| Renewable energy has become a major strategic move to transform the world's energy and to meet its climate change goals.The basic trend of global energy transformation is to achieve the transformation of fossil energy system to low-carbon energy system,and finally enter the renewable energy-based sustainable energy era.For our country,the domestic power grid is relatively mature,but there are still some difficulties in admitting large-scale new energy sources to access the power grid,and it is also unable to meet the requirements of large-scale transmission of new energy power.Due to its flexible operation control and high intelligence,flexible DC transmission can enhance the stability of power system,enhance the system's ability to absorb clean energy,improve the reliability and flexibility of distribution network,Grid interconnection,island power transmission and power supply in the big city center has been widely used.At present,many ends and flexible multi-terminal HVDC projects have been put into operation at home and abroad.At the same time,the successful development of DC circuit breakers makes it possible to build a DC grid.In this paper,the flexible DC system model is studied first,including its topology,AC system grid,DC system architecture,control system and modulation strategy.Among them,the basic control system of DC system includes the pole control layer and the bipolar control control layer.The pole control layer includes the outer loop power control,the inner loop power control,the inner loop current limit and the capacitor voltage balance control.The control system also includes the system's circulation suppression and start-up control.Based on the actual parameters of the project,this paper builds a detailed flexible HVDC transmission system model on the power system simulation program PSCAD/EMTDC simulation platform,including AC and DC system modeling,control system and modulation strategy function realization.A novel rapid simulation model based on Thevenin's equivalent MMC is established.The hundreds of sub-modules on a single bridge arm are simplified as two components on the upper and lower arms,which greatly reduces the order of the equations to be solved during electromagnetic transient calculation.This significantly increases the simulation speed.Based on the model of the flexible HVDC transmission system,the system simulation analysis was carried out.Realizes the simulation of MMC type soft and straight system under many working conditions.The type of system fault is divided into AC side fault of converter transformer,AC side fault of converter transformer and DC fault according to the location of fault in the converter station.Among them,DC faults are divided into unipolar grounding short-circuit faults in DC lines,inter-electrode short-circuit faults in DC lines,and ground fault short-circuit faults in the valve side of the DC reactor.Analysis of the above-mentioned fault caused by the system operation over-voltage and the corresponding energy received by the corresponding arrester,the calculation results for the converter station related equipment selection,test provides a basis.Finally,DC circuit breakers are installed on the DC line.When a short-circuit fault occurs on the DC line side,a DC overvoltage appears on the DC output port of the faulty converter,and corresponding suppression measures are proposed.This DC overvoltage will only occur in the installation.The DC circuit breaker in the DC grid has a great engineering reference value. |
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