Optimal coordination of directional overcurrent relays consideringdefinite time backup relaying

A methodology is presented for the consideration of definite-time backup relays in the optimal coordination of directional overcurrent relays using linear programming. It is shown that the influence of considering second-zone distance relays and breaker failure relays impose important requirements for the determination of the time dial settings of directional overcurrent relays. The paper introduces a revised formulation of the optimization problem. Results are presented for the application of the methodology on a power system with 2 generators, 9 buses, 2 transformers and 7 transmission lines     

Presolve analysis and interior point solutions of the linear programming coordination problem of directional overcurrent relays

A linear programming interior point algorithm is proposed for the solution of the problem of coordinating directional overcurrent relays in interconnected power systems considering definite time backup relaying. The proposed algorithm is a variation of the primal–dual approach that uses multiple correctors of centrality. Pre-solution problem filtering simplification techniques are used prior to the application of the linear programming algorithm. Results are presented for the application of the methodology on a realistic test case, a 115–69 kV power system with 108 buses, 86 lines, 61 transformers, and 97 directional overcurrent relays. Optimal solutions are found in an automatic fashion, using the algorithm for the settings of the ground relays as well as for the phase relays. The application of the pre-solution problem simplification techniques is highly recommended, resulting in a significant reduction of the size and complexity of the linear programming problem to be solved. The interior point approach reaches a feasible point in the close vicinity of the final optimal result in only one or two iterations. This fact represents an advantage for on-line applications. The proposed methodology and in particular the use of the presolve problem simplification techniques is shown as a new valuable tool for the setting of directional overcurrent relays in interconnected power systems.     

Optimal coordination of directional overcurrent relays ininterconnected power systems

The authors present a new methodology based upon the principles of optimization theory, to treat the problem of optimal coordination of directional overcurrent relays in interconnected power systems. With the application of the proposed technique, this coordination problem is stated as a parameter optimization problem, which in general, is of a large dimension, especially when many different system configurations and perturbations are to be considered. Several optimization procedures, including direct methods and decomposition techniques, for solving this large scale coordination problem are described, and results of optimally coordinating directional overcurrent relays in power systems with up to 30 buses are presented     

Applying CTs with digital ground relays

Industrial power distribution system substation transformers and generator step-up transformers in power stations often use resistance-grounded wye secondary windings for medium-voltage power distribution. The purpose of this is to limit damage due to ground-fault currents, while providing sufficient fault current for the operation of ground-fault relaying. The relaying used to protect against ground faults in the system may not provide sufficient protection of the transformer winding against internal faults because the backup ground overcurrent relay in the transformer neutral-to-ground connection must be set to coordinate with downstream relays. In order to protect the winding itself, special relays are utilized. Ground differential protection can be provided by digital overcurrent relays in conjunction with auxiliary ratio matching transformers. Ground differential protection can also be provided in multifunction digital relays. Transformer protection relays may include this feature with one of the schemes used with component relays. If a feeder-protection relay is used on the secondary, in some cases, this may have a ground-directional feature that can be utilized for ground-differential protection     

A novel scheme for current-only directional overcurrent relay

Overcurrent relays are widely used as main protection in sub-transmission and distribution systems. In mesh and multi-source networks, application of directional relay is unavoidable. Traditional directional overcurrent relays use the reference voltage phasor as the polarizing quantity to estimate the direction of the fault. Traditional direction distinguishing scheme is unreliable in the case of close-in faults. In this paper, a novel algorithm for directional overcurrent relay is proposed. The new algorithm uses only current signals for determining the fault direction. It uses superimposed component of the current signal and does not require phasor estimation. This new algorithm uses pre-fault current signal as the polarizing quantity. The proposed method is tested on simple power system in different situations. The results show it leads to fast and reliable directional protection.     

Protection of bulk regular network system with dispersed generators

In urban areas there is a growing need to ensure bulk power distribution, service reliability and an interconnection to dispersed generators. It is effective to improve the network distribution system for the foregoing view. Regular network systems often are used for the area, but coordinations of the protection are complex as usual. This report describes the protective coordinations of the regular network system that performs a bulk power distribution and includes dispersed generators. The interconnections influence the coordination of substation protective relays and network protectors at the primary feeder fault. The increase of the network currents associated with the bulk network system makes the protection difficult at low-voltage grid cable fault.     

High current low ratio

This article is one of the few references describing high current testing of protective relays using low ratio current transformers (CTs). Maximum fault currents on power plant auxiliary buses are routinely 40 kA and can be as high as 80 kA. Accurate measurement of high-current magnitudes requires high-ratio CTs. The rests show the limitations of Fourier and cosine filters used in microprocessor relays that extract the fundamental phasors and eliminate harmonics. The tests validate the operation of a cosine-peak adaptive filter designed to cope with the highly distorted saturated waveforms produced by the low-ratio CTs subjected to high current. The details of relay operation are shown in unfiltered event records of the test cases. This article reports on the results of primary high-current tests of overcurrent, motor, and distance relays using low-ratio CTs. The test currents ranging from 6-50 kA were used with CTs with ratios of 50:5, 300:5, and 600:5. This article compares the internal unfiltered event records with MATLAB simulations.     

Overcurrent protection for distribution feeders with renewable generation

Distribution systems in Taiwan are typically radial type or normally open loop type. Distribution feeders have a simple protection system that usually employs overcurrent relays. When renewable generation (RG) is connected to the distribution feeder, the feeder changes from a single-source system to a complex multi-source system, which causes loss of the original coordination among feeder protection devices. The operation of RG units can cause protection failure. Fault current produced by RG units may reduce the current measured by the feeder relay. Therefore, this paper proposes connecting RGs to the feeder via four-way circuit switches with overcurrent relays, and dividing the feeder into several protection zones. Each protection zone is capable of isolating fault itself.The algorithm process and design procedure of overcurrent protection are also proposed for distribution feeders with RG. Results of this research provide a valuable reference for overcurrent protection that improved protection coordination and system reliability.     

C.A.E. Software for Transmission Protection System: Puget Power Experience

One major concern of a protection engineer is to achieve successful coordination of all relays in a transmission network. This tedious and laborious process is usually accomplished manually at present. In this paper we present the results of a Computer-Aided Protection System based on our experimentation with the entire 115 kV transmission network of Puget Sound Power and Light Company (Puget Power), which consists of 38 buses and 61 lines. The protection package entails algorithms for both overcurrent and distance relays. Effective man-machine interaction is provided by means of computer-graphics. Such interaction is invaluable in providing the protection engineer with both the details of coordination process and the control over this process. A novel feature of the package is the performance and analysis feature for overcurrent relays. The results for the above system are provided and the execution times for different test cases using an IBM 4341 computer system are also presented.     

Optimal computation of distance relays second zone timing in amixed protection scheme with directional overcurrent relays

This paper describes a technique to determine the optimal time setting for the second zone of distance relays when used in a mixed protection scheme with directional overcurrent relays. The technique consists of including the second zone operation time as a new variable in the original problem statement of optimal computation of directional overcurrent relays settings. It is shown that the influence of distance relays and directional overcurrent relays must be considered when the settings of these relays are computed. Numerical results obtained with the proposed method for a realistic power system are presented     

Influence of harmonics on power distribution system protection

The effects of nonsinusoidal voltages and currents on the performance of static underfrequency and overcurrent relays are experimentally studied. The tests were conducted in such a way that the frequency, amplitude and phase shift of individual harmonics could be adjusted in a controlled manner by using a waveform generator with a phase-locking circuit. The relationship between the harmonic currents and voltages was modeled in terms of the power system impedance within residential distribution systems. It was found that for harmonic voltage and current amplitudes (as they occur in distribution systems), underfrequency relays and the time delay operation of overcurrent relays show a marked deterioration in performance. The instantaneous operating characteristics of overcurrent relays, however, are hardly affected by the presence of harmonic currents     

A novel fault protection system using communication-assisted digital relays for AC microgrids having a multiple grounding system

This paper develops a novel fault protection system for AC microgrids having a multiple grounding system. Communication-supported digital relays which have different protection modules are used for this novel microgrid protection system. The protection modules in a digital relay have various functionalities to protect AC microgrids from various fault types, such as: low/high-impedance ground faults and short-circuits. To effectively use the developed novel microgrid protection system, the first step is to reconfigure different microgrid structures into a standard microgrid configuration having a multiple-grounding system through using delta/wye-grounded or wye-grounded/wye-grounded transformers. Then, a fast–dependable–adaptable (FDA) fault protection algorithm is developed to protect AC microgrids from faults occurring at trunk lines, common buses or branch lines. At an islanded operation mode of AC microgrids, by using digital relays, a new protection module is designed to detect, locate, and classify the faults occurring at trunk lines and common AC buses, while other protection modules can use the existing protection methodologies (e.g. directional/non-directional overcurrent protection principles, etc.) to detect and clear the faults at source branches or load branches of the microgrids. At a grid-connected operation mode, standard overcurrent protection modules are applied for the FDA fault protection system. Simulation and experiment results obtained from various fault cases at a real low-voltage AC microgrid have validated the effective operation of the FDA fault protection system.     

Trade-Offs in Overload Protection for Low Voltage Motors

Running overcurrent protection for motors rated 600 V or less is available in a variety of devices. Thermal overload relays, thermal protectors, magnetic and solid-state overload relays, multifunction solid-state motor protectors, and even short-circuit protective devices selected according to certain rules of the National Electrical Code (NEC) can provide varying degrees of protection. Selection of the running overcurrent protective device is based upon application requirements, cost, convenience, and performance. The advantages and disadvantages of each method of providing overload protection for three-phase low-voltage motors in a concise manner are outlined, so that an individual selecting overload protection for a motor circuit can be aware of the many considerations and select the overcurrent protective means that optimizes the concerns he feels are most important.     

基于VSC的直流配电系统过电流保护研究

针对低压直流配电网,本文利用电压源型换流器的自关断功能来代替直流断路器限流的作用,进行保护方案配置。本文主要聚焦故障中断和隔离,利用继电器检测本保护区域故障信息并把动作信号传输到换流器,换流器动作隔离故障。在PSCAD软件中搭建两端供电的直流配电模型进行仿真,结果表明,在采用继电器的情况下,采用过电流的保护方案可以满足快速响应的要求,为下一步直流配电系统保护方案的研究奠定了良好的基础。     

Microprocessor based inverse-time multiple overcurrent relays

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.     

基于用户自定义特征的反时限有源配网保护方案

由于分布式电源输出功率的随机性与波动性,当分布式电源大量接入配电网后,传统的反时限过电流保护方案很难满足配电网保护选择性和速动性的要求。为此,文章通过研究数字反时限过流保护继电器,提出一种基于用户自定义特征的反时限过电流保护方案。保护方案将反时限继电器的特性常数(A)和反时限的类型常数(B)同继电器的时间整定系数、启动电流共同作为待优化的连续变量,并以最小故障电流时所有保护动作时间之和最小为目标函数,利用内点法求解保护的最优配置。方案保证了线路出口处故障时保护能快速动作,满足了配电网保护选择性和速动性的要求。最后,通过仿真分析,对保护方案进行了验证。     

Operation Logic Proposed for Time Overcurrent Relays

This paper presents a new logic of operation for time overcurrent relays. With the application of the proposed technique, the sensitivity of time overcurrent relays is increased. In order to obtain this, the relay operates with a dynamic pickup setting, such as load current. Additionally, fast operation of overcurrent relays is obtained. The functional structure is described and the logic of overcurrent relay is evaluated under different operative states. We describe the functional structure of the proposed relay and its implementation in laboratory tests.     

An on-line relay coordination algorithm for adaptive protectionusing linear programming technique

An adaptive system for protecting a distribution network should determine and implement relay settings that are most appropriate for the prevailing state of the power system. This paper presents a technique for determining coordinated relay settings. The technique uses the Simplex two-phase method; Phase I determines whether the constraints selected for illustrating the conditionality between primary and back up relays are feasible, and Phase II finds the optimal relay settings. A looped distribution system, protected by directional overcurrent relays, was used for testing the technique. The tests were conducted in a laboratory environment; some results from those tests are reported in the paper     

Optimum coordination of directionalovercurrent relays for combined overhead/cable distribution system with linearprogramming technique

Heavy fault currents flow in the event of fault at the loads connected in distribution system. To protect these loads, circuit breakers and relays are required at appropriate places with proper coordination between them. This research paper focuses on finding optimum relay setting required for minimum time to interrupt power supply to avoid miscoordination in operation of relays and also investigates effect on time multiplier settings (TMS) of directional overcurrent relays in a system with combined overhead lines-underground cables. Linear programming problem (LPP) approach is used for optimization. It is interesting to know the quantitative variations in TMS as the underground cables have different characteristics than overhead lines.     

矿用隔爆型真空馈电开关中的过电流保护

从煤矿井下低压电网中装设过电流保护的重要性入手,描述了矿用隔爆型真空馈电开关中过电流保护的工作原理。重点分析了对称短路相敏保护特性曲线的确定方法,讨论了影响保护动作可靠性的各种因素。介绍了以单片机为中央控制单元所构成过电流保护系统的硬件结构和软件框图,下井前对保护系统进行了测试试验,动作指标符合设计要求。经现场运行表明该保护系统性能稳定,动作可靠,具有重要的应用价值。