The Senate, pushing aside northeastern states' fears of new air pollution problems, last night approved a plan to force 80 electric power plants to convert from oil to coal. The vote was 86 to 7.

The conversion plan, part of President Carter's effort to reduce America's dependence on oil, could cost taxpayers more than $4 billion through grants and loans to the utilities shifting to coal.

But in its rush to produce tangible election-year evidence of dealing with energy matters, the Senate turned out a bill with a number of imponderables.

For example, the measure would exempt oil-burning power plants from converting under certain circumstances, including evidence of economic hardship. The savings in oil also was in dispute, although the Energy and Natural Resources Committee estimated that 80 conversions could save up to 300,000 barrels of oil per day.

The Senate's approach fell short of Carter's proposal (he sought 107 plant conversions), but had strong support of the Senate leadership and the administration.

The idea of weaning 80 selected plants from oil met no resistance, although critics said the legislation was glaringly deficient with its proposed reversal of an earlier congressional ban on the burning of natural gas by utilities after 1990.

Debate yesterday centered on the potential environmental consequences of moving to coal.Fears of New Englanders about more air pollution were augmented by the Canadian government's concern about acid-laced rainfall.

Environmentalists and legislators from the Northeast urged inclusion of language that would assure no increases in sulfur dioxide and nitrogen oxide pollution as a result of burning more coal.

Of particular concern was acid rain, which many feared would intensify as power plants upwind from their region spewed damaging emissions.

The move to amend the committee bill was led by Sens. Paul E. Tsongas (D-Mass.) and Robert T. Stafford (R-Vt.) who warned of severe health and economic hardship for their states if additional pollution safeguards were not imposed.

They and others from the Northeast pushed amendments that would have prevented conversions to coal until assurances were provided that the plants would produce no more air pollution than they did while burning oil.

Opponents contended that the bill already ensured compliance with present air standards, and that any strengthening efforts more properly should be raised next year when the Clean Air Act is due for reauthorization.

Faced with a solid front of coal-state senators, including Majority Leader Robert C. Byrd (D-W.Va.), who actively worked for the committee version, the northeasterners were turned back.

By a vote of 63 to 31, floor manager J. Bennett Johnston (D-La.) won approval of his motion to table a Tsongas-Stafford amendment which, among other things, would have ban ned increases in air pollution from any plant converted with public money.

Stafford, angered by poor attendance during the debate, contended that committee and administration assurances that pollution would not be allowed to increase were not good enough.

"The question is whether we pay to do the conversion properly. We can't be sure what the pollution increase will be, but before EPA Administrator Douglas Costle was muzzled by the White House, he said there would be a 16 percent increase in New England," Stafford said.

"We are talking about the precedent of downgrading a region's environment. Pollution can be held at present levels and it can be done at relatively small cost -- at about the price of 1 1/2 aircraft carriers. If the Senate can sacrifice the environment of New England, it can do the same with the rest of the nation."

Johnston, reflecting the Senate's haste to move along, described the Tsongas-Stafford approach as "a killers amendment" that would "nullify the whole mechanism of this bill."

While Johnston was the coal lobby's hero of the day, he was not its hero by acclamation. Earlier, in committee, he had been instrumental in winning approval for power plants to continue to use natural gas as their main fuel beyond 1990.

Congress two years ago ordered that most power plants end their use of gas by 1990 -- an order issued during a period of natural gas shortages. The coal industry had argued vigorously for the ban on gas use, with an eye toward winning new markets as new power plants were built. The industry opposed the Johnston-orchestrated exemption.

Johnston, from gas-rich Louisiana, maintained that gas supplies had become more abundant since 1978 and that continued use of the fuel, principally in the Southwest and California, was justified.

Under the Senate conversion bill, four power generating stations with 10 units in Virginia and three stations with six units in Maryland would be required to switch from oil to coal by 1985.