Inverse Time Overcurrent Research Articles

A proposed coordination strategy for meshed distribution systems with DG considering user-defined characteristics of directional inverse time overcurrent relays

In this paper, coordination strategy that considers using user defined characteristics for the inverse time overcurrent relays is proposed. Typically, the coordination between relays operating times within meshed systems are achieved by adjusting two relay settings; pick up current and time multiplier setting (TDS and Ip). The equation that models the digital inverse time overcurrent relay operation has two constants; one of them represents the constant for relay characteristics (A) and the other one represents the inverse time type (B). The proposed coordination strategy considers the two relay characteristics constants as continuous variable settings that can be adjusted. These (A and B) values are chosen optimally in addition to (TDS and Ip) to achieve coordination. The coordination problem is formulated as a nonlinear programming problem where the main objective is to minimize the overall time of operation of relays during primary and backup operation considering faults at different locations. The results are compared against the relay coordination using the conventional settings. The problem is applied to the meshed power distribution network of the IEEE 30 bus systems equipped with synchronous based DGs. The results show that the proposed strategy can significantly reduce the overall relay operating time and thus making it an attractive option for meshed distribution systems with DG.

Allowable capacity of ACDG considering inverse time overcurrent protection

The integration of distributed generation (DG) into distribution system has a lot of impacts on the original current protection. The influences of alternating current (AC)-connected DG on inverse time overcurrent protection are analyzed in this paper. Based on the setting and coordination principles of protection, four constraints for the allowable capacity of AC-connected DG are advised to ensure the reliability of protection. A specific example is given to show the calculation process. In order to increase the allowable capacity, some improvement methods are proposed. Two different integration points are compared, and finally the one requiring less of protection modification is chosen. The effectiveness of the measures is also verified through the example. Copyright © 2014 John Wiley & Sons, Ltd.

Table Based Algorithm for Inverse-Time Overcurrent Relay

Abstract This paper presents a new table based algorithm for overcurrent relay with inverse-time characteristic. The algorithm is based on loading the adequate time vector through which inverse-time characteristic is modeled. It uses samples of the current and calculates rms value. The rms current represents an input value for the index estimation what determines corresponding element from already loaded time vector. Since the calculations used in this algorithm are based on simple mathematical operations, short processing time is achieved. Performances of the algorithm are tested by several computer-generated signals. Furthermore, comparative analyses showed indisputably that suggested procedure possesses significant advantages compared to other solutions: simplicity, high speed of operation and accuracy.

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

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.

Extending Motor Life With Updated Thermal Model Overload Protection

Thermal issues shorten motor life. Modern motor relays must protect for these effects, which can occur for motors from small to large (50 hp and greater) and for all voltage operational levels. However, being too conservative on thermal limits decreases motor productivity. An ANSI/IEEE 49 thermal element creates a realistic thermal model of the motor because the element takes the load level and negative-sequence currents into account. The 49 thermal element is superior to older methods using inverse-time overcurrent elements (51). This paper discusses updates in the existing 49 thermal model implementation. Also discussed are resistance temperature detector (RTD) biasing, RTD voting, and the role of transducers in the modern motor relay.

An Intelligent Algorithm for the Protection of Smart Power Systems

The paper proposes a new concept of protection philosophy for Dutch distribution grids with high penetration of renewable energy. Conventional protection concepts are normally based on protection schemes that consist of definite or inverse time overcurrent relays. In future power systems, these schemes can lead to high fault clearing times, unselective tripping and massive DG disconnections, which are not acceptable in a deregulated multi-owner energy market. This research work proposes an efficient intelligent communication-based protection algorithm that implements different multi-functional protection principles supported by blocking schemes. The real-time organization of both the hardware architecture and communication infrastructure of the protection algorithm is illustrated in detail. Special emphasis is given to the network reconfiguration scenario cases and detailed simulation results of such illustrative studied cases are provided. The new protection strategy guarantees protection selectivity and provides DG-unit availability during and after the fault. The intelligent algorithm is applied on existing Dutch distribution network and meaningful conclusions are derived.

Adaptive Protection Algorithm for Overcurrent Relay in Distribution System with DG

This paper proposes the new adaptive protection algorithm for inverse-time overcurrent relays (OCRs) to ensure their proper operating time and protective coordination. The application of the proposed algorithm requires digital protection relays with microcontroller and memory. The operating parameters of digital OCRs are adjusted based on the available data whenever system conditions (system with distributed generation (DG)) vary. Moreover, it can reduce the calculation time required to determine the operating parameters for achieving its purpose. To verify its effectiveness, several case studies are performed in time-domain simulation. The results show that the proposed adaptive protection algorithm can keep the proper operating time and provide the protective coordination time interval with fast response.

Mitigation of Distributed Generation Impact on Protective Devices in a Distribution Network by Superconducting Fault Current Limiter

Protection of radial distribution networks is widely based on coordinated inverse time overcurrent relays (OCRs) ensuring both effectiveness and selectivity. However, the integration of distributed generation (DG) into an existing distribution network not only inevitably increases fault current levels to levels that may exceed the OCR ratings, but it may also disturb the original overcurrent relay coordination adversely effecting protection selectivity. To analyze the potentially adverse impact of DG on distribution system protective devices with respect to circuit breaker ratings and OCR coordination fault current studies are carried out for common reference test system under the influence of additional DG. The possible advantages of Superconducting Fault Current Limiter (SFCL) as a means to limit the adverse effect of DG on distribution system protection and their effectiveness will be demonstrated. Furthermore, minimum SFCL impedances required to avoid miss-operation of the primary and back-up OCRs are determined. The theoretical analysis will be validated using the IEEE 13-bus distribution test system is used. Both theoretical and simulation results indicate that the proposed application of SFCL is a viable option to effectively mitigate the DG impact on protective devices, thus enhancing the reliability of distribution network interfaced with DG.

Limit Curves by Power System's Transient Stability for the Inverse-Time Overcurrent Relays

This paper presents the limit curves by transient stability for the inverse-time overcurrent relays. Two cases were analyzed: (1) faults in radial lines and (2) faults in lines which are fed from both ends. A novel time-current limit curve by stability was obtained for the radial case; this curve could be compared with the inverse-time overcurrent curve of the relay. Another limit curve by stability was obtained for lines which are fed from both ends; this curve represents the combination of the maximum time-dial settings at both line ends. The nonsimultaneous clearing for lines which are fed from both ends, the dynamic evolution of the fault current, and different prefault conditions were considered in the examples.

Study of Solving the Coordination Curve Intersection of Inverse-Time Overcurrent Relays in Subtransmission Systems

This paper presents a practical and effective novel approach to solve the coordination problem caused by the curve intersection of overcurrent relays in subtransmission systems. Based on the IEEE and IEC standard characteristic curve equations of digital overcurrent relays, as well as the simulation curve equation for electromagnetic overcurrent relays, lever setting adjustment equations for a selected point on the characteristic curve is derived. Then, centered on the derived equations, a dedicated software program is developed to compute the corresponding relay lever setting with different tap value in order that, passing through the selected point, another curve with the new lever setting is chosen. Finally, assisting with the graphical capability of the commercial ASPEN OneLiner, the proposed fixed-point coordination curve adjustment procedure can completely eliminate the coordination curve intersections. In addition, an actual case of a subtransmission network in Taiwan Power Company is simulated to validate the feasibility of the technique proposed.

On the Protection of Thermal Processes

Standards specify an equation for the time-current characteristic of a thermal relay. When the equation is published without reference to its derivation, it may be mistaken for the characteristic of an inverse-time overcurrent relay which if implemented would provide only marginal protection. This paper explains the ambiguity introduced in the standard and addresses thermal protection from the perspective of a thermal model that provides optimum protection. The paper employs MATLAB simulations to compare the dynamics of the thermal model with that of an overcurrent implementation during cyclic overloads.

Time overcurrent adaptive relay

In this paper we recommend an inverse time overcurrent adaptive relay. This adaptive relay has a greater sensitivity and reduced operating times using only the information available at the relay's site. We herein describe the functional structure of the overcurrent adaptive relay, its implementation and the results obtained in laboratory tests with real time acquired signals.

Microprocessor based inverse-time multiple overcurrent relays

Abstract Inverse-time overcurrent relays are used to protect transmission lines and other power system equipment. The use of microprocessors to implement single and multiple overcurrent relays has been reported in the literature. In this paper it is shown that several independent overcurrent relays and other tasks can be implemented using a single microcontroller: the Intel 8051. An interrupt-driven array based program is designed to monitor and control up to 128 overcurrent relays from a single monitor/control board. The system consists of a personal computer and several monitor and control interface boards, each built around the 8051 microcontroller. The proposed system surpasses the previously published systems in its efficiency, response time, and its ability to implement hundreds of different overcurrent relays of various types in addition to performing other tasks.

Combined use of definite and inverse time overcurrent elements assists in transmission line ground relay coordination

In some cases the coordination of ground time overcurrent relays in transmission line protection applications can be difficult. The combined use of definite and inverse time overcurrent characteristics sometimes provides significantly improved protection speed and sensitivity. Two cases are discussed where the use of hybrid characteristics has achieved improvements. The application of hybrid characteristics is limited by difficulties in dynamic coordination. The technology to overcome the dynamic problems would appear to be technically feasible. >

A UNIVERSAL INVERSE-TIME OVERCURRENT RELAY.USING INTEGRATED CIRCUITS

ABSTRACT In this paper the development of a static universal inverse time overcurrent relay using itnegrated circuits is presented. The relay is designed with adjustable slope characteristics to have versatile application in modern complex power system to match with those of the system components like fuses, transmission lines and motors. The slope control feature of the characteristics is expressed mathematically over the whole designed range by an analytical equation which makes it convenient for automatic coordination, with other protective devices, using digital computers. The principle of the design and the calculated performance characteristics are given. A prototype of the proposed relay is built having characteristics of the form (I)n.t = constant where n ranges from 0.43 to 8.1 and I is the multiple of current setting. Test results conducted on a prototype are shown to corroborate the theoretical analysis.

Flexible inverse overcurrent relaying using a microprocessor

The fundamentals of inverse time overcurrent relaying, recent changes in transformer characteristic curves, and a flexible microprocessor-based relay that has wide application possibilities are described. Insight into the process of selecting a particular time-current operating characteristic shape for a given application is provided. Guidelines are presented for the selection of a setting for an instantaneous-trip device based upon its transient reach tendencies. It is concluded that the availability of a microprocessor-based inverse time overcurrent relay that has selectable characteristics built in will greatly relieve the application difficulties that have been associated historically with their fixed-characteristic electrochemical counterparts. >

A Microprocessor Based Inverse Time Overcurrent Relay

The application of microprocessors to the design of the Inverse Time Overcurrent (ITOC) Relay is discussed. Details are given of the design and performance of an experimental microprocessor ITOC relay which was recently developed at the University of Canterbury. Comparison is made with the conventional electromechanical version and with the various static relay designs now appearing on the market. The improvements in performance as far as accuracy and flexibility are highlighted together with the potential for reduced manufacturing costs. The paper also describes how the capabilities of the microprocessor can be used to enhance and extend the functions of the relay. In particular the ability to store a running record of input current waveforms which can be ‘frozen’ in the event of a fault condition for future analysis is believed to be an original concept in relay design. The paper concludes with a discussion on the problems associated with achieving adequate reliability and ensuring correct operation in the power system environment. Additional comments are also made on the general impact of microprocessors in the field of protection and the directions this is likely to go in the future.

A Solid-State Protective Relay for Use in Educational Laboratories

This paper presents the design and operation of a solid-state, directional, inverse-time overcurrent protective relay. One of the unique features of this relay is the ease of adaptability to various situations because of the variety and ranges of setting. This feature, and the very low cost of manufacturing the relay, make it ideal for use in a laboratory setup for educational purposes, as in a university, although its use is certainly not limited to such an application.

Inverse time overcurrent relays using linear components

Inverse time overcurrent relays with various time-current characteristics are required for matching the characteristics of other protective devices and thermal characteristics of the protected equipment. The authors have presented, in this paper, a method wherein any time-current characteristic curve of overcurrent relays can be synthesized from small segments which are realized using only linear components and, therefore, follow a simple and definite mathematical law. Since, in the proposed approach, the characteristic curve is not obtained by fitting nonlinearities of components into it, the accuracies with which the characteristic can be produced and repeated both are very high.

Printed-disc inverse-time overcurrent relay

The paper describes a new type of electromechanical inverse-time overcurrent device for the protection of power-supply circuits. The various time/current characteristics of the new relay and the methods by which they are obtained are given, and some basic features are discussed. A method of compensating for the timing errors caused by ambient-temperature change is described.