Overvoltage mitigation and insulation coordination of ±1000 kV and above DC transmission project

Overvoltage suppression and insulation coordination for converter station equipment are key techniques in ±1000 kV ultra-high voltage direct current (UHVDC) transmission projects, and they are very important for lowering the insulation level and manufacture difficulty of equipment. Thus, means for equipment overvoltage suppression in ±1000 kV UHVDC converter stations and for the parameter selection of high-performance metal oxide arrester (MOA) were discussed. It is indicated that increasing charge rate and reducing residual voltage ratio is an important means to lower the protection level of MOA. Moreover, the feasibility of lowering equipment insulation level by adopting high-performance MOA was introduced in detail. The definitiveness method, as the insulation coordination method for equipment in ±1000 kV and above UHVDC transmission projects, was proposed. Based on simulations, the method can offer reasonable coordination currents and current waveforms which make the insulation coordination accurate. By using the method, recommended values of surge insulation level for ±1000 kV and above UHVDC equipment can be given: the switching/lightning impulse insulation levels of ±1000 kV and ±1100 kV DC pole line are 1900 kV/2250 kV and 2050 kV/2400 kV, respectively; the witching impulse insulation levels of ±1000 kV and ±1100 kV Y winding terminal blocks/D winding terminal blocks on valve side of high voltage converter transformers are 1950 kV/1500 kV and 2050 kV/1600 kV, respectively. Meanwhile, according to the overvoltage distribution along transmission lines, insulation configurations of towers along a line are advised to be determined by comprehensively analyzing the amplitude and distribution characteristics of the line overvoltage and the altitude along the line, and using principle of subsection design is also suggested for making towers cost more economic and more reasonable.