Shijun Xie

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.

Observation of the positive leader propagation in the inverted rod-plane gap

The characteristics of the positive leader propagation in the inverted rod-plane gap are investigated in this paper. The discharge experiments in 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. The aspects to describe the positive leader propagation characteristics of the inverted rod-plane gaps are obtained and compared with the traditional rod-plane gaps. The critical corona radius and continuous leader inception voltage of rod derived in the 3-m inverted rod-plane gap are lower than that of the traditional rod-plane gaps.

Influence of smoothing reacteor arrangement on transients of converter station for ±500kV double-circuit HVDC system

For HVDC transmission system, the smoothing reactor has two structural types; oil-immersed core reactor and air-core reactor. The former needs to concentrate installed on DC-line, and the latter can be arranged flexibly on both pole line and neutral line. According to the different arrangements of smoothing reactors, the system steady-state performance and over-voltage of converter station have some difference. In this paper, the influence of different smoothing reactor arrangements on the system steady-state performance and converter station over-voltage performance of Xiluodu-Guangdong ± 500kV double-circuit HVDC project is analyzed by using ATP/EMTP. A reasonable arrangement of smoothing reactor is proposed, which aims at both maintaining reliability and reducing costs.

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.

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.

Observation on the attachment processes of competition tests under switching impulse voltage

The attachment processes of competition tests under switching impulse voltage were studied in this paper. A series of competition tests with rod-rod gap configuration for these studies were carried out. In these competition tests, a high-speed CCD video camera was used to clarify the attachment processes, assisting by current measurement of the grounded rods and the voltage measurement of the discharge gap. The experimental results indicated that the processes can be divided into two stages related to the downward discharge and the upward discharge respectively, both in positive and negative competition tests. However, the scale of the two stages and the discharge process in the two stages are different, and these differences affect the striking probability finally.