Analysis on Insulation Resistance Performance in Airport Lighting Circuit DONG Hui Fen a, GAO Qing Ji b and LIU Jian Shan c

Transcription

1 Advanced Materials Research Online: ISSN: , Vol. 668, pp doi: / Trans Tech Publications, Switzerland Analysis on Insulation Resistance Performance in Airport Lighting Circuit DONG Hui Fen a, GAO Qing Ji b and LIU Jian Shan c Robotics Institute, Civil Aviation University of China, Tianjin, a hfdong@cauc.edu.cn, b gaoqingji@vip.sohu.com, c jsliu_07@cauc.edu.cn Keywords: airfield lights; lighting circuit; insulation performance; curve fitting; parameter identification; simulation. Abstract. The airfield lighting circuits are the important part of the airfield lighting system, and the insulation performance of the circuits directly affects the aircraft flight safety. The side insulation performance of the lighting circuit and the effect of the temperature and humidity on the insulation are analyzed in the paper. Depending on the measure data provided by the Shanghai PUDONG international airport, the curve of circuit insulation resistance changed with the time is fitted through computer, and the insulation resistance mode based on the parameter identifying is obtained. The theoretical foundation is laid for the forecast decline of insulation resistance, and a theoretical basis is provided for the maintenance of the lighting circuit. Introduction Airfield lighting system is an important part of airfield lighting system, so reliable operation of the airfield lighting system is very important, a direct consequence of the runway lighting circuit insulation resistance drop is that the airfield lighting systems can t run normally, a significant security risks has been left to the aircraft navigational aids, and even lead to catastrophic accidents. Airfield lighting circuit is an important part of airport navigation system. It provides the power supply and signal transmission for a variety of electrical equipments of navigation system. Lighting circuit of airport navigation system runs in the environment of wet and high temperature all year round, the more stringent requirements of the insulating properties of the circuit is then put forward. Because of electrical, mechanical, chemical and other factors, light cables will gradually aging and resulting in the cable insulation performance gradual decline or even damage, how to judge the insulation resistance change and actual life of the lighting circuit must be done. Whether the airfield lighting work reliably for a long time, or it is out of operation due to the insulation aging is of great concern to the airport management [1]. The lighting system maintenance and replacement of related accessories is based on the insulation resistance of lighting circuit in the airport. Airfield Lighting Circuit Structure Airport airfield lighting circuit is a suspension loop constituted by a series of isolation transformers. Lamps are connected to the secondary class of isolation transformer, whose luminous intensity is positive proportional to the loop current I. Therefore the brightness of the bulb can be changed by changing the current value of the loop, when the loop current in a numerical constant, the lamp brightness will remain constant. The airfield lighting circuit is mainly composed by the main transformer, cables, isolation transformers and lamps as shown in Fig. 1. In Fig. 1, T is the step-up transformer, where have two connectors between primary winding of the isolation transformer and the cable for each lamp which is formed with a period of cable, two cable connectors, and an isolation transformer. J1 and J2 are the cable connectors. If the lighting circuit insulation resistance drops to a certain value, the ground leakage current i increases, the current flowing through the bulb drops, which affects the lights intensity and reduces the brightness of the lights, then the navigation lights wouldn t work normally and give the flight a security risk, so it is necessary to analyze the insulation resistance performance of the lighting circuit cable. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,02:17:06)

2 Advanced Materials Research Vol Static model of the lighting circuit insulation resistance From the lighting circuit structure, the lighting circuit insulation mainly includes lighting cable insulation, cable connector insulation, and isolation transformer insulation and so on. So the main impact factors on the circuit insulation resistance value are the quality of cable, isolation transformer, cable joint production, isolation transformer box, the length of the cable and the number of cable connectors (or the number of lamps), as well as the attenuation of insulation resistance value over time with the airport geological conditions, climatic conditions and construction technology level. In order to analyze the insulation performance of the entire lighting circuit, the model of circuit insulation resistance is analyzed at the first, including the cable, connectors and isolation transformer. Cable Insulation. The cable material quality can fundamentally ensure that the circuit insulation resistance value is great [2,3,4]. High voltage power cables buried in the ground are very easy to damp, resulting in the damage of the cables in varying extent, which is a direct threat to the safe operation of the navigation lamps [5,6,7]. Therefore, in order to safeguard security of power supply and ensure the safe operation of the airfield lighting systems, it is necessary to do the preventive experiment on the lighting circuit cable regularly. Here the cable insulation is analyzed at the first. The total current flowing through the cable is i 0 under the DC voltage, which is the sum of the conduction current I 1, the capacitive current I 2 and the absorption current I 3, that isio = I1 + I2 + I3. I 3 and I 2 decay to near zero after a sufficiently long time, only the conduction current I 1 remains stable, for the cable insulation resistance value, it gradually increases as the time pass by.a cable is equivalent to the insulation resistance as shown in Fig. 2, that is: T J1 J2 R11 U UL 升压变压器 I 灯泡 隔离变压器 Isolation Transformer C11 R12 C12 step-up Lamp transformer Z eq 1 Fig. 1 Diagram of airfield lighting circuit Fig. 2 Equivalent circuit of Cable resistance i I I I I Ie o t/ T1 = + + = + (1) o t/ T1 In which: Io = U/ R11, Ie = I2 + I3, Absorb the time constant: T 1 = ( R 11 // R 12 )( i C 11 + C 12 ) (2) The resulting expression for the cable insulation resistance is 1 1 Z eq1 R11// //( R12) (3) Then thez eq1 stable value is: Zeq1 = R11 (4) Cable Connector Insulation Resistance. The cable connector production quality is another important factor affecting the lighting circuit insulation resistance value [8], the connector production must be done in strict accordance with the operational procedures to ensure the insulation quality. With the analysis on the equivalent circuit of cable insulation, we can get the equivalent circuit of the cable connector insulation, which is shown in Fig. 3. In Fig. 3, R 21 and C 21 are equivalent resistance and capacitance of the cable connector respectively, R 22 and C 22 are equivalent resistance and capacitance of Teflon insulation tape and self-adhesive insulating tape used in the cable connectors, R 23 and C 23 are equivalent resistance and capacitance of the PVC insulation tape used in the cable connectors. Then the insulation resistance is Z = R // //( R // + R // ) (5) eq The stable value is: Zeq2 = R21//( R22 + R23) = R21i ( R22 + R23)/( R21 + R22 + R23) (6) The isolation transformer insulation resistance. The quality of isolation transformer box has a great impact on the circuit insulation resistance value[9], Inlet and lid are sealed to ensure the box dry. If they are not well sealed, it will result in high temperature and humidity after the system has

3 502 Materials Science and Engineering II been used, which can accelerate the aging of isolation transformer, cables and connectors, and thus the circuit insulation resistance value decreases rapidly. For the isolation transformer, the insulation circuit to earth of primary side can be equivalent to the circuit shown in Fig.4 as follows: R 31 is equivalent to the insulation resistance of brace or pad, R 32 and C 32 is equivalent to the resistance and capacitance of the insulating material between the primary and secondary side respectively, R 33 and C 33 is the equivalent resistance and capacitance of insulating varnish respectively[10]. C21 R31 R22 R21 R23 R32 R33 C22 C23 C32 C33 Z eq2 Fig.3 Equivalent circuit of cable connector Fig.4 Equivalent circuit for transformer of primary side Z eq3 Insulation Impedance: 1 1 Z = eq3 R31//( R32// + R33// ) (7) The stable value of Z eq3 is Zeq3 = R31//( R32 + R33) = R31 ( R32 + R33)/( R31 + R32 + R33) (8) Insulation equivalent circuit for the total lighting circuit. With the analysis above, we can acquire the lighting circuit shown in Fig.1, which is isolated from earth, the equivalent insulation circuit between the secondary side of transformer T and the connector J 2 of the first group of lamps is shown in Fig. 5, which can be termed as the equivalent insulation resistance to earth of single group lighting circuit.the equivalent insulation impedance of single lighting circuit can be known from Fig.5, Z = Zeq 1// Zeq2// Zeq3// Z (9) eq4 The capacitor is equivalent to open loop under the steady state, so the total impedance is: Z = R // R // R // R = R //0.5 R // R (10). eq Considered that the spaces between the N groups of lamps are equal, the insulation impedance to earth of all lighting circuits are equal to parallel connection insulation impedance to earth of single lighting circuit, that is lighting circuit insulation impedance to earth is equal to the parallel connection insulation impedance for N single lighting circuits, that is: ZeqN = Zeq/ N (11). Generally, the insulation impedance of test is the steady-state value and there is only resistance in the equivalent insulation impedance model. Connectors and transformers are placed in the transformer box, so their insulation can be equivalent to a resistance, which is collectively referred to as isolation transformer resistance R t, the equivalent resistance circuit is simplified shown in Fig. 6: Cable connected with the isolation transformer Joint section Isolation and its accessory Joint section Req1 Req2 J1 Req3 Req2 J2 R12 C12 C11 R11 R21 C21 R22 R23 C22 C23 R31 R33 R32 C33 C32 R21 C21 R22 R23 C22 C23 Rc1 Rt1 Rc2 Rt2 RcN RtN Fig.5 Equivalent resistance to earth of single lighting circuit Fig. 6 Simplified equivalent circuit The equivalent insulation resistance of the entire lamp circuit is shown in Fig. 6, the result of N connected lights in parallel, in which the single lamp insulation resistance is the parallel resistance between the cable insulation resistance R c and the isolation transformer insulation resistance R t.

4 Advanced Materials Research Vol Analysis on the effect factor model of lighting circuit insulation resistance Airfield lighting circuits are buried in the ground, the circuit insulation resistance is affected by the temperature, humidity, quality of connectors, cable length, transformer status and others, it is nonlinear and high coupling, there are some difficulties in accurately reflecting the real situation of circuit resistance. The influence of temperature on the circuit insulation resistance. Circuit insulation resistance varies with the temperature changing; the changing degree is different in insulation types [11]. In general, the insulation resistance of the insulating medium is decreased with the temperature increase. One reason is that the ionic number will increase with the temperature increasing, especially when the temperature increase rate is great, the number of medium ions are index doubled, while the ion mobility also increases which makes ion motion increases, so that the insulation resistance decreases; Secondly, the media surface conductance is inversely proportional to the distance of two electrodes, when the temperature rises, the volume conductivity is more than the surface conductivity of the medium, which is enhancing the performance of the conductivity of the insulating medium, so the insulation resistance value declines exponentially with the temperature increasing. Influence of humidity on the circuit insulation resistance. The influence of humidity on the insulation resistance is very great, when the insulating material is wet, its volume resistivity and surface resistance should be reduced significantly, the dielectric loss will be increased and the compressive strength is also reduced. The degree of damp for insulation materials is related to the ambient air relative humidity and temperature, the relative humidity is greater and the temperature is lower, the humidity of the material itself is also greater, and then the insulation resistance is greatly reduced, insulator s resistance and its hygroscopicity are closely related, as the humidity increases, a layer of water film on the insulator surface is come into being. If the easily ionized substance is dissolved in the water film, because the water film has a high conductive, the insulation resistance is reduced significantly; insulation resistance is very sensitive to the change of humidity. In the case of high humidity, moisture in the air or dirty layer is dissolved by the moist and soluble conductive material, the conductance of the contamination layer on the insulator surface increases greatly; the insoluble absorbent material retains moisture and promotes a filthy layer conductance increasing. Under applied voltage, the surface leakage current of insulator has also come to increase substantially, then the insulating properties of materials are reduced significantly, and the resistance is lowered [12]. Analysis on influencing factors by parameter identification model. The impact of temperature and humidity on the circuit insulation resistance is mainly analyzed in front. For the impact of cable length, the longer the length is, the smaller the insulation resistance will be, the better the quality of cable connector is, the greater the insulation resistance will be, the better the insulating material is, the greater the resistance will be. For humidity k 1, materials k 2, joint quality k 3, cable length k 4 and other factors, the identification model of the insulation resistance is: R = K( Rt N) (12). The coefficient K is a function of the impact factor coefficientk = ak1 + bk2 + ck3 + dk4. According to the measured data (Table 1), the measured data is the insulating value of different temperatures converted to room temperature value under 22. The parameters a, b, c, d values are estimated by using the method of system parameter identification. Table 1 The measured data humidity k 1 materials k 2 joint quality k 3 cable length k 4 coefficient K

5 504 Materials Science and Engineering II Insulation resistance parameter identification model by using the least squares method is: R = ( k k k k )( R N) (13) Lighting circuit insulation resistance measurement and model verification The static model of the lighting circuit insulation resistance is analyzed above, in order to verify the correctness of the mathematical model, the third runway lighting circuit is analyzed in the Shanghai PUDONG International Airport. Analysis on the lighting circuit insulation resistance measured in short-term. Glide Road edge lights circuit is tested in one day, the loop cable is through the pipe lying, the lights circuit has 185 lamps and 185 isolation transformers, which is 100W for each transformer. The number of the cable connector is 370. The insulation resistance is above 20000MΩ, the primary insulation to earth of isolation transformer is for more than 2500 MΩ, the cable insulation resistance to earth is 2000 MΩ or more. When the circuit is tested, the loop is disconnected at both ends, and the measurement is done from one end, the number of the isolation transformer is gradual increased and followed by measuring, surface temperature is 37 C, surface humidity in the morning is 50% and 35% at noon, the average humidity is 42%. The data curve is shown in Fig. 7. From the Fig.7 because of the joint production process, the cabling and the insulation of isolation transformer tank, there are different results between the measured insulation resistance and the ideal calculated value in actual measurement, but the whole circuit insulation resistance value ultimately measured is reduced with the number of lamps increasing. There are 185 lights in taxiway edge. From the measured data, the insulation resistance of the first ten lights is 1400MΩ, one side of the insulation resistance approximate linear decline for the first 80 lamps with the increase of the lamp number, the whole lamp insulation resistance declines slowly with the increase of the lamp number after 80 fixtures and remains approximately constant, the entire circuit insulation resistance drops to 95MΩ for 185 lights. Analysis on the lighting circuit insulation resistance measured in Long-running. Insulation resistance value of the taxiway center line lights circuit in the first runway lighting of the Shanghai PUDONG International Airport is analyzed, the fitting curve of the resistance value changing with the time is obtained by analyzing the laws of the resistance value changing over time, shown in Fig. 8: Isolation resistance MΩ Primal data Fitting curve Is olatio n res is ta nc e M Ω t the lamp number Time (day) Fig. 7 Taxiway edge lights (185 lights) Fig.8 Insulation resistance variation over time (day) The actual variation curve of insulation resistance value for taxiway center line lights recorded in the last three years shown in Fig.8. From the January to May (150 days), the value of insulation resistance for the taxiway center line lights is gradually reduced due to the temperature and humidity gradually increase, the insulation resistance value is the minimum from June to August (150 to 250 days), and the temperature and humidity have reached the highest level in the past year; From September to November (250 to 400 days), the temperature decreases slowly and the weather is dry, the humidity drops, and then the taxiway center line resistance value will gradually increase, and the value is the maximum in the first year of winter and the spring of the second year (400 to 450 days).

6 Advanced Materials Research Vol Conclusion Whether the lighting circuit can operate reliably in the long period is of great concern to airport management department. Analysis on the insulation performance of the first side of the lighting circuit is done in this paper, and the static model of the insulation resistance is established. The insulation resistance value decreases as the temperature and humidity increase, the change of circuit insulation resistance over the time approximates to sinusoidal in one year; The parameter identification model of insulation resistance to earth is given according to the analysis of lighting circuit insulation resistance measured in 3 rd runway of 2 nd flight area of Shanghai PUDONG International Airport. The law of circuit insulation resistance changing over time is simulated and fitted by the computer, the simulation results show that the lighting circuit model of insulation resistance to earth is reasonable, and the theoretical foundation is laid for the forecast of insulation resistance decline, a theoretical basis is provided for the maintenance of the lighting circuit. References [1] Nelson Theethayi, Member, IEEE, Vladimir A. Rakov, Fellow, IEEE, and Rajeev Thottappillil, Senior Member, IEEE, Responses of Airport Runway Lighting System to Direct Lightning Strikes: Comparisons of TLM Predictions With Experimental Data. (IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, 2008). [2] Zhou Li, Shen Niyong, Discussion and measurement about insulation resistance of high voltage cable. (FERRO-ALLOYS, China, 2006). [3] Ouyang Ben hong, Kang Yi. Influence of Accelerated Water Tree Aging on Insulation of XLPE Cables, (High Voltage Engineering, China, 2010). [4] Yue Bo, Xie Heng kun, Evaluating the aging condition of main insulation in large generator based BP artificial neutral network with fuzzy output. (Proceedings of the CSEE, China, 2005). [5] Zhao Qiang, Liang Zhi yong, Analysis of Test on Insulation of High-voltage on Site [J].Power System Technology. 2006(8): p [6] A. J. Alsammarae A. K. Behera S. Akhtar, An approach to calculate insulation resistance at different temperatures and voltages, (IEEE TRANSACTIONS ON NUCLEAR SCIENCE, APRIL 1990). [7] C Dang and M Chaabm, Thermal performance modeling of power cable joints, (Power Cables and Accessories, 1993). [8] Tang Zheng sen, Li Jin lu, Li Zhi juan, Analysis of Test on Insulation of High- voltage on Site, (Insulating Materials, China, 2009). [9] Liu Ze hong, Guo Xian Shan, Insulation Structure of UHV Power Transformer. (High Voltage Engineering, China, 2010). [10] Leng Yong, The Analysis of Insulation Resistance and Absorption Ratio of Transformer. (High Voltage Engineering, China, 2002). [11] Wang Xu hui, Sun Ya ling, Liu Yang, and Zhang Dong lin. Study on Temperature Characteristics of Insulation Resistance of Stator Winding of High-Voltage Motor. (EXPLOSION-PROOF ELECTR ICMACH INE, China, 2006). [12] Xu Ping, Insulating Resistance Measurement and Main Factors Influencing on the Insulating Resistance. (Coal Mining Technology. China, 2000).

7 Materials Science and Engineering II / Analysis on Insulation Resistance Performance in Airport Lighting Circuit /