Analysis of steady-state power transfer capability and dynamic performance of VSC-HVDC with impedance-compensated synchronisation method connected to weak AC grid

This paper investigates the steady-state power transfer capability and dynamic performance of the voltage source converter high voltage direct-current (VSC-HVDC) system connected to a weak AC grid that relies on the impedance conditioned phase locked loop (IC-PLL). The IC-PLL has recently been proposed to address the issue of synchronisation with weak AC grid by supplementing conventional synchronous reference frame phase locked loop (SRF-PLL) with additional virtual impedance term. The resulting IC-PLL aims to synchronise the converter to a remote and stronger point in the grid, hence increasing the upper bound on the achievable power transfer achieved by the VSC converter connected to the weak grid. The effectiveness of this approach is examined by comparing its steady-state power transfer capability and the dynamic performance with that of VSC-HVDC that relies on conventional SRF-PLL based synchronisation method. The small signal model is used to examine the stability limits in the time domain and the dynamic performance in the frequency domain. The results indicate that VSC-HVDC system utilizing IC-PLL synchronization method is capable of recovering theoretical maximum steady-state power transfer capability even in the presence of weak grid connection. Furthermore, this recovery is possible for the range of values of virtual grid impedance. Therefore, accurate estimate of the true grid impedance is not necessary in order to reach theoretical maximum steady-state power transfer capability. However, it is shown that dynamic performance of the overall VSC-HVDC system defined in terms of active power tracking does improve in terms of both reducing oscillations and speeding up response time if the grid impedance is accurately estimated and used by the IC-PLL synchronization method.