Lightning Current Observation on UHVAC Transmission Lines by Improved Magnetic Steel Rod Method

Sensors & Transducers, Vol. 179, Issue 9, September 2014, pp. 149-156 Sensors & Transducers © 2014 by IFSA Publishing, S. L. http://www.sensorsportal.com Lightning Current Observation on UHVAC Transmission Lines by Improved Magnetic Steel Rod Method Anfeng Jiang, Zhengcai Fu, Wei Sun, Zhen Xu Key Laboratory of Power Transmission and Power Conversion Control Shanghai Jiaotong University, Shanghai, 200030, China Tel.: +86 18818238624, fax: +86 021 622932313 E-mail: Received: 12 May 2014 /Accepted: 29 August 2014 /Published: 30 September 2014 Abstract: Lightning stroke is the main threaten to the safe operation of UHVAC transmission lines. The magnetic steel rod method had been widely used as a traditional method for lightning accident and lightning current investigation in high voltage transmission lines. For the application of the magnetic steel rod method on UHVAC transmission lines, this paper proposed a practical and easy improved method of installing the magnetic steel rods directly on the transmission towers. The residual magnetism curve and accuracy verification of the proposed method was obtained through complex steel structure impulse current experiments. The magnetic steel rod location on UHVAC transmission tower was analyzed by the numerical analysis software MagNet. The application results of the proposed method on the first demonstration and test UHVAC transmission line is presented. The obtained datasets after one storm season was analyzed through Electromagnetic Transients Program. It is shown that the proposed method is feasible. Copyright © 2014 IFSA Publishing, S. L. Keywords: Lightning current observation, UHVAC transmission line, Magnetic field, Numerical analysis, Impulse current experiment, EMTP, Improved magnetic steel rod method. 1. Introduction Due to rapid growing of power load demand and long distance of electric energy transmission, the first demonstration and test Ultra High Voltage Alternating Current (UHVAC) transmission line had been constructed in China [1]. The operating experiences in Russia and Japan show that lightning is the major hazard to the safe and stable operation of UHVAC transmission lines [2-3]. For example, the survey of UHVAC transmission line in Japan show that lightning trip out occupied 98 % of total number of system outages [3]. The lightning current parameters and stroke characteristics (stroke to the overhead ground wire or phase conductor) are the fundamental requirements http://www.sensorsportal.com/HTML/DIGEST/P_2414.htm for rational lightning protection design of UHVAC transmission systems [2-10]. The lightning overvoltage and protection countermeasures of the first UHVAC transmission system in China are considerable designed [4-7]. In order to assess and control the lightning performance of UHVAC transmission systems more accurately in future, it is crucial to observe the lightning current parameters and lightning stroke characteristics on UHVAC transmission lines [8-10]. The parameters of lightning current obtained by the Lightning Location System (LLS) were used in design of UHVAC transmission system in China [4-7]. The minimum observational square grid 20 km×20 km was taken by the LLS, but the lightning attractive radius of the UHVAC 149 Sensors & Transducers, Vol. 179, Issue 9, September 2014, pp. 149-156 transmission line is only hundreds of meters [4-6], which means the lightning current of LLS is different from actual stroke on UHVAC transmission line. Therefore, the measurements of more accurate lightning current parameters on UHVAC transmission lines are needed. The magnetic steel rod, magnetic tape, shunt resistance, current transformer, B-dot probe and Rogowski coil had been used for measuring lightning current on transmission lines [11-18]. Lightning rods with length 2~6 m were often raised up on transmission line tower tops and then the devices mentioned above were installed around them for the purpose to get an idea environment for the installation of the devices and relatively accurate measurement of the lightning current [12-18]. However, those lightning rods increase the equivalent height and change the electric field distribution of the towers. So the measured dataset of lightning current are different from the actual operational transmission lines. Compared with the method of direct measurement of lightning current by sensor coil with digital recording system and other methods of lightning accident and lightning current investigation [11-19], the magnetic steel rod method is economic and could be installed on a large number of transmission line towers. In this paper, an improved magnetic steel rod method is proposed to observe the lightning current on the first demonstration and test UHVAC transmission lines. The new magnetic steel rods with preferable performance are adopted and the impulse current calibration experiment is carried out to obtain the residual magnetism curves. Based on numerical analysis of magnetic field distribution around transmission tower due to lightning current, the magnetic steel rods are directly installed on the transmission line towers’ angle steel structures. The proposed method is applied to the first UHVAC transmission line in China. The observation results of one storm season are analyzed through the Electromagnetic Transients Program (EMTP). dB = μ0 Idl sin θ μ0 I = sin θ dθ , 4π r2 4π a (1) where a is the vertical distance between the conductor and magnetic steel rod, r and θ are shown in Fig. 1. Then B at the position P generated by current through the whole conductor is given by θ2 μ0 I μI sin θ dθ = 0 ( cos θ1 − cos θ 2 ) , 4π a θ 4π a B= (2) 1 If the conductor length l >> a, than the magnetic field intensity H is given by H = B μ0 = I 2π a , (3) Fig. 1. The principle of improved magnetic steel rod method. Based on the equation (3) and flow chart in Fig. 2, the amplitude and polarity of the lightning current could be obtained by measuring the residual magnetism curve of the magnetic steel rod, and then the lightning current parameters and stroke characteristics will be concluded. 2. The New Magnetic Steel Rods and Impulse Current Calibration Experiment 2.1. The Theory of Improved Magnetic Steel Rod Method The improved magnetic steel rod method is based on the residual magnetism characteristic of magnetic steel rod. The lightning current recording principle of magnetic steel rod is shown in Fig. 1. As shown in Fig. 1, lightning current pass through the conductor and induce a strong ambient magnetic field. Based on the Biot-Savart law, the lightning current I pass the conductor with length l, the current element Idl generate magnetization intensity dB at the magnetic steel rod installation position P is given by 150 Fig. 2. The flow chart of improved magnetic steel rod method. 2.2. The Residual Magnetism Curve of the Magnetic Steel Rod Comparing with the previous magnetic steel rod, the new developed magnetic steel rods used i (...truncated)