Showing papers on "Power-system protection published in 2015"

A Comprehensive Review of Catastrophic Faults in PV Arrays: Types, Detection, and Mitigation Techniques

Abstract: Three major catastrophic failures in photovoltaic (PV) arrays are ground faults, line-to-line faults, and arc faults. Although there have not been many such failures, recent fire events on April 5, 2009, in Bakersfield, CA, USA, and on April 16, 2011, in Mount Holly, NC, USA, suggest the need for improvements in present fault detection and mitigation techniques, as well as amendments to existing codes and standards to avoid such accidents. This review investigates the effect of faults on the operation of PV arrays and identifies limitations to existing detection and mitigation methods. A survey of state-of-the-art fault detection and mitigation technologies and commercially available products is also presented.

Distance Protection of Lines Emanating From Full-Scale Converter-Interfaced Renewable Energy Power Plants—Part I: Problem Statement

Abstract: The fault ride-through (FRT) requirement of modern grid codes results in interactions between full-scale converter-interfaced renewable energy power plants (CIREPPs) and the protection systems of high-voltage transmission grids, which normally involve distance elements either as the primary or the backup relay. Such interactions are influenced by the CIREPPs' exclusive fault behavior, and have been left largely unnoticed in relaying literature. Part I of this paper develops a CIREPP model suitable for relaying studies, and highlights the CIREPP properties by which the protection system is endangered the most. Then, the operating scenarios leading to the malfunction of a distance relay that is located at a CIREPP substation and protects the adjacent line are unveiled. These scenarios include in-zone short circuits missed by the relay and incorrect tripping for out-of-zone faults, which would, in turn, neutralize FRT schemes implemented inside CIREPPs. The findings of this study also hold true for the ac lines emanating from voltage-sourced converter-based HVDC connections.

Detection and Location of High Impedance Faults in Multiconductor Overhead Distribution Lines Using Power Line Communication Devices

Abstract: An effective power system protection scheme has to be able to detect and locate all occurring faults corresponding to low and high impedance values. The latter category poses the greatest challenge for the protection schemes due to the low values of the related fault current. This paper extends previous work by the authors on the subject, aiming to achieve detection and location of high impedance faults (HIFs) in multiconductor overhead distribution networks utilizing power line communication (PLC) devices. Fault detection is proposed to be performed by a PLC device installed at the starting point of the monitored line and by using differences to the values of metrics related to input impedance at frequencies utilized by narrowband systems. Moreover, fault location can be derived by a response to impulse injection procedure utilized by all installed PLC devices along the line. The method is evaluated and validated in various simulation test cases concerning its ability to effectively detect and locate HIFs.

Protection Strategies for Medium-Voltage Direct-Current Microgrid at a Remote Area Mine Site

Abstract: This paper presents protection strategies for a medium-voltage dc (MVDC) microgrid at a remote area mine site. The microgrid is operated to provide high power quality and reliability to sensitive loads and to improve the energy efficiency of the mining equipment. In the MVDC microgrid, various local distributed energy resources have been used, including photovoltaic arrays, wind turbines, a fuel-cell stack, an energy storage system, and mobile diesel generators. For the protection of transmission lines, a communication-based differential protection scheme with solid-state electronic relays is employed to isolate the faulted part of the MVDC microgrid. This is further reinforced by dc overcurrent protection as a backup. Earlier research work had neglected the backup protection for dc systems. In addition, communication-based dc directional overcurrent protective relays are used for both source protection and load protection to support a bidirectional power flow. MATLAB/Simulink modeling and simulation results are presented and discussed to illustrate the proposed system's dependability and security.

A new HVDC grid test system for HVDC grid dynamics and protection studies in EMT-type software

Abstract: This paper proposes a new HVDC grid test system for electro-magnetic transient analysis, suitable for HVDC power system studies ranging from protection to dynamic studies investigating converter be ...

Fault-Location Scheme for Power Distribution System with Distributed Generation

Abstract: This paper presents a novel fault-location scheme for unbalanced power distribution system in the presence of distributed generation (DG). The proposed scheme first identifies the possible fault locations using a new formulation of the impedance-based method. The new formulation overcomes the requirement of fault-type identification by using only one fault-location equation. The proposed equation is applicable to all shunt fault types. From the possible fault locations, the exact fault location is then identified by matching the measured voltage at the substation bus and each DG unit bus with calculated ones. The proposed scheme is applicable for all DG types without the need for their individual parameters. The balanced and unbalanced laterals and the capacitive effect of distribution line are also considered. The proposed scheme was evaluated and tested on a modified IEEE 34-bus distribution system using PSCAD/EMTDC software.

Protection of AC and DC distribution systems. Embedding distributed energy resources: a comparative review and analysis

Abstract: The integration of distributed generation (DG) units into distribution networks has challenged the operating principles of traditional AC distribution systems, and also motivated the development of emerging DC systems. Of particular concern are the challenges associated with the operation of conventional protection schemes and/or devices. This paper first analyses the fault current characteristics in AC and DC distribution systems; it then presents a comprehensive review of the latest protection methods proposed for distribution systems embedding DGs. In addition, the advantages and disadvantages of each method are outlined and compared. The differences between the protection algorithms employed in/proposed for AC and DC systems are also discussed. Finally, this study identifies the future trends and provides recommendation for researches in the field of protections of DC distribution networks.

A New Whole-Line Quick-Action Protection Principle for HVDC Transmission Lines Using One-End Current

Abstract: HVDC transmission systems have found wide applications in modern power systems and have produced a useful influence on the stability and security of power systems. The currently used protections for HVDC systems operate on the principles which have poor reliability and inadequate sensitivity. To overcome these problems, this paper presents a new whole-line quick-action protection principle for HVDC transmission lines. The proposed method, based on the boundary characteristics of the line, can distinguish internal faults from external ones using single-terminal current measurements only. The criterion of this protection has been formulated. Simulations on frequency-dependent parameter-line model and the field-recorded data of a $\pm$ 500-kV HVDC system show that this method is valid and feasible. The proposed protection, sound and clear in principle and easy to implement, can be expected to become a primary protection of HVDC transmission.

DC Transmission Grid With Low-Speed Protection Using Mechanical DC Circuit Breakers

Abstract: This paper introduces a dc transmission grid with fault-tolerant inductor-capacitor-inductor (LCL) voltage-source converters (VSCs) and using a slow protection system based on mechanical dc circuit breakers (CBs). LCL VSC inherently regulates dc fault current to levels that converters can sustain for prolonged periods which avoids insulated-gate bipolar transistor tripping and brings significant advantage to security and reliability aspects. Simple mechanical dc CBs are used at dc busbars and connecting points of each dc cable, in the same manner as it is normal practice used with ac transmission protection. The protection logic is based on differential methods which gives excellent selectivity and reliability. The fault clearing time is in the order of 30–60 ms which allows for reliable protection decision making. The simulation results obtained from a four-terminal dc grid modeled on the PSCAD platform confirm successful dc fault isolation and grid recovery for a range of severe dc fault scenarios.

Assessment of Robustness of Power Systems From a Network Perspective

Abstract: In this paper, we study the robustness assessment of power systems from a network perspective. Based on Kirchhoff's laws and the properties of network elements, and combining with a complex network structure, we propose a model that generates power flow information given the electricity consumption and generation information. It has been widely known that large scale blackouts are the result of a series of cascading failures triggered by the malfunctioning of specific critical components. Power systems could be more robust if there were fewer such critical components or the network configuration was suitably designed. The percentage of unserved nodes (PUN) caused by a failed component and the percentage of noncritical links (PNL) that will not cause severe damage are used to provide quantitative indication of a power system's robustness. We assess robustness of the IEEE 118 Bus, Northern European Grid and some synthesized networks. The influence of network structure and location of generators are explored. Simulation results show that the connection with short average shortest path length can significantly reduce a power system's robustness, and that the system with lower generator resistance has better robustness with a given network structure. We also propose a new metric based on node-generator distance (DG) for measuring the accessibility of generators in a power network which is shown to affect robustness significantly.

Multiterminal Hybrid Protection of Microgrids Over Wireless Communications Network

Abstract: Communications network plays a key role in the next-generation microgrid protection schemes, as the protective relays need to communicate with each other to dynamically track their tripping currents due to the time-varying nature of the loads and the distributed generation (DG) outputs. Hence, communications cost and reliability are some of the key factors that need to be considered before deploying a microgrid protection scheme. In this paper, we introduce the concept of a hybrid microgrid protection system, which implements traditional differential protection scheme along with an adaptive microgrid protection scheme. While the differential scheme is more sensitive, yet relies on extensive communications, the adaptive scheme is less sensitive but has low communications load. The joint deployment of these two schemes has the potential to increase the accuracy and precision of the whole protection scheme, while reducing the overall communications cost. Moreover, we propose a preemptive switching algorithm for the microgrid relays, which ensures a seamless transition from differential protection to adaptive protection based on the bit error rate of the communications links. Some illustrative results are provided based on an IEEE 802.16/WiMAX network to justify the proposed hybrid protection scheme.

Synchrophasor-Based Wide-Area Backup Protection Scheme with Data Requirement Analysis

Abstract: This paper presents a fast and robust wide-area backup protection scheme to detect the faulty condition and to identify the faulted line in a large power network The proposed methodology uses positive-sequence synchrophasor data captured by either digital relays with synchronization capability or phasor measurement units dispersed over the network The basic idea behind the new protection scheme is the comparison of bus voltage values calculated through dissimilar paths Upon occurrence of a fault, the faulty condition is first detected and the bus(es) connected to the faulted line is(are) determined Among transmission lines connected to the suspected bus(es), the faulted one is thereafter identified In addition to two-terminal transmission lines, multiterminal lines are also incorporated The performance of the proposed method is validated on the IEEE 57-bus test system in different fault conditions (fault type, fault location, and fault resistance) Discrimination of faulty and normal conditions is simulated by examining various stressed conditions, for example, load encroachment, generator outage, and power swing The data requirement of the proposed technique is analyzed as well To do so, a mathematical model for the optimal placement of measurement devices is developed and applied to different IEEE standard test systems

Event-Based Protection Scheme for a Multiterminal Hybrid DC Power System

Abstract: In this paper, we investigate an event-based protection scheme for a multiterminal dc power system, which includes hybrid energy resources and various loading schemes. The proposed protection scheme transfers less data when compared with commonly used data-based protection methods, and does not require high-speed communication and synchronization. Each protection unit is able to autonomously identify the type of event using the current derivative fault identification method, employing an artificial inductive line impedance. In order to accurately set the protection relays, detailed fault current analysis considering low pass resistor capacitor filter effects are presented. The decision for fault isolation is made based on the unit judgment and the data received through high-level data communication from other interconnected units. The performance of the proposed protection scheme was evaluated under different dc feeder and bus faults. The results show that this scheme is able to accurately identify the type of fault, isolate the faulted area, and restore the system quickly while limiting the load voltage drop to its preset limit.

A new protection scheme for DC microgrid using line current derivative

Abstract: Protection scheme for AC transmission systems are well understood and matured On the other hand, DC system is still facing a challenge in developing proper protection scheme because of its natural characteristics A protection scheme, which utilizes some of the developed techniques for AC system, and modified to suit the DC system characteristics, is proposed in this paper The scheme is based on the concept of DC current profile under transients, which depends on the fault location This property is combined with the directional feature to achieve the protection of a DC microgrid The proposed scheme is demonstrated on the ring type DC microgrid system, which is able to detect the fault in the DC system, and also ensure its backup protection The proposed concept is verified and tested through MATLAB/Simulink simulations

Adaptive fault identification and classification methodology for smart power grids using synchronous phasor angle measurements

Abstract: Smart power grids (SPGs) entail comprehensive real-time smart monitoring and controlling strategies against contingencies such as transmission line faults. This study proposes a novel methodology for identifying and classifying transmission line faults occurring at any location in a power grid from phasor measurement unit measurements at only one of the generator buses. The proposed methodology is based on frequency domain analysis of equivalent voltage phase angle and equivalent current phase angle at the generator bus. Equivalent voltage and current phase angles are the angles made by three-phase equivalent voltage and current phasors with respect to reference axis. These angles are estimated through Park's transformation and frequency domain analysis is performed over a fixed time span equal to inverse of system nominal frequency using fast Fourier transformation. The proposed methodology can be utilised for relaying purposes in case of single transmission lines as well as for system protection centre (SPC) applications in power grid. The significance of the fault information from the methodology is for assisting SPC in SPGs for transmission line fault detection and classification to restore the transmission lines at the earliest and initiate wide-area control actions to maintain system stability against disturbances generated by occurrence and clearance of fault.

A protection strategy for micro-grids based on positive-sequence component

Abstract: In recent years, the concept of micro-grid has appeared as an appropriate way for the integration of distributed energy resources (DERs) in the distribution networks. However, micro-grids have encountered a number of challenges from control and protection aspects. One of the main issues relevant to the protection of micro-grids is to develop a suitable protection technique which is effective in both grid-connected and stand-alone operation modes. This study presents a micro-grid protection scheme based on positive-sequence component using phasor measurement units and designed microprocessor-based relays (MBRs) along with a digital communication system. The proposed scheme has the ability to protect radial and looped micro-grids against different types of faults with the capability of single-phase tripping. Furthermore, since the MBRs are capable of updating their pickup values (upstream and downstream equivalent positive-sequence impedances of each line) after the first change in the micro-grid configuration (such as transferring from grid-connected to islanded mode and or disconnection of a line, bus or DER either in grid-connected mode or in islanded mode), they can protect micro-grid lines and buses against subsequent faults. Finally, in order to verify the effectiveness of the suggested scheme and the designed MBR, several simulations have been undertaken using DIgSILENT PowerFactory and MATLAB software packages.

Distance Protection of Lines Emanating From Full-Scale Converter-Interfaced Renewable Energy Power Plants—Part II: Solution Description and Evaluation

Abstract: Having unveiled the problems associated with distance protection of lines connected to full-scale converter-interfaced renewable energy power plants (CIREPPs) due to remote and intermediate infeed in Part I of this paper, Part II describes why the countermeasures currently deployed by commercial relays to tackle these problems in a conventional power system fail in the presence of a CIREPP. Then, a new formula to calculate impedance is devised for the phase element of the relay to prevent maloperation in the event of line-to-line to ground faults. In addition, a communication-assisted method with minimal bandwidth requirement is proposed for balanced and line-to-line faults. The issues that should be taken into consideration for single-line-to-ground faults are also discussed. The solutions presented in this paper are simple and independent of the host system grid code, and can be applied to industrial memory-polarized distance relays. A comprehensive performance evaluation is presented to verify the reliable operation of the proposed methods.

Risk Assessment in Extreme Events Considering the Reliability of Protection Systems

Abstract: This paper presents a new risk assessment method that is applicable to extreme cases in power systems. This paper analyzes the interactions among protection system components and the power grid in extreme events pertaining to simultaneous faults and cascading failures. The hidden failures of protection systems could exacerbate power system conditions if cascading events tend to follow a path to a blackout. The proposed risk assessment considers detailed reliability models of protection system components including circuit breakers (CBs) and protective relays. The failure probability of a CB is formulated considering its component degradation rate and operation times. The failure model of a protective relay is constructed using the dynamic fault tree. The evolution of cascading failures of power systems in extreme conditions, which deteriorates due to protection system malfunctions, is modeled based on the actual physical system behavior. The effectiveness of the proposed risk assessment method is demonstrated using a modified 9-bus system and the IEEE 68-bus system.

Impedance-Differential Protection: A New Approach to Transmission-Line Pilot Protection

Abstract: Differential protection is able to provide the most selective protection compared with other protective schemes. In this paper, a new differential scheme is proposed for pilot protection of transmission lines. The concept of differential impedance ( ${\rm Z}_{\rm diff}$ ) is introduced using the synchronous voltages and currents of both terminals of the line. The proposed scheme is capable of fast discriminating between the internal and external faults, even in the worst operating conditions. The dependence on the line capacitive charging current and the source strength are common problems in conventional pilot protection methods which have been removed in the proposed scheme. Moreover, compared with the conventional differential protection, the proposed scheme can exactly determine the fault location. Both computer and experimental simulation studies have been performed to evaluate the proposed scheme. The obtained results approve the high efficiency of the proposed scheme as powerful pilot protection.

Improved fault location algorithm for multi-location faults, transforming faults and shunt faults in thyristor controlled series capacitor compensated transmission line

Abstract: Fault location estimation in series compensated transmission lines is quite difficult because a non-linear current dependent circuit appears between the substation and fault point. In particular, the faults which occur at different locations at the same time in different phases known as multi-location faults has not been addressed by researchers. Other types of fault that may occur in transmission lines are transforming faults where one type of fault transforms to another type fault after some time. In this study, a fault location estimation scheme using artificial neural network (ANN) is presented for multi-location faults, transforming faults as well as for commonly occurring shunt faults in thyristor controlled series capacitor (TCSC) compensated transmission line. DB-4 wavelet is used for pre-processing of the three-phase current and voltage signals. The shunt capacitance of the line is considered based on distributed parameter line model. Feasibility of the ANN-based fault location algorithm is tested under a wide variation of parameters, such as fault type, location, fault resistance and fault inception angle. Fault location errors are within 0.001–1% range.

A new formulation for coordination of directional overcurrent relays in interconnected networks

Abstract: Summary In large and interconnected networks, the coordination between all protective devices, without any violation, is very complicated and sometimes impossible. For obtaining reasonable and optimal coordination with suitable operating time (OT) for relays, some constraints are obliged to violation. In order to reduce the related violation and also the relays OTs, some objective functions (OFs) are proposed in literatures in which the discrimination times of the primary and backup relays is added to the OF. In this paper, for obtaining the optimal coordination of directional overcurrent relays, the OF is improved to overcome the drawbacks of the existing OF. In other words, as the main contribution, a new problem formulation is proposed so that not only the optimal settings of relays are obtained but also the OTs of the primary and specially backup relays are reduced considerably. In order to solve the proposed problem formulation, the effects of both near and far-end faults are considered. Also, regarding the protection system flexibility, the program is capable of choosing the best characteristic for each overcurrent relay. The genetic algorithm is used as an optimization tool, and the effectiveness of the proposed method is demonstrated by comparing the coordination results for the two sample networks with those obtained by using the recent developed OFs. Copyright © 2013 John Wiley & Sons, Ltd.

Investigation of the sympathetic tripping problem in power systems with large penetrations of distributed generation

Abstract: This study contains an investigation into sympathetic tripping – the undesirable disconnection of distributed generators (DGs) (in accordance with the recently-introduced G83/2 under voltage protection) when a network fault occurs in the vicinity of the DG and is not cleared quickly enough by the network protection (i.e. before the DG's under voltage protection operates). An evaluation of the severity of and proposal of solutions to the problem of sympathetic tripping on a typical UK distribution power network is presented. An inverter model (as the majority of DGs will be inverter-interfaced) that characterises the fault response of the inverter and its associated protection functions has been developed for use in simulation through exhaustive laboratory testing of a commercially-available 3 kW inverter for DG application; the observed responses have been modelled and incorporated in a power system simulation package. It is shown, when using presently-adopted DG interface and network protection settings, that the risk of sympathetic tripping is high in several future scenarios. To mitigate this risk, the impact of modifying network protection settings is evaluated. This study has two key findings – determination of the conditions at which the risk of sympathetic tripping is high and evaluation of a technique to mitigate this risk.

A UFLS Scheme for Small Isolated Power Systems Using Rate-of-Change of Frequency

Abstract: This letter presents a centralized underfrequency load-shedding (UFLS) scheme using the initial rate-of-change of frequency (ROCOF) to decide whether and how much load needs to be shed. The applicability of the centralized UFLS scheme is shown for a Spanish small isolated power system.

Fault Diagnoses for Industrial Grid-Connected Converters in the Power Distribution Systems

Abstract: The reliability of power electronics converter systems (PECSs) is of paramount importance in industrial, commercial, aerospace, and military applications. Therefore, the knowledge about the fault-mode behavior of a converter system is extremely important from the perspective of improved system design, protection, and fault-tolerant control. Faults of power switches in PECSs are classified as short circuit (S-C) faults, open circuit (O-C) faults, and degradation faults. S-C faults in most cases cause an overcurrent condition that is readily detected and acted upon by standard protection systems such as overcurrent, undervoltage, or overvoltage protection. However, the degradation faults and O-C faults often do not trigger fault protection but rather cause system malfunction or performance degradation. Since the standard protection system may not detect these fault types, their diagnoses become critical for PECSs. This paper presents new methods for fault detection, localization, and diagnosis for grid-connected power converters and the identification of the unbalance input voltage to the converter. The proposed fault diagnostic algorithms are verified in both the simulation and the experimental environments in order to evaluate their robustness and effectiveness. The power converter under the study consists of three main subsystems: the three-phase uncontrolled rectifier, the boost chopper, and the single-phase inverter circuits.

A network approach for power grid robustness against cascading failures

Abstract: Cascading failures are one of the main reasons for blackouts in electrical power grids. Stable power supply requires a robust design of the power grid topology. Currently, the impact of the grid structure on the grid robustness is mainly assessed by purely topological metrics, that fail to capture the fundamental properties of the electrical power grids such as power flow allocation according to Kirchhoff's laws. This paper deploys the effective graph resistance as a metric to relate the topology of a grid to its robustness against cascading failures. Specifically, the effective graph resistance is deployed as a metric for network expansions (by means of transmission line additions) of an existing power grid. Four strategies based on network properties are investigated to optimize the effective graph resistance, accordingly to improve the robustness, of a given power grid at a low computational complexity. Experimental results suggest the existence of Braess's paradox in power grids: bringing an additional line into the system occasionally results in decrease of the grid robustness. This paper further investigates the impact of the topology on the Braess's paradox, and identifies specific substructures whose existence results in Braess's paradox. Careful assessment of the design and expansion choices of grid topologies incorporating the insights provided by this paper optimizes the robustness of a power grid, while avoiding the Braess's paradox in the system.

Superconducting Fault Current Limiter Placement for Power System Protection Using the Minimax Regret Criterion

Abstract: In a power grid, the distributed generations (DGs) are not generally controlled by the system operator. Fault current levels in the power grid may change because of unpredictable changes in the status of noncontrollable DGs. These changes in the fault current can have an impact on the relay coordination. In this paper, the uncertainty associated with the connection status of DGs is reflected in the relay coordination. Superconducting fault current limiter (SFCL) placement can be effective for reducing the fault current while eliminating misoperation of the relays caused by DGs. However, an SFCL may also have a negative impact on the operation of the relays. An effective SFCL placement method is needed because of the high cost of the SFCL and the impact on the power system protection. In this paper, an SFCL placement method based on the minimax regret criterion is proposed for power system protection considering the uncertainty of the DGs.

Comprehensive protection strategy for an islanded microgrid using intelligent relays

Abstract: In this paper, a comprehensive protection strategy is proposed for insuring dependable and secure operation of an islanded microgrid system. This is implemented using microprocessor based relays to prevent unnecessary loss of critical loads and distributed generators (DGs). Several improvements are proposed to clear the way for plug-and-play of distributed generators (DGs) and loads. Furthermore, recommendations are presented to tackle the elusive high impedance fault problem, commonly encountered in distribution systems. In addition, the insufficient power generation due to DG-drop off under temporary fault conditions is also investigated. An optimal directional overcurrent bus protection is also presented. Several case studies and analyses are carried out to demonstrate the proposed protection strategy. Results from simulation using MATLAB/Simulink are also shown.