This paper presents the 3-D CFD calculation results of the pressure rise due to the High Energy Arcing Faults (HEAFs) in the metal-clad switchgears. The calculations were performed considering the came-oﬀ of the roof panel that was observed in the arc tests. The calculated pressure development approximately agreed with the measured one. Furthermore, the energy of hot gases exhausted from the broken roof panel was calculated to investigate the thermal eﬀect of hot gases

Goda, Y.

Iwata, M.

Miyagi, T.

Shirai, K.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2017.2.116Plasma Physics and Technology 4(2):116–119, 2017 © Department of Physics, FEE CTU in Prague, 2017CALCULATION OF PRESSURE RISE AND ENERGY OF HOT GASES DUETO HIGH ENERGY ARCING FAULTS IN THE METAL-CLADSWITCHGEARM. Iwataa,∗, T. Miyagia, Y. Godaa, K. Shiraiba Central Research Institute of Electric Power Industry (CRIEPI), Electric Power Engineering ResearchLaboratory, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japanb Central Research Institute of Electric Power Industry (CRIEPI), Nuclear Fuel Cycle Backend Research Center,1646 Abiko, Abiko-shi, Chiba-ken 270-1194, Japan∗ m-iwata@criepi.denken.or.jpAbstract. This paper presents the 3-D CFD calculation results of the pressure rise due to theHigh Energy Arcing Faults (HEAFs) in the metal-clad switchgears. The calculations were performedconsidering the came-off of the roof panel that was observed in the arc tests. The calculated pressuredevelopment approximately agreed with the measured one. Furthermore, the energy of hot gasesexhausted from the broken roof panel was calculated to investigate the thermal effect of hot gases.Keywords: switchgear, internal arc, pressure rise, energy of hot gases.1. IntroductionHigh Energy Arcing Faults (HEAFs) can be causedby the failure within electrical installations due toeither a defect or an exceptional service condition ormaloperation [1]. The HEAFs lead to the temperaturerise and the pressure rise in the installations and thenthe exhausting of hot gases from the installations. Asa result, the successive fire incident owing to the igni-tion of flammable materials may occur inside and/oroutside the installations.In Japan, the Tohoku District-off the Pacific OceanEarthquake attacked the Onagawa Nuclear PowerStation of Tohoku Electric Power Co., Inc. on March11, 2011 and the successive fire incident due to theHEAFs occurred in the electrical cabinet. In order toclarify the pressure rise in the non-arc proof electricalcabinets due to HEAFs and understand the criticalcondition for the successive fire occurrence in thecabinets, a series of 3-phase internal arc tests with6.9 kV switchgears were performed [2]. In the tests,the arc current Iarc was around 20 kA, the arcingduration tarc was 0.1–2 s, and the electric arc energyWarc was 3–45MJ.In this paper, the calculations of the pressure risedue to the HEAF in the switchgear were carried out us-ing a CFD (Computational Fluid Dynamics) method.The calculated pressure rise was compared with themeasured one. The propagation of the hot gases in theswitchgear and the energy of the hot gases exhaustedfrom the switchgear through the broken roof panelwere also investigated.2. Calculation method and conditionsIn the 3-D calculations, a CFD program (CFD-ACE+)was applied to solve the full set of basic hydrodynamicequations (i.e., conservation equations of mass, mo-mentum and energy). Those conservation equationsin terms of partial differential equations were solved ateach location in the switchgear iteratively, while theenergy input, a portion of the arc energy, was set lo-cally. The general form of the conservation equationscan be written as:∂∂t(ρ φ) +∇ · (ρvφ) = ∇ · (Γ∇φ) + S (1)where φ represents the conserved variables under con-sideration, ρ the gas density, v the gas velocity, Γ thetransport coefficient of φ and S the source term for φ[3].Figure 1 shows the schematic drawing of theswitchgear used in the tests. The switchgear hastwo cable compartments (CCs), a bus compartment(BC) and two circuit-breaker compartments (CBCs).The compartments are separated by the metal plates.For the ventilation in the switchgear, the metal platesof the rear wall as well as the metal plates betweenthe CC and BC have some rectangular slits. Figure 2shows the sketch of the upper CC considered in thecalculations. The volume of the CC was 1.31m3. Inorder to simplify the calculations, the pressure andthe temperature of the outside of the upper CC were0.1MPa and 293 K, respectively. As the arc was ig-nited in the upper CC, the energy input Winput wasapplied locally in the CC. Winput was given as kptimes Warc. kp is a fraction of the Warc leading to thepressure rise. kp changes depending on the arc currentand the arcing electrode material, e.g. kp decreasesfrom 0.6 to 0.53 gradually with increasing arc currentfrom 1 kA to 12.5 kA in the case of copper electrodes[4]. The values of kp estimated in other publication[5] is in the same range as those mentioned above.When estimating the values of kp, the energy balance116vol. 4 no. 2/2017 Pressure Rise due to the HEAFsLower ＣＣ Upper ＣＣ BＣ Upper ＣBＣ Lower ＣBＣ Slits Slits Slits Slits Slits Front side Rear side Figure 1. Lateral view of the compartments in theswitchgear [8]. [CC: Cable compartment, BC: Buscompartment, CBC: Circuit-breaker compartment]Point 1 BC Rear wall Lower ＣＣ Energy input region CBC (on the lateral wall) Roof panel Figure 2. Geometrical arrangement considered in thecalculations (upper CC). [8]in the closed container was investigated [4–7], wherethe “input“ energy in the container was the electricarc energy and the energy of chemical reactions likeoxidation of electrode vapor in the case of exother-mal reactions. Conversely, the output energy in thecontainer was the energy causing pressure rise in thecontainer and the energy losses due to arc radiationenergy and the energy portion for electrode meltingand evaporation.The pressure rise in the CC was measured at point1 in the test. The gas inside was air with an initialpressure and temperature of 0.1MPa and 293K, re-spectively. Iarc was 21 kA, tarc 0.1 s (100ms), Warc3.1MJ, the current frequency 50Hz. kp was takenas 0.53, because the arcing electrode material wascopper in the tests, and kp did not change so muchwhen the arc current increased from 4 kA to 12.5 kA[4]. The metal plate with the slits was simply con-sidered as a medium of gas flow resistance and ofheat resistance/capacitance in the same way as in [9].The walls of the CC were assumed to be adiabatic,because the arcing duration was short (0.1 s), and the0 5 1 0 1 5 2 0- 1 001 02 03 04 05 0 Pressure rise dP  (kPa)T i m e  t  ( m s )E x p .C a l .Figure 3. Calculation and experimental pressure de-velopments at point 1 in the upper CC.CC was embedded in ambient air, which was a goodheat-insulating medium.During the test, the tightening bolts of the roofpanel of the upper CC broke owing to the pressurerise in the CC and the roof panel came off. In thecalculations, it was assumed that a part of the roofpanel began to open when the pressure rise reachedabout 5 kPa, which was estimated using the ultimatetensile strength (420N/mm2) and the cross-sectionalarea (14.2mm2) of the bolt, and the area of the roofpanel. In our previous paper [8], as a first step, theopened area of the roof panel was constant (0.064m2).In this paper, the opened area increased gradually(0.036–0.295m2) with time by considering the motionof the roof panel. The opened area was estimated byusing the following method. Supposing that the roofpanel is rigid and the roof panel comes off with therear eyebolts as the supporting points, the motionof the roof panel due to the internal pressure rise inthe upper CC was calculated by using the motionequation. The gap between the roof panel and theupper CC was calculated, and then the opened areaof the roof panel was estimated by using the gap area.3. Calculation results3.1. Pressure riseFigure 3 shows the calculated and measured pressuredevelopments of point 1 in the CC. The measuredpressure rise dP increased and reached the maximumvalue of about 23 kPa at time t of about 8.7ms, anddP showed a tendency to decrease although the arccontinued to burn in the CC. In this paper, the cal-culation was carried out until t = 15ms, where dPdecreased gradually. The calculated pressure develop-ment agreed well with the measured one.Figure 4 shows the 3-D pressure distribution in theCC. With time t, dP near the energy input regionincreased and the pressure wave propagated to anotherregion, e.g. the rear of the CC. Also, the hot gases inthe CC went out mainly through the opened area of117M. Iwata, T. Miyagi, Y. Goda, K. Shirai Plasma Physics and Technology  dP60 kPa- 10 kPa(a) t = 0 ms(c) t = 10 ms (d) t = 12 ms (e) t = 14 ms(b) t = 5 msFigure 4. Time dependence of 3-D pressure distribution in the upper CC.the roof panel after t of about 4.4ms when dP of theroof panel reached about 5 kPa.3.2. Energy of hot gases exhausted from roofpanelWhen investigating the thermal effect on personneland flammable materials near the electrical equipmentin which fault arcs occur, it is necessary to estimatethe energy of hot gases exhausted from the equipment.In this section, the energy flow density EFD of hotgases exhausted from the opened area of the roof panelwas calculated using the same method as in [10]. Theenergy flow density EFD of hot gases is given byEFD = Hex ρex vex (2)where Hex, ρex, and vex are the specific enthalpy,gas density, and velocity of the gas at the exit of theopened area of the roof panel, respectively.Figure 5 shows the energy flow density EFD of hotgases exhausted from the broken roof panel. EFDincreased rapidly at time t of about 4.4ms when theroof panel began to open owing to the pressure risein the CC.Figure 6 shows the measured arc energy Warc andthe calculated energy Wex of the exhausted hot gases.The energy Wex increased gradually because bothEFD and the opened area increased with time.Figure 7 shows the ratio kex of Wex to Warc. Theratio kex increased approximately in proportion to theopened area Aop of the roof panel. This result suggeststhat the ratio kex can rise more as the opened areaincreases. It is necessary to consider the development0 5 1 0 1 5 2 001 02 03 04 05 0 Energy flow density EFD (MW/m2)T i m e  t  ( m s )Figure 5. Calculated energy flow density of hot gasesexhausted from the opened area of roof panel.of the energy of hot gases exhausted from electricalequipment when investigating the thermal effect ofhot gases on personnel and flammable materials nearthe equipment.4. ConclusionsThis paper described the 3-D CFD calculation re-sults regarding the pressure rise due to HEAFs in themetal-clad switchgears and the energy of hot gasesexhausted from the switchgears thorough the brokenroof panel. The energy input took place locally inthe upper cable compartment of the switchgear. Thegas inside was air with an initial pressure of 0.1MPa.The arc current Iarc was 21 kA, the tarc 0.1 s (100ms),118vol. 4 no. 2/2017 Pressure Rise due to the HEAFs0 5 1 0 1 5 2 00 . 0 00 . 0 20 . 0 40 . 0 60 . 00 . 20 . 40 . 6Arc energy Warc  (MJ)T i m e   t  ( m s )Hot gases energy  Wex  (MJ)Figure 6. Calculated energy of the exhausted hot gasesand measured arc energy.0 . 0 0 . 1 0 . 2 0 . 3 0 . 40 . 0 00 . 0 20 . 0 40 . 0 60 . 0 8kex  (=Wex / Warc)O p e n e d  a r e a    A o p  ( m 2 )Figure 7. Dependence of the ratio kex = (Wex/Warc)on the opened area of roof panel.Warc 3.1MJ. The calculations were performed con-sidering the propagation of the hot gases from thecable compartment to the outside and the came-offof the compartment roof panel that was observed inthe test. As a result, the calculated pressure devel-opments of the cable compartment agreed well withthe test results. In addition, the calculated energy ofhot gases exhausted from the opened area of the roofpanel increased gradually because both the energy flowdensity of hot gases and the opened area increasedwith time. The above properties of hot gases mustbe considered when investigating the thermal effect ofhot gases on personnel and flammable materials nearthe switchgears.References[1] IEC Std.62271-200. AC metal-enclosed switchgear andcontrolgear for rated voltages above 1 kV and up to andincluding 52 kV, 2011. https://archive.org/details/gov.in.is.iec.62271.200.2003.[2] K. Shirai, Y. Suzuki, Y. Goda, M. Iwata, andT. Miyagi. High energy arcing faults fire tests usinghigh voltage metal-clad switchgear components (1)preliminary tests for fire evaluation. In 2013 FallMeeting of the Atomic Energy Society of Japan, 2013.[3] S.V. Patankar. Numerical Heat Transfer and FluidFlow. Hemisphere Publishing Corp., 1980.[4] M. Iwata, K. Anantavanich, and G. Pietsch. Influenceof arc current on fraction kp of electric arc energyleading to pressure rise in a closed container. In Proc.17th Int. Conf. on Gas Discharges and theirApplications, pages 189–192, 2008.[5] X. Zhang, G. Pietsch, and E. Gockenbach.Investigation of thermal transfer coefficient by theenergy balance of fault arcs in electrical installations.IEEE Trans. Power Delivery, 21(1):425–431, 2006.doi:10.1109/TPWRD.2005.852274.[6] T. Miyagi, S. Tanaka, M. Iwata, T. Tadokoro,T. Ohtaka, and T. Amakawa. Influence of arc currenton energy balance due to high current arc in a closedchamber. IEEJ Trans. Power and Energy, 128(12):1561–1568, 2008. doi:10.1541/ieejpes.130.232.[7] S. Tanaka, T. Miyagi, T. Ohtaka, M. Iwata,T. Amakawa, and Y. Goda. Influence of electrodematerial on pressure-rise due to arc in a closed chamber.IEEJ Trans. Power and Energy, 130(2):232–238, 2010.doi:10.1541/ieejpes.128.1561.[8] M. Iwata, T. Miyagi, Y. Goda, Y. Suzuki, andK. Shirai. Numerical approach to pressure rise due toshort-circuit fault arc in the metal-clad switchgears. InProc. 20th Int. Conf. on Gas Discharges and theirApplications, pages 171–174, 2014.[9] M. Iwata, S. Tanaka, T. Miyagi, T. Amakawa, andG.J. Pietsch. Simulation of pressure rise due to faultarc in a closed container with a perforated metal plate.In Proc. IEEJ Technical Meeting, pages ED–11–70/SA–11–43/ SP–11–16, 2011.[10] M. Iwata, S. Tanaka, T. Miyagi, T. Amakawa, andG.J. Pietsch. Influence of perforated metal plate onpressure rise and energy flow due to internal arcing in acontainer with a pressure-relief opening. 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Calculation Of Pressure Rise And Energy Of Hot Gases Due To High Energy Arcing Faults In The Metal-clad Switchgear

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Calculation Of Pressure Rise And Energy Of Hot Gases Due To High Energy Arcing Faults In The Metal-clad Switchgear

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