Voltage source converters (VSCs) generate harmonics under unbalanced conditions. With the wider applications of VSCs, the harmonics produced by VSCs will provide new challenges of harmonic problems in the modern power system. The harmonics generated by VSCs are caused by the switching characteristics of power electronic devices and the interaction between the ac and dc sides, which is a non-linear and complex problem. Dynamic phasor sequence components (DPSCs) are applied into the analysis of the harmonic produced by VSC in this study. With selecting the DPSCs of current harmonics on ac side and voltage harmonics on dc side as the state variables, a harmonic analysis model of VSC is presented as a linear time invariable system in the state space. The model considers both the detailed switching dynamics and the interactions between the ac and dc sides of VSC. The model also shows the analytical expression of the relationship between harmonics and sequence components of supply voltages. By solving the model, the harmonics generated by VSC under unbalanced conditions can be calculated mathematically. The comparison with simulation and experiment results can testify that the analysis model is very useful to analyse the harmonics of VSCs directly.
Increasing penetration of distributed generators (DGs) brings new challenges to the control and protection of distribution networks. This article proposes a control and protection framework based on Multi-agent System (MAS), in which situation awareness of zone agents plays an important role. The main protection is designed to locate and isolate faults based on the peer-to-peer communication among the zone agents. Fault-tolerant mechanisms, including self-inspection and backup protection, are developed to cope with the malfunction of individual MAS and local communication failure. An automatic parallel restoration scheme that doesn't require complete grid information is proposed to restore the power supply. Comprehensive simulation studies are carried out on a 71-bus distribution network with DGs using DIgSILENT, and the results have shown the effectiveness of the proposed self-healing framework.
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