The application of power factor correction capacitors to reserve spare capacity of existing main transformers

As a result of major revamp and several expansion projects to a 30-year old world-class petrochemical refinery, the total refinery power demand increased from 67MVA to 100MVA with a corresponding lagging power factor decrease from 93% to 89% at the 34.5kV system. The refinery has a secondary selective system and gets its power from utility company via two main power transformers. Each transformer is rated 60/80/100MVA, ONAN/ONAF/OFAF, with 115kV primary and 34.5kV secondary. The latest expansion project was committed to reserve at least 10MVA in each transformer for future growth in a single-ended condition while supporting the total refinery load. Several options were considered and evaluated to enhance the transformer´s forced-cooled (OFAF) capacity (100MVA). One option was to improve the existing cooling system of each transformer. Another option was to replace both transformers with new ones having higher MVA ratings. The option of installing capacitor banks was evaluated and accepted by the project management because of its cost effectiveness, practicality, reliability, and constructability. This option relieves transformers a certain amount of reactive load (VAR) from the total apparent load (kVA) by installing two banks of power factor correction capacitors at their 34.5kV secondary sides. As a result, each main transformer can provide the extra 10 MVA capacity for future load growth.