Dielectric Strength Restoration Velocities of Multi-Break Vacuum Circuit Breakers

Transcription

1 Engineering an Technology 2015; 2(4): Publishe online June 20, 2015 ( Dielectric Strength Restoration Velocities of Multi-Break Vacuum Circuit Breakers E. Saafan Electrical Engineering Department, University of El-Mansoura, Mansoura, Egypt aress Keywors Vacuum Circuit Breaker, Multi-Break Vacuum Interrupters, Sf 6 Circuit Breaker, Dielectric Strength, Velocity of Dielectric Strength, Transient Recovery Voltage, Breaking Capacity Receive: May 28, 2015 Revise: June 4, 2015 Accepte: June 5, 2015 Citation E. Saafan. Dielectric Strength Restoration Velocities of Multi-Break Vacuum Circuit Breakers. Engineering an Technology. Vol. 2, No. 4, 2015, pp Abstract A multi-break interrupters of vacuum circuit breakers has become the most requirements of high voltage circuit breakers with high breaking capacity that not environmentally harmful. In this article the ielectric properties of multi-break vacuum circuit breakers were consiere. Velocities of ielectric strength restoration of multi-break vacuum interrupters were analyze. 1. Introuction A circuit breaker is an electrical switch esigne not only to interrupt the high magnitue short-circuit currents; switching of ifferent loas, capacitor banks, reactors an unloae cables or transmission lines are some of the uties whose requirements are not only ifferent but might even conflict with those of the high current interruption [1]. During switching-off processes the movable contacts of circuit breakers begin to separate an the arc is prouce. This arc nees to be extinguishe, in orer to retain the breaking capacity of the circuit breakers. SF 6 circuit breakers stretch the arc an use the ielectric strength of the sulfur hexafluorie (SF 6 ) to quench the stretche arc. In vacuum circuit breakers (VCB), the electrical contacts are enclose in a vacuum out of which one is fixe an the other is movable. During switching-off processes, the movable contacts pulls away from the fixe contacts an minimal arcing is prouce [1, 2]. Accoring to the ielectric strength, SF 6 has better behavior than vacuum. That is why SF 6 has generalize as insulating an as arc quenching meium. Uner normal conitions, SF 6 is an inert, non-flammable, non-corrosive, oorless, an non-toxic gas. However, at temperatures over 1000 o C, SF 6 ecomposes to gases incluing S 2 F 10 which is highly toxic. Fortunately, the ecomposition proucts recombine abruptly after arc extinction (when the temperature goes own) [2, 3]. SF 6 has been labele as one of the major global warming gases, since the 3 r Session of the Conference of the Parties to the Unite Nations Framework Convention on Climate Change [4, 5]. Hence, there is an urgent nee to stuy new generation of high voltage circuit breakers that not environmentally harmful. Note in this view that, there are many efforts being mae to fin an alternative gas to SF 6 an to minimize the utilization an leakage of SF 6 in circuit breakers. The attempts to fin an alternative gas to SF 6 have not yet borne fruit. It means that vacuum an SF 6 circuit breakers are staying the preferable ones [5, 6]. As it is known in VCBs the quenching meia is a vacuum, so there is no risk for the environment. Another avantage of VCBs is the higher ielectric strength restoration after current zero in comparison with other types of circuit breakers. The problem of using VCB in high voltage applications is relate to the high voltage capability of a single gap between electroes. The breakown characteristics have very high epenence of electroe area, an the ielectric strength with the contact gap. On this way, in vacuum

2 216 E. Saafan: Dielectric Strength Restoration Velocities of Multi-Break Vacuum Circuit Breakers breakown, the breakown voltage is proportional to the square root of the gap length. Thus, a longer gap is necessary for the vacuum interruption. But this technology makes the circuit breaker bigger an leas to the problem of arc control. It appears a high arc voltage noise, which inicates that the vacuum arc is unstable in a long gap [2, 4]. The stuy of circuit breakers base on the vacuum interrupters in series began in the 1960s. During the past perio the patent applications were mae by several manufacturers in the USA an Japan but none was applie in the inustry because of technical conitions. Note that, series connection of vacuum interrupters has also a few isavantages like complex mechanism an non-uniform voltage istribution across each interrupter ue to the stray capacitance. To keep the uniform voltage istribution, graing capacitors woul require to be connecte across each break [5, 7, an 8]. With the evelopment of large-capacity VCBs, a new roun of research is proceeing in the 21 st century [4, 5, an 7]. The main purpose of this research is to compare between multi an single breaks of 110 kv VCBs from the point of view ielectric properties an velocities of ielectric strength restoration. 2. Vacuum Circuit Breaker Dielectric Strength Restoration It is known that when the circuit breaker contacts starts to separate from each other the ielectric strength of the gap starts to increase. After the arc has been extinguishe, the race between the transient recovery voltage (TRV) an the ielectric withstan of the circuit breaker begins. When the TRV excees the ielectric withstan of the circuit breaker a breakown occurs an creates a conucting path between the two circuit breaker contacts. Then the TRV jumps back to zero an oes not start to rise again before the arc is extinguishe [3, 9, an 10]. Therefore the ielectric withstan of the circuit breaker is a significant parameter for the switching analysis. For VCBs the most of authors use linear restoration law [11, 12]. But this law is not quite suitable to the physical nature (ecreasing of strength at increasing of inter-contact istances) of vacuum inter-contact gaps [13, 14]. Therefore for moeling the ielectric properties of the inter-contact spaces we use the logarithmic law of ielectric strength restoration in VCBs presente in [15]. This law takes into account both inertia of contact an inconstancy of strength of vacuum gaps. The logarithmic restoration law for ielectric strength of 110 kv VCB with single-break is given by the following empirical formula; =191.43log cos! %&', *++ < * /00 (1) where: V str (t) is the acceptable law of circuit breaker s ielectric strength restoration; X m is the maximum istance between contacts of single-break VCB; t is time; T full is the full switch-off time of circuit breaker; t off is the initial instant of contact separation [13, 15]. In orer to reach the high ielectric strength, ouble-break or more connecte in series of VCBs are use. In case of consiering ouble-break of VCBs, the gap length of each break assume equal to X m /2. Then the formula of ielectric strength restoration is given by computing the ielectric strength of each break multiplie by the number of breaks as following; 1*/203 = log cos! %&' (2) Theoretically to get the same ielectric strength obtaine from formula (2) while using of VCB with single-break, the contact istance suppose have twice the value of X m. So the formula of ielectric strength restoration of single-break VCB have twice the gap length is given by, 89:03 =191.43log ; 1 cos! %&' (3) In orer to get the ielectric strength gain, a comparison between ouble an single breaks ielectric strength can be obtaine as following. By iviing formulas (2) an (3) the ielectric strength gain of ouble-break VCB (K 2 ) is given =CDE$B = F 7 = P GHIJKLM.NM O 6 P QKRHSTUV>W> X GHIJKLM.NM75 6 QKRHST UV>W> X (4) In general form the formulas (2) an (4) for VCB with multi breaks connecte in series can be written as following,

3 Engineering an Technology 2015; 2(4): /08 =191.43]log cos! %&' (5) 6#$>C =CDE$B = F 9 = D GHIJKLM.NM O 6 D QKRHSTUV>W> X GHIJKLM.NM95 6 QKRHST UV>W> X (6) where: n is the number of breaks connecte in series of VCB; K n is the ielectric strength gain of the n breaks of VCB. From formula (5), it is clear that the maximum value of the ielectric strength were got at full time of contact separation (t-t off = T full ). So for 110 kv VCB (T full = 40 ms, X m = 4cm), the ielectric strength gains of ouble an triple breaks are 1.402, an respectively. These results have a great agreement with that obtaine in [16, 17]. From the analysis above, ouble or triple breaks have higher ielectric strength characteristic than single break for 110 kv VCB. 800 ielectric strength restoration of 110 kv VCB with single, ouble, an triple breaks are presente in figure (1). From the curves shown in figure (1), it is clear that the triple or ouble breaks connecte in series have higher breakown voltage than single break of VCBs with the same equivalent spacing. At the same time, the triple or ouble breaks of VCBs are not breakown simultaneously at a moment. Thus, the multi breaks of VCBs are more reliable in high voltage applications than single break that have larger gap istance. 3. Velocities of Dielectric Strength Restoration Breakown voltage [kv] single break ouble break triple break Time [ms] Fig 1. Dielectric strength restoration of 110 kv VCB with single, ouble, an triple breaks. Since the single break of VCBs with longer gap istances have a few isavantages that mentione previously, a comparison from the point of view ielectric strength restoration characteristics between multi-break an single-break with the same equivalent spacing were performe below. In other wors for multi-break it is assume that, the gap length of a single-break (X m ) is ivie into n parts an the length of every gap is X m /n. The curves of In moern high voltage vacuum interrupters it s very important to have ability to withstan the steep rising part of the TRV, in orer to minimize the probability of repeate re-ignitions. It means that the vacuum interrupters shoul have a high rate of velocity of ielectric strength restoration more than the TRV. This unerlines the importance of ielectric strength restoration velocities of the circuit breakers. So in this section a comparison between triple, ouble, an single breaks of VCBs from the point of view ielectric strength restoration velocities were performe. Since the ielectric strength restoration formula of VCB has a non-linear character, the mathematical ifferentiation is the easy way can be use to get the expressions of velocity an acceleration. In other wors ifferentiation of ielectric strength an velocity with respect to time allows etermination of the rate of change of ielectric strength (velocity), an the rate of change of velocity (acceleration) respectively. For multi-break VCB, the function of ielectric strength restoration velocity can be obtaine by ifferentiating the formula (5) with respect to time as following; /08 = /08 =191.43] _ logj1+5.75`; _ ] a1 cosbc *++! '\. +/00 =83.136] g]j1+5.75`; ] a1 hijbc *++!. +/00 '\ Then use a rule on ifferentiation the logarithm a function of function such as 1 1 ln [m]=+o + an get,

4 218 E. Saafan: Dielectric Strength Restoration Velocities of Multi-Break Vacuum Circuit Breakers /08 = J 89b U>W> KLM.NM O 6 D bkq*u>w> \ (7) To get the maximum value of ielectric strength restoration velocity, it nees to get at first the function of acceleration an then fining the time instant of maximum velocity which occurs at zero erivatives. /08 = r c; s _ w u jx]b c *++! {. +/00 u. +/00 _v u1+5.75v ; Xb1 hij c *++! t ]. u z = }~i +/00 y At this instant of time the numerator of the erivative obtaine equal to zero, then hij An corresponingly, jx] = M.NM5 6 9LM.NM5 6 (8) = KLKK.MO 6 D KLM.NM O 6 D (9) Directly put (8) an (9) into the formula (7) an obtain expression for maximum velocity such as, /08, = KLKK.M O 6 D (10) From the formula (8) fin now the time instant correspone to the maximum value of velocity. c * ; = ~hhij. +/00 ]+5.75; ƒ6 = *++ =V X ~hhij M.NM5 6 9LM.NM5 6 (11) The curves of corresponing velocity functions are presente in the figure (2). This figure shows the velocity curves of triple, ouble, an single breaks of 110 kv VCB with the same equivalent spacing. Fig 2. Dielectric strength restoration velocities of triple, ouble, an single breaks of 110 kv VCB. As shown in figure (2), the character of triple or ouble breaks has a high rate of ielectric strength restoration velocity more than single break of VCB with the same equivalent spacing. It means that, the multi-break can withstan the steep rising part of the TRV efficiently more than single-break of VCBs an consequently have higher breaking capability uring switching-off processes. The velocities an corresponing time instants for 110 kv VCB with single, ouble, an triple breaks shown in table (1). Table 1. Velocities (maximum, average) an corresponing time instants for 110 kv VCB. number of breaks, n maximum velocity [kv/ms] average velocity [kv/ms] instant of maximum velocity [ms] Dielectric strength restoration velocity [kv/ms] single break ouble break triple break Time [ms] From the results shown in table (1) it s evient that, triple breaks of 110 kv VCB have the higher values of velocities. Note that, the average velocities have little values ue to the non-uniformity of ielectric strength in inter-contact gap. It means that the ielectric strength of VCB is conitione by taking into account ecreasing of ielectric strength of vacuum gap with increasing of its length. 4. Conclusion Multi-break of vacuum circuit breakers has a high rate of ielectric strength restoration velocity. It means that it has an excellent ability to eal with the steep rising part of the transient recovery voltage more than single break, which makes it faster in the current interruption processes. The success an evelopment of multi-break vacuum technology,

5 Engineering an Technology 2015; 2(4): give confience to meet the ielectric requirements of vacuum circuit breakers to use it with high voltage applications. References [1] Paul G. Slae, Vacuum interrupter: Theory, Design an Application, CRC-Press, [2] A. Iturregi, E. Torres, I. Zamora, ano. Abarrategui, High voltage circuit-breakers: SF6 vs. vacuum, International Conference on Renewable Energies an Power Quality, Valencia (Spain), [3] T. Lazimov, E. Saafan, Transitional processes at capacitive currents switching-off by single an ouble-break interrupters of vacuum circuit breakers, Present Problems of Power System Control, Wroclaw, Polan, No. 5, PP , [4] C. Xian, D. Xiongying, anl. Minfu, The Voltage istribution characteristics of a hybri circuit breaker uring high current interruption, Plasma Science an Technology, vol. 15, No.8, PP , [5] S. Kulkarni, A. Kahane, an U. Sanvatsarkar, Prospects of application of vacuum switchgear at transmission voltages, proceeing of Gritech th International Exhibition & Conference, New Delhi, Inia, [6] Berlin, Germany, CIGRE SC B3 Colloquium, Tutorial, Panel an Annual Meeting, Electra, No. 239, [7] M. Homma, M. Sakaki, E. Kaneko, ans. Yanabu, History of vacuum circuit breakers an recent evelopments in Japan, IEEE transactions on ielectrics an electrical insulation, vol. 13, No. 1, PP.85-92, [8] E. Schae, E. Dullni, Influencing factors of breaking capacity of ouble-break vacuum circuit breakers, Telkomnika, Vol. 11, No. 6, PP , [9] E. Schae, E. Dullni, Recovery of breakown strength of a vacuum interrupter after extinction of high currents, IEEE Transactions on Dielectrics an Electrical Insulation,Vol. 9, No. 2, PP , [10] S. Giere, H. C. KÄarner, anh. Knobloch, Dielectric strength of ouble an single-break vacuum interrupters, IEEE Transactions on Dielectrics an Electrical Insulation Avances in Dielectric Applications, Vol. 8, No. 1, PP , [11] G. A. Yevokunin, A. A. Korepanov, Over-voltages an their limitation at vacuum circuit-breakers switching, Electricity, No. 4, PP.3-10, [12] S. M. Wong, L. A. Snier, an E.W.C.Lo., Over-voltages an re-ignition behavior of vacuum circuit-breaker, proceeings of IPST - International Conference on Power Systems Transients, New-Orleans, USA, PP.1-6, [13] [13] T. Lazimov, S. Imanov, an E. Saafan, Transitional recovery voltages at capacitive currents switching-off s by vacuum an SF6 circuit-breakers, Energy an Power Engineering, Chicago, USA, Vol.6, No. 5, [14] M. Bue, A. Horn, F. Körner, M. Kurrat, ank. Steinke, Dielectric behaviour of vacuum circuit-breakers, IEEE/PES Transaction,Vol. 3, PP , [15] H. Mammaov, T. Lazimov, ans. Imanov, Improve moel of circuit-breaker s ielectric strength restoration, proceeings of ELECO 09-International Conference on Electrical an Electronic Engineering, Bursa, Turkey, [16] Liao Minfu, Duan Xiongying, an Zou Jiyan, Dielectric strength an statistical property of single an triple-break vacuum interrupters in series, IEEE Trans. on Dielectric an Electrical Insulation, Vol. 14, No. 3, PP , [17] Xie Jiuming, Sun Dengyue, Wang Jianzhong, Cao Jianbo, an Liu Wenping, New structure esign for 126kV HV vacuum circuit breaker, International Journal of Control an Automation, Vol. 7, No. 12, PP. 1-10, Biography Esam Saafan was born in El-Mansoura, Egypt in He receive the B.Sc. an M.Sc. egrees in Electrical Engineering from Faulty of Engineering, University of El-Mansoura, Egypt in 2001 an 2007 respectively. He obtaine the Ph.D. egree in High Voltage Engineering in 2012 from Azerbaijan Technical University, Baku. From 2001 to 2012 he worke in the Electrical Engineering Department, University of El-Mansoura, Egypt as a Lecturer Assistant. Since 2012, he has been a Lecturer in the same university. Dr. Esam Saafan research areas inclue transitional processes in power electric systems an their computer simulation, power systems electromagnetic compatibility.