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A Model to Evaluate the Regulatory Reliability Indexes Impact on the Subtransmission and Distribution Network Expansion Planning

The structure of the distribution system designed and implemented in Brazil makes use of concession contracts with distribution companies in order to establish the responsibility for the power supply to consumers in their geographical regions, including the expansion and operation planning of their networks. As a rule, the structure of contracts and investment compensation mechanisms follow different procedures for distribution and transmission services. These differences have created in some cases mismatches between the transmission network and distribution network planning and they are far more evident in the “boundary” of these networks. Usually, the boundary network between transmission and distribution networks is identified as the subtransmission network, with lower voltage levels if compared with the transmission voltage levels, and in many cases a significant part of this network also belongs to distribution companies. In practice, the mentioned mismatches in the planning processes of both networks have created several disadvantages like overloads in equipments, increases in operational and investment costs and low reliability to end consumers. In literature, several studies show efficient tools in the expansion planning of transmission networks and distribution networks but in general they do not consider both networks simultaneously. Due to the aforementioned reasons, one way for the distribution companies to improve its financial situation and operation performance is to consider the expansion planning involving the subtransmission and distribution networks simultaneously looking for planning solutions as a whole.
Other important aspect to be considered in the expansion planning of subtransmission and distribution networks is the fact that in recent years with the presence of electricity markets, regulatory agents have imposed strict rules on distribution companies looking for more efficient services. For this reason, financial penalties are imposed when the established minimum reliability indexes (CID, CIF, SAIDI, SAIFI) are violated. In the current regulation, the distribution company revenues are affected not only when the financial penalties are applied but also because the reported indexes are used to calculate future tariff adjustments. Several authors have proposed models considering the impact of the reliability indexes in the expansion planning solutions but in an exogenous way. In other words, only after obtaining the minimum cost planning solutions, the impact of the reliability indexes is evaluated. This strategy has the disadvantage of obtaining planning solutions that do not measure in the same basis the cost equilibrium between the robustness obtained due to improving reliability and the reduction in investment and operation costs. The model presented here allows incorporating in the formulation of the cost minimization expansion-planning problem a performance criteria based on several reliability indexes in order to evaluating the impact of recognizing (or not) adequate reliability.
The model contributes in two main aspects:
a) A mixed integer linear formulation that considers the simultaneous expansion planning of the subtransmission and distribution networks belonging to the same distribution company.
b) Integration of reliability indexes and performance criteria as economic factors in the expansion and operation planning of distribution and subtransmission networks.


João Daniel Andrade Cascalho    
University of Brasilia

Pablo Cuervo    
University of Brasilia


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