  High-voltage direct-current (HVDC) systems for constant or intermittent power
delivery have recently been developed further to support grid frequency
regulation (GFR). This paper proposes a new control strategy for a
line-commutated converter-based (LCC) HVDC system, wherein the DC-link voltage
and current are optimally regulated to improve real-time GFR in both rectifier-
and inverter-side AC networks. A dynamic model of an LCC HVDC system is
developed using the DC voltage and current as input variables, and is
integrated with feedback loops for inertia emulation and droop control. A
linear quadratic Gaussian (LQG) controller is also designed for optimal
secondary frequency control, while mitigating conflict between the droop
controllers of the HVDC converters. An eigenvalue analysis is then conducted,
focusing on the effects of model parameters and controller gains on the
proposed strategy. Simulation case studies are also performed using the
Jeju-Haenam HVDC system as a test bed. The results of the case study confirm
that the proposed strategy enables the HVDC system to improve GFR, in
coordination with generators in both-side grids, by exploiting the fast
dynamics of HVDC converters. The proposed strategy is also effective under
various conditions for the LQG weighting coefficients, inertia emulation, and
droop control.
