Luís M. R. Oliveira

A Permeance-Based Transformer Model and Its Application to Winding Interturn Arcing Fault Studies

This paper investigates the behavior of power transformers under the occurrence of permanent or intermittent winding insulation faults. For the study of these phenomena, a simple and efficient permeance-based electromagnetic transformer model is proposed, which is based on the simultaneous consideration of magnetic and electric equivalent circuits. To incorporate the internal faults in this model, a suitable equivalent circuit of the faulty winding is described. With the aid of this transformer model, the onload exciting current Parks Vector Approach will be applied for diagnosing the occurrence of permanent and intermittent winding faults. Experimental and simulation tests results are presented in this paper, which demonstrate not only the adequacy of the digital transformer model for winding fault studies, but also the effectiveness of the proposed technique for detecting winding interturn insulation faults in operating three-phase transformers.

Prognostic value of CA125 in advanced heart failure patients

INTRODUCTION Serum levels of CA125 are often high in advanced heart failure (AHF) patients. AIM To determine the predictive value of CA125 in forecasting the occurrence of death or cardiac transplantation in an AHF population. METHODS 88 AHF patients referred for heart transplantation were divided into 2 groups based on CA125 levels: normal (CA125<38 U/mL) and elevated (> or = 38 U/mL). Events (death or heart transplant) were monitored over a period of 13+/-7 months after CA125 determination. RESULTS Patients with elevated CA125 (n=65) had significantly lower blood pressure, body mass index, serum sodium and peak exercise oxygen consumption, while B-type natriuretic peptide levels were significantly higher. The combined primary endpoint (death or heart transplant) rate was 39.4% and 62.3% in normal and elevated CA125 groups, respectively (p=0.029). Multivariate regression analysis showed that CA125 and sodium levels were the only independent predictors of the combined endpoint. CONCLUSION In AHF patients, plasma CA125 was an effective prognostic marker. Its determination may contribute to better risk stratification in this population.

Leakage Inductances Calculation for Power Transformers Interturn Fault Studies

This paper presents a general approach to compute the leakage inductances of power transformers with turn-to-turn winding faults. The leakage inductances are obtained by using well-known conventional formulas, adapted to the asymmetrical conditions produced by the interturn fault in the leakage flux distribution. Two methods for calculating the leakage inductances are presented: the first requires detailed information about the geometry of the windings whereas the second uses simplifying assumptions, allowing to express the leakage inductances as a function of the nameplate short-circuit inductance, the number of turns, and core dimensions. A leakage inductance model of the transformer with faulty turns is also proposed, which is based on the three-winding transformer theory. This leakage inductance equivalent circuit can be easily integrated into other electromagnetic transient simulators. The results obtained from the application of the analytical methods and the equivalent circuit are validated using data obtained from finite-element analysis and experimental short-circuit tests.

Intermittent turn-to-turn winding faults diagnosis in power transformers by the on-load exciting current Park's Vector Approach

This paper presents the application of the on-load exciting current Parks vector approach for diagnosing permanent and intermittent turn-to-turn winding faults in operating power transformers. A digital model for the simulation of the behavior of three-phase transformers affected by the presence of winding faults is also proposed. Experimental and simulated results demonstrate the effectiveness of the proposed diagnostic technique, which is based on the on-line monitoring of the .on-load exciting current Parks Vector patterns.

Power transformers behavior under the occurrence of inrush currents and turn-to-turn winding insulation faults

This paper intends to provide a detailed characterization of the transformer behavior under the influence of inrush currents and/or incipient winding faults. Experimental and simulation results are presented and discussed. Finally, a promising new method to identify inrush currents, and to distinguish them from internal faults, is suggested.

Comparing power transformer turn-to-turn faults protection methods: Negative sequence component versus space vector algorithms

This paper presents a comparison of two of the most sensitive methods to detect low-level turn-to-turn faults in the windings of three-phase transformers. The performance of the negative sequence component and of the space-vector protection algorithms is tested under several internal and/or external fault conditions. The results indicate that the space-vector approach is slightly more sensitive for detecting low-level turn-to-turn winding faults. For more severe defects, the fault detection sensitivity of both methods is similar.

Comparing Power Transformer Turn-to-Turn Faults Protection Methods: Negative Sequence Component Versus Space-Vector Algorithms

This paper presents a comparison of two of the most sensitive methods to detect low-level turn-to-turn faults in the windings of three-phase transformers. The performance of the negative sequence component and of the space vector protection algorithms is tested under several internal and/or external fault conditions. The simulation results indicate that the fault detection sensitivity of both methods is very similar.

Modeling and Characterization of Leakage Inductances for Transformer Winding Fault Studies

This paper presents an analytical method to compute the leakage inductances of power transformers with a turn-to-turn winding fault. A leakage inductance model to represent the transformer with faulty turns is also proposed. The results obtained from the application of the analytical method are validated by using data obtained from finite-element analysis and experimental tests.

Correction to “A Permeance-Based Transformer Model and Its Application to Winding Interturn Arcing Fault Studies” [Jul 10 1589-1598]

In the above titled paper (ibid., vol. 25, no. 3, pp. 1589-1598, Jul. 10), there were some textual errors that are corrected here.

Development of an Energy Management System for the Charge Scheduling of Plug-in Electric Vehicles

As the number of Plug-In Electric Vehicles continues to rise, existing electrical grids need to adapt to support the expected charging demand of such vehicles. Fortunately, a growing number of renewable energy sources are also being introduced in current electrical grids, reducing the dependency on fossil fuels. Leveraged by the self-consumption legislation in several countries, the introduction of renewable energy sources continue to happen well beyond the end of the feed-in tariff rates. However, due to their variable nature, renewable energy sources are frequently characterized as intermittent resources, which cause mismatches in the required equilibrium between production and demand. In this scenario, the role of end users is very important, since they are not only required to participate in energy generation - becoming the so-called prosumers – but also they should allow the adjustment of the consumption, according with the generation levels. Plug-In Electric Vehicles, due to their power requirements, just exacerbate this problem. Following our previous work concerning scheduling algorithms for self-consumption scenarios, in this paper we describe the implementation of an Energy Management System for the charge scheduling of Electric Vehicles. The proposed system considers several requirements, including electrical grid limitations, present time and subsequent tariff costs, actual and predicted renewable energy generation levels, and user preferences. It then runs an optimization algorithm that decides when the charging of such vehicles should happen and controls the power delivered to charge them, accordingly.