Due to the environment concerns, energy risks, fossil fuel loss reduction, controllable loads reduction and also the PV ac-tive power reduction. The validity and the effectiveness of the proposed method are verified by the simulation with the modified IEEE123-bus system. The paper by G. Zhang and J. Yu from Central South University, China presents an open circuit fault diagnosis method for cascaded H-bridge multilevel inverter based on the level-shifted pulse width modulation. The fault diagnosis is done based on the logical judgement instead of the conventional algorithms, thus improving the detection time. The proposed fault diagnosis method can also work for more number of levels and higher switching frequency of the multilevel inverter compared with the prior state-of-the-art scheme. As the interharmonic generated by the grid-connected PV system has attracted widespread attention recently, the paper by Q. Luo and his co-authors from South China University of Technology, China and Guangdong Power Grid Corp Guangzhou Power Supply Bureau Co Ltd, China presents an order-reduction method of interharmonics analysis model based on the principle of the interharmonic interaction. The proposed reduced-order interharmonic analysis model shows substantial computational savings while keeping accuracy. The paper by Y. Qiu and his co-authors from South China University of Technology, China and Shenzhen Power Supply Bureau Co., Ltd., China presents a method for analyzing the voltage deviation isolation (VDI) performance for a two-stage high-frequency isolated AC-DC converters in LVDC systems, which can isolate not only the faults but also the voltage deviation at the AC side and improve the voltage quality at the DC side. The range of VDI can be determined with considering the conditions for stable operation of AC-DC converters, constraints on power transmission limit and static stability limit. Simulation model is established to verify the feasibility and validity of the proposed method. The paper by C. Gong and his co-authors from the University of Macau, Macao, China presents a H∞ optimal controller design based on harmonic state-space modeling for the static var compensator coupling hybrid active power filter in reactive power and current harmonics compensation. The proposed controller achieves high steady-state accuracy, fast transient response, and fixed switching frequency simultaneously, which shows superior control performances in comparison with the recent developed state-of-the-art multi-quasi-proportional-resonant controller. Both simulation and experimental results verified the effectiveness and performance of the optimal controller design. The paper by D. Xiao and her co-authors from Southwest Jiaotong University, China addresses the power quality problems caused by freight railways in weak grids with high penetration of wind generation. This paper proposes a comprehensive control method for negative sequence current suppression and reactive power compensation for a co-phase traction power supply system connected with doubly fed induction generators. The proposed approach can fully solve the power quality issues and effectively reduce the capacity of the co-phase traction power supply system. The paper by G. Lv and his co-authors from Nanjing Institute of Technology, China presents a voltage sag source location estimation method based on optimal configuration of system monitoring points and fault parameter estimation. The adaptive cuckoo algorithm is used to solve the three optimization variables of fault line, fault distance and fault resistance. The validity and the effectiveness of the proposed estimation method is verified by the simulation with the IEEE-30 node standard test system. • Ken King Man Siu, University of North Texas, USA • Minwu Chen, Southwest Jiaotong University, China • Qing Zhong, South China University of Technology, China • Xiaorong Xie, Tsinghua University, China • Yongheng Yang, Zhejiang University, China We would like to express our deep gratefulness to Prof. Jinjun Liu, Editor-in-Chief, for his tremendous support from the initiative throughout the final stages of this Special Issue. Finally, we would like to thank the CPSS TPEA staffs involved with the production and technical support of this Special Issue. We look forward to seeing more great papers on Power Quality Conditioning in Modern Power Grids Integrated Emerging Power Electronic Systems in CPSS Transactions on Power Electronics and Applications in the time yet to come and your continuous support to the journal.