Zhizhao Li

Electric field step in air gap streamer discharges

Electric field (E-field) in air gap streamer discharges under positive lightning impulse was measured by specifically developed integrated electro-optic sensors. An E-field step phenomenon was observed. The E-field firstly agreed with the Laplace field, then suddenly increased with a rise time of μs. The occurrence probability of this phenomenon increased as the applied voltage increased. The discharge current waveforms and photos taken by a fast camera prove the E-field step was caused by the space net charge. From the E-step rise time and the corona area range, the average electron drift speed under the experiment situation was estimated about 0.2 × 106 – 0.6× 106 m/s.

Study on the Influence of the DC Voltage on the Upward Leader Emerging From a Transmission Line

The evaluation of the lightning shielding failure of a transmission line is very important in power transmission system design, and the operation voltage has rarely been considered before. However, when the operation voltage reaches 800 kV and the tower height is near 100 m, the influence of operation voltage may not be ignored. In this paper, an experimental system consisting of an air gap between a plate/rod and a conductor was established, where the dc voltage could be applied to the conductor and was accompanied by a negative impulse voltage on the plate/rod electrode. An upward leader up to 1.54 m in length was observed, and the leader velocity and charge density were obtained in a 11-m-long air gap, whereas the voltage on the conductor was 800-kV dc. The influence of dc operation voltages on the upward leader was studied. The experimental results showed that the upward leader will emerge much earlier and may progress longer with dc operation voltages. That is, the lightning attachment to the conductor may occur more easily when the voltage of the conductor is opposite in polarity to that of the downward lightning than when the conductor has no operation voltage.

Research on characteristics of surface electric field on transmission lines under lightning stroke

Lightning is a major cause of power interruptions in power transmission lines. Nowadays the Leader Progression Model is an advanced method to evaluate the shielding failure probability of transmission lines. Calculating the surface electric field on phase conductors and ground wires precisely is important, because the surface electric field has great effect on the upward leader inception. In this paper, the influence of lightning current and some other factors on surface electric field were analyzed.

Study on the Influence of Space Charge on the Upward Leader Emerging From the Conductors of UHVDC Transmission Lines

The space charge produced by the glow corona from tall objects is known to inhibit the initiation of upward leaders. However, in contrast to consideration of the glow corona at the surface of conductors caused by thunderclouds, the inhibitive effect of ion flow on the initiation of upward leaders from conductors due to dc operation voltage has been relatively neglected in previous studies of the leader progression model. In this paper, the glow corona at the surface of a conductor due to the thundercloud and the operation voltage have been analyzed quantitatively and compared with each other. It is found that the influence of the operation voltage on the distribution of ion flow is the largest in the area near the conductor. Since the operation voltage has a greater influence on the inception of the upward leader than the thundercloud, it should therefore also be considered in the leader progression model. A new simplified model is hence proposed to account for the distribution of ion flow under thundercloud conditions. In addition, a simulation experiment is designed to study the influence of ion flow on the initiation of the upward leader from a 6-bundled conductor, and the results are used to verify the validity of the simplified model. Finally, the model is applied to assess the influence of ion flow on the inception of upward leaders under different lightning currents. The calculation results show that the ion flow causes the upward leader to incept later, and reveal that the influence of the thundercloud on the inception of the upward leader is small, although it influences the distribution of the ion flow significantly.

Upward leader inception criterion considering gas kinetic process and heat conduction

Modelling of upward leader inception is important for calculating lightning shielding failure rates of transmission lines. The leader inception model proposed by Gallimberti I. has been widely adopted for about 30 years and applied in the analysis of unstable upward leader inception. However, the gas kinetic process and heat transfer via conduction are not considered in this model. Theoretical analysis and simplified simulation show that the gas density variation and heat conduction cannot be neglected. In this paper, the gas kinetics and heat conduction have been considered, and a simplified one-dimensional thermofluid dynamics (1D-TFD) model has been proposed as an upward leader inception criterion. A Mach-Zehnder interferometer was set up to observe the gas density variation in leader channel and the channel expansion. Experimental results indicate that the gas density reduction in leaders is significant, and the calculation of the channel diameter expansion has a good agreement with that obtained by experiments.

Comparison of the Characteristics of Upward Leader Emerging From the Ground Wire and Conductor

An understanding of the characteristics of upward leaders from transmission lines is crucial to the analysis of shielding failure. Since data on these characteristics are difficult to capture under natural circumstances, experiments are needed. In this paper, 7-m plane/rod to conductor and plane/rod to ground wire air gap have been used for studying the characteristics of an upward leader from ground wire and conductor, and the differences in the upward leaders initial characteristics between these two were compared. The experimental results show that the streamer and upward leader from the ground wire were incepted much earlier, but the difference in their velocity is small compared with that from the conductor. These conclusions may suggest that the length of the upward leader from the ground wire is longer than that from the conductor, and this condition may lead to the ground wire being struck by lightning more often compared to the conductor when no operational voltage is applied.

Experimental research of the upward leader inception from transmission line

Electro-Geometric Model (EGM) has been applied widely all over the world to evaluate the lightning shielding failure of transmission line. But with the increase of operation voltage to 1000 kV and tower height to more than 100 meters, some shortcomings of the traditional EGM model were found by many researchers. Leader Progression Model (LPM) was proposed to solve these problems. But some fundamental works, such as when the upward leader initiates and how it propagates, is still indistinct. An experiment was designed and implemented in this paper to simulate the electric field circumstance near the test transmission line while the lightning downward leader approached. The upward leader from the test transmission line was clearly observed. The inception voltage and characteristics was analyzed. The streamer and leader initiated from the ground wire and conductor bundle were compared. The experiment results show that the leader initiating from the ground wire is more active in the inception stage. The velocity of the upward leader acquired is about 1.2–3.0 cm/μs.

Analysing on characteristics of lightning shielding failure of UHVAC double circuit transmission line based on leader progression method

Shielding failure of lightning is the key problem for safe operation of UHV transmission lines. Taking the feature of 1000KV AC double circuit transmission lines into account, the leader progression method with proper criterion was employed. The lightning shielding failure rate of top phase, middle phase and bottom phase of two different transmission line tower types were compute, in three different geography conditions. The research work is helpful for improving the lightning performance of UHV transmission lines.

Research on the characteristics of upward leader emerging from the transmission lines influenced by operation voltage

The problem whether the DC operation voltage can influence the characteristics of upward leader emerging from transmission lines is still in question since experiments and observation data is insufficient. It is necessary to study the relationship between the characteristics of the upward leaders and operation voltage. This paper presents some simulated experimental results of a combined plane-rod (PR) electrode to conductor air gap of 7 meter length. A current measuring instrument with frequency response of from DC to 6.5MHz was developed and used to measure the leader current. Besides, high speed CCD camera was used to observe the optical phenomenon of the whole process. The 200/2000μs impulse voltage up to 4MV and positive DC voltage up to 800kV was applied to the electrode and conductor respectively. The higher charge per unit in the leader channel and earlier inception time has been found along the higher DC voltage. These results have been compared with those in the gap that only impulse voltage was applied. Finally, the influence of positive DC voltage on the characteristics of upward leaders was discussed.

Analysis on shielding failure of large lightning current of transmision lines

Shielding failure of lightning is the key problem for safe operation of transmission lines. Using Electro-geometry Method to simulate the process of shielding failure, large currents hit a conducting wire hardly, however actually they did sometimes. So, Leader Progression Method is used to analyze the relationship between amplitude of lightning current and other parameters, such as wires position, ground tilt angle and so on.