With the development of high voltage direct current (HVDC) transmission technology in China

parallel converters are drawing ever-increasing attention to their structure. The HVDC transmission system with parallelly connected converters is particularly advantageous when the transmission is built by stages or in the de-icing operation mode. The characteristics of a 4-terminal HVDC system with parallel converters are analyzed at length. A new multi-terminal dc system coordinated mode is proposed for rectifier control of DC current and inverter control of DC voltage. And an additional special DC balancing control function is applied to avoid an unambiguous operating DC voltage. This coordinated mode has not only retained the relative independence of the bi-parallelly connected DC system as best it can

but also it has simplified the control system design for the 4-terminal HVDC transmission. In addition

it is now possible to realize flexible power allotment between parallelly connected converters. De-blocking the second valve and blocking the first valve being of special interest for parallel operation

methods of doing this are also described. Finally

a case study of 4-terminal HVDC system with parallelly connected converters has validated the control strategies using PSCAD/EMTDC. This work is supported by State Grid Corporation of China.