Showing papers on "Power-system protection published in 2008"

Summary of Distributed Resources Impact on Power Delivery Systems

Abstract: Because traditional electric power distribution systems have been designed assuming the primary substation is the sole source of power and short-circuit capacity, DR interconnection results in operating situations that do not occur in a conventional system. This paper discusses several system issues which may be encountered as DR penetrates into distribution systems. The voltage issues covered are the DR impact on system voltage, interaction of DR and capacitor operations, and interaction of DR and voltage regulator and LTC operations. Protection issues include fuse coordination, feeding faults after utility protection opens, impact of DR on interrupting rating of devices, faults on adjacent feeders, fault detection, ground source impacts, single phase interruption on three phase line, recloser coordination and conductor burndown. Loss of power grid is also discussed, including vulnerability and overvoltages due to islanding and coordination with reclosing. Also covered separately are system restoration and network issues.

Restoration of Directional Overcurrent Relay Coordination in Distributed Generation Systems Utilizing Fault Current Limiter

Abstract: A new approach is proposed to solve the directional overcurrent relay coordination problem, which arises from installing distributed generation (DG) in looped power delivery systems (PDS). This approach involves the implementation of a fault current limiter (FCL) to locally limit the DG fault current, and thus restore the original relay coordination. The proposed restoration approach is carried out without altering the original relay settings or disconnecting DGs from PDSs during fault. Therefore, it is applicable to both the current practice of disconnecting DGs from PDSs, and the emergent trend of keeping DGs in PDSs during fault. The process of selecting FCL impedance type (inductive or resistive) and its minimum value is illustrated. Three scenarios are discussed: no DG, the implementation of DG with FCL and without FCL. Various simulations are carried out for both single- and multi-DG existence, and different DG and fault locations. The obtained results are reported and discussed.

Reliability-Constrained Optimum Placement of Reclosers and Distributed Generators in Distribution Networks Using an Ant Colony System Algorithm

Abstract: Optimal placement of protection devices and distributed generators (DGs) in radial feeders is important to ensure power system reliability. Distributed generation is being adopted in distribution networks with one of the objectives being enhancement of system reliability. In this paper, an ant colony system algorithm is used to derive the optimal recloser and DG placement scheme for radial distribution networks. A composite reliability index is used as the objective function in the optimization procedure. Simulations are carried out based on two practical distribution systems to validate the effectiveness of the proposed method. Furthermore, comparative studies in relation to genetic algorithm are also conducted.

Analysis Results of Output Power Loss Due to the Grid Voltage Rise in Grid-Connected Photovoltaic Power Generation Systems

Abstract: This paper describes the connected photovoltaic (PV) power generation system's grid overvoltage protection function and summarizes the occurrence of the output power loss due to the grid voltage rise. Power injection from the PV system will raise the voltage at the power distribution line. A power conditioning subsystem (PCS) needs to regulate its output if the voltage becomes higher than the upper limit in order to avoid the overvoltage at the power grid. Thus, a PV system cannot generate electricity under the high grid voltage. There are 553 residential PV systems installed in Ota, Japan, for the demonstration research project of clustered PV systems. Measurement data of these 2.1-MW grid-connected PV systems are used for the analysis. Only the limited number of PV systems experienced a significant amount of output energy loss due to the high grid voltage in a particular day, whereas the other system's outputs also raise the grid voltage. The causes of this maldistribution of the output energy loss are the difference of the line impedance, the difference of the starting voltage of the PCS's grid overvoltage protection function, and the imbalance of the load in single-phase three-wire power distribution systems. The present control of the PCS successfully avoids the overvoltage on the grid but cannot share the loss.

A Centrality Measure for Electrical Networks

Abstract: We derive a measure of "electrical centrality" for AC power networks, which describes the structure of the network as a function of its electrical topology rather than its physical topology. We compare our centrality measure to conventional measures of network structure using the IEEE 300-bus network. We find that when measured electrically, power networks appear to have a scale-free network structure. Thus, unlike previous studies of the structure of power grids, we find that power networks have a number of highly-connected "hub" buses. This result, and the structure of power networks in general, is likely to have important implications for the reliability and security of power networks.

Prevention of Reliability Degradation from Recloser–Fuse Miscoordination Due To Distributed Generation

Abstract: Distributed generation (DG) may bring about reliability degradation, instead of reliability enhancement, due to recloser-fuse miscoordination. For the prevention of this reliability degradation, this paper proposes a method to find the threshold value of the DG capacity, beyond which recloser-fuse coordination is lost. Mathematical equations for protective devices are used to set up the protection settings and calculate the threshold value. Three reliability indices-SAIFI, SAIDI, and ENS-are evaluated and compared with the study cases in the reliability test system RBTS bus 2. The results show that the proposed method is effective to prevent the deterioration of system reliability. In addition, a simple modification to the protection system is presented as a preliminary solution.

A Novel Scheme to Identify Symmetrical Faults Occurring During Power Swings

Abstract: A novel, fast unblocking scheme for distance protection to identify symmetrical faults occurring during power swings has been proposed in this paper. First of all, the change rate of three-phase active power and reactive power being the cosine function and the sine function with respect to the phase difference between the two power systems during power swings has been demonstrated. In this case, they cannot be lower than the threshold of 0.7 after they are normalized. However, they will level off to 0 when a three-phase fault occurs during power swings. Thereafter, the cross-blocking scheme is conceived on the basis of this analysis. By virtue of the algorithm based on instantaneous electrical quantities, the calculation of the active and reactive power is immune to the variation of the system power frequency. As the integration-based criterion, it has high stability. Finally, simulation results show that this scheme is of high reliability and fast time response. The longest time delay is up to 30 ms.

A Study of Self-Organized Criticality of Power System Under Cascading Failures Based on AC-OPF With Voltage Stability Margin

Abstract: From the perspective of self-organized criticality, this paper develops a novel model with AC-OPF and AC grid upgrade to study the cascading failures and blackouts in power systems, which overcomes some shortcomings of existing blackout models. The proposed model contains two types of dynamics, one is fast dynamics which simulates the serial blackouts in power systems, the other is slow dynamics which reflects the tendency of the power systems time evolution. This model also has voltage stability analysis function and can reveal critical characteristics from reactive power and voltage viewpoint. Simulation results of the IEEE 118-bus system with this model show that the fast dynamics can capture the cascading process and the criticality property in micro scale. Besides, the macro scale of self-organized criticality of power systems can be revealed from the mean value of fractional overloads and the ratio of total load demand to the total network transfer capability. Furthermore, the voltage stability criticality status could be detected from the eigenvalue with the smallest magnitude through reactive power and voltage relevant modal analysis.

Adaptive-neuro-fuzzy inference system approach for transmission line fault classification and location incorporating effects of power swings

Abstract: In the present milieu, changes in regulations and the opening of power markets have manifested in the form of large amount of power transfer across transmission lines with frequent changes in loading conditions based on market price. Since conventional distance relays may consider power swing as a fault, tripping because of such malfunctioning would lead to serious consequences for power system stability. A frequency domain approach for digital relaying of transmission line faults mitigating the adverse effects of power swing on conventional distance relaying is presented. A wavelet-neuro-fuzzy combined approach for fault location is also presented. It is different from conventional algorithms that are based on deterministic computations on a well-defined model for transmission line protection. The wavelet transform captures the dynamic characteristics of fault signals using wavelet multi-resolution analysis (MRA) coefficients. The fuzzy inference system (FIS) and the adaptive-neuro-fuzzy inference system (ANFIS) are both used to extract important features from wavelet MRA coefficients and thereby to reach conclusions regarding fault location. Computer simulations using MATLAB have been conducted for a 300 km, 400 kV line and results indicate that the proposed localisation algorithm is immune to effects of fault inception, angle and distance. The results contained here validate the superiority of the ANFIS approach over the FIS for fault location.

A Cumulative Sum-Based Fault Detector for Power System Relaying Application

Abstract: In this paper, a cumulative-sum-based fault detection algorithm is proposed for the power system relaying application. Literature suggests the successful application of this method to process control systems where the deviation of parameters is tracked to indicate any abnormal conditions. The effectiveness of the algorithm is found to be better than the traditional methods in the presence of noise, system frequency deviation, and other uncertainties. It is also not affected by load change in a system. Above all, it provides relatively higher index values without compromising detection speed.

Identifying Single-Phase-to-Ground Fault Feeder in Neutral Noneffectively Grounded Distribution System Using Wavelet Transform

Abstract: A scheme of single-phase-to-ground fault feeder identification in distribution networks with the application of a wavelet transform technique is presented in this paper. The scheme uses zero-sequence current traveling waves to identify the faulted feeder, and the busbar residual voltage to determine an event caused by fault or switch operation. The current traveling waves measured by zero-sequence current transducers are decomposed using wavelet multiresolution analysis. The local modulus maxima of the wavelet transform are extracted to determine the time of the initial traveling wave. The wavelet transforms on all feeders at the time are compared in magnitude and polarity with each other to identify the faulted feeder. The feeder identification is independent of the network neutral-point grounding mode. The proposed scheme was implemented and verified using Electromagnetic Transients Program (EMTP)-generated signals. The scheme proved to be robust against transients generated during normal events, such as feeder energizing and de-energizing as well as capacitor bank switching.

Harmonics content based protection scheme for Micro-grids dominated by solid state converters

Abstract: Protecting Micro-grids containing micro-sources equipped with power electronics interfaces is a major challenge for engineers developing techniques to exploit renewable energy sources for electricity generation Conventional techniques based on overcurrent protection have major limitations and new techniques have to be explored The method described in this paper provides reliable and fast detection for different types of faults within the micro-grid This proposed protection scheme consists of two main stages The first stage is mainly used to identify the type of the fault and the second stage which utilise the THD to discriminate between the faulted zones and as a result the relay will trip and isolate the faulted section leaving the rest of the network un-affected Analysis and simulation results are presented for different types of faults within the micro-grid

An Investigation on the Nondetection Zones of Synchronous Distributed Generation Anti-Islanding Protection

Abstract: Anti-islanding protection is an important technical requirement when interconnecting distributed generators. Unfortunately, most anti-islanding protection schemes cannot detect islanding situations under certain system operating conditions - there are nondetection zones within which anti-islanding protection schemes are ineffective. This paper investigates these nondetection zones associated with the common anti-islanding protection schemes of synchronous distributed generators: frequency and voltage-based relays. Moreover, the characteristics of the nondetection zones and the key factors that influence them are analyzed. The results are useful for utility companies and distributed generators owners to design and to evaluate the effectiveness of anti-islanding protection schemes proposed for various distributed generation interconnection projects.

Ground Distance Relaying With Fault-Resistance Compensation for Unbalanced Systems

Abstract: Fault resistance is a critical variable in distance relaying If not considered due to underreaching phenomenon, it may cause the misoperation of ground distance relays for internal faults Still, as a consequence of the overreaching phenomenon, the unbalanced nature of loads and asymmetry of lines can affect the distance protection operation efficiency Mainly due to these aspects, there is low precision in protection zone limits of ground distance relays In this paper, a new algorithm is proposed to increase the precision of these limits, improving efficiency in the distance protection process The proposed method is based in phase coordinates and uses a fault resistance estimate to develop the trip decision procedure The results show that the algorithm is suitable for online applications, and that it has an independent performance from the fault resistance magnitude, the fault location, and the line asymmetry

Lightning and Surge Protection in Photovoltaic Installations

Abstract: The aim of this paper is to give scientific background and essential assumptions to be introduced into the design of lightning and surge protection in photovoltaic installations (PVIs), with particular emphasis on the aspects of standardization to be covered. For this purpose, the relevant protective measures given in the standards for conventional low-voltage power distribution systems (CLVPDSs) are adapted in part. This revision is required because the peculiar characteristics of PVIs are different from those of CLVPDSs. The resulting protection approach that determines the advisable protective measures by a risk management has been applied to an actual grid-connected PVI (GCPVI), Univer Project. The extra cost of this protection in this PVI (approximately 3.6% of the system cost) is of secondary importance because of the increase of safety and availability. Furthermore, in order to fulfill with this protection, the surge withstand capability (SWC) of PV modules has been investigated as well.

Evaluation of Rocof Relay Performances on Networks with Distributed Generation

Abstract: Due to the increasing levels of distributed generation in the distribution network, the correct operation of loss-of mains protection is of utmost importance. Utilities have voiced their concern relating to the false operation of ROCOF relays, which are one of the most commonly employed loss-of-mains detection method. Whilst 'standard settings' are used, the performances of commercially available ROCOF relays are reported to vary considerably. This paper presents an investigation on the characteristic of ROCOF relays that contribute to their diverse operating responses. The factors investigated are frequency measuring techniques, measuring windows, time delays and the under-voltage interlock function. With the increasing penetration of DG into the network, it is becoming common to have multiple DG units connected to the same network location. Two generators connected at similar location and employing ROCOF relays with the same setting but different characteristics were simulated. When subjected to the same network disturbances, the possible interference between the two relays was analyzed.

From Packaging to "Un"-Packaging - Trends in Power Semiconductor Modules

Abstract: Power semiconductor modules play a key role in power electronic systems. Their inherent advantage of integrating different power chips, circuits and sense, drive and protection functions into one sub-system with electrically insulated cooling has lead to a wide range of products, being different in size, power and function. This paper will provide an overview of today's power modules and packaging and interconnect technologies. Trends towards next generations of power modules will be highlighted. In the growing market of hybrid and electrical vehicles, products are emerging where power modules are "un-packaged" to arrive at highly integrated, compact sub-systems which are better suited for the harsh environmental conditions and the required power density than the classical power modules.

Identifying the Primary Fault Section After Contingencies in Bulk Power Systems

Abstract: This paper deals with the problem of fault section estimation in electric power systems, undertaken at a control center level and using information about the operation of protection relays and circuit breakers. The developed methodology should be used after the occurrence of contingencies with definitive disconnections, and before beginning the process of network restoration. Due to the absence of an analytic formulation, the problem calls for the use of artificial-intelligence techniques, such as neural networks and expert systems. Neural networks are employed to model the protection systems, dealing with the uncertainties involved with relay and circuit-breaker operation messages. An expert system is used to complement the results provided by the neural networks, considering the network topology. The results show that the developed methodology is applicable to real large-scale power systems. In addition, it is capable of noise suppression in relay and circuit-breaker trip messages, treats multiple faults naturally, and infers a solution even in cases when remote backup protection action occurs.

Blocking of Zone 3 Relays to Prevent Cascaded Events

Abstract: Defense systems are needed to prevent catastrophic failures of a power grid due to cascaded events. Cascaded events can be attributed to improper operations of protective relays. The most challenging problem for the design and implementation of a defense system is the performance in accuracy and speed in a real-time environment. Protective devices are normally designed to operate fast in order to isolate the fault(s). This paper proposes a new methodology to distinguish line overloads from actual faults for distance relays. In order to distinguish between line flow transfers from a line outage and an actual fault, the line outage distribution factor (LODF) and generation shift factor (GSF)-based power flow estimation method, and a secure peer-to-peer (P2P) communication structure are adopted. Computer simulations of cascaded events for a six-bus system and the Korean power grid have been performed to establish the feasibility of the proposed scheme.

Impact of distributed generation units with power electronic converters on distribution network protection

Abstract: An increasing number of distributed generation units (DG units) are connected to the distribution network. These generators affect the operation and coordination of the distribution network protection. The influence from DG units that are coupled to the network with a power electronic converter (PEC) is different from that of conventional DG units with an electrical machine directly coupled to the network. This contribution shows that PEC-based DG units can be controlled in such a way that the negative influence on the distribution network protection is limited. A decision tree and a flow-chart are presented that show how a PEC-based DG should respond to network faults. The flow-chart can be used in real time to bring the proper control algorithm in operation during a fault.

Islanding protection of distribution systems with distributed generators — A comprehensive survey report

Abstract: Anti-islanding protection schemes currently enforce the DGs to disconnect immediately for grid faults through loss of grid (LOG) protection system. This greatly reduces the benefits of DG deployment. For preventing disconnection of DGs during LOG, several islanding protection schemes are being developed. Their main objectives are to detect LOG and disconnect the DGs from the utility. This allows the DGs to operate as power islands suitable for maintaining uninterruptible power supply to critical loads. A major challenge for the islanding protection schemes is the protection co-ordination of distribution systems with bi-directional fault current flows. This is unlike the conventional overcurrent protection for radial systems with unidirectional fault current flow. This paper presents a comprehensive survey of various islanding protection schemes that are being developed, tested and validated through extensive research activities across the globe.

Design Strategies for Load-Shedding Schemes Against Voltage Collapse in the Hellenic System

Abstract: Analytical description of the design of individual load-shedding schemes against voltage collapse in the Hellenic system is presented in this paper. Analysis and simulation results that validate the efficiency of these protection schemes are provided. This information may prove helpful for the design of similar load-shedding schemes in the future. Two event-driven load-shedding schemes based on design strategies proposed in this paper have been already implemented in the Hellenic system.

A Distance Protection Relay for a 1000-kV UHV Transmission Line

Abstract: A traditional distance protection relay uses a lumped parameter model of the transmission line. For a long 1000-kV UHV line with large distributed capacitance, such a simplified line model is inadequate and can cause malfunction of the relay due to large errors in the impedance measurement. This paper presents a new line model for a distance relay and the procedure of reliable fault impedance measurement and relay setting. It describes the proposed application of the relay developed to a 645-km 1000-kV UHV transmission line including shunt reactor compensation, which is currently being constructed in China. Tests are carried out on an RTDS (real time dynamic system) to evaluate the relay performance, showing significant improvement compared to a more traditional distance relay. The new distance relay is shown to satisfy the requirement of 1000-kV UHV transmission line protection.

Real-Time Power System Control Using Synchrophasors

Abstract: To date, synchronized phasor measurements have been used mainly for power system model validation, postevent analysis, real-time display, and other similar activities. However, synchrophasors have a greater potential than monitoring and visualization. Synchrophasors will increasingly contribute to the reliable and economical operation of power systems as real-time control and protection schemes become broadly used. Synchronous phasor measurements are now available in relays and meters; however, a practical means of processing the data in real time had been lacking. This paper describes a synchronous vector processor and several practical applications, including automated diagnostics, remedial action schemes, direct state measurement, and stability assessment.

Protection of micro-grids dominated by distributed generation using solid state converters

Abstract: The protection of micro-grids containing distributed generation using solid state converters can not depend on monitoring the fault current to detect short circuit faults. In order to protect the semi-conductor devices, the output current from the converter is generally limited to about twice the unit's full load current. This paper describes an alternative and novel approach based on monitoring disturbances in the voltages seen at the generator's terminals to protect the generator and the associated network. It considers the application of this technique to a three-phase network and presents simulation results to a variety of different faults. These demonstrate the operation of the protection scheme and its ability to handle the different types of faults.

Power system restoration dynamics issues

Abstract: Recognizing that power system blackouts are likely to occur, it is prudent to consider the necessary measures that reduce their extent, intensity and duration. Immediately after a major disturbance, the power systempsilas frequency rise and decay are arrested automatically by various control and protection schemes. The success rate of these automatic mechanisms has been about 50%! The challenge is to coordinate the control and protective mechanisms with the operation of the generating plants and the electrical system. During the subsequent restoration phases, plant operators in coordination with system operators attempt to manually maintain a real and reactive balance. The duration of these manual procedures has invariably been much longer than equipment limitations can accommodate. In this paper several restoration dynamic phenomena are presented and the required research and development identified.

Multi-load level reactive power planning considering slow and fast VAR devices by means of particle swarm optimisation

Abstract: The problem of optimal allocation of fast and slow reactive power VAR devices under different load levels is addressed. These devices are supposed to be utilised to maintain system security in normal and contingency states, where corrective and preventive controls are implemented for the contingency cases. Load shedding and fast VAR devices are used in the corrective state in order to restore the system stability very quickly, even though they are highly expensive, whereas cheap slow VAR devices can be used in the preventive state to obtain the desired security level. The main objective is to establish a trade-off between economy and security by determining the optimal combination of fast and slow controls (load shedding, new slow and fast VAR devices). To meet the desired steady-state security limits, a variety of constraints have to be considered during the investigated transition states. The overall problem is formulated as a large-scale mixed-integer nonlinear programming problem. Particle swarm optimisation as an efficient method for solving such problems is applied to solve the problem. The proposed approach has been successfully tested on the IEEE-14 as well as IEEE-57 bus systems.

Bidirectional Front End Converter for DG With Disturbance Insensitivity and Islanding-Detection Capability

Abstract: This paper introduces and develops a new control strategy for the bidirectional front end converter for grid-connected distributed generation units. In the method, the switching technique directly controls the current of the voltage-source converter. The switching method is based on the evaluation of the inductor-flux error trajectory which allows to predict the next commutation instant. The control is fast and the line-current waveform is insensitive to the grid voltage perturbations or harmonics, and dc bus disturbances. A new frequency detection algorithm, robust against disturbances and with fast response in case of islanding occurrence, is introduced and applied. The proposed control method has been examined under various scenarios, including mains supply faults, distorted and unbalanced grid, and IEEE 1547 anti-islanding test. The results show the feasibility and applicability of the proposed control for the considered electronically-interfaced distributed generation units.

Coordination of generator protection with generator excitation control and generator capability

Abstract: This paper was written by a working group of the IEEE power system relay committee to provide guidance to the industry to better coordinate generator protection with generator control. The paper discusses specific calculation methods that can be used to ensure that generator protection and excitation system control are fully coordinated. It also specifically addresses the coordination of relays with generator full load capability and machine steady state stability limits. Because of recent blackouts, NERC (North American electric reliability council) is developing standards for the coordination of generator protection and control. This paper provides practical guidance on providing this coordination.