AC filter for UHVDC transmission system is used to filter harmonic generated by the converter and to compensate the reactive power of the converter which is absorbed during the operation. For assuring safe and reliable operation of AC filter

it is usually equipped with unbalance protection of high voltage capacitors. Based on the unbalance protection principle of high voltage filter capacitor

the calculation formula of unbalance protection setting is derived. The simulation model in PSCAD/EMTDC is set up in accordance with parameters of equipment for certain UHVDC transmission project

such characteristic as overvoltage

unbalance current and total current flowing through filter of the elements of filter capacitor after breakdown is studied. On the premise of effectively clearing faulty element of inner fuse

it is shown by both theoretical analysis and simulation calculation that the ratio K of unbalance current of HV capacitor to the total current of AC filter increases with the increase of element numbers and the calculation of both is in agreement. Finally

the simulation analysis and the breakdown fault record of the capacitor is compared and analyzed with combination of AC filter switching fault of UHVDC transmission project. It is shown by the result that the value K obtained from analysis of site fault record and the analysis result is in agreement

which proves further the effectiveness of the simulation model.