J. L. Naredo

A Comparative Study of Neural Network Efficiency in Power Transformers Diagnosis Using Dissolved Gas Analysis

This paper presents a comparative study of neural network (NN) efficiency for the detection of incipient faults in power transformers. The NN was trained according to five diagnosis criteria commonly used for dissolved gas analysis (DGA) in transformer insulating oil. These criteria are Doemenburg, modified Rogers, Rogers, IEC, and CSUS. Once trained, the NN was tested by using a new set of DGA results. Finally, NN diagnosis results were compared with those obtained by inspection and analysis. The study shows that the NN rate of successful diagnosis is dependant on the criterion under consideration, with values in the range of 87-100%.

An improved arc model before current zero based on the combined Mayr and Cassie arc models

A computer model that describes the dynamic arc behavior in the high- and low-current regions before current zero is proposed. The model divides the current and voltage waveform in two regions. A differential equation for both regions which unifies current and voltage time derivatives is obtained by means of a generalized function method. The computer waveforms reproduced with the model show good agreement with measured results published in in the low and high current regions, but further comparison with other test measurements are required to know if the model has any feature of predictability.

Algorithmic evaluation of underground cable earth impedances

A comprehensive analysis of Pollaczeks integral is presented in this paper. It reveals the difficulties found when attempting its numerical solution. This analysis further provides a strategy to avoid these difficulties. A numerical algorithm is then proposed and implemented. Solutions to Pollaczeks integral are provided in graphic form for a broad range of applications. Normalized earth impedances also are provided for underground cables within the same broad application range. Finally, the normalized impedances are compared against those from a widely used approximation.

Electromagnetic transients in underground transmission systems through the numerical Laplace transform

This paper describes a method for the analysis and simulation of electromagnetic transients in underground power transmission and submarine cables. This method uses the frequency domain technique known as the numerical Laplace transform [Int. J. Electl Engng Educ., 15 (1978) 247]. This method can be used, as a principal analysis tool or as a backup for time domain tools such as the EMTP [Electromagnetic Transients Program Reference Manual, Portland, OR, USA (1986)]. Two application cases taken from specialized literature included here, are taken from Wedepohl and Wilcox [Proc. IEE, 120 (1973) 253] and L. Marti [IEEE Trans. Power Del., 3 (1988) 1099], respectively.

Electromagnetic transients in overhead lines considering frequency dependence and corona effect via the method of characteristics

In order to take into account the frequency dependence of transmission line parameters, the propagation equations with transient parameters have to be solved. In this work these equations are integrated by means of the method of characteristics and a convolution procedure using synthesized rational functions. The effects of corona are included using voltage-dependent shunt capacitances and an iterative process in the finite difference solution of the line equations.

Full frequency-dependent line model for electromagnetic transient simulation including lumped and distributed sources

This paper describes a new full frequency-dependent line model based on finite differences and the method of characteristics from the theory of partial differential equations. This model is well suited for handling distributed sources and is proposed as a complement to the existing traveling wave-based approaches, particularly for studies requiring line subdivisions. Three application examples are included in the paper. The first consists of a line excited by an externally radiated field. Comparisons with the Electromagnetic Transients Program (EMTP) illustrate the advantages of the new model over the traveling wave methods when lines must be subdivided. The next two examples demonstrate the applicability of the new model to long homogeneous lines and cables. In these cases, comparisons with the numerical Laplace transform show its remarkable accuracy.

Frequency-domain transient analysis of nonuniform lines with incident field excitation

In this paper, two methods for analyzing nonuniform multiconductor transmission lines in the frequency domain are considered. The first one is based on the cascaded connection of chain matrices of uniform line segments. In the second method, a space-variant linear system is solved. Besides, a technique to introduce the effect of incident electromagnetic fields into the nonuniform line model is presented. To obtain the solution in the time domain, the numerical laplace transform (NLT) is applied. Several application examples are analyzed, and comparisons with the ATP/EMTP and with experimental measurements published elsewhere are provided.

A Wideband Line/Cable Model for Real-Time Simulations of Power System Transients

This paper describes the implementation of a full frequency-dependent model for transmission lines and cables in a state-space-based solver for electromagnetic transients. This implementation is for real-time simulation of power system switching transients. It is based on the wideband universal line model with modifications being incorporated to meet the computational speed requirements of real-time applications. The real-time performance of the implementation is demonstrated through application examples.

An Advanced Real-Time Electro-Magnetic Simulator for power systems with a simultaneous state-space nodal solver

This paper presents a simulation method that combines state-space analysis with a nodal method for the simulation of electrical systems. This paper extends the concept of a discrete companion branch equivalent of the nodal approach to state-space described systems, and enables natural coupling between them. The flexible clustering of state-space described electrical subsystems into a nodal method has the following advantages: first, the nodal admittance matrix can be constrained in size while still permitting the solution of a switched network by nodal admittance matrix on-line triangularisation. Also, each group can have a precalculation of all internal modes (caused by switches, for example) within itself, an important feature for real-time applications. Secondly, the state-space formulation enables the use of higher-level discretization methods with L-stability properties. Finally, the approach enables the coupling of complex nodal-based models like FD-line into a state-space based solver. The method is implemented in a commercial real-time simulation software tool, the Advanced Real-Time Electro-Magnetic Simulator (ARTEMiS).

Fast transients analysis of nonuniform transmission lines through the method of characteristics

A new method for the simulation of fast transients on nonuniform transmission lines is proposed in this paper. The method is based on the method of characteristics of partial differential equations theory. It is shown that the proposed method can easily include resistive losses and wave speed variations. In addition, in order to bring the proposed method to practical usefulness, a Norton equivalent for transmission line ends is presented. At any given time, the Norton equivalent of each transmission line end is independent of the lines interior behavior at that time, therefore the model can readily be included into any electrical network transients simulation program. The advantages of the method are shown here by applying them to two problems. The first one consists of analyzing the effects of nonuniformities caused by the sagging of conductors on an aerial line. For this case, results from electromagnetic transients program and finite differences time domain simulations are included for comparison purposes. The second application consists of the simulation of a fast impulse propagating along a transmission tower modeled as a network of vertical and horizontal transmission lines.