Standard network transformers are meticulously designed in accordance with guidelines stipulating requirements, technical evaluation criteria, and testing protocols established by each utility. These instructions serve to foster consistency in assessing quality attributes for the selection and procurement of oil transformers, as well as in drafting tender documents. They facilitate coordination and transparency throughout the production and procurement processes, while fostering a competitive environment aimed at elevating the quality standards.
In Iran, the general specifications of standard 20 kV oil transformers are as follows:

• Rated voltage: 20 kV
• Rated power: 25 to 2000 kVA
• Frequency: 50 Hz
• Casualty level: ’AB
• Impedance percentage: 4% up to a power of 200 kVA and 6% for powers higher than 200 kVA
• Vector grouping: Yzn5 for up to 200 kVA and Dyn5 for power rating that is higher than 200 kVA

• Tank type in terms of cooling and connection with the outside air: Corrugated wall transformers with a conservator or hermetically sealed.
• Installation height: 1000 meters
• Maximum ambient temperature: 45 degrees Celsius
• Color type: RAL7032
• Cooling system: ONAN
• Oil: according to IEC60296 standard
• Protection equipment: Oil thermometer (for power higher than 630 kVA) – Buchholz relay (for power higher than 1000 kVA)