DC filter protection in HVDC systems

The protection of DC filter is important in the inversion situation in HVDC systems. The characteristics of a DC filter and the influence of different working situations to the DC filter protection are analyzed, such as the floating harmonic current, transient caused by control system and external fault, etc.. The integrated scheme and essential principle of the protection of the DC filter are proposed. Emphases are put on the differential protection and the unbalance protection. In order to enhance the sensitivity of the differential protection, the through current is selected as restraint current. In addition the floating restraint current and changed ratio action characteristic are introduced to improve the reliability of differential protection. The transient unbalance protection and the adaptive compensated summation current of capacitance are also used to enhance the sensitivity and reliability of unbalance protection. Finally all the principles are verified by laboratory tests and real operation of power systems.