider area substations stepping weatherproof cabinets enhance cooling protective enclosure magnetic path key components residential commercial transmission system vital component top seller voltage ratio primary winding secondary windings secondary winding core voltage transformation primary winding compared secondary windings determines secondary winding results reducing highvoltage electricity lower voltage output lower voltage suitable large transformers located businesses substation transformers utility poles commonly lowvoltage conductors types laminated silicon steel