Impulse Corona Characteristics And Their Influence On Transmission Line Transient Computation

In the lightning transient computation of overhead lines,the corona effect is taken into account by employing the charge-voltage characteristics(q-u curves).The existing researches mainly focus on the lightning surges with double exponential waveforms.However,field tests and numerical simulations show that most lightning surges incoming to substations have the damped oscillation waveforms due to refraction and reflection.Therefore,the practical need exists in studying the q-u curves under this non-standard type damped oscillation surge and their influence on the lightning transients.For this purpose,a systematic research is performed by laboratory measurement and model computation,it covers the following aspects.An impulse corona experimental system is built,which consisted of a modified impulse voltage generator,corona cage and measurement devices.The experimental system can be used to measure the q-u curves under damped oscillation surge.Effective anti-interference measures are taken and the stray capacitance and inductance of the experimental system are greatly reduced,thus a high degree of accuracy is attained.The effects of the frequency and oscillation depth on the q-u curves are studied by varying the waveforms of applied voltages.The measured results show that the corona charge increases with the frequency and the size of the minor loops is dependent on the oscillation depth.The q-u curves on bundle conductors are measured in a reduced-scale model under damped oscillation surge,which indicates that the corona discharge grows weak and the minor loops tend to shrink with the increase in number of sub-conductors.Based on the microcosmic physical processes of gas discharge,a mathematical physical model of the impulse corona is proposed.The effects of collision ionization,drift of the charged particles,space photoionization,attachment and recombination are taken into account in the proposed model.The q-u curves under damped oscillation and double exponential surges are computed.The computed results are compared with the measured ones and a better agreement is shown between them,which verified the validity of the proposed model.The corona discharge process under damped oscillation surge is analyzed by computationally exploring the space electric field distribution varying with time.Additionally,the formation mechanism of the minor loops is also discussed.The computed space electric field distribution indicates that the opposite polarity corona might be one of the reasons for the formation of minor loops.Multiphase overhead line model is developed by using the finite difference time domain(DFTD)method to solve the telegraphers’ equation in time domain.In order to take the frequency dependence of the line parameters into account,the vector fitting(VF)technique and recursive convolution are employed.The validity of the line model is verified by the simulation results obtained from EMTP-RV and measured ones reported in literature.Typical numerical examples are given for applying the overhead line model to lightning transient computation and a comparison is made between the computed surge waveforms on single and multiphase overhead lines.The effects of the waveform parameters(i.e.frequency and oscillation depth)are also studied on the attenuation and distortion of damped oscillation surge.The Suliciu’s fitting method is implemented into the proposed overhead line model.It has the capability of computing the lightning transient under double exponential surges with corona effect taken into account.The computed results are compared with the measured ones to verify the feasibility of the Suliciu’s fitting method and the proposed line model.Then,the fitting method is extended to simulate the q-u curves with minor loops under the damped oscillation surge.The extended Suliciu’s fitting method is employed to investigate the behavior of attenuation and distortion for lightning surges.The computed results reveal that considerable error may be caused for negative polarity,when the minor loops are neglected.However,the attenuation and distortion of positive damped oscillation surge is less affected by the minor loops.