Method and compensating device for compensating current oscillations

Abstract

To reduce unwanted current oscillations such as, for example, of the second harmonic of the system frequency (f.sub.o), the firing angles (.alpha.1, .alpha.2) of a link-circuit rectifier (1) and of a link-circuit inverter (2) are compensatingly acted on in push-pull mode. For this purpose, a direct-current link-circuit current signal (S5, S5'), which contains the instability or the current oscillation to be compensated, is fed by means of a current detector (5, 5') to a bandpass filter (16, 16', 16') which is tuned to the frequency (f.sub.o, f.sub.x) of the respective current oscillation. The phase of a bandpass filter output signal (S16) is shifted by 90.degree. trailing in a 90.degree. phase-shifting section (15, 15', 15'). A compensating signal (S15,S15', S15') obtained in this manner is fed to first and second function generators (13, 13', 13'; 14, 14', 14'). The first function generators allow the positive component (S13) to pass and the second function generators allow the negative component (S14) of the compensating signal to pass. The output signal of the first function generator passes via a switch (12') to a summing section (11') for forming the cosine of the firing angle (.alpha.2) of the link-circuit inverter (2). The output signal of the second function generator passes via a switch (12) to a summing section (11) for forming the cosine of the firing angle (.alpha.1) of the link-circuit rectifier (1). By these means, the firing angle (.alpha.2) of the link-circuit inverter (2) is reduced when the direct-current link-circuit current signal is dropping and the firing angle (.alpha.1) of the link-circuit rectifier (1) is increased when it is rising.