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OptNet
Transmission expansion planning model
Objective
OptNet is a computational tool for determining the least-cost transmission network reinforcements required to ensure the supply of the forecasted load along the study horizon, taking into account the N-1 security criterion (adequate supply under any single circuit outage). An alternative objective function is to minimize the sum of investment costs plus the cost of expected unserved energy due to circuit outages (reliability worth criterion). It is also possible to represent several generation dispatch scenarios for each load scenario (due, for example, to the existence of renewable sources such as hydropower and wind). This allows for a more robust expansion plan and for a better tradeoff between investment costs and supply reliability. The transmission network is represented by a linearized power flow model; different power flow limits can be used for the base case and for the post-contingency situations. The planning can be carried out either sequentially for each time stage (“forward planning”) or by determining the optimal expansion for the final year and going backwards in time to determine the optimal timing for each reinforcement (“horizon year planning”).
System characteristics
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Execution through a user-friendly graphical interface with resources for visualization of the network; |
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Visualization of results in spreadsheets; |
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Network data plus load and generation dispatch scenarios can be directly imported from the transmission-constrained dispatch model SDDP, developed by PSR; |
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Ranking of candidate reinforcements by cost-benefit indices; |
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Detailed performance analysis of a user-provided expansion plan. |
Solution methodology
Advanced optimization techniques are used to solve the transmission planning problem. A major difficulty is that, due to Kirchhoff’s second law, the problem has to be formulated as a nonlinear mixed integer programming model. Initially, the nonlinearities are removed by the use of a new disjunctive formulation developed by PSR. Logical constraints for candidate circuits in parallel and topological constraints are introduced in a pre-processing phase, to reduce computational effort. Finally, there is the option of using incremental reinforcement strategies, where candidate circuits are ranked by cost-benefit indices related to the effectiveness of reducing load curtailment due to overloads in both the base-case and the post-contingency situations.
Some recent applications
The OptNet model has been recently applied in the following studies:
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Five-year transmission plan for El Salvador (34 contingencies, 200 monthly dispatch scenarios, 47 candidate circuits); |
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Five-year transmission plan for Venezuela (36 contingencies and 125 candidate circuits). |
Tela 1.2.2
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