Reliability aspects of network planning for meshed subtransmission networks

As the intermediate part of the power system connecting transmission and distribution systems, the subtransmission networks incorporate a great number of intrinsic features. Despite their ambiguous role, sometimes being treated as distribution or as transmission, the subtransmission networks have a clear and direct effect on power quality issues for end-customers. Therefore, they must be handled and appropriately modeled, because of their particularities and operational practices. This thesis proposes modifications to the generic network cost function which is adjusted to the subtransmission networks. These modifications are located in the outage cost component, in the inclusion of the voltage sag cost component, and in the network assessment of two-dimensional time-related scenarios. More particularly, this work is focused on urban meshed topology with underground cable and overhead line connections. This type of network is well represented by the typical 110-kV Nordic subtransmission network which constitutes a realistic example in cold countries. Furthermore, this thesis develops four modules that complement the subtransmission network assessment algorithm. The Reliability Module assesses networks in a block-layer structure from the perspective of the delivery point. The Voltage Sag Module determines the voltage sag distribution function and its severity coefficients. The Network Cost Module compiles statistical data and results from the previous modules to estimate the total network cost over a project period. As an ancillary tool, the Sensitivity Module identifies zones of low reliability within the tested network and suggests remedial actions. The results show that this sort of subtransmission network with short and medium-length lines contains large voltage sag and outage costs. For instance, overhead networks are susceptible to meteorological phenomena, thus producing high rates of failures, interruptions and voltage sags. Conversely, underground networks are virtually immune to these, consequently resulting in an extremely low number of interruptions and voltage sags. Despite the aggregated investments, the gradual replacement of overhead networks with underground networks would most likely yield several operational benefits, including reinforcement of grid resilience and reliability, as well as supporting power quality to end-customers.