Tatiane C. C. Fernandes

Application of extended participation factors to detect voltage fluctuations in distributed generation systems

The dynamics of distribution systems has been significantly modified by the increasing penetration of distributed generation in distribution grids. Distribution systems, with distributed synchronous generators, may exhibit dynamic characteristics similar to those observed in transmission systems, such as, for example, low-frequency electromechanical oscillations. The impacts related to electromechanical oscillations raise important concerns about the system stability and power quality across the distribution grids. Tools usually employed to analyze such kind of oscillations in transmission systems (such as participation factors, for example), may not be fully appropriate for analyzing this same phenomenon in distribution systems. Due to the aforementioned issues, this work proposes the use of extended participation factors as a tool to detect voltage fluctuations caused by electromechanical oscillations in distributed generation systems. The results of a case study show that, in some cases, the traditional participation factors are not able to detect these fluctuations, while the extended participation factors performed well with respect to this detection.

Statistical analysis for the detection of voltage fluctuations in distributed synchronous generation using extended participations factors

This work proposes the use of a new concept of participation factors as a tool to detect voltage fluctuations caused by electromechanical oscillations in distributed generation systems, in order to evaluate the impact of these oscillations in power quality. While the originally proposed participation factors (here called traditional ones) may not appropriately detect the mentioned impacts (which manifest themselves in the form of voltage fluctuations), the extended participation factors used in this paper are able to detect an quantify them from a statistical viewpoint. To calculate these factors, a statistical approach based on Monte Carlo Simulations is adopted. The results of a case study presented in this paper illustrate the effectiveness of the extended participation factors in the detection of the mentioned voltage fluctuations.

Estimation of synchronous generator model parameters operating under unbalanced three-phase conditions

In this paper, a trajectory sensitivity method (TSM) is used to estimate parameters of a synchronous generator model suitable for unbalanced conditions which are typical of distribution systems. The current formulation and mechanical equations which represent the synchronous machine have parameters whose value modification cause significant changes in systems output. Trajectory sensitivity method is used to estimate the parameters which most affect the behavior of the system and whose trajectory sensitivity functions have comparable values. The obtained results show the method is able to estimate the parameters with great accuracy, once the initial values of the parameters are inside the convergence region, even when the system is liable to a high level of imbalance. It can also be noticed through the results that, even considering great modifications in the parameters, the convergence region is large enough to guarantee the correct estimation of the parameters.

An approach for statistical quantitative analysis of voltage fluctuations using extended participation factors

In this paper, a new concept of participation factors is used to investigate the presence of the voltage fluctuations induced by the electromechanical oscillations in systems with distributed generation. Unlike the originally proposed participation factors (which will be called traditional participation factors), the extended participation factors is calculated taking into account a set of initial conditions, which makes them able to detect and quantify the voltage fluctuation from a statistical viewpoint. In order to obtain a set of initial conditions that efficiently reflects the random nature of the plausible disturbances in the system, a Monte Carlo method and a normal distribution of the disturbance parameters are used to build this set. The obtained results show the efficiency of extended participation factors in detecting and statistically quantifying voltage fluctuations. In addition, the results indicate that it is possible to infer the probability of occurrence of voltage fluctuation in the studied system considering a statistical approach.

A study on voltage fluctuations induced by electromechanical oscillations in distributed generation systems with power factor control

This paper focuses on analyzing the impact of electromechanical oscillations caused by distributed synchronous generators (with power factor control) on distribution systems from the power quality point of view. The electromechanical oscillations are natural responses of synchronous generators to a mismatch in their net torques, and thus, are present in every distribution system equipped with such generators. In this work, a previously established methodology is applied to analyze the induced phenomena, which are related to voltage fluctuations. This methodology is used to the power quality assessment of a real power distribution system from Brazil. The obtained results show that electromechanical oscillations in distribution systems can indeed be a significant source of power quality problems, especially regarding voltage fluctuations, even when power factor (instead of terminal voltage) control is performed on the field circuit of the machine.

An approach to estimate parameters of a three-phase synchronous generator model

In this paper, in order to estimate the operational parameters of synchronous generator model suitable for unbalanced conditions, an approach which uses Trajectory Sensitivity Functions (TSF) and is based on Unscented Kalman Filter (UKF) is developed. In this framework, iniatilly, TSF are used to identify the parameters which most affect the behavior of the system. After the ordination of the parameters according to the TSF values, an iterative process which uses an estimation method based on the UKF is employed. Analyzing the results obtained by the approach when it estimates the parameters of a model of synchronous generator connected to a distribution system, we can verify that the proposed approach works efficiently.

Application of modal estimation techniques for small signal stability assessment in distribution systems considering measurement noise

The increase of distributed generation in the global scenario has motivated the study of the electromechanical oscillation phenomenon in distribution systems with load imbalance, an intrinsic characteristic of this type of system. In this paper, after showing that typical linearization procedures cannot be applied to the assessment of the small-signal stability of unbalanced distributed generation systems, this assessment is done with the application of a modal estimation technique: the Prony method. This approach enables the calculation of oscillation frequencies and damping ratios under unbalanced conditions, as earlier works from the authors have shown. However, under realistic conditions, measurement noise patterns with different spectra may degrade the results of the proposed application. The main contribution of this paper is the evaluation of the impact of noise over the small-signal stability assessment technique proposed by the authors for unbalanced distribution generation systems, aiming at a better suited technique for practical applications. The results show that, while frequency remains mostly unaffected by the presence of noise, the identification of the damping ratio can be significantly influenced by this factor.

Stability Analysis and Control of Synchronous Generators Operating in Distribution Systems under Unbalanced Load Conditions

Abstract Most of the existing tools for stability analysis of power systems are based on the assumption that the system operates under balanced conditions, i.e., the amount of power consumed in each of the three phases is approximately the same. However, recent changes in the operating paradigms of power systems (such as, for example, the introduction of generation at the distribution level, where load imbalance is an inherent feature) require a revision of the previously mentioned assumption. This paper presents a new set of tools designed to enable both large- and small-signal stability analysis of power systems, when the source of imbalance is the load and the generators connected to it are synchronous machines. The results of the two main approaches proposed in this paper are compliant with the ones given by commercial-grade stability analysis software in the balanced case (which is the one that commercial software can handle), and are also matching in the unbalanced case, which provides a cross-validation to both of the proposed approaches.

An approach to refine linear models used in small signal stability assessment

In this paper, an approach is proposed to improve linear models adopted in small-signal stability studies using information extracted from measured outputs of real system, as small disturbance data recorded by phasor measurement units (PMU). With this aim, a trajectory sensitivity technique is applied on the mismatch between the output of the simulated model and the response of the real system. Using this error signal and the sensitivity curves, this method can refine linear model in order to obtain a good match between the model and the measurements. In addition, this technique allows to identify which coefficients of the linear model can be refined from available data. Results obtained for a test case show that the proposed approach, which uses signals that can be directly measured by PMUs, is effective to calibrate the coefficients of interest in the system linear model.