Sub-transmission Networks Research Articles

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-Load Network De-Icer Specification for a Large Transmission Network

This paper presents a feasibility study conducted with a view to installing an on-load network de-icer (ONDI) for de-icing HV transmission lines forming part of the Matapedia subtransmission network operated by Hydro-Quebec. The ONDI concept makes use of a phase-shifting transformer (PST) to induce very large ac currents to heat line conductors by the joule effect. A single ONDI can handle a number of lines of different length with no need to transfer loads to other lines or disconnect them. In the study presented, more than 900 km of 230- and 315-kV HV circuits can be de-iced with one ONDI installed at Rimouski substation. To put the ONDI to work, only existing circuit breakers need to be operated so that no open-air (ice-sensitive) disconnecting switches are involved. The adequacy of this planning study is also discussed here with regard to taking into account uncertainties related to the weather conditions expected for severe ice storms.

A Novel Approach to Fast Eliminating the Characteristic Curves Crossing Problem of Relay Coordination in Radial Subtransmission Networks

A novel approach is presented to solve the coordination problem of overcurrent relays in radial subtransmission networks resulting from the intersection of characteristic curves. Based on the ANSI/IEEE standard characteristic curve equation of digital overcurrent relays, a slope adjustment equation for a selected point on the characteristic curve is derived and then used as the core of a dedicated software program developed to compute the corresponding relay setting with different tap value in order that, passed through the selected point, another curve with steeper slope is chosen and thus able to escape from the crossed position. Finally, with the assistance of the graphical capability of commercial package OneLiner in drawing the upstream and downstream relay coordination curves after adjustment, the curves intersection problem in protection coordination can be readily eliminated. In addition, an actual case of a radial subtransmission network with six buses in Taiwan Power Company is simulated to validate the feasibility of the approach.

Maclaurin–Newton algorithms in 132 kV subtransmission security simulations

This paper examines how applicable approximate Jacobian inversions are when implemented in the security analysis simulations of 132 kV power subtransmission. The complete Scottish 400/275/132 kV power transmission network was simulated, including the 132 kV subtransmission network with its high r/ x ratios. Both the coupled and decoupled Maclaurin–Newton load flow algorithms were tested. It was proved that high r/ x ratios found in the 132 kV level, five times higher than in 400/275 kV, have an important influence on convergence and accuracy of the inversion Jacobian load flow algorithms. It was found that the decoupled inversion load flow was applicable for 132 kV, it converged regularly, but had worse convergence and accuracy characteristics, compared to 400/275 kV applications, while the coupled inversion load flow was not applicable at all for 132 kV, it always diverged.

Using Pumped Storage Loads in Restoration Paths: A Field Test in the Italian National Grid

This paper deals with power system restoration and discusses the connection of pumped storage loads to restoration paths. This new strategy allows avoiding any disturbance to customers during field tests of restoration paths and improves as well the reliability of restoration plans. The first application of the new strategy concerned the field test of a new path in central Italy. This paper reports the test and analyzes the main novel features of the considered path: complete separation from subtransmission and distribution networks and use of pumped storage loads during its energization. These aspects and their consequences are discussed with reference to both field testing of restoration paths and effectiveness of system restoration.

Optimal power flow evaluation of distribution network capacity for the connection of distributed generation

Distributed generation capacity will increase significantly as a result of UK Government-led targets and incentives. Whereas the technical problems arising from distribution-level connections may be mitigated for individual connections, the anticipated connection volumes imply a potential risk of conflict between connections, in that inappropriately sized or located plant could constrain further development of the network and consequently threaten the achievement of renewable energy targets. One means of addressing this risk is to encourage development at sites that are more suitable and at the same time discouraging those at inappropriate ones. First of all network operators must be able to evaluate the available capacity on the system (i.e. the headroom). A technique is presented that facilitates such an analysis. Termed ‘reverse load-ability’, the approach models fixed-power factor distributed generation as negative loads and uses the optimal power flow to perform negative load shedding that effectively maximises capacity and identifies available headroom. The technique is applied to an extensive distribution and sub-transmission network. It rapidly identifies available headroom within the imposed thermal and voltage constraints. Furthermore, its use is demonstrated in examining the consequences of a sequence of connections in terms of the impact on available headroom and in sterilising the network.

SRAT—Distribution Voltage Sags and Reliability Assessment Tool

Interruptions to supply and sags of distribution system voltage are the main aspects causing customer complaints. There is a need for analysis of supply reliability and voltage sag to relate system performance with network structure and equipment design parameters. This analysis can also give prediction of voltage dips, as well as relating traditional reliability and momentary outage measures to the properties of protection systems and to network impedances. Existing reliability analysis software often requires substantial training, lacks automated facilities, and suffers from data availability. Thus it requires time-consuming manual intervention for the study of large networks. A user-friendly sag and reliability assessment tool (SRAT) has been developed based on existing impedance data, protection characteristics, and a model of failure probability. The new features included in SRAT are a) efficient reliability and sag assessments for a radial network with limited loops, b) reliability evaluation associated with realistic protection and restoration schemes, c) inclusion of momentary outages in the same model as permanent outage evaluation, d) evaluation of the sag transfer through meshed subtransmission network, and e) simplified probability distribution model determined from available faults records. Examples of the application of the tools to an Australian distribution network are used to illustrate the application of this model.

An easy-to-use software for subtransmission network analysis

An easy-to-use software for subtransmission network analysis

An easy-to-use software for subtransmission network analysis

This paper describes a graphical easy-to-use power system simulation software. The software is specially designed to meet the requirements of saving time and effort in its use. Visualization, automation of some manual procedures and solution of special problems in subtransmission networks has been realized. An efficient network diagram editor is introduced. The combined use of the network diagram and the automated substation diagrams facilitates network operation state setting and interpretation of results. Some practical techniques have been adopted to improve the convergence of load flow and var optimization computations. Furthermore, the automatic network topology analysis function has contributed a lot to error finding in the built network database and the input data of network status. It has been successfully used in two Chinese electric utilities.

Final implementation of SEACON: new scope and new open system platform

SEACON is a hybrid (heuristic and algorithmic) expert system for contingency analysis and preventive or corrective control of power transmission networks, that has been recently redesigned to implement new end user requirements and to increase its performance. This paper describes the main results and the operational experience from the development and implementation of a new system based on the initial idea. Software reuse strategies became capital in this project. The new design includes advanced functionalities demanded by the practical use of the former system. Because of the high performance of the new system (up to fifteen times faster than the original) and its friendly user interface, SEACON has become a very valuable tool among dispatchers. SEACON is in operation in the Red Electrica's National Control Centre and in Iberdrola's Central and Regional Control Centres (Spain), dealing with transmission and subtransmission networks (ranging between 400 kV and 66 kV), attached to the online state estimators and screening over 2000 possible contingencies. Results are presented using natural language, and a client-server scheme provided for corporate access.

A comprehensive fault diagnostic system using artificial intelligence for sub-transmission and urban distribution networks

This paper describes an intelligent diagnostic system for an interconnected distribution network developed to assist the system operator with fault identification during restoration. The intelligent process utilizes only those data available in a standard SCADA system such as the post fault network status, the list of the tripped breakers, main protection alarm, and the conventional event log. The fault diagnostic system is implemented by three independent mechanisms, namely the generic core rule, the generic relay setting inference and the specific post-fault network matching and learning. The generic core rule generates various possible fault locations and the generic relay inference examines whether each possible fault location is logical and valid. The specific network matching compares whether the post fault network and the related tripped breakers are identical to a previous fault event. Test results obtained from two distribution networks confirm that the developed system is practical, reliable and accurate.

二次系統の供給信頼度解析システムの開発

二次系統の供給信頼度解析システムの開発

A test system for teaching overall power system reliability assessment

This paper presents the concept of overall power system reliability evaluation using an educational test system. The paper extends an existing test system by developing the necessary distribution and subtransmission networks. The extended test system has all the main facilities, such as generation, switching stations, transmission, sub-transmission and radial distribution networks found in a practical system. The test system, is however, sufficiently small that students can analyze it using hand calculations or by developing small computer programs to fully understand the reliability models and evaluation techniques. Overall power system reliability evaluation is concerned with providing acceptable customer service. This is an important concern in today's electric utility environment. This should therefore be an essential element in teaching power system reliability evaluation at either the graduate or undergraduate level, The extended test system presented in this paper and the concepts presented assist in satisfying this requirement.

Real-time simulator for power system dynamics studies

The addition of dynamic models of loads, machines and subtransmission networks to the existing three-phase real-time simulator of Hydro-Quebec (IREQ), has made available a tool appropriate for the study of power system dynamics in real time. The valid frequency range of the simulator, which already covered harmonics up to 2000 Hz, has been extended to include machine dynamics in the region of less than 1 Hz. The paper describes the models added and also presents some results of the integration of these dynamic models, showing how they compare favorably with equivalent results obtained using a transient stability simulation program.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

95/01543 A heuristic linearized line outage contingency ranking of integrated multiterminal AC-DC power systems

This study presents a reliability assessment tool for meshed subtransmission networks in a three-layer structure. In this model, the three critical zones of influence, containing four blocks, are determined, described and quantified as reliability parameters. Moreover, this work is framed on the distinction between subtransmission networks consisted of underground cables (UGC) and overhead lines (OHL), accrediting attributed failure rates, repair times and intrinsic features under multiple circumstances and the seasonal variation of load and co-generation within them. For this achievement, the current analysis ascribes differentiation between adverse and normal weather conditions and the impact on considered equipment, as well as division of time period of analysis into different scenarios. This method is tested in the IEEE 30-bus network, as underground and as overhead networks, and in a typical Nordic 25-bus subtransmission network; and the influence of each of the four blocks is described at the end of this paper. As results, underground parts of the network exhibit more homogeneous outage time throughout the year than the overhead parts and the presence of co-generation improved reliability both in the directly connected substations and in the entire network. Furthermore, the block constituted by the substation equipment presents the largest influence on the system reliability. Similarly, common-mode failures in overhead networks introduce significant decrease on network availability.

95/01532 Distributed approach to black point

This study presents a practical and effective method for inspecting and repairing curve intersection situations that occur in upstream and downstream coordinated overcurrent relay pairs in subtransmission networks. This type of miscoordination can cause backup relays to trip before their paired primary when faults occur, resulting in unexpected expansions of blackout areas. The proposed method is based on the IEEE and IEC standard characteristic curve equations of digital overcurrent relays and a customized curve equation for electromechanical overcurrent relays. Consequently, the proposed method is applicable to both digital and electromechanical relays. Two subtransmission network cases for the Taiwan Power Company were simulated to validate the proposed method.

Rotor heating as an indicator of system voltage instability

Voltage stability hinges on the continuing control of the total power system's supply and demand of reactive power within the overall transmission, subtransmission and distribution networks which connects the power stations to the consumers. This is complicated by the elements interspersed within this extensive network that both supply and consume reactive power. Extensive collapses have followed disturbances more severe than considered at the planning stages, but there is a puzzling delay of minutes between the disturbance and collapse. By simulating the physical process, the critical factor has been found to be the action of rotor overcurrent protection in curtailing rotating unit reactive power output. The puzzling delay has been solved by the mathematical modelling of the generator rotor's thermal response which shows that above rated rotor current would take minutes to reach the rotor's temperature limit. The paper describes the heat run tests from which the parameters for mathematical rotor thermal models for 500 MW and 660 MW generators have been obtained. The algorithm that has been devised is both simple and reliable and allows computer evaluation, in a fraction of a second, of the time for rotor temperature limit to be reached. When integrated into SCADA, this information would give an early warning and so provide a valuable tool for the control system voltage stability. >

Training the restoration of electricity supply systems in the netherlands

AbstractUsing a stand‐alone training simulator developed at the Gerhard‐Mercator‐ University Duisburg/Germany, a sequence of training seminars was staged in cooperation between KEMA‐ECC, the Power Systems Institute of the University, and with support of the Dutch Electricity Generating Board (Sep). A group of twelve utility operators of Sep and the different regional transport and production companies attended each seminar, with the aim of getting experience on the performance of their systems under the adverse conditions of network‐restoration. For that aim, the simulator models the Dutch 380 kV/220 kV transmission grid and eight connected subtransmission networks on 150 kV and 110 kV.

Optimum location of resources in distributed planning

The concept of installing dispersed resources in an electric utility distribution or subtransmission system is gaining interest in the industry. Such resources could be photovoltaic cells, wind generation, battery storage, fuel cells or demand-side management solicitations. Research by others has suggested that the benefits of distributed resources could be substantial. However, these distributed benefits are site specific. A method to optimally locate such resources in a meshed network for maximizing the potential benefits is outlined in this paper. The proposed second order algorithm computes the amount of resources in selected nodes to make up a given total to achieve the desired optimizing objectives. The network under consideration could be a transmission, subtransmission, or distribution network. The benefit expressed as a performance index can be the minimization of losses, VAr losses, or loadings in selected lines. >

A line-current measurement based state estimator

The authors present a method for state estimation when the measurement set consists basically of node voltages and line currents. The key idea is that P- theta observability can be reached provided inequality constraints are added to the estimation process. These constraints simply state that P>or=0 for a generator bus and P<or=0, for a load bus. Simulation results confirm that the method may be applicable to medium-sized subtransmission networks, the only major exception being radial networks with more than one feeder bus.<<ETX>>