An Improved Model of Total Supply Capability for Distribution Systems
The Total Supply Capability (TSC) model is newly proposed to evaluate the maximum load a distribution system can supply under the N-1 security guideline. This TSC model and a linear programming algorithm can calculate the accurate TSC of a distribution system; but the results of transformer loading rates calculated by this model are usually unbalanced, which is unacceptable in the practical operation. In order to solve this problem, this paper discusses the solution properties of the TSC model first. It is found that the transformer loading rates vector is not unique under the same TSC level; and only one of the optimal solution can be obtained from the linear programming software such as lingo. Then, an improved TSC model is proposed, which takes variance of loading rates and heavy-load areas into account. This new model is divided into two parts, the first part is the TSC calculation process, and the second part is the most balanced loading rate calculation process. Finally, a practical case is given to illustrate the effect of the improved model. It has shown that the loading rates calculated through this method are more balanced than the original one under the same TSC. The method can be useful tools to help system planners to get the best configuration of transformers in distribution systems.
Total supply capability and its extended indices for distribution systems: definition, model calculation and applications
This study systematically proposes and defines a set of new quantitative indices named total supply capability (TSC) and its associated indices, such as available supply capability, substation supply capability, network transfer capability, maximum supply capability and expandable supply capability. The concepts in the TSC family provide new approaches to analyse and optimise distribution networks. TSC for distribution systems is comparable to the total transfer capability for transmission systems, but there are significant differences, such as the radial configuration and service restoration in TSC consideration. TSC is defined as the maximum load to serve under the expanded N−1 security guideline for the distribution system, taking into account the capacities of the substation transformers, network topology, link capacity and other constraints. Here, the expanded N−1 security means a successful, near-immediate service restoration to all loads except for the faulted section. A mathematical model for TSC is then set up and an accurate method is presented to calculate the TSC with lingo. Sample applications are provided to illustrate a new TSC-based approach to evaluate the link capacity and obtain the reconductoring proposal. The results from a real distribution system demonstrate the effectiveness of the proposed TSC modelling and TSC-based approach.
Numerical analysis of surface crack propagation in flexible pavements using XFEM and cohesive zone model
Surface initiated (top-down) cracking in the longitudinal direction of pavements has become a predominant mode of failure in thick asphalt pavements. Most previous researchers studied crack propagation using the traditional Linear Elastic Fracture Mechanics (LEFM) method by calculating the stress intensity factor around the crack tip. In this paper, Extended Finite Element Method (XFEM) was used to study the key factors causing surface initiated crack propagation under surface tension and/or shear. A two-dimensional (2-D) finite element (FE) model for a two-lane flexible pavement was built to predict pavement responses and crack propagation potential under various scenarios. It was found that surface tension is the major driving force for crack propagation when the crack is at some distance away from tire loading. The results show that the distance from the location of crack to tire loading, the depth and direction of the crack, and the modulus ratio between the hot-mix asphalt (HMA) layer and the base layer have significant influences on crack propagation under surface tension. On the other hand, this study shows that shear stress can induce surface initiated crack propagation near the loading area as the shear resistance of material becomes low at the high temperature.