A Suppression Method of Harmonic Instability in Line-Commutated Converters Applying Active Harmonic Filters

Harmonic instability of line-commutated converters (LCC) may cause restriction in grid operations or increase of running costs. Recently, voltage-source converters (VSC) are applied to HVDC transmission system. Some of them are installed neighboring to the existing LCCs. In such case, VSC’s fast control characteristics can be utilized to contribute to LCC’s stable operation. This paper proposes a novel solution of harmonic instability in LCCs, where adjacent VSC is operated as an active harmonic filter. The VSC improves the high system impedance resulting from parallel resonance in the ac grid. The authors analyze and demonstrate its effectiveness to the harmonic instability based on a stability criterion and electromagnetic transient (EMT) simulations. The required compensation current for the active filter is much smaller than the rated current of the LCC. Moreover, it is confirmed that the active filter control can be applied to normal VSC based BTB, HVDC, or STATCOM systems as an additional function without interrupting their original functions.