Congressional tax experts estimated yesterday that President Carter's proposed "windfall profits" tax would raise up to $3.4 billion a year in new revenues for the Treasury, not $1.7 billion as the White House has projected.

However, the congressional estimates were calculated on the assumption that oil prices would rise to$17.50 a barrel after adjustment for inflation, not $16 a barrel, the figure the White House had used.

The estimates, compiled by the Joint Committee on Taxation in preparation for action on the bill by the House Ways and Means Committee, showed that while Carter's tax would be mild, it would produce huge revenues if prices soared.

The panel also published estimates of rival congressional proposals designed to stiffen the Carter plan. One such proposal, it said, could boost the maximum yearly gain to the Treasury to $7.6 billion.

The congessional experts estimated, using the $17.50 - a - barrel oil price, that Carter's proposal would tax away 34.1 percent of the roughly $9.4 billion in "windfalls" the oil companies would reap after paying federal income tax.

The White House has not issued comparable figures using its $16 - a - barrel price estimate. The Treasury has said instead that the tax would reduce the industry's after - tax income by 30.8 percent. Other experts question these figures.

It was not immediately clear what impact the new estimates would have on the Ways and Means Committee's deliberations. The panel could begin next week to draft its windfall tax legislation, but it may delay a week or two.

Carter told a news conference yesterday he viewed prospects for passage of his tax proposal as "a serious problem," in which * he said his best hope was to "take my case to the public" to drum up support.

However, sentiment in the Ways and Means Committee so far has been stiffen Carter's proposal, not to block it. Rep. Al Ullman (D - Ore.), the panel's chairman, has indicated publicly the committee will toughen Carter's plan.

The Carter proposal actually would impose a tax rate of 50 percent, rather than 34.1 percent, but the plan would exempt large portions of profits resulting from lifting price controls on old oil and other categories.

The White House previously had reffered casually to "a 50 percent tax" in describing its plan. More recently, however, officials have become more conservative, pointing out that the rate would not apply to all new earnings.

The alternative proposal that would boost the "windfall" tax to $7.6 billion would raise the actual tax rate under the plan to 85 percent, a rate the administration considers overly harsh.

Other alternatives proposed by Ways and Means members - from reducing the exemptions that Carter has proposed to allowing an offset for higher state severance taxes - would raise from $3.2 billion to $5.8 billion.

The revenue estimates all apply to 1982, the first year prices will be fully decontrolled. The panel's estimates of new revenues from Carter's plan in other years are $42 million in 1979; $1.2 billion in 1980 and $2.9 billion in 1981.

In comparison, Carter had projected that his plan would yield no new revenues in 1979, $500 million in 1980, $1.5 billion in 1981 and $1.7 billion in 1982. Both estimates show the revenues declining gradually after 1982.

The higher oil price figure used by the Joint Committee was regarded as more realistic. Carter had used the $16 - a - barrel figure as the price in effect when the plan was announced. However, crude oil prices have risen since.

The reason $1.50 in higher crude - oil prices would produce so much in added tax revenues is that the price boost covers a category of oil that already is free from price controls and is not subject to exemptions.

Apart from the difference in price estimates, the Joint Committee's figures included a small tax hike resulting from a move to deny a writeoff for the depletion allowance on the so-called "windfall"profit

However, any such move would affect only the small, so-called "independent" oil drillers. The major oil companies are not now allowed to take the writeoff anyway. The difference in total revenues amounts to less than $200 million.