This paper analyses the reason of open line voltage protection when an open line test was executed in Longzheng HVDC.The main reason is that the open line voltage formula in Longzheng HVDC can only be applied to the ideal situation when the firing angle of the converter is less than 60° and the valve is not connected to the DC transmission line.The marginal time constant of open line test can be calculated according to the structural parameters and the insulation requirement of DC transmission line on the basis of succeed open line test.The threshold of open line voltage protection
under satisfying the sensitivity requirement
can be set which can improve the HVDC reliability.