Search

In this paper, the problem of minimum losses reconfiguration in weakly meshed medium voltage (MV) distribution systems is dealt with. The presence of meshes can either be a cause of the presence of particular loads, favoured customer (FC) nodes, requiring a double supply path or of distributed generation (DG) nodes, that cannot work in islanded mode. The problem has been brought back to the more general problem of radial networks reconfiguration with the major aim of minimising power losses subject to constraints on voltage drops at load nodes and current in branches as well as on the partition of power generated among supply nodes in the network. The problem is non-linear and combinatorial considering the objective function, the constraints and the unknowns; for its solution, the authors have used a genetic algorithm. For the creation of the first generation as well as of the subsequent generations, a set of new generation and perturbation operators have been set up. They guarantee that all solutions represent a radial structure while supplying all the loads. In this way, the generation of unfeasible solutions is avoided and the verification step afterwards is no longer necessary. The applications show that such operators allow a wider exploration of the search space, while their implementation is quite easy and does not require a particular computational effort.

In this paper, a methodology to solve radial distribution networks, with constant current and/or impedance loads, is proposed. The techniques currently available to solve such systems are based either on iterative methods or on the bus impedance matrix. The method developed is the extension of a technique that is valid to solve networks made of impedances with one supply point. The methodology can be applied to directly and rapidly solve large distribution systems in which the loads are modeled as constant current/impedance. It is also able to solve meshed systems having voltage-dependent loads, inside an iterative backward/forward method.

The single-line equivalent model is commonly used for voltage stability studies in both distribution and transmission systems. Referring to a single-line system, in the companion paper [1] the authors presented the theoretical aspects of voltage stability related to the load characteristics. In particular, expressions of the stability index for constant power, constant impedance and constant current loads, singularly and in all the possible combinations have been obtained. This index indicates how far the load node is from its voltage collapse point, allowing the maximum load power beyond which voltage collapse takes place to be evaluated. In this paper a simple method to evaluate the parameters which define the single-line equivalent system of a radial distribution network is presented, allowing to extent to it the expressions of the stability index previously found. Finally, the results of a detailed analysis of the influence of load models on the voltage stability of a realistic network, obtained with different methodologies, are reported for comparison and discussed.

The two-bus equivalent model is commonly used for voltage stability studies in both distribution and transmission systems. The paper presents a simple method to evaluate, for each bus, the parameters which define the equivalent circuit of a radial distribution network. In particular, a straightforward way for determining the Thévenin equivalent impedance behind a load node is proposed, which allows to better identify the maximum loading point beyond which the voltage collapse takes place in the network. Simulation results show that the proposed method is significantly more accurate than other existing methods on evaluating the critical power at a particular node (i.e. the weak node of the network) starting from any operating point.

In the paper, after having presented the general backward/forward methodology for radial systems analysis, a new b/f procedure showing some interesting features that improve its performance in terms of convergence speed and calculation effort is presented. The features that fundamentally are responsible for such improvements concern the main steps of the b/f procedure. The starting voltage profile solution is different from the flat profile and is suitably modified. In the backward phase and starting from the second iteration, the branch currents variations due to the loads changes are evaluated; the latter variations are calculated on the basis of the difference of nodal voltages at the beginning and at the end of each iteration. Finally, the adopted convergence criterion is based on the entity of the difference between each load node current in two subsequent iterations. The results of the applications of the proposed methodology to a set of networks taken from the literature on the topic are reported. In this way, the performance of the proposed methodology has been evaluated in terms of computational efficiency. The results of other tests have evidenced the influence of each feature of the modified b/f methodology over its performances.

This paper shows a technical-economical analysis of a novel Uninterruptible Power Systems (UPS) configuration, in which the backup batteries would be replaced by a photovoltaic (PV) system. Many interesting technical and economical issues concerning the proposed configuration are discussed against the traditional UPS and backup batteries solution. Moreover the technical solutions concerning the proposed integrated system realized using a double conversion UPS are motivated. Finally the operating costs of the proposed integrated PV-UPS system and the classical system with UPS and batteries with different sizes, and as the back-up time varies, are also compared and interesting results have been observed. Of course, the costs estimation for systems including renewables is not easy and cannot be precise, especially on the mid and long term, but still the analysis carried out shows that the proposed system seems to be an economically convenient choice at present in many cases and in the near future almost in all cases.