| With the vigorous development of new energy power,the installed capacity of wind power,photovoltaic is constantly improving.Northwest China,represented by Qinghai Province,gradually presents the characteristics of ultra high proportion of new energy.China’s new energy center and load center do not coincide.There is an urgent problem that how to send a large number of new energy power safely,efficiently and economically.New energy has the characteristics of volatility,randomness,and a large number of power electronic equipment,which brings great challenges to the selection of transmission topology,the design of control strategies,and the safe and stable operation of the system.In view of the above three problems,the main contents include:The feasible topology for the collection and delivery of ultra high proportion of new energy is proposed,and the technical and economic analysis of the AC collection mode and the VSC-HVDC collection mode is carried out.From the technical point of view,a simulation was established to analyze the power quality of the two collection methods under the influence of output fluctuations,grid-connected capacity,line distance and system strength.The results show that when the grid connection conditions are bad,there are some limitations in AC collection mode.From the economic point of view,the equivalent distance of AC and VSC-HVDC is calculated by using the annual fee method,which provides a basis for the topological selection of new energy collection.Based on the proposed topology,a stable operation control strategy is designed.The mathematical model and station-level control of VSC-HVDC and LCC-HVDC is analyzed,and the basic control strategy for the system according to the demand of collection and delivery is designed.In PSCAD,an example of ultra high proportion of new energy collected by hybrid mode and delivered by LCC-HVDC is established which verifies the effectiveness of the control strategy.The stability of the weak sending-end system with ultra-high proportion of new energy is studied from two aspects:the static voltage stability and transient overvoltage characteristics of the sending-end converter bus.From the perspective of static voltage stability,using sensitivity analysis,the factors affecting the voltage stability of the converter bus are deduced.Aiming at the problem that short circuit ratio and effective short circuit ratio can not reflect the critical steady state of voltage under the scenario of ultra high proportion of new energy collection,the sending-end static voltage stability index(SSVS)index is proposed,and the effectiveness of SSVS is verified.Using SSVS,the voltage support effect of VSC with constant reactive power control and constant AC voltage control is analyzed.From the perspective of transient overvoltage,a fast analysis method of overvoltage amplitude caused by commutation failure is proposed.Compared with the existing calculation formulas,it is found that the accuracy of the proposed method is higher and the adaptability is better,especially in the weak system.In order to improve the transient characteristics of the sending end,prevent the high-voltage disconnection of the new energy power stations,and give full play to the over-current and over-voltage ability of VSC,an active voltage support control strategy is designed,and the effectiveness of the control strategy is verified. |
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