Paulo M. Silveira

A New Method of Current-Transformer Saturation Detection in the Presence of Noise

The nonlinear characteristics of magnetization of a current transformer (CT) can cause waveform distortion in the secondary current, which may result in malfunction of protective relays. This phenomenon is known as CT saturation. The accurate saturation detection, by the use of digital signal processing techniques applied to secondary current, plays an important function in solving the problems caused by the saturation. This paper presents a new method of saturation detection, based on the first difference of the low noise Lanczos filter. The formulation of the adaptive threshold according to the level of the SNR is presented and the detection algorithm is implemented, tested and compared with the third difference function method. The tests were performed using distorted secondary current signals generated by the RTDS (Real Time Digital Simulator) and corrupted by several levels of White Gaussian Noise (WGN). The results showed a high detection capability of the proposed algorithm regardless the CT characteristics, the secondary burden and the fault characteristics. Additionally, the method showed to be robust against noise up to 40 dB SNR and suitable for real-time implementation.

New Approach to Improve High-Voltage Transmission Line Reliability

This paper presents a remote fault detection and identification system for transmission lines, permitting elimination, or at least minimizing, the use of maintenance methods employed by power utilities, as a result of its technical potential and capability to reduce operational costs. It consists of a data acquisition system, capable of acquiring and storing high frequency signals present in the transmission lines (TLs). The signals, after their storage, are treated and identified through signal processing techniques such as digital filters and neural networks. This system has been installed in the reception terminal of one power line carrier system, in an electric power utility in Brazil. Many fault-simulating tests were carried out in the transmission line for pattern definition. Therefore, it is possible to develop an algorithm capable of identifying any potential transmission line fault. With the results obtained in this first part of the research, and with the continuity of the project, new signals will be obtained, identified and trained by the neural network.

Loss-of-Excitation Protection and Underexcitation Controls Correlation for Synchronous Generators in a Real-Time Digital Simulator

This paper presents the results of a study that analyzes the relationship between the excitation system of a synchronous machine and the settings of the loss-of-excitation (LOE) relay applied to protect this machine. A complete generation system taking into account all the physical components was modeled in a real-time digital simulator, including a new excitation system model (ST7B). By considering a method to protect the generator against partial and total LOE, the settings of the LOE protection were made taking into account the technical data of the synchronous machine. A hardware-in-the-loop simulation using a numeric relay was implemented in a laboratory. Thus, several tests were performed in order to evaluate the coordination between the protective relay and the limits of the modeled excitation system during underexcited operation.

Current-Transformer Saturation Detection Using Savitzky-Golay Filter

A new application of the Savitzky-Golay (SG) filters in current-transformer saturation detection is proposed. The ability of the SG filters to handle noisy signals improves the robustness of the detection process in the presence of noise while requiring only low computational effort.

Review of signal processing techniques for time-varying harmonic decomposition

The trend towards the implementation of Smart Grids results in a higher complexity of power signals and their voltage quality implication. Thus the need for more advanced signal processing techniques is required. A number of techniques have been recently developed and proposed in order to analyze the new environment and phenomena. Among these techniques this paper discusses different time-frequency methods used in power systems disturbance analysis. Filter Banks, Sliding-Window Recursive-DFT (SWR-DFT) and Wavelet Packet Transform (WPT) are compared. The methods are discussed based on their advantages and disadvantages. Signals with harmonic and inter-harmonics distortion are used in order to compare performance of the methods related to fundamental frequency variations, fast magnitude oscillations and inter-harmonics. The results presented in this work should assist quality expert to select the appropriate method according to application.

Comparative Analysis of Power Quality Instruments in Measuring Power under Distorted Conditions

Abstract This paper aims to evaluate the performance of commercial power quality (PQ) instruments. By using a high precision programmable voltage and current source, five meters from different manufacturers are analyzed and compared. At first, three-phase voltage signals are applied to those PQ instruments, considering harmonic distortion, voltage dip, voltage swell, as well as unbalanced voltages. These events are measured, compared and evaluated considering the different instruments. In addition, voltage and current signals are applied under different conditions, with and without harmonic and unbalances, in order to obtain the measurements of electrical power. After analyzing the data generated in all these tests, efforts are focused on establishing a relationship between the accuracy of the measurements. Considering the percentages of errors, a score is assigned to each instrument. The tests have shown “excellent” and “good” results regarding measuring active and apparent power, as well as power factor. However, for non-active power, almost all instruments had lower performance. This work is relevant considering that Brazil is deploying standardization in terms of power quality measurements aiming towards regulatory procedures and index limits.

Comparative analysis of power quality instruments measuring voltage and power

This paper aims to evaluate the performance of commercial power quality meters. By using a high precision programmable voltage and current source, several meters are analyzed and compared. At first, At first, distorted three-phase voltage signals are applied to several kinds of PQ instruments, considering harmonics, sags, swells, as well as unbalanced voltages. Next, the features of these events are measured, compared and evaluated taking into account the different instruments. In further analysis, voltage and current signals are applied in different situations, with and without harmonic and imbalances, in order to obtain the measurements of electrical power. After analyzing the data generated in all these tests, efforts are focused on establishing a relationship between the error values of the measurements. Considering the percentages of errors, a score is assigned to each measurement instrument. This work is justified since Brazil is deploying a standardization in terms of power quality measurements aiming regulatory procedures and limit values of indices.

Impact of electricity theft on power quality

Electricity theft is responsible for economic problems for the electric utility due to revenue loss caused by electricity consumers that are not paying for it. The stealer has a tendency to consume more energy, resulting also in power quality problems. An increase in power demand to values greater than the transformer rated power can result in different quality deviations, like transformer overload, voltage unbalance and steady state voltage drop on system buses. The objective of this paper is to analyze, by using MatLab simulations and considering different grid configurations, how electricity theft results in power quality issues, specifically voltage drop in steady state. Additionally, it is shown how the steady state voltage drop can result in economic penalties for the electric utility when the proper voltage exceeds the network operational standards.

Observing time-varying unbalances and asymmetries at harmonic frequencies

Traditional harmonic distortion analysis assumes balanced and steady-state conditions. Moreover, harmonic components are often associated with certain sequences (positive, negative or zero). However, in reality unbalance causes the existence of all sorts of sequences for every frequency. Additionally, time-varying conditions complicate analysis and require appropriate models. Many of the time-varying unbalances, especially at higher frequencies, have not been discussed in detail before. This paper uses the Slide Window Recursive Fourier Transform to compute the time-varying harmonic components and then calculates and plots the positive, negative and zero sequences and parameters for each frequency. Recorded signals from a real system are used for this analysis. These time-varying parameters can then be used to further investigate the nature of some transient phenomena and for providing information for protection and control applications as the grid becomes smarter and more complex.

Visualizing power quantities under non-sinusoidal and time-varying conditions

Traditional analysis of waveform distortion assumes balanced and steady-state conditions. However, due to the widespread use of power electronics, non-linear loads and a multiplicity of harmonic-producing loads, power signals are becoming increasingly distorted and time varying. This complex scenario demands appropriate analysis. This paper illustrates the visualization of power quantities under time-varying conditions based on the defitions of IEEE Standard 1459. The Sliding Window Recursive Fourier Transform is used to decompose real signals in time-varying harmonics and subsequently it calculates the main parameters as function of time. These include symmetrical components, total harmonic distortions and apparent power. This visulization can lead to a better understanding of the nature of power signals under time-varying conditions.