The federal agency that insures the nation's banks asked Congress yesterday for broad new powers to cut the cost of salvaging or liquidating problem banks.

The chairman of the Federal Deposit Insurance Corp., L. William Seidman, told the Senate Banking Committee that the insurance agency wants to double insurance premiums for banks making riskier investments.

He also asked that the FDIC be empowered to arrange for the purchase of troubled banks before they actually collapse and that the FDIC be allowed to take over and run failing banks until healthy buyers can be found.

Current law allows the FDIC to step in only when a bank has failed but bars it from owning or operating a financial institution. To preserve depositor confidence under those limitations, the FDIC is forced to liquidate a closed bank within a few days if a buyer hasn't been found.

The legislation Seidman seeks is intended to ease such pressures on the FDIC, whose funds are being squeezed by record bank failures.

The chairman's request comes one day after the FDIC, the Office of the Comptroller of the Currency and the Federal Reserve Board -- the three federal agencies that regulate banks -- announced they have softened rules for bank funding and bookkeeping in an effort to encourage banks to restructure and get rid of troubled loans.

Last year, a record 120 banks failed. Seidman said that at least that many will be closed in 1986. He said that on March 3 the FDIC had 1,196 banks on its problem list, a number that has increased steadily since the spring of 1981, when 200 banks were on the list.

"This dramatic change has provoked many questions about the condition of the banking system, the handling of bank failures, the role that deposit insurance has played and the role that it should play," Seidman said.

Seidman told the Banking Committee that several large banks were on the problem list, but he would not name them nor say how many there were. He cautioned, however, that banks large and small are likely to continue to suffer from bad loans in a variety of business sectors and that the FDIC lacks the "tools" to head off a crisis.

"Weaknesses are likely to persist during the next year or more in energy and agriculture [markets] and in several foreign countries that are significant bank borrowers, and parts of the banking system will continue to be hurt by these strains," Seidman said.

Seidman said that liquidations cost the FDIC, which insures bank deposits up to $100,000, more than mergers because a healthy institution that buys a failing one assumes part of the cost.

He said that if the FDIC could own and operate failing banks for a limited period of time -- essentially establish "bridge banks" -- the fund would have more time to find buyers for the institution and spare itself the cost of reimbursing depositors.

Seidman also asked Congress to extend but amend the Garn-St Germain Act of 1982 when it expires April 15. The act allows regulators to arrange for emergency acquisition of troubled banks, but only those with assets of $500,000 or more and only once they have failed.

Seidman wants the FDIC to be able to step in only when a bank is in trouble but well before the problems have resulted in failure. He also wants the asset floor of banks eligible for the emergency program sliced to $250 million.

Seidman and Edwin Gray, chairman of the Federal Home Loan Bank Board, who also testified yesterday, joined in asking the banking committee to lobby on their behalf before the Senate Finance Committee, which is considering dropping tax breaks for healthy institutions that buy failing banks.