A Novel Cluster Balance Strategy of the Shunt Chb for Delta-Type Serial Shunt Soft-Normally Open Points

As the proportion of renewable energy integrated into the distribution networks increases, issues such as flow congestion and voltage violations arise in the distribution network. To achieve these problems, a novel delta-type serial shunt soft-normally open point (D-S <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{3}$</tex> NOP) is proposed. Owning to the serial-shunt structure, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D-S^{3} N O P$</tex> can achieve a wide range of power flow control with a relatively small capacity requirement. Thus, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D-S^{3} N O P$</tex> offers the advantages of lower cost and smaller size, compared with the back-to-back modular multilevel converter (BTB-MMC). However, the cluster balance strategy of CHB in D-S <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{3}$</tex> NOP has not been thoroughly researched. The original strategy relies on the fundamental frequency circulating current injection. The extra circulating current magnifies the current stress of the power device. Therefore, this paper proposes a novel cluster balance strategy for the CHB in D-S <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{3}$</tex> NOP, which can maintain the cluster energy balance without extra circulating current. The control degree of freedom gets fully utilized. This method reduces the current stress on power devices, which can reduce the device cost and improve its economic viability. The effectiveness of the proposed strategy is verified through simulation.