Plans for an Hawaiian "wind farm" using 20 windmills to generate electricity were announced yesterday by state officials and executives of two companies that are to build the $350 million alternative energy facility.

Hawaii Governor George Ariyoshi said the windmill project, the biggest ever, would allow the island state to cut its use of imported oil, which is used to generate 97 percent of Hawaii's electric power.

The project will be a joint venture of Windfarms Ltd. of San Franciso and the Hamilton Standard Division of United Technologies Corp. Hawaiian Electric Co. has signed a contract to buy all the power generated by the project.

The windfarm will be built on the island of Oahu. "The island's steady trade winds make that one of the best places in the world to begin commercialization of wind energy," said Richard E. Bell, vice president of Hawaiian Electric.

United Technologies is to build the windmills, giant air-driven turbines with fiberglass blades 260 feet long. The hawaiian windmills will each be capable of producing four megawatts of electricity and will be similar to ones United Technologies is building for the Department of Interior at Medicine Bow, Wyo.

Bechtel Power Corp. of Los Angeles will construct the towers and power facilities.