HVDC converters impedance modeling and system stability analysis

Abstract

Harmonic linearization is used as the fundamental tool to model the sequence impedances of MMC and LCC as well as the frequency cross-coupling terms. Different from two level-VSC, the steady-state operation of MMC and LCC contains significant steady-state harmonics and linearization must be performed upon the harmonic trajectories. Finite number of MMC steady-state harmonics are modeled using harmonic matrices. In addition, the MMC impedance models capture the dynamics of arm capacitor voltage and circulating current. Double Fourier series is utilized to model the infinite number of steady-state harmonics generated from LCC operation. Moreover, the phase control of LCC is modeled and included in the impedance models. The stability of the offshore ac collection bus is studied from sub-synchronous to super-synchronous frequency range. Different control designs are proposed to mitigate the potential of resonance and instability. In the two-level VSC system, both the HVDC rectifier control and the wind inverter control are leveraged to enhance the system stability. In the MMC system, the internal circulating current control is found to be significant to its output impedance responses and its damping effects on the system stability are presented. In the LCC system, the stability of ac bus is improved through the modification of the STATCOM voltage control. The frequency cross-coupling effects are qualitatively analyzed and found not to affect the stability analysis.

High voltage direct current (HVDC) transmission has been used in power systems for decades. There are three types of converters used in HVDC systems: line-commutated converters (LCC), voltage source converters (VSC), and modular multilevel converters (MMC). Recently, HVDC has become attractive for power transmission from offshore wind farms to onshore grids. The control of the offshore ac collection bus creates potential instability and resonance issues in a wide frequency range. The impedance-based stability theory has been proven to be a useful tool for analyzing and solving such problems This thesis presents the impedance modeling and system stability analysis for the three types of HVDC converters.