Investigations on the Control of Over pressure

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					                                     Investigations on the Control of Overpres-
Institut für Hochspannungstechnik




                                     sure Development in Case of Fault Arcs in
                                     Electrical Installations
                                     Fault arcs in electrical installations are extremely rare, however, can not be excluded fully.
                                     Therefore the effects of fault arcs must be considered during designing electrical installa-
                                     tions and their buildings. If installations have to be included into already existing buildings
                                     a special problem arises: the control of overpressure in the case of a fault. It can be solved
                                     by pressure relief channels and / or fault arc absorbers.


                                                 Fault Arcs in Electrical In-                   for the calculation of the pressure de-
                                                 stallations                                    velopment averaged in space. This
                                                                                                is sufficient for most applications.
                                                 Insulation failure, flashover, malfunc-         With the program based on the ’Com-
                                                 tion or maloperation by service per-           puter Fluid Dynamics’ (CFD) software
                                                 sonal in electrical installations can lead     ’CFX5’ the overpressure- and temper-
                                                 to powerful fault arcs. In this case the       ature development is determined in
                                                 fault arc will burn until the failure is cut   space and time. The adaption of an ap-
                                                 out by a protection device. In medium          propriate calculation grid and the cal-
                                                 voltage systems the order of magni-            culation procedure are however com-
                                                 tude of the electric power dissipated          plex and time consuming. This pro-
                                                 in fault arcs may reach some ten MW.           gram is used e. g. for optimization of
                                                 The fault arc delivers energy to its sur-      pressure relief channels.
28                                               roundings so that conducting and insu-         Calculations of the pressure rise in
                                                 lating materials may vaporise and even         electrical installations caused by fault
                                                 burn. This can lead to thermal and me-         arcs are necessary to estimate the risk
                                                 chanical stress of the installation, due       of fault arcs, to reduce the costs for de-
                                                 to the fact that the heating of the sur-       velopment and testing and in the case,
                                                 roundings causes a pressure rise. This         that tests can not be performed, e. g. in
                                                 pressure rise has to be controlled in or-      already existing buildings.
                                                 der to protect people and to minimize
                                                 damages.
                                                                                                Experimental               Investiga-
                                                                                                tions
                                                                                                A series resonant circuit with a
                                                                                                battery of 1500 electrolytic capac-
                                                                                                itors (Cges = 0,3 F, UN = 1000 V)
                                                 Overpressure Development                       is used to perform short circuit
                                                                                                tests.   The capacitors (each with
                                                 The calculation of the overpressure de-        C = 820 µF, U = 500 V) are fit with
                                                 velopment as a result of fault arcs            a diode network in order to improve
                                                 is performed with two different pro-           the operational safety. The discharge
                                                 grams. The program package ’PRES-              diodes Da and the diodes with the
                                                 SURE’ already developed [1] is used            charging current Db , see fig. 1, limit

                                    JAHRESBERICHT 2003
Research activity: Fault Arcs




                                                                                                   Institut für Hochspannungstechnik
the fault energy to the energy content     The attached tube will be assembled
of a capacitor. The split-up of the        with arc absorbers. Fault arc absorbers
charging voltage on U1 and U2 guar-        are cooling meshes and therewith pas-
antees a symmetric charging of the ca-     sive elements detracting energy from
pacitors in series. By changing the        the gas heated up by the fault arc
inductance L, the arc duration can be      through thermal conduction. They do
varied between 20 and 100 ms. The          not need maintenance. By means of
experiments are carried out in a four      pressure and temperature measurement
flange of 70.9 volume containing the        the utility of expanded metal layers and
electrodes with attached tube.             other materials, e. g. special ceramics
                                           [2], will be examined.



                                               anode
                                                                   absorber
                                                         cathode
                                               P1 , T1                           P2 , T2




                                           Fig. 3: Test vessel
Fig. 1: High current circuit

The transmission of the measurement
values of fault arc current, burning
                                           References
voltage and test vessel overpressure out
of the high current range is accom-
                                             [1] G. Friberg, Berechnungsver-                       29
                                                 fahren zur Bestimmung der
plished by fibre optics. A temperature            Druckbelastung in elektrischen
measurement system will be included.             Anlagen im Störlichtbogenfall,
                                                 Dissertation RWTH Aachen,
                                                 1999

                                             [2] P. Glasmacher, Neue Störlicht-
                                                 bogenabsorber - Lösung für
                                                 MS-Schaltanlagenräume, etz,
                                                 Heft 18/2003, S. 42-43


                                           Contact
                                           Dipl.-Ing. Michael Schmale
Fig. 2: Measured curves of the burning Schmale@ifht.rwth-aachen.de
        voltage and arc current        ++49/(0)241/80-94958




                                                                              JAHRESBERICHT 2003