Adaptive Virtual Inertia Provision for AC and MT HVDC Grids Based on Converters' Capabilities

Abstract

This paper presents a novel perspective on providing adaptive virtual inertia (AVI), aimed at improving DC voltage stability in Multi-Terminal High Voltage DC (MT-HVDC) grids while simultaneously enhancing frequency response in AC grids. The proposed approach introduces an innovative Virtual Synchronous Generator (VSG) that supplies AVI for the AC systems. Additionally, a new control strategy for the Power Electronics Converters (PECs) that supply the MT-HVDC grid is presented, referred to as dcVSG, to provide AVI for this grid. Utilising both controllers concurrently enables adaptive and simultaneous virtual inertia provision on both DC and AC grids, while effectively leveraging the operational capabilities of the PECs. In this regard, the DC voltage and the AC grid frequency are considered as control parameters. The AVI is dynamically adjusted according to the PEC operating point. Specifically, the calculated maximum AVI is sensitive to the increase and reduction of the control parameter, demonstrating appropriate distinct values in response. This behaviour aims to utilise the PEC's maximum power capacity. The small-signal stability of the proposed system is analysed by focusing on the influence of virtual inertia on overall stability. Also, to assess the stability of the proposed controllers, Lyapunov stability theory, alongside a series of detailed simulation analyses, is conducted utilising the Cigre-DCS3 test grid. The simulation outcomes indicate that the proposed coordinated strategy yields a 20% reduction in DC voltage deviation while also enhancing frequency nadir. Additionally, it achieves over a 60% decrease in the rate of change of voltage (RoCoV) on the DC side and a 68% reduction in the rate of change of frequency (RoCoF), specifically when compared to methods that rely solely on the headroom power of the PEC to deliver maximum virtual inertia.