Extra High Voltage Overhead Lines Research Articles

The problem of the actual insulation coordination for 500 kV with overvoltage in substations and the needed for more surge arresters in high voltage grid

The new extra high voltage overhead lines and substations have increased the overvoltage phenomenon in the electrical interconnected system, especially in high voltage circuit breakers operate in a failure condition. This research develops a new methodology for transitory analysis, due to limits in the insulation coordination, available in the international standards IEC 60071, however it is not enough, more surge arresters should be required with the new overhead lines caused by overvoltage. The influence by lightning, circuit breaker failure (50BF), grounding system, towers and insulation chains are affected, in three-phase reclosing, maneuvers and transformer energization, finally a new assessment of the performance develops an accurate calculation of the overvoltage. Our findings are in the three-phase reclosing, the maneuvers have introduced overvoltage through the transmission line, higher values ​than 2.3625p.u. However, the insulation coordination is less than 2.32p.u.; compared with the limit of 2.28p.u. Finally, the overvoltage has increased from 160% to 258% according to the distance in the overhead line in 500 kV, with the needed of surge arresters, with the new equipment the overvoltage has decreased from 258% to 100%.

Mathematical simulation of extra-high voltage overhead transmission lines for travelling-wave-based relay protection devices

The complexity of transient processes during travelling-wave propagation in extra high voltage overhead lines is deter-mined by the fact that many frequency components are presented in the electromagnetic wave front. For the correct analysis of the aforementioned transient processes and for further development of travelling-wave-based relay protec-tion and fault location devices, it is necessary to take into account the parameters of the environment where electromagnetic wave propagation occurs. For the transmission line, these parameters are longitudinal inductance, longitudinal resistance, shunt capacitance and shunt resistive conductance. It is worth mentioning that for a variety of reasons unit-area longitudinal inductance and resistance are the parameters which depend on frequency, thus travelling wave velocity is different for different frequency components of the electromagnetic wave front. The popular researches which address this problem either fail to consider the dependence of overhead line inductance and resistance on frequency or this dependence is considered approximately for the high-frequency range, which can cause significant errors in the transient processes analysis during travelling wave propagation. In view of this, the development of the approach to overhead transmission lines simulation, which will allow both determining the line parameters in a wide frequency range and evaluating steady-state behaviour and transient processes in long-distance transmission lines, arrears relevant. The main research method to establish extra-high voltage transmission line parameters employs the simulation study in COMSOL Multiphysics. A number of assumptions were made in the simulation process: homogeneous soil layer, entire transmission line section and absence of transmission line conductor sag. The usage of finite elements method (FEM) for differential equation solution in the above-mentioned software is also seen as an assumption. The approach to extra-high voltage transmission lines simulation has been offered. This approach consists of the usage of Maxwell’s equations in combination with numerical integration with finite elements method (FEM). Frequency response for transmission line parameters and travelling-wave propagation velocity rate for different frequency components have been obtained. The results obtained can be used in updating of electrical power system models for further research in the relay protection field. The suggested approach will allow evaluating line parameters of other voltage types and obtaining more accurate values of transmission line parameters and travelling wave propagation speed along transmission lines for high-frequency range. The use of these transmission line models will enable to formulate approaches to improve the existing algorithms of travelling-wave-based relay protection and fault location devices.

Electrostatic performance of EHV overhead lines with asymmetrical bundles

This study deals with the computation of the electric potential and field in the vicinity of overhead extra high voltage (EHV) power transmission lines with asymmetrical bundles. The values of the electric field at the surface of the bundles as well as at the ground wires and the ground surface will also be obtained. The computer model is based on finding the potential coefficients and then solving a set of complex equations, whose number depends on the number of subconductors per phase, number of 3-phase circuits involved and the number of existing ground wires. The method is applied to a 6-bundle, 750 KV, single circuit line with flat configuration and two ground wires, resulting in a total of 20 complex equations to be solved, in order to get the charge on each of the subconductors and ground wires. The electrostatic potential and the electric field at any point can then be readily found. A comparison of the subconductor field of both symmetrical and asymmetrical bundles is made. It will be shown that the use of asymmetrical bundles will result in a reduction of the surface gradients at the bottom subconductors. At the same time, however, the field at the surface of the other subconductors will increase. This indicates that the suggested application of these asymmetrical lines for reducing the audible noise due to the line corona activity should be investigated more critically. No general statement is possible on this issue, since the numerical values of the subconductor fields will largely depend on the particular geometrical data of the line under consideration.

Controllable Static Reactive-Power Compensators in Electrical Supply System

Long-distance transmission of electrical power through extra-high voltage overhead lines and power distribution through cable system call for controllable reactive-power compensating circuits on an increasing scale. The transmission losses are thereby reduced, voltage deviations kept within permissible limits and the system stability is improved The achievements in the field of thyristor techniques made it possible to build steplessly controllable compensating reactors which are combined with a three-phase thyristor power controller. Selection of optimum voltages, current and reactor reactance of the static compensator is dealt with and the current and voltage ratings of the thyristors are explained. The problem of harmonics, the influence of unbalanced conditions, and the protective and monitoring arrangements are also referred to.