Search

Basic energy and electricity concepts are often misunderstood by first-year engineering students. One of the main reasons for this is that these concepts are perceived as abstract, making it challenging for students to relate them to real-life situations. Experimental laboratories are designed to help students delve deeper into these concepts. However, the demand for more visual aids by students, coupled with their limited ability to grasp complex ideas, often leads to the perception that some labs are outdated and reinforces misconceptions. To address this issue, this article introduces a novel laboratory workbench and methodology aimed at enhancing basic electrical energy and instrumentation laboratory sessions. The approach utilizes a more visual and interactive platform that enables students to connect electrical concepts with real-life elements. Prior to attending the class, students watch instructional videos that demonstrate the practical procedures, following the Flipped Learning strategy. The results of this study reveal that students felt more confident during the laboratory sessions and exhibited a more active attitude, actively asking questions and defending their viewpoints. Professors also observed that time was utilized more effectively, allowing for a better understanding of topics and clarification of confusing content. Overall, the proposed lab sessions significantly improve the learning experience of first-year Physics students and foster their autonomy in learning. [ABSTRACT FROM AUTHOR]

A synchronized current-comparator bridge for calibrating analog merging units at power frequencies of 60 and 50 Hz is described. Analog merging units take analog signals from instrument transformers, convert them into digital form, pack the sampled values into Ethernet frames conforming to the IEC 61850-9-2 LE specification, and stream the frames to other equipment in a digital substation for further processing. Since the sampled values are used for protection and metering, the merging units need to be calibrated. The expanded uncertainties ($k = 2$) of the described calibration system are estimated to be better than $15~\mu \text{V}$ /V for voltage magnitude, $15~\mu \text{A}$ /A for current magnitude, and $60~\mu $ rad for phase of either voltage or current. [ABSTRACT FROM AUTHOR]

In the frame of the constantly increasing in electric power systems (EPSs) complexity, the challenge of ensuring adequacy of relay protection (RP) devices operation becomes more and more urgent. The authors propose to use the detailed mathematical models of the combination «instrument transformers (IT)—RP» with modern EPS simulators as the solution. It is very important to adequately simulate IT, in particular, the magnetic core magnetization process, because IT largely determines the shape of the RP-controlled signal and affects on its operation. However, in practice simplified models are currently used due to the lack of an accurate mathematical description of the IT core magnetization characteristics. Such models do not reflect all processes in the magnetic core. The aim of this work is to develop a hysteresis mathematical model, which will have a high accuracy in reproducing the magnetization processes of transformer core. The main research method is mathematical modeling of the ferromagnetic material magnetization processes in the MathCAD software. The article presents main development principles of the mathematical model with magnetic hysteresis memory based on the Preisach theory, which reproduces with high accuracy both the major and minor hysteresis loops. [ABSTRACT FROM AUTHOR]

Highlights • Analysis of MV instrument transformers in transient-state. • Overvoltages transferred from primary windings to the secondary ones when a fast voltage impulse excites machines. • Comparison of the results obtained through experimental tests and a theoretical study. Abstract This study illustrates how selected instrument voltage transformers (VTs-I) behave in a transitory state. Some laboratory tests are conducted on medium voltage (MV) inductive voltage types to evaluate the voltage transferred from primary to secondary windings when a fast voltage impulse (1.2/50 μs) excites three machines constructively in different ways. Possible effects of metal parts present in the laboratory on the VTs-I behavior are analyzed, and other tests are conducted to determine a suitable shielding for the VTs-I. In order to interpret the experimental results, a theoretical study is performed using a simplified model of the instrument transformer. Finally, characteristic parameters are assigned values that were derived from experimental tests. [ABSTRACT FROM AUTHOR]

This paper describes the influence of magnetization of an instrument current transformer (ICT) core on ICT errors, and presents a procedure for demagnetizing an ICT. The dependence of ICT errors on the magnetization of the ICT core for different magnetic materials is given in the paper, together with a detailed procedure for ICT demagnetization. The results of experiments are summarized, and conclusions are drawn on when ICT demagnetization is necessary, and on how to prevent the destruction of an ICT due to its winding being punctured. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized method for estimation of dynamic states of a power system. The method remains robust to time-synchronization errors and high noise levels inmeasurements. Robustness of the method has been achieved by incorporating internal angle in the dynamic model used for estimation and by decoupling the estimation process into two stages with continuous updation of measurement-noise variances. Additionally, the proposed estimation method does not need measurements obtained from phasor measurement units; instead, it just requires analog measurements of voltages and currents directly acquired from instrument transformers. This is achieved through statistical signal processing of analog voltages and currents to obtain their magnitudes and frequencies, followed by application of unscented Kalman filtering for nonlinear estimation. The robustness and feasibility of the method have been demonstrated on a benchmark power system model. [ABSTRACT FROM AUTHOR]

State-approved test centres in Germany supplying accuracy tests for instrument transformers must, in the future, provide measurement uncertainty budgets for their quality management systems. In this work, the cause of ratio error and phase displacement of instrument transformers are therefore discussed. The traceability to the national standards of PTB, the attainable uncertainty, and the permitted error limits of test equipment for testing instrument transformers are presented. Finally, an example of an uncertainty budget for a current transformer of the class 0,2 S is given. [ABSTRACT FROM AUTHOR]

This paper presents the study of the propagation of the effects of uncertainties in a typical measurement system for modern power networks. Nowadays there is a strong demand from the electricity industry of simple and ready-to-use solutions for calculating uncertainty in such “new” measurement systems. To this purpose a simplified approach is presented whose results are shown to be in good agreement with those provided by conventional but more complex methods. Two applications, rather simple but very common, are considered in this paper. In the first one, the measurement of a voltage phasor is considered and the measurement chain is made by an low power instrument transformer (LPIT) and a converter used for adapting the level of signals from the LPIT to the inputs of intelligent electronic device. The second application deals with the measurement of active power of a system made by a low power voltage transformer (LPVT), a low power current transformer and a powermeter. [ABSTRACT FROM AUTHOR]

In this paper a wavelet packet transform (WPT)-based algorithm is proposed to develop and improve the backup wide area protection for power systems equipped with phasor measurement units (PMUs). Faults on power systems such as high impedance faults (HIFs) have specific characteristics that include high frequency components that can be extracted using the WPT, which is one of the important methods in signal processing. The proposed idea uses power system voltage and current, measured by voltage and current transformers, respectively, and calculates high frequency information of voltage and current waveforms. Afterwards, the differences of voltage coefficients of each phase at each bus with remote buses are compared with a threshold value to detect HIFs in a power system. In addition, differences of current coefficients of each phase of transmission lines in local and remote terminals are compared with a threshold value for discovering the faulted line. The sum of the current coefficients of all feeders connected to the same bus, for each phase, is compared with a threshold value for discriminating the faulted bus. This algorithm is able to detect the regular fault types. It should be noted that for using PMUs and their time-synchronized systems with high-speed GPS communication links, it is necessary to access the power system's real-time data at local and remote terminals. The presented method is simulated by MATLAB software on the New England Power Network. Simulation results show that the algorithm has sufficient security and dependability. [ABSTRACT FROM AUTHOR]

We consider the following two estimation problems from synchrophasor measurements: first, estimation of positive sequence transmission line parameters and second, estimation of ratio correction factors for instrument transformers (ITs), also known as remote meter calibration (RMC). These two seemingly distinct problems are actually interrelated because incorrect parameters of any one set adversely affects the results of another. Hence, a simultaneous line parameter and RMC approach is proposed. Following extensions and adaptions viz., implicit RMC, explicit RMC, solving an individual line problem, and solving a network level problem are proposed. Open-circuit tests on transmission lines are proposed to improve accuracy in RMC. A decibel-scale-based statistical score is introduced for detection of ITs which require calibration. Extensive simulation results are presented to justify the claims. [ABSTRACT FROM AUTHOR]

Transformer differential protection application has always been a challenge to protection engineers and commissioning personnel, even with older electromechanical and solid-state technologies. The emergence of microprocessor-based protection, where the intelligence is implemented numerically in software algorithms, and the fact that different relay models, even from the same manufacturer, apply different algorithms and compensation techniques and use different terminology and notation, has made it increasingly more complicated to apply the protection and even more challenging to correctly test them during commissioning. This paper reviews how phase differential protection principles are implemented in numerical technology by different relay models, provide guidance in their application, and guide test personnel in performing their work. Test methods are proposed for use with modern test systems that can be applied to test all transformer phase differential protection functions easily and effectively. [ABSTRACT FROM AUTHOR]

Current Transformers (CTs) are prone to saturation due to large amplitude of fault current and existence of decaying Direct Current (DC) offset. Since the CT saturation leads to mal-operation of protective relays, detection and correction of saturated currents is one of the most important challenges in the power systems protection. In this paper, a new algorithm is presented for detection of CT saturation time interval by using the Improved S-Transform (IST). Simultaneously, IST can yield a complete visualization of signal in both time and frequency domains. An index based on instantaneous power of the CT secondary current is calculated using the IST output matrix. Then, the saturation region is estimated by determining the maximum and minimum values of the proposed index in the time domain. Comprehensive simulations are implemented using PSCAD/EMTDC software. Main parameters which have direct effect on the saturation levels are considered in simulation studies. As extremum points of the proposed index are determined for detection of saturation regions, the proposed method is not affected by CT parameters. The obtained results show that the proposed algorithm can precisely detect CT saturation time intervals even in noisy conditions. [ABSTRACT FROM AUTHOR]

Presents a study which examined the technical and economical problems of applying amorphous magnetic cores in medium-frequency transformers for induction heating applications. Core material characteristics; Experiments on a power transformer; Conclusion.

In industrial facilities with on-site generation, there are two major approaches to connect generators to the main distribution busbar. One is to connect generators through step-up delta-wye transformers with low-resistance grounded wye on the busbar side. The second common method is to connect generators directly to the main distribution busbar and use low-resistance grounding or hybrid grounding in the generator neutral. A grounding resistor in the step-up transformer’s wye side neutral or in the generator neutral will limit the ground fault in the main distribution system. When busbar differential protection is used on the main busbar, sensitivity for bus ground faults has to be evaluated to ensure detection of limited low-level ground faults. While step-up transformer grounding resistors can be selected to allow ground faults of higher magnitudes detectable by bus differential protection, the choice of grounding resistor is more complex when generators are connected directly to the main busbar. In such systems, a grounding resistor in the generator neutral limits the generator fault to protect the alternator core from excessive damage. At the same time, busbar differential protection needs to detect a limited low-magnitude ground fault on the bus. This paper discusses considerations to select the main distribution busbar differential protection type, CTs, and relaying scheme to enable detection of busbar ground faults limited by the generator low-resistance grounding resistor. [ABSTRACT FROM PUBLISHER]

This paper aims at the appropriate applications of two important current differential protection criteria by quantitative analyses. Combining theoretical analysis with simulation tests, the overall performances of two typical percentage differential protection criteria are compared in terms of the operation sensitivity and security. First, with appropriate parameter settings, the comparative sensitivity analysis between the phase-current-based criterion and the superimposed-component-based criterion is done. Further, the security of these criteria is compared, and some disadvantages of the criterion based on phase current directly are reported. The performance of the criteria during the current transformer (CT) saturation when experiencing heavy external faults are studied, and some valuable findings are reported as well. With the conclusion drawn from this paper, the applications of both criteria can be arranged more reasonably. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

The present article dwells upon the modern stage in development of current transducers which are a link between controlled (protected) multi-motor drive system on the one side, and its protection, control and automation systems, on the other side. Conducted research makes it possible to demonstrate, on reasonable grounds, main disadvantages of using current transformers, Hall effect sensors and optical transformers of new designs as primary current transducers. Based on the information provided in the article, one can conclude that in the nearest future the most promising type of primary current transducer for relay protection and automation systems (including control systems) are Rogowski coils. Design peculiarities and various implementation schemes of such coils make it possible to greatly extend the application field of diagnostic and protection systems in multi-motor drive applications. The concluding section of the article contains analysis of widely used software for mathematical modeling of primary current transducers. Main advantages of using Micro-Cap program in modeling of current transformers and Rogowski coils are investigated. [ABSTRACT FROM AUTHOR]

In recent decades, conventional electric instruments have already been widely used to monitor the performance of geotechnical structures. However, there are several inherent limitations of electric instruments for engineering including: electromagnetic interference, a large number of cables for multipoint measurement, signal loss in long distance transmission, and poor durability. Since the first Fiber Bragg Grating (FBG) sensor was fabricated in 1978, a significant progress has been made on the commercialization of optical fiber sensing technologies. In 1980s, a fully distributed sensing technology named Brillouin Optical Time Domain Analysis (BOTDA) has been proposed and developed for measuring strain and temperature. In this paper, the authors review previous studies on the development and application of fiber optic sensors. Based on the measured strains, various analysis methods were transferred to required parameters such as displacement, force and pressure which can more directly reflect the safety of geotechnical structures under complex engineering stress condition. [ABSTRACT FROM AUTHOR]

The need for testing at HVDC and UHVDC levels has increased dramatically over the last few years. Typically test objects include valves, cables, line insulators, cable accessories, optical instrument transformers and station post insulators. Some of these tests are dielectric tests and in many cases the tests include also pollution testing, which is a main dimensioning parameter for outdoor insulation in DC. This publication focuses on the experience that STRI has gained throughout years of investigating and testing insulators under HVDC in polluted conditions. Examples from service and field stations and laboratory investigations of artificially and naturally polluted ceramic and polymeric insulators are presented. Examples of practical implementation of laboratory results in design practice are also given. [ABSTRACT FROM PUBLISHER]

The state estimator constitutes the cornerstone of the supervisory control and data acquisition system since it provides the power system operating situation in consecutive time intervals. Furthermore, the output of the state estimator is used by other tools responsible for the monitoring and control of the power system. Therefore, there is a need for the power system state estimator to be as accurate and reliable as possible. The main source of uncertainty that may deteriorate the accuracy of a weighted least squares (WLS) state estimator, provided that the network parameters are perfectly known, is the uncertainty that is encompassed in the measurements. It is well known that the measurement chain is not ideal and this information is passed to the state estimator through the measurement weights. In this paper, the effect of the measurement weights, which are calculated by considering both the standard uncertainties associated with the measurement devices and the instrument transformers, on the WLS state estimator is examined. [ABSTRACT FROM PUBLISHER]

This paper addresses the power system state estimation problem considering the correlations among instrument transformer signals. The correlations among processed measurements are estimated using the Unscented Transformation given the correlation and basic statistical information of internal measurements. In addition, the paper analyzes the impact of these correlations on the estimated states using the IEEE 118-bus test system under various operating conditions and measurement configurations. [ABSTRACT FROM PUBLISHER]

Highlights: [•] NCIT uses the low-power current transformers for metering high-voltage parameters. [•] NCIT allows for a greater accuracy of the captured data even at higher frequencies. [•] The NCIT is equally precise as the conventional voltage and current transformer. [•] Currents and voltages errors correspond to accuracy class of 0.2. [ABSTRACT FROM AUTHOR]

The calibration of the protective current transformer (CT) is of particular importance, since its accuracy at high currents is crucial to the correct operation of the subsequent relay protection devices. Conventional calibration methods have been using an electromagnetic CT which contains an iron core as the standard CT. The iron core is big and difficult to manufacture for high-current measurement, and the serious residual magnetism of the iron core at high currents can lead to excessive measurement errors. This paper proposes a calibration system based on indirect comparison of CT and Rogowski coil, i.e. using an iron-core CT to correct the error of the Rogowski coil at low currents, which may be caused by the position of the current-carrying conductor and so on, and then using the calibrated Rogowski coil as the standard transformer at high currents for its good linearity and wide dynamic range, and there is no magnetic saturation. Since the output of the Rogowski coil needs to be integrated, an improved digital integrator based on direct current (dc) negative feedback is adopted, which can effectively eliminate the influences of temperature drift, time drift and dc offset caused by the analogue circuit. The measurement errors of each part of the calibration system have also been discussed, and the test results show that the accuracy of the system can reach up to the 0.05S Class and the uncertainties are 0.038% for ratio and 0.68′ for phase in the range 500 A to 50 kA. [ABSTRACT FROM AUTHOR]

Abstract: Operational experience from electrical transmission networks has demonstrated that human error represents a key factor in operating inefficiencies, equipment damage, and accidental incidents. The awareness of the importance of human factor has increased significantly over the last few years primarily owing to the fact that some major incidents have had significant human error contributions. The analysis of these incidents has led to recognition of the fact that more information about human actions and errors is needed to improve the safety and operation of electrical transmission networks. Thus, power grid companies are placing renewed emphasis on approaches to prevent the occurrence and limit the effects of human errors. This paper analyzes human errors in the electrical transmission networks of the Romanian Power Grid Company. It is important to understand how human operators supervise these networks and the challenges they face in the monitoring task. In the paper an analysis of incidents related to human errors, over a time span of 10years (2000–2009) was performed. The analysis is based on partitioning the electrical transmission network of Romania using a clustering method. The aim is to provide design engineers with a better understanding of the electrical network operation so that they can respond to the needs of the operators in when new human–machine interfaces for monitoring tasks are implemented. [Copyright &y& Elsevier]

The understanding of high-current events such as short-circuit transients and impulse currents is important for power quality management. The work carried out and presented in this paper is focused on the characterization of commercial current transducers for traceable on-site measurements of 8/20 \mu \s (rise/fall time) impulse currents up to 50 kA peak value with a target uncertainty of 0.1% for the complete measurement system. The investigation method consists of using two current transducers based on different technologies that will detect simultaneously the same impulse current. [ABSTRACT FROM PUBLISHER]

This paper presents two methods for determining the ratio error and the phase displacement of a protective current transformer (PCT) with a transformation ratio of 10 kA/1 A and a measuring current transformer (MCT) with a transformation ratio of 20 kA/5 A. The primary conductor passed by a current (10 or 20 kA) placed in the center of the toroid is replaced by a current loop with ten turns placed in the center of the toroid. In the second case, the conductor is replaced by ten turns uniformly distributed along the circumference of the toroid. The primary current (2 or 1 kA) is measured in both cases using a toroidal current comparator, and the secondary current difference is evaluated by means of a current transformer (CT) error measurement system. The errors were also measured using a CT analyzer. [ABSTRACT FROM PUBLISHER]

Recently, electronic instrument transformers (EIT) meet some problems such as high error and instability in practical application, and some new EITs and the calibration system of electronic instrument transformers (CSEIT) are attracting wide concern. In this paper, some key factors affecting CSEIT error, including sampling synchronization, digital signals processing and error estimation of the calibration system itself, are analysed, and the corresponding solutions are also offered. Sampling synchronization is realized by outputting second pulse and trigger pulse at the same time, and the length of pulses response has been controlled in a short period. Digital signal processing synthesizes some special design such as FIR digital filter, industrial frequency measurement and zero-phase filter to overcome the effects of harmonics and noise, and uses FFT interpolated algorithm based on Hanning window to eliminate the calculation error when fundamental frequency spectrum leakage happens. A novel error estimation scheme of CSEIT is proposed based on the instruments with lower measuring error. Results of the error estimation of the CSEIT itself show that the system can reach the error of 0.01 % in magnitude and 30 second in phase at rated operation state. [ABSTRACT FROM AUTHOR]

In the paper an analysis of the influence of the objective function during the process of metrological optimal design of an electromagnetic measurement system is made. The case study is a combined measurement transformer. The optimal design of the transformer is made by using an original program based on the genetic algorithm. Four different objective functions comprising the main metrological parameters of the transformer (voltage and phase displacement error of the voltage measurement core and current and phase displacement error of the current measurement core) are posed for the optimal design. Two metrological approaches are embedded in the objective functions: 1) the conservative (with certain measurement errors) and 2) the Guide EA-4/02 "Expression of the Uncertainty of Measurements in Calibration" (GUM) statistical approach. The optimal design solutions derived for the different objective functions are compared and discussed. [ABSTRACT FROM AUTHOR]

This paper deals with detailed analyses of a current transformer (CT) and a voltage transformer (VT) under transient conditions. The aim of these analyses is to investigate instrument transformers' behavior during the fault operation in medium voltage networks. Accurate mathematical models of equivalent circuits for CT, respecting nonlinear B-H curve, and VT, respecting parasite capacities of secondary windings, are described. All parameters of defined equivalent circuits are investigated by means of FEM and by measurements. Obtained parameters are used in transient analyses of CT and VT. The simulated results are compared with measured ones on real CT and VT, obviously used in medium voltage networks. On the base of simulated and measured transients of both CT and VT, some important recommendations are given which transformer is more suitable to give a signal for a new fault location method based on triangulation principle. [ABSTRACT FROM AUTHOR]

To equivalently and effectively test the shielding effect of a heavy current transformer (HCT) with shielding coils in a laboratory, an eccentric bus-bar testing method has been used to simultaneously simulate the effect of primary bus bar and adjacent bus bar since 1970s. However, there has not yet been an effective way to determine the crucial parameters, particularly the eccentric distance, which means the distance from the center of an eccentric bus bar to the center of an HCT. This paper presents several analytical formulas to determine the eccentric distance and relevant parameters in the testing method, which may overcome the difficulty at present. The formulas are derived out from a magnetic circuit model and further verified by a finite-element method. Numerical results show that these formulas are effective and easy to use in the testing of an HCT. Most importantly, the source of an equivalent relative error has been also investigated in detail. Furthermore, if the eccentric current, which should be equal to the original primary current, is still too high to be reproduced in a laboratory, auxiliary and compensating windings could be adopted to reduce it. [ABSTRACT FROM PUBLISHER]

The results of an estimate of an algorithm for constructing a mobile instrument for the operational measurement of phase-current strength with an error of 0.2% in high-voltage (greater than 10 kV) circuits without disturbing the couplings in the phase conductors are presented. The problems involved in designing such instruments and possible technical solutions are considered. [ABSTRACT FROM AUTHOR]

Abstract: Modern electronics found in various measuring equipment is sensitive to the effect of transient over-voltages. This paper treats the measurement procedure for the estimation of Finite Impulse Response (FIR) models of voltage instrument transformers, dedicated for the study of fast electromagnetic transients, with the special concern to its influence on the equipment connected to the secondary. A black-box approach is applied, and the model identification is based solely on a single measurement with the impulse excitation. The paper proposes two different procedures for the estimation of the analytical expression of the excitation, based on the parameters of the impulse generator or its estimation using a least-squares procedure. The frequency response of the transformer is used for the design of an initial FIR model, which was further reduced using corrected Akaike information criterion. This way the duration of the transient response calculation is further decreased, and the computation complexity reduced. Once determined, the FIR model allows, through the digital filtering operation (closely related to the concept of recursive convolution), a very easy time-domain calculation of the system’s response at any excitation. [ABSTRACT FROM AUTHOR]

Transformers are the main components in distribution systems that failure of them has many disadvantages for the system. The reason of most of these failures is increase of transformer's loss due to increase of harmonics particularly, current harmonics and environment or Ambient temperature. These losses cause increase of temperature in different parts of transformer especially; its hot spot and increase of this temperature can reduce useful life of transformer's insulations. Therefore, be aware of harmonics values and contents, reduction of loading capacity of transformer and also, calculation of transformer's hot spot led to approximate remained life time of transformer seems essential. In this paper, additional to evaluation of equations related to loss and estimation of life time of transformer, measuring and sampling of a 400 kVA transformer's current and voltage has been done. Then, losses, remained life and load ability of that have been estimated under these situations according to equations and by using MATLAB/SIMULINK software. [ABSTRACT FROM AUTHOR]

Abstract: For better performances, electronic instrument transformers are used to replace conventional electro-magnetic inductive current or potential transformers based on ferrous cores and windings to measure heavy current and high voltage in power system. But unpredictable stability, high price and unsatisfactory life hold back their industrializations. Employing the simple but reliable insulation structure of conventional transformer and the unconventional sensing methods of electronic transformers at the same time, an innovative combined electronic instrument transformer is presented in this paper. Performances of the prototype are estimated through relative tests, and the novel design has low price, high accuracy, satisfactory stability, very wide frequency response and long operation life, and therefore is well adapted to any voltage level up to 750kV. [Copyright &y& Elsevier]

In the paper a thorough electromagnetic transient analysis of a combined instrument transformer will be given. A CAD procedure for transformer optimal design by using a genetic algorithm original program will be accomplished. The transient analysis will be fulfilled for the initial and optimal design as well as for a realised transformer prototype. The transient analysis will be non-linear, since the results from previous FEM-3D electromagnetic field study as well as the non-linearity of the two transformer measurement cores will be incorporated. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new control scheme for multi-terminal HVDC (MTDC) systems to improve the performance during the system transients such as AC line faults. The new method is called parallel multi-proportional-integral (MPI) control scheme. This new control scheme provides wide range of controller parameters and has the ability to response quickly. To evaluate the performance of the proposed control scheme, a four-terminal HVDC system is considered with the detailed modeling of converters, commutation transformers, source impedances, line parameters, and filters. Simulation results from various transient conditions are compared between the proposed controller, the fuzzy logic-based auto-tuning PI controller, and the conventional fixed-parameter PI controller. The SimPowerSystems and the fuzzy logic toolboxes in the MATLAB/SIMULINK software package are used to conduct the simulations. Simulation results show that the MPI controller has better performance than the conventional fixed-parameter PI controller and has very similar responses compared to the fuzzy logic-based auto-tuning PI controller without any added complexity in controller design procedure for a MTDC system. [Copyright &y& Elsevier]

Partial discharge (PD) classification in power cable accessories and high voltage equipment in general is essential in evaluating the severity of the damage in the insulation. In this article, the PD classification was realised as a two-fold process. Firstly, measurements taken from a high-frequency current transformer (HFCT) sensor were represented as features by means of a transformation to the classifier and secondly, the probabilistic neural network (PNN) classifier itself was capable of effectively recognising features coming from different types of discharges. The feature that was used as a fingerprint for PD characterisation was extracted from the moments of the probability density function (PDF) of the wavelet coefficients at various scales, obtained through the wavelet packets transformation. The PNN classifier was used to classify the PDs and assess the suitability of this feature vector in PD classification. Four types of artificial PDs were created in a high voltage laboratory, namely corona discharge in air, floating discharge in oil, internal discharge in oil and surface discharge in air, at different applied voltages, and were used to train the PNN algorithm. The results obtained here (97.49, 91.9, 100 and 99.8% for the corona, the floating, the internal and the surface discharges, respectively) are very encouraging for the use of PNN in PD classification with this particular feature vector. This article suggests a feature extraction and classification algorithm for PD classification, which when combined together reduced the dimensionality of the feature space to a manageable dimension, and achieved very high levels of classification. [ABSTRACT FROM AUTHOR]

This paper presents an extension of real-time tests of a wavelet-packet-transform-based technique for the differential protection of three-phase power transformers. The proposed technique is implemented using a DS1102 digital signal processor board and tested on two different three-phase power transformers with neutral resistance grounded. Different magnetizing inrush and internal fault currents are investigated with current transformer (CT) saturation for different loading conditions, including capacitive loads. The results show a complete independence from transformer parameters, load types, grounding method, or CT saturation. Furthermore, the proposed technique has high speed, good accuracy, small required memory, and reliable responses with reduced computational burden. In all cases of the investigated internal faults, the proposed algorithm is capable of identifying the fault and generating a trip signal in less than a quarter cycle based on a 60-Hz system. [ABSTRACT FROM AUTHOR]

A hybrid algorithm for phasor estimation is proposed that is immune to dc offset and current transformer (CT) satura- tion problems. The algorithm utilizes partial sum (PS)-based and multistage least-squares (MLS)-based methods before and after CT saturation is detected, respectively. The MLS-based method is initiated when the third difference of the secondary current detects the start point of the first saturation period. The determination of each saturation period is based on the sum of the secondary current from the start point of the first saturation period. A least-squares (LS) technique estimates the dc offset parameters from the single-cycle difference of the secondary current in the unsaturated periods. Removal of dc offset from the secondary current yields the sinusoidal waveform portion. Finally, the LS technique is used once again to estimate the phasor from the sinusoidal waveform portion. The performance of the algorithm was evaluated for a-g faults on a 345-kV 100-km overhead transmission line. The Electromagnetic Transient Program was used to generate fault current signals for different fault angles and remanent fluxes. The performance evaluation shows that the proposed algorithm accurately estimates the phasor of a current signal regardless of dc offset and CT saturation. The paper concludes by describing the hardware implementation of the algorithm on a prototype unit based on a digital signal processor. [ABSTRACT FROM AUTHOR]

This article describes the use of an improved top oil temperature rise model of a distribution transformer. The IEEE Standard C57.91-1995 model for transformer thermal calculations is discussed briefly, and a loss of life calculation based on the “10-degree rule” is formulated. This loss of life formulation is used with the improved model for top oil temperature rise to assess the impact of ambient temperature change on distribution transformer life. A probabilistic model for ambient temperature rise is used, and an increased ambient temperature of 3.5°C in 100 years is studied. Seven artifact transformers and four loading test beds are considered. The service life calculated for a base case and an increased ambient temperature case shows that there is a reduction of the order of 3-6 years for the cases studied. [ABSTRACT FROM AUTHOR]

This article presents an extensive survey of current research on the transformer loss problem, particularly from the view of practical engineering applications. It reveals that the transformer loss problem remains an active research area. This article classified the transformer loss problem into three main groups: (a) tank losses due to high-current bushings, (b) losses in transformer core joints, and (c) stray losses in the transformer tank. It is based on over 50 published works, which are all systematically classified. The methods, the size of transformers, and other relevant aspects in the different works are discussed and presented. [ABSTRACT FROM AUTHOR]

Relations for determining the optimum frequency step when measuring the conductance of a piezoceramic element are presented. An instrument-attachment to the frequency-characteristic analyzer is developed, which enables the piezoelectric modulus to be measured and monitored. The value of the piezoelectric modulus is converted into a dc voltage, transmitted to a digital indicator and is used to control the technological equipment. [ABSTRACT FROM AUTHOR]