High-Voltage Test and Measuring Techniques by Wolfgang Hauschild & Eberhard Lemke

High-Voltage Test and Measuring Techniques by Wolfgang Hauschild & Eberhard Lemke

Author:Wolfgang Hauschild & Eberhard Lemke
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg


6.2.2.2 Polarity Reversal and Switch-off

A reversal of the voltage is a very hard stress for HVDC insulation. Therefore, several standards require it in type tests. The used HVDC test system must have a motor-driven polarity reversal as, e.g. indicated in Fig. 6.9. The principle of the polarity reversal for a column of three rectifiers is shown in Fig. 6.13a: It is presumed that the rectifiers are connected to a column of smoothing capacitors and a capacitive test object (Frank et al. 1983; Schufft and Gotanda 1997). The reversal cycle starts at t 1 with switching-off the feeding AC voltage and a turn of the rectifiers. The capacitors are slowly discharged via the resistive part of the voltage divider (compare Fig. 6.8a). When the rectifiers approach the opposite electrodes at t 2, sparkovers between their electrodes and the related opposite fixed electrodes occur. Within a very short time of some milliseconds, the capacitances are rapidly discharged to zero via the rectifier column. Now, the AC voltage is switched on again, and the capacitances are charged into the opposite polarity (t 3). The charging time depends on the time constant determined by the capacitances to be charged (generator plus test object), the impedances of the charging circuit and the available power of the HVAC feeding circuit. It may range from few seconds to few 10 s which is sufficient for most test applications.

Fig. 6.13Polarity reversal. a Switching process of the rectifiers. b Definition of the reversal time



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