The 7-bus system below has a low-cost generator at bus 2, which cannot be fully dispatched due to congestion on lines 2-5 and 5-4 (the 100% pie charts on the figures indicate that those two facilities cannot accommodate any additional flow). The lowest cost dispatch to serve all the demand without overloading lines 2-5 and 5-4 requires use of the very expensive generator at bus 4.
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Without any topology changes, congestion on lines 2-5 and 5-4 leads to the nodal price contour shown in the same 7-bus system below. Price in bus 4 are high (red) since additional demand is served by the expensive generator at bus 4 (it cannot be served by the low-cost unit at bus 2 due to congestion). In contrast, prices in buses 2 and 6 are low (blue) since additional demand at those two buses can be provided by the low-cost unit at bus 2. Note that there is flow (15 MW) on the line going from bus 4 to bus 3. That flow exacerbates congestion on lines 2-5 and 5-4, since a portion of it goes through those two lines. Also note that line 4-3 carries flow from a high price bus to a low price bus, indicating that such flow is not economical.
If the flow on line 4-3 is interrupted by removing the line from service, the two congested facilities (2-5 and 5-4) become relieved. Opening line 4-3 forces flow away from lines 2-5 and 5-4, and into uncongested lines 1-3 and 2-3. The congestion relief opens up room to increase the dispatch of low-cost generator 2 and decrease the dispatch of higher cost units, including generator 4, while still meeting transmission limitations. Thus, opening line 4-3 enables generation cost savings by allowing a lower cost dispatch that meets the reliability criteria. The resulting dispatch and new price contour is shown in the 7-bus system below. Note that there is still congestion in the system (line 2-5 is at its limit), but generation costs went down (from $18,186 to $17,733) and prices are more uniform than in the case without topology changes.