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Even though shunt compensation of transmission lines (TLs) is a common practice to limit the overvoltages stemming from the Ferranti effect, it may cause transient and steady‐state resonant overvoltages when one phase is left open due to the single‐pole‐open condition following the temporary phase‐to‐ground (Ph‐g) fault. Furthermore, the existing shunt reactors potentially increase the duration of the secondary arc current at the fault point and hence prevent offering fast single‐pole reclosing. To exploit the mentioned issues, in recent studies, a neutral reactor is introduced, which is installed in the neutral point of the shunt reactors; nonetheless, obtaining the appropriate amount of it is remarkably challenging. In this paper, an optimal neutral reactor (ONR) is determined based on an optimization problem, which minimizes the neutral reactor cost subject to limiting both resonant overvoltages and the duration of the secondary arc current. In modelling, the Laplace domain is employed rather than the phasor domain to consider both transient and steady‐state conditions since this technique allows having better control over transient overvoltages as well as transient secondary arc. [ABSTRACT FROM AUTHOR]

Mal-operation of distance protection relays during power swing (PS) condition can endanger the power system stability. Therefore, it is crucial to distinguish between short-circuit faults and PS by a power swing blocking (PSB) function to prevent undesired tripping of distance relay. This paper develops a novel and straightforward PSB method based on the rate of impedance angle change (RIAC). Not only can the proposed approach fairly discriminate both symmetrical and asymmetrical faults from PS, but also it can appropriately preserve protection scheme security during PS under single-pole tripping (SPT) on a transmission line. Furthermore, there is no need to perform extensive dynamic stability studies for setting the PSB function parameters. Unlike the rate of impedance change approaches utilized two blinders, the developed approach only needs an inner blinder. Thus, it can be used for networks running near their operational limits. The proposed method is evaluated through a modified IEEE-PSRC benchmark system in PSCAD/EMTC environment. The obtained results verify superior performance of the proposed method to traditional impedance rate of change method under different scenarios. [ABSTRACT FROM AUTHOR]

This article presents a novel straightforward and real-time algorithm to measure power system frequency. The main idea of this approach is to improve accuracy of the estimated frequency during off-nominal frequency conditions. The proposed approach includes two stages: 1) the DFT algorithm is used to estimate voltage amplitude and phase angle by assuming that the frequency is the same as its value at the previous step, and 2) based on Taylor approximation and using the least error square algorithm, the error due to the frequency difference is calculated and thus the estimated frequency would be updated. One of the main features of the proposed approach is that it can be easily implemented on the available industrial hardware requiring low computational burden. The proposed approach is also robust against noise and harmonics and can track the deviations of the power system frequency as well. The performance of the proposed approach is evaluated through simulation studies and also verified based on implementing it on a prototype synchronizing relay. [ABSTRACT FROM AUTHOR]

This study describes an actual problem with circulating current at the ground grid of an actual power system that has an air-core reactor. Such reactors generate a large amount of magnetic flux whose path can be closed through its underneath ground grid. This magnetic flux induces a voltage on the ground meshes and thus results in considerable circulating currents that pass through the ground meshes. Indeed, the main novelty of this study is investigating the effect of air-core reactors on the other nearby equipment and considering the air-core reactor's flux underneath ground meshes. This phenomenon occurred in a steel industrial plant and practically leads to severe damages to equipment through which the current has been flowing, including ground grid conductors, cable shields, and foundation armatures. This study addresses the origins of problems raised by the manufactured equipment in a reputable company based on both simulations and experimental studies. Based on the obtained results, several practical approaches are proposed and implemented to overcome the problems mentioned above. [ABSTRACT FROM AUTHOR]

This study describes an actual problem with circulating current at the ground grid of an actual power system that has an air‐core reactor. Such reactors generate a large amount of magnetic flux whose path can be closed through its underneath ground grid. This magnetic flux induces a voltage on the ground meshes and thus results in considerable circulating currents that pass through the ground meshes. Indeed, the main novelty of this study is investigating the effect of air‐core reactors on the other nearby equipment and considering the air‐core reactor's flux underneath ground meshes. This phenomenon occurred in a steel industrial plant and practically leads to severe damages to equipment through which the current has been flowing, including ground grid conductors, cable shields, and foundation armatures. This study addresses the origins of problems raised by the manufactured equipment in a reputable company based on both simulations and experimental studies. Based on the obtained results, several practical approaches are proposed and implemented to overcome the problems mentioned above. [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]

This paper introduces a new approach, based on faster-than-real-time (FTRT) analysis, to predict out-of-step (OOS) condition of a turbine-generator (T-G) unit subsequent to a fault scenario. The proposed FTRT analysis is based on 1) solving a state-space model of the T-G electromechanical system and corresponding controllers without the need for online measurements, and 2) an estimated Thevenin equivalent of the rest of the power system. The measurement-free FTRT analysis is carried out after the Thevenin parameters are estimated subsequent to the fault clearance instant. Equal area criterion is applied on the predicated variables to detect OOS condition. A reliable decision can be made based on this approach, since it does not involve simplifying assumptions contrary to the existing approaches. The FTRT approach accurately considers the impacts of the detailed model of the generator, automatic voltage regulator, prime mover, governor system, and the host power system. This paper provides detailed mathematical formulation for the proposed OOS detection method and verifies its feasibility and accuracy based on single machine and large test systems. The proposed approach is also implemented on an industrial platform and evaluated. The results confirm that the proposed approach can accurately identify OOS conditions, earlier than the rotor angle noticeably increases. [ABSTRACT FROM AUTHOR]

This paper 1) introduces a new approach for dependable and secure detection of loss-of-field (LOF) events based on estimating rotor signals, 2) implements digital algorithm of the corresponding LOF relay in an industrial hardware platform, and 3) evaluates performance of the LOF relay in a real-time-digital-simulator (RTDS) based control-hardware-in-the-loop (CHIL) environment. The main features of the proposed relay as compared with the conventional approaches include 1) capability to discriminate between system disturbances and LOF events, and 2) dependability and higher speed of the LOF detection mechanism. To evaluate the proposed approach performance for various types of LOF events, a detailed excitation system is modeled and implemented in the RTDS-based CHIL environment. This paper also verifies superior performance of the proposed LOF relay as compared with that of a conventional impedance-based LOF relay, using the RTDS-based CHIL platform. [ABSTRACT FROM PUBLISHER]

A new problem formulation for islanded microgrids (MG) is introduced and developed by employing a multi-objective function approach where the objectives include: i) fuel consumption cost, ii) voltage stability index, and iii) total voltage variation). A hybrid optimization algorithm is proposed to solve the proposed algorithm by combining the harmony algorithm (HS), mutation, and crossover operators of the genetic algorithm (GA). To find the best solution for the non-dominated results, a fuzzy logic method is employed. The performance of the proposed approach is compared with those of the other optimization and non-optimization methods in MG using 33-bus test network in a MATLAB environment. The obtained results show that the proposed algorithm is capable of: i) significantly reducing the islanded MG customer interruptions and ii) improving the islanded MGs stability. In addition, selecting appropriate parameters of droop facilitate the successful implementation of the islanded MG concept in distribution systems. [ABSTRACT FROM AUTHOR]

The excitation system of a synchronous generator has a major role in proper operation and control of the generator. Loss of excitation (LOE) may cause harmful effects on the both generator and power system. This paper presents a new approach to detect LOE based on calculating the synchronous generator internal voltage. This parameter is considerably sensitive to LOE and can be used as a straightforward index to discriminate LOE events from system disturbances more quickly and robustly than the conventional indices. A novel and practical technique with minimum computational burden is presented to calculate the generator internal voltage. To evaluate the proposed algorithm, LOE events in a multimachine test system are investigated. In addition, the proposed approach is experimentally evaluated on a 10 kVA synchronous generator. Performed studies demonstrate that the proposed strategy not only noticeably decreases the LOE relay operation time, but also desensitizes its maloperation to system disturbances. [ABSTRACT FROM PUBLISHER]

Concerned with deploying zero-emission energy sources, reducing energy wasted through transmission lines, and managing power supply and demand, monitoring and controlling microgrids have found utter importance. Accordingly, this paper aims to investigate the efficacy of state estimation fusion for a synchronous generator as well as an induction motor in order to ameliorate system monitoring. A third-order nonlinear state-space model, that operates based on actual input data taken from the Smart Microgrid Laboratory, is assumed for each of the electrical machines. The model parameters are set according to the parameters of the electrical machines. A fusion structure based on the internet of things communication network, which is modified to increase uncertainty, is presented for fusing the state estimates. The data fusion topology is distributed and relies on two data fusion models. The first model is a set of state estimators, referred to as data input-feature output model. The second one fuses the estimators' outputs based on supervised machine learning methods, referred to as feature input-feature output model. The simulation results in MATLAB and Python show the efficiency of linear regression methods compared with other leveraged methods for data fusion. By comparing the results obtained from both simple and complex estimation filters, it can be deduced that combining simple filters, extended Kalman filter in this case, with simple data fusion methods, linear regression in this case, can produce much more accurate results in a short period of time. Besides, this study shows that the averaging operators are unsuitable for estimation fusion by referring to their convexity condition. • Regression methods can outperform other methods for the purpose of data fusion. • Simple filters combined with simple fusion methods boost state estimation accuracy. • The averaging operators are unsuitable for estimation fusion due to performing convex fusion. [ABSTRACT FROM AUTHOR]

Faults in the power system should be detected as soon as possible. Otherwise, some defects in the protection system can result in imposing considerable destructive damage to the system equipment. The excitation system of a synchronous generator plays a significant role in the proper operation and control of the generator. Loss-of-excitation (LOE) may cause harmful effects on both the generator and power system. If LOE is not correctly detected, an unnecessary trip signal may be issued, resulting in the overloading of parallel generators and even a system blackout. Conventional techniques for detecting this fault are based on impedance measurement. Despite the simple structure of these techniques, they are sometimes unable to detect some faults correctly. The presence of flexible alternating current transmission system devices can significantly affect the impedance characteristic of the system and as a result, the operation of impedance relays. In this article, first, LOE and other bugs causing failures and errors in the protection of LOE are simulated in the presence and absence of a unified power flow compensator (UPFC), and parameters with different behavior are determined. Then, the most sensitive parameters are sorted using an intelligent method based on their sensitivity. Once the sort order table is obtained, the first two parameters are applied to a ground data handling neural network method, and LOE is detected. The simulation results show that the proposed algorithm is significantly reliable in detecting LOE faults. Moreover, LOE is detected very quickly in the presence or absence of the UPFC in the system by preventing the tolerance of electrical and mechanical stresses by the machine and system; thus, the maloperation of the protection system is avoided. [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]

• An effective protection method for i) detecting electric faults using current variations, ii) discriminating them from other disturbances based on the transient response of PV cells, and iii) locating the faulty string within PV arrays. • To maximize reliability and ensuring applicability, dynamic and static models of PV cells are both taken into account whilst formulating the method. • A minimal number of measurement sensors will be used for all the functionalities offered. Quality degradation, power loss, and fire hazards are amongst the consequences of uncleared electric faults within photovoltaic (PV) arrays. This paper proposes an effective protection method for i) detecting electric faults using current variations, ii) discriminating them from other disturbances based on the transient response of PV cells, and iii) locating the faulty string within PV arrays with a minimal number of measurement sensors. To accomplish these, both dynamic and static models of PV cells are taken into account whilst formulating the problem. To evaluate the performance of the proposed method, a prototype solar power plant is simulated under a wide variety of normal and abnormal conditions in PSCAD-EMTDC software. The simulation results verify the proposed method's high speed and desirable accuracy. Independency from meteorological data and the need for a minimal number of measurement sensors are the salient advantages of the proposed method. Therefore, the implementation of this method would be affordable for PV systems of different scales. [ABSTRACT FROM AUTHOR]

Small-scale synchronous generators (SSSGs) are widely utilized as distributed generators in power systems. A large fault current due to the SSSG phase-to-ground short circuit may result in excessive damages to the stator core and windings, in a way that major repairs, such as core disassembling and rewindings, become necessary. Thus, the fault current should be properly restricted to prevent such damages and reduce the required capital and time for SSSG repairs. In this research work, a model to calculate the SSSG internal fault current is developed. Meanwhile, an appropriate index to quantify the damages to the faulty SSSG is presented. Afterward, influencing parameters on reduction of the damages are investigated when either frequently used TT or TN grounding systems are applied for SSSGs. Studies show that the stator damages are mainly caused by the SSSG fault current rather than that of the system. Accordingly, to reduce the SSSG vulnerability to the stator earth fault, not only some applicable strategies are proposed for the both grounding systems, but also the favorable grounding system is determined. [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 transformers are of the most critical and expensive equipment in the power system industry. Meanwhile, they are exposed to a variety of faulty and abnormal operating conditions. Therefore, to prevent extension of these events they are protected by different protective schemes. Among the various types of the transformer faults, detecting turn-to-turn faults is the most challenging one for the protection systems. Industrial experiences and standard documents reveal that the existing protection systems include severe shortcomings, and still more efforts are required to offer a reliable method for detecting such faults. To this end, this article puts forward the proposal of a novel turn-to-turn fault detection method to satisfy the basic protection requirements of dependability, security, and speed. This method properly detects the low-level turn-to-turn faults while is stable during system transients and external faults. Performance of the proposed method is evaluated through various experimental and simulation studies. The obtained results verify superior operation of the proposed method under different faults and conditions. [ABSTRACT FROM AUTHOR]

• Suggesting an innovative probabilistic framework for modeling and evaluating the protection systems. • Proposing new probabilistic models for fault characteristics and protection system elements based on their realistic behavior. • Determining the most suitable setting value of zone-2 intentional time delay. • Improving the protection system operation without missing security and selectivity. The time delay of distance relay zone-2 has considerable impacts on the protection system performance. Traditional deterministic techniques for adjustment of this parameter only consider the worst cases, even with a low probability of occurrence. This leads to the increment of the fault clearing time for most of the high-probability scenarios and endangers the system's stability. To diminish this issue and achieve a suitable tradeoff between protection system selectivity and operation time, a novel probabilistic framework is proposed in this paper for proper adjustment of zone-2 time delay. This framework includes two major parts; the fault and protection system are appropriately modeled in the first part, and then in the second part, the Monte Carlo method is employed as the computational engine for analyzing the system attributes under different conditions considering local and pilot protection schemes. Finally, two indices are proposed for the quantitative assessment of operation speed and selectivity of the protection system. The simulation studies fairly reveal that the operation speed of zone-2 is significantly improved using the proposed settings. In addition, the obtained results indicate that the proposed probabilistic approach not only improves the dependability, but also does not endanger the security of the protection system. [ABSTRACT FROM AUTHOR]

Here, an optimization framework is proposed for the service restoration (SR) problem in distribution networks based on mixed‐integer second‐order cone programming (MISOCP). The objective function is to minimize (i) curtailed loads, (ii) the number of switching operations, (iii) crew dispatching cost, and (iv) operational cost. A novel approach is developed for the optimal switching crew routing (SCR) to perform the manual switching operations in a short time. Self‐healing actions including (i) network reconfiguration, (ii) load shedding, (iii) adjusting the output power of the dispatchable distributed generations (DGs), and (iv) the optimal tap setting of the voltage regulation devices are adopted in the SR strategy. Both the voltage dependency and the uncertainty nature of loads are modelled. Besides, the switch failure scenario is considered in the proposed model to deal with real operating conditions. Hence, the developed optimization framework offers the most robust SR solution. The simulation studies are performed on an actual 87‐bus system in MATLAB/Yalmip environment by adopting Gurobi solver. The obtained results verify that the proposed model enhances quality of the solution in terms of SR process time compared to the other existing models. [ABSTRACT FROM AUTHOR]

• Determining the proper size of the SCB to minimum risk of the self-excitation. • Detecting and preventing the SCB self-excitation. • Limiting the SCB switching transients. • The proposed strategies for limiting SCB transients are verified using comprehensive simulation studies. Medium voltage shunt capacitor banks (SCBs) are widely used for improving voltage profile and providing reactive power in electrical networks. Transient oscillations caused by SCBs, e.g., switching and self-excitation phenomena, may damage sensitive equipment in electrical networks. This paper provides an analytical description of the SCB transients and proposes efficient techniques to limit such transients, which include (i) determining the proper size of the SCB to minimum the risk of the self-excitation, (ii) detecting and preventing the SCB self-excitation using a novel protection scheme, and (iii) limiting the SCB switching transients by developing some approaches for medium voltage application. The proposed strategies for limiting SCB transients are verified using comprehensive simulation studies conducted on an actual power system. [ABSTRACT FROM AUTHOR]

• The impacts of UPFC on the conventional protection scheme is scrutinized. • Features are extracted based on simulation observance and combined with SOM method. • Classification is carried out with LVQ NN. • Proposed method compared to conventional impedance based approach. • The proposed scheme improves the LOE protection function in presence of UPFC. Loss-of-excitation (LOE) conditions may cause devastating effects on both the generator and power system. If LOE is not meticulously detected, two dangerous events may emerge: 1. unnecessary trip signal, 2. not detecting this abnormal condition or detecting it with time delay. Both mentioned flaws have considerable harmful consequences on the generator and power system. Conventional techniques for detecting LOE conditions are based on impedance measurement. The presence of flexible alternating current transmission (FACT) system devices can considerably affect the impedance characteristic of the system and as a result, the operation of impedance-based relays. This paper simulates LOE and stable power swing (SPS) conditions causing failures and errors in the LOE protection in the presence or absence of a unified power flow compensator (UPFC), and parameters with different behaviors are determined. Then, the most sensitive parameters are combined using an intelligent method. The single parameter, achieved from a combination of features in the previous process, is applied to a learning vector quantization (LVQ) neural network. The LVQ is a type of artificial neural network that is based on a prototype supervised learning classification algorithm and trained its network through a competitive learning algorithm. The simulation results show that, in a single machine infinitive bus (SMIB) test system, R-X directional scheme has 2 mal-operations and the proposed algorithm has no mal-operation without UPFC in the system. With UPFC in the system, the R-X directional scheme has four mal-operations and the proposed algorithm has no mal-operation. In the IEEE 9-bus test system, the R-X directional scheme has five mal-operations and the proposed algorithm has no mal-operation. Moreover, applying the proposed algorithm, LOE is detected very quickly in the presence or absence of the UPFC in the system and this approach prevents electrical and mechanical stresses enforced to the machine and power system. © 2017 Elsevier Inc. All rights reserved. [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]

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]

While in many applications, multiterminal dc (MTDC) systems are potentially appropriate substitutes for their ac counterparts, their protection problems still require more attention. This paper proposes a novel traveling-wave-based methodology for fault location in MTDC systems. The traveling-wave principle, along with two graph theory-based lemmas, is deployed to locate the fault by sectionalizing the graph representation of the MTDC system. Accordingly, the system of equations relating the fault inception time, fault point, and first arrival time at different converter locations would be derived and solved. The method merely needs the first surge arrival times, thereby eliminating the practical problems in relation to identifying subsequent traveling waves. More important, it successfully determines the fault location, regardless of the network topology complexity, that is, the number of its meshes and radial lines. To demonstrate the effectiveness of the method, it is applied to some complicated MTDC systems containing meshes and radial lines. Numerous simulation studies carried out for different conditions verify high accuracy, robustness against fault impedance, and noise immunity of the proposed method. [ABSTRACT FROM AUTHOR]