Showing papers in "IEEE Transactions on Power Delivery in 2005"

Micro-grid autonomous operation during and subsequent to islanding process

Abstract: This paper investigates (i) preplanned switching events and (ii) fault events that lead to islanding of a distribution subsystem and formation of a micro-grid. The micro-grid includes two distributed generation (DG) units. One unit is a conventional rotating synchronous machine and the other is interfaced through a power electronic converter. The interface converter of the latter unit is equipped with independent real and reactive power control to minimize islanding transients and maintain both angle stability and voltage quality within the micro-grid. The studies are performed based on a digital computer simulation approach using the PSCAD/EMTDC software package. The studies show that an appropriate control strategy for the power electronically interfaced DG unit can ensure stability of the micro-grid and maintain voltage quality at designated buses, even during islanding transients. This paper concludes that presence of an electronically-interfaced DG unit makes the concept of micro-grid a technically viable option for further investigations.

Phasor measurement unit placement techniques for complete and incomplete observability

Abstract: This paper presents techniques for identifying placement sites for phasor measurement units (PMUs) in a power system based on incomplete observability. The novel concept of depth of unobservability is introduced and its impact on the number of PMU placements is explained. Initially, we make use of spanning trees of the power system graph and a tree search technique to find the optimal location of PMUs. We then extend the modeling to recognize limitations in the availability of communication facilities around the network and pose the constrained placement problem within the framework of Simulated Annealing (SA). The SA formulation was further extended to solve the pragmatic phased installation of PMUs. The performance of these methods is tested on two electric utility systems and IEEE test systems. Results show that these techniques provide utilities with systematic approaches for incrementally placing PMUs thereby cushioning their cost impact.

Modeling and analysis of noise effects on broadband power-line communications

Abstract: Power line noise is known to affect the performance of broadband power-line communications significantly. This paper presents a frequency-domain approach to characterize and model the statistical variation of power-line noise. The model considers both the background noise and the impulsive noise. The background noise model is based on statistical analysis of the results from two long-term measurements of noise spectrum conducted at two separate sites of a laboratory and a residential apartment. On the other hand, the impulsive noise model is obtained by direct measurements from the noise sources (i.e., various electrical household appliances). The amount of impulse noise reaching a power-line communications (PLC) receiver can then be determined with consideration of the channel transfer characteristics between the noise sources and the PLC receiver. Using these noise models, the performance of two major classes of digital modulation schemes, namely single-carrier modulation and multicarrier modulation, are analyzed and compared. It is found that the multicarrier scheme performs better than the single-carrier scheme when subjected to the observed power-line noise with non-Gaussian statistics.

Artificial neural network and support vector Machine approach for locating faults in radial distribution systems

Abstract: This paper presents an artificial neural network (ANN) and support vector machine (SVM) approach for locating faults in radial distribution systems. Different from the traditional Fault Section Estimation methods, the proposed approach uses measurements available at the substation, circuit breaker and relay statuses. The data is analyzed using the principal component analysis (PCA) technique and the faults are classified according to the reactances of their path using a combination of support vector classifiers (SVCs) and feedforward neural networks (FFNNs). A practical 52 bus distribution system with loads is considered for studies, and the results presented show that the proposed approach of fault location gives accurate results in terms of the estimated fault location. Practical situations in distribution systems, such as protective devices placed only at the substation, all types of faults, and a wide range of varying short circuit levels, are considered for studies. The results demonstrate the feasibility of applying the proposed method in practical distribution system fault diagnosis.

Comparative analysis between ROCOF and vector surge relays for distributed generation applications

Abstract: This paper presents a comprehensive comparative analysis between rate-of-change-of-frequency (ROCOF) and vector-surge (VS) relays for distributed generation islanding detection. The analysis is based on the concepts of detection-time versus active power-imbalance curves and critical active power imbalance. Such curves are obtained through dynamic simulations. The performance of these devices considering different scenarios is determined and compared. Factors such as voltage-dependent loads, generator inertia constant, and multidistributed generator systems are analyzed. False operation of these relays due to faults in adjacent feeders is also addressed. Results show that ROCOF relays are more reliable to detect islanding than vector surge relays when the active power imbalance in the islanded system is small. However, ROCOF relays are more susceptible to false operation than VS relays.

Dynamic thermal modelling of power transformers

Abstract: The aim of this paper is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states based on data received in a normal heat run test (i.e., the top oil in the tank of the transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber-optic test) results obtained at varying load current on a 250-MVA-ONAF-cooled unit, a 400-MVA-ONAF-cooled unit and a 605-MVA-OFAF-cooled unit. The results are also compared with the IEEE-Loading guide (1995) Annex G method.

Performance analysis of OFDM systems for broadband power line communications under impulsive noise and multipath effects

Abstract: The impulsive noise and multipath effects are the main reasons to cause bit errors in power line communications. In this paper, the bit error rate (BER) performance of the orthogonal frequency division multiplexing (OFDM) system under the impulsive noise and multipath effects are theoretically analyzed in terms of closed form formulas. Through the analysis, it is shown that OFDM can mitigate the adverse effect of the impulsive noise and only the heavily disturbed impulsive noise will interfere the BER performance of the OFDM system. It is also shown that the adverse effect of multipath is more serious than that of impulsive noise. In this paper, the guard interval is used to improve the BER performance of the OFDM system. As the longer guard interval is inefficient in using the signal power, the optimum guard interval that can achieve the best BER performance is studied.

Autonomous load sharing of voltage source converters

Abstract: An autonomous load-sharing technique for parallel connected three-phase voltage source converters is presented. An improved power-frequency droop scheme computes and sets the phase angle of the voltage source converter (VSC) directly to yield more rapid real power sharing without sacrificing frequency regulation. Reactive power sharing in the presence of a mismatch between the VSC output interface inductors is achieved by having each VSC regulate the high side voltage with a drooped voltage reference. Dynamics of the reactive power control can be tuned without interfering with steady-state reactive power sharing. Simulation results that validate the proposed technique are also provided.

Power system frequency estimation using least mean square technique

Abstract: Frequency is an important parameter in power system monitoring, control, and protection. A least mean square (LMS) algorithm in complex form is presented in this paper to estimate power system frequency where the formulated structure is very simple. The three-phase voltages are converted to a complex form for processing by the proposed algorithm. To enhance the convergence characteristic of the complex form of the LMS algorithm, a variable adaptation step-size is incorporated. The performance of the new algorithm is studied through simulations at different situations of the power system.

Parameters of lightning strokes: a review

Abstract: The paper presents the statistical data of the significant parameters of lightning flash, collected by many researchers over many years around the world. The significant parameters of a lightning flash are: peak current, waveshape and velocity of the return stroke, the total flash charge and /spl int/I/sup 2/dt. Negative first strokes have traditionally been considered to produce the worst stress on the system insulation. The subsequent negative strokes have significantly lower peak current but shorter wavefronts. This may stress the system insulation more. The positive strokes have about the same median current value as the negative first strokes and longer fronts, thus producing less stress. However, their duration is longer than that of the negative strokes. Therefore, the system insulation may be damaged because of the lower volt-time characteristic for long-duration waves. The positive strokes may also cause more thermal damage because of their significantly higher charge and /spl int/I/sup 2/dt. The relationship between the return-stroke velocity and the current peak is a significant parameter in estimating lightning-induced voltages and also in estimating the peak current from the radiated electromagnetic fields of the lightning channel. For better accuracy, the current and the velocity should be measured simultaneously. Better methods to measure the stroke current need to be developed. Correlation coefficient between various lightning parameters is another important parameter which will affect the analysis significantly. Lightning characteristics should be classified according to geographical regions and seasons instead of assuming these characteristics to be globally uniform.

A novel approach to the modeling of the indoor power line channel-Part II: transfer function and its properties

Abstract: In Part I of this work, we introduced multiconductor-transmission-line (MTL) theory to model the indoor power-line (PL) channel. We have also shown that the proposed MTL approach can also be used to take into consideration both the topology of the link and particular wiring practices such as bonding. In this contribution, we continue our bottom-up approach to indoor PL channel modeling and we show that the circuit model found in Part I can be represented in terms of cascaded two-port networks (2PNs) coupled through a single modal transformer. Once the equivalent 2PN representation is obtained, it is possible to represent the whole PL link by means of transmission (ABCD) matrices only. The results presented here allow us to reveal that the PL channel is a more deterministic media than commonly believed and also allows us to unveil interesting properties of the PL channel, such as symmetry, that were previously unknown.

HEM: a model for simulation of lightning-related engineering problems

Abstract: This paper presents the assumptions and formulation of a hybrid electromagnetic model, developed for general application in numerical solutions of lightning current-related problems. Results provided by this model show very good agreement with experimental data for the transient response of a transmission-line tower to impulsive currents.

VSC transmission operating under unbalanced AC conditions - analysis and control design

Abstract: This work presents an analysis and a new control design of a voltage-source converter (VSC) transmission system operating under unbalanced network conditions. The system is analyzed in the positive and negative synchronous reference frames. The proposed control strategy contains a main controller and an auxiliary controller. The main controller is implemented in the positive d-q frame using decoupling control without involving positive/negative-sequence decomposition. The auxiliary controller is implemented in the negative-sequence d-q frame using cross-coupling control of negative-sequence current. Simulation results using the SIMULINK power system blockset show good performance of the proposed control strategy for a 300-MW 300-kV dc VSC transmission system during both balanced conditions and unbalanced conditions as may be caused by a solid single-phase-to-ground fault.

A method of a lightning surge analysis recommended in Japan using EMTP

Abstract: EMTP has been widely used in Japan to analyze switching and lightning overvoltages to design power stations and substations from the viewpoint of insulation coordination. The Japanese standard of high voltage testing, JEC-0102-1994, and the insulation design and the coordination of an 1100-kV line of Tokyo Electric Power Company were mostly based on EMTP simulations. This paper presents a method of a lightning surge analysis using the EMTP recommended in the Japanese standard and related guidelines, and suggests further work to be done based on discussions of applicable limits and problems of the recommended models.

Deriving an equivalent circuit of transformers insulation for understanding the dielectric response measurements

Abstract: Preventive diagnosis and maintenance of transformers have become more and more popular in recent times in order to improve the reliability of electric power systems. Dielectric testing techniques such as return voltage measurement (RVM) and polarization-depolarization current (PDC) measurement are being investigated as potential tools for condition assessment of transformer insulation. A better understanding and analysis of the dielectric test results are only possible with a clear understanding of the physical behavior of the insulation system in response to moisture and aging. A circuit model, which describes the dielectric behavior of the transformer's main insulation system, has been parameterized in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been developed between the physical condition of the insulation and the equivalent model parameters that enable a clear and transparent interpretation of the dielectric test results.

Fuzzy-logic-based fault classification scheme for digital distance protection

Abstract: In this paper, a fuzzy-logic-based algorithm to identify the type of faults for digital distance protection system has been developed. The proposed technique is able to accurately identify the phase(s) involved in all ten types of shunt faults that may occur in a transmission line under different fault resistances, inception angle, and loading levels. The proposed method needs only three line-current measurements available at the relay location and can perform the fault classification task in about a half-cycle period. Thus, the proposed technique is well suited for implementation in a digital distance protection scheme.

An adaptive Kalman filter for dynamic harmonic state estimation and harmonic injection tracking

Abstract: Knowledge of the process noise covariance matrix Q is essential for the application of Kalman filtering. However, it is usually a difficult task to obtain an explicit expression of Q for large time varying systems. This paper looks at an adaptive Kalman filter method for dynamic harmonic state estimation and harmonic injection tracking. The method models the system as a linear frequency independent state model and does not require an exact knowledge of the noise covariance matrix Q. As an alternative, the proposed adaptive Kalman filter switches between the two basic Q models for steady-state and transient estimation. Its adaptive function allows for the resetting of the Kalman gain to avoid Kalman filter divergence problems under steady-state and allow fast tracking of system variations in transient conditions. Simulation results on the 220 kV network of the lower South Island of New Zealand are presented to validate this approach.

Radiometric location of partial discharge sources on energized high-Voltage plant

Abstract: Partial discharges (PD) generate wideband radio frequency interference and, consequently, can be detected using radio receiving equipment. Due to the advances in ultra-high-speed sampling equipment, it is possible to accurately measure the propagation of the PD wavefront as it passes through a 4 element antenna array. From these measurements, the three-dimensional position of the PD source can be calculated using an iterative algorithm. The locating equipment is suitable for use within the vicinity of energized high-voltage plant and can locate sources up to 15 m from the array. Results are presented showing the location ability of the equipment under laboratory and field conditions. A significant advantage is the ability to detect PD sources in energized plant without the need for outages or electrical connections.

High impedance fault detection based on wavelet transform and statistical pattern recognition

Abstract: A novel method for high impedance fault (HIF) detection based on pattern recognition systems is presented in this paper. Using this method, HIFs can be discriminated from insulator leakage current (ILC) and transients such as capacitor switching, load switching (high/low voltage), ground fault, inrush current and no load line switching. Wavelet transform is used for the decomposition of signals and feature extraction, feature selection is done by principal component analysis and Bayes classifier is used for classification. HIF and ILC data was acquired from experimental tests and the data for transients was obtained by simulation using EMTP program. Results show that the proposed procedure is efficient in identifying HIFs from other events.

Travelling wave based fault location for teed circuits

Abstract: This paper describes a fault location algorithm for three terminal lines using wavelet transform of the fault initiated transients. The results presented in are extended to the case of three terminal configuration and a new single ended procedure is developed for teed circuits. The algorithm gives accurate results for the case of three terminal lines including series compensated branch, mutual coupled line section and different values of fault resistances. The performance of the algorithm is tested on different scenarios by using ATP/EMTP program and MATLAB Wavelet Toolbox.

High-impedance fault detection using discrete wavelet transform and frequency range and RMS conversion

Abstract: High-impedance faults (HIFs) are faults which are difficult to detect by overcurrent protection relays. Various pattern recognition techniques have been suggested, including the use of wavelet transform . However this method cannot indicate the physical properties of output coefficients using the wavelet transform. We propose to use the Discrete Wavelet Transform (DWT) as well as frequency range and rms conversion to apply a pattern recognition based detection algorithm for electric distribution high impedance fault detection. The aim is to recognize the converted rms voltage and current values caused by arcs usually associated with HIF. The analysis using discrete wavelet transform (DWT) with the conversion yields measurement voltages and currents which are fed to a classifier for pattern recognition. The classifier is based on the algorithm using nearest neighbor rule approach. It is proposed that this method can function as a decision support software package for HIF identification which could be installed in an alarm system.

Transformer modeling for low- and mid-frequency transients - a review

Abstract: One of the weakest components of modern transient simulation software is the transformer model. Many opportunities exist to improve the simulation of its complicated behaviors, which include magnetic saturation of the core, frequency-dependency, capacitive coupling, and topologically correctness of core and coil structure. This paper presents a review of transformer models for simulation of low- and mid-frequency transients. Salient points of key references are presented and discussed in order to give an accessible overview of development, implementation and limitations of the most useful models proposed to date. Techniques for representation of the most important behaviors are examined, with the intent of providing the needed foundation for continued development and improvement of transformer models.

On the definition of total harmonic distortion and its effect on measurement interpretation

Abstract: The existence of two different definitions for total harmonic distortion (one in comparison to the fundamental and one in comparison to the signal's root mean square) might cause ambiguity and misinterpretation of measured data. The difference between those definitions is stressed out in this letter. It is suggested that total harmonic distortion measurements in the context of power systems should always adopt the first definition and never the second.

On high-frequency circuit equivalents of a vertical ground rod

Abstract: Vertical ground rods have been used extensively from the early days of electrical engineering for earth termination of electrical and lightning protection systems. They are usually represented with equivalent circuits with lumped and distributed parameters based on quasistatic approximation, which limits the upper frequency of their validity domain. However, lightning-related studies often require modeling in the megahertz frequency range. Also, emerging technologies, such as power-line communications, require analysis in frequency ranges even up to a few tens of megahertz. The rigorous electromagnetic (EM) field theory approach may be used for such frequency ranges, but equivalent circuits are needed for the usual network analysis methods. In this paper, we look at possibilities to construct simple equivalent circuits that can approximate or match results from the EM model. In particular, we compare a usual homogenous distributed parameter circuit with a nonhomogenous one determined by curve matching with results from the EM model. The analysis is illustrated using numerical simulations.

Measurement of harmonics/inter-harmonics of time-varying frequencies

Abstract: A novel method of extraction and measurement of individual harmonics of a signal with time-varying frequency is presented. The proposed method is based on a nonlinear, adaptive mechanism. Compared with the well-established techniques such as DFT, the proposed method offers (i) higher degree of accuracy, (ii) structural/performance robustness, and (iii) frequency-adaptivity. The structural simplicity of the algorithm renders it suitable for both software and hardware implementations. The limitation of the proposed method as compared with DFT-based methods is its slower transient response. Based on simulation studies, performance of the method is presented and its accuracy and response time are compared with a DFT-based method.

Electromagnetic transients simulation models for accurate representation of switching losses and thermal performance in power electronic systems

Abstract: This work presents an electrothermal model of an insulated-gate bipolar transistor (IGBT) switch suitable for the simulation of switching and conduction losses in a large class of voltage-sourced converter (VSC)-based flexible ac transmission systems (FACTS) devices. The model is obtained by mathematical derivation of loss equations from the known submicrosecond device switching characteristics, and through the selection of appropriate differential equation parameters for representing the thermal performance. The model is useful in determining the device's heat generation, its junction temperature, as well as the cooling performance of the connected heat sinks. The model provides accurate results without recourse to an unreasonably small time step.

Improvement of power quality using adaptive shunt active filter

Abstract: Problems caused by power quality have great adverse economical impact on the utilities and customers. Current harmonics are one of the most common power quality problems and are usually resolved by the use of shunt passive or active filters. In this paper, a new control design using artificial neural networks is proposed to make the conventional shunt active filter adaptive. The proposed adaptive shunt active filter can compensate for harmonic currents, power factor and nonlinear load unbalance. A self-charging technique is also proposed to regulate the dc capacitor voltage at the desired level with the use of a PI controller. The design concept of the adaptive shunt active filter is verified through simulation studies and the results obtained are discussed.

Voltage control with inverter-based distributed generation

Abstract: This letter proposes a method to use the voltage-source inverters with distributed generation to control the voltage in a distribution network A droop line is used to prevent hunting between controllers Analytical expressions are derived for the voltage along a feeder with uniformly distributed load/generation The proposed algorithm will alleviate the voltage-control problem reported when installing distributed generation

Two control schemes to enhance the dynamic performance of the STATCOM and SSSC

Abstract: The paper presents an in-depth investigation of the dynamic performance of the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC) theoretically and by exact digital simulation. A 24-pulse GTO dc-ac converter model is designed to represent the operation of the STATCOM and SSSC within a power transmission system. Two major factors of the STATCOM instability are analyzed and a new Automatic Gain Controller (AGC) is proposed to ensure the stable operation of the STATCOM under various load conditions. It is shown that the Phase-Locked Loop (PLL) inherent delay has a great effect on the dynamic operation of the SSSC and a new auxiliary regulator is proposed to enhance the dynamic performance of the SSSC. The proposed control schemes are validated by digital simulation.

Sensitivity of AC adjustable speed drives to voltage sags and short interruptions

Abstract: This paper discusses the sensitivity of adjustable speed drives (ASDs) to voltage sags and short interruptions on the basis of extensive test results. Existing standards and previously published work are critically reviewed, and a description of test procedures needed for appropriate assessment of ASD sensitivity is presented. The following tests were performed: sensitivity to rectangular three-phase, two-phase, and single-phase voltage sags with ideal and nonideal supply characteristics, as well as sensitivity to nonrectangular-balanced three-phase voltage sags similar to those caused by the starting of large motors. The results show that although the behavior of this equipment has a rather complex pattern, a simple representation of ASD sensitivity to various types of voltage sags and short interruption can be established.