Research On Stability In Multi-send HVDC Systems

The reverse distribution of power production and consumption in our region has laid an important position in the High Voltage Direct Current(HVDC)transmission.Compared with AC/DC systems with single-HVDC transmission or multi-infeed HVDC transmission,multi-send direct current(MSDC)system's sending sides are concentrated in energy-intensive areas,thus the electrical distances of the DC rectifier are close.Besides,the grid structure of the sending system is single and weak,which makes the interaction between AC and DC to a greater extent affect the stability of the MSDC system,one HVDC operation stability may be affected the stability of other HVDC,even the entire MSDC system.Therefore,it is of great significance to study the stability of MSDC system,and it is urgent to put forward the index to measure the system strength of MSDC system.The existing research on multi-DC coupling and the stability of AC-DC systems are aimed at multi-infeed direct current(MIDC)transmission system.While this paper deduces from the perspective of equivalent power,for the first time,the general formula and simplified formula for multi-send short circuit ratio(MSSCR),which accounts for the coupling between multi-dc projects,is proposed,in order to evaluating the system strength in MSDC transmission system.Then,considering the control characteristics of rectifier in sending-side and combining the maximum power curve method,the criticality of MSSCR is proposed.Multi-send critical short circuit ratio(MSCSCR)is deduced by the quasi steady-state equation of rectifier,to determine the MSDC system is strength or not,and the influence of each parameter in the system on MSSCR and MSCSCR is studied,the parameter setting suggestions are given.After that,the over-voltage level and resonance in the system are effectively evaluated by using MSSCR and MSCSCR,and a reasonable range is provided for the strength requirements of the MSDC system as reference.Finally,the PSD-BPA data of Yunnan Power Grid,which contains 7 HVDC transmission,is used as the simulation test object to validate the conclusion of the stability judgment index MSSCR I proposed in this paper.The simulation and calculation of one DC blocking on the other DC project operation to support the conclusion of evidence.The simulation results show that the proposed MSSCR can well characterize the mutual coupling relationship between multiple DC transmission,and evaluate the static stability of MSDC system also.