According to the data of energizing unloaded converter transformer

the precharge of converter transformers

which is caused by grading capacitor of double-break circuit breaker

affects the amplitude of magnetizing inrush current

whose effect law needs to be studied.Consequently

firstly

the characteristics of grading capacitor and the flux linkage performance of transformers during precharge and breaker closing are analyzed theoretically.Secondly

a simulation model was built on PSCAD/EMTDC based on the parameters of the DC project

including the double-break circuit model and converter transformers model.Meanwhile

the influence of grading capacitor on magnetizing inrush current of the converter transformer was analyzed from three aspects

including the voltage at the grid side

the constant value

and the amplitude of inrush current.The results show that the amplitude of voltage at the grid side and its increase rate will increase with the increase of the grading capacitor value when the converter transformer is energizing unloaded; the value of time constant will also increase with the increase of grading capacitor value while its changing rate will slow down

and the amplitude of inrush current will decrease with the increasing grading capacitor or the lengthening duration of precharge.Therefore

for the purpose of weakening the amplitude of inrush current

it is suggested that operators should control the time between dis-connector closing and double-break circuit breaker closing on the basis of the time constant of converter transformers during precharge.