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This article shows practical rules which were developed for the adjustment of Numerical Overcurrent Functions (NOCF) of microprocessor-based protective relays in order to avoid their maloperation due to Transformer Magnetizing Inrush Currents (TMIC). The TMIC waveforms are decaying and nonsinusoidal, and their effects are different on instantaneous, definite-time, or inverse-time NOCF; thus, specific rules are necessary for each type of NOCF. Experimental methods, specifically developed to perform tests of NOCF with TMIC, were applied to four commercial relays from different manufacturers. As each relay has different types of curves, a total of 26 inverse-time curves were included in the set of tests. In practice, the specific behavior of a relay cannot be assumed as known during the coordination of protection; therefore, the rules are based on the worst cases of results (i.e., in order to avoid the incorrect trip of any of the analyzed relays). The input data for the developed rules are the estimated cumulative RMS values of TMIC, assumed as known since there is a long tradition of their estimation. The estimated cumulative RMS values of TMIC, at different time intervals, should be divided by the numerical factors which were specifically proposed for each type of NOCF (to obtain suitable limits to avoid their inappropriate operation). In order to illustrate the application of the proposed rules, the solutions to two cases related to a simple example are shown at the end of this article. [ABSTRACT FROM AUTHOR]

Protective relays in electric power grids recognize the types of electrical faults in a few seconds. The most common detection method to detect the types of electrical faults is based on measuring the angle between the zero and negative sequence currents. However, it is not completely accurate because the phase-to-phase-ground and phase-to-ground electrical faults could have the same detection conditions. Therefore, engineers need to plot the events after an electrical fault to observe the nature of the incidents in detail. In this study, the phase-to-ground fault apparent (PGFA) admittance method with phase/ground boundaries identified the types of electrical faults located in distribution power lines and feeders. This method was based on measuring the PGFA admittance magnitudes for the faulted and nonfaulted phases, resulting in greater than zero and near zero, respectively. The PGFA admittance algorithm was built with MATLAB/Simulink software and tested with signature library and grid simulation events. The PGFA method with phase/ground boundaries was evaluated with the confusion matrix. The measured and predicted values matched in more than 90% of the tests, and the PGFA admittance method with phase/ground boundaries presented an accuracy of 94.3% and a precision of 100%. [ABSTRACT FROM AUTHOR]

Transformation of the distribution network, integrated with Distributed Energy Resources (DER), into a MicroGrid (MG) is an attractive approach in improving the reliability and resilience of the network. In this paper, by introducing the important and necessary aspects of the MG formation problem, an effective methodology for transformation of a distribution network into the MG is proposed. In the presented methodology, optimal allocation of DERs within MG is obtained by optimization of power losses and voltage stabilization along feeders. Moreover, the short-circuit constraints are also taken into account simultaneously while allocating DERs. The reason for the latter is that the integration of DERs alters the amplitude and direction of the short circuit current, which can cause the overall fault current to exceed the designed fault-level capability of the circuit breakers. This issue may put CBs at risk of failure and degrading MG resilience. In this regard, resilience-oriented evaluation of the MG formation, from the perspective of considering short circuit level and coordination of OCRs, is also performed. Another advantage of short circuit studies is devising complete and economic overcurrent-based protection plan for MG in both operating modes (grid-connected and islanded). It should be pointed out that adopting an appropriate control routine is a key element in the MG formation. Hence, a control strategy, based on Energy Storage System (ESS) utilization, is suggested in this paper. This feature, which is another contribution of the paper, will guarantee seamless transition to island mode and stable operation of the MG. The proposed formulation of the MG formation is solved using a modified multi-objective PSO and the Pareto-optimal set is obtained. However, since the optimal solutions are nondominated to each other, a Fuzzy Satisfaction Method (FSM) is utilized to find the optimal solution among the Pareto-set. Utilization of the proposed methodology leads to the successful transformation of the distribution network into a MG, which not only considers the load flow constraints but also preserves the optimal coordination of the protective relays and resilience of the MG. In addition, seamless transition and stable operation of the MG in island mode is achieved. [ABSTRACT FROM AUTHOR]

Summary Unintentional malfunctioning of the distance relay and other protective relays is the main cause of cascading outage and leads to power system failure during power swing. During prevention from power swing, power swing block (PSB) blocks the protective relay where the power system remains unprotected against the faults. Protection challenges such as fault diagnosis with fault section identification for multi‐terminal lines are very complex during PSB operation or failure of protective schemes during power swing where conventional schemes are not able to resolve aforementioned issues. Such complex situations motivate to design the protection algorithm for a three terminal line which identifies the fault section. To deal with this challenge, a communication assisted superimposed positive sequence currents (SPSCs) based technique is proposed to identify the fault section and differentiate the internal and external faults in three terminal lines during power swing. This scheme compares the maximum of SPSCs recorded at three terminals with maximum of estimated SPSCs at T‐point. Estimated SPSCs at T‐point corresponding to each particular terminal are obtained using a long line model ABCD parameter method, which requires voltage and current data measured at respective terminals. The proposed technique has been found accurate and reliable. It works irrespective of types of faults, CT saturation, noise inclusion and reclosing operation of breakers. Its superiority over existing and commercially available schemes are also proven in this paper. It has been tested in a 400 kV double circuit three terminal system and a modified form of 400 kV multi‐machine 9‐bus system using EMTDC/PSCAD. [ABSTRACT FROM AUTHOR]

Summary: This paper aims to present a new linearization approach to finding the optimal protective relay settings in power systems. The traditional protective relay coordination problem (CP) involving directional overcurrent relays (OcRs) is a nonconvex, nonlinear constrained optimization problem that is widely addressed in the literature. In linear programming (LP) based solution methods, the current plug settings (CPS) are fixed, and the time dial settings (TDS) are found, which though reasonably adequate but do not fetch the global solution. In nonlinear programming (NLP), both TDS and CPS are deemed to be decision variables, but NLP requires a proper initial‐point to get the best optimal solution. In this paper, the nonconvexity in CP formulation is convexified using bilinear relaxations, and global optimum is obtained. It is done by writing each bilinear term in CP formulation as a set of four linear inequalities using McCormick envelopes, and the error generated due to the linear approximations is reduced iteratively by updating the variable bounds. The proposed approach is programmed in MATLAB coding platform, and theoretical validation has been performed. The proposed approach has the advantages of being independent of initial‐point and attain the global optimum without the help of global optimization solvers. [ABSTRACT FROM AUTHOR]

Summary: The power system protection scheme must detect faults as soon as possible and isolate the faulty section from the rest of the network in the shortest possible time. Protection in medium voltage distribution networks is usually performed through applying coordinated protective overcurrent relays. Changes in the network topology and operational parameters may cause some uncertainties, which can have deteriorative impacts on the fault current measuring by overcurrent relays, hence lead to in‐coordinated operation of the overcurrent relays. In this paper, it is aimed to optimally solve the problem of the overcurrent relays coordination considering the fault current uncertainty. To this end, a new risk‐averse robust optimization portfolio is proposed based on information gap decision theory to derive the optimistic coordination scheme under severe uncertainties. The presented model is mathematically formulated and implemented effectively to some medium voltage test power systems. The illustrative numerical results demonstrate the effectiveness of the conducted optimization model in both risk‐neutral and risk‐averse strategies in procuring the robust coordination solution of the overcurrent relays. [ABSTRACT FROM AUTHOR]

The number of constraints in coordination of distance and directional overcurrent relays (D&DOCRs) in power systems increases significantly by considering changes in network topology, especially in large power systems. By increasing the number of coordination constraints, the complexity of the D&DOCRs coordination problem is increased. Therefore, in this paper, a new method is proposed to reduce the number of coordination constraints in the D&DOCRs coordination problem. In the proposed method, redundant constraints are identified before solving the coordination problem. The proposed method is independent from D&DOCRs settings and operating characteristics of directional overcurrent relays. In order to evaluate the proposed method, two different test systems, namely, an 8-bus and the IEEE 14-bus test systems, are used. Based on the presented results, it can be seen that the proposed method is efficient, and by utilizing it, many redundant coordination constraints are identified and removed, helping to obtain effective solutions for the optimal settings with less computational effort. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Summary: An exploration of the faulty signal of three‐phase overhead transmission lines can provide versatile information about the presence of fault during power swing (PS) and its type in a specific time domain to accustom protective relaying scheme for real‐time application. In this paper, an illustrative feature extraction technique, maximal overlap discrete wavelet transform (MODWT), has been used to extract the attributes from the faulty signals during PS condition. The standard deviated values of the MODWT coefficients of the current signals only have been used as input features for fault detection and classification in an Indian power transmission network. The proposed scheme is based on a fault triangle with different fault planes. The performance of the proposed scheme has been assessed with real‐time field fault data using "Wavewin" software environment and further appraised with other existing schemes too. Moreover, the proposed scheme is not affected by zero sequence mutual impedance between the two inductively coupled parallel circuits and independent of communication latency affect because it employs one terminal data only. [ABSTRACT FROM AUTHOR]

Summary: Saturation of the current transformer (CT) results in maloperation of current‐dependent protective relays. While enhancing in the core area may prevent the saturation, it simultaneously increases the size, the weight, and, subsequently, the price of the CT, which is desirable for neither the CT manufactures nor the operators. This paper conducts a probabilistic analysis to investigate the impact of influential factors on CT saturation for CT dimensioning. The main aim of the probabilistic analysis is to identify the busses of the power network in which they may be highly exposed to the CT saturation phenomena. The proposed probabilistic framework is performed based on the time‐domain fault analysis. In the probabilistic time‐domain fault analysis, the effects of fault location, fault type, and fault resistance are investigated. Also, the effects of load and generation are considered probabilistically. On the basis of the selected random variables (RVs), the probability density function (PDF) of the voltage of the knee point (VKP) is obtained. Also, a new criterion so‐called depth of saturation (DofS) is introduced that shows the level of exposure to saturation phenomena for each bus. The results show that the proposed probabilistic framework helps to find a general observation to VKP of each CT from the CT‐dimensioning aspect. Also, the proposed probabilistic framework provides some useful information about the probability of saturation area in the power system. [ABSTRACT FROM AUTHOR]