ngxin He

Observation of the Streamer–Leader Propagation Processes of Long Air-Gap Positive Discharges

A novel observation method is proposed to study the streamer-leader propagation processes of long air-gap positive discharges. Experiments of 3-12 m air-gap discharges have been conducted in this paper. The observed images could clearly and particularly describe the characteristics of the positive streamer-leader propagation processes. Some important phenomena of the propagation processes are indicated and discussed, such as the leaders velocity that varied with the time, the different lengths of inception leaders, the leaders branching phenomena, the continuous streamer-leader propagation processes simultaneously with the voltages descent, and the upward leaders propagation processes.

Attachment Processes and Influencing Factors in Competition Tests Under Switching Impulse Voltages

Research on the attachment processes in competition tests can provide insights into the influencing factors involved in the selection process that determines the lightning strike point. In this paper, a series of competition tests with a rod-rod gap configuration are designed and carried out. The test results indicate that the strike mechanism is decided by two discharge processes, namely the free development of the downward-propagating discharge and the interception process of the upward and downward discharges. If the first process prevails, the strike probabilities for different grounded rods will tend to be equal. For the second process, the strike probabilities reflect the different performances of the grounded rods for intercepting downward discharges. As the scale of the discharge gap increases, the development of the downward leader becomes more and more dominant in selecting the strike point. In natural lightning strikes, the scale of the downward leader is normally more than 90% of the discharge gap, hence the protective effect of lightning rods with different tips will not be significant for low rod heights and small lightning currents. To reflect the effect of the downward leader development on the choice of the strike point, a minimum discharge air gap of 10 m is suggested.

Comparison of Positive Leader Propagation in Rod–Plane and Inverted Rod–Plane Gaps

In order to investigate the different features of the positive leader propagation in the rod-plane and inverted rod-plane gaps, the discharge experiments in a 3-m rod-plane gap and a 3-m inverted rod-plane gap were carried out under switching impulse voltage. The positive leader propagation process was recorded by using a high-speed CCD camera. Some aspects to describe the positive leader propagation characteristics of the rod-plane and inverted rod-plane gaps are obtained and compared in this paper, including the critical corona radius, the average continuous leader inception voltage, the axial leader velocity, and the final jump length. The possible reason resulting in those differences is also discussed here.

Experimental Study on Lighting Shielding Performance of ±500 kV HVDC Transmission Lines

(3-7) . The credibility of the lightning simulation experiment depends on how exactly it reconstructs the actual lightning attachment process. Over the past three decades, many experiments have been carried out in high-voltage laboratories to develop a more valid way to simulate the lightning attachment process by using long sparkle discharge. Most of these experiments were carried out by using two typical configurations. One is a rod-plane gap that the rod electrode is energized with positive impulse voltage and the plane is grounded potential (8) . It results in a positive leader incepting from the rod. Another typical configuration is an inverted rod-plane gap (5) . In this configuration, the overhead plane electrode is energized with negative impulse voltage and the rod is located on the ground at zero potential. It results a positive upward leader inception from the rod. In this paper, a traditional rod-plane gap is adopted. The rod electrode is energized with negative impulse voltage. The test object is replaced by a scaled model. Therefore, the encounter process of the downward leader and upward leader can be simulated by the negative streamer inception from the rod electrode and the upward streamer inception from the scaled model, respectively. The purpose of this paper is to investigate the lightning shielding performance of the HVDC transmission line concerning the effectiveness of dc operation voltage by employing a simulation experiment approach. Firstly, the equivalency of the scaled experiment is discussed from the following aspects: (a) the determination of test scale factor; (b) the electric field simulation of downward leader; (c) nominal field simulation of the HVDC transmission line. Secondly, a test circuit is established concerning the isolation of the dc charge circuit and surge generator. The exposure space distribution of the HVDC transmission line obtained from a large amount discharge experiments and the typical discharge phenomenon are described in section 3. Finally, the effect mechanism of the dc voltage on the lightning shielding performance of HVDC transmission line is preliminary analyzed according to the recorded phenomenon.

Characteristics of long air gap discharge current subjected to switching impulse

Measuring the pre-breakdown current of long sparks in air is important for investigating the discharge mechanism. Since the breakdown of long air gaps is conducted by a series of streamer-leader processes, the corresponding current signals cover a bandwidth of 0 to more than 20 MHz. Measurement accuracy of the current from the high voltage side is affected by the displacement current and impulse electromagnetic interference. In this paper, a coaxial current sensor with a DC bandwidth of 74.45 MHz is developed. A displacement current-restrained electrode structure is proposed to reduce the equivalent capacitance between the current sensor and the ground over 30 times. Combined with the digital optical fiber synchronous acquisition unit, a current measurement system for long air gap discharge is established. For the purpose of the UHV systems external insulation optimization design, the discharge current waveform of a 6 m rod-plane air gap under positive switching impulse voltage with 250 μs and 1000 μs time to crest is obtained. Discharge images and stressed voltage are combined to analyze the continuous feature of a current waveform under critical time to crest impulse and discontinuous feature under long front duration impulse. For the purposes of a lightning protection study, the current waveform of a 10 m rod-plane air gap is subjected to negative switching impulse. Finally, the pulse characteristics of the current corresponding to the single channel and branching stepped negative leader are discussed.

The Effect of Corona Discharge on Leader Initiation in Long Air Gaps

In this paper, the effect of the corona space charge on the leader initiation is experimentally and theoretically investigated. The critical corona charge for the stem-leader transition is reviewed and a more proper charge criterion is recommended based on the experimental measurements and theoretical analysis. It is shown that the chocking effect of the corona space charge for the second corona inception should not be ignored, which decreases the electric field strength of the stem tip and hinders the initiation of the leader. A simple physical model is proposed to calculate the leader initiation time, which can consider the thermodynamic mechanism for stem-leader transition and the chocking effect of the corona space charge, and a good agreement with the experimental results was obtained.

Assessment of lightning shielding performance of double-circuit UHV overhead transmission lines

Field study indicates that more than 90% lightning stroke trip-out accidents for UHV transmission lines are caused by lightning shielding failure flashover. The large scale of the tower and the high operation AC voltage increase the probability of direct lightning strokes attaching to the phase conductor of double circuit UHV transmission line. The unexpected high lightning shielding failure flashover rate of double circuit UHV transmission line in Japan is obtained which has been in 500kV-operation for 10 years. Moreover the characteristics of direct lightning strokes attaching to phase conductor conflicts to the prediction result of classical EGM method. A new double circuit UHV transmission line is planned to be under construction in eastern China and the lightning shielding design is one of the key aspects to ensure a well reliability of this double circuit UHV system. Firstly, a new 2-D analysis model based on leader propagation concept for lightning shielding performance assessment is proposed in this paper. The extrapolation utility of existing upward leader inception criterion is discussed and the critical corona radius method is used in this study. A new upward leader continuous progression criterion is developed based on the classical long air gap discharge theory. The typical terrain including plain area, mountain top, valley and side slope is established in this model by using Schwarz-Christoffel mapping. Secondly, the lightning shielding performance of double circuit UHV transmission line in Japan is analyzed by employing this model, and the calculation result is in a good agreement with the field observation by TEPCO which verifies the utility of this model. Finally, the lightning shielding performance assessment of double circuit UHV transmission line in China is carried out. The lightning shielding performance of designed double circuit UHV transmission line in China is better than that of Japan in typical terrain corresponding to ground flash density is 2.8 flashes/km2/year which is recommended in Chinese standard of design technology for overvoltage Protection of power apparatus. The lightning shielding failure flashover rate will rise taking into account the lightning activity frequency is much stronger in the partial line segments.

Number and dimensional distribution of stems near the anode in a 1m air gap under positive pulse

Characteristics of stem play an important role in the leader inception and development, and deserve comprehensive studies. In order to observe the stem experimentally, a schlieren system was established. Experiments in a 1m cone-plane gap under positive impulse voltage were carried out and the discharge current and schlieren photographs of discharge process were recorded synchronously with associated analysis. Experimental results showed that the number of stems were always more than one at the initial stage of the discharge in long gaps. Only 3% of the cases had one stem in our experiments. The number of stems followed a normal distribution with mean 4.01 and standard deviation 1.26. The first corona charge did not have a significant impact on the number of the stems. These stems were concentrated near the cone tip in the range -4 to 4 pixel. Using the method for the position of the stems described in this paper, the distribution of the stems followed a normal distribution with an average of -0.44 and standard deviation 4.31.

Lightning performance assessment of 500kV transmission lines in Southern China

The disproportion in natural resource and power load results in lots of 500kV transmission lines in southern China. These transmission lines are seriously influenced by lightning because of the complex terrain and frequent lightning activities in southern China. In this paper, lightning performance of six 500kV transmission lines in southern China is analyzed with the help of Lightning Protection Analyzing System for Transmission Line (LPTL). The simulation is based on the method recommended by IEEE standard, considering the geographical environment and structure of each tower. The results show the existing difference in lightning performance of each tower. Then comparative analysis is made between the simulation results and operation experience on flashover rate of entire line. It indicates that the IEEE method considering practical situation is available to the lightning performance assessment of 500KV transmission lines in southern China.

Experimental study on the positive upward leader inception condition of horizontal conductors

It is believed that the lightning strike point of a large scale structure is dominated by the conjunction of the downward stepped leader and the upward connecting leader. The average height of conductors of the UHV transmission line is above 100m, and there is rarely report on the observation of positive upward leader initiated from ground wires or large bundle conductors in lightning attachment process to UHV transmission lines. In this paper, a series of experiments were carried out in high voltage laboratory to investigate the positive upward leader initiated from the real scale ground wire and bundle conductors. The positive upward leader emerging from horizontal conductors is recorded by high speed camera and the corresponding current is measured by using a broadband digital acquisition device. The stabilization field of ground wires and bundle conductors is derived from experiment results. It is concluded that the grounding resistance has no impact on the stable leader initiation, and the positive continuous upward leader incepted from the large bundle conductor is more difficult than the ground wire.