Research On Operating Characteristics And Stability Of Intensive Multi-Send HVDC Systems

Appliction of HVDC technology eases the contradiction of electricity production and consumption, but it casues many stability problems for the grid. The maximum transmission power of HVDC system is limited. The greater the transmission capacity, the better stability of rated running point is. In terms of HVDC transmission capability, many researches have been achieved. The HVDC transmission capability can be restricted by the strength of AC system connected with the converter station, so short circuit ratio of AC bus, which is known as SCR, becomes an important quantitative indicator to evaluate the voltage support capability and accepting DC poewr capability of the AC grid.Compared with receiving end, the network structure of sending end is weaker, as it usually consisits of some power stations and converter stations. What’s more, once sending end becomes multi-send HVDC system, converter stations usually distribute in narrow areas with rich resources, which leads to closely electric coupling and strong interact effect among the converter stations.The present study generally believed that the DC system requirement for AC system strength is more outstanding when at inverter runtime, therefor, researches on HVDC transmission capability often use inverter stations as research object.Aiming at the problem that current research on multi-send HVDC system stability lacks, this paper analyses the characteristics of HVDC rectifier and inverter connected to various strength AC system. It is pointed out that when the ratio of SCR between rectifier and inverter reaches a specific value, the transmission capacity of HVDC system is limited by the specific side. What’s more, this paper puts forward multi-send interaction factors and multi-send short circuit ratio index to evaluate stability of multi-send HVDC system, and the effectiveness of the indicators has been proved by simulation. Based on the quantitative relationship between maximum value of transmission power and SCR, this paper defines the SCR ratio of rectifier and inverter under the same maximum value of transmission power as the critical ratio of SCR. Study in this paper shows that the strength of both inverter and rectifier sides would affect the transport capacity of HVDC system. When the ratio of SCR between rectifier and inverter is less than a threshold value, the rectifier will limit the maximum value of transmission capacity.