Fabiano F. Costa

A Method for Averting Saturation From Series Transformers of Dynamic Voltage Restorers

This paper proposes a technique for preventing saturation in series transformers from dynamic voltage restorer (DVR) systems. The method consists in correcting the voltages which are injected through the transformers into the power system to compensate voltage sags. It restricts the compensating voltages during the sag whenever it predicts that a maximum limit for the flux linkage is about to be exceeded. The prediction is carried out at the beginning of a stabilized voltage sag. Moreover, the technique allows a certain level of sag compensation even when the estimated flux is expected to exceed the saturation limit. The voltage sag level and phase are computed through an adaptive recursive least squares (RLS). The RLS estimation incorporates a transient period before it achieves a stable state whenever there is a sag event. The DVR is not supposed to operate in this period. Therefore, this paper also outlines a simple procedure for detecting the RLS estimation stable level. Simulation of different scenarios of voltage sags and the results from the experimental implementation of a DVR show the effectiveness of the method.

Optimal positioning of geo-referenced short circuit sensors for faster fault finding using genetic algorithm

The performance of electric energy distribution system is strongly affected by faults. System restoration is critical due to difficulties in searching process by the utilities maintenance crew. Usually fault searching methods employed by utilities may lead to different locations to the same fault, mainly if the feeder is dense connected. There have been many approaches in order to overcome this problem. One popular method applies short-circuits indicators to be allocated along the feeders. Nevertheless, their cost can be a limitation for the method. In this context, this work issues the problem of optimal short-circuit indicator allocation along electrical network. The allocation is optimally accomplished in order to minimize the fault searching area of the utility maintenance crew. As the the problem formulation leads to a multimodal discrete nonlinear optimization, a genetic algorithm has been the optimizer tool selected to accomplished it. The technique has been tested on a model of a feeder of an Brazilpsilas Utility. The results showed a significant improvement over ad-hoc methodologies usually adopted by utilities.

Modeling and state-space feedback control of a DC-DC converter for photovoltaic systems

In this work is proposed a model of a photovoltaic (PV) system comprised by PV-arrays connected to a dc-dc converter. Toward this purpose, the PV panels, dc-dc converter and the controller are modeled as a set of state-space equations suitable for adjusting the transient and steady-state performances of the system. The photovoltaic system is evaluated with the average state variable method and the controllers are designed by means of concepts of the characteristic polynomial and the system controllability. These concepts in state-space formulation allow the controllers can be determined by the ackermanns method. The modeling and proposed control are tested by simulations and experimental procedures. The results corroborate the performance of the method.

A Computer Aided Design for optimization of optical networks

HiperionCAD is a computer aided design (CAD) software which allows the design and optimization of optical networks. The tool has been developed by means of the model driven development (MDD) approach. A graphical modeling framework (GMF) has been employed to model the domain, automatic code generation and graphic edition of the object models. We introduce the idea of a different CAD tool for design and optimization of optical network. The optimization of each component parameters comprising the network is performed using a genetic algorithm (GA) and the feasibility of this GA implementation is discussed by considering the different procedures. This paper is focused on describing the main characteristics and a demonstration of the implemented HiperionCAD tool for the optimization of optical networks by GA implementation.

Sensitive load voltage compensation with a suitable control method

This work proposes the usage of a repetitive-based control to dynamically restore the voltage applied to grid sensitive loads. The control is able to wipe off harmonic distortion and relies on simple transfer function. As a consequence, there is no need to apply harmonic selective filters. Moreover, the filter is able to work on sinusoid references and avoids employment of the dq transform. The design of the control parameters along with the system stability are discussed. A Simulink-based simulation has been built to verify the designed control performance. The experimental results are extracted from a setup of three-phase compensator which copes correctly with different scenarios of sagged and harmonic distorted voltages.

A sequence components estimation technique applied for distributed generation

This paper proposes to apply an instantaneous sequence estimation technique to enhance the control systems applied in distributed generation. It relies on a recursive least-square estimation of the αβ voltage components which are directly linked to the sequence components. The method was originally proposed to serve a dynamic voltage restorer controller. Here, this technique has been expanded to improve it robustness with regards to harmonic distortions, presented on the point of common coupling (PCC) between the mains and the distributed generator. The technique has been tested on synthetic signals and on signals produced through a simulation of a grid-connected inverter. The results endorse the proposed technique.

Coupled model for three-phase three-wire grid connected inverters through LCL filters

This paper proposes a state-space modeling for a three-phase three-wire voltage source inverter (VSI) connected to the grid through an LCL filter. The main feature of this model is to fully represent the coupling among the phases. The proposed model can be employed to develop controllers for dealing with power-grid unbalance with more accuracy using the natural reference frame. The results demonstrate that, under unbalance conditions, the currents injected into the grid are quite different from those obtained from the traditional single-phase decoupled model approach, which is usually applied to design controllers for VSIs connected to the grid through LCL filters.

Sensitive Load Voltage Compensation Performed by a Suitable Control Method

This paper proposes the usage of a repetitive-based control to dynamically restore the voltage applied to sensitive and critical loads of power system. The control intrinsically is able to wipe off harmonic distortion and relies on simple transfer function. As a consequence, there is no need to apply harmonic selective filters. Furthermore, the control system is able to work out on sinusoid references and, thus, avoids the need of employing the dq transform. A recursive least-squares is also included to the control system in order to assure the synchronization of the voltages to be restored. The design of the control parameters along with the system stability is discussed. The experimental results are produced with a setup of a three-phase series compensator. The scenarios for emulating faulty voltages are the same for experimental and simulated results. The results corroborates the usage of the proposed method.

Voltage quality enhancement with minimum power injection

This work proposes the usage of a repetitive-based control to dynamically restore the voltage applied to sensitive and critical loads of power systems. Besides, sag and voltage swells, the proposed control can intrinsically mitigate harmonic distortions. Furthermore, the filter is able to work out on sinusoid references and, thus, avoids the need of employing the dq transform. A recursive least-squares is also included to the control system in order to assure the synchronization of the injected voltages to be restored. The design of the control parameters along with the system stability are discussed throughout the paper. Also, this work aims to analyze two different types of voltage insertion into the grid through the series compensator. In the first one, the voltage injection is in synchronization with the pre-faulted grid voltage, while the second injects the voltage in a manner that the compensator spends a minimum of active power. Both types produce reference voltages that are used by the repetitive control. Additionally, simulated and experimental results are presented and corroborate the method efficacy.

A Novel Prony Approach for Synchronous Generator Parameter Estimation

In this paper we propose to use a novel Pronys approach, relying on suitable time-segmentation of current oscillograms and proper model order choice, to estimate transient and sub-transient parameters of synchronous generators associated to sudden three-phase short-circuit tests. Comparison between the proposed approach and a traditional graphical method, described in the IEEE-115 standard, has been performed. The current data used here have been generated from characteristic equations of short-circuit armature current. Based on these results, we conclude that the Pronys algorithm is a suitable alternative to the graphical method for parameter estimation in synchronous machines, providing more accurate estimates than the later approach.