The article will present the combination of a low-voltage fuse with a pyrotechnic switch in compact design. The limited switching capacity of the very simple pyrotechnic switch is increased considerably when combined with a fuse which has classical passive time/current characteristic and high switching capacity. FEM calculations will demonstrate the functional principle and the passive and active characteristics of functional models will be examined. Switching behaviour with different overcurrents in case of passive or active triggering will be discussed on the basis of measuring results. The behaviour of such a fuse with high impulse currents will also be presented

Ehrhardt, A.

Wolfram, S.

English

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

doi:10.14311/ppt.2019.1.91Plasma Physics and Technology 6(1):91–94, 2019 © Department of Physics, FEE CTU in Prague, 2019PYROTECHNIC SWITCH WITH FUSE FUNCTIONA. Ehrhardta,∗, S. Wolframba DEHN + SÖHNE GmbH + Co.KG., Hans-Dehn-Straße 1, 92318 Neumarkt, Germanyb Institute for Electric Power and Control Engineering, Technische Universität Ilmenau, Gustav-Kirchhoff-Str. 1,98693 Ilmenau, Germany∗ arnd.ehrhardt@dehn.deAbstract. The article will present the combination of a low-voltage fuse with a pyrotechnic switchin compact design. The limited switching capacity of the very simple pyrotechnic switch is increasedconsiderably when combined with a fuse which has classical passive time/current characteristic andhigh switching capacity. FEM calculations will demonstrate the functional principle and the passiveand active characteristics of functional models will be examined. Switching behaviour with differentovercurrents in case of passive or active triggering will be discussed on the basis of measuring results.The behaviour of such a fuse with high impulse currents will also be presented.Keywords: fuse, pyrotechnic switch, impulse current, numerical simulation, SPD.1. IntroductionFor more than 100 years, overcurrent protective de-vices in the form of fuses and switches have beensuccessfully enhanced and adapted for the most di-verse networks, consumers and protective objectives.The massive modification of the power supply net-works which is currently underway also places newdemands on these protective devices. As numerouspower generators feed electricity into the grid, the flowof power in the distribution network is no longer justunidirectional, but frequently bidirectional. What ismore, the source characteristics of numerous electric-ity generators and the new regulating mechanismsfor controlling the power flow reduce the short-circuitcurrents in the grid. Both effects influence, amongother things, the tripping conditions for overcurrentprotective devices in the power grid. In order tokeep up with requirements, additional compact andlow-cost functions are needed alongside the optimumconfiguration and selection of devices.Fuses have the advantage over switches that theyare smaller while ensuring an extremely high short-circuit breaking capacity and that they have a veryhigh current limitation and thus low total clearingintegrals. In addition, they are inexpensive and do notexhaust ionised gases. The disadvantage, however, isthat fuses only protect devices and systems once andthat they respond slowly in case of small overcurrentswhich results in long melting times and also longarcing times over some half waves. In case of suchsmall currents, the installation conditions and theambient temperatures frequently have an impact onthe disconnection behaviour. With the modificationof networks described above, these disadvantages maycome to the fore.Switches, in contrast, have the advantage that theovercurrent performance is easily adjustable and thatthey can often be externally triggered. This externaltriggering by additional monitoring devices allowssafe disconnection in a relatively short period of time,even in the event of faults which do not directly causehigh short-circuit currents or in networks where theoperating and short-circuit current are almost equal.In networks with active power regulation, however, thepassive protection characteristic of protective devicesis frequently only decisive if these triggering systemsfail. As with high short-circuit currents, the one-time switching function in combination with externaltriggering is therefore sufficient in case of such faults.For this reason, a combination of separate circuitbreakers and backup fuses has become an establishedpart of medium-voltage systems to get the benefit ofthe advantages of both devices [1]. Measures aimed atdisconnecting small currents faster have been availablefor fuses for many years. To this end, e.g., exothermalmasses are used which passively or actively heat thesolders on fuse-elements or directly heat subareas [2].These procedures can considerably reduce the naturalmelting time, however, disconnection within 10 ms,as is common with switches, cannot be achieved. It isalso common knowledge that disconnection of fusescan be accelerated by a short-circuit enabling fastdisconnection times, at least directly after the short-circuit has been generated [3]. This, however, requiresconventional networks with a high short-circuit power.An overview of the different approaches can be foundin [4].The principle of the IS - Limiter, namely parallelconnection of a pyrotechnic switch and a fuse, is knownfrom the field of high-voltage [5]. This principle canalso be used for low-voltage applications at a smalldesign size. Similar devices have also been developedfor the automotive sector and DC systems. However,these devices require a trigger signal and have nei-ther a passive short-circuit performance nor, withoutadditional programming, a defined time-current char-91A. Ehrhardt, S. Wolfram Plasma Physics and Technologyacteristic. The design targets for the switching devicefor low-voltage applications were: High passive short-circuit breaking capacity of alow-voltage fuse Time/current characteristic of a low-voltage fusewithout triggering Size and costs similar to those of a fuse Additional feature: External triggering with a dis-connection time < 30ms High capacity to carry impulse current, particularlylightning currents Nominal current up to approx. 250A Nominal voltage up to approx. 400V AC2. Set-up and components of theapproach examinedThe sample solution consists of components of an en-closed fuse, size 22x58. This size is borderline in everyway with regard to the desired values, however, thisapproach allows a realistic assessment of the physi-cal limits and the construction volume required forimplementation.Conventional compacted silica sand was chosen as aquenching medium. Two parallel copper fuse-elementswith modulation, which were, as far as possible, in-stalled straight through the housing, were used forthe required high impulse current carrying capabil-ity. The nominal current of the two fuse-elements is250A. The fuse can carry lightning impulse currentswith an amplitude of 25 kA (wave form 10/350 µs withI2t = 156 kA2s). Thus a configuration was selectedby which the nominal current and impulse current areat the upper limit, resulting in maximum requirementson the switching capacity. Consequently, lower valuescan be used for the same size or the same values canbe used for larger sizes.A very simple pyrotechnic switch was integratedbetween two separated sand-filled parts of a fuse. Therestricted sections of the fuse-element required fordisconnecting the short-circuit current are distributedevenly between the two parts of the fuse-link. Figure 1shows the basic test set-up with an electric match.A hollow space a2 is located between the two sep-arated sand-filled parts a1 of the fuse-link. The twoparts a1 of the fuse-link filled with silica sand are sepa-rated from the hollow space a2 by a thin wall < 1mmso that no arc extinguishing medium can enter thehollow space. A cutting blade c is situated in thehollow space between the two separated parts of thefuse-link nearby the fuse-elements b.Consequently, only little force is required to actuatethe cutting blade. The overall width of the hollowspace is smaller than 5mm. The two copper fuse-elements b routed directly through the two sand-filledparts of the fuse-link and also through the hollowspace a2. The two fuse-elements b are connected withFigure 1. Test set-up with actuator.Figure 2. Fuse-element with restricted sections andadditional section for cutting.the end caps f. The fuse-elements have an additionalrestricted section in this hollow space which is dimen-sioned in such a way that it does not trip in case ofthe common tripping integral of the fuse when short-circuit currents are interrupted. Figure 2 shows asingle fuse-element.This additional restricted section does not melt overthe entire time/current range as long as the fuse showsnormal passive behaviour. It simply reduces the forcerequired for the interruption of the fuse-elements b.The arrangement of the hollow space, the thin walls,the cutting blade, the additional restricted section andthe electric match constituted a simple pyrotechnicswitch. This pyrotechnical switch has a limited self-switching capacity. The electric match can be ignitedwith a short current flow (5A, 10µs). The cuttingblade disconnects the fuse-elements. An arc can burnback the fuse-element up to the two parts of the fuse-link filled with silica sand where then the arc safelyquenched at the latest to the next current zero.3. Functional principle3.1. Passive functionThe time/current performance (heating until meltingof one restricted section of the first fuse-element) of92vol. 6 no. 1/2019 Pyrotechnic Switch with Fuse Functionthe sample arrangement was simulated based on FEMcalculations of transient impulse currents up to thecurrent which melts the fuse after 1 h and comparedwith a conventional fuse [6]. Figure 3 shows the tem-perature rise of the fuse-element when impulse currentflows through it. The calculated time/current char-acteristics were compared with real currents in therange of about 600A to 50 kA. The characteristics areillustrated in Figure 4 and show enough concordance.Figure 3. Calculated maximum temperature at 25 kA,10/350 µs impulse.Figure 4. Comparison of the measured with the calcu-lated passive time/current characteristic.The passive switching performance (no triggeringover the ignition cable) of the sample arrangementwas tested at 440V in the current range of 600A to50 kA. Figure 5 shows the current and voltage charac-teristic at 50 kA compared with the characteristic ofa conventional fuse design.3.2. Active functionIn addition to the passive function, the function ofthe pyrotechnic switch was also tested under load(overcurrents). The ignition of the electric matchand the current load of the test arrangment weremanaged via a time-dependent sequential control. Inthis context, the trigger time TDelay and the timeuntil both fuse-elements are disconnected Tmove arerelevant. The resulting delay TV defines the maximumcurrent load which has to be mastered. Figure 6ashows the disconnection of a current of 600A andFigure 6b the disconnection of a current of approx.2 kA.In principle, extremely high short-circuit currentsare only switched passively. According to the pas-sive time/current characteristic, currents > 3 kA areonly interrupted passively. The pyrotechnic switchwas modified for accelerated triggering for the testFigure 5. Measured voltage/current characteristic atIp = 50 kA AC of the conventional fuse and the py-rotechnic switch.in Figure 6b. The test current in Figure 6b andthe switching capacity of the pyrotechnic switch thusalready exceed the maximum capacity of the switch.This high switching capacity of the simple pyrotechnicswitch and the combination with the passive protec-tion characteristic of the fuse thus allow safe andreliable series connection both in case of active andpassive functions.Figure 7 shows the passive time/current charac-teristic and the actual active characteristic. It be-comes evident that the sample arrangement examinedensures safe disconnection at 440V over the entirecurrent range within a duration of < 30ms. In caseof a longer delay or non-triggering, the arrangementshows the time/current performance of a fuse withthe relevant characteristic.4. Condition after impulse current loadIn addition to the time/current performance and theswitching capacity, the condition of the fuse-elementwas assessed after a sequence of impulse current loadswhich correspond to a normative load of type 1 surgeprotective devices [7]. Figure 8 shows such a fuse-element. The restricted sections have changed colour,but this is uncritical. No partial melting resultingfrom excessive current densities or deformations ofthe fuse-element caused by electrodynamic forces werefound.After the switching loads, which were mastered withactive and passive function, the impulse withstandvoltage (insulation) was also tested by determiningthe impulse sparkover voltage of the sample set-up. Itbecame evident that in case of active disconnectionthe impulse sparkover voltages were always > 6 kVindependent of the load current. This is identicalwith the value achieved with short-circuit currents incase of passive function. Without active function, onlyelectric strengths which are partly below 2 kV occurredwhen passively switching currents by means of fuse-93A. Ehrhardt, S. Wolfram Plasma Physics and TechnologyFigure 6. Measured voltage/current characteristic in case of disconnection for: a) Ip = 600A (UC = 434V,cos(ϕ) = 0.92); b) Ip = 2 kA (UC = 434V, cos(ϕ) = 0.83).Figure 7. Measured time/current characteristic of thepyrotechnic switch.Figure 8. Colour change of the restricted sections afternormative load.elements in the seconds range (interruption of onlyone restricted section per fuse-element). The samplearrangement examined thus safely prevents repeatedcurrent flow through the fuse after disconnection evenin case of higher voltages.5. SummaryThe sample arrangement presented in this paper isa space-saving and cost-effective compromise for net-works, plants or equipment for which a one-timeswitching function is sufficient. It was demonstratedthat high switching capacities and low total clearingintegrals similar to fuses of comparable size can beachieved by means of additional triggering. The dis-connection time in case of external triggering is thesame as in case of conventional externally triggeredswitching devices. If triggering fails, the protectiveeffect is equivalent to that of a normal passive fuseso that at least conventional overcurrent and short-circuit protection is ensured. Moreover, the samplearrangement can safely carry high transient impulsecurrents. Safe active disconnection is ensured inde-pendent of the current load, namely from zero currentto the short-circuit current range, thus achieving ahigher and at the same time defined electric strengthcompared to passive fuses.References[1] IEC 616272-105 High-voltage switchgear andcontrolgear – Part 105: Alternating current switch-fusecombinations, March 2002.[2] R. Ranjan. Current surge protector for power fuses.U.S. Patent 4807082(A), May 14, 1987.[3] M. Koprivsek. Triggered fuse. In 10th ICEFA, pages69–78, September 2015.[4] H. Bessei and F. Glinka. Smart fuses for smart grids:Considerations about the need, potential productfeatures and feasibility. In 10th ICEFA, pages 79–86,September 2015.[5] K. H. Hartung and V. Schmidt. Limitation of shortcircuit current by an IS - Limiter. In 10th EPQU,September 2009.[6] D. Rochette, W. Bussière, and R. Touzani. Modelling ofthe pre-arcing period in hbc fuses including solid - liquid- vapour phase changes of the fuse element. In 64th IEEEHolm Conference on Electrical Contacts, pages 87–93,September 2007. doi:10.1109/ICEFA.2007.4419972.[7] IEC 61643-11 Low-voltage surge protective devices –Part 11: Surge protective devices connected tolow-voltage power systems – Requirements and testmethods, March 2011.94

Pyrotechnic Switch with Fuse Function

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Pyrotechnic Switch with Fuse Function

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