Showing papers on "Power-system protection published in 2014"

Survey on Fault-Tolerant Techniques for Power Electronic Converters

Abstract: With wide-spread application of power electronic converters in high power systems, there has been a growing interest in system reliability analysis and fault-tolerant capabilities. This paper presents a comprehensive review of conventional fault-tolerant techniques regarding power electronic converters in case of power semiconductor device failures. These techniques can be classified into four categories based on the type of hardware redundancy unit: switch-level, leg-level, module-level, and system-level. Also, various fault-tolerant methods are assessed according to cost, complexity, performance, etc. The intent of this review is to provide a detailed picture regarding the current landscape of research in power electronic fault-handling mechanisms.

Graphical Methods for Defense Against False-Data Injection Attacks on Power System State Estimation

Abstract: The normal operation of power system relies on accurate state estimation that faithfully reflects the physical aspects of the electrical power grids. However, recent research shows that carefully synthesized false-data injection attacks can bypass the security system and introduce arbitrary errors to state estimates. In this paper, we use graphical methods to study defending mechanisms against false-data injection attacks on power system state estimation. By securing carefully selected meter measurements, no false data injection attack can be launched to compromise any set of state variables. We characterize the optimal protection problem, which protects the state variables with minimum number of measurements, as a variant Steiner tree problem in a graph. Based on the graphical characterization, we propose both exact and reduced-complexity approximation algorithms. In particular, we show that the proposed tree-pruning based approximation algorithm significantly reduces computational complexity, while yielding negligible performance degradation compared with the optimal algorithms. The advantageous performance of the proposed defending mechanisms is verified in IEEE standard power system testcases.

High speed differential protection for smart DC distribution systems

Abstract: This paper presents a high speed current differential implementation approach for smart dc distribution systems capable of sub-millisecond fault detection. The approach utilizes the natural characteristics of dc differential current measurements to significantly reduce fault detection times compared to standard applications and hence meet requirements for dc converter protection (around 2 ms). Analysis is first developed to help quantify protection implementation challenges for a given dc system. Options for implementing the proposed technique are then illustrated. Results of scaled hardware testing are presented which validate the overall protection operating times in a low voltage environment. These results show the implementation approach can consistently achieve protection system operating within the order of a few microseconds .

A Differential Sequence Component Protection Scheme for Microgrids With Inverter-Based Distributed Generators

Abstract: The protection of a microgrid containing inverter- based distributed generators (IBDGs) presents several problems if traditional techniques which rely on the current (fuses and overcurrent relays) are used. A possible solution to these problems is the use of a new type of the relay which takes advantage of the enhanced processing techniques and communication infrastructure, both of which are recently becoming available for power networks application. This paper proposes a new communication-based protection scheme for isolated microgrids where a data mining approach is used to identify the relay settings and parameters. A feature selection technique is implemented to help identify the most relevant electrical features required for the fault detection and to establish the best communication strategy to use between relays. The proposed approach is tested using a MATLAB simulation of a facility scale isolated microgrid embedded with IBDGs. The results show that a differential protection scheme that relies on symmetrical components is the most effective strategy for protecting microgrids with IBDGs.

Time-frequency transform-based differential scheme for microgrid protection

Abstract: The study presents a differential scheme for microgrid protection using time-frequency transform such as S-transform Initially, the current at the respective buses are retrieved and processed through S-transform to generate time-frequency contours Spectral energy content of the time-frequency contours of the fault current signals are calculated and differential energy is computed to register the fault patterns in the microgrid at grid-connected and islanded mode The proposed scheme is tested for different shunt faults (symmetrical and unsymmetrical) and high-impedance faults in the microgrid with radial and loop structure It is observed that a set threshold on the differential energy can issue the tripping signal for effective protection measure within four cycles from the fault inception The results based on extensive study indicate that the differential energy-based protection scheme can reliably protect the microgrid against different fault situations and thus, is a potential candidate for wide area protection

An advanced protection scheme for enabling an LVDC last mile distribution network

Abstract: Low voltage direct current (LVDC) distribution systems have the potential to support future realisation of smart grids and enabling of increased penetration of distributed renewables, electric vehicles, and heat pumps. They do however present significant protection challenges that existing schemes based on DC fuses and conventional electro-mechanical circuit breakers (EMCBs) cannot manage due to the nature of DC faults and slow device performance. Therefore, this paper presents an advanced protection scheme that addresses the outstanding challenges for protecting an LVDC last mile distribution network. The scheme takes advantage of advanced local measurements and communications that will be naturally integrated in smart grids, and the excellent level of controllability of solid state circuit breakers. It thus provides fast DC fault detection and interruption during DC transient periods in addition to achieving fault limitation and fast reliable restoration. The introductory part of the paper quantifies the potential benefits of LVDC last mile distribution networks, and discusses the potential LVDC architectures that best utilise the existing plant. Based on the new LVDC architectures, a typical UK LV network is energised using DC and modelled, and used as a case study for investigating the protection issues and evaluating the new protection scheme performance through simulation.

Traveling Wave-Based Protection Scheme for Inverter-Dominated Microgrid Using Mathematical Morphology

Abstract: Inverter-dominated microgrids impose significant challenges on the distribution network, as inverters are well known for their limited contribution to fault current, undermining the performance of traditional overcurrent protection schemes This paper introduces a new protection scheme based on the initial current traveling wave utilizing an improved mathematical morphology (MM) technology, with simplified polarity detection and new logics introduced for meshed networks and feeders with single-end measurement The proposed protection scheme provides ultrafast response and can be adapted to varied system operational modes, topologies, fault conditions, and load conditions Only low-bandwidth communication is required to achieve high-speed operation and adequate discrimination level in meshed networks Simulation in PSCAD/EMTDC verifies both the sensitivity and stability of the proposed protection scheme under different microgrid operational scenarios

Revealing cascading failure vulnerability in power grids using risk-graph

Abstract: Security issues related to power grid networks have attracted the attention of researchers in many fields. Recently, a new network model that combines complex network theories with power flow models was proposed. This model, referred to as the extended model, is suitable for investigating vulnerabilities in power grid networks. In this paper, we study cascading failures of power grids under the extended model. Particularly, we discover that attack strategies that select target nodes (TNs) based on load and degree do not yield the strongest attacks. Instead, we propose a novel metric, called the risk graph, and develop novel attack strategies that are much stronger than the load-based and degree-based attack strategies. The proposed approaches and the comparison approaches are tested on IEEE 57 and 118 bus systems and Polish transmission system. The results demonstrate that the proposed approaches can reveal the power grid vulnerability in terms of causing cascading failures more effectively than the comparison approaches.

The Influence of Inverter-Based DGs and Their Controllers on Distribution Network Protection

Abstract: The ever growing penetration of distributed generation (DG) in a distribution network has a profound impact on network protection and stability. Traditional protection schemes and algorithms need to be extensively investigated as more and more DGs get introduced into the network. The current version of IEEE Standard 1547 does not present a comprehensive solution for fault current detection in the presence of various kinds of DGs. Power electronic inverter-based DGs (IBDGs) are of special concern in distribution network protection as they are often incapable of providing sufficient fault current and their controllers play a principal role in the DG behavior. In this paper, the effects of voltage and current controllers for IBDGs on industrial and commercial power system protection schemes are investigated. It is shown that the inverter control mode has a direct impact on its fault current levels and duration. A simplified distribution network model with IBDG operating under voltage and current control modes was tested to verify the effects of these controllers. This paper also proposes an adaptive relaying algorithm to detect the faults in the presence of IBDGs with various types of controllers.

Development of a Novel Protection Device for Bipolar HVDC Transmission Lines

Abstract: A novel nonunit transient-based protection scheme for bipolar high-voltage direct current (HVDC) lines is proposed. The protection scheme is composed of a starting unit, a boundary unit, a directional unit, a faulty pole identification unit, and a lightning disturbance identification unit. The prototype based on the proposed scheme is developed and its performance is evaluated through a hardware-in-the-loop test using real-time digital simulator (RTDS) and real HVDC control devices. Better than the presently used line protection, the proposed protection not only can detect faults on HVDC lines more rapidly and accurately in different HVDC operation modes, but is also more effective in detecting high-impedance grounding faults. In addition, the novel protection is designed to identify lightning disturbances from short-circuit faults correctly, which avoids the unnecessary restart of HVDC systems due to protection misjudgment.

Preventive control approach for voltage stability improvement using voltage stability constrained optimal power flow based on static line voltage stability indices

Abstract: Voltage stability improvement is a challenging issue in planning and security assessment of power systems. As modern systems are being operated under heavily stressed conditions with reduced stability margins, incorporation of voltage stability criteria in the operation of power systems began receiving great attention. This study presents a novel voltage stability constrained optimal power flow (VSC-OPF) approach based on static line voltage stability indices to simultaneously improve voltage stability and minimise power system losses under stressed and contingency conditions. The proposed methodology uses a voltage collapse proximity indicator (VCPI) to provide important information about the proximity of the system to voltage instability. The VCPI index is incorporated into the optimal power flow (OPF) formulation in two ways; first it can be added as a new voltage stability constraint in the OPF constraints, or used as a voltage stability objective function. The proposed approach has been evaluated on the standard IEEE 30-bus and 57-bus test systems under different cases and compared with two well proved VSC-OPF approaches based on the bus voltage indicator L - index and the minimum singular value. The simulation results are promising and demonstrate the effectiveness of the proposed VSC-OPF based on the line voltage stability index.

Mitigating cascading failures in interdependent power grids and communication networks

Abstract: We study the interdependency between the power grid and the communication network used to control the grid. A communication node depends on the power grid in order to receive power for operation, and a power node depends on the communication network in order to receive control signals. We demonstrate that these dependencies can lead to cascading failures, and it is essential to consider the power flow equations for studying the behavior of such interdependent networks. We propose a two-phase control policy to mitigate the cascade of failures. In the first phase, our control policy finds the unavoidable failures that occur due to physical disconnection. In the second phase, our algorithm redistributes the power so that all the connected communication nodes have enough power for operation and no power lines overload. We perform a sensitivity analysis to evaluate the performance of our control policy, and show that our control policy achieves close to optimal yield for many scenarios. This analysis can help design robust interdependent grids and associated control policies.

Wide-Area Measurement-Based Backup Protection for Power Network With Series Compensation

Abstract: This paper presents a synchronized phasor measurement-based wide-area backup protection scheme which compares the magnitude of sequence voltages of buses at a system protection center to identify the bus closest to the fault. Then, the sign of cosine of the angle between voltage and current at both ends of all lines connected to this bus are compared to identify the faulted branch. The technique is tested for various faults resulting in voltage/current inversion in a series-compensated line and during load encroachment and power swing conditions using data simulated through EMTDC/PSCAD for a nine-bus, three-machine power system. The scheme is found to be accurate and fast. With today's synchronized phasor measurement technology and communication system, the proposed scheme can overcome the limitations of the conventional backup protection scheme.

Transmission-Line Fault Analysis Using Synchronized Sampling

Abstract: An automated analysis approach, which can automatically characterize fault and subsequent relay operation, is the focus of this paper. It utilizes synchronized samples captured during transients from both ends of the transmission line to detect, classify, and locate transmission-line faults and can verify that the tripped line has indeed experienced a fault. The proposed method is tested for several faults simulated on an IEEE 118-bus test system and it has been concluded that it can detect and classify a fault using prefault and postfault recorded samples within 7 ms of fault inception and can accurately locate a fault with 3% accuracy. This time response performance is highly desirable since with the increasing use of modern circuit breakers, which can open the faulty line in less than two cycles, the time window of the captured waveforms is significantly reduced due to the unavailability of measurement signals after breakers open.

A Novel Fault Current Control Scheme to Reduce Synchronous DG's Impact on Protection Coordination

Abstract: Synchronous-machine DGs are well known to cause miscoordination of distribution system protections because of the generator's ability to contribute large fault currents to the fault. This paper proposes a field discharge circuit to limit the generator's fault current, thus leading to a synchronous-machine DG with little impact on distribution system protection. In this paper, the operation of a solid-state switch-based field discharge circuit is studied, and its effects on the generator's output current during the fault are investigated. It is shown that the proposed scheme removes the steady-state component of the fault current and accelerates the decay of the transient ac component of the current. The results demonstrate that the proposed field discharge circuit is sufficient to prevent miscoordination of the feeder protections when short time-delay and/or inverse-time overcurrent relays are involved in the protection scheme.

Review on protection coordination strategies and development of an effective protection coordination system for DC microgrid

Abstract: In this paper, common DC-fault detection methods have been reviewed. Effects of two line-to-line and line-to-ground fault types from various fault locations to operation of the DC system are presented. In addition, operation principle, strong and weak points of four main types of protection devices (including fuses, no-fuse circuit breakers, power-electronic protection devices and protective relays) are mentioned. Each different type of protection devices has the ability to protect and isolate different components of the DC microgrid (e.g. power converter, PV system, battery system, capacitor and others) under fault occurrences. The paper analyses possible protection coordination strategies of protection devices to ensure safety of any components in the DC microgrid. These summarized coordination strategies can be suitable for any DC microgrid configurations. In the next content, an effective protection coordination system of a real community-sized DC microgrid is developed, which use fast-acting fuses to replace no-fuse circuit breakers already installed at some certain locations in the DC microgrid. Aims of this improved protection coordination system are to shorten critical fault clearing time and get the cost effectiveness while still ensuring high selectivity, dependability and safety of the DC microgrid. As a result, the no-fuse circuit breakers placed at locations such as: output of PV arrays, output of the battery, output of the fuel-cell system, and terminals of power converters are efficiently replaced by the fast-acting fuses. Additionally, protection devices located at load feeders are suggested to use the relays to optimise the coordination time between main and back-up protection in case of faults occurring at the load feeders.

Application of synchronised phasor measurements to wide-area fault diagnosis and location

Abstract: This study introduces a novel approach to power system fault diagnosis by synchronised phasor measurements. Conventionally, faults are diagnosed through the status of protective relays and circuit breakers which are activated following a fault. However, the hidden failures of the protection system has itself often been among the main suspects of partial or widespread blackouts. This study proposes an alternative fault diagnosis approach independent of the function of the protection system. An analytical method is introduced for power system fault diagnosis using dispersed synchronised measurements and bus impedance matrix ( Z bus). Fault inception is first detected by local phasor measurement units (PMUs). Fault diagnosis is then carried out in a hierarchical manner so that first the faulted zone of the system is diagnosed, next the faulted line in the faulted zone is diagnosed and finally the fault point along the diagnosed line is located by gradient descent. The proposed method is applied to the WSCC 9-bus, where fault incidents on all of the transmission lines are examined. Moreover, the proposed method is successfully applied to the IEEE 118-bus test system consisting of 28 PMUs, which demonstrates successful fault diagnosis and location for a large-scale power system despite the limited coverage of PMUs.

New support vector machine-based digital relaying scheme for discrimination between power swing and fault

Abstract: This study presents a new support vector machine (SVM)-based identification method which effectively discriminates between various types of fault and power swing conditions. Different power swing cases, fault cases and fault during power swing cases have been generated with varying fault and system parameters using PSCAD/EMTDC software package. The performance of the developed algorithm has been tested over 3510 testing dataset. Using three phase current samples for half cycle duration of each simulation case of post-fault/power swing conditions, an overall classification accuracy of 98.71% is achieved. The proposed scheme is capable to detect faults during power swing condition accurately as the current waveform/signatures for both the cases are entirely different. On the other hand, the conventional scheme do not provide effective discrimination in the said situation as they compare magnitude/phase of the current signals (require pre/post-processing of signals). In addition, conventional scheme gives poor accuracy during unknown system/unseen dataset whereas the proposed scheme provides promising accuracy (97.61%) in the said situation.

A Fault Detection and Classification Technique Based on Sequential Components

Abstract: This paper presents a fault classification technique for transmission lines based on the fault sequence components, for fast and reliable operation of protective relays. First, symmetrical components of fault current and voltage signals are extracted. Next, the fault type is classified using the zero and negative sequences. To realize the faulted phases in ground faults, a criterion index based on the zero and negative sequences is defined. The imaginary part of the defined criterion index is used. This index is maximum in the faulted phase in single-phase-to-ground faults, and it is minimum in one of the faulted phases in phase-to-phase-to-ground faults. In addition, to identify ungrounded faults, a different criterion index using the positive and negative sequences is defined. The results show that the scheme provides a fast fault classification, without the need for a threshold to operate.

Designing Cold Ironing Power Systems: Electrical Safety During Ship Berthing

Abstract: Cold ironing power system design requires unique components to supply shore power to ships for cold ironing operation. Currently, the development of new standards is in progress, and operating procedures are being written to maximize electrical safety, standardization of the process, and interchangeability from one location to another. This article describes the power system design, including a power system protection scheme, which should enhance the electrical safety by design. The power system grounding, equipment grounding, and touch potential that can impact personnel safety are described. A very basic outline of the operating procedures and training needed for the operators to maximize electrical safety during cold ironing operation are also included in this article. In addition, this article provides the current status of the draft International Electrotechnical Commission (IE C)/International Organization for Standardization (ISO)/IE Standards 80005-1 [5] and 80005-2 [6].

Automated analysis of power systems disturbance records: Smart Grid big data perspective

Abstract: Analysis of faults and disturbances play crucial roles in secure and reliable electrical power supply. Digital fault recorders (DFR) enable digital recording of the power systems transient events with high quality and huge quantity. However, transformation of data to information, expectedly in an automated way, is a big challenge for the power utilities worldwide. This is a key focus for realizing the `Smart Grid'. In this paper, the architecture and specifications for the primary and the secondary information for the automated systems are described. This provides qualitative and quantitative guidelines about the information to derive out of the disturbance data. A quantified estimate of big data for the substations, has been estimated in the paper. Possible ways of reducing the big data by utilizing intelligent segmentation techniques are described, substantiated by real example. Utilization of centralized protection and remote disturbance analysis for reducing big disturbance data are also discussed.

Multifunctional Coreless Hall-Effect Current Transformer for the Protection and Measurement of Power Systems

Abstract: This paper presents a multifunctional coreless current transformer based on the Hall effect (multifunctional HCT), and designs its actual electronic circuit that can discriminate load and fault currents to perform both metering and protective functions. The multifunctional HCT has two primary circuits: 1) distinguishes the load current from the fault current and 2) delivers the current value using an analog multiplexer. This HCT can eliminate the 2.5±0.05 V offset voltage of commercial Hall sensors. It possesses two amplifiers with different gains. Therefore, this multifunctional HCT accurately measures power currents during the operations in measurement systems and protection systems. Using a low-pass filter, the multifunctional HCT is not only capable of reducing noise interference, but also improves the accuracy of power current measurements. The experimental results show that the multifunctional HCT can exhibit an accuracy class of 0.5 for both the measurement and protection of power systems within IEC standard 60044-8. Because the proposed multifunctional HCT does not have core and saturation problems, the accuracy class and limit can achieve the standard accuracy limit factor grade of 63.

A review on Microgrid protection

Abstract: Incorporation of Microgrid (MG) comprising of distributed generators (DGs), in the existing bulk power system has become an inevitable trend. The traditional distribution network designed to operate radially is protected by simple protective devices such as fuses, reclosers and over-current relays (OCRs). This changes the distribution system from single supply radial system to multi-source network which causes the multidirectional power flow and also having significant impact on protection coordination issues. The presence of DG and MG changes the Load-Flow of the overall network and also influences the fault current magnitude and direction. A lot of research work is carried out for improvement of the grid connected distributed generators for the safe and reliable operation of the overall system. This paper proposes a comprehensive review on the traditional protection scheme of the distribution system and the challenges associated with the MG protection system along with the discussion of some alternate protection strategies.

A Novel Backup Distance Protection Scheme for Series-Compensated Transmission Lines

Abstract: Conventional distance protection applies the positive-sequence impedance to protect a line against short-circuit faults. Series-compensated lines may considerably change the positive-sequence impedance of the fault path and cause the distance relays to maloperate. To overcome this problem, the mutual impedance between phases of a transmission line is used for the design of a new distance protection scheme. The voltages and currents of both ends of the line are applied to compute the mutual impedance between the relay and the fault point. The proposed scheme has a reliable performance in protection against single-phase and double-phase-to-ground faults and, therefore, it can be used as a backup protection. Simulation results approve the efficiency of the proposed method in protection of series-compensated transmission lines.

Real Time Parameter Estimation for Power Quality Control and Intelligent Protection of Grid-Connected Power Electronic Converters

Abstract: This paper presents a method to identify power system impedance in real-time using signals obtained from grid- connected power electronic converters. The proposed impedance estimation has potential applications in renewable/distributed energy systems, STATCOM, and solid state substations. The method uses wavelets to analyze transients associated with small disturbances imposed by power converters and determine the net impedance back to the source. A data capture period of 5 ms is applied to an accurate impedance estimation which provides the possibility of ultra fast fault detection (i.e., within a half cycle). The paper describes how the proposed method would enhance the distributed generation operation during faults.

Apparatus, system and method for total protection from electrical faults

Abstract: Electrical fire risks and consumer safety are major concerns driving regulators worldwide to implement more stringent electrical code and enforcement. While there are many commercially available protective devices available, none affords total protection, and they are made and classified only to protect circuits from specific or limited number of faults. This invention is about an apparatus, system and method for protection of electrical circuits and consumers from literally all known electrical faults in Direct Current (DC) and Alternating Current (AC) single and multi-phase systems such as and including Arc Faults, Ground and Leakage Faults, Surge, Overload, Short Circuit, Glowing Connections, Miswired Connections, overvoltage, undervoltage and Phase-Loss. Said apparatus, system and method come in the form of or in a housing as a Circuit Breaker, Receptacle, Convenience Outlet, Attachment Plug, Equipment Controller, a circuit, or a system integrated with another system, device or apparatus.

Protection of Microgrid through Coordinated Directional Over-current Relays

Abstract: Microgrids integrate distributed energy resources to provide reliable, environment friendly and economic power to small/medium sized urban communities or to large rural areas. Due to the existence of generators at all levels of the distribution system and two distinct operating modes, i.e. grid connected and islanded modes, the fault currents in a system vary substantially. With appropriate relay coordination they gain the ability to island generators and loads together. This has a potential to provide a higher local reliability than that provided by the power system as a whole. The protection scheme implemented addresses the existing protection challenges encountered due to the presence of both distributed generators and inverter interfaced distributed generators in the same system. The proposed protection scheme equipped with directional overcurrent relays is tested using ETAP on a microgrid that consists of distributed energy resources like photovoltaic arrays, wind, diesel generator and micro hydro turbine for various fault locations.

Simulation and analysis of faults in high voltage DC (HVDC) power transmission

Abstract: Modern civilization depends heavily on the consumption of electrical energy for industrial, commercial, agricultural, domestic and social purposes. However, for the current HVDC system, proper protection devices and logic are not yet as mature as the AC counterpart. This paper presents the fault analysis for the protection of the HVDC (65–765 kV range) grid, using PSCAD. Faults in the DC transmission line are analyzed. This paper also looks into the response of the system to each kind of faults. It is observed that the AC and DC faults have different signatures allowing us to tell them apart. The rise time of the fault current is presented here. Analysis of load changes is also done comparatively with the fault cases.

Protection for DC Distribution System with Distributed Generator

Abstract: DC distribution system has advantages of high power quality, large transmission capacity, high reliability, simple structure, economy and low energy consumption, and so forth. It has been a key part of smart grid nowadays. However, the development of DC distribution system is constrained by the lack of operational experience in DC system, the small interrupting capacity of DC circuit breaker (CB), and the lack of protection schemes for system itself. In this paper, protection for DC distribution system with distributed generator (DG) is fully investigated and verified. Firstly, the electromagnetic transient model of DC distribution system with DG is presented. Simulation based on the electromagnetic transient model is carried out. Both the step response and the steady-state performance verify the accuracy of the model. Then the fault characteristic mechanism is analyzed, and the protection principles and scheme are investigated in detail, including voltage mutation principle as protection starting component, differential current protection principle for DC bus, and two-section current protection for distribution lines. Finally, transient responses with protection scheme are analyzed during faults. The results present that the protection principles and scheme are feasible for DC distribution system with DG.