Jonas R. Pesente

Evaluation of Volts/Hertz and over-excitation limiters acting under unbalanced load conditions

Currently, a worldwide tendency of decentralization of electric energy production (called Distributed Generation) is observed, where small generators - sometimes composed by synchronous machines - are connected mainly to distribution grids. Due to the intrinsic characteristic of load imbalance between phases in these systems, there are a series of questions about operation of these generators and its respective control loops. Some of these questions are related to the imbalance impact over the actuation of limiters in the excitation systems of these generators. In this context, this paper presents an analysis of the actuation of the Volts/Hertz and over-excitation limiters, when the synchronous generator is submitted to extreme situations under unbalanced load conditions. Additionally, the effect of different terminal voltage transducer representations over the actuation of these limiters was evaluated. The obtained results demonstrate that the unbalance condition, associated with an inappropriate specification of the voltage transducer, may have a significant impact on the actuation of these limiters.

Modeling and evaluation of the protection of distributed synchronous generators connected to unbalanced systems

The study of the imbalance effect on the behavior of protection functions used in synchronous generators, which are directly connected in the distribution and sub transmission systems, is the subject of this work. Very few studies are available for this type of generation, commonly known as Distributed Synchronous Generation (DSG), in spite of its great potential in countries like Brazil, for instance. Within this context, this work presents a detailed modeling for DSG protection and its corresponding evaluation with a full representation of the three-phase system. The results from simulations show the effect of imbalance on the performance and behavior of the types of protection present in distribution systems with DSGs. It was verified that there are cases where the representation of the unbalanced system is necessary for the correct analysis of the protection system performance and its adjustments.

Determination of disturbance patterns using fault recorder data and regression trees

This paper proposes a new method to train a regression tree for disturbance classification based on long term records considering the most probable operational conditions obtained by historical data of the analyzed system. Unlike the usual, deterministic approach of simulating a set of predetermined conditions varying fault resistance and position, the decision making process is now supported by a decision tree that was trained taking into account a set of probable initial conditions and disturbance parameters. This makes the classifier more accurate to identify the most common shapes of records for particular classes of disturbance, resulting in an appropriate balance between the number of records being generated and sensitivity of the classifier to a particular type of perturbation. The database for training was created by a large set of simulations, generated randomly by the Monte Carlo method until a target value for the confidence interval in the classification of the variables is reached. Results obtained showed that simple indices, together with a comprehensive set of rules, lead to a highly accurate classifier and highlight relations between input quantities and operational conditions with their effects on the nature of the corresponding disturbance.

MultiAgent Systems in Power System Protection: Review, Classification and Perspectives

Unlike traditional software paradigms, which usually are specified with a rigid software structure, the Multi-Agent Systems (MAS) prioritize the specification of the tasks of computational routines, and the interaction between them. This less rigid software structure results in a set of benefits from a computational point of view. Exploring this perspective, several studies of its application to solve electric power systems problems have been published in the last two decades. This paper reviews the applications of MAS to the area of power system protection, and analyses issues related to the coordination between the operations of protection devices - which is a key feature to ensure the integrity of system. Most of these studies, however, are dispersed and do not clearly indicate the benefits of using the MAS. In order to collaborate in solving this issue, this paper presents a broad review of relevant published research, proposes a delimitation of its main subclasses and points out the potential benefits of SMA in each group.

Evaluation of under-excitation limiter actuation under unbalanced load conditions

The increase of distributed generation in a worldwide level, combined with its characteristic of operating under load imbalance between phases, motivated the study of various aspects of its operation, such as the analysis of possible low-frequency oscillations and the generation penetration level that meets the minimum energy quality criteria. Among the features that deserve a careful scrutiny, there is the investigation of the distributed synchronous generator dynamics under severe operating conditions, including the relevant dynamic associated to its control loops. In this context, this paper presents an analysis of the actuation of the under-excitation limiter, when operating in synchronous generators subject to load imbalance between phases. In addition, the effect of different representations of the generator terminal voltage transducer (which is used as a feedback signal to the excitation system and its associated elements) on the actuation of the under-excitation limiter was evaluated. The obtained results show that significant levels of imbalance, associated with certain terminal voltage transducer representations, may have an adverse impact on the actuation of the under-excitation limiter.

Guidelines resulting from modeling, evaluation and coordination enhancement of Itaipu hydro power plant protection

Generator protection misoperations may intensify a disturbance or even lead the power system to a major outage, especially when it occurs in a large power plant. Additionally, the power system expansion changes the short circuit currents and, thus, from time to time the system requires a re-evaluation of protection coordination. In this paper, Itaipu power plants modeling, analysis and coordination evaluation of the protection functions performed due to scheduled technological update are presented. Lessons learned and undesirable situations concerning operation of large generators connected to large transmission systems observed during this process were used to propose a set of guidelines for overexcitation protection coordination evaluation for generators with the same type of configuration.

A comparative study among three methods for classification and location of faults

Currently, given the high amount of data generated by digital fault recorders, several studies on the processing and classification of events (such as faults, for example), are routinely performed. In this context, several methods have been developed and are often used interchangeably. This work is a constituent part of a larger project with the objective of developing an integrated computational tool for disturbance analysis in power systems, and the main goal of the study reported here is the definition of the most suitable method for identification and classification of faults to be integrated to the mentioned tool. The method based on changes in the current, the apparent impedance method and a classifier based on a set of rules built using fuzzy logic are compared among each other. The result of this work is a report synthesizing the benefits and limitations of each method, which was used to choose the method that will be used in the final disturbance analysis tool.

A comparative analysis between two approaches for the simulation of power system electromechanical transients

Traditionally, due the high effort involved in the simulation of power systems electromechanical transients, this simulations is carried out the so-called phasor model approach. This approach considers de following main simplifications: equal distribution of loads among the phases, representation of elements by lumped and identical parameters in all phases, invariance of this elements with respect to the frequency, representation the sinusoidal quantities by phasors and the representation of the system by its one-line diagram. These assumptions are valid for the bulk power system because utilities do their best to balance the loads at substation level, so the transmissions circuits operate with almost identical power in each of their phases. Recently, however, there has been an expansion in the number of synchronous generators connected directly to distribution feeders. In this context, some of the assumptions in the phasor model approach are no longer valid, mainly with respect to the unbalanced operating conditions. Therefore, a detailed three-phase representation is mandatory for the simulation of electromechanical transients originated from de mentioned connections. For this reason, the main objective of this paper is compare the results of simulations using the two previously mentioned approaches for a synchronous generator connected to a balanced distribution feeder. Once an agreement is reached between these two results, the confidence provided by this agreement will make it possible to proceed using the detailed three-phase model to simulate electromechanical transients under unbalanced conditions.