Study on fault ride-through capability of wind farm integration using MMC-HVDC

Due to various advantages, high voltage direct current transmission based on voltage source converter (VSC-HVDC) is gradually coming into practice. It realizes the independent control of both active and reactive power, and has a good dynamic ability of reactive power support. Moreover, if used in wind power transmission, VSC-HVDC decouples the wind farm and the AC grid, which effectively enhances the fault ride-through (FRT) capability of wind power system. There are mainly three VSC circuits for HVDC application at present: HVDC Light of ABB, HVDC Plus of SIMENS, and alternate-arm multilevel converter (A2MC) of Alstom. Schemes from ABB and SIMENS have been widely used, which both apply VSC for ac/dc conversion. HVDC Light uses two-level converter with serial structure of insulated gate bipolar transistors (IGBT), which has advantages like small volume, light weight, and simple control of converter arm. However, it also has some disadvantages, such as more switching loss, higher harmonics in output power, and difficulty in implementing synchronous driving technology for serial IGBTs. HVDC Plus uses modular multilevel converter (MMC) topology, which has obvious advantages in reducing switching loss, lowering harmonic content, upgrading capacity, and managing fault problems. Besides that, MMC has shown superiority in improving FRT capability of wind power system, as studied in the paper. For large-scale wind farm integrated through VSC-HVDC system, when a short-circuit fault occurs in AC grid, the grid-side inverter station transmits less power while there is no change in wind power, which leads to imbalance of power in HVDC system, rise of dc voltage, and disturbances in wind farm. Worse still, the whole HVDC system may collapse under more serious conditions. Converter station built in MMC topology can improve the fault characteristics of wind farm integrated through VSC-HVDC system, and alleviate its disturbances after fault. The paper briefly introduces the basic principles and method of choosing the capacitors of traditional two-level VSC circuit and MMC circuit. To study on FRT problem for wind farm integrated through VSC-HVDC system, a two-terminal VSC-HVDC simulation model of doubly-fed wind farm connected to AC grid respectively through MMC station and traditional VSC station is established in PSCAD/EMTDC, and control strategy of each part is discussed. When there is a three-phase short-circuit fault in AC grid, the FRT capability of VSC-HVDC system with wind farm that integrates through two different schemes is analyzed, and control effects of prevalent FRT method on the wind power system under the two schemes are compared, thus obtaining the influence of converter station built in MMC topology on FRT capability of wind farm integrated through VSC-HVDC system. Capacitors in MMC modules play a crucial role in enhancing the system's FRT capability. The conclusions can provide reference for promoting application of MMC in VSC-HVDC system for transmitting wind power.