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The proposed technique is based on extracting the windowed wavelet transform of fault generated transients to distinguish between faults in a busbar protection zone from those outside the zone, particularly in case of early and severe current transformer (CT) saturation. Most given techniques are tending to block the differential measurement during the portion of the cycle that a current transformer is saturated. Some other techniques bring more meaning to the breakpoint settings of the operating characteristic. The traditional differential concept is based on the current magnitude for estimating the restraining and biasing values. The significant imbalance comes from the secondary current of a current transformer with a resistive burden which collapses to zero when saturation occurs and it remains at zero until the time when the next zero crossing would have occurred. The study describes a new technique with improved characteristic and the differential concept does not depend on the current magnitude. The wavelet transform (WT) depends on high frequencies of the faulted signal produced owing to CT saturation. The proposed WT is based on Mexican basis function that was found to be an excellent discriminant for identifying the fault signals during the CT saturation. The time and frequency information during saturation are obtained rather than the current magnitude. [ABSTRACT FROM AUTHOR]

This study presents a new support vector machine (SVM)-based fault zone identification scheme, which correctly identifies faults occurring inside and outside the protection zone of a busbar. A 220 kV substation with double busbar scheme present in an existing Indian system is considered for validation of the proposed fault zone identification algorithm for busbar protection. Fault simulations are performed using PSCAD/EMTDC software package and the outcome is then interfaced with MATLAB, where fault zone identification algorithm is implemented. A total of 44 650 internal and external fault cases with varying fault resistances, fault inception angles, fault locations, types of faults and source impedances have been used for validation of the proposed scheme. Even though the proposed scheme utilises only 29.63% of the total simulation cases (63 450) for the training of SVMs, it gives an overall fault zone identification accuracy of more than 90% when tested with using remaining 70.37% simulation cases for validation. The proposed scheme successfully discriminates between in-zone and out-of-zone faults with high fault zone identification accuracy for different fault and system conditions. Moreover, the proposed scheme remains stable during cross-country faults and severe current transformer (CT) saturation condition. [ABSTRACT FROM AUTHOR]

Derivative-based approaches have been used both in the literature and in the industry to identify fault current distortions caused by current transformer (CT) saturation. This study presents a new simple, but efficient, derivative-based technique to detect CT saturation. First a difference function for the samples on the two sides of the current local extremum is defined. Based on the values of this difference function for various current samples, a variance function is defined. By comparing this variance function with a pair of pre-set thresholds, CT saturation can be determined. This technique is not affected by CT parameters such as CT burden, core remanence or magnetisation curve. The independence of this method from the fault current characteristics, such as decaying DC time constant, noise and so on, is investigated. Numerous simulations were carried out using PSCAD/EMTDC program. The performance of this algorithm for simulated fault currents together with a set of experimental results verify the accurate operation of the proposed method for different conditions including mild and fast CT saturations. [ABSTRACT FROM AUTHOR]

When a transformer is initially energised, its iron core is driven into saturation, which will result in high-amplitude magnetising inrush current. For lessening this transient phenomenon, a voltage compensation-type inrush current limiter (VCT-ICL) is presented in this study. The VCT-ICL can automatically insert high impedance to restrain the sudden change in line current. After completing the restriction of inrush current, the VCT-ICL freewheels, and acts as a short circuit. Thus, other than performing the restriction, the VCT-ICL has a negligible effect in the circuit and the transformer seems to connect directly with the voltage source. When a load is connected at the secondary side of the transformer, to adjust the magnitude of DC reactor current so that it is equal to or higher than the peak of line current, there is almost no distortion to the load voltage and current waveforms during the steady state. A 5 kVA single-phase power transformer is used as an example to demonstrate the performance of the proposed limiter. According to the simulated and experimental results, the feasibility of the VCT-ICL has been verified. Moreover, increasing the insulation levels of the related components in the proposed limiter allows it to be applied in high-voltage distribution transformers. [ABSTRACT FROM AUTHOR]

This study proposes a novel pilot protection for transmission lines by using enhanced transmission line pilot impedance (ETLPI). The ETLPI is defined by the ratio of the voltage difference of fault-superimposed component to the current summation of fault-superimposed component at both terminals of the protected line and used to distinguish the internal faults from the external faults. For the external fault, the amplitude of ETLPI is greater than the positive-sequence impedance of the protected line, whereas it is smaller than the positive-sequence impedance for the internal faults. The pilot protection scheme based on the ETLPI has the feature of avoiding the influence of line distributed capacitance and the saturation of current transformer (CT) as well as the transient response of coupling capacitor voltage transformers (CCVT). Therefore the pilot protection is suitable for different length transmission lines. Both theoretical analysis and test results obtained from EMTP digital simulation and the physical simulation have demonstrated that the pilot protection is of high reliability and good adaptability. [ABSTRACT FROM AUTHOR]

This study presents a new support vector machine (SVM)-based fault zone identification scheme for busbar which correctly identifies faults occurring inside and outside the protection zone of busbar. The proposed scheme utilises one cycle post-fault current signals of all the lines as an input to SVM. In order to achieve the most optimised classifier, Gaussian radial basis function has been used for training of SVM. Feasibility of the proposed scheme has been tested by modelling an existing 400 kV Indian busbar system in PSCAD/EMTDC software package. More than 28 800 fault cases with varying fault resistances, fault inception angles, fault locations, types of faults and source impedances have been generated and used for validation of the proposed scheme. The proposed scheme effectively discriminates between in-zone and out-of-zone faults with very high fault classification accuracy for different fault and system conditions. Moreover, the proposed scheme remains stable during an early and severe current transformer (CT) saturation condition giving an accuracy of 99% for all the fault cases. [ABSTRACT FROM AUTHOR]

An exciting current resulted from the hysteresis characteristics of the core causes an error of a measurement current transformer (CT). To produce the high accuracy measurement CT, it is designed to minimise the exciting current. This requires a large cross-section of the core and the high permeability core. If the exciting current can be estimated exactly and compensated, the accuracy of the measurement CT can be improved significantly. This study describes the development of a compensation algorithm for a measurement CT. The proposed compensation algorithm decomposes the exciting current into the core-loss current and the magnetising current. The algorithm uses not the hysteresis curve but the flux-magnetising current curve to minimise the number of required curves for interpolation. The exciting current at every sampling interval is obtained by summing the core-loss current and the magnetising current and then added to the measured secondary current to obtain the correct primary current. The test results using Electromagnetic Transients Program (EMTP)-generated data indicate that the proposed algorithm can improve the accuracy of the measurement CT significantly even with a couple of the curves. The experimental test results indicate that the compensator can improve the accuracy of the CT significantly from 2.5 to 0.1%. [ABSTRACT FROM AUTHOR]

Different from the existing voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system, this article proposes a new voltage source inductive filtering converter (VSIFC)-based HVDC transmission system. It uses the inductive filtering transformer and the related full-tuned branches to replace the commutating reactor and/or transformer and the high-pass damping filters, which makes the VSIFC-HVDC with more obvious characteristics on integrated design and installation. This article concerns on the converter operating characteristics of the VSIFC-HVDC system, which include harmonic suppression in the AC valve winding of the transformer, and the active and the reactive power (P and Q) operating ability of the VSIFC. The equivalent circuit model and the mathematical model of the VSIFC are established. Based on this, the theoretical analysis is performed. Then, within the PSCAD/EMTDC environment, the filtering and the P and Q characteristics of the new VSIFC and the traditional VSC-based HVDC transmission systems are simulated, respectively. By comparison, it validates the theoretical analysis, and shows that the VSIFC-HVDC not only reduces the polluted area of the harmonic, but also has the better transmitted power ability and the wider reactive power control range. [ABSTRACT FROM AUTHOR]

Protective relay is designed to make a trip decision based on calculation of collected measurements, and thus, the quality of data plays an important role in ensuring the reliability of protective relay. However, abnormity of any part of the data acquisition system (DAS), especially cyber risk, may make the protection system receive fake measurements and misoperate. Besides those efforts spent on the reliability of DAS and cyber security, protective relay itself should be able to tolerate fake measurements and actively minimise the impacts associated with bad data. Based on evidence theory, this study proposes an approach taking advantage of abundant information in digital substations to identify fake measurements and avoid misoperation of protective relay. When protective relay is prone to misoperate after the analysis of fake measurements from several current transformers (CTs) or voltage transformers (VTs), measurements from other CTs/VTs are rapidly collected to indicate whether there is a real fault in the power system. In this way, misoperation because of fake measurements can be prevented and CTs/VTs providing bad data can also be identified. Simulation tests demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

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]

In designing high-current injection transformers (CITs), many factors need to be considered for its compatibility with the internal and external propagation and generation of disturbances within electromagnetic environment, i.e. the magnetic leakage field must be reduced to a level that will make the operation of CIT system safe. On the other hand, inrush current (transient current) may inadvertently affect the operation of the protective relay system and necessitate strengthening of the mechanical structure of the CIT. The inrush current and leakage field do not favour each other, but they are considered as two aspects of EMC considerations. The prior art has only addressed asymmetric winding configuration (S-P-S-P) for inrush current reduction. For comparative purposes, the authors propose a novel hybrid approach that is a composition of auxiliary windings and S-P-S-P structures based on an active shielding concept for EMC improvement. To design an optimal choice of the distributive ratios of S-P-S-P of the main transformer windings, the formulae of the leakage inductance are calculated from the structural parameters of the transformer. Hence, as a case study, there comes the need for advanced numerical techniques for correctly calculating the leakage flux distributions and EMC considerations within a 25 kA CIT enclosure. [ABSTRACT FROM AUTHOR]

Network and protective current transformer designers at present are especially interested in the TPZ class current transformers. The behaviour of protective current transformers in a transient state is very important because it influences the proper functioning of the protection system. The basic problem in the design process is to accurately determine the instantaneous current error versus time. The application of field analysis to estimate the influence of the type of core construction, especially the distribution of air gaps, on the transformation errors of the TPZ class protective current transformers is presented. The transient state analysis, as well as the harmonic steady-state analysis, of protective current transformers was carried out using the field-and-circuit method. The 3D analyses were performed for the magnetic cores of current transformers with different numbers of air gaps. The computational results were compared with the test results. The presented application of the field-and-circuit method using professional software offers constructors the possibility of determining the properties for designing the protective current transformer during the design process before making the prototype. The investigation results presented enable one to estimate the influence of the number of magnetic core air gaps on the effective total length, which is of the highest importance for the design of linearised magnetic circuits, applied not just in current transformers. [ABSTRACT FROM AUTHOR]

A two-step optimisation procedure for the design of a passive shield aimed at decreasing the low-frequency magnetic field using a transformer is presented. After having introduced the relevant characteristics of the medium size three-phase transformer, the procedure is described in detail. It is based on the decomposition in the harmonic component of the radiated field, which is associated with the direct, inverse and homopolar sequences of the current of the transformer. The effect of the optimised shield is verified by measurements. [ABSTRACT FROM AUTHOR]

The conventional current transformers (CTs) in the form of a bar primary and secondary winding on a highly permeable ring core suffer from ratio error and phase angle errors, which are primarily due to the no-load current of the transformer. A modified design of a CT has been described. In this design, a feedback technique has been used in order to reduce the no-load current to a negligibly small value so that the said error may be minimal. Using this technique, a CT has been designed and fabricated with the commercially available material. The performance of the transformer has been tested experimentally and the output and input AC waves have been observed in a storage cathode ray oscilloscope (CRO) and the experimental results are reported. [ABSTRACT FROM AUTHOR]

Advantages of using a current transformer (CT) for current sensing in switched mode power converters are that galvanic isolation is inherent, losses are low, the bandwidth is high and a high-amplitude output signal may be obtained. However, ‘droop’ occurs as a result of the magnetising current drawn. Peak current droop is defined as the instantaneous per-unit shortfall in sensed current at the end of a rectangular pulse. Average current droop is defined as the per-unit shortfall in the average current sensed. The CT inherently operates in a resonant mode when sensing a unidirectional current pulse. This is advantageous as some of the current–time product lost to the magnetising branch may be recovered thereby alleviating the average current droop. Average current droop is investigated when diode rectification is used. Three operating modes are identified and described. These are designated the discontinuous magnetising current, continuous magnetising current and discontinuous secondary current modes (DSCM). It is shown that the CT's core losses may predominately influence the average current droop. Provided the DSCM is avoided, simple correction factors are shown to be appropriate for substantially correcting the sensed current to allow for these losses. [ABSTRACT FROM AUTHOR]

A wavelet-based digital directional relay for power transformer protection, implemented using field programmable gate array. The general purpose SPARTAN3E FPGA kit was employed for developing the prototype relay, with all the coding done using the hardware description language VHDL. The relay logic consists of two parts: disturbance detection based on first-level high-frequency details of the voltage signals and fault discrimination using a power-based directional signal derived from the first-level high-frequency details of both voltage and current signals. The real-time windows target Toolbox of MATLAB was used to apply the current and voltage input signals to the prototype relay in real-time. The logic is deterministic, computationally efficient, fast, secure and highly reliable. The operating time is 6 ms, about one-third of power frequency cycle (20 ms). The scheme uses only the sign of the directional signals, rather than the difference in their magnitudes, hence it can work reliably in the presence of fault resistance and current transformer saturation. The validity of the proposed logic was exhaustively tested by simulating various types of internal and external faults, energisation conditions and load variations on a 132 kV system modelled in EMTP/ATP with a 31.5 MVA, 132/33 kV, Y-Δ transformer. The relay was able to correctly discriminate between internal faults, external faults and non-fault disturbances for the entire 880 test cases. [ABSTRACT FROM AUTHOR]

The design, evaluation and implementation of a busbar differential protection relay that operates in conjunction with a current transformer (CT) compensating algorithm are described. Prior to saturation, the secondary current of a CT is not compensated. The compensating algorithm detects the start of first saturation on the basis of the third-difference function of the current and estimates the core flux at the first saturation start by inserting the negative value of the third-difference function of the current into the magnetisation curve of a CT. Thereafter, it calculates the core flux and then the corresponding magnetising current in conjunction with the magnetisation curve. The calculated magnetising current is added to the measured secondary current to obtain the correct secondary current. The algorithm can estimate the correct current irrespective of the level of the remanent flux. In the proposed busbar protection scheme, a current differential relay with the single-slope operating characteristic is used on the basis of the compensated current of the saturated CT. Test results indicate that the relay shows satisfactory performance for the various external and internal faults with CT saturation, particularly in the case of a progressive fault from a feeder fault to a busbar fault. The algorithm is implemented in a prototype relay based on a digital signal processor. The relay achieves greater stability on external faults, enhanced sensitivity on internal faults and fast operation on internal faults with CT saturation. [ABSTRACT FROM AUTHOR]

The performance of line current differential protection algorithms during current transformer (CT) saturation is analysed. The study covers popularly used algorithms and presents their degree of immunity to CT saturation. The degree of error in phasor measurement due to CT saturation is estimated through simulation studies and this information is used to decide the CT dimensioning for the relay. [ABSTRACT FROM AUTHOR]

Direct current (DC) flowing through the earthed neutrals of transformer windings causes a DC component in the magnetising current. Owing to non-linearity, the waveform of this current is strongly distorted. A method based on the finite element analysis of a transformer is presented that is capable of predicting the waveform of the magnetising current with the voltage of the winding as well as the magnitude of the DC bias given. The relationship between the flux linkage of the winding and the value of the magnetising current is determined, with static conditions assumed. With the aid of this flux–current curve, the dependence of the waveform and thus of the DC component of the magnetising current upon the bias flux is established. Hence, the flux bias yielding the given DC component is obtained by suitable iterative techniques, arriving at the waveform of the magnetising current with the prescribed DC value. Measurement of the current waveform at various voltages and DC bias has been carried out on a single phase transformer. The computed waveforms agree well with the measured ones. [ABSTRACT FROM AUTHOR]

A systematic procedure to synthesise power gyrators for constant switching frequency operation is presented. The equivalence between ideal sliding dynamics and PWM zero dynamics is used to derive the control law of the duty cycle. As a consequence, any power gyrator with stable behaviour in sliding-mode results in a stable equivalent gyrator at constant switching frequency with the same stability conditions. The design of a PWM G-gyrator with controlled output current based on a buck converter with input filter illustrates the procedure. Simulations and experiments are in good agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

The detection of broken-rotor-bar faults based on common steady-state-analysis techniques, such as the fast Fourier transform (FFT), is known to be significantly dependent on the loading conditions of the induction motor. At light load, it is difficult to distinguish between healthy and faulty rotors because the characteristic broken-rotor-bar fault frequencies in the stator current are very close to the fundamental-frequency component and their amplitudes are small in comparison. As a result, detection of broken bar faults and classification of the fault severity under light load is very difficult. To overcome this problem, the analysis of the envelope of the transient starting-current waveform using the wavelet-transform has been investigated. The envelope extraction is used to remove the strong fundamental-frequency component, which would otherwise overshadow the characteristic differences between a healthy motor and a faulty motor with broken rotor bars. The wavelet-transform results are processed to develop a normalised parameter called the ‘wavelet indicator’ which is sensitive to the presence of broken-rotor-bar faults. The results are verified using tests on machines with a varying number of broken bars, as well as partially broken rotor bars, over a wide range of loading conditions. The effects of initial rotor position and supply imbalance are also investigated. [ABSTRACT FROM AUTHOR]

Improving the utilisation of transformers requires that the hot-spot and top-oil temperatures be predicted accurately. Using measured (noisy) data to derive equivalent linear dynamic thermal models yields performance that is superior to the ANSI standard model, but the reliability of these model coefficients must be assessed if the user is to have confidence in the model. By adding arbitrarily large amounts of data in the modelling process it was expected to make the reliability measures of these models arbitrarily small. When this did not happen, an investigation began that showed why there is a limitation to the accuracy of models derived from noisy data. It is also shown that a standard technique for assessing the reliability of model coefficients is invalid because of the absence of unmeasured driving variables. An alternative method for assessing transformer model reliability is provided. [ABSTRACT FROM AUTHOR]

Requirements to avoid current-transformer (CT) saturation result in impractically large CTs. The paper describes the design, evaluation and implementation of a compensating algorithm for the distorted secondary current of the CT caused by saturation. The algorithm can compensate the distorted current irrespective of the level of the remanent flux and is suitable for use with measurement-type CTs for protection. A second-difference function detects when a CT first starts to saturate. At the start of first saturation, the negative value of the second-difference function corresponds to the magnetising current; which is used in conjunction with the magnetisation curve to obtain the core flux. This value is then used as an initial flux to calculate the core flux. The magnetising current is determined by inserting the calculated core flux into the magnetisation curve and added to the secondary current. The resultant compensated current is equal to the secondary referred primary current. Test results indicate that the algorithm can compensate accurately the severely distorted currents caused by the small size of measurement CTs and is not affected by the remanent flux. The paper concludes by describing the experimental test results on a small CT using a prototype compensation system based on a digital signal processor. [ABSTRACT FROM AUTHOR]

Protective current transformers (CTs) are integrated into electrical power systems for assessment of the health of the system. They are intended to provide a scaled-down value of the line current. During major faults, excessive fault currents with decaying DC components could flow through the primary of the CT, causing core saturation and a consequent malfunction of the protection system. This problem is addressed in the paper, and a signal-processing scheme is proposed for fast predetermination of the peak value of the steady-state AC component, instant of occurrence of fault and the time constant of the decaying direct current. Samples of the CT secondary current are collected until the onset of saturation, and are operated upon using a covariance-Prony algorithm to estimate the aforementioned critical parameters. Simulation results in support of the proposed technique are also presented. [ABSTRACT FROM AUTHOR]

When the magnetic circuit of a conventional protection current transformer (CT) enters saturation, the wave-shape of the secondary current distorts due to the increase in the excitation current. The distortion can be detected by analysing the secondary current using an algorithm that evaluates the first, second and third difference functions. The first difference contains points of inflection, which correspond to the start and end of each saturation period. The second and third differences convert the discontinuities at the points of inflection into pulses that can be used to detect saturation. The design and evaluation of an algorithm for detecting CT saturation using the third difference function is described. A low-pass filter, required for anti-aliasing and noise rejection, softens the discontinuities and reduces the magnitude of the pulses seen in the second and third difference signals. The softening effect is pronounced when the cut-off frequency of the filter is reduced. The sampling rate of 64 samples/cycle is used and the currents are passed through a first-order low-pass RC anti-aliasing filter with a cut-off frequency of from 1920 to 480 Hz. Experimental test results clearly demonstrate that the algorithm successfully detects the start and end of each saturation period. Results are obtained when a prototype detector based on a digital signal processor was used on a high current test-rig. Results indicate that the detector can correctly detect when the secondary current signal is distorted and in all the test cases the detector operated correctly. [ABSTRACT FROM AUTHOR]

A power-converter circuit for a switched reluctance motor is proposed. It consists of one switching device per phase and a dump capacitor, auxiliary switch and flyback transformer. In this converter circuit, the demagnetising current of an off-going phase is stored in the dump capacitor and is returned to the DC link capacitor through the transformer. The dump capacitor voltage can be controlled by the duty ratio control of the auxiliary switch. The auxiliary switch is PWMcontrolled in high frequency, so that the size of the transformer can be reduced. The voltage rating of the auxiliary switch and dump capacitor is lower than that of the main switches and DC link capacitor. The modes of operation are described and the effectiveness of the proposed converter circuit is verified through simulation and experiment. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented on the creation of a battery charger by a reader of the magazine during lunch breaks of his apprentice training, with the help of metal plate rectifier, a transformer with various tappings and rotary switch, as of July 2014.

The article reports on developments in renewable energy development in Great Britain citing the operations of the NaREC's Clothier Laboratory and renewable energy center in the northeast. The laboratory has equipment that can test electrical equipment performance under extreme conditions. It also houses a transformer to test electrical equipment for optimum grid operation.