Multiple-circuit Transmission Lines Research Articles

Distance protection of multiple‐circuit shared tower transmission lines with different voltages – Part I: fault current magnitude

Multiple-circuit transmission lines combining different voltage levels in one tower present extra challenges when setting a protection philosophy, as faults between voltage levels are possible. This study presents a detailed theoretical analysis of such combined faults, including the development of a formula for estimating the magnitude of the short-circuit current. It is demonstrated that if the faulted phase from the higher voltage level leads the faulted phase from the lower voltage level, a distance relay at the higher voltage level sees the fault in the forward direction, whereas a distance relay at the lower voltage level sees the fault in the reverse direction. The opposite happens if the lower voltage level leads the higher voltage level. It is also demonstrated that the magnitude of fault currents of combined faults is normally slightly larger than of equivalent single-phase-to-ground fault at the higher voltage level. Part II will continue the research work and focus in the fault loop impedance RX diagrams.

Distance protection of multiple‐circuit shared tower transmission lines with different voltages – Part II: fault loop impedance

Multiple-circuit transmission lines combining different voltage levels in one tower present extra challenges when setting a protection philosophy, as faults between voltage levels are possible. In this study, the fault loop impedance of combined faults is compared with the fault loop impedance of single-phase-to-ground faults and it is demonstrated that they are similar for high short-circuit powers; however, the fault loop impedance of a combined fault may increase substantially as the short-circuit power of the system decreases, a behaviour that is less noticeable for single-phase-to-ground faults. It is also demonstrated that the fault loop impedance of combined faults is more resistive, when compared with equivalent single-phase-to-ground faults. It is concluded that the settings used to protect a line against single-phase-to-ground faults are capable of protecting the line against combined faults, being advised to increase the resistive limit of the protection zone, if the network has lower short-circuit power. It is recommended to assure that the fault can only happen for cases where the faulted phase from the higher voltage level leads the faulted phase from the lower voltage level, if the length of the line at lower voltage level is smaller than of the line at higher voltage level.

Method to measure the unbalance of the multiple‐circuit transmission lines on the same tower and its applications

This study proposes a new method for measuring the unbalance of the multiple-circuit transmission lines (MCTLs) on the same tower, which can characterise the physical natures of the line unbalance. First, the impedance and the sequence current of the MCTLs on the same tower are calculated under the consideration of the impact of ground wires. Then, the unbalance suppression methods of the optimal phase sequence arrangement and asymmetry parameter compensation are presented. Finally, the model of the six-circuit transmission lines on the same tower is simulated in PSCAD according to a real project in China. The line unbalance, the impacts of the connection patterns of ground wires on unbalance and the unbalance suppression are simulated and analysed. Simulation results show that the proposed method is applicable and convenient to accurately calculate the parameters of the line unbalance and to select the optimal way to suppress the unbalance. The work can provide the theoretical guidance for the optimal design and the operation of the MCTLs on the same tower.

交直流并行线路雷击时电磁暂态仿真研究

为了节约资源和环境保护，我国特高压输电线路将形成大容量多回输电线路、交直流并行的格局。雷击是特高压线路跳闸的主要原因，论文搭建了交直流并行输电线仿真模型，对线路遭受雷击时的电磁暂态进行了仿真分析。首先采用PSCAD仿真软件建立了特高压直流输电工程的仿真模型，建立了特高压交直流并行输电线路模型。基于以上模型对雷击特高压直流线路进行了暂态仿真，分析了参数的暂态变化。结果表明线路电压与电流均迅速上升，两极各项参数发生了畸变和振荡。 In order to save resources and protect the environment, the power grid of China will have more multiple-circuit transmission lines and high capacity AC/DC parallel transmission lines. This paper builds the simulation model of AC/DC parallel transmission lines and studies the transient process of transmission lines when lightning, which is one of the main reasons of line fault. The electromagnetic transient simulation software PSCAD/EMTDC is employed to establish the simu-lation model of UHV DC model, and the AC/DC parallel transmission model is also set up. Based on the model established above, we simulate the transient simulation of lightning striking on the DC lines and AC lines, and parameters’ changes are analyzed. It turns out that the voltage and current of the transmission lines will rise abruptly when the lightning happens and many parameters of DC lines will have strong distortion and oscillation.

Power transmission cost allocation using power flow tracing and reliability benefit

Power transmission cost allocation methods have been varied depending on local context of electricity supply industry. There is a common principle that transmission line capacity should be properly allocated to accommodate actual power delivery with adequate reliability margin. This paper therefore, proposes a method that allocates transmission embedded cost to both generators and loads in an equitable manner, incorporating probability indices to allocate transmission reliability margin among users in both supply and demand sides. Probabilistic indices so called transmission internal reliability margin (TIRM) and transmission external reliability margin (TERM) decomposed from Transmission reliability margin (TRM) are introduced, making true cost of using overall transmission facilities. A numerical example on a simple six-bus system with multiple-circuit transmission lines which represent a characteristic of practical system is also presented to illustrate the application of the proposed method. Key words: Cost allocation, transmission pricing, reliability benefit, power flow tracing and reliability margin.

Supercomponentes Contingency Modeling for Security Assessment in Power Systems

The utilization of the power systems to its maximum capacity is a condition that produces economic benefits to all agents involved in the chain of energy supply and thus to society. However, under these conditions, operators can take risks that could cause the loss of system stability and, in the worst case, blackouts. Thus, operators require analysis tools that give them information in order to take preventive and corrective actions, to operate the system in a reliable way and to check the behaviour of the same. Generally, in this type of analysis, simple component's contingencies are used. However, the practice has shown that more than one component (generation unit, circuit line, single transformer, among others) of the system can fail, either by simultaneous outages; events in cascade or by a major outage that means a complete installation with several components gets out of service. So, a super component refers to complex electrical facilities as: substations, generation plants and multiple circuit transmission lines. An outage of the super component implies the multiple and simultaneous outages of many elements. These kinds of events are not, generally, considered in the mentioned analyses; however, it would be very important to keep them in mind in the security evaluation of power systems. This paper presents a methodology to evaluate the severity of this kind of contingencies in the operative planning scenario. The super components that are taken into account are the multiple circuit lines (in the same structure), generation plants and substations. The impact of contingency on power system security is evaluated in a 24-hour ?day ahead? planning horizon through the computation of the energy not supplied, the number of overloaded lines and the percentage of load loss The Energy not supplied is computed as function of a load shedding based on operator's rules.

Common-Cause Outages In Multiple Circuit Transmission Lines

Reliability evaluation of a power system involving both generation and transmission elements is extremely complex. Outages of these elements are usually considered to be s-independent events. Recent investigations, however, have indicated that common-cause outages of multicircuit transmission configurations can appreciably affect the predicted reliability. Closed form expressions for steady state probabilities in 2- and 3-line cases (including certain common-cause failures) are developed. These expressions provide transmission-line state probabilities for composite generation and transmission system reliability studies. The procedure can also be used to develop state probabilities for other line models and for systems with four or more lines on the same right-of-way. The examples show the influence of the common-cause outage rate on the state probabilities. There is a definite need to include commoncause outages in reliability evaluation of transmission systems. This will require a more comprehensive approach to collecting transmission line outage data than has previously been used by most utilities.

Common-cause outages in multiple circuit transmission lines : R. Billinton, T. K. P. Medicherla and M. S. Sachdev. IEEE Trans. Reliab.R-27, (2) 128 (June 1978)

Common-cause outages in multiple circuit transmission lines : R. Billinton, T. K. P. Medicherla and M. S. Sachdev. IEEE Trans. Reliab.R-27, (2) 128 (June 1978)