Algorithmic-knowledge-based adaptive coordination in transmission protection

Changes in power system operating conditions can affect both the reach point of distance relays and the coordination of distance and overcurrent relays. To improve the performance of the protection system, an adaptive scheme of relay settings is proposed. That is possible since the protection relays became digital . This paper describes a system for online adaptive setting and coordination of protection relays in meshed networks where the backup protection is achieved in remote form. After changes in the state of the network, an expert system (ES) performs the detection and proposes the correction of erroneous settings of protection zones and miscoordinations between relays at different stations; through an algorithm based on a fast determination of a nonminimal set of "break-points" (BPs) relays , new time characteristics for the coordination are found. The results of the application of the developed system to a test network of 34 nodes, 55 branches, and 110 relays are shown.     

New coordination approach to minimize the number of re-adjusted relays when adding DGs in interconnected power systems with a minimum value of fault current limiter

The presence of DGs in power networks tends to negatively affect relays coordination. Adding fault current limiters FCLs is one of the possible solutions to mitigate negative impacts of DGs addition on protection systems. Traditional schemes have estimated the minimum value of FCL to restore relays coordination when adding DGs without resetting of any relays. That minimum value of FCL in such case is called a critical value, where below this value the relays coordination will be lost.Nowadays, designing FCL to simultaneously achieve two conflicted objectives of good performance and low cost is considered a great challenge. The paper introduces a new scheme to determine to what extent we could decrease FCL impedance value below its critical value with re-adjusting the original settings of only one adaptive relay to get relays coordination. Decreasing FCL value below its critical value will reduce the cost especially for superconductivity FCL. The proposed scheme can determine the location of that selected relay to be an adaptive one and estimate its re-adjusted new settings to be applied when DGs are added while inserting the reduced value of FCL.Actually the proposed scheme can be applied for any networks irrespective of the number of added DGs and their capacities; while having an adaptive relay is the only requirement to implement it.The proposed approach is implemented and effectively tested on the large well-known interconnected IEEE-39 bus test system with 84 relays. Its results are compared with other approaches where, no re-adjusted relays settings are applied. A noteworthy advantage of the proposed scheme is the ability to implement a reduced FCL value than the critical value, by adjusting only one relay settings in the whole network. The proposed scheme may also be extended to re-adjust settings of more than one relay and get further reduced value of FCL. Furthermore, it is also shown that a more optimum value of the total operating time of all primary relays for near end faults is achieved when applying the proposed method rather than other traditional schemes.     

Selection of minimal tripping times for distance protection using fault trees with time dependencies

Correct coordination between protection relays is an essential requirement to maintain stability and integrity of electrical power systems. Time settings of protection relays have to be adapted to time characteristics of electrical power systems and time settings of other protections. In the paper, fault trees with time dependencies (FTTD) are used for selecting time delay settings of remote back-up distance protections. In FTTD, events and gates are characterized by time parameters, which are derived from time characteristics of protection devices and the inherent delays of protection equipment. Two case studies are presented. First, the coordination of protections for the simple transmission line composed of two identical sections is studied. Next, the transmission line that contains two sections of different lengths and transformer between them is analyzed. In both cases, formulae for time delays of protections are derived. According to calculations executed for the first case, our method gives time delays that often are significantly smaller than values suggested by generally recommended and accepted rules.     

Efficient coordination of ROCOF and frequency relays for distributed generation protection by using the application region

This paper proposes a method for determining the coordination of the rate of change of frequency (ROCOF) and under/overfrequency relays for distributed generation protection considering islanding detection and frequency-tripping requirements. The method is based on the concept of application region, which defines a region in the trigger time versus active power imbalance space where frequency-based relays can be adjusted to satisfy the anti-islanding and frequency-tripping requirements simultaneously. This method can be used to optimally determine the instantaneous and time-delay settings of different frequency-based relays in a unified manner. The application region can be also employed to evaluate how much an existing frequency-based protection scheme violates one of the above-mentioned requirements. In addition, it can be used to set different combinations of frequency-based relays in order to make the distributed generator frequency-based protection system as efficient as possible, since it respects the region that satisfies both requirements.     

一种新型故障定位方法的研究

论述了利用因果网技术进行故障定位的方法。因果网描述了故障元件、继电器、开关之间内在的动作关系,利用了比传统专家系统更深的知识、简单、明确。提出了利用面向对象技术形成因果网的方法,所形成的因果网能适应各种不同的网络,通用性强,且能考虑故障扩大的情形。     

Identifying the Primary Fault Section After Contingencies in Bulk Power Systems

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.     

Non-communication protection method for meshed and radial distribution networks with synchronous-based DG

This paper presents an efficient protection method which can be used for both meshed and radial distribution networks (DNs) with synchronous-based distributed generation (SBDG) units. The method does not require any communication system in the both grid-connected and islanded modes of operation. The microprocessor-based relays used in the DNs are programmed with a new time-current-voltage characteristic utilising only local fault voltage and current magnitudes. The proposed method is verified by simulation study on the DN of IEEE 30-bus test system as a meshed network in grid-connected mode. The method is also tested on an Iranian practical radial DN in the both grid-connected and islanded modes of operation. The test cases include different fault conditions, with SBDG at various locations and different DG penetration levels, and also without any SBDG in the networks. It is shown in a comparative study that the new time-current-voltage characteristic achieves a notable reduction in total relay operating times without any communication links. In addition, the method uses the same protection settings for the both grid-connected and islanded modes of operation.     

A new approach for calculating zone-2 setting of distance relays and its use in an adaptive protection system

This paper presents a method for calculating zone-2 setting of distance relays without causing coordination problems. The proposed method is based on the impedance seen by distance relays when faults are simulated on the reach of zone-1 of primary relays for the maximum and minimum generation outputs of the power system. It is shown that the proposed method increases the reach of zone-2 relays without causing coordination problems. The proposed method is modified for use in an adaptive protection system. It is shown that further improvements can be achieved when settings are calculated using the proposed method and the prevailing system conditions. Measures to alleviate the impact of communication failure are discussed. The proposed method and its adaptive version were applied to an existing power system and some results are reported in the paper.     

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.     

Ground Fault Protection for Bus-Connected Generators in an Interconnected 13.8-kV System

This paper is based on modifications implemented in the protection and grounding systems of a large paper mill and describes selective ground fault protection for a 13.8-kV system with multiple bus-connected generators, synchronous bus ties, and utility interconnections. The ground fault current in the system is reduced from the existing 3400 A to 500 A, and a hybrid grounding system is implemented for each of the generators. As the ground fault currents are reduced to limit the fault damage, the sensitivity and selectivity of the ground fault protection become important. Directional ground fault relays with coordinating pickup settings are applied to achieve this objective. The new platform for directional elements (numerical relays) drives its performance from sequence impedance measurements     

An Expert System for Fault Section Estimation Using Information from Protective Relays and Circuit Breakers

This paper deals with an expert system which can estimate possible fault sections using information from protective relays and circuit breakers. This system is applicable to dispatching centers and can help dispatchers to judge emergency situations as the first step in restoration procedures. When some faults occur, the system makes inferences based on both knowledge about protection systems and information on the operating protective relays and tripped circuit breakers. The system can give possible answers even in the case of multiple faults and false operations of relays and circuit breakers. This expert system is written in Prolog.     

一种新的基于图论确定所有最小断点集方法

在复杂环网的继电保护整定计算过程中,必须确定方向过流保护和距离保护的整定配合顺序,而整定计算的起点即断点的确定尤为重要。利用图论方法确定了一些旨在降低求取最小断点集问题复杂度的原则,如网络分割、最少断点数的确定、不考虑相邻保护、对称、无平行线网络选择断点等原则,并利用这些原则提出了逐步求取网络所有最小断点集的方法。算例表明该方法降低了计算最小断点集问题的复杂度,且具有一定的有效性和通用性。     

Adaptive centralized protection scheme for distribution systems with DG using risk analysis for protective devices placement

Conventional electric distribution systems are radial in nature, supplied at one end through a main source. These networks generally have a simple protection system usually implemented using fuses, re-closers, and over-current relays. Recently, great attention has been paid to applying Distributed Generation (DG) throughout electric distribution systems. Presence of such generation in a network leads to losing coordination of protection devices. Therefore, it is desired to develop an algorithm which is capable of protecting distribution systems that include DG, through diagnosis and isolation of faults. A new approach for the protection of distribution networks in the presence of DGs is presented in this paper. The algorithm is based on dividing an existing distribution network into several zones, each capable of operating in island operation. In the suggested method, risk analysis is used to optimize the protection zones by optimal placement of protective devices. Multilayer Perceptrons (MLPs) neural networks are used for determination of faults. The proposed scheme has been implemented on a selected part of a real distribution network of a large city and a MATLAB based developed software has been used to implement the proposed algorithm on the real network data.     

Implementation of Directional Over-current Relay Coordination Approaches in Electrical Networks

As the smart grid concept is employed in the electric power system, network load flow and topology changes intensively to meet the best generation-demand balancing point. These changes must account for protection devices to enhance their performance. The coordination of directional over-current relays is most commonly studied based on fixed network operation and topology within a mesh power system. But sub-transmission and distribution systems constantly operate differently to satisfy the variety of load demand levels throughout the day and year. Hence, if the setting of directional over-current relays changes according to the different operations of the system, then relay operation time and sensitivity can both be enhanced. This can potentially improve the protection performance in a smart grid system. Therefore, this study is carried out based on the comparison among three coordination approaches: conventional (fixed settings), discrete (groups of settings), and continuous or real time (dynamic settings).     

Development of an expert system for estimating fault section incontrol center based on protective system simulation

The authors describe a practical expert system which estimates the fault section in an electric power system by inferring dynamically the protective coordination of the protective relays. The expert system represents the characteristics of protective relays by using numerical figures corresponding to the protection zone and time setting. This method provides high maintainability of the system, and also allows the system to display inference processes in a user friendly manner. The expert system described will be used at three new control centers of Shikoku Electric Power Co., Inc., in 1991     

Communication-based Adaptive Protection for Distribution Systems Penetrated with Distributed Generators

In this article, a communication-based adaptive over-current protection scheme for distribution systems penetrated with distributed generators is proposed. A communication network between the over-current relays, the distributed generators, and the utility grid is employed to automatically update the settings of the protective relays. Moreover, the communication reliability is increased through the addition of a backup communication system. The proposed scheme has the advantage of operating during grid-connected and islanding modes of operation. The scheme employs two simultaneous algorithms. The first algorithm works when the system configuration is changed due to the connection/disconnection of a distributed generator or the utility grid. The second algorithm efficiently uses the exchanged information between the relays to identify the faulted section and hence speeds up fault clearance. The proposed scheme is tested for difierent fault conditions as well as for different system configurations. The results demonstrated that relay operating times, including the communication delay, are greatly reduced when the faulted section is identified and relay settings are adjusted accordingly. In addition, a negligible time delay was experienced when the backup communication network was put in service.     

Dual-setting directional over-current relays: An optimal coordination in multiple source meshed distribution networks

To guarantee a safe operating scheme against the probable faults, an accurate and fast relaying scheme is of high priority. This challenge seems to be more sophisticated in multiple source meshed distribution networks hosting distributed generations (DGs). In this case, the current experiences bidirectional flows. More recently, dual-setting over-current relays are evolved as competent countermeasures for such cases. In this way, establishing an optimal coordination strategy is recognized as the first prerequisite in assuring a safe protection scheme. To this end, the present study aims at minimizing the overall operating time of primary and backup relays. Typically, the coordination problem is carried out by adjusting two parameters namely, pick up current (Ip) and time dial setting (TDS). In contradiction, the proposed approach follows a user-defined settings supported by some operating rules. Thus, besides the aforementioned settings, the coefficients of the inverse-time characteristics are also optimized. In other words, more flexibility is attained in adjusting relay’s characteristic in regard of the network’s loading and topological changes. Furthermore, inclusion of operating rules in main objectives, not only reduces the sum of operating time of all relays, but also increases the efficiency of backup relays regarding the break points in meshed networks. In other words, the covering zone by each backup relay is increased. The proposed approach demonstrates a non-linear programming fashion which is tackled based on genetic algorithm (GA). Several numerical studies are carried out to interrogate the validity of the proposed approach encountering different fault scenarios. The obtained results are encouraging.     

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     

电力变压器保护设计专家系统的结构

提出了一个用于变压器保护辅助设计的专家系统，不仅可以实现保护系统配置、保护方式选 择、保护装置选型及继电器产品确定，还能表示保护设备之间、保护系统与一次系统之间的 关系。     

Overcurrent Protection from Earth Faults Based on Higher Harmonics for Compensated 6- to 10-kV Cable Networks

Devices based on the method of absolute measurement of the level of higher harmonics in zerosequence currents of connections of a protected object are the most commonly used for protection from earth faults in compensated 6- to 10-kV cable networks of industrial and urban power-supply systems. However, many years of operating experience have shown that the technical refinement (the selectivity and the sensitivity) of overcurrent protection from earth faults based on the method of absolute measurement of higher harmonics is not always sufficient. One of the reasons for the low technical refinement of these protection devices is the inaccuracy in estimating the maximum and minimum possible higher harmonics levels in the earth-fault current of compensated 6- to 10-kV cable networks, which are required for the selection of the operating current settings and the sensitivity determination. The main factors influencing the maximum and minimum higher harmonics levels in the earth-fault current and their ratio were revealed on the basis of the computational experiments using the imitation models for 6- to 10-kV cable networks, the limiting values of these levels were determined, an determination of the harmonics-level instability in the fault current for cable networks with different total load compositions was given, and its interrelation with the fluctuations of the total coefficient of harmonic voltage components on 6- to 10-kV buses of network power centers was established. The conditions of applicability and the area of application of overcurrent protection from this type of failure based on the use of higher harmonics of the zero-sequence current for different objects of 6- to 10-kV compensated cable networks were determined taking into account the above estimates. Specifications for a method for selecting the operating current settings were proposed.