Modeling and analysis of DC-link harmonic instability in LCC HVDC systems

Harmonic instability may occur in high-voltage dc (HVDC) links due to dynamic interactions between HVDC terminals and the impedance of the dc lines or cables. Such instability has been studied for HVDC systems based on line-commutated converters (LCC) by using state-space models. This paper proposes a different approach using the dc terminal impedance of the HVDC converters. The small-signal output impedance model of the LCC rectifier and the input impedance model of the LCC inverter are developed by harmonic linearization while taking into account the converter-level control, ac and dc filtering elements, as well as the impedance of the ac grid at each HVDC terminal. An impedance-based stability criterion is then applied to determine the stability of the dc link under various control designs. The effects of different control functions on the dc link stability are also studied based on the impedance model to provide guidelines for converter-level control design.