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NAME : Richter, Frank COUNTRY : Germany REGISTRATION NUMBER : CE113578 GROUP REF. : A3 PREF. SUBJECT :PS1 QUESTION N° : Q1-9 HV Vacuum circuit-breakers: Challenges of capacitive load switching In general, capacitive current switching is a crucial requirement for circuit breaker applications. Switching currents with capacitive attribute constitutes a major challenge for high-voltage vacuum circuit breakers with single-break vacuum interrupters. This is especially the case concerning very low restrike behavior, which is mandatory for class C2 classification of IEC-requirements [1]. The difficulty of fulfilling requirements increases from switching unloaded lines and cables to the switching of single capacitor bank to back-to-back capacitor bank applications (see table 1). Load Inrush current Load current Cap. voltage factor Unloaded lines and cables negligible Small up to 1.7 p.u. Single capacitor bank small Small up to 1.4 p.u. Back-to-back capacitor bank high Small up to 1.4 p.u. Table 1 : Overview on different capacitive loads The capacitive current switching of unloaded lines and cables is an application using requiring comparably low currents up to a few hundred amps. The dielectric stress after current zero represents the main challenge for the circuit breaker. This dielectric stress results from the nearly fully charged capacitance on the load side at the time of current zero and the oscillating voltage of the supply side. The circuit breaker must withstand the differential voltage at the terminals. This results in an oscillating direct voltage stress in (1-cos(ωt)) wave shape. The dielectric stress is not a transient load, but starts with zero steepness and reaches the maximal value after 10 ms for a 50 Hz and 8.3 ms for a 60 Hz configuration. I UCB LN ~ UN CN UA UL CL Neglecting : 1 ωL N << ⇒ U A ≈ U N ; C N << C L ωC N Fig. 1. Switching of capacitive loads [2] Single capacitor bank and back-to-back applications present the same dielectric loads with slightly higher breaking currents. For back-to-back applications, the making operations may cause different issues because the circuit breaker contacts close under significant influence of high-frequency inrush currents. This could result in welds between the contact surfaces. These welds break in the ensuing opening operation and can influence the capacitive switching behavior [3]. The welding behavior is a contact material property. The cause for voltage restrikes in vacuum can be manifold and initiated by various circumstances. One reason could be the exceedance of the maximum permissible field strength between the arcing contacts of the vacuum interrupter. This is not only a phenomenon in the phase of contact separation, but also possible with an insufficient final contact stroke (dielectric design of vacuum interrupters). Results indicate delayed voltage breakdowns in vacuum interrupters despite sufficient opening velocity and contact distance. This phenomenon is termed late breakdown and can be observed even after several ten milliseconds periods. The explanation for this behavior could vary. Zadeh [4] measured field emission currents in vacuum interrupters, which occurred during the period, when the (1-cos(ωt)) voltage was applied. Her research revealed high field emission currents shortly just prior to the restrike event. This would seem to imply that field emission currents initiate the breakdown (dielectric design of vacuum interrupters). There is another theory which states that particles, activated by mechanical shocks, are responsible for initiating the restrike phenomenon. Gebel and Hartmann [5] proved a close connection between mechanical shocks in the kinematic chain and late breakdowns. Even if these investigations were carried out with high current arc interruption, it is conceivable that this might also appear during capacitive current switching. Giere et al. [2] investigated the influence of different closing velocities and the capacitive voltage factors on the restrike performance. The summarized result is shown in Figure 2 (kinematic chain). 25 20 15 Relative restrike probability [%] 10 5 2 1,7 0 0 100 Closing velocity [%] 1,4 140 Capacitive voltage factor [p.u.] 200 Figure 2: Results of investigation of the influence of closing velocity and cap. voltage factor on the restrike performance [2] References: [1] [2] [3] [4] [5] IEC 62271-100, 04-2008, "High Voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers," Edition 2.0. Giere et. al: „Capacitive Current Switching Capability of 72.5 kV High-Voltage Vacuum Interrupters”, ISDEIV 2012, Tomsk T. Delachaux, F. Rager, D. Gentsch, "Study of vacuum circuit breaker performance and weld formation for different drive closing speeds for switching capacitive current," XXIVth International Symposium on Discharges and Electrical Insulation in Vacuum, September 2010. M. Koochack Zadeh, V. Hinrichsen, R. Smeets, A. Lawall, "The Impact of Capacitor Bank Inrush Current on Field Emission Current in Vacuum," XXIVth International Symposium on Discharges and Electrical Insulation in Vacuum, September 2010. R. Gebel, W. Hartmann, "Mechanical shocks as cause of late discharges in vacuum circuit breakers," IEEE Transactions on Insulation, Vol. 28, No. 4, 1993, pp.468.