Multi-circuit Lines Research Articles

Estimation of the Values of Electrical Shock Currents during Live-Line Work in Multi-Circuit, Multi-Voltage HVAC Transmission Lines

This article covers the analysis of voltages induced on the conductors of a de-energized circuit of a multi-circuit, multi-voltage HVAC transmission line. As a result of the multiplied interactions between the circuits in such lines, the expected electrical shock currents (touch currents) to which a lineman performing live work on such a line may be exposed are determined. A number of supporting structures of three- and four-circuit lines with various degrees of geometric asymmetry are analyzed. Analyses have shown that in multi-circuit lines in which circuits of different voltages are carried on a common structure, despite the outage of one of the circuits, touch voltages and electrical shock currents (touch currents) exceeding the permissible values can be expected on its conductors, endangering the safety of the lineman. The arrangements of s in such lines that provide the smallest values of touch currents are indicated.

Comprehensive research on hybrid energy storage strategies of park microgrids with optical storage and DC microgrids

The multi-circuit coupling transmission mode is an inevitable trend of power grid construction, mainly with partially coupled multi-circuit lines. Affected by the local coupling between lines, the existing fault location methods have large errors. To this end, fault analysis and fault location methods are carried out on partially coupled double-circuit transmission lines. According to the structure and characteristics of the partially coupled double-circuit transmission line on the same tower, the voltage and current interface equations at the coupling demarcation point of the partially coupled double-circuit transmission line are established using line decoupling theory. On this basis, a time-domain analysis model of fault location for each coupling section is constructed, and a time-domain method for fault location of partially coupled double-circuit transmission lines is proposed. Finally, the electromagnetic transient simulation software is used to construct a partially coupled double-circuit transmission line model on the same tower, and a comprehensive simulation verification is performed. The results show that the proposed method has high accuracy.

Mitigation of common mode failures at multi-circuit line configurations by application of line arresters against back-flashovers

Due to the limited number of corridors multi circuit line configurations are often applied. These overhead lines frequently consist of high towers that are subject to lightning strokes. In case of higher current amplitudes and higher footing resistances due to bad earthing conditions back-flashovers are caused leading to common mode failures and to severe outages. The paper describes investigations performed by means of computer simulations to identify the towers of a multi-circuit line consisting of voltage levels 380 kV, 220 kV and 110 kV that are endangered by back-flashovers of the 110-kV double-circuit lines. The footing resistance of towers of the targeted line section has been measured by an instrument at high-frequency. Influence of various factors on the back-flashover over 110 kV insulator strings has been studied by means of EMTP-ATP simulations. Different current waveforms of the lightning stroke have been used to represent the first stroke and subsequent strokes. The towers are represented by the models described in [3], [8]. Available flashover analysis methods [7], [8], [12], [13] like leader development method by Pigini et al and by Motoyama, and voltage-time integration method by Kind have been applied. The towers at which back-flashover is more likely to occur than at other towers are identified by the time integral of voltage according to Kind. Various factors like tower footing impedance, tower surge impedance and tower height are considered. Application of line a surge arrester is shown to be a successful mitigation technique to reduce the back-flashover rate of those 110 kV lines. The lightning overvoltage performance of surge arresters has been analyzed by means of digital simulations. Based on the results of investigations line arresters were installed on the towers in question. Since the installation no further common mode failure has been observed.

Lightning protection performance of quadruple-circuit 500 kV transmission lines on the same tower with composite cross arm

In recent years, composite materials cross-arm (CMCA) towers have been gradually used in 10 kV, 35 kV or other transmission lines with lower voltage level in China. However, it is rarely used in higher voltage level transmission lines, especially in multi circuit lines on the same tower. Because of its higher height, we should pay more attention to the lightning protection performance of high voltage level multi circuit lines on the same tower. In this work, the simulation and calculation analysis of Lightning stroke model of quadruple-circuit 500 kV transmission line is done to compare CMCA tower with steel tower. The research shows that the tolerance lightning level of CMCA tower is superior to conventional steel tower, The lightning withstand level of single circuit flashover is 45% higher than that of steel tower and the minimum shielding failure lightning withstand level is 47% higher than that of steel tower. The data can provide the theoretical basis for the construction and optimization design of high voltage CMCA tower.

A Novel and Robust Model of the GUPFC Controller System Based on Adaptive Fuzzy Logic- PI Controller to Enhance the Control System Performance in Following Reference Active and Reactive Power

The optimal electrical power transmission problem in electrical energy transmission lines has led to increased attention to the use of flexible alternating current transmission systems (FACTS) and the design of double- and multi-circuit lines. Hence, recently, multi-converter FACTS devices have been utilized in the literature to control voltage and power of multi-circuit transmission lines. A generalized unified power flow controller (GUPFC) is one of such emerging FACTS devices that can manage voltage and power control crisis in multi-circuit lines. The GUPFC is the most advanced generation of FACTS, which will be able to control active and reactive power in at least two circuits and voltage in one circuit with the best quality possible and satisfy the operator’s all requests. This paper, for the first time, presents the use of an adaptive control system design based on the proportional-integral (PI) controller and fuzzy system to enhance the fast and dynamic responsiveness of the system. PI systems alone cannot control the GUPFC under different operation conditions such as when the default reference values of active and reactive power are changed, or transient faults occur, or a transmission line experience outage. Thus, the use of a fuzzy controller, as a powerful tool, is very efficient in solving the mentioned problems. To analyze the proposed algorithm’s results, a test system and a GUPFC based on a 48-pulse voltage source converter (VSC) are implemented in the MATLAB/Simulink environment. The satisfactory results obtained in the simulation section verify the correct performance of the suggested method.

Mutual Coupling Compensation Techniques Used for Distance Protection of Parallel Lines

When a distance relay protects a transmission line located on a dual circuit tower, a coupling effect will occur between the two circuits. Transposition of the circuits can reduce the mutual impedances, but this does not cater to the zero-sequence mutual coupling impedance during earth faults. As a result, the impedance measured by a distance relay under phase-to-earth fault conditions in these circumstances will not represent the correct impedance to the fault point unless these effects are taken into account. On multi-circuit lines, primarily if they operate in parallel, a zero-sequence mutual coupling should be considered when calculating settings for distance protection function. A 220 kV parallel line sharing the same tower was analysed using DigSilent Power Factory in the simulations. Phase-to-earth faults in different configurations were analysed on this system, and the reach of the protection relay was then estimated for operation. The results confirm how a protection relay can overreach and underreach in a distance protection scheme due to the influence of mutual coupling.

Unbalanced currents of EHV multi-circuit lines and coordination of zero-sequence overcurrent relayings

With features of large transmission capability and high utilization efficiency of transmission corridor, multi-circuit transmission lines sharing tower or corridor are widely applied in extremely high voltage (EHV) systems. However, mal-operation of zero-sequence overcurrent relaying for multi-circuit transmission lines could occur, and this paper is focused to solve this problem. Overview of typical multi-circuits transmission lines, including phase arrangement and erection form, are introduced first. Then, simulation model of a typical 500 kV triple-circuit transmission line in real project is established in Power Systems Computer Aided Design (PSCAD), and it is verified by real record data provided by power utility in China. By using this simulation model, factors on current unbalances are discussed in detail, and the simulation results have shown that mal-operation of zero-sequence overcurrent relaying is caused by large unbalanced current in normal operation and under faults. Consequently, a lateral coordination scheme for zero-sequence overcurrent relayings among multi-circuit lines is proposed to avoid the mal-operation caused by unbalanced current to the greatest extent without receiving samples from remote end or adjacent circuits, and the reliability of protective relaying is improved significantly.

Lightning protection of a multi-circuit HV-MV overhead line

This paper deals with the assessment of the lightning performance of a medium voltage (MV) power line sharing the same poles of a high voltage (HV) line. We focus on the case of a 15-kV line with compact configuration that is supported by the concrete poles of a 69-kV or a 138-kV transmission line. Due to the taller HV pylons and conductors, the number of direct events to the multi-circuit line is higher than for a MV line alone. The presence of the HV conductors and of the overhead ground wire reduces the amplitudes of the induced overvoltages in the MV conductors due to indirect lightning events. For the considered configurations, with a compact MV line, the number of flashovers per year expected in the double-circuit line due to both direct and indirect flashes is lower than in a MV line alone. It is shown that such a number depends mainly on the line withstand voltage, on the grounding resistance and on the number of surge arresters. The dependence is quantitatively discussed in the paper. The analysis of the overvoltages transferred on the LV side of the distribution transformers is also performed.

Mechanism and solution of zero-sequence directional protection maloperation caused by broken line faults in multi-circuit transmission lines on the same tower

When one loop line of the multi-circuit transmission lines on the same tower is in states of non-full-phase operation or asymmetric line-breaking faults, the zero-sequence direction of functioning loop lines is likely to maloperation due to zero-sequence voltage compensation. The multi-circuit transmission lines on the same tower with different voltage classes will induce the same zero-sequence voltage distribution trend as the fault line due to the mutual inductance between loop lines, and thus may conduct improper operation as well. This paper analyses the causes of maloperation of zero-sequence directional protection caused by non-full phase operation or line-breaking fault of multi-circuit lines on the same tower, and puts forward a method to prevent maloperation by extracting line status from station domain information. In the case of multi-circuit lines on the same tower with same voltage class, the crossing point of zero-sequence voltage of the line can be calculated and the zero-sequence voltage can be compensated in reverse to prevent maloperation. In the case of multi-circuit lines on the same tower with different voltage class, fault characteristics can be identified according to substation information and the zero-sequence direction protection can be automatically blocked. RTDS is used to simulate and verify the electrical characteristics, malfunction process and the effectiveness of prevention methods of multi-circuit lines on the same tower when a non-full-phase operation or abnormal line breakage occurs.

Research on Capacitance Current Compensation Scheme of Current Differential Protection of Complex Four-Circuit Transmission Lines on the Same Tower

Current differential protection is the main protection of transmission lines which include multi-circuit lines on the same tower, and whose sensitivity and reliability of differential protection is mainly affected by the distributed capacitive current. For the four-circuit line on the same tower, due to the influence of coupling between the loop road, the distributed capacitance current increases significantly when compared with ordinary lines, affecting the sensitivity of the current differential protection, especially for different voltage levels throughout the four-circuit lines on the same tower. The relationship of the electrostatic coupling between the circuits is more complex, and increases the difficulty of the compensating the distributed capacitance current. This paper is based on the electrostatic coupling principle of four-circuit lines on the same tower, establishes the distributed parameter model of four-circuit transmission lines on the same tower, and discusses the effect of circuit operation mode on the compensation of capacitance current differential protection when different faults occur on the complex four-circuit transmission lines on the same tower. A new compensation scheme suitable for capacitive current compensation is proposed. Simulation results show that this capacitive current compensation scheme can effectively improve the performance of current differential protection.

Earth wires currents calculation by tableau analysis

The paper presents a new method to compute a phase-to-earth fault current distribution in overhead transmission line. The method is based on the tableau analysis of an unbalanced, multi-wire model of a transmission line. It allows for the calculation of current distribution in overhead earth wires along the transmission line. Additionally, it may be used to calculate currents in a tower’s earthing system as well as voltages over every tower of the transmission line. The method can be used for analysis of any type of transmission line (single circuit, double circuit and multi-circuit lines) with any number of earth wires. The calculation results may be used for complex designs of transmission line earthing systems, earth wire thermal rating selection, and the evaluation of shock hazards.

Research on Total Electric Field for DC Lines Converted from Double-Circuit AC Lines

With rapid growth of power demand, transmission capacity is also in urgent need of upgrading. In some cases, converting existing AC transmission lines to DC lines can Improve the transmission capacity and reduce the construction investment. In this paper, the upstream finite element method was expanded to calculate the total electric field of same tower multi-circuit DC lines converted from double-circuit AC lines, and the validity of the algorithm was confirmed by experiments. Taking a DC line converted from a typical same tower 500 kV double-circuit AC transmission line as an example, the surface electric field and the ground total electric field in different pole conductor arrangement schemes were calculated and analyzed, and the critical height of pole conductors for DC lines in residential and non-residential area were determined. Then, the corridor width of DC and AC lines at critical height in residential and non-residential areas before and after AC-DC line transformation were compared. The results indicate that for DC lines converted from common 500 kV double-circuit AC lines, the ground total electric field can meet the requirements of corresponding standard with appropriate pole conductor arrangement schemes.

Analysis of Possibilities to Reduce the Voltages Induced in Multi-circuit and Multi-voltage Overhead Lines

Analysis of Possibilities to Reduce the Voltages Induced in Multi-circuit and Multi-voltage Overhead Lines

Improving the lightning protection effect of multi-circuit tower by installing coupling ground wire

Erecting coupling ground wire is a type of effective lightning protection measure to reduce trip-out rate of back flashovers. Nowadays coupling ground wires has been applied on some multi-circuits transmission lines in China. However, various setting positions of coupling ground wire have different impacts on the lightning protection effects, especially the multi-circuit lines. Taking the typical 500/220kV and 220kV quadruple circuit transmission lines for example, the paper presents and analyzes the effects of setting coupling ground wire on the center of cross-arm at different heights. The shunt coefficients of tower, ground wires and coupling ground wire, and the coupling coefficients are also compared and discussed in situations that the coupling ground wire is mounted in different positions.

Ion-Flow Field Calculations of AC/DC Hybrid Transmission Lines

Large-scale integration of new renewables requires substantial increases of transmission capacity on existing transmission corridors. To limit land use, overhead transmission lines have been widely built as multicircuit lines. On these, changing one ac circuit to dc to increase transmission capacity leads to lines with hybrid ac/dc towers. Since practically no operational experience with such hybrid lines exists to date, questions such as superimposed ac and dc fields intensified by corona-generated space charges arise. In this paper, a method to simulate these ion-flow fields using the discontinuous Galerkin method is for the first time applied to hybrid towers. Ways to considerably reduce the computational effort of the problem are discussed. The results indicate that hybrid lines are feasible in reference to space-charge-related problems. Shielding the ground is possible at the expense of increased coupling between the ac and dc circuits.

A useful methodology for analyzing distance relays performance during simple and inter-circuit faults in multi-circuit lines

A methodology for evaluating and visualising the response of power system distance protection units most commonly used is presented. A general case, with different topologies, of a double-circuit transmission line is considered and different types of inter-circuit faults are treated. Single line and simple faults are simplified cases of the general one. Faults are treated by means of the adequate connection between the sequence networks, where different types of faults are represented by a resistance star associated to both lines and ground. Different combinations of these resistances result in different types of faults. Fault conditions are introduced to sequence networks of the rest of the system through phase current controlled current sources, and the resulting sequence voltages at the fault point are applied to fault resistances through sequence voltage controlled voltage sources. To calculate faults the mesh analysis method has been modified to take into account the dependencies fixed by the controlled sources. Real previous state of the power system (load flow, sequence source impedances at both ends and sequence impedance between ends) is taken into account. A useful and reliable way of representation is presented to obtain the expected response of different types of distance units in multi-circuit lines in presence of inter-circuit simultaneous faults.

Guidelines For Determining Parallel Resonance On EHV Transmission Lines

Generalized curves are derived which allow the utility engineer to easily determine the possibility of parallel resonance on EHV transmission lines. The curves are given for transposed and untransposed multi-circuit lines and for single-circuit lines where the misoperation of a single phase of a circuit breaker is to be considered. The parallel resonance problems associated with typical reactor and transformer configurations are evaluated.

USSR study of electrical transmission lines of increased capacity and reduced ecological influence

A study of AC electrical transmissions with increased capacity and reduced ecological influence is described These transmissions relate to overhead multicircuit lines where field concentration in the area near the circuits is achieved by bringing the phase wires closer together and creating a phase shift between the voltages of the corresponding phases. Between 20 and 50% more natural power is provided than with the usual double-circuit line of the same voltage. Several transmission systems of this kind are studied, their parameters are given and electrical field tensions below the phases are calculated. The results on the first semi-commercial double-circuit lines of increased capacity are discussed.

Circulating Currents in Paralleled Untransposed Multicircuit Lines: I - Numerical Evaluations

The current unbalance in a 50O-kV double-circuit and a 138-kV triple-circuit, paralleled and untransposed transmission line is analyzed comprehensively. This paper establishes that the magnitudes of individual circuit current unbalances due to currents circulating between the paralleled circuits are more significant than the net through system unbalance. Since the former were not considered in previous investigations, the criteria established there for proper phase configurations is shown in this paper to lead to maximum circulating current unbalance. The influence of series capacitor line compensation is also shown and the ramifications of these unbalances are discussed and evaluated on a present worth of annual loss basis.

Circulating Currents in Paralleled Untransposed Multicircuit Lines: II - Methods for Estimating Current Unbalance

The current unbalance in a 500-kV double-circuit and a 138-kV triple-circuit, paralleled and untransposed transmission line is analyzed comprehensively. This paper establishes that the magnitudes of individual circuit current unbalances resulting from currents circulating between the paralleled circuits are more significant than the net through system unbalance. Since the former were not considered in previous investigations, the criteria established there for proper phase configurations is shown in this paper to lead to maximum circulatimg current unbalance. The influence of series capacitor-line compensation is also shown and the ramifications of these unbalances are discussed and evaluated on a present worth of annual loss basis.