Research On UHV DC Converter Secondary System Surge Protection Measures

Lightning current through the lightning rod or lightning into the ground network will cause the ground potential rise,resulting in ground potential counterattack.Ground potential counterattack is easy to cause harm to the secondary equipment.There have been many DC blocking accidents due to the secondary system disturbed by lightning.Therefore,it is necessary to carry out research on the lightning protection.Firstly,the paper researched the structure,principle and characteristics of the discharge tube and varistor used in the low voltage arm of the DC voltage divider in the converter station.The advantages and disadvantages of series-parallel connection of discharge tube and varistor are analyzed.At the same time in order to understand the working condition of the discharge tube and varistor in the extremely cold area,the low temperature test of discharge tubes and varistors were done.Then the working condition of DC voltage divider is analyzed and the relevant transfer function is deduced.The transient response characteristic of DC voltage divider is expounded from three working conditions.Then the discharge tube and varistor simulation module are built by EMTP software,so we obtain the simulation model diagram of the voltage divider secondary circuit.It can be seen from the simulation that the voltage of the DC voltage divider low-voltage arm needs to go through a transient process of 100 milliseconds after the discharge tube is broken down.And this 100 millisecond recovery time beyond the DC under voltage protection setting,which will cause DC under voltage protection malfunction.So the surge protection measures of the DC voltage divider side need to be improved.Next,the paper introduces a combined model of discharge tube and varistor.The simulation results show that the voltage recovery time of the low voltage arm is shortened greatly when lightning causes the discharge tube to move,and the DC under-voltage protection can be prevented from maloperation.Finally,in order to verify the correctness of the chosen surge protection scheme,a test circuit was set up in the Electromagnetic Compatibility Laboratory of North China Electric Power University.The experimental results show that different types of combination of discharge tubes and varistors has a great influence on the residual voltage and voltage recovery time.Through comparison and analysis,the series combination which the recovery time is shortest,the recovery voltage is closest to the normal work voltage of low-voltage arm is chosen,and the series combination is applied to a converter station.