This paper describes a three-phase cascade Static Synchronous Compensator (STATCOM) without transformer. Lt presents a control algorithm that meets the demand of load reactive power and also voltage balancing of isolated dc capacitors for H-bridges. The control algorithm used for inverter in this paper is based on a phase shifted carrier (PSC) modulation strategy that has no restriction on the cascaded number. The performance of the STATCOM for different changes of loads was simulated.
3. Cascaded Multilevel Inverter
Fig1.cascaded multilevel STATCOM.
EXPECTED SIMULATION RESULTS:
F ig. 2 Source voltage, source current and inverter current far inductive load(sourece current gain-5 and Inverter current gain-8).
Fig. 3 Load & Inverter Reactive componenets of current for Inductive load.
F ig. 4 Response of DC link voltage for inductive load.
Fig. 5 Source voltage and inverter current for the change of inductive load to half of the load at I sec(lnverter current gain-8)
Fig. 6 Load & Inverter Reactive componenets of current for the change of Inductive load to half of the load at I sec.
Fig. 7 Source voltage and inverter current for the change of inductive load to standby at 1 sec (Inverter current gain-8).
Fig. 8 Load & Inverter Reactive componenets of current for the change of Inductive load to standby at 1 sec
F ig. 9 Inverter Output Voltage
Fig. 10 Harmonie spectrum ofInverter line voltage.
Fig. 11 Load & Inverter reactive component for the change of Inductive to
capacitive load at 1.5 Sec.
Fig. 12 Response of oe link voltage for change in mode of operation from
inductive to capacitive load at 1.5 Sec.
Fig. 13 Inverter reactive component for the change of Inductive to capacitive load at 2 Sec
Fig. 14 Response of OC link voltage for change in mode of operation from inductive to capacitive at 2 Sec
The cascaded H-bridge multilevel topology is used as one of the more suitable topologies for reactive-power compensation applications. This paper presents a new control strategy for cascaded H-bridge multilevel converter based STATCOM. By this control strategy, the dc-link voltage of the inverter is controlled at their respective values when the ST A TCOM mode is converted from inductive to capacitive. The dc link voltages of the inverter are kept balanced in all the circumstances, and the reactive power that is produced by the STATCOM is equally distributed among all the H-bridges.
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 Hirofumi Akagi, Shigenori Inoue and Tsurugi Yoshii, "Control and Performance of a Transformerless Cascade PWM ST A TCOM With Star Contiguration," IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 1041-1049, July/ August 2007.
 H. Akagi, H. Fujita, S.Yonetaniand Y. Kondo, "A 6.6-kV transformerless ST ATCOM based on a tivelevel diode-clamped PWM converter: System design and experimentation of a 200-V 1 O-kV A laboratory model," IEEE Trans. Ind. Appl., vol. 44, no. 2, pp. 672-680, Mar./Apr. 2008.