Pablo Arboleya

Unified AC/DC Power Flow for Traction Systems: A New Concept

This paper adds some new contributions to the unified alternating current/direct current (ac/dc) power-flow method that is applied to railway power supply systems. These contributions are mainly focused on the way that the unified power-flow problem is implemented. The authors propose a new technique based on graph theory to model the motion of the trains without varying the system topology and dimensions, as well as the solution vector. Furthermore, a new matrix formulation is developed to provide an easy way of setting out the problem. The combination of these techniques makes the unified ac/dc power flow easier to implement, and the comparison among different instants can directly be done, representing the major contribution of this paper. Finally, as a minor contribution, a new technique based on previous matrix formulation is developed to easily obtain all active and reactive power magnitudes in compact form.

Efficient Energy Management in Smart Micro-Grids: ZERO Grid Impact Buildings

In a smart micro-grid (MG) each generator or load has to take part in the network management, joining in reactive power supply/voltage control, active power supply/frequency control, fault ride-through capability, and power quality control. This paper includes a new concept for building integration in MGs with zero grid-impact so improving the MG efficiency. These aims are shown to be achievable with an intelligent system, based on a dc/ac converter connected to the building point of coupling with the main grid. This system can provide active and reactive power services also including a dc link where storage, generation, and loads can be installed. The system employed for validation is a prototype available at ENEA Laboratories (Italian National Agency for New Technologies). A complete and versatile model in MATLAB/SIMULINK is also presented. The simulations results and the experimental test validation are included. The trial confirms the model goodness and the system usefulness in MG applications.

An improved control scheme based in droop characteristic control for microgrid converters

This paper analyzes the effects of the decoupling of the cross-coupling terms when PI synchronous regulators are used in droop characteristic converter. The PI synchronous regulators are implemented for both voltage and current control. An analysis of the different working modes of the converter are presented. Three sub-modes for the island model, Conventional droop mode, power quality mode and Sync mode are analyzed. One mode for grid connected mode, Grid supporting mode is analyzed.a

Analytical Interpretation and Quantification of Rotational Losses in Stator Cores of Induction Motors

We propose an approach to allocate and delimit a region in which the rotational losses are of most importance in the stator core of induction motors. The delimitation is based on the analysis of points at which the minimum flux density is not null. The analysis of flux paths and values of flux density over a number of motors allows a model of flux density to be proposed for the chosen rotational region. We conducted the process by post-processing finite-element results. A comparison with bench test results shows that the approach can confine the effects of rotational losses within a region allocated in the tooth roots without significant loss of accuracy. We give analytical expressions based on geometrical data. The approach provides a quick method to evaluate the rotational losses by analytical means, bypassing the use of numerical methods at those design stages at which is preferable to reduce the accuracy in favor of computational speed.

Improved model of photovoltaic sources considering ambient temperature and solar irradiation

The aim of the present work is to propose a complete Photovoltaic (PV) Direct Current (DC) source model, considering non linear effects with ambient temperature and solar irradiance. The main advantage of this model is that all non ideal characteristics of the PV source are taking into account, and complex weather conditions patterns can be considered. This model includes: The PV array and boost-buck DC-DC converter that operates to assure maximum power extraction. The Maximum Power Point Tracking (MPPT) algorithm, which is based on the incremental conductance method, is also described. The model can be used for example, as a DC source to supply grid connected or islanded inverters, to study the interaction of PV generators with the power system.

Unbalanced Power Flow in Distribution Systems With Embedded Transformers Using the Complex Theory in

This paper presents three new contributions to power flow analysis of unbalanced three-phase distribution systems. First, a complex vector based model in αβ0 stationary reference frame is developed to state the power flow equations using a compact matrix formulation. The proposed model is based on Kirchhoffs current law (KCL) and Kirchhoffs voltage law (KVL). Then, a general and exact power transformer model in the αβ0 reference frame is proposed. Finally, this transformer model is incorporated into the power flow problem. It will be shown that the use of an orthogonal reference frame simplifies the modeling of the distribution network components. In this work, both the network and the power transformer, as well as PQ type loads, PQ and PV type generators and a slack bus are modeled. By using the node incidence matrix instead of the admittance matrix, the information about the grid topology and the grid parameters (including power transformers) is separately organized. As it will be demonstrated, the proposed formulation is ready to incorporate other complex models of loads, generators or storage devices. The model is tested by using the IEEE 4-Node and the IEEE 123-Node Test Feeders with different transformer connections and balanced and unbalanced lines and loads.

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In this article, multiresolution analysis (MRA) and wavelets are applied to the study of the existence of an internal short circuit in a transformer. An analytical support is given to the proposal of a relaying technique based on the derivation of the existence of specific singularities in the modulus of the space vector as obtained from the three differential signals. The proposal stems from the quite different travel angles of the space vector: maximum 120 degrees for healthy inrush and 360 degrees for fault. The sharp notches in the modulus of the space-vector are then detected using de first detail in MRA.

Stationary Reference Frame

A new method for simulating faulted transformers is presented in this paper. Unlike other methods proposed in the literature, this method uses the data obtained from any sound transformer simulation to obtain the damaged condition by simply adding a set of calculated currents. These currents are obtained from the definition of the fault. The model is fully based on determining the incremental values exhibited by the currents in phases and lines from the prefault to the postfault condition. As a consequence, data obtained from simulation of the sound transformer may be readily used to define the damaged condition. The model is described for light and severe faults, introducing this latter feature as a further add-on feature to the low-level faults simulation. The technique avoids the use of complex routines and procedures devoted to specially simulate the internal fault. Of prompt application to relay testing, the proposed analytical model also gives an insight into the fault nature by means of the investigation of symmetrical components. In contrast with its low complexity, the method has shown to present large accuracy for simulating the fault performance.

A Solution to the Dilemma Inrush/Fault in Transformer Differential Relaying Using MRA and Wavelets

In the past years, there has been an increasing concern about turn‐to‐turn faults in power transformers, due to the high costs that unexpected outages cause. It is not always possible to analyse the transformer behaviour under such faults at rated conditions, since the tests are largely destructive. Therefore, models are of great importance to avoid severe damage to machines. In this paper, a model based on flux rearrangement around the damaged turn is presented. Two roles are assigned simultaneously to the damaged turn – i.e. the turn is seen as damaged but segregated from winding, and virtually undamaged belonging to the winding – and the damaged turn leakage flux is considered to be a coupling flux between the two roles. Thus, complex relations of magnetic couplings between the turn and the rest of the transformer are avoided, resulting in a very fast model, easy to develop further, which has been validated through a comparison with a finite element model.

Analytical approach to internal fault simulation in power transformers based on fault-related incremental currents

A literature survey reveals that the study of rotational power losses in stator cores has been conventionally performed by means of finite-element analysis. This paper proposes an alternative characterization of rotational power losses in the tooth roots - a region that has been long known as a seat of flux rotation - relating some geometric dimensions to the values of flux density of interest to compute the rotational losses over the region. A simplified computation of maximum and minimum flux densities is proposed based on the trajectories of flux at singular time intervals. This enables the calculation using analytical expressions. For that reason, the proposed approach improves the power losses analysis in early design stages, showing the impact of projected geometries on the total loss estimate. Moreover, it consequently provides a tool to include the rotational losses estimate within optimization-oriented iterative searches.