Showing papers on "Power-system protection published in 1994"

Improved techniques for modelling fault arcs an faulted EHV transmission systems

Abstract: A realistic simulation of fault arcs is extremely important in the successful design and development of alternative and improved transmission system equipment such as adaptive auto-reclosure and novel protection schemes It is also particularly important in relation to the design of single pole switched systems This paper discusses, in some detail, techniques that have been developed to model fault arcs more realistically than has been achieved hitherto A time dependent dynamic resistance representation of the primary arc is proposed with emphasis on an empirical approach which is used to determine the parameters concerned In particular, significant improvements have been made to the dynamic conducting characteristic of secondary arc models The paper concludes with a presentation of some interesting studies relating to typical 400 kV single circuit and double circuit systems when the new are models are incorporated into the electromagnetic transients program (EMTP)

Decision trees for real-time transient stability prediction

Abstract: The ability to rapidly acquire synchronized phasor measurements from around the system opens up new possibilities for power system protection and control. This paper demonstrates how decision trees can be constructed offline and then utilized online for predicting transient stability in real-time. Primary features of the method include building a single tree for all fault locations, using a short window of realistic-precision post-fault phasor measurements for the prediction, and testing robustness to variations in the operating point. Several candidate decision trees are tested on 40,800 faults from 50 randomly generated operating points on the New England 39 bus test system. >

Adaptive relay setting for stand-alone digital distance protection

Abstract: Reach accuracy of a distance relay on transmission lines is adversely affected by fault resistance combined with remote-end infeed which is not measurable at the relaying point. Different network conditions correspond to different remote-end infeed behaviour and in conventional setting a safety margin is necessary so as to avoid maloperation. In this paper an adaptive setting concept which can overcome this disadvantage is proposed. A microprocessor based distance relay with this new technique can respond to network conditions that change from time to time and computer simulation has confirmed the validity of this new concept. >

The influence of motor re-acceleration on voltage sags

Abstract: The assumption that a voltage sag is rectangular is not correct in a power system with large induction motor loads. The motors decelerate during the short circuit. After fault-clearing, they will accelerate again, drawing a high reactive current from the supply, causing a prolonged postfault voltage sag. This is aggravated by the removal of branches by the protection. The resulting shape of some voltage sags in an example power system is shown and discussed. For the example power system, a stochastic voltage sag table is determined. This table gives the expected number of sags of different depth and duration. The influence of faster protection and of reduced transformer impedance on the table is presented. A simple motor model is implemented in a method for including interruptions due to voltage sags in the reliability analysis of power systems. This model is presented briefly and used to show the influence of motor parameters on the number of sags that lead to an interruption of plant operation. >

A fuzzy-set approach to fault-type identification in digital relaying

Abstract: Sometime the operations that a relay is required to perform cannot be easily described in a deterministic way. A significant example of this situation is given by the operations a line relay must perform in order to detect the type of fault (line-to-ground, line-to-line, line-to-line involving ground). Recently, the mathematical theory of fuzzy sets featured many practical applications, mainly in industrial controls, and fuzzy-set processors are available to allow real time applications. This paper shows that a fuzzy-set approach can be useful even in digital relaying, whenever "fuzzy" decisions have to be under-taken. A possible application to the detection of the type of fault when symmetrical component relaying techniques are adopted is proposed and the results of simulation tests are given. >

Adaptive distance protection of a double-circuit line

Abstract: Due to changes in the power system, such as generator and line outages and changes in load and generation, the performance of distance relays can vary. In the case of a distance relay protecting a phase of a double-circuit line, the state of the parallel circuit is of major importance. Simulations show that, depending on the power system state, a distance relay can cover from less than 50% up to far more than 100% of the total line length. This is demonstrated with a double-circuit line under the single-line-to-ground fault (SLG) fault condition, since this is the most common type of fault. In this paper the distance protection of a double-circuit line under the SLG fault condition is formulated. To achieve correct operation, the relay does not only use the measured quantities of the circuit-to-be-protected, but also the zero sequence current of the parallel circuit. Such a relay requires extra measuring equipment, and, moreover, the zero sequence current of the parallel circuit cannot always be measured. Therefore, another approach is chosen. A correction factor is introduced, set adaptively according to the actual power system state. In this way, the appropriate setting of the relay is provided, in relation with the actual power system state. A side-effect of the adaptive setting of the relay is that the safety margin in the relay settings is decreased, due to the uncertainty in the power system state. By adapting the relay to the actual power system state, maximum selectivity is achieved, and the protection system as such will be more reliable. >

Development of a new fault location system for multi-terminal single transmission lines

Abstract: Conventional fault location systems which use one-terminal AC voltages and currents are difficult to apply to multi-terminal power systems. This paper discusses a new fault location system for multi-terminal single transmission lines. Asynchronous sampling at each terminal is preferred in order to simplify the transmission equipment and an algorithm for synchronizing the asynchronous sampling data is presented. Another algorithm is presented which converts the original multi-terminal power system by progressive conversion to a system with one fewer terminals to arrive at a 2-terminal system containing the fault. An effective fault locating system can be constructed by combining these algorithms with existing reactive power locating operations. EMTP simulation results are presented. >

Non-unit protection technique for EHV transmission systems based on fault-generated noise. Part 1: signal measurement

Abstract: The paper describes the signal processing techniques developed for a new protection technique for EHV transmission lines. The signal processing unit, which forms a major part of the protection scheme, utilises fault-generated high-frequency signals derived from the signal measurement circuit to determine whether the fault is internal or external to the protected zone. The protection scheme has been designed using computer-aided techniques which closely emulate the hardware currently under development and which is being implemented using present-generation hardware.

An approach to the analysis and detection of crosstalk faults in digital VLSI circuits

Abstract: The continuous reduction of the device size in integrated circuits and the increase in the switching rate cause parasitic capacitances between conducting layers to become dominant and cause logic errors in the circuits. Therefore, capacitive couplings can be considered as potential logic faults. Classical fault models do not cover this class of faults. This paper presents a logic level characterization and fault model for crosstalk faults. The authors also show how a fault list of such faults can be generated from the layout data, and give an automatic test pattern generation procedure for them. >

Transients computation for relay testing in real-time

Abstract: This paper discusses characteristics of a new digital simulator for protection relay testing. The most demanding design requirement is computation of fault transients under the condition of real-time change of power system configuration due to relay operation. This problem is solved using EMTP computational techniques enhanced with novel numerical solutions for dynamic power system configuration change and nonlinear element modeling. An advanced computer architecture is utilized to achieve further optimization of the execution time for the transients computation code. The main advantage of this design is the use of conventional single processor computer architecture in combination with advanced digital signal processors (DSPs). This makes this simulator an off-the-shelf product with all the benefits of commercially available computers priced at a relatively low cost. >

Fault impedance estimation algorithm for digital distance relaying

Abstract: Fault impedance is one of the major parameters that must be estimated accurately in digital distance relaying application. In this paper, a new algorithm is proposed based on symmetrical components theory. The proposed algorithm has computational advantage over previously suggested symmetrical components based algorithms. A procedure for applying shunt fault conditions to the sequence equations to estimate fault impedance of the protected transmission line is discussed. The Alternative Transient Program (ATP) that is available on personal computers was used in evaluating the proposed algorithm. ATP models a power system and simulates many fault conditions on a selected transmission line. Fault data obtained were used in calculating fault impedance using the proposed algorithm. Fault impedance estimates were inserted in relay characteristics to determine suitability of the proposed algorithm for first zone distance protection. Sample results of these studies which show stable fault distance estimates are presented and discussed in the paper. >

Protection system representation in the Electromagnetic Transients Program

Abstract: This paper concerns the addition of the few critical elements of a protection system to the Electromagnetic Transients Program (EMTP), which is one of the most widely used programs for the simulation of transients in power systems. It contains models for almost every major power system component. A protection system consists of instrument transformers, relays, and circuit breakers. Models for current transformers (CTs) and capacitor voltage transformers (CVTs) are developed, validated, and incorporated in the EPRI/DCG EMTP Version 2.0. The user can define the values of the CT and CVT parameters. Total FORTRAN capability has been added to the EMTP; new subroutines and an inbuilt structure to allow the linking of user-defined FORTRAN subroutines with the main EMTP are explained. This capability is necessary to simulate computer relay algorithms. These new features in the EMTP together constitute the critical elements of a protection system. Thus, it is now possible to simulate the dynamic interactions between a power system and a protection system. >

Modeling the protective system for power system dynamic analysis

Abstract: Mathematical models for power system dynamic analysis including the dynamics of the protective system are presented. The formulation of the equations is based on establishing explicit relations between the protection system and the rest of system dynamic elements through the node admittance matrix Y/sub BUS/. This relationship is achieved by representing the circuit breakers of interest as part of the transmission network elements. The protective relays, reclosing relays and circuit breakers are modeled as dynamic devices for tracking the dependence of these devices on the voltage and current inputs using the phasor state variables. A simple example illustrates the concepts. As a direct consequence of the analytical model, the concept of a protection success region is introduced and the implications for stability analysis, relay coordination, adaptive relaying and cascade tripping are discussed. >

Applying low-voltage circuit breakers in direct current systems

Abstract: Low-voltage, direct current equipment in electric utility control systems, uninterruptible power systems (UPS), telecommunication systems, and elsewhere has received increasing attention in recent years. Power conditioning needs are one reason. Closer attention by regulatory agencies is another. This paper is intended to help fill a gap in available material on ratings and application characteristics of low-voltage circuit breakers in DC power systems. >

An expert system for power systems fault analysis

Abstract: This paper describes an expert system for fault analysis which has been put into field tests in the dispatch centre of the North East China Electric Network. Previous expert systems for fault analysis on transmission power systems are mainly based on information about the operation of protective relays, whereas this new expert system mainly uses information on tripped circuit breakers, which is more readily available. For some complicated faults, a small number of relay signals are needed. Data concerning the distribution and characteristics of protective relays are concealed in the fault models in the knowledge base of the expert system. And consequently the large database normally required for these data is unnecessary. This also enables the expert system to be more easily transplanted to other networks. Furthermore, because it can distinguish the operational performances of different relays, it analyses faults more accurately. >

High-resistance faults on a multi-terminal line: analysis, simulated studies and an adaptive distance relaying scheme

Abstract: The presence of a T-connection to a third terminal can drastically affect the performance of a distance relay at the other terminals. Fault resistance, especially high fault resistance, makes this problem more severe and complicated. Detailed analysis of the apparent impedance for these cases as seen from the relaying point is derived and, based on extensive simulations of the infeed/outfeed and fault-resistance effects on the relay characteristics, an adaptive distance relaying scheme is proposed. A microprocessor based distance relay using this new technique can quickly respond to very-high-resistance faults with maximum coverage of the protected line. The validity of this new scheme has been confirmed by real-time testing on a prototype hardware scheme. >

New approach to distance protection for resistive double-phase to earth faults using adaptive techniques

Abstract: Distance protection is prone to mal-operation under resistive double-phase to earth fault conditions. This is caused by the relay measuring the incorrect impedance to the fault due to the influence of infeed from the remote end of the line. The paper describes two adaptation techniques, one for use with the earth elements, and one for use with the phase elements, which enable the correct impedance to be measured, and hence improve distance protection performance under such fault conditions.

Analysis of grounded and ungrounded photovoltaic systems

Abstract: Grounding has always been a subject of controversy during discussions of electrical systems. Grounding techniques and requirements, like language, vary from region to region and country to country. Optimized grounding for personnel protection does not optimize the fire safety of a system and grounding for fire safety does not optimize personnel safety. Grounding to provide protection for equipment requires a third set of requirements. Photovoltaic (PV) power systems are current sources and require different grounding techniques than conventional voltage sources. Distributed leakage paths, multiple fault paths, and new roles for fuses and circuit breakers are among a few of the new issues that need careful consideration. This paper presents and analyzes the grounding issues associated with PV energy sources. Grounding configurations, faults, personnel safety, fire safety, and surge protection are addressed.

Critical considerations for utility/cogeneration inter tie protection scheme configuration

Abstract: Special considerations must be given to protection systems for inter ties between cogeneration plants and existing host utility power systems. The considerations are reviewed in light of contemporary experience with mid size cogenerating plants. Typical inter tie configurations are deliberated with emphasis on implications on the design of relevant protection systems. Protection schemes are presented and evaluations to their advantages and disadvantages are given. The importance of examining the protection scheme requirements during the conceptual design stage of a cogenerating plant is demonstrated. >

Reduction in distribution transformer failure rates and nuisance outages using improved lightning protection concepts

Abstract: This paper presents a case study of an overhead distribution feeder in a very high keraunic area. The feeder was experiencing approximately twice the number of lightning caused transformer failures per year than any other distribution feeder in the area. In an attempt to lower the outage rate and the repair costs, the lightning protection for this feeder was studied and a new protection system was installed. The first section of this paper discusses the outage history of this feeder and existing transformer overvoltage protection. It then details the steps that were taken to improve the protection so acceptable customer service could be achieved. The interruption data results for one year after the protection changes were implemented are then presented. The results from the field trial of the new lightning protection scheme exhibit a significant improvement in the feeder service reliability. The last section of this paper is a compilation of teardown data of failed distribution transformers. The analysis includes failed transformers from the study feeder, and failed transformers from the same electric utility, but not part of the feeder study. The causes of the transformer failures were tabulated. The results from the teardowns indicate that a high percentage of distribution transformers with interlaced and noninterlaced secondary windings fail from low-side voltage surges. >

Protection of dispersed storage and generation units against islanding

Abstract: This paper deals with islanding protection which is the most important protection requirement of dispersed storage and generation units (DSGs). A power based digital algorithm has been developed for the protection of DSG units against islanding. Computer simulation results show that, in addition to effectively detecting the islanding, the new algorithm trips for the load changes that occur during the islanding operation but remains unstable for the load fluctuations while the DSG unit is operating in parallel with the utility network. The algorithm also detects the condition when the utility supply is reconnected to the power island while the two supplies are out of phase. >

Potential applications of expert systems to power system protection

Abstract: Working Group D10 Applications of Expert Systems to Power System Protection of the Line Protection Subcommittee, Power System Relaying Committee, was organized in 1989 with the following assignment as finally adopted: "Survey expert system technology to identify suitable and significant applications to power system protection. Write a technical paper that introduces protection engineers to expert systems and describes several potential applications". This paper represents the work performed by the Working Group in fulfilling its assignment. The paper is divided into sections that describe a number of potential applications thought to be significant by the members of the Working Group. >

Description of a Micro-SMES system for protection of critical customer facilities

Abstract: The system configuration, theory of operation, computer simulations, and important application experiences of a (750 kVA) superconducting magnetic device (Micro-SMES) for application on power distribution circuits to enhance customer power quality are presented. The functions of the major components of the Micro-SMES are separately discussed and the operation of each component is illustrated with an Electro-Magnetic Transients Program (EMTP) simulation example. Application performance characteristics obtained from the first practical installation of a Micro-SMES are summarized and its protection effectiveness is evaluated. >

Shunt capacitor effect on electrical distribution system reliability

Abstract: To improve the security and reliability of a distribution system, as much power as feasible must go through a given transmission line. This can be achieved by using shunt capacitors as compensators. These shunt capacitive compensators improve the load carrying capability of the line by controlling the reactive power flow. Consequently, the capacitor existence cannot be ignored in evaluating system reliability. The paper applies the state-space method to calculate the reliability indices for compensated and uncompensated systems with different success criteria. The importance of using shunt capacitors to improve the level of distribution system reliability is illustrated in addition to their original function as reactive power controllers. The authors procedure, based on a Markov process, is applied to a numerical example, and indicates that system reliability is improved when using shunt capacitors. >

Potential Applications of Expert Systems to Power System Protection Working Group D10 of the Line Protection Subcommittee, Power System Relaying Committee Mark Enns, Chairman; Larry Budler, T.W. Cease, Ahmed Elneweihi, Ed Guro,

Abstract: Working Group D10 Applications of Expert Systems to Power System Protection of the Line Protection Subcommittee, Power System Relaying Committee, was organized in 1989 with the following assignment as finally adopted: Survey expert system technology to identify suitable and significant applications to power system protection. Write a technical paper that introduces protection engineers to expert systems and describes several potential applications. This paper represents the work performed by the Working Group in fulfilling its assignment. The paper is divided into sections that describe a number of potential applications thought to be significant by the members of the Working Group.

Extensive evaluation of high performance protection relays for the Hydro-Quebec series compensated network

Abstract: This paper reports on an extensive study to compare the performance of 8 preselected high performance power line protection systems on the Hydro-Quebec series compensated network. The paper first presents an evaluation of the commissioning aspects and manufacturing quality of the tested relays. It then describes the simulated network, the relay testing procedure, and the laboratory test results obtained on a modern real time power system simulator. Published results of relay performance on series compensated lines are scarce and this paper provides valuable information for utility protection engineers. A total of 115,000 network disturbances were simulated and recorded on 20 different 735 kV lines; fractional test results for 84,030 cases are presented in the paper. The approach using real time power system simulation proved to be an efficient technique for evaluating protection relay performance. The laboratory test results demonstrated that the two best protection principles to be applied on the Hydro-Quebec series compensated network are those based on current differential and segregated phase comparison. The analysis does not take into account telecommunication requirements or other external constraints. >

An expert system for integrated protection design with configurable distribution circuits. II

Abstract: For pt.I see ibid., vol.9, no.2, p.1108-14 (1994). An example protection system design is presented using an integrated expert system developed for an engineering workstation. The example problem consists of three interconnected distribution circuits. Dynamic knowledge, created by the expert system from the rules and system data, is used in the design. System data is presented pertaining to the circuits along with the created dynamic knowledge. The implemented rules dealing with the coordination, placement and selection of protective devices are presented, along with all associated parameter values. The final design and results are shown. >

A novel scheme for protection of power semiconductor devices against short circuit faults

Abstract: Power semiconductor devices find wide application in modern power electronic converters. Protection of these devices against overload/short circuit conditions is of paramount importance. Present day protection topologies employing different circuits have invariably one main drawback in that the fault current reaches the set value before action is initiated to trip the system. This poses a severe stress on the device. Hence an adequate safety margin has to be necessarily provided to prevent excessive device stresses and care has to be taken to see that the device is operated well within its safe operating areas. The present paper proposes a method wherein the slope or rate of rise of the fault current is detected and once the slope exceeds the set reference, action is initiated to trip the system much before the fault current reaches dangerous levels. The method provides a fast means of detection of overload and short circuit currents and can be conveniently adopted for the protection of devices in power transistor/IGBT based inverters against short circuited load conditions or shoot through faults. The possible reduction of stresses in the power devices are also highlighted. >

On site relay transient testing for a series compensation upgrade

Abstract: This paper describes tests on the relays on a long 500 kV AC line carried out on site using the RTDS (Real Time Digital Simulator) of the Manitoba HVDC Research Centre. The purpose of the tests was to examine the relay behaviour when series compensation is inserted in the line in 1993. New settings for the relays have been found which will give adequate cover for all faults although some faults will be entirely dependent on the communication link for short clearance times. >

Realtime fault detection and classification in power systems using microprocessors

Abstract: A method is proposed for real-time fault detection and classification in power systems. The method can be implemented in low-cost units based on 8-bit microprocessors. These units are part of a multiprocessor system which allows the detection of single line to ground faults, double line and double line to ground faults, and three-phase faults. The detection of an overcurrent triggers two simultaneous processes executed in real time: the waveforms of the voltage and current signals are registered and a probabilistic criterion is used for fault confirmation. The process of confirmation is based on the estimation of the three phase voltage phasors by means of a set of Kalman filters, and on the calculation of the fault probability. If the fault has been classified correctly, a recording is made with the samples acquired, including five cycles of the prefault signals and 150 cycles of the postfault signals. This recording is use by the multiprocessor system to calculate the fault distance and to generate an accurate fault recording. The algorithm has been verified using a computer simulation of the different types of faults, and has been optimised so as to detect and correctly classify faults in less than 20 ms.