OLT Analysis and Protection Control Design for ±800kV UHVDC System

In this paper, based on basic protection and control principle of the open line test in UHVDC transmission system, the influences of the variation of firing angle of the converter on open line voltage is analyzed in detail, when the firing angle of the converter is reduced to less than 60°, the DC voltage can be still risen to its rated value in the condition that the intensity of the corona discharge in DC transmission line is not broken. In the practical engineering test, when the firing angle of the converter is reduced to 45.1°, the open line voltage can be risen to the rated value 800kV. At the same time, we can know that the current protection principle with line open circuit test can only use direct current and differential current more than 0.025pu as the main criterion.