Origin and e ffect of surges
3.2 a) When the switch opens, the instantaneous value of the supply
voltage on the line results in a high potential difference between the system and the disconnected line. The potential difference, which is established in only a few milliseconds, can cause a flashback between the switch contacts that are yet to close. The line voltage then balances at a level equal to the instantaneous value of the supply
Table 3.1.4 a Impulse flashover voltages/impulse breakdown voltages
(1.2/50μs) in electrical systems and equipment up to 1000V
voltage and the arc between the switch contacts is quenched. This process can occur several times. The switching overvoltage generated by the equalization of the appropriate instantaneous value of the supply voltage has the characteristic of a damped oscillation with a frequency of several 100 kHz. The initial amplitude of these switch- ing surges always corresponds to the potential difference between the switch contacts at the moment of the flashback and this amplitude can be a multiple of the nominal supply voltage.
(b) Disconnection of an open-circuit transformer. If an open-circuit transformer is disconnected from the network, its self-capacitance is loaded by the energy of the magnetic field. The inductive–capacitve circuit now oscillates until all of the energy in the ohmic resistance of the circuit is converted into heat. The resulting switching over- voltages can reach amplitudes of several times the value of the nom- inal supply voltage.
(c) Earth fault in the floating (earth-free) network. If an earth fault occurs at the outer conductor of a floating network, then the poten- tial of the complete conductor system will be altered by the value of the voltage of the affected conductor with respect to earth. If the earth fault arc interrupts, the effect is similar to that of an open- circuit conductor or capacitor being disconnected: switching over- voltages will develop with damped oscillations.
In addition to switching overvoltages from power plants of this nature, which capacitively influence low-voltage systems, rapid variations in cur- rent can also generate surges in low-voltage systems by inductive coup- ling. Such sudden current variations can be due to either the connection or disconnection of a heavy load, or a short circuit, an earth fault or double earth fault.
Switching overvoltages can also be generated within the low-voltage systems themselves due to the following:
•
the disconnection of inductances connected in parallel with the source of voltage, such as transformers, inductors or coils of contactors andFigure 3.2 a Switching surges on disconnection of a capacitance
relays. (In this case, switching overvoltages are generated in a similar way to that described above for the disconnection of an open-circuit power transformer.)
•
the disconnection of inductances in the series arm of the current cir- cuit such as conductor loops, series inductors, or the inductances of the actual conductors. (These inductances try to maintain the flow of current, even if the circuit is interrupted. The magnitude of the switch- ing overvoltages that arise depends on the value of the current at the time of disconnection.)•
intentional disconnection of circuits by means of switches, or un- intentional disconnection brought about by the tripping of fuses or circuit breakers, or by line discontinuity before the natural current zero-axis crossing. (Rapid changes in current resulting from occur- rences such as these give rise to switching surges, normally with damped oscillations, which are a multiple of the nominal voltage of the system.)•
by phase control circuits, commutation effects in brush collector systems, and by sudden unloading of machines and transformers. Extensive measurements taken in different low-voltage networks have shown that the most remarkable surges have been caused by interfering radiation of arcs generated in switchgear.Electromagnetic interference by switching in power technical systems is usually more frequent than lightning interferences.
For conducted, broadband interference a difference is made in the EMC standards between high and low energy impulses or pulses depend- ing on the type of switching operation. It is possible that switching inter- ference is generated outside a building and enters through the power lines or it can be generated internally. This is either defined analogously to the lightning interference as combined surge voltage and surge current inter- ference or as impressed surge voltages.
In part the broadband, high energy, conducted interference of switch- ing processes is equated to the conducted lightning interference inside the building (with a duly carried out lightning protection equipotential bonding). So interference for different types of environment with corre- spondingly adjusted peak values is defined in the VG standards (Tables
3.2 a and 3.2 b).
An impressed surge voltage due to disconnection processes or overcur- rent protective components is defined in DIN VDE 0160. The surge voltage 0.1/1.3 ms (0.1 ms rate of rise, about 0.15 ms front time) with a peak value uN/max will be superimposed on the peak value uN/max of the
alternating current voltage.
Broadband, low energy switching voltage interference, (i.e. bursts) are shown in DIN VDE 0847 Part 4-4. These impressed voltage impulses 5/50 ns (5 ns rate of rise, about 7.4 ns front time) with peak values
depending on the severity of testing are supplied as pulse packages into power and communication lines through coupling capacities.
Apart from conducted interference, considerable interfering radia- tion can also be due to the switching processes themselves (e.g. arcs generated by the withdrawing of disconnectors) inducing more con- ducted interferences.
Sources
FGH MANNHEIM: ‘Transiente Überspannungen’, Fachberichte der FGH Mannheim, etz-a Elektrotech. Z., 1976, 97(1), pp. 2–27
MENGE, H.-D.: ‘Ergebnisse von Messungen transienter Überspannungen in Freiluft-Schaltanlagen’, etz-a Elektrotech. Z., 1976, 97(1), pp. 15–17
Table 3.2 a Lightning interference ‘1.2/50μs’
Table 3.2 b Lightning interference ‘10/700 μs’
LANG, U., and LINDNER, H.: ‘Überspannungen in Hochspannungsschaltan- lagen – Schutz von Sekundäreinrichtungen’, Elektrizitätswirtschaft, 1986, 22, pp. 680–683
SCHWAB, A. J.: ‘Elektromagnetische Verträglichkeit’ (Springer Verlag, Berlin, Heidelberg, New York, 1990)
HASSE, P.: ‘Überspannungsschutz von Niederspannungsanlagen – Einsatz elektronischer Geräte auch bei direkten Blitzeinschlägen’, 3. aktualisierte Auflage (Verlag TÜV Rheinland, Köln, 1993)
HASSE, P., and WIESINGER, J.: ‘EMV – Blitz-Schutzzonen-Konzept’ (Pflaum Verlag, München; VDE Verlag, Berlin-Offenbach, 1994)
VDEW: ‘Hinweise für die Messung von transienten Überspannungen in Sekundärleitungen innerhalb von Freiluft-Schaltanlagen’ Vereinigung Deutscher Elektrizitätswerke – VDEW e. V., Ausgabe Oct. 1975
DIN EN 61000-4-5 (VDE 0847 Teil 4-5): 1996-09: ‘Elektromagnetische Ver- träglichkeit (EMV)’. Teil 4: Prüf- und Messverfahren. Hauptabschnitt 5: Prüfung der Störfestigkeit gegen Stossspannungen (IEC 1000-4-5: 1995); Deutsche Fassung EN 61000-4-5: (VDE Verlag, GmbH, Berlin/Offenbach, 1995)
DIN EN 61000-4-4 (VDE 0847 Teil 4-4): 1996-03: ‘Elektromagnetische Ver- träglichkeit (EMV)’. Teil 4: Prüf- und Messverfahren. Hauptabschnitt 4: Prüfung der Störfestigkeit gegen schnelle transiente elektrische Störgrössen/Burst; EMV-Grundnorm (VDE Verlag, GmbH, Berlin/Offenbach, March 1996)
DIN VDE 0160: 1988-05: ‘Ausrüstung von Starkstromanlagen mit elektro- nischen Betriebsmitteln’ (VDE Verlag, GmbH, Berlin/Offenbach, May 1988) DIN-VDE-Taschenbuch 515: ‘Elektromagnetische Verträglichkeit 1. DIN- VDE-Normen’ (VDE Verlag, GmbH, Berlin/Offenbach, 1991)
VG 96 903 Teil 76/08.89: ‘Schutz gegen Nuklear-Elektromagnetischen Impuls (NEMP) und Blitzschlag’. Prüfverfahren, Prüfeinrichtungen und Grenzwerte. Verfahren LF 76: Prüfung mit Direkteinspeisung eines Span- nungsimpulses 1,2/50s und eines Stromimpulses 8/20s (Beuth-Verlag, GmbH, Berlin, Aug. 1989)