Insurance and real estate executive Flaxie M. Pinkett yesterday was elected to the board of directors of Potomac Electric Power Co. - the first woman to serve as a director of the Washington-based utility since it was founded in 1895.

Pepco also reported increased profits during the past year and the company's directors approved a 1978 construction budget of $180 million, compared with $140 million this year.

In a separate interview with Washington Post reporters, Pepco chairman and president W. Reid Thompson revealed that the electric utility is considering so-called interruptable rates for some operations of the federal government, the company's largest customer.

During periods of peak power demand under the structure of interruptable rates, facilities involved would have their power supply cut and would have to shut down. In exchange, normal power rates for such large-volume users would be somewhat lower.

Pepco faces an enormous demand for electricity during the summer months when air conditioners are used and must build generating facilities or contract for outside power to meet peak-hour consumption.

Under the plan being studied by Pepco, some of this demand could be curtailed with a new rate structure. Obviously, Thompson noted, the plan could be applied only at facilities where a temporary shutdown is feasible.

Pinkett, the new director, replaces Lee D. Butler, a board member since 1948 and president of Lee D. Butler Inc., a Lincoln-Mercury dealership.

Chairman and president of John R. Pinkett Inc., an insurance and real estate agency in Washington, Pinkett is secretary and trustee of the Fort Lincoln New Town Corp., and a director of the Washington Board of Realtors, Metropolitan Washington Board of Trade, Columbia Federal Savings & Loan Association and Union First National Bank.

Pepco's profits for the first 11 months of 1977, reported yesterday, were $76.8 million ($1.63 a share) compared with $69.1 million ($1.54) for the same period last year. Revenues rose to $619 million from $507 million.

The utility's officers currently expect to spend about $820 million on new facilities and construction in the five years, 1977 through 1981. Thompson said yesterday. This total is somewhat above a forecast last June of $775 million for the five years, primarily because of additional requirements for pollution controfl equipment.

Thompson said spending for environmental controls alone would total $46 million this year, $32 million in 1978 and $185 million over the five years.