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Transformer inrush current occurs due to the saturation of magnetic core. The magnetic characteristic of the core directly affects the inrush current waveform. Second harmonic and higher‐order harmonics are present in the inrush harmonic spectrum. A harmonic‐based method analysing the amplitude and phase of different harmonics is presented here. Based on the proposed method, the differences between the harmonic spectrum of new low‐loss and previous‐generation materials are investigated. Then, using the patterns of 1st to 10th harmonic phases, a novel method for inrush current detection is proposed. In the proposed method, possible patterns for the harmonic phases related to the inrush current are specified. The harmonic phases are compared with those specified patterns. In the case of matching, the inrush current condition is detected. To evaluate the theoretical principles, simulations are performed, also, the proposed method is applied to the field data to assess the practical performance, and results are presented. [ABSTRACT FROM AUTHOR]

Electrical Arc Furnaces (EAFs) consume time‐varying power. This causes mechanical torque variations in the nearby small scale synchronous generators (SSSGs) and reduces their life expectancy. In this paper, an appropriate EAF model is implemented and verified by using field measurements. Moreover, an actual distribution system to which an SSSG is connected to supply industrial loads including an EAF is investigated based on time‐domain simulation studies. To enhance dynamic performance of the SSSG adjacent to an EAF load, two novel approaches are presented in this paper. The first method is utilized for proper adjustment of governor controller parameters. Analytical and simulation‐based studies are accomplished to determine proper parameters of the SSSG conventional governor controller to mitigate the mechanical oscillations caused by EAF. These comprehensive investigations reveal that the governor PI controller proportional parameter should be adjusted as small as possible to mitigate the SSSG oscillations; while, the oscillations are fairly independent of integrator parameter. Besides, the integrator parameter should be set as high as possible to improve the SSSG dynamic performance. Moreover, a new modified SSSG governor controller including steady‐state and transient control loops is developed and a novel control strategy is proposed to enhance steady‐state and transient performance of SSSG. Finally, robustness and effectiveness of the proposed controller and its parameters are investigated using field measured EAF current signals for various transients. It should be noted that the proposed strategies of this paper can be also applied to similar cases where a nonlinear time varying load is connected to an SSSG. [ABSTRACT FROM AUTHOR]

An effective robust fuzzy model predictive control (RFMPC) method for secondary voltage control in islanded microgrids (μGs) is presented here. In contrast to the existing techniques, which require a detailed model of μG and an ideal communication network between μG central controller and primary local controllers, RFMPC is synthesized for a non‐linear model of the μG with various time delays, uncertainties, and bounded disturbances. The famous Takagi‐Sugeno fuzzy approach is adopted to approximate the inherently non‐linear model of μG by locally linear dynamics. The Lyapunov–Razumikhin functional method is exploited to deal with time delays. In this regard, sufficient conditions are provided in the form of linear matrix inequalities (LMIs). Then, a sequence of control laws corresponding to a set of terminal constraints is computed offline. Doing so, the online stage is reduced to solving a convex problem with LMI constraints considering the sequence of constraint sets obtained in the offline stage, thereby reducing the computational burden significantly. Robust positive invariance and input‐to‐state stability property concerning communication network deficiency are then speculated. The effectiveness of the proposed RFMPC is verified via a comprehensive suite of simulations in the MATLAB/SimPowerSystems environment. [ABSTRACT FROM AUTHOR]

Unsecured operation of backup distance relay during an external fault results in removing a needed energy link and can jeopardise overall system stability. This problem that is well known as distance relay overreach is very likely during coupling capacitor voltage transformer (CCVT) transients mainly under high system impedance ratio (SIR) conditions. To tackle this challenging problem, a new methodology consisting of three stages is proposed in this study. It is capable of solving the overreaching problem which occurs under high SIR conditions, without sacrificing the operation speed of distance relay for inside the first zone faults. Comprehensive simulation studies are carried out to test the proposed methodology. The obtained results demonstrate its feasibility and effectiveness for practical applications in the power system. [ABSTRACT FROM AUTHOR]

This study introduces a predictive auto‐reclosure scheme based on faster‐than‐real‐time (FTRT) analysis for synchronous generator type of distributed generation (DG) units to decrease DG oscillations after successful reclosure and also enhance transient stability margin of the unit after unsuccessful reclosure. The FTRT analysis, as a new concept for the protective relaying, represents a powerful predictive auto‐reclosure scheme which is based on a state‐space model of the DG unit. After a fault scenario and the first operation of the associated breaker, the FTRT analysis predicts energy level of the DG unit and determines an optimum dead‐time corresponding to the minimum energy level of the DG unit for reclosure that can provide an appropriate margin for transient stability. The effectiveness and accuracy of the proposed method are verified based on a test system. The results confirm that the proposed approach can effectively enhance transient stability for unsuccessful reclosure and reduce the corresponding oscillations after reclosure. [ABSTRACT FROM AUTHOR]

Load dynamics play an important role in the voltage stability. Grid-side short-circuit faults may induce severe voltage drops at motor load buses. Once the fault is cleared, depending upon its clearing time, the load bus voltage may either return to its permissible range or degrade gradually until it collapses. From the short-term voltage stability viewpoint, the former is a stable phenomenon, while the latter is an unstable circumstance and the load should be shed in order to prevent voltage instability from being spread over the grid. To cope with this problem, this study presents a new under-impedance load shedding (UILS) scheme. A quadratic distance relay is used as the load shedding element. The trajectory of load impedance is monitored by the proposed UILS relay. Following the grid-side fault clearance, if the impedance trajectory enters the load shedding zone and remains inside it for a predefined time, voltage instability is detected and the load is curtailed by the UILS relay. The proposed approach is tested on various networks, including a real power system model. The obtained results approve the efficiency of the proposed UILS scheme. [ABSTRACT FROM AUTHOR]

Single‐phase neutral reactors are used in shunt compensated transmission lines (TLs) to accelerate the extinction of the secondary arc through suppression of the arc current. Application of similar shunt reactors with the same capacities is prevalent in the transmission systems. Using the traditional method, however, for each TL, a specially designed neutral reactor is needed that could be in some cases extremely large and practically infeasible. This study investigates the feasibility of application of a properly designed neutral reactor, which is independent of the line length. This solution only depends on the ratio of positive‐ and zero‐sequence capacitances of the line as well as the shunt reactor inductance. Accordingly, for all TLs at the same voltage level equipped with shunt reactors of the same capacities, the proposed neutral reactance would be almost identical which helps to minimise the cost and design complexity. Meanwhile, the required insulation level would be kept sufficiently small for any compensation degree. Based on the steady‐state analysis and time‐domain simulations conducted using the electromagnetic transient program, it is shown that for the majority of TLs, it is possible to extinguish the secondary arc within an acceptable short period of time by applying the proposed neutral reactor. [ABSTRACT FROM AUTHOR]

This paper presents a new algorithm for discrimination of short-circuit faults from power system stressed conditions. The proposed algorithm is based on the combination of distance relay zone-3 operation and traveling waves generated by either the fault or circuit-breaker switching. By pick-up of the distance relay zone-3, the algorithm looks for traveling waves arriving from the fault point in the first stage. If a fault generated traveling wave is detected, it would be concluded that a fault has occurred. Otherwise, the algorithm enters the next stage and waits for arriving traveling waves caused by open-switching of the faulted line remote circuit-breaker. If any traveling wave is detected, the system is under fault condition. Otherwise, the reason of zone-3 pick-up is due to a stressed condition. Training a decision tree-based classifier with the proposed features and simulating the method demonstrate that the proposed algorithm is able to provide a reliable remote back-up scheme for protection of transmission lines. [ABSTRACT FROM AUTHOR]

This study presents an adaptive phasor estimation approach for fast numerical distance protection. The proposed method is based on signal condition assessment and choosing the proper phasor estimation method. Effects of errors caused by decaying DC component and coupling capacitor voltage transformer transients, as the main problems for distance relay impedance measurement procedure, are alleviated using the proposed adaptive method. Meanwhile, a new short window DC removal method is introduced. Thanks to the fast DC value estimation method, a short window phasor estimation method is utilised for current phasor estimation. Using the proper phasor estimation method based on voltage and current signal conditions results in more accuracy, fast decision making procedure and consequently a more reliable protection scheme. To verify the performance of the proposed method, numerous simulation studies are carried out. The results assert that the proposed adaptive method not only accelerates the relay operation, but also improves the relay accuracy which enhances numerical distance protection from security and dependability viewpoints. [ABSTRACT FROM AUTHOR]

Loss of excitation (LOE) event threatens both the generator and power system stability. Numerous approaches have been proposed for LOE detection, most of which use the impedance trajectory. Such methods may maloperate under system disturbances, especially power swings. This study presents a new approach for LOE detection by estimating the field flux linkage participation (FFLP) in the stator flux linkage. Intrinsically, this newly defined index mainly depends on the generator excitation system and thus highly reduces when an LOE takes place. The FFLP parameter can easily be calculated using stator voltage and current signals. All kinds of LOE events cannot be studied using most available simulation software. To cover all of them, utilising state variables, a generator model connected to a power system is implemented. Performed simulation studies demonstrate that the proposed strategy not only considerably decreases the LOE relay operation time, but also improves the LOE relay security against system disturbances. In addition, the proposed approach is experimentally evaluated on a 10 kVA synchronous generator. [ABSTRACT FROM AUTHOR]

This study describes a new fault-location technique using negative-sequence voltage for three-terminal lines. The ratios between the negative-sequence voltage magnitudes measured at each terminal are utilised to first determine the faulted section and then to estimate the exact fault location within the section. Since the current data is not deployed, the influence of inherent errors of current transformers can be avoided. The proposed method can accurately locate the unbalanced faults, that is, single-phase-to-ground, double-phase-to-ground, and phase-to-phase faults, regardless of the fault resistance and pre-fault conditions and without any need to identify the fault type. The method requires only the negative-sequence reactance behind each terminal which can be estimated by the short- circuit analysis with an acceptable accuracy. Reliability and practicality of the proposed method make it an attractive option to include in numerical protective relays. Simulation experiments with different fault cases reveal the capability of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a fuzzy logic-based fault-type classification algorithm for digital relays is proposed. Using this algorithm, the fault type is recognized amongst 10 types of shunt faults that may occur on transmission lines under different fault resistances, inception angles, line lengths, and system operating conditions. The proposed method uses the phase sequence components of three phase voltages and currents that are available in most of the power system protection relays. An improved technique is used to estimate the current and voltage signals. A fuzzy method is used to identify the type of fault from the current and voltage signals separately and then combines the results to provide more accurate fault-type recognition. This combination in the proposed method leads to overcome the uncertainty associated with each individual signal. In addition, the fuzzy method is rearranged for ease of implementation in the digital relays while maintaining the acceptable results. To test the performance and robustness of the proposed algorithm, various simulation studies have been carried out under wide variations of the parameters. The algorithm is also tested using some real test data recorded from a high-voltage power system. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This study presents a technique to identify current transformer (CT) saturation based on waveshape properties of the CT secondary current. First, a variable‐length window, designated as the symmetry assessment window (SAW), is defined. Certain distinctive features of the distorted and undistorted currents inside the SAW are extracted by defining two variance functions. Using the extracted features, a new criterion is introduced to detect CT saturation. The proposed method is unaffected by fault current characteristics and CT variable parameters, such as CT burden, core remanence and its magnetising curve. One major contribution of this method is allowing cheaper measurement systems for busbar protection by easing the relay CT requirements. Analysis of real power system fault currents in a real‐time manner using a hardware setup is accompanied by PSCAD/EMTDC simulation results to verify the accuracy and high speed of the proposed technique in various conditions. [ABSTRACT FROM AUTHOR]

Following large disturbances, the conventional under frequency load shedding (UFLS) relays may not operate properly, as they are designed to merely preserve the frequency stability of the system, independent of its voltage stability. In this study, an adaptive load shedding scheme is proposed to maintain the power system stability when the conventional scheme fails. In doing so, the frequency and its falling rate along with a properly defined voltage falling rate are exploited to determine closeness to the disturbance area and its severity. Accordingly, a proper amount of load is shed from the locations with the highest influence on the system stability. The proposed scheme exclusively utilises locally measured quantities and does not require a communication link. The associated logic is practical and can be easily appended to the existing UFLS relays to enhance their performance. To demonstrate the efficiency of the proposed UFLS scheme, it is applied to two large‐scale power systems where the exact load model is taken into account. The obtained results verify that a large number of blackouts because of inappropriate operation of conventional UFLS relays can be prevented using the proposed scheme. [ABSTRACT FROM AUTHOR]

Power transformer differential relays may incorrectly operate due to inrush condition or transformer external faults lead to current transformer (CT) saturation. In this paper, a new technique is presented to distinguish internal fault current from magnetizing inrush current in power transformers. The proposed technique applies two moving windows which both of them estimate magnitude of transformer differential current using different methods. The first moving window method is based on full-cycle Fourier algorithm (long window) which uses one cycle samples of the current and provides precise estimation. The second one is originated from least error square (LES) method that has five sample point input data (short window). LES method has fast response, but its estimation might not be quite accurate. The proposed technique (which is a combination of two mentioned methods) presents a new criterion for discrimination between internal fault currents and inrush currents using the differences between output values of these two estimators. Meanwhile, obtained results demonstrate precise operation of the proposed algorithm for different conditions such as CT saturation and over-flux condition; therefore, it can be a multi-objective technique for transformer protection. Also, a general scheme is presented in order to improve differential protection. In this paper, a real part of 400/230 kV Iranian national grid is simulated using PSCAD software for evaluating the performance of the proposed algorithm. Also, additional validation of the proposed technique is tested offline using data collected from a three-phase, 5 kVA, 60 Hz, laboratory prototype transformer. [ABSTRACT FROM AUTHOR]

SUMMARY To block auto-reclosing operation for permanent earth faults on extra-high-voltage transmission lines, an accurate numerical algorithm is developed in this paper. The earth fault location and its nature (arcing or arc-less) are estimated, only using the sequence quantities of the local terminal and their harmonics. The proposed algorithm calculates the amplitude of arc voltage and compares it with a given threshold value; then, decision is made if it is transient or not. A square wave arc model is considered to represent the long electric arc in the free air. One main contribution of the algorithm is its independence from the remote terminal information such as phase difference and infeed current. The successful simulation results using the square wave and dynamic arc models show that this simple and powerful algorithm can be effectively utilized for implementing a single-pole auto-reclosing scheme. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY: This article presents a new approach for power transformer protection based on a hybrid pattern recognition scheme. The hyperbolic S‐transform (HST) is a very powerful tool for analysis of the nonstationary waveforms because it is able to extract the information from transient signals simultaneously in both time and frequency domains. Magnitude, frequency, and HST contours are the main attributes obtained from the output matrix of HST. At first, differential current waveforms of different conditions such as normal, internal and external faults, inrush, and over‐excitation conditions are analyzed by HST and some potential useful features are extracted from the abovementioned contours. To decrease the dimension of feature vector and to increase the classification accuracy of the proposed algorithm, the most effective features are selected by using well‐known feature selection methods namely sequential forward selection, sequential backward selection, and genetic algorithm. Selected features are trained by a probabilistic neural network as an effective classifier core, which has advantages regarding learning speed and generalization capability compared with feed‐forward neural network. The classification accuracy of the proposed algorithm has been used as a criterion function for the selection of the best subset features. The proposed protection scheme is evaluated for various operating conditions of three different transformers using the PSCAD/EMTDC package. Extensive simulation results show that the proposed algorithm relies only on the waveshape properties, and it is independent of the value of transformer parameters and consumed power. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Summary: A wide‐area backup protection (WABP) method is proposed in this paper based on the zone status of distance relays. The time difference between the operation of zone‐3 and wide‐area based methods makes it possible to employ wide‐area data for improvement of zone‐3. In case of zone‐3 is directly backup for zone‐1, the proposed method can decrease the time delay of zone‐3 with a complete reliability. Moreover, the proposed method is capable of discriminating between the actual faults and power system stressed conditions. Considering the operational time of wide‐area methods, the algorithm could also be applied for zone‐2 backup protection. The proposed method is also developed to consider the availability of wide‐area data for all busses within the disturbance area, as well as data missing for one or two adjacent busses, which are remarkable advantages over the existing WABP methods. Meanwhile, the wide‐area data used in the proposed method are only the pickup/release status of distance relays zones which are binary data and can be communicated readily through existing wide‐area infrastructures. Simulation results on the IEEE 57‐bus test system demonstrate the effectiveness of the proposed method as a promising WABP in improving the performance of zone‐3 in distance protection schemes. [ABSTRACT FROM AUTHOR]