The four-year debate about the Big Stone II coal-fired power plant was supposed to come to an end Thursday, but its future remains undecided.

After five hours of deliberations, the Minnesota Public Utilities Commission (PUC) decided that it wanted more numbers before deciding whether to approve two power lines from the proposed South Dakota plant into Minnesota.

The move was recommended by Commissioner Dennis O'Brien, as it became clear that he would be the swing vote.

Chairman LeRoy Koppendrayer and David Boyd had expressed their intent to approve the lines, and Thomas Pugh and Phyllis Reha had indicated that they would not.

The commissioners' next step is to hear cost projections from an independent expert, although they did not decide when or how.

O'Brien said he was frustrated by the dueling cost models offered in testimony by the two sides: five regional power companies led by Otter Tail Power, which want permission to build the lines to their Minnesota customers; and five environmental groups, which oppose them based largely on the proposed plant's emissions of carbon dioxide -- a leading cause of global warming.

The cost projections in the models matter, because Minnesota law says the power companies must demonstrate that coal is a cheaper way to meet their energy needs than are either conservation or renewable energy sources. The developers have said that, without the transmission lines into Minnesota, they might scrub Big Stone II altogether.

The delayed vote introduced new concerns. Todd Guerrero, an attorney for the power companies, said that they hope the commission can make a decision by September, when regulatory actions in other states should be finished. Koppendrayer, who vacates his chairman position at the end of this month, said he is worried that more delay could kill the project.

Janette Brimmer, an attorney for the environmental groups, said the board's reluctance to make a decision proves that the project developers have not met their burden of proof.

Koppendrayer and Boyd opposed the delay. O'Brien, Pugh and Reha voted for it.

H.J. Cummins • 612-673-4671