Abstract
Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.
Highlights
With large-scale distributed power and diverse load growth, power systems have become increasingly complex and large in capacity
The proposed feedforward calculation method uses instantaneous leg output phase voltages and currents to directly calculate zero-sequence voltage for leg energy balancing in the system, and this computational process can be accomplished with the help of an embedded system such as a digital signal processor (DSP)
The zero-sequence voltage can be injected for leg energy balancing in the wye-connected cascaded H-bridge (CHB) converter
Summary
With large-scale distributed power and diverse load growth, power systems have become increasingly complex and large in capacity. By applying multilevel VSC-high voltage direct current (HVDC) composed of numerous voltage levels to the STATCOM, it is easy to increase voltage level and rated power and significantly reduce the size of the coupling transformer and filter Due to these benefits, multilevel converters are increasingly being applied to VSC-STATCOMs. Due to these benefits, multilevel converters are increasingly being applied to VSC-STATCOMs Among these multilevel converters, the cascaded H-bridge (CHB) converter is the most representative and suitable topology for the VSC-STATCOM. The delta-connected CHB converter uses a zero-sequence circulation current for DC capacitor voltage balancing and leg energy balancing. The proposed feedforward calculation method uses instantaneous leg output phase voltages and currents to directly calculate zero-sequence voltage for leg energy balancing in the system, and this computational process can be accomplished with the help of an embedded system such as a digital signal processor (DSP). A 50MVA full-scale powersim (PSIM) simulation was used to verify the proposed method
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