Virginia Electric and Power Co. yesterday trimmed $16 million from its request to raise Virginia electric bills this fall, but revealed new problems with its nuclear power plants that could lead to higher monthly charges next year.

At a hearing before the State Corporation Commission in Richmond, Vepco reduced from $49.6 million to $33.5 million the amount it wants to add to customers' bills in October, November and December.

The $33.5 million increase would raise the bill of a typical residential customer who uses 1,000 kilowatt hours of electricity a month from $50.39 a month to $55.33 a month. The nearly $50 million increase would have hiked the bill to $57.89 a month.

Vepco is asking for the temporary increase to make up for the additional cost of oil the company is burning because its nuclear power plants are shut down for repairs.

The $33.5 million is the amount Vepco has spent so far on higher fuel bills, said William Berry, executive vice president of the utility. By the end of the year, Berry said, the added fuel costs will total $84 million. Vepco will ask the SCC to add the remaining $50.5 million to Virginia electric bills next year.

Vepco earlier estimated its increased fuel costs would total $91 million, but scaled that figure down, to $84 million this week without explanation.

As the Corporation Commission opened hearings on the request, Vepco disclosed that problems at three of its nuclear plants will keep them shut down longer than expected.

The extended delays will force Vepco to keep burning expensive oil, adding to the fuel costs the company is asking to pass on to customers.

Vepco said its Surry Unit 1, which was shut down by the Nuclear Regulatory Commission on March 13 so engineers could check its plumbing to see if it could withstand an earthquake, will not be back in operation on Oct. 1 as expected. The NRC has now ordered Vepco to check piping support plates in the plant, located in Surry, Va., 180 miles south of Washingotn. Vepco officials said they don't know when the agency will allow the plant to return to service.

A sister plant, Surry Unit 2, which is shut down for repairs that were supposed to be completed by Nov. 1, "may not return to full service before January, 1980," Vepco said. Company officials said the complex repair work-- some of it never before done on a nuclear plant-- is taking longer than expected.

Vepco also said delays caused by the NRC will prevent its new North Anna 2 plant, in Louisa County, from starting up on Dec. 1 as planned.

The delays are the result of an NRC moratorium on starting up new nuclear plants that was imposed after the Three Mile Island accident. "The recent developments at the NRC may prevent the unit [at North Anna] from going into commercial operation in 1979," Berry told SCC officials.

How much longer the three plants will remain idle and how much the delays will eventually cost Vepco and its customers is unknown, utility company executives said.

A month ago, Vepco asked the SCC to allow it to pass the full $91 million on to customers. Vepco said it could either collect that amount in the last few months of the year-- requiring a 30-40 percent increase in electric bills-- or collect part of the money-- $49.6 million-- this year and the remainder in 1980.

On Wednesday, the SCC staff recommended Vepco be allowed to increase bills only by the amount it has actually spent so far on higher fuel costs, about $30 million as of the end of July. Conceding that Vepco might have higher fuel costs for the remainder of the year, the SCC staff said consumers shouldn't have to pay for that oil until Vepco burns it.

To the surprise of its critics, Vepco yesterday accepted the staff recommendation and reduced its requests to $33.5 million-- the amount Berry said has been spent on higher fuel costs through the last week of August. But he said Vepco will ask to charge the rest of the $84 million-- $50.5 million-- to customers next year.

The $33.5 million would have been added automatically to Vepco customer's bills under the "fuel adjustment clause" that was repealed by the Virginia General Assembly last year.

Under the new law that replaced automatic fuel adjustment increases, Virginia utilities must peition the SCC for approval to pass on the higher costs.

After Vepco made its pitch for higher rates at yesterday's hearings, the session was recessed until Sept. 19, when critics will have a chance to cross-examine Berry and other Vepco executives on the request.