Refunds of health insurance premiums for nearly 2 million federal workers and retirees have moved a step closer to reality with the approval yesterday by the Senate of legislation that would make retirees eligible for the same payments as active workers.

But because the Senate bill differs from one already approved by the House, the checks -- expected to range from $20 to $400 -- can't be mailed yet. The House bill also raises the government's share of health premiums, while the Senate bill doesn't make any change in the employer- employe premium split.

Congressional sources say they hope to iron out the differences quickly so the refunds can be mailed. About 250,000 people here are eligible for the rebates, which are being made by health insurance carriers to 1985 policyholders.

Those plans include Blue Cross-Blue Shield, Aetna, the National Association of Letter Carriers, the American Federation of Government Employees, Government Employees Hospital Association, Government Employees Benefit Association and the Foreign Service plan.

The General Accounting Office released a report yesterday that says the size of refunds being proposed for employes should be increased, while the refunds proposed to the government should be cut back. GAO made the study for Rep. Mary Rose Oakar (D-Ohio), an advocate of bigger rebates for policyholders.