The Virginia Electric and Power Co. yesterday blamed mechanical problems with its Surry nuclear power plant and the weather for recent increases of 15 to 20 per cent in charges to customers for electricity.

The company said it also has experienced a series of other difficulties, including an unstable market for purchasing coal and oil to fuel generating plants and failure to achieve the authorized rate of return on its financial base.

These problems, plus reduced projections for consumers needs in a decade, resulted in cancellations earlier this month of two reactor units at the Surry plant.

Vepco's recitation of setbacks came in response to an order from the VIrginia State Corporation COmmission to demonstrate that "utility inefficiencies are not passed on to the rate payers" in higher fuel adjustment charges.

Utilities are allowed to charge customers directly for increases in the cost of fuel to generate electricity.

The SCC said there had been "significant flutuations" in fuel adjustment charges and "a growing disparity" between fuel price increases and fuel adjustment increases.

In its order to Vepco on March 9, the SCC said, "This large difference has not been due basically to changes in the costs of fuel, but primarily to the price of facilities used to generate the energy."

The Vepco statement yesterday did not directly address the specific SCC questions. But Vepco officials will testify April 6 on the matter in greater detail. Vepco issued a strong defense of the idea of fuel adjustment charges and said the only states in the nation that don't have them use primarily hydroelectric power.

It said that its nuclear problems included leaks in steam generator tubing, a difficulty experienced by many other nuclear plants.

Vepco said that there has been a 4.4 per cent increase in the use of electricity in the year ending February, 1977, over the previous year. The extreme cold was at least partially responsible for a large part of the increases in consumer bills, the statement said.

Vepco had been ordered to respond to the SCC by yesterday. A hearing on the response will be held April 6 in Richmond.