The District of Columbia Public Service Commission has decided to go along with an agreement that would allow Potomac Electric Power Co. to sell electricity at low cost to the owners of Three Mile Island, saying that the agreement has been changed enough to protect D.C. consumers from losses.

The commission voted not to object to a settlement agreement among General Public Utilities and other utilities in the Pennsylvania-New Jersey-Maryland pool (PJM) pending before the Federal Energy Regulatory Commission that would allow GPU to pay less than the standard rate for supplemental energy purchased from pool members.

The agreement represents a revision of an earlier GPU request to buy supplemental energy at even lower costs and a decision by the Public Service Commission that it was better to allow Pepco to make some money off lower-cost sales than for Pepco to lose the business altogether.

An accident at GPU's nuclear-powered facilities in Three Mile Island, Pa., in March 1979 shut the nuclear power plant down and forced GPU to go elsewhere for power for its Pennsylvania customers. The Pennsylvania Public Service Commission directed the utility to try to get it at cost.

GPU and other utilities in the pool entered into an interim agreement to allow GPU to pay cost plus 10 percent for extra energy -- a lower rate than that at which members of the pool normally buy and sell supplemental power.

Before that agreement became effective, however, GPU went to the FERC and reinstated a request to buy supplemental power at cost. Pepco opposed the plan.

The Public Service Commission's previous position on the issue of sales at cost was to decline to find it in the public interest. The recent decision -- made by two commissioners voting for it -- was somewhat more positive, saying in effect that the PSC doesn't object.

"The commission notes that the new agreement is substantially different from the original agreement that it examined and declined to find in the public interest in May 1980," a release from the PSC said.

The agreement still must be approved by the FERC.

While the debate has been going on over how supplemental energy for GPU should be priced, GPU has been buying power from outside the pool, which has cost the customers of pool members money, according to a Pepco spokesman. Pepco said it traditionally has made money from its transactions within the pool.