Showing papers in "IEEE Transactions on Power Delivery in 1998"

Fault location using wavelets

Abstract: This paper describes the use of wavelet transforms for analyzing power system fault transients in order to determine the fault location. Traveling wave theory is utilized in capturing the travel time of the transients along the monitored lines between the fault point and the relay. Time resolution for the high frequency components of the fault transients, is provided by the wavelet transform. This information is related to the travel time of the signals which are already decomposed into their modal components. The aerial mode is used for all fault types, whereas the ground mode is used to resolve problems associated with certain special cases. The wavelet transform is found to be an excellent discriminant for identifying the traveling wave reflections from the fault, irrespective of the fault type and impedance. EMTP simulations are used to test and validate the proposed fault location approach for typical power system faults.

IEEE Standard for Synchrophasors for Power Systems

Abstract: IEEE Standard 1344, Synchrophasors for Power Systems, was completed in 1995. It sets parameters required to ensure that phasor measurement will be made and communicated in a consistent manner. It specifies requirements for the timing signal used for phasor synchronization and the time code needed for input to a measurement unit. GPS is the recommended time source and IRIG-B is the basic format used for time communication. The standard requires correlating phasors computed from unsynchronized and synchronized sampling to a common basis. Timetagging accurately and consistently is essential for wide area comparison of phase. The standard specifies information exchange and control message formats. These include data output, configuration, and command messages. It includes 7 annexes that discuss the concepts covered in the body of the standard.

SSSC-static synchronous series compensator: theory, modeling, and application

Abstract: This paper describes the theory and the modelling technique of a flexible alternating current transmission systems (FACTS) device, namely, static synchronnous series compensator (SSSC) using an Electromagnetic Transient Program (EMTP) simulation package. The SSSC, a solid-state voltage source inverter coupled with a transformer, is connected in series with a transmission line. An SSSC injects an almost sinusoidal voltage, of variable magnitude, in series with a transmission line. This injected voltage is almost in quadrature with the line current, thereby emulating an inductive or a capacitive reactance in series with the transmission line. The emulated variable reactance, inserted by the injected voltage source, influences the electric power flow in the transmission line.

An universal active power line conditioner

Abstract: The power circuit of a general active power line conditioner (APLC) is based on series and shunt power converters that share a single DC link. In the present paper, a generic control concept for these series and shunt converters is proposed. It is based on the instantaneous real and imaginary power theory. In fact, the resulting equipment deals with the custom power and FACTS concepts. This equipment incorporates not only the compensation functions at the fundamental frequency like a unified power flow controller (UPFC), but also provides active harmonic mitigation capabilities. For these reasons, the compensator proposed here is called the universal active power line conditioner (UPLC). Simulation and experimental results are presented to confirm that the new approach has better performance than those obtained by controllers based on traditional concepts of active and reactive power.

A measurement method based on the wavelet transform for power quality analysis

Abstract: The paper presents a measurement method for power quality analysis in electrical power systems. The method is the evolution of an iterative procedure already set up by the authors and allows the most relevant disturbances in electrical power systems to be detected, localized and estimated automatically. The detection of the disturbance and its duration are attained by a proper application, on the sampled signal, of the continuous wavelet transform (CWT). Disturbance amplitude is estimated by decomposing, in an optimized way, the signal in frequency subbands by means of the discrete time wavelet transform (DTWT). The proposed method is characterized by high rejection to noise, introduced by both measurement chain and system under test, and it is designed for an agile disturbance classification. Moreover, it is also conceived for future implementation both in a real-time measurement equipment and in an off-line analysis tool. In the paper firstly the theoretical background is reported and briefly discussed. Then, the proposed method is described in detail. Finally, some case-studies are examined in order to highlight the performance of the method.

Operation of the unified power flow controller (UPFC) under practical constraints

Abstract: The UPFC is the most versatile and complex power electronic equipment that has emerged for the control and optimization of power flow in electrical power transmission systems. It offers major potential advantages for the static and dynamic operation of transmission lines, but it brings with it major design challenges, both in the power electronics and from the perspective of the power system. As the UPFC transitions from concept to full-scale power system implementation, the control and protection of this sophisticated equipment are of primary concern. This paper describes the basic control, sequencing and protection philosophies that govern the operation of the UPFC, subject to the practical constraints encountered in an actual high power installation. The operation of the UPFC is illustrated with representative results from a TNA study, undertaken jointly by the Electric Power Research Institute (EPRI), Western Area Power Administration (WAPA) and Westinghouse Science and Technology Center (STC).

UPFC-unified power flow controller: theory, modeling, and applications

Abstract: This paper describes the theory and the modeling technique of a flexible alternating current transmission systems (FACTS) device, namely, unified power flow controller (UPFC) using an Electromagnetic Transients Program (EMTP) simulation package. The UPFC, in this paper, consists of two solid-state voltage source inverters which are connected through a common DC link capacitor. Each inverter is coupled with a transformer at its output. The first voltage source inverter, known as a static synchronous compensator (STATCOM), injects an almost sinusoidal current, of variable magnitude, at the point of connection. The second voltage source inverter, known as a static synchronous series compensator (SSSC) injects an almost sinusoidal voltage, of variable magnitude, in series with the transmission line. This injected voltage can be at any angle with respect to the line current. The exchanged real power at the terminals of one inverter with the line flows to the terminals of the other inverter through the common DC link capacitor. In addition, each inverter can exchange reactive power at its terminals independently. The functionalities of the models have been verified.

Statcom controls for operation with unbalanced voltages

Abstract: Voltage sourced static VAr compensators such as the Statcom need to be able to handle unbalanced voltages. Mild imbalance can be caused by unbalanced loads while severe short-term imbalance can be caused by power system faults. A synchronous frame voltage regulator is presented that works even when three phase symmetry is lost. This regulator addresses voltage imbalance by using separate regulation loops for the positive and negative sequence components of the voltage. The proposed regulator allows the Statcom to ride through severe transient imbalance without disconnecting from the power system and, further, to assist in rebalancing voltages. The regulator maintains sufficient bandwidth to perform flicker compensation. The controller's performance is simulated for a Statcom in a model distribution system where it is subjected to a severe single line to ground fault and a rapidly varying three phase load.

A combined ANN and expert system tool for transformer fault diagnosis

Abstract: A combined artificial neural network and expert system tool (ANNEPS) is developed for transformer fault diagnosis using dissolved gas-in-oil analysis (DGA). ANNEPS lakes advantage of the inherent positive features of each method and offers a further refinement of present techniques. The knowledge base of its expert system (EPS) is derived from IEEE and IEC DGA standards and expert experiences to include as many known diagnosis rules as possible. The topology and training data set of its artificial neural network (ANN) are carefully selected to extract known as well as unknown diagnosis correlations implicitly. The combination of the ANN and EPS outputs has an optimization mechanism to ensure high diagnosis accuracy for all general fault types. ANNEPS is database enhanced to facilitate archive management of equipment conditions, trend analysis and further revision of the diagnosis rules, Test results show that the system has better performance than ANN or EPS used individually.

Simulation of transmission line transients using vector fitting and modal decomposition

Abstract: This paper introduces a fast and robust method for rational fitting of frequency domain responses, well suited for both scalar and vector transfer functions. Application of the new method results in increased computational efficiency for transmission line models using modal decomposition with frequency dependent transformation matrices. This is due to the fact that the method allows the fitted elements of each eigenvector to share the same set of poles, and that accurate fitting can be achieved with a relatively low number of poles.

Fuzzy-neuro approach to fault classification for transmission line protection

Abstract: This paper presents a new approach to real-time fault detection and classification in power transmission systems by using fuzzy-neuro techniques. The integration with neural network technology enhances fuzzy logic systems on learning capabilities. The symmetrical components in combination with three line currents are utilized to detect fault types such as single line-to ground, line-to-line, double line-to-ground and three line-to-ground, and then to define the faulty line. Computer simulation results are shown in this paper and they indicate this approach can be used as an effective tool for high speed digital relaying, as the correct detection is achieved in less than 10 ms.

A new approach for distribution feeder reconfiguration for loss reduction and service restoration

Abstract: This paper presents a new approach to solve the distribution feeder reconfiguration problem for loss reduction and service restoration. By using the proposed algorithm, a more efficient network configuration can be obtained to reduce loss. Three switching indices were defined in this paper. Branch voltage-drops and line constants were used with all the electrical constraints. Meshed networks were considered instead of the radial topology by closing all the tie switches. By considering only the largest switching index in each loop, this algorithm can reduce the number of feasible states drastically. The switching index can also be used for service restoration. Many tests have been run to show its effectiveness.

AEP UPFC project: installation, commissioning and operation of the /spl plusmn/160 MVA STATCOM (phase I)

Abstract: American Electric Power (AEP) has selected its Inez Substation in eastern Kentucky for the location of the world's first Unified Power Flow Controller (UPFC) installation. Comprising two /spl plusmn/160 MVA voltage-sourced GTO-thyristor-based inverters, this installation is not only the first practical demonstration of the UPFC concept, but also by far the highest power GTO-based Flexible AC Transmission Systems (FACTS) equipment ever installed. The installation will be the first demonstration of this type of equipment connected in series with a transmission line. The project is a collaborative effort between AEP, the Westinghouse Electric Corporation, and the Electric Power Research Institute (EPRI), and has been divided into two phases corresponding to the installation of the shunt- and series-connected inverters respectively. Phase I (installation and commissioning of the shunt inverter (STATCOM)) was completed successfully in July, 1997, and the STATCOM is now operational. Phase II is scheduled for completion in early 1998. This paper gives a brief background to the project, describes the installation and special features of the equipment, and reports on the Phase I commissioning and STATCOM operation.

Experimental testing of the artificial neural network based protection of power transformers

Abstract: This work presents a novel technique to distinguish between magnetizing inrush and internal fault currents of a power transformer. The proposed differential algorithm is based on an artificial neural network (ANN) and unlike the existing relaying techniques, this method is independent of the harmonic contents of the differential current. A novel neural network is designed and trained using the back-propagation algorithm with experimental data. After training the network, simulation and on-line tests are carried out to evaluate the performance of the ANN based algorithm under different fault and energization conditions. Both simulation and experimental results are quite satisfactory.

A novel technique for high impedance fault identification

Abstract: A novel technique using wavelet analysis filter banks (WAFB) to identify distribution high impedance faults (HIFs) is presented. A new model of HIF is used. HIFs and capacitor bank switching operations are simulated by the Electromagnetic Transients Program (EMTP) and their current signals are studied. High frequency components with the time localization information of both HIFs and capacitor bank switching operations are obtained using WAFB and their behavior is differentiated clearly. Results demonstrate that WAFB can be used as an element in a HIF detector for fast and accurate identification of distribution HIFs.

Control design and simulation of unified power flow controller

Abstract: The unified power flow controller (UPFC) is a solid-state controller which can be used to control active and reactive power flows in a power transmission line. In this paper, the authors propose a control strategy for UPFC in which they control real power flow through the line, while regulating magnitudes of the voltages at its two ports. They design a controller for this purpose which uses only local measurements. The control strategy is evaluated using digital simulation for a case study.

Indices for assessing utility distribution system RMS variation performance

Abstract: For many years, electricity distribution companies have used sustained interruption indices as indicators of the reliability of service provided on their systems. Today, however, many electricity consumers are adversely affected by more subtle voltage disturbances such as sags and swells. Many utilities are well aware of such service quality concerns and are implementing extensive monitoring systems to detect such disturbances and assess service quality in this regard. This paper presents a subset of work completed which provides utilities with tools to make more complete service quality assessments. Indices developed to reflect system service quality with respect to all RMS variations are presented. Example values for the indices are calculated using data from a national distribution power quality data collection project. Finally, an example application of the indices currently being made by a distribution utility is discussed.

An iterative technique for fast and accurate measurement of power system frequency

Abstract: This paper describes the design, computational aspects and implementation of an iterative technique for measuring power system frequency. The technique provides accurate estimates to a resolution of 0.01-0.02 Hz for near-nominal, nominal and off-nominal frequencies in about 20 ms. Computation requirements are modest and the technique has been implemented on a modern digital signal processor. The proposed technique was extensively tested using voltage signals obtained from a dynamic frequency source and from a power system. Some test results are presented in the paper.

A new non-communication protection technique for transmission lines

Abstract: This paper proposes a new noncommunication protection technique for transmission line protection. The technique relies on firstly the detection of fault generated high frequency current transient signals. A specially designed multi-channel filter unit is then applied to the captured signals to extract desired bands of high frequency signals. Comparison between the spectral energies of different bands of the filter outputs determines whether a fault is internal or external to the protected zone. In addition to the saving in costs through negating the need for a communication link, the technique also retains many advantages of the 'transient based protection' technology, such as insensitivity to fault type, fault position, fault path resistance and fault inception angle. It is also not affected by CT saturation, the power frequency short-circuit level at the terminating busbar or the precise configuration of the source side networks.

Experimental evaluation of STATCOM closed loop dynamics

Abstract: This paper presents a new approach for the dynamic control of FACTS apparatus, such as the STATCOM and UPFC, which utilize voltage source inverters (VSI) as their main building block. The control concept is based on a linearization of the dq inverter model. Feedforward techniques which are traditionally used for the approximate decoupling of d and q-axis control are discarded, in favour of a high gain full state feedback approach which assigns both closed loop system poles and, more importantly, their associated eigenvectors. Experimental validation of the approach is carried out on a laboratory STATCOM setup. Due to the nonlinear nature of the VSI equations and the uncertainty of AC system parameters, actual closed loop system dynamics can stray quite dramatically from those desired. Root locus analysis is therefore performed to investigate the small signal system dynamic behaviour. The loci demonstrate that the effect of system nonlinearity on the closed loop poles is virtually eliminated by the proposed control. The effect of AC system parameter variations is also shown to be minimal.

System and customer impact: considerations for series custom power devices

Abstract: The growing interest in power quality has led to a variety of devices designed for mitigating power disturbances, primarily voltage sags. One class of these devices is the series device. The size of a series device (SD) is usually described as having a certain percent RMS magnitude voltage injection capability. The required RMS magnitude is usually taken from measured data or the results of a circuit model simulation. These results can be graphically illustrated using magnitude versus duration matrices or curves. Due to the one cycle averaging effect of the RMS calculation, waveshape, transients, and phase shift are not adequately described, and thus does not accurately predict the necessary injection capability required of the series device. Careful consideration must also be taken when applying a series device onto a system. The series device acts as an additional energy source on the system. The device must coordinate with other protective devices to prevent damage to other loads on the system, particularly upstream loads. The series device requires a continuous current path to insert energy, so protective devices must be arranged accordingly. However, a load upstream of the series device and downstream of the protective device creates a potential for reverse current flow through the upstream load.

Artificial neural network approach to distance protection of transmission lines

Abstract: A distance relay for the protection of transmission lines is usually designed on the basis of fixed settings. The reach of such relays is therefore affected by the changing network conditions. The implementation of a pattern recognizer for power system diagnosis can provide great advances in the protection field. This paper demonstrates the use of an artificial neural network as a pattern classifier for a distance relay operation. The scheme utilizes the magnitudes of three phase voltage and current phasors as inputs. An improved performance with the use of an artificial neural network approach is experienced once the relay can operate correctly, keeping the reach when faced with different fault conditions as well as network configuration changes.

An adaptive protection scheme for advanced series compensated (ASC) transmission lines

Abstract: This paper presents an adaptive protection scheme for advanced series compensated (ASC) transmission lines. The scheme is based on Kalman and adaptive Kalman filters. The adaptive scheme utilizes the differences in the transient current signals for faults encountering and not encountering the ASC to determine the fault location with respect to the ASC and the faulted phases. Equations to determine the line impedance to the fault on these lines are developed. The adaptive scheme is tested for faults along and outside the protected line using EMTP simulated data.

Multiterminal HVDC systems in urban areas of large cities

Abstract: Subtransmission and distribution systems for large cities are usually built up of underground cable systems of considerable length. With regards to both cost and performance, a multiterminal HVDC system might be an interesting alternative of the conventional HVAC system. The performances of such a multiterminal system have been studied. The simulation results have shown that great improvement might be offered by the introduction of such a multiterminal HVDC system especially with regard to the power quality and earth fault disturbances in the system.

A statistical analysis of the effect of electric vehicle battery charging on distribution system harmonic voltages

Abstract: This paper presents a statistical method for predicting the effect that widespread electric vehicle (EV) battery charging will have on power distribution system harmonic voltage levels. The method uses a statistical model for nonlinear load currents to generate the probabilities of specific harmonic voltage levels. The statistical model for the harmonic currents produced by a concentration of EVs accounts for partial harmonics cancellation introduced by uncertainty and variation in charger start-time and initial battery state-of-charge. A general solution technique is presented along with several examples using data from a commercially-available EV charger and an actual power distribution system. The results show that there is a definite threshold penetration below which EV charging has negligible impact on the number of buses whose voltage total harmonic distortion (THD/sub V/) exceeds 5%. During the late evening of a summer day, the authors' example distribution system can accommodate EV penetration levels as high as 20%. A similar analysis of the system in the spring or fail indicates that the system can accommodate a 15% EV penetration before THD/sub V/ exceeds 5% at an unacceptable number of buses.

Pitfalls of electric power quality indices

Abstract: Several indices are in common use for the quantification of electric power quality. These indices are convenient for condensing complex time and frequency domain waveform phenomena into a number. However, a variety of commonly encountered circumstances are not readily accommodated (e.g. quasiperiodic and aperiodic signal components and noninteger multiples of the power frequency). Also, the power acceptability curves have been used as convenient measures of power quality and these curves do not capture three-phase detail of power quality problems. In this paper, the general subject of pitfalls of power quality indices is discussed and suggestions for alleviating these problems are made.

Modelling geomagnetically induced currents produced by realistic and uniform electric fields

Abstract: The methods used to model geomagnetically induced currents (GIC) on power systems depend on the nature of the electric field used as input. A uniform electric field, often used to simplify the modelling, is shown to have different properties from realistic nonuniform fields. Realistic fields which go to zero at infinity can be uniquely represented by the sum of the gradient of a scalar potential and the curl of a vector function. The scalar potential term is conservative and irrotational, while the vector term is nonconservative and solenoidal. In contrast, a uniform electric field can be represented simply by the gradient of a scalar potential. These different mathematical properties mean that modelling techniques derived for uniform fields may not work for realistic fields. This is examined using, as an example, the modelling of GIC produced in a conducting network at the surface of the Earth. It is shown that uniform electric fields can be represented by voltage sources in the transmission lines or at power system ground points. However, realistic electric fields, such as those produced by the auroral electrojet, cannot be represented by voltage sources at ground points and have to be represented by a voltage source in the transmission lines.

The application of ultra-high-frequency partial discharge measurements to gas-insulated substations

Abstract: This paper deals with the application of the ultra high-frequency method (UHF-method) for the detection of partial discharges (PD) in gas-insulated substations (GIS). First, a review of the GIS-relevant part of the theory of electromagnetic wave propagation in the UHF-range (300 MHz-3 GHz) is given. New information considering the attenuation due to reflections of UHF signals at spacers is presented. For the sensors used to pick up the UHF signals inside the GIS, a new description of an antenna valid in the UHF range is introduced. The second part of this paper presents measurements with the UHF method to compare the sensitivity to the conventional PD detection method according to IEC-270. Practical conclusions are drawn from these new results for the application of the UHF method for onsite testing and online PD monitoring of GIS.

A cooperative protection system with an agent model

Abstract: The authors propose a new concept, the "relay agent", to realize a cooperative power system protection system, which consists of distributed equipment combined with a communication network to cooperate in the realization of adaptive protection functions. Relay agents move between equipment to utilize data and functions distributed over the power system. In the paper, relay agents are classified by their roles and protection functions under some typical conditions are simulated. Consideration based on the simulations indicate that the concept enables the protection system to keep an isolated zone minimum against any changes in power system conditions and to secure high reliability of the protection system with less redundant hardware. The new concept "relay agent" will provide a powerful means of advanced protection functions by combination of microprocessor technology and information communication technology.

Time-varying harmonics. I. Characterizing measured data

Abstract: This paper represents Part I of a two-part article which summarizes the probabilistic aspects of harmonics in electric distribution systems. This first part reviews the problems associated with direct application of the fast Fourier transform to compute harmonic levels of nonsteady state distorted waveforms, and various ways to describe recorded data in statistical terms. Each statistical description is applied to a set of recorded data for illustration purposes.