A Study Of Optimization For Discharge And Its Security Strategy Of A Long HVDC Submarine Cable

High voltage submarine cables are the main way of power transmission from offshore wind farms to the onshore grid.Nowadays,cross linked polyethylene(XPLE)is widely used as insulation material of high voltage direct-current(HVDC)submarine cables.DC withstand voltage test is one of the test items after XLPE DC cable installation.Fast discharge for a submarine cable should be carried out after the withstand voltage test due to the cable insulation damage caused by the test.In addition,there is a probability of falling off of the high voltage wire connecting the submarine cable terminal and the high voltage device during the process of withstand voltage test or discharge,causing direct ground discharge of high voltage devices,which is a security threat to the staff and devices at the test site.This paper takes the power transmission project of China Three Gorges Rudong offshore wind farm as the study object,and carries out a study of fast discharge strategy for the power transmission submarine cable with multi-section series resistor discharge device,and the security strategy at the test site during the discharge process.The main study work in this paper is as follows.An optimization model of fast discharge for the submarine cable is put forward.This paper proposes a discharge circuit variable resistance response model based on the first-order RC equivalent circuit of submarine cable discharge circuit,and gets the calculation method of voltage,current and power of discharge resistors in the discharge process.A temperature calculation model and its numerical difference scheme is also derived.The fast discharge process is instructed by the optimization method by calculating voltage,power and temperature of discharge resistors in advance.This method determines the short-circuit time of each section of resistors by calculating the status quantities of resistors takes of fast discharge under the constraints of temperature,power and voltage.The temperature of resistors is estimated with lumped parameter temperature calculation method to improve the calculation efficiency and meet the real-time requirement of fast discharge.This paper also proposes an optimization model of security strategy at the test site.The main risk factor at the test site is the step voltage distribution caused by the high voltage wire falling off in the discharge process.The step voltage distribution is the main analysis basis for the security strategy at the test site.According to the conditions of the test site,this paper puts forward a calculation model of electrical potential distribution and its numerical difference scheme,and the calculation of step voltage distribution is based on the potential distribution.The step voltage is the potential difference between two points.However,there is a multitude of combinations of two different points.Therefore,this paper puts forward a safe area delimiting method of test site.In this method the step voltage distribution is calculated with electric field strength in this paper.By dividing the test site into a sequence of small cells and calculating the upper limit of step voltage in each cell with its electric field strength distribution,the step voltage distribution is calculated,so as to divide the safe area of the test site.This method does not generate a large number of point combinations due to the difference of the number and positions of the points in the calculation of the step voltage by its definition formula,and reduces the calculation complexity while ensuring the accuracy of the delimitation of the safe area.Finally,this paper carries out verification experiments of the calculation models of discharge resistor temperature and test site potential distribution which is the basis of the optimization models of fast discharge and security strategy.Both calculation models are numerical calculation methods.By carrying out the verification experiments,the accuracy of those calculation models is verified by the comparing the consistency of the experimental and calculation results to ensure the validity of the models of fast discharge and security strategy.