On the Provision of Frequency Regulation in Low Inertia AC Grids Using HVDC Systems

This paper proposes an improved model of a high voltage dc (HVDC) link capable of providing frequency support to networks with no inertia or near-zero inertia; for instance, ac grids with small synchronous generators. The model is useful to carry out steady-state and dynamic simulations of ac grids described by their positive-sequence representation using phasorial information. The core of the frequency control scheme put forward in this paper uses the angular aperture that exists between the internal phase-shifting angle of the voltage source converter (VSC)-HVDC rectifier and the voltage angle at its ac terminal. This is amenable to power flow regulation in the dc link and, hence, to frequency control in the low-inertia grid. A feature of this model is that the developed VSC-HVDC link model may also be used to feed an island system; for instance, a system with dead load, where the inverter station provides the electrical angular reference. Hence, the inverter acts as a virtual synchronous generator with frequency regulation capabilities as seen from the low-inertia ac grid. The dynamic control scheme that enables the VSC-HVDC to provide frequency control in such operating environments has been comprehensively investigated under a wide range of credible scenarios. Overall, the dynamic system of equations describing the VSC-HVDC and the synchronous generators are discretized using the trapezoidal rule, and the ensuing equations are combined together with the algebraic equations of the ac and dc grids in a linearized reference frame amenable to iterative solutions using the Newton-Raphson method.