The paper presents the application of Delta- connected cascaded H-bridge multilevel SVG under unbalanced compensation currents or unbalanced supply voltages. Clustered balancing control for delta-connected SVG can be realized by injecting a zero-sequence current to the delta-loop. But zero-sequence current injection may cause the high peak phase current which may break converter switches. The aim of this paper is to analyze the key factors that affect the maximum output current of the SVG with injecting zero-sequence current and acquire the quantitative relationship between unbalance compensation capability, the unbalance degree of the supply voltage, the initial phase of negative-sequence voltage, the unbalance degree of the compensation current and the initial phase of negative-sequence current. On this foundation, the valid compensation range of delta-connected SVG under unbalanced conditions is obtained. Furthermore, the compensation characteristic of the negative-sequence current is deduced with the certain supply voltage and the influence of supply voltage variation on the maximum output current for SVG is also considered with the certain compensation current. Finally, the correctness of the relevant theoretical analysis is verified by simulation and experiment.
- Delta-connected cascaded H-bridge multilevel SVG
- Zero-sequence current
- Unbalance degree
Fig.1. Delta-connected cascaded H-bridge multilevel SVG system configuration
EXPECTED SIMULATION RESULTS:
(a) Ki increased from 0 to 20%
(b) Ki increased from 20% to 50%
(c) Ki decreased from 50% to 20% and Ku increased from 0 to 10%
(d) Ku increased from 10% to 40%
Fig.2 Partial enlargement waveforms during sudden change of unbalance degree
In this paper, the effect of unbalanced supply voltage and compensation current on the delta-connected SVG has been analyzed. Injecting zero-sequence current into the delta-loop allows maintaining cluster voltage balancing for the SVG. However, it has been shown that zero-sequence current injection may cause the high peak phase current which may break converter switches. In order to guarantee safe and reliable operation of the delta-connected SVG, whose maximum output current level Imax/Ip is chosen as the standard to measured its unbalance compensation capability and the valid compensation range under unbalanced conditions can also be obtained. The unbalance compensation range of the delta-connected structure is limited by the unbalance degree of the supply voltage, the initial phase of negative-sequence voltage, the unbalance degree of the compensation current and the initial phase of negative-sequence current. The quantitative relationship between unbalance compensation capability and other influence factors derived in this paper can provide a good theoretical basis for the parameter design and device selection of the delta-connected cascaded H-bridge multilevel SVG. In addition, the delta-connected SVG is more sensitive to the unbalance degree of the supply voltage than the unbalance degree of the compensation current， and it will be better way for industrial applications aiming at improving the power quality. The simulation and experimental results further verified the rationality and accuracy of the analysis.
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