GB1252572A - - Google Patents

Abstract

1,252,572. Static converters. NATIONAL RESEARCH DEVELOPMENT CORP. 28 April, 1969 [6 May, 1968], No. 21339/68. Heading H2F. Current harmonics produced by a polyphase static converter and which can cause serious interference in A.C. mains connected thereto, are reduced by injecting current into the converter circuit at 3 times the A.C. line frequency or a multiple of three times. In the rectifier arrangement shown in Fig. 1 the injected current is provided by a harmonic generator 9 connected in parallel with an interphase transformer between the star points of two secondary windings coupled to a common primary winding. The generator may be a motor-generator set or a static frequency generator. The interphase transformer 19, 20 has a selectively high impedance to the injected current which will thus flow mainly through the secondary windings in a path depending upon which rectifiers (e.g. diodes or thyristors) are conducting. For instance, with rectifiers 21, 22 conducting the injected current will flow in a path indicated by dotted line 10. An equivalent balancing current will flow in the corresponding primary winding phases thus modifying the primary waveform. The peak value (y) of the injected current should be in the order of one half the converter load current (2h) and should be phase displaced from the interphase transformer magnetizing current, e.g. by an angle of 90 degrees for unretarded rectification. The angle is adjusted for any change in the firing angle of the rectifiers. A table 1 (not shown) gives the harmonics present as percentages of the fundamental with various values of the injection ratio (y/h). In a polyphase full wave bridge rectifier arrangement (Fig. 2, not shown) the interphase transformer is dispensed with. The arrangement described is also used when inverting.