Transient Overvoltage Study on LCC-MMC Hybrid UHVDC Transmission System

This paper is a study on the transient overvoltage across the converter valve in the hybrid ultra high voltage direct current(UHVDC) transmission system in which the rectifier adopts Line commutated Converter(LCC), and the inverter adopts voltage Source Converter(VSC). The arrester configuration for hybrid UHVDC transmission system is proposed, and the electromagnetic transient model of hybrid UHVDC transmission system is built. Based on the model, the overvoltage and arrester stress across valve in the hybrid UHVDC transmission system are obtained in the conditions of three-phase grounded fault at the ac system, single-phase grounded fault at valve side of upper bridge converter transformer and dc line grounded fault. It shows that the overvoltage and the stress of arresters across converter valve is very large in the conditions of single-phase grounded fault at valve side of upper bridge converter transformer and it is a decisive case for the overvoltage calculation of converter valve and the stress design of the arrester across the converter valve. Moreover, the effect of protection strategy on overvoltage is studied, and the investigation shows that the MMC converter adopts the strategy of controlling the DC voltage to zero, which can reduce the stress of arrester across the converter valve in the condition of single-phase grounded fault at valve side of upper bridge converter transformer. The study can provide the basis for insulation coordination of hybrid UHVDC transmission system.