Xiao-xing ZHANG, Yang CHEN, Jun-zhong Tang State Key Laboatoy of Powe Tansmission Equipment & System Secuity and New Technology, Chongqing Univesity A Complex Stuctual Hon Antenna fo Patial Dischage Detection Abstact. This pape pesents a small complex stuctual Hon antenna senso on which cetain effective stuctues such as dielectic filled hon, wedge-shaped substate, and backing cavity ae utilized. Model calculation and expeimental data show that the antenna possesses a good adiating pefomance and multi-band popety in UHF band. Steszczenie Opacowanie pzedstawia kompletny stuktualny, małych ozmiaów czujnik antenowy hon. Wykozystano w nim pewne efektywne stuktuy takie jak: wypełnienie dielektyczne hona, podłoże klinowe i wnękę odbiciową. Obliczenia modelu i dane doświadczalne wykazują, że jest to wielopasmowa antena w zakesie UHF, z dobymi własnościami pomieniowania. Kompleksowa stuktualna antena Hona do detekcji wyładowań niezupełnych Keywods: dielectic filled, micostip antenna, TEM hon antenna. Słowa kluczowe: wypełnienie dielektyczne, technika mikopaskowa, antena Hona TEM (popzecznej fali magnetycznej) Intoduction Gas insulation substation (GIS) has been inceasingly applied in powe systems because of its small size, high eliability, and othe advantages. Howeve, GIS faults may cause accident enlagement, which can lead to a huge economic loss. Moeove, GIS equies a long maintenance time, theeby affecting a lage aea of powe supply and causing seious consequences. Investigations fom the Confeence Intenational des Gands Reseaux Electiques indicate that vaious insulation poblems ae the main easons fo GIS failue [1]. Infomation on insulation states can be chaacteized by patial dischage (PD) signals. The ultahigh fequency (UHF) method tests electomagnetic (EM) wave signals (300 MHz to 3000 MHz) excited by PD in space, to avoid intefeence signals fom both electical system and outside. Testing UHF signals cannot only detect the pesence of GIS defects, but also estimate the insulation faults and its seveity. Sensos ae of two types. Inne sensos possess high sensitivity and stong anti-intefeence ability [2], but ae impossible to install in a GIS aleady in opeation. In compaison, oute sensos o antennas have little influence on the GIS intenal electic field and can be easily installed, as such they ae moe conducive to on-site pactice. Commonly used extenal UHF antennas fo PD detection have a low cente fequency that cannot simultaneously satisfy both bandwidth and gain equiements. This chaacteistic is a disadvantage when eceiving high-ode mode waves, and thus PD cannot be entiely detected. Fo this eason, the pesent pape developed a wide-band, small volume, and cavity-backed TEM hon antenna aimed at UHF detection fo GIS PD. The micowave simulation and tests using GIS defect imitation shows that the antenna possesses good high fequency pefomance and suitability fo online monitoing and defect identification of GIS. The Popagation and leakage of EM waves in GIS A PD occuence in GIS geneates a steep cuent pulse which, despite a ise-time of only a few nanoseconds, can excite EM waves with seveal GHz of fequency. The UHF technique measues these EM waves fo the ealy detection of equipment insulation faults. Leakage occus when the EM wave tavels acoss the waveguide gaps, which ae egaded as slot antennas (Figue 1), and its adiation chaacteistic is detemined by the gap stuctue. The adiation lobe can be distinguished clealy in this figue. As a tansmitting antenna, the slot antenna obtains enegy fom the waveguide, which, in tun adiates enegy back into space. This antenna is adopted with the outlay senso installed at the GIS gaps to intecept pat of the leaky adiation. Fig.1. Leakage pogess of EM waves excited by the PD in GIS The UHF signals which caused by PD tavel though the waveguide as TEM, TE, and TM waves inside the GIS. Nondispesive TEM wave can spead in GIS at any diffeent fequency. But TE and TM waves have cut-off fequencies. The wave tavels though the waveguide with size a<<b. Among all TE and TM waves, TE 11 has the lowest cut-off fequency as follows: (1) c f c TE11 ( a b) whee: f c cut-off fequency, c speed of light, a,b the adius of the conducto and waveguide cavity, espectively. Consideing 500 kv GIS as an example, a=0.089 m and b=0.254 m, thus f c (TE 11 )=278 MHz. In othe wods, longitudinal wave popagation will occu only when the PD signal fequency is highe than 278 MHz. Fo those devices of lowe voltage class, cut-off fequency could be even highe coespondingly. Only TEM wave popagation exists in the aea below the cut-off fequency of the TE 11 wave. Thee is no cut-off fequency because f c (TEM)=0, c(tem) =, and any fequency can satisfy the popagation equiement. The fequency esponse cuve has some wave ipples because PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1b/2013 51
new waves tavel at highe fequencies and when above cut-off fequencies of each mode it ascends quickly and peiodically [3]. In pactice, fequency esonance might cause esonance anges neaby the cut-off fequencies because of the wave eflection o efaction in the chambe. The fequency of PD signals caused by tip defects in GIS can each beyond 1 GHz. The pulse of coona dischage in the ai and its ise time can last fo a elatively long peiod. Geneally, coona dischages appea at fequencies below 150 MHz, with a quick attenuation when its fequency inceases. Thus, PD can be detected by monitoing excited EM waves, especially those of high fequency (300 MHz to 3000 MHz). Thus, intefeences when using conventional electical testing methods can be avoided, such as coona dischage in electical powe system, to incease the signal-to-noise atio of PD detection. The Optimization of hon antenna The hon antenna can be consideed as an opened waveguide, which has a high degee of diectionality. Fo many yeas, TEM hon antennas have been widely used to adiate and eceive pulse signals as a kind of ulta-wide band high powe antenna, with optimized pefomance though basic stuctue modifications such as changing the shape of plates and adding load impedance. When the basic fom of a TEM hon matches a 50 coaxial cable, the feed point should be located at the head of the paallel waveguide. Howeve, if it is an optimized hon, the new feed point matching with the hon design should be e-found unde these cicumstances. Foeign scholas studied the chaacteistics of the dielectic filled hon as a special fom of loading [4]. The hon section between the paallel waveguide and the hon apetue is constucted with a tapeed shape, appoximating to an exponential ate, to minimize the eflection of the waveguide taveling wave and to decease the antenna standing wave atio. This method can likewise educe sidelobe level. Using a dielectic filled antenna enables the eception of EM waves of shote wavelengths. The EM waves ae tansmitted though the hon when c (cut-off wavelength). Theefoe, the dielectic filled antenna can eceive moe high-ode mode signals, allowing eceived EM signals to maintain good bandwidth and gain despite having a small antenna size. Odinay micostip patch antennas have a naow bandwidth, and the distance between patch and gound plane is fa shote than the opeating wavelength, so EM popagation can be appoximately consideed as bound in this space. Inceasing the distance to half wavelength o longe, the EM waves will no longe be confined to the dielectic mateial, but will adiate o eceive thei enegy fom the patch edge, which foms a quasi-tem antenna [5]. Micostip antennas can be equivalent to the paallel esonance cicuit with high quality facto, so pope bandwidth can be obtained by vitually edesigning the antenna esonance. This can be achieved by inceasing the thickness of the dielectic mateial o using a specialshaped dielectic substate [6,7]. Stepped stuctue and wedge shape ae usually adopted to ceate special-shaped substates, inceasing the substate thickness to expand the bandwidth. Moeove, to feed at a thinne edge, this shape can shoten the pobe length, educing the effect of inductive eactance on antenna efficiency. Figue 2 shows the wedge-shaped dielectic stuctue. The cente fequency of the wedge-shaped dielectic micostip antenna is: (2) c fc 2L cos 1 h1 h2 sin whee: L, an exta facto elated to the electic field nea the edge. In the cuent pape, the antenna design follows a multistage micostip stuctue aiming to boaden the band. Each patch section has a diffeent slope coefficient. The antenna positioned in the cavity foms the back cavity stuctue, and pefoms bette in tems of bandwidth and adiation efficiency compaed with antennas without back cavity. The depth of back cavity loaded antenna is about one quate wavelength of the cente fequency wave. Mateials absobing EM waves fill the cavity, which can lead to loss of adiation powe, gain eduction, and inceased antenna weight. Chaacteistics of TEM hon antenna with composite stuctues In this pape, the antenna has a semi-closed integal stuctue. Apat fom the diection to which the hon apetue points, the antenna is coveed with a metal shell in all othe diections, filling dielectic in the back cavity, and fed by a coaxial line. Loaded dielectic in the hon has highe dielectic coefficient than that in the micostip stuctue. Thus, the whole antenna can be seen as a dielectic loaded TEM hon antenna with back cavity. ' h 2 L dielectic Fig.2. Wedge-shaped dielectic micostip stuctue h 1 Fig.3. Configuation of the antenna The design diffes fom odinay ai micostip antenna owing to the metal shell. Half the TEM hon section is loaded with a dielectic mateial with about 6 7 pemittivity and the thee-dimensional (3-D) section of the micostip stuctue is loaded by a dielectic mateial with pemittivity ' = 2. With the composite stuctue combining stepped stuctue with wedge shape, the antenna will have a multiband popety. The 3-D micostip and cavity shell foms a quasi-tem hon, which can still maintain good adiation chaacteistics in high fequency; thus, the antenna can wok in a wide band. Simulation esults show that the antenna cente fequency is about 930 MHz. This esult is elated to the idealized and appoximate pocess used to deal with the simulation model and the vaiation of antenna flae angle. Simulation esults show that the antenna cente fequency is about 930 MHz. This esult is elated to the 52 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1b/2013
idealized and appoximate pocess used to deal with the simulation model and the vaiation of antenna flae angle. equiements fo eceiving EM signals can still be met with a 3.97 db gain. As fequency inceases, the antenna diectionality gadually wosens and the backwad lobe distots. In paticula, when fequency eaches 3 GHz, the gain will decease and the main lobe will divide into two pats. This effect may be caused by the ability of the antenna to eceive and adiate highe wave modes at this fequency. Table 1 shows the measued gain. Fig.4. VSWR chaacteistic with a fequency 0.5 1 GHz Fig.5. VSWR chaacteistic with a fequency 1 3 GHz The antenna status appoaches full-wave esonance when neaing 930 MHz, which means eactance is close to zeo and the impedance values fo pue esistance at the feeding point because voltage and point cuent ae in the same phase. Figue 4 shows that the standing-wave atio is no moe than 2:1 in the band between 880 and 980 MHz, whee additional loss can be neglected and the antenna can be consideed as completely matched. Figue 5 clealy shows that a 42 esonance exists. The antenna also shows acceptable standing-wave atio pefomance in the ange neaing 1.8 GHz. Fig.7. Display of 3-D fa-field ealized gain patten Fig.6. Measued VSWR chaacteistic of the antenna In many cases, the value of antenna impedance is unknown, but the measued voltage standing-wave atio (VSWR) can seve as a substitute. Thus, the module value of eflection coefficient can be calculated fom VSWR. Figue 6 shows the measued standing-wave atio in the ange of 45 MHz to 3000 MHz. The antenna has pefect standing-wave atio within 780 900 MHz, 1.25 1.65 GHz, and 2.15 2.3 GHz, but impefect atio below 600 MHz. Geneally, the less eflective bands and standing-wave atio cuve tendency ae consistent with simulation esults. Antenna gain and patten Simulation of adiation pattens fom 1 GHz to 3 GHz show that, along with inceasing fequency, fa-field adiation pefomance is enhanced and ealized gain can then damatically incease. If antenna adiation fequency ises fom 1 GHz to 2.5 GHz, the ealized gain would incease fom 0.316 db to 5.07 db. Although the gain slightly declines when fequency inceases to 3 GHz, Table 1. Testing ecod of gain 500 MHz 800 MHz 1 GHz -12.6dB -1.4dB 1.3 db Figue 8 shows measued gain patten using the logaithmic coodinate system. The adii of the concentic cicles epesent the signal voltage logaithm. This coodinate inceases the significant level of the side-lobe, eflecting the omnidiectional pattens of the antenna. In the gain patten, half powe (-3 db) points can be seen at about 45ºand 135º, which means the half powe beam width is 90º. Diectional diffeence in all degees is less than 15 db. Within 1 GHz fequency, diectionality will gadually enhance along with inceasing fequency. The measuements of the antenna In this expeiment, the hon antenna was used to detect atificial GIS defects in the device filled with 0.3 MPa SF 6 gas. Signal wavefoms wee ecoded with a high-speed digital oscilloscope. Futhemoe, the cuent sensing signal eceived by Rogowski coil was compaed fo efeence. PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1b/2013 53
with 31 mv amplitude descibes the signal measued by the small hon antenna. 0.03 0.02 Amplitude/ V 0.01 0.00-0.01-0.02-0.03 0 1000 2000 3000 4000 5000 0.004 0.003 sampling point a) By Rogowski coil 0.002 Amplitude/ V 0.001 0.000-0.001-0.002 0 1000 2000 3000 4000 5000 sampling point b) By the hon antenna Fig.9. Expeiment detected PD signals Voltage 20 mv/div Fig.8. Antenna gain patten Needle-plate dischage was used to simulate PD defects. At dischage inception voltage, digitizing voltage signals on the feede wee collected with 5 GS/s sample ate, using both Rogowski coil and hon antenna fo synchonous detection. The small hon antenna obtained signals with lowe amplitude, but could detect distinct PD signals, satisfying the need fo PD detection with high sensitivity. The human body effect may advesely affect the sensitivity of the small antenna. When the cuent diection on the antenna is paallel to a human body, which can be consideed as a semi-infinite plane eflecto of a cetain coefficient, the mio effect geneates a cuent towad the opposite diection, educing the electic field and inceasing the magnetic field. When the small antenna was placed on the gound linked metal o on the suface of GIS cavity, the antenna E-plane should also be pependicula to the metal suface to obtain the maximum signal amplitude. A 5 kv ac high voltage was applied to the defect imitation, simultaneously using both micostip patch antenna (390 MHz cente fequency, 25.6% elative bandwidth, 5.38 db highest ealized gain) and dielectic filled hon antenna to eceive EM signals adiated fom PD. Figue 10 shows that the fist wavefom cuve with 22 mv maximum value epesents the signal measued by micostip antenna, wheeas the second wavefom cuve 电 Time 200 ns/div Fig.10. Measued wavefoms of UHF signals In contast to the micostip antenna, the hon antenna has a smalle volume and lowe gain, but a wide bandwidth. The poposed antenna can eceive UHF signals enegy fom a wide band, eflecting less EM enegy and having highe signal attenuation, yet eceiving UHF signals with highe amplitude. Conclusions Antenna adiation popeties wee detected though theoetical analysis and simulation on a small quasi-tem hon antenna. In contast to the effect of micostip antenna laboatoy measuements, the conclusions can be summaized into the following: a) The use of TEM hon stuctue can boaden the antenna bandwidth and maintain good eception pefomance when in the fequency of 1 GHz and above. The antenna can also eceive intact EM signals of PD. b) Cetain effective stuctues such as dielectic filled hon, wedge-shaped substate, and backing cavity can be utilized duing the antenna design pocess to enable miniatuization as well as optimize standing-wave atio chaacteistics. The latte pesents a fequency chaacteistic of the multiband micostip antenna due to the adoption of multistage wedge-shaped micostip stuctue. 54 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1b/2013
c) Antenna gain tends to incease with antenna fequency within the fequency spectum of UHF band. Antenna diectivity is nealy omnidiectional, and antenna gain meets the equiement of a small eceiving antenna. Moeove, compaed with nomal micostip patch antennas, the esponse wave amplitude is highe when eceiving PD signals. REFERENCES [1] Gilles B az a nney, Recent Developments in Insulation Monitoing Systems of GISs, Intenational Electic Powe Fo China, 6 (2002), No. 4, 41-43 [2] Dong-suk Kim, Chul-min Hwang, Young-noh Kim, Development of an intelligent space built into the intenal-type UHF patial dischage senso, 2008 IEEE Intenational Symposium on Electical Insulation. Vancouve, Canada: ISEI 2008, 396-399 [3] M.D.Judd, O.Faish, B.F.Hampton, Modeling Patial Dischage Excitation of UHF Signals in Wavepde Stuctues using Geen s Functions, IEE Poceedings, Science, Measuement and Technology, 143 (1996), No. 1, 63-70 [4] Y.Huang, M.Nakhkash and J.T.Zhang, A Dielectic Mateial Loaded TEM Hon Antenna, Twelfth Intenational Confeence on Antennas and Popagation. Exete, UK: IEEE, 2 (2003), 489-492 [5] Feng-Wei Yao, Shun-Shi Zhong, Xian-Ling Liang, Ulta-boadband patch antenna using a wedge- shaped ai substate, Micowave Confeence Poceedings, 2005. Asia- Pacific Confeence Poceedings. Suzhou, China: APMC, 4 (2005), 2698-2700 [6] Young-Min Jo, Boad band patch antennas using a wedgeshaped ai dielectic substate, Antennas and Popagation Society Intenational Symposium, 2 (1999), 932-935 [7] Debatosh Guha, Sudipta Chattopadhyay, Jawad Y. S i d d i qui, Estimation of Gain Enhancement Replacing PTFE by Ai Substate in a Micostip Patch Antenna, IEEE Antennas and Popagation Magazine, 52 (2010), No. 3, 92-95 Authos: pof. d Xiao-xing ZHANG, State Key Laboatoy of Powe Tansmission Equipment & System Secuity and New Technology, Chongqing Univesity, Chongqing 400044, China, E- mail: zhxx@cqu.edu.cn; Yang CHEN, Guangdong Electic Powe Design Institute, Guangzhou 510663, Guangdong Povince, China, E-mail: shenyangcdqz@163.com. PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1b/2013 55