To improve the power transfer capability and reliability of the power grid, the unified power flow controller (UPFC) project has been put into operation in the western Nanjing power grid. Based on the practical project, the influence of UPFC on the distance protection is analyzed. To cope with that, the novel three-zone distance protection scheme is put forward. The zone 2 and zone 3 of the novel three-zone distance protection scheme are the same with those of conventional distance protection; however, the novel distance protection zone 1 is based on the parameter model of transmission line. According to the average value and fluctuation degree of the fault distance calculation results, the proposed distance protection zone 1 can effectively distinguish between the internal faults and external faults. More importantly, the proposed distance protection zone 1 can realize effective cooperation with the conventional distance protection zone 2 and zone 3, to construct a complete three-zone distance protection scheme and guarantee the correct operation of the novel three-zone distance protection scheme. The simulation cases in the PSCAD/EMTDC software environment verifies that the proposed three-zone distance protection has excellent performance, and would not be affected by the operational mode and control parameters of UPFC.

The unified power flow controller (UPFC) based on modular multilevel converter (MMC) is the most creative flexible ac transmission system (FACTS) device. In theory, the output voltage of the series MMC in MMC-UPFC can be regulated from 0 to the rated value. However, there would be relatively large harmonics in the output voltage if the voltage modulation ratio is small. In order to analyze the influence of MMC-UPFC on the harmonics of the power grid, the theoretical calculation method and spectra of the output voltage harmonics of MMC are presented. Subsequently, the calculation formulas of the harmonics in the power grid with UPFC are proposed. Based on it, the influence of UPFC on the grid voltage harmonics is evaluated, when MMC-UPFC is operated with different submodular numbers and voltage modular ratios. Eventually, the proposed analysis method is validated using digital simulation. The study results would provide guideline for the design and operation of MMC-UPFC project.

The fault current characteristics of inverter interfaced distributed generators (IIDGs) are very different from those of traditional synchronous generator. Prior to the fault current analysis of IIDG, a control strategy for low voltage ride-through (LVRT) of IIDG is proposed to meet the grid code. Then the fault current characteristics of IIDG are analyzed based on the study of dynamic responses of the inverter under conditions of symmetric faults and asymmetric faults. Meanwhile, the analytical expressions of the fault current of IIDG are obtained. Finally, the fault current characteristics of IIDG are validated with simulation studies.