IEEE Transactions on Power Delivery, Vol. 13, No. 4, October 1998 1257 zy Capacitor Commutated Converter Circuit Configurations for Dc Transmission K. Sadek M. Pereira D.P. Brandt A.M. Gole A. Daneshpooy Siemens, Brandt Consultants, Inc. University of Manitoba, Erlangen, GERMANY Winnipeg, MB., CANADA Winnipeg, MB., CANADA zyx Abstract zyxwvutsrqpon - Two non-conventional HVdc converter arrange- ments are compared. These include the Capacitor Commutated Converter (CCC) in which series capacitors are included between the converter transformer and the valves, and the Con- trolled Series Capacitor Clonverter (CSCC), based on more con- ventional topology, in which series capacitors are inserted between the ac filter bus and the ac network. Results show that both options have comparable steady state and transient perfor- mance. Danger of ferroresonance with the CSCC option is eliminated by controlling the amount of series compensation. Kevwords: HVdc Transmission, Capacitor Commutated Con- verter, Controlled Series Capacitor Converter, Thyristor Switched Series Capacitor. I. INTRODUCTION The conventional Graetz bridge converter used in HVdc transmission relies on the ac bus voltage for the com- mutation process. At the inverter side, this means that the incoming valve must be triggered sufficiently in advance of the line-line voltage zero crossing to provide sufficient com- mutation margin after the end of the overlap period. This zyxwvut typ- ically results in the converter consuming a large amount of reactive power at rated conditions. The reliance on line volt- age for commutation also makes the converter susceptible to commutation failure in the event of ac system voltage depressions and operation into weak ac systems is particu- larly difficult. The conventional arrangement also presents special problem with large dc cables. Any reduction in the ac bus voltage causes a corresponding dc voltage decrease and thus an increased current when the cable’s capacitance dis- charges. The converter’s inherent characteristic that causes the extinction angle y to decrease with an increase in current, increases the probability of commutation failure. The capacitor commutated converter (CCC), shown schematically in Fig. 1 is one topology that has been pro- zyxwvu PE-045-PWRD-0-12-1997 A paper recommended and approved by the IEEE Trafismission and Distribution Committee of the IEEE Power Engineering Society for publication in the IEEE Transactions on Power Delivery. Manuscript submitted April 15, 1997: made available for printing December 12, 1997. posed to remedy some of these drawbacks [1,2,3,4,5]. Recently, the CCC was studied in considerable detail with respect to its steady state and transient performance[4]. Another possible circuit configuration called the ‘Controlled Series Capacitor Converter’ abbreviated CSCC, has the series capacitors inserted at the connection of the fil- ter bus to the ac system as shown in Fig. 2. Relatively little has been reported on this scheme. A similar topology has been proposed by others [6,7], in which fixed capacitors are used instead of controlled capacitors @& [qf+ Ac Filters - Fig. 1. Capacitor Commutated Converter (CCC) Fig. 2. Controlled Series Capacitor Converter 11. CIRCUIT DESCRIPTIONS A. zyxwvuts CCC ConJiguration Fig. 1 shows a schematic diagram of a basic six pulse CCC group. The dc current flows through each phase capacitor in either forward or reverse direction during the conduction of the corresponding upper or lower group thyris- tor in that phase. The capacitors are charged with a polarity that aids in the commutation process. Indeed, they can be selected so that the firing angle zyxw ci can be increased to a value well beyond 1 80°, which would be impossible with a conven- tional converter. However, this greatly increases the peak voltage on the valve as compared with the conventional Gra- 0885-8977/98/$10.00 zyxwvu 0 1997 IEEE