The U.S. banking system, while fundamentally sound, faces major threats in the coming years, including a recession of unknown severity, a Federal Reserve governor predicted yesterday.

Unusually high debt ratios leading to more individual bankrupticies, and increased energy prices affecting the ability of international borrowers to pay also pose serious future problems, Fed Governor J. Charles Partee told a Senate Committee.

In discussing the state of the industry yesterday before the Senate Banking Committee, Partee urged Congress to pass legislation permitting interstate takeovers of failed banks in certain emergency situations and quick financial assistance to others. The other federal financial regulators present at the annual oversight hearing concurred and outlined their agencies' rescue plans.

Comptroller of the Currency John G. Heimann expressed concern, as did committee Chairman William Proxmire (D-Wis.), about the decline in equity-to-assets ratios for large banks.

"Inflation has contributed to a level of asset growth which has outpaced the ability of most . . . to augment their capital accounts, either from retained earnings or from external sources," Heimann said. Bank stocks haven't been favored recently by investors.

As a result, the ratio of equity capital to assets for the 10 largest national banks has dropped to 4.02 percent, while the ratio for smaller banks was 8.07 percent at year end. With greater leverage, big banks can make loans with smaller margins and therefore gain a competitive advantage over smaller banks.

Proxmire suggested the ratio be set by law at 8 percent, but Heimann and the other regulators said this would be too inflexible. However, they did support legislation authorizing them to set the rate where they thought best.

Jay Janis, chairman of the Federal Home Loan Bank Board, said his agency expects to provide $3 billion to $4 billion in advances to savings and loans during the first half of 1980. He also predicted the number of problem associations would rise somewhat and that the number of supervisory mergers might increase. At the end of 1979 there were 79 S&Ls, with assets of $13.2 billion, on the problem list.

Janis predicted that earnings for the first half of 1980 would be less than half those of the second half of 1979, a return on assets of 30 instead of 65 basis points. Though the decline in interest rates makes the picture somewhat brighter, he added it would be "unrealistic" to expect a dramatic shift of funds from securities to savings accounts.

Total savings flow for 1980 now are predicted to be $36.5 billion, up from an earlier forecast of $27 billion but well under the $38.5 billion of 1979. Janis said loan repayments would decline to $40.2 billion, down from $49.7 billion in 1979.Mortgage lending will drop from $99 billion last year to $61 billion this year. However, S&Ls' $32 billion in reserves will see them through this crisis, he added.

Lawrence Connell, chairman of the National Credit Union Administration, said its recently created Central Liquidity Facility was preparing to provide over $100 million in protracted adjustment credit to credit unions that got into trouble speculating in Ginnie Mae securities. Nine credit unions were involved heavily. Connell, along with the other regulators, supported pending legislation to regulate Ginnie Mae dealers.

In addition, member credit unions borrowed $37.7 million last year to counteract disintermediation. Federal credit unions, which were the fastest-growing financial institutions in 1978, were the second-slowest in 1979, as savings growth dipped 85 percent. During 1979, 45 cases of special assistance, totaling $8 million, were authorized to avoid liquidation.