Pacific Gas & Electric Co. offers a filter program to remove moisture from transformers. The program removes dissolved water in oil to less than 10 ppm (5 lpm) on a single pass and particulates greater than 0.5 micron during vacuum filling to meet the transformer manufacturer's specifications.

Four Superdri cartridges are installed in the filter vessel, through which oil is processed at 5 gpm (19 lpm). The system is compatible with components that detect leakage, low oil level and high operating pressure. It also purges air during start up and after filter changes. The time to install and achieve full operation averages 30 min. The power supply operates at 115-V alternating current (ac), 12 A, and includes a 0.5-hp motor that drives a 5-gpm positive displacement pump for oil circulation through the filter, with a 3-cfm vacuum pump to evacuate the system during start up. For monitoring, the system has an oil flow meter (which shows gallons per minute and the number of gallons circulated through the system), an oil temperature gage and pressure gages.

Designed for safe, unattended 24-hr operation, the system has an inlet line that connects a 1-inch (2.5 cm), bonded, 25-ft (8-m) hose to the bottom drain valve. The oil returns from the dry-out system to the upper oil fill valve connection. A test of water in oil determines extraction rate and dryness of the insulation. Oil temperature should range from 50øC to 85øC (122øF to 185øF) to allow water to migrate from the cellulose insulation and be absorbed by the oil. A Doble Domino moisture probe monitors the status of the filter cartridges and shows the amount of water removed. The filter cartridges can process 5000 gal (18,927 l) of oil, which may vary depending on the initial level of dissolved water, the amount of free water in oil and the temperature in the transformer.