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Ground faults are the main cause of electrical failure in transmission and distribution systems. In ungrounded distribution networks, it is difficult to detect where a first ground fault has happened as only capacitive currents are available. Also, it is important to trip only the faulty line in order to ensure the system's reliability. Directional residual overcurrent relays are the standard solution for these systems, but their difficult commissioning and high costs make it not trivial. This paper introduces a new selective and directional protection method to determine the faulty line in ungrounded medium voltage networks with three or more feeders. The method evaluates the residual currents in the feeders coupled to the same busbars in combination with a reference current created by the sum of a combination of residual currents of all feeders. Both simulation and laboratory results validate this new method, which is easy to start up, as it does not need previous configurations for its implementation, and it presents an economical solution to this problem. [ABSTRACT FROM AUTHOR]

Harmonic blocking (HB)-based differential relays, generally employed in the transformer protection, are likely to fail to interrupt internal short-circuit faults that occur under geomagnetically induced current (GIC) conditions. This paper proposes a waveform-based approach that enhances the differential protection dependability under the GIC conditions by detecting the GIC and overriding the HB signals for the internal faults. A novel algorithm is developed for the GIC detection, which employs three-phase instantaneous differential current waveforms and recognizes a GIC condition if the waveforms are all asymmetric in one direction. The proposed waveform-based approach also utilizes trip request signals of the differential relay's fault detection module to unblock the relay for the internal faults during the GIC. The results show that the proposed waveform-based approach (i) can successfully unblock the differential protection for the internal faults under the GIC conditions, (ii) does not malfunction for the external faults during the GIC, (iii) discriminates transformer energization cases from the GIC conditions, and (iv) is immune to the measurement noise. The current transformer (CT) saturation is also considered in the studies. The proposed approach's effectiveness is verified for different transformer types, including three-phase and single-phase transformers. [ABSTRACT FROM AUTHOR]

The use of power converters as solid state transformers is an attractive solution to modernize the power network, but this solution has not been fully addressed for MV and HV substations. This paper presents a customized and simple control for the Modular Multilevel Matrix Converter (M3C), specially conceived for its operation on synchronous ports, which is the case of AC substations. The control allows bidirectional power transfer, generation/absorption of reactive power and provisions of ancillary services. The converter is compared to the back-to-back Modular Multilevel Converter (B2B-MMC) where the key performance indicators to carry out the comparison are power efficiency, number of semiconductor devices, passive components required, footprint, voltage cell balance, fault blocking capability and stress of components. The simulation results show the features, performance and attractiveness of the M3C topology in a 33/11 kV, 16 MW substation under different operating conditions, including grid faults and dynamic operation. The M3C presents similar efficiency and performance than the B2B-MMC, but it uses fewer semiconductor devices, passive components and total cell capacitor energy than the B2B-MMC, reducing cost and footprint. The experimental results show the performance of the M3C under less ideal conditions including a substation transformer saturation and power step response. [ABSTRACT FROM AUTHOR]

Stray fluxes passing through the current transformer (CT) core produced by adjacent bus-bars and conductors or eccentric installation of primary conductors results in asymmetric magnetic flux distribution along the core. This may lead to exceeding the magnetic flux from its appropriate saturation level in some parts of the core, which is called partial saturation (PS) phenomenon. Under such a condition, CT error increases, and some relays maloperations may take place. This paper proposes a novel analytical approach to design flux equalizing winding (EW) for resolving such a problem. Moreover, PS occurrence in a core balance CT (CBCT) is analyzed using the finite element method (FEM), and its negative influence on a sensitive earth fault (SEF) relay is scrutinized as a case study. Simulation studies reveal that the proposed approach can prevent PS-related undesirable effects in a real power system. [ABSTRACT FROM AUTHOR]

Current transformer (CT) saturation is one of the dominant causes of relay protection devices’ malfunctions, which pose a threat to the safe operation of the power system. To address this problem, we propose a hybrid physical model- and data-driven method. The method firstly detects the CT saturation and then compensates it to reproduce the real waveform. Considering the multi-factor and strong nonlinearity of CT saturation, a data-driven model, namely the Fully Convolutional Network (FCN), is built to detect the operation status of CT. As for the compensation, a physical model of short-circuit current is used for its conciseness and universality. Through tactfully integrating the data model and the physical model, the proposed method is endowed with two major merits: the arduous adjustment of universal thresholds and parameters in existing methods is avoided, and the deficiency in generalization and interpretability of the data-driven method is assuaged. Simulation and experimental results verify the effectiveness of the proposed method. Furthermore, its application potential to future protection is explored. [ABSTRACT FROM AUTHOR]

The single-phase-to-ground (SPG) fault at the valve side of the converter transformer is one of the most critical faults threatening the operation of bipolar half-bridge modular-multilevel-converter-based multi-terminal direct-current (HB-MMC MTDC) systems. Following a valve-side SPG fault, the upper-arm submodule (SM) capacitors in the faulty converter may suffer a serious overvoltage, and the grid-side fault currents may experience no zero-crossings. The existing protection methods generally rely on tripping the grid-side alternate-current circuit-breaker (ACCB), which cannot fully isolate the fault. The healthy poles and stations may encounter severe disturbance and be all blocked, which will lead to a power outage of the entire MTDC system. This paper proposes an ACCB-independent scheme based on thyristor-pairs and damping SMs to address these problems. Antiparallel thyristor-pairs are connected in series within the upper and lower arms in each phase. Serial damping SMs are connected within the lower arms to help the lower-arm thyristor pairs cut off the fault currents. Although the proposed additional devices inevitably cause conduction losses, they can effectively protect the faulty converter and healthy circuit in the MTDC system. Simulations in PSCAD/EMTDC have been conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results. [ABSTRACT FROM AUTHOR]

Bushing plays a significant role in insulation and mechanical support for power transformers. Its working state is closely related to the reliable operation of power transformers. To overcome the electromagnetic interference and security risks during sensor installation, a Sagnac interference optical technique for partial discharge (PD) detection is proposed in this paper. By establishing a simulation model of the sensing structure, it is verified that output signal amplitude varies periodically with the length of sensing and delay fibers. Moreover, the output value decreases with increasing probe radius. The results show that a delay fiber of 1.5 km, a sensing fiber of 3 m, and a bending radius of 5 mm are optimal parameter combinations to detect PD in a 72.5 kV defective bushing. In addition, consistent detection waveforms induced by both internal and external defects are obtained by the optical sensor and high frequency current transformer (HFCT) simultaneously, then the effectiveness of the optical sensor on detecting PD signals for bushing is verified. It is of great help to carry out online monitoring of bushing insulation defects through optical non-destructive installation. [ABSTRACT FROM AUTHOR]

Integration of wind-based renewable energy sources into long Extra High Voltage/Ultra High Voltage uncompensated/compensated power transmission network poses significant problems in terms of proper detection of power swings and effective discrimination between symmetrical faults and power swing situations. As the existing protection strategies are unable to detect the said circumstances, a novel technique, based on the difference between sending end and receiving end positive sequence currents angles of the transmission line, is proposed in this paper. The required data is collected with the help of Phasor Measurement Units placed on both sides of the line. The scheme results in effective discrimination between distinct faults and circumstances of asymmetrical/symmetrical power swing and achieves satisfactory outcome during current transformer saturation condition. The proposed algorithm is evaluated on the wind-integrated IEEE-9 bus system by producing power swings, various cases of faults, and faults during power swings. Validation of the suggested technique was carried out by the execution of hardware-in-loop simulation on a Real-Time Digital Simulator. The achieved outcomes disclose higher sensitivity and better discriminating ability of the presented technique in comparison with those of numerous prevailing methods. [ABSTRACT FROM AUTHOR]

This paper proposes a robust pilot protection scheme based on waveform similarity that restricts the influence of outliers in fault line identification. By comparing two refactoring currents constructed using the time-domain sampling values of currents at the two terminals of a transmission line, a waveform similarity algorithm called the Kendall's Tau coefficient (KTC) is employed to distinguish two different datasets of a protected transmission line. A reliable fault identification is realized under different fault types, distances, and resistances. Proposed criteria that are robust against to current transformer saturation, synchronization error, and noise are proposed. The KTC is based on simply the rank order instead of the metric value and thus limits outliers to a moderate level, ensuring protection safety in cases where outliers that cannot be eliminated completely are included in the current waveforms. The simulation results demonstrate the excellent performance of the proposed robust pilot protection scheme. [ABSTRACT FROM AUTHOR]

This paper presents a time-domain differential protection method based on the Bayes’ Theorem and on the current samples to determine the fault occurrence probability as a function of time. It also describes the theory involved in developing the proposed protection method when considering the differential protection of transmission lines. The authors performed fault simulations using an existing electrical system modeled in EMTP varying the fault type, phases involved, fault resistance, inception angle, and fault location, and carried out a sensitivity analysis regarding high impedance faults, current transformer saturation, noisy data, misalignments between terminal samples and power swings. The authors also tested the proposed method against actual fault events from four existing transmission lines of the Brazilian interconnected power system. All results indicate that the proposed solution is fast, accurate, secure, and dependable. [ABSTRACT FROM AUTHOR]

Converter transformers are one of the most important and costly equipment in High Voltage Direct Current (HVDC) transmission systems. They require sensitive and reliable protections. However, the maloperation of restricted earth fault (REF) protection of converter transformers caused by unloaded energization of converter transformers has been reported. Aiming at this problem, the saturation and measurement distortion of the current transformer on neutral line (CTN) and the resulting maloperation mechanism of REF protection are analyzed firstly. Then, an auxiliary restraint scheme of REF protection of converter transformers is proposed exploiting the similarity between the waveforms of the zero-sequence current on neutral line and the calculated zero-sequence current from measured phase currents under magnetizing inrush and fault conditions. The proposed auxiliary restraint scheme is based on normalized dynamic time warping distance (DTWD), which has strong robustness against CT saturation. The proposed auxiliary restraint scheme is verified by simulations and practical data recorded on-site. [ABSTRACT FROM AUTHOR]

In general, a power transformer is equipped with differential protection relay with the second harmonic restraint criterion to improve reliability. However, this criterion may fail in some cases, resulting in incorrect operation of protection relay. Therefore, to further upgrade the performance of differential protection, this paper proposes a duty cycle-based scheme as complement. Firstly, the waveform of the differential current in one cycle and its two derived waveforms, constructed by translating and taking absolute, are transformed into corresponding square waves, respectively. Then, duty cycles of these square waves are calculated, and three criteria are presented to quantify the differences between the duty cycles of disturbances and faults. Finally, the criteria and the second harmonic restraint are combined through three logic gates to form the proposed scheme for protection. A large number of simulation experiments have verified the effectiveness of the proposed scheme, which outperforms the frequently used protection schemes. [ABSTRACT FROM AUTHOR]

This paper presents an advanced differential protection scheme for protecting transformers during geomagnetic disturbance (GMD) conditions and the flow of geomagnetically induced current (GIC) in the transformer. In this study, the inaccuracy of the existing harmonic blocking (HB) methods of the differential protection is investigated under the GIC conditions. It is shown that there are scenarios in which the HB-based differential relays fail to clear the internal faults since they are blocked due to the GIC-related harmonic currents. By detecting both GIC conditions and internal short-circuit faults, a solution scheme is proposed which overrides the harmonic inhibition signals issued by the HB module and makes the differential relay capable of tripping the transformer. Extensive time-domain simulations within the EMTP-RV verify the effectiveness of the proposed scheme in (i) unblocking the transformer differential protection for the internal faults during the GIC, (ii) preventing the malfunction for the external faults, (iii) distinguishing the transformer inrush and sympathetic inrush currents from the GIC conditions, and (iv) immunity to current transformer (CT) saturation. Furthermore, the simulation results prove that the differential protection equipped with the proposed approach outperforms the existing HB methods under the GIC conditions. [ABSTRACT FROM AUTHOR]

Protection of microgrids at the Point of Interconnection (POI) is a challenging task. Particularly, single-line-to-ground (SLG) faults on interconnection lines are difficult to detect using the fault current detection relays if the microgrid is ungrounded at utility side of its interconnection transformer; such microgrid contributes very low fault current, which disappears once the utility feeder breaker opens. Other common schemes for protection of microgrid interconnection line against SLG faults include Direct Transfer Trip (DTT) and over-voltage relay (59G). The main challenges associated with these methods are installation and maintenance costs of high-speed communication system, sensitivity of relay settings, and delayed fault clearing time due to selectivity requirements. To address such concerns, this paper proposes the use of a distance relay at the microgrid/LV side of the interconnection transformer to provide protection against SLG faults on the interconnection line at the utility/HV side. The designed distance relay executes an enhanced apparent impedance calculation using residual voltage compensation to correctly detect the fault, properly measure the fault location, and timely isolate the fault. In this paper, various scenarios are analyzed using ASPEN and PSCAD simulation tools to demonstrate the effectiveness of the proposed method for protecting the microgrid interconnection line under different fault scenarios. [ABSTRACT FROM AUTHOR]

Inverter transformers are one of the key components in a PV plant and their operating patterns are considerably different from that of transformers in a substation. Moreover, these transformers experience higher level of harmonics generated by the inverters. Therefore, investigating the ageing profile of the inverter transformer is a timely requirement. In this paper, the ageing of inverter transformers in a medium-scale solar PV plant is studied by utilizing various real-time measurements including load current, top-oil temperature (TOT), ambient temperature, moisture in top oil, solar irradiance and individual current harmonics. An updated moisture-dependent thermal model (MDTMH), with consideration of current harmonics is developed to estimate the hot-spot temperature (HST). The influence of weather conditions, which directly determine the load current of inverter transformers on the loss of life (LOL) of an inverter transformer is also investigated. Furthermore, the effect of current harmonics levels on the inverter transformer's HST and LOL is explored. Finally, comparisons of HST and LOL of inverter transformers connected to PV arrays with three different tracking technologies, i.e., fixed-tilt, single-axis and Dual-axis PV arrays are performed. [ABSTRACT FROM AUTHOR]

This paper presents the possibilities of reducing emergency situations and reducing the time of failure in the power grid. The paper contains a detailed description of the method (algorithm) of operation in emergency situations on the example of a Gas-Insulated Switchgear (GIS) located in one of the system-forming (node) stations in the power grid. The level of detail contained in this paper, which has not been previously encountered in previous publications, favors an in-depth understanding of the possible solutions that result from these emergencies. The above method (algorithm) of activities has been developed based on three levels of research: simulation, theoretical and practical. This means that the simulation analysis that was performed based on theoretical information was experimentally tested as part of an emergency situation in one of the system-forming (nodal) stations in the power system. In order to achieve the result described in this article, one of the most promising types of devices used in power systems was selected - the GIS. This has been confirmed by numerous analyzes of the effectiveness of these devices included in this article. The paper contains a detailed description of the individual steps in the field of corrective actions and aimed at minimizing downtime. In addition, this article provides guidance to monitor possible emergencies that are designed to increase the reliability of the power grid. [ABSTRACT FROM AUTHOR]

The inrush current generated by power transformer energization (TE) causes voltage sag of the nearby bus, the typical residual voltage of the voltage sag caused by transformer energization (VSTE) is about 0.7 p.u.--0.9 p.u. and lasts for a longer duration, which may lead to the sensitive equipment trips. This paper proposes an estimation method of voltage sag frequency considering VSTE. Firstly, based on the maximum entropy method, the residual flux distribution is calculated by considering the relationship between the residual flux and closing angle at the moment of TE. The voltage sag waveform caused by TE is calculated based on the estimation of the probability distribution of residual flux, to evaluate VSTE objectively and comprehensively. Secondly, this paper proposes a non-rectangular voltage sag equivalent frequency statistical method based on multiple magnitude-duration (MMD) function, to avoid the overestimation of voltage sag frequency by the traditional methods. Furthermore, this paper proposes an assessment algorithm of voltage sag frequency. Finally, the simulation of IEEE-30 test system verifies the necessity and correctness of the proposed method. [ABSTRACT FROM AUTHOR]

High speed trains supply is, nowadays, performed by 2 × 25 kV power systems. If a ground fault occurs, its location and correction are crucial to minimize downtime. One of the methods for locating ground faults in power systems is the impedance method. This was a good solution for 1 × 25 kV railways power systems, as the ratio between reactance and fault distance is linear. However, the use of autotransformers between the catenary, feeder and rails makes this ratio non-linear in 2 × 25 kV configuration. This work presents a novel ground fault location method for 2 × 25 kV power systems. The new method is based on the ratio of the currents of the autotransformers and the substation transformer. The main contribution, compared to previous methods developed, is that this new method is insensitive to the fault current variations produced by the network voltage fluctuations or by the main transformer tap changer position, among others for 2 × 25 kV power systems. The novel approach has been corroborated by numerous experimental tests in the laboratory and computer simulations, with satisfactory results. [ABSTRACT FROM AUTHOR]

A distributed zero-sequence-current (ZSC) carrier signal injection via current transformer (CT) is proposed for underground cable single-phase-to-ground fault location in neutral isolated medium voltage (MV) distribution networks. High-frequency ZSC carrier signal is directly injected to a high-voltage underground cable of an energized distribution network via CT. The injected carrier signal travels along the cable to the receiving center in the substation, and is processed for quick fault line selection and fault location. No external power supplies or signal generators are needed due to the self-contained nature of the injector. The device can be constructed as a sleeve to be mounted around the underground power cable. This permits economical retrofitting to existing installations of power line communication (PLC) devices. In this paper, the operational principle and the application considerations for such an injection device and fault location tricks are explained, followed by the theory behind the proposed technique. Finally, MATLAB based numerical simulations are performed and a low-voltage prototype is tested to verify the validity of the proposed approach. [ABSTRACT FROM AUTHOR]

Line impedances estimation is of great importance in power system studies. Different software are used in utilities to calculate line impedances using the line conductor and structure data. The calculated values cannot be reliable due to inaccurate information or change in atmospheric condition. Also, since there is a lack of communication between different departments such as system protection, line construction, etc., a line conductor/structure may be changed while the affected departments are not notified. Most of the impedance estimation methods use phasor measurement units (PMUs) in the protection relays which are connected to protection class current transformers (CTs) that do not provide accurate currents compared to metering class CTs. These methods not only require PMUs in the substations, but also fail to measure zero-sequence voltages and currents due to inadequate zero-sequence components under normal operation. In parallel transmission lines, zero-sequence impedance estimation is even more challenging. This paper introduces a method for estimating the self and mutual zero-sequence impedances for mutually coupled transmission lines using recorded fault data. Extensive simulation and protective relay test results in RSCAD/RTDS indicate that the proposed approach has high accuracy in estimating the self and mutual zero-sequence impedances as well as the fault location in parallel transmission lines. [ABSTRACT FROM AUTHOR]

Methods and research for preventing utility-caused wildfires have significantly advanced in the past few decades, including development of criteria on reduction of energy release in an electrical fault to mitigate the probability of ignitions. Technologies to minimize energy release during electrical faults and achieve these ignition benefits are being applied at the substation level. However, the benefits can be targeted to a high-risk area by resonant grounding an isolation transformer on a distribution circuit. By doing this it is possible to reduce single line to ground fault current beyond the transformer to a level unlikely to ignite fires. This system was constructed and shown capable of reducing the power into a phase-to-ground fault by a factor of more than 10,000 and detect ground faults smaller than 0.5 A. This allowed it to meet the ignition criteria mandated for distribution network fire safety in the Australian state of Victoria. It was also tested for typical ignition scenarios such as downed phase conductors and phase conductors contacting with a tree branch. In all cases the probability of ignition was reduced by 90% or more. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of instrument current transformers technical state assessment based on machine learning methods. The introductory parts of the paper provide a detailed analysis of modern methods and approaches for technical state assessment of high-voltage power equipment of power plants and substations as well as a review of modern software tools and the latest trends in the given field of study. Justification of the relevance of the presented research aimed at instrument current transformers technical state assessment is provided along with the motivation for machine learning methods application for improvement of the accuracy and quality of high-voltage equipment state classification. Within the framework of the study, a comparative analysis of gradient boosting on decision trees and random forest algorithms was carried out for a given mathematical problem formulation. The main stages of processing the initial dataset are proposed as a step-by-step procedure, including feature extraction, feature transformation, feature interactions, etc. The outperforming efficiency of gradient boosting on decision trees algorithm was validated for real power equipment fleet. The resulting classification quality metrics of current transformers technical state assessment, Precision and Recall, are estimated to be 87.1% and 83.7%, correspondingly. [ABSTRACT FROM AUTHOR]

Thermal stress plays a prominent role in the reliability of bushings and contributes to the reliability of power transformers, especially during overload conditions. Thus, exploring the temperature distribution in the bushings is essential. This paper proposes a new thermal model to estimate the hotspot temperature (HST) of oil-impregnated paper (OIP) bushings, based on a modified thermal-electrical analogy model. The proposed model is developed based on the finite element method (FEM) to accurately model all fluid flow and internal convection as well as the thermal conduction mechanism. To this end, convection thermal resistances are defined, and their nonlinear characteristics are calculated for different overloading conditions. The proposed approach is applied on a 245 kV, 800 A OIP bushing to analyze not only the normal loading condition but also short-term overloading beyond the rated current. In addition, the oil flow condition with different load currents and transformer top oil temperatures (TTOT) are investigated. The results show a consistent temperature rise with the typical test conditions with no identified problematic situation. However, during overloading, the temperature rise exceeds beyond the permissible limits recommended by IEEE C57.19. Hence, the overloading of transformers may deteriorate the bushing insulation and reduce its lifetime. [ABSTRACT FROM AUTHOR]

Energization of the transformer leads to large magnetizing current called inrush current. It results in reduction in transformer life, reduction in circuit breaker life, and power quality issues in the system. For low and medium voltage level systems in generation and distribution, usually, gang operated circuit breakers are employed, which do not allow independent pole operation. This makes controlled switching more challenging in such systems. This paper proposes a controlled switching technique to minimize the inrush currents for a 3-phase transformer using gang operated circuit breaker. Optimum points for controlled closing and opening are obtained from a flux error function calculated from residual and prospective fluxes of the transformer. Practical challenges due to error in voltage measurements, variation in mechanical closing time, transients in flux, etc. and their solutions are also discussed in this paper. The proposed technique is validated by Controller Hardware-In-Loop (CHIL) test and a scaled-down laboratory test on 5 kVA, 210/400 V, Delta-Star transformer. In both cases, the inrush current is limited to 1pu, demonstrating the effectiveness of the proposed scheme. Furthermore, a field test is carried out on a 500 kVA, 6.6 kV/433 V, Delta-Star distribution transformer to confirm the viability of the developed switching technique. [ABSTRACT FROM AUTHOR]

A new faulty section identification and fault distance estimation technique for three-terminal transmission line (TTL) is proposed in this paper. It is based on the derivation of three indices by solving non-linear equations with iterative method utilizing synchronized measurements from three ends of the line. In the absence of synchronization, an angle synchronization operator is derived for building a common reference among the three terminals. The efficacy of the technique is verified by generating several fault cases on an existing Indian 400 kV TTL using PSCAD/EMTDC software. Reported results demonstrate the capability of the presented technique for all types of fault on three-terminal transposed/untransposed as well as the non-homogeneous transmission line. It also provides accurate fault location under comprehensive variation in type of fault, line length, fault inception angle, complex fault impedance, source impedance, and current transformer (CT) saturation condition. Its accuracy remains almost constant despite errors in line parameters, as well as synchronization and noise in the measured signals. The initial guess has a minimal impact on the accuracy and convergence of the presented technique. The attained results reveal higher accuracy and better robustness of the proposed technique in comparison with those of several existing techniques. [ABSTRACT FROM AUTHOR]

High inrush current severely threats the safe operation of the power system and transformers, which may lead to mal-operation of overcurrent protection device, internal short-circuit fault in transformers. Phase-controlled switching technology is an alternative method to reduce inrush current effectively by controlling the front-mounted circuit breaker (CB) to break or make at the predetermined phase angles of current or voltage. In this paper, a new simplified phase-controlled switching strategy is proposed for three-phase unloaded transformer with Y-connected windings in the primary side, which can limit the inrush current under 46.7% of the captured maximum inrush current. Meanwhile, this strategy is independent on the residual fluxes in the iron cores. The optimal making angle is obtained by theoretical analysis based on an equivalent circuit model of unloaded transformer. The comparison of the proposed method and the random switching approach is also presented with considering residual flux. A simulation model is established to calculate the amplitudes of inrush currents against various residual fluxes when using the proposed phase-controlled switching strategy and random switching method. The simulation results show that the proposed strategy can limit the inrush current under a low value. A test platform with high and low voltage instruments is built and some measurements are carried out. The experimental results are in good agreement with the theoretical analysis and simulation results, which verifies the effectiveness and practicability of the proposed strategy. [ABSTRACT FROM AUTHOR]

In some conditions, the flux in the core of current transformers (CT) increases. If this increase in flux leads to saturation of the protective core, it can create a lot of problems in power system protection. One of the hardware solutions to prevent the occurrence of core saturation in CT is the fixed switched resistor demagnetization method. Due to disadvantages such as dependence on CT parameters and inefficiencies in severe core saturation, the use of this method has not been expanded. This paper proposes a new method to improve the fixed switched resistor demagnetization method. In the proposed method, instead of using a fixed resistor in the circuit topology, an adjustable resistor with an auxiliary transformer is used. In the previous method, the control process is based on flux estimation from the secondary terminal voltage. The control process in the proposed method is based on the second derivative of CT secondary current. To verify the effectiveness of the suggested method, first simulation is done using COMSOL and MATLAB softwares and then all necessary electronic and control circuitry are designed and built. Experimental test is carried out on a 20 kV- 500/5 A CT. The simulation and experimental results show that this method prevents the CT from saturation in case of short circuit faults. The main advantages of this method are independent to CT parameters and efficient in severe core saturation. [ABSTRACT FROM AUTHOR]

A new technique for power transformer protection, that depends on the calculation of fault detection ratio (FDR) based on quartile of superimposed differential currents is presented. Internal faults are detected by comparing FDR with the predetermined threshold derived through analytical analysis and verified based on simulation results. The performance of the presented scheme is tested by modeling an existing Indian power transmission network using PSCAD/EMTDC software. Various types of abnormal conditions such as magnetizing inrush (including sympathetic and recovery) and over-excitation along with several types of internal (winding, inter-winding and turn-to-turn) faults have been investigated. Its validity has also been checked on the data of magnetizing inrush and simultaneous inrush with the internal fault in the transformer, which was obtained from the real network. The results reveal the effectiveness of the suggested technique in differentiating internal faults from other abnormal conditions. Further, it remains stable during heavy through fault with current transformer saturation. It is equally applicable for different ratings and winding connections of power transformers. Finally, comparative evaluation of the proposed scheme with the conventional and several existing techniques shows its superiority in terms of higher sensitivity during internal faults and improved stability in case of non-internal faults. [ABSTRACT FROM AUTHOR]

This article presents a method based on graph theory and Boolean algebra, to obtain dynamic zone selection for busbar protection applications. The solution is an offline software-based tool capable of calculating logic equations for any bus configuration in terms of the status of switching devices. Therefore, the equations can be set up and executed in real time when using existing relays. The method accounts for normal bus-switching sequences, in-service transfer, bypassed current transformers, breaker substitution, and paralleled buses. One of the main benefits is that neither logic equations nor switching analysis are required as inputs when using this method. The obtained logic equations can be set up in most existing bus relays in a straightforward manner. The method was tested in two bus arrangements, and the results have proven its effectiveness. [ABSTRACT FROM AUTHOR]

With the capability of limiting the increasing short-circuit current of power grid, high voltage built-in high-impedance transformer (HVBHT) has been widely used in recent years. However, due to its special structure, the magnetizing inrush with high magnitude and slow decay is likely to occur during no-load energization, which may lead to the mal-operation of zero-sequence overcurrent protection of the upstream transmission line. This phenomenon has become a serious problem in the actual production in China. To solve this problem, a modified time difference method is proposed based on the substation area information, so as to accurately identify the magnetizing inrush, internal fault as well as current transformer saturation. Subsequently, a countermeasure on preventing line zero-sequence overcurrent protection from mal-operation due to HVBHT energization is proposed. Simulation results based on PSCAD indicate that the proposed protection scheme can cope with various kinds of energization and fault situations effectively. Thus, the correct operation of zero-sequence overcurrent protection can be ensured. Compared with other existing identification methods of magnetizing inrush, the proposed method shows great superiority. [ABSTRACT FROM AUTHOR]

Current Transformers (CTs) saturation leads to distortion of current waveform and operational malfunction of protection relays. This paper introduces a practical and simple method to prevent CTs saturation. The method uses a negative voltage feedback and applies it to CT secondary side. The injected voltage opposes the excitation voltage of CT in a manner that the magnetic flux of CT remains almost constant under saturation level for any faults. The method is categorized as a hardware solution, does not need to detect the CT saturation time and works either for digital or non-digital relays. The method is verified in a wide range of simulations and experimental results. The obtained results show that the method is reliable, effective, and accurate. [ABSTRACT FROM AUTHOR]

Switching-on a power transformer creates energizing inrush current (EIC) which in turn leads to sympathetic inrush current (SIC) in adjacent in-service transformers (ITs). Sympathetic inrush current changes the waveform and prolongs the duration of its origin EIC. This makes the analysis and estimation of EIC complicated. Occasionally, the stress of EIC can cause internal short circuit in energizing transformer. Detecting short-circuit fault current during EIC is a challenge for transformer differential protection. On the other hand, SIC may mislead the earth-fault protection of in-service-transformer. This paper, first, proposes a new dc equivalent-circuit based on the dc-component of inrush current, which is efficient in analyzing concurrent EIC and SIC. Next, based on this equivalent circuit, new predictive formulas for EIC and SIC are derived. By employing these formulas, the transformer internal fault during inrush current can be detected based on the comparison between the predicted and measured waveforms, which prevents differential protection from fail-to-trip. Also, the predicted SIC waveform can be compared with the measured waveform to prevent IT earth-fault protection from mal-trip, during inrush current. Finally, the proposed equivalent circuit and formulas are verified using the EMTP simulation results of a real grid, under different system conditions. [ABSTRACT FROM AUTHOR]

This paper proposes a new three-phase to five-phase transformer, with a 30-pulse AC–DC converter for power quality improvement in vector-controlled induction motor drive. Here, three isolated transformers (T1, T2, and T3) are used and each transformer is designed to convert three-phase AC-supply into five-phase AC supply, to achieve ten-pulse AC–DC conversion. A unique configuration and angle among these three transformers is designed in such a way that the resultant input current of this transformer shows 30 pulses in it. A hybrid multilevel inverter is proposed in this paper, which is cascading a three-level neutral point clamped inverter with a two-level voltage source inverter to drive an induction motor (IM). A modified level shifted carrier-based sinusoidal pulsewidth modulation and a 3rd harmonic injection techniques are utilized to modify the modulating signal, to enhance the fundamental magnitude, and to reduce the harmonics than traditional technique. The responses of proposed medium voltage induction motor drive (MVIMD) are evaluated by simulating in MATLAB-Simulink. The proposed MVIMD performance is then verified using a scaled down laboratory prototype under various operating conditions such as steady state, starting, load perturbation, speed control, field weakening, varying load conditions, and over a wide speed range of IM. [ABSTRACT FROM AUTHOR]

The current differential protection is introduced into the active distribution network to improve the selectivity and reliability of protection. However, it is difficult to meet the synchronous requirement of differential protection for distribution networks. Therefore, this paper proposes an impedance differential protection for the active distribution network. The principle and criterion of the impedance differential protection are established according to the significant differences between differential impedance and restraint impedance during normal operation, external fault, and internal fault. Furthermore, an auxiliary criterion is constructed by the amplitude characteristic of fault current to cope with the existing dead-zone fault. A complete protection scheme based on the proposed principle is designed. Several major factors are taken full consideration in the scheme including transition resistances, current transformer saturation, unsynchronized data, various penetrations, and underground cables. Extensive digital simulations are conducted to verify the principle. Theoretical analyses and simulations demonstrate the effectiveness and advantage of this principle. Compared with current differential protection, the impedance differential protection significantly reduces the requirement of data synchronization and transmission bandwidth, which plays an important role in adapting to the communication infrastructure of distribution network. [ABSTRACT FROM AUTHOR]

Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential protection failure to trip for converter transformer internal faults. The main factor responsible for the protection failure to trip is half-cycle saturation of the converter transformer core, which results from the dc component in the fault current. This suggests that harmonic blocking may not be suitable for differential protection of converter transformers to identify inrush conditions. In this regard, this paper proposes a novel method using mathematical morphology to effectively discriminate between the conditions caused by inrush and internal faults. This method implements the waveform symmetry criterion and improved morphological gradient criterion to distinguish between these two conditions. The effectiveness of the proposed method is evaluated based on extensive simulation, and the presented results show that the proposed method can ensure operation for all types of internal faults. Additionally, this method improves the accuracy of inrush identification, even in extreme cases, such as sympathetic inrush and inrush with current transformer saturation. [ABSTRACT FROM AUTHOR]

This two-part paper presents a method to mitigate half-cycle saturation of transformers caused by monopolar operation of neighboring HVDC transmission lines while keeping the ability to detect the zero-sequence fault currents (ZSFC) when ground faults occur. Part I of this paper has presented the mitigation principles and device design of the proposed neutral current blocking switch. In Part II, the performance of the proposed method to permit the circulation of zero-sequence current is investigated. An operation strategy is proposed that simultaneously allows the mitigation of half-cycle saturation and the detection of ZSFC. Simulations on a widely-used 500 kV system show that the proposed mitigation technique, using a sub-synchronous switching frequency (no higher than 30 Hz), can effectively mitigate the half-cycle saturation while allowing the circulation of ZSFC. The novel mitigation method exploits the characteristic differences between half-cycle saturation (dc) and asymmetric faults (ac). The method provides an implementable solution to the dc-bias phenomenon because it delivers concurrently dc-bias mitigation, minimal impact on ground fault detection, and no switching stresses on the power electronic switches. The proposed technique can also be applied to the mitigation of geomagnetically induced currents. [ABSTRACT FROM AUTHOR]

This paper presents a new protection algorithm for double circuit transmission lines using the sum of ratios of modal currents. Relay-operating characteristics is proposed using the complex ratio of modal currents. The proposed algorithm estimates modal domain line parameters and identifies the faulty phases for single pole-tripping applications even during inter-circuit faults. The scheme is sensitive to high resistance internal faults and also detects internal faults with outfeed. The proposed scheme is immune to mutual coupling and is found robust to uncertainties in parameter estimation due to measurement inaccuracies. The scheme is tested for its effectiveness to deal with current transformer saturation and synchronisation error. The scheme protects series compensated double circuit transmission lines even in the presence of current inversion. Exhaustive case studies in electromagnetic transient program (EMTP)/alternate transient program are performed on 400 kV, 300 km, double circuit transmission lines and the proposed protection algorithm is implemented in MATLAB. The results confirm the superiority of the proposed scheme. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) testing of high-voltage (HV) cables and cable accessories has been implemented predominantly using high-frequency current transformers (HFCTs) as PD sensors. PD currents initiating at PD sources are coupled onto cable conductors. They travel away from the PD sources and are detected by HFCTs installed at cable terminations. In this paper, based on combining finite-difference time-domain (FDTD) modeling and transfer function theory, a hybrid modeling approach is proposed to investigate the processes of PD coupling and detection involved in HFCT-based PD testing of HV cables. This approach allows exciting a PD event anywhere in FDTD models of the cables and predicting output from HFCTs some distance away. Implementation of the method is illustrated using an 11 kV XLPE cable. Moreover, a “direct measurement” method of obtaining original PD pulses as the excitation source waveform is presented. The modeling approach introduced here will facilitate studies on the relationship between measured PD signals and those excited at PD sources, which can potentially give useful insight into the basic mechanisms behind PD detection in cables. [ABSTRACT FROM AUTHOR]

An on-line method is proposed for cable transfer function (TF) identification. For a cable's TF identification with this method, no knowledge about the cable geometry and material specification is needed. Moreover, it does not require that the cable system go out of service. An identification signal is injected through a high-frequency current transformer (HFCT) to the shield of the cable. By measuring the injected signal via HFCTs clamped on the shield at two distinct locations along the cable, the TF of the cable can be identified. A novel partial discharge (PD) localization method is also introduced, which is based on asynchronous PD measurements. In this method, travel distance of the signal is estimated by the reconstruction of a reference signal from the measured signals, using the obtained cable TF. Reference signal refers to the typical waveform of PD signal at its source location. Both the cable identification and PD localization methods are validated using two experimental setups to simulate a diverse range of PD source distance. The test cables are a 666-m long, single-core, low-voltage (2 kV) signal cable with PE insulation, and a 51-km long, single-core, medium-voltage (24 kV) power cable with XLPE insulation. [ABSTRACT FROM AUTHOR]

Substation asset health has always been one of the utmost concerns for utilities. The substation secondary assets such as current transformers, potential transformers, and coupling capacitor voltage transformers play vital roles for system situational awareness and grid operations. This paper proposes a novel on-line substation instrument transformer health monitoring system (SITHMS), which merely uses time-synchronized and high-resolution phasor measurement unit (PMU) data. The SITHMS can provide early warning and alert utility personnel to diagnose the equipment malfunction and take proactive actions for asset management to prevent equipment failure. The core component of the SITHMS is a model-based three-phase substation-level linear state estimator, which features a built-in largest normalized residual (LNR) module that can detect anomalies in PMU data by using substation model and configuration, so as to probe if there is malfunction or potential failure of these secondary equipment. In addition, we present the system's implementation architecture and testing environment setup. Case studies with the PSCAD simulated data and field PMU data from actual substations at American Electric Power (AEP) demonstrate the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

In this paper, a new technique for low-severity inter-turn fault detection under online operating conditions of dry-type transformers is proposed. The proposed methodology requires just two inexpensive flux sensors and is based on the fundamental component of the core-leakage fluxes at the transformer's outer limbs. Theoretical analysis using the proposed analytical models and experimental validation on a 10-kVA experimental prototype transformer are carried out to evaluate the performance of the proposed methodology under various fault severities. The real-time fault-detection capability of the proposed technique is also evaluated using a fabricated prototype health monitoring device, which reveals that as low as 2-turns bolted fault out of 150 turns can be detected within 3 fundamental cycles. The methodology is immune to loading and supply voltage unbalance conditions and on par with the classical differential negative sequence current based technique with an additional advantage of 78% reduction in the total number of sensors required for a three-phase three-winding transformer. [ABSTRACT FROM AUTHOR]

In this paper, an efficient method based on the Kalman filter (KF) theory is proposed for fast and accurate detection and compensation of current transformer (CT) saturation. During the unsaturated regions, the algorithm uses the current sample points together with extended Kalman filter (EKF) for obtaining a model, which suitably represents the current signal behavior. The acquired model is used to reconstruct the original current waveform during the saturated regions. The onset and end points of the CT saturation intervals are detected by defining a criterion based on the estimation error of the EKF algorithm. The proposed method is very profitable as it requires only one sample point to detect the CT saturation and it also copes with the problems of measurement noises and harmonics. These advantages are confirmed by a large number of test cases obtained using PSCAD/EMTDC software as well as by real-time hardware implementation. [ABSTRACT FROM AUTHOR]

With smart fault indicators (SFIs) being widely deployed in distribution networks, availability of SFIs statuses and their current measurement provides an opportunity to locate distribution system faulted line sections in real time. However, the integrity of SFIs data together with complex fault scenarios poses a challenge for fault diagnosis, especially with the presence of distributed generators (DGs). To solve this issue, this paper proposes a new method based on mixed integer linear programming (MILP) for identification of the faulted line sections using incomplete and incorrect SFIs statuses. By minimizing the discrepancy between the expected SFI statuses and the available evidence at the distribution operating center, the proposed approach is able to handle complex fault scenarios with multiple faults and failures or malfunctions of SFIs. When DGs are present, the current measurement of SFIs and their statuses are jointly used. The result is a robust algorithm for detection of faulted line sections of distribution systems with DGs in real time. In addition, a new technique is proposed to convert nonlinear logical constraints into a linear combination of decision variables. The resulting linearity of the developed optimization model ensures its optimality. Simulation results based on a utility feeder demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore a low step-ratio dc–dc converter at multi-megawatt scale is a crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc–dc converter configuration with only a single stack of half-bridge sub-modules (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step ratio is small ($1 - {\boldsymbol{v}_{\boldsymbol{L}}}/{\boldsymbol{v}_{\boldsymbol{H}}}$). Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc–dc topologies. Further, the requirement for SM capacitive energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with a down-scaled prototype. The SMs are operated with an adjustable phase shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft switching for all power switches to increase efficiency. [ABSTRACT FROM AUTHOR]

The current transformer (CT) saturation phenomenon has been one of the main problems for the power transformer differential protection, leading to incorrect current measurements and relay misoperation. This paper proposes a fast and efficient transformer differential protection scheme with additional differential CT saturation and cross-country fault detection modules after the external fault detection, all of them based on the differential wavelet coefficient energy with border distortions in order to stabilize the relay during external faults and distinguish accurately CT saturation from cross-country internal faults. The proposed method was assessed by using representative simulations of internal faults, transformer energizations, and external faults with CT saturation followed by cross-country internal faults, and good results were achieved. [ABSTRACT FROM PUBLISHER]

Current transformer (CT) saturation can cause the maloperation of transformer differential protection under external faults. To avoid this problem, a method based on the histogram of the normalized rotated current (NRC) is proposed. NRC is obtained by a symmetric waveform transformation of a part differential current. An index based on the histogram is defined to evaluate the NRC frequency distribution. The external faults with different degrees of CT saturation, internal faults, and transferring faults can be clearly distinguished by the proposed method. Moreover, the detection time is invariably less than a half cycle after the obvious increase of the differential current. Simulation results validated the proposed method. [ABSTRACT FROM PUBLISHER]

This paper investigates the performance of phasor measurement units (PMUs) under interference conditions, which can appear in a power system and are not tested by the C37.118.1 standard. Three PMUs from different vendors configured for the M-class requirements were used to test three possible interference condition scenarios. In the first scenario, noise is added to the PMU input signal. The test runs a sweep of signal-to-noise ratios and the accuracy versus the noise level is obtained. The second scenario injects multiple harmonics with the input to test the influence on accuracy. The last scenario focuses on instrument transformer saturation, which leads to a modified waveform injected in the PMU. This test goes through different levels of current transformer saturation and analyzes the effect of saturation on the accuracy of PMUs. The test results show PMU measurements will be degraded when the input signal is distorted by high noise or a saturated current waveform, but is not particularly affected by multiple harmonics. This information can be used when selecting a PMU to ensure that it will provide a reliable measurement for the intended use. It can also be used for developing more robust PMUs and applications resistant to degraded measurements. [ABSTRACT FROM PUBLISHER]

Sympathetic inrush is a magnetizing inrush phenomenon between two or more transformers when one of them is switched on under no load condition. This phenomenon has caused transformer differential protection maloperation in several cases due to the failure of the second harmonic restraint algorithm. Sympathetic inrush is very different in both the mechanism and characteristics from general inrush generated by a single transformer. Hence, a special identification method is required. The physical mechanism of sympathetic inrush between transformers is thoroughly studied to understand its characteristics, especially the alternate saturation feature. A substation-area information-based identification method is proposed to identify sympathetic inrush of the in-service transformer (IT). The alternate opposite saturation characteristic is used in the method. Both locking and unlocking criteria are designed for current differential protection to prevent the IT protection from maloperation. The proposed method is verified to be effective under different system conditions by simulations conducted in PSCAD and RTDS. [ABSTRACT FROM PUBLISHER]

Smart Grids employ intelligent control applications that require high quality data: fast, secure, and error free. Several researchers have focused on providing techniques for low latency and secured data links for these applications. Bad data detection is however generally provided only at the central level due to limitations in legacy technologies employed in many substations. With the introduction of IEC61850 data sharing within the substation becomes more flexible and transparent allowing more sophisticated management of data quality. Hence, this paper proposes a substation level bad data detection algorithm to facilitate also these types of requirements from applications. The algorithm is based on automatically detecting the substation topology by parsing standard substation description files and online state of circuit breakers and disconnectors. By applying linear weighted least square based state estimation algorithm, bad data from failing current transformers (CT) can be detected. By conducting the verification of different types of bad data, the results show the output of bad data detection algorithm provides higher accuracy than output from both measurement and protective CT in both static and faulty situations. [ABSTRACT FROM PUBLISHER]