A Coordinated Control Strategy of LCC HVDC Systems for Frequency Support with Suppression of AC Voltage Fluctuations

In this paper, a new control method for a line-commutated converter-based (LCC) high-voltage direct current (HVDC) system to support the frequency and AC voltage was proposed. For coordinated control of the inverter and rectifier, droop-based controllers are adopted for the outer loop of conventional controllers, and DC voltage is used to transfer the frequency information so that communication is not required. A method to determine the droop constants of the proposed controller is presented to emulate the conventional $f$ / $P$ droop characteristic and suppress the AC voltage fluctuations. Furthermore, a small-signal state-space model of the LCC HVDC system with the proposed scheme is derived, and root locus analysis is carried out to evaluate its stability. The effects of the proposed method are verified based on case studies, using the well-known CIGRE BENCHMARK model. The advantages of the proposed method can be summarized as follows: 1) system reliability is enhanced because there is no risk of communication failure during frequency information transfer and 2) voltage stability at the inverter side is enhanced as AC voltage fluctuation during frequency support is minimized.