Javier Gómez-Aleixandre

Scheduling of Droop Coefficients for Frequency and Voltage Regulation in Isolated Microgrids

This paper details a procedure based on bifurcation theory to evaluate the impact that droops and primary reserve scheduling have on the microgrid stability. The methodology is based on finding the worst primary reserve share-that is, the share closest to instability-that can be found after rescheduling the droops of selected generating units that support frequency (and voltage) regulation. The solution-which consists of a measure of the distance to instability in a given direction-is found in a multi-parameter space endowed with coordinates corresponding to the droop coefficients. Two stages are proposed to achieve the solution. First, an investigation of the distance to bifurcation is computed in a one-dimensional parameter space in a defined search direction. Then the direction of this search is updated by calculating the normal vector at the found bifurcation point. The procedure is iteratively repeated until the closest bifurcation is found. The proposed approach is analyzed in a 69-bus and 11-generation unit isolated microgrid. It is shown through the analysis of some scenarios how the distances and normal vectors provide valuable insight on the correct scheduling from the stability point of view, giving advice on how the primary reserve should be more reliably scheduled.

Complex-Valued State Matrices for Simple Representation of Large Autonomous Microgrids Supplied by

This paper focuses on representing the state space model of a microgrid in which power regulated (PQ ) and voltage/frequency regulated (Vf) generation units share a distribution system. The generation units considered in this paper are inverter interfaced. This introduces some interesting modeling problems which are treated in the paper, such as the decoupled cascaded control schemes or the non-negligible grid dynamics. A modeling approach is proposed based on four defined complex vectors. These vectors allow for complex-valued system matrices to be formed in a quite automated way. Moreover, a convenient partition of the system matrices is proposed, which in turn allows fast and easy modifications. Additionally, a multivariable methodology is proposed to simultaneously find the control system gains in an optimal sense. A 69-bus radial system, supplied by 20 generation units, is used to demonstrate how the proposal is of easy implementation to conduct small-signal stability analyses.

PQ

This paper details a modeling procedure that incorporates composite loads in stand-alone microgrids in which, because of the low system inertia provided by inverter-interfaced generation units, the grid dynamics is not neglected. The paper introduces a methodology based on 1) separately treating the plants (RL grid elements) from reference frames and control systems; and 2) establishing a vector valued function to methodologically describe all plants in a similar way. Induction motors equations are rearranged to be integrated within the model, giving as a result a highly structured, compact system model. Next, bifurcation theory is adapted to the problem to show that composite loads are a need in the microgrid modeling if more realistic results about oscillations and mainly about load margin are pursued. Thanks to the modeling procedure, this is proven by means of a series of analyses conducted in a microgrid of considerable larger dimensions than those presented to date in the literature.

and

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

Vf

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.

Generation

Distributed generation systems composed of non-renewable and renewable power sources is one of the best approaches for reducing greenhouse gas emissions. Nevertheless, uncontrolled integration of power sources in the distribution system may have negative effects on efficiency and working parameters. Global optimization of distributed generation in the system is not available, but microgrids arrangements make it possible to the design management systems which are able to control their working parameters and give fast responses to internal events without affecting the distribution system. This work presents a new active-power dispatch algorithm capable of adjusting microgrid generation to demand on-line in grid-connected-mode. It also reduces the greenhouse gas emissions to a minimum, and optimizes running costs of microsources. The algorithm uses a heuristic approach based on cost functions of microsources and has been tested to solve different power dispatch cost optimization problems. The results obtained are superior to those obtained by applying state-of-the-art optimization methods, in terms of global cost and emissions, system stability, and computational resources requirements. This reduction of the requirements of computational resources makes it possible to run the algorithm on-line, using an off-the-shelf programmable logic controller or microcontroller. Thus, the infrastructure requirements and new investments are reduced and the penetration of microgrids based on renewable energies is improved.

Composite Loads in Stand-Alone Inverter-Based Microgrids—Modeling Procedure and Effects on Load Margin

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.

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

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.

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

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.

On-line minimization of running costs, greenhouse gas emissions and the impact of distributed generation using microgrids on the electrical system

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.