Junjia He

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.

The initiation phenomena of electrical treeing in XLPE cable insulation

Since the XLPE cables are widely used in power system and electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses, its of great importance to study the electrical treeing phenomena in XLPE cable insulation for the improvement of safety and reliability of power system. The process of electrical treeing includes three stages: initiation, propagation and breaking down. In this paper, the initiation phenomena of electrical treeing in XLPE cable insulation are studied. XLPE samples are obtained from a commercial XLPE power cable, in which electrical trees have been grown from pin to plane. The process of electrical treeing in XLPE samples are uninterrupted recorded by an online microphotograph system. The growing time and structure of tree initiation subjected to various voltages, frequencies and pin-plane spacings are analyzed. The results show that trees in different structures have the similar initiation structure. With the larger voltage and shorter pin-plane spacing, the time for initiation was longer and less dispersed. The influence of frequency on initiation is relatively small.

Harmonic measurements and analysis in a modern steel manufacturing facility

Harmonics is a growing concern to both electric utilities and their customers. Nonlinear loads in large-scale industries are important causes for the harmonic currents and voltage distortion. In modern steel manufacturing facilities, Arc furnaces and rolling mill loads are the two major nonlinear loads. They generate significant harmonic currents that flow through the plant and utility power system. These harmonic currents cause system voltage distortion, power loss, additional heating and equipment failures. This paper presents a harmonic analysis in a modern steel manufacturing facility for ac electrical arc furnace (EAF) loads with static vary compensation(SVC) in both off and on states, and rolling mill loads with high frequency filters in both off and on states. Total harmonic distortion(THD) in voltage, 2nd–25th harmonics content in current are presented. Curves of RMS voltage, active power, and reactive power are also given. Box fractal dimensions are computed as an charateristic parameter for both evaluation of the complexity of measurement curves and characterization of EAF and rolling loads.

Effects of Superconducting Fault Current Limiter on Power Distribution Systems

Mathematical models for the studied power distribution system and its superconducting fault current limiter (SFCL) are given. Based on these models and EMTP, the influence on the peak value of short circuit current, the non-fault line voltage of feeder, and the voltage over SFCL due to the limiting resistance of SFCL and the fault detection time is analyzed. The results show that SFCL can drastically limit the peak value of fault current, and it is helpful when switch faults occur. In addition, SFCL is in favor of maintaining the transient non-fault line voltage and increases the power quality. Keywords-power distribution system; superconducting fault current limiter; fault current limiting resistance; fault detection time

Realization of Self-Defined Control System for Constant Extinction Angle Control Using PSCAD/EMTDC

In order to model real complex systems and provide designers with the possibility to study and improve strategy in PSCAD/EMTDC, the paper presents a new gamma measurement method and models a self-defined control system for constant extinction angle control on the basis of the requirement of the operation in HVDC system and the control principle of constant extinction angle. According to the comparison of two HVDC systems using different control system, the self-defined control system and the control system of CIGRE, it is proved that the self-defined control system has good control performance. The system parameters of self-defined control system can be defined, and the structure of system can be optimized according to faults, as will make the system have fault protecting function. This system provides more powerful and efficient methods for designers to model real complex systems in PSCAD/EMTDC.

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.

Study on the Trigger System Design of STATCOM

In the study of STATCOM, the control theory and method has become increasingly mature, but how to make use of the high performance microprocessors and new devices as the trigger circuit hardware is still a focus. Digital trigger system, including the pulse generator and its peripheral circuit, is a key technology of STATCOM. As the trigger angle changes in the range of only several degrees in the operation of the STATCOM, there is a high requirement on the design and implementation of trigger system. Aimed at this status, the paper compares several popular design schemes of STATCOM trigger system, sums up the characteristics and requirements of the trigger system, and gives out the design content, logic realization and attentions of a general design mode of the digital trigger system. This study may be also a reference for the design of other types of FACTS controllers and trigger systems.

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.

Effect of wind on the distribution of the corona discharge ionized field generated from grounded wire during thunderstorms

The properties of corona discharge initiated from a long grounded wire suspended horizontally above the ground with regard to the effect of wind during the charging process of a negative thundercloud and the propagation of a descending downward stepped leader are studied. A two-dimensional (2D), time-dependent model is developed to evaluate the corona discharge ionized field quantitatively. It is found that the corona current increases almost linearly with the increase of the wind speed. The corona current decreases rapidly when the background electric field reaches constant in the absence of wind but not that fast at strong wind. Wind can only affect the outer region of the corona space charge cloud instead of the adjacent region of the grounded wire. Consequently, different wind speeds have no effect on the initiation condition of the streamer discharge of the grounded wire but can affect the development of the connecting upward leader in the space.