Evaluation of inertia emulation strategies for DC- Voltage-controlled and power-controlled converter stations in HVDC interconnections

This paper presents an evaluation of two control schemes for virtual inertia support from the converter terminals of an HVDC interconnection. A point-to-point interconnection between asynchronous power systems is assumed, where one terminal controls the dc-side voltage while the other controls the power flow. A virtual synchronous machine (VSM) based on a simulated swing equation is considered for the power-controlled terminal, but this approach is not directly applicable to the dc- voltage-controlled terminal. Thus, an alternative control strategy is presented and evaluated, where the capacitive dc-side voltage dynamics are utilized to emulate the swing equation dynamics for inertia emulation while ensuring regulation of the steady-state dc voltage. Simulation results are presented to illustrate how these control strategies can provide inertial response at both converter terminals and how this inertial response influences the dynamics of the dc voltage and the frequency of the ac grids. The presented schemes for inertia emulation are assessed and compared against conventional control strategies for power and dc-voltage control with grid synchronization by a phase locked loop. Moreover, the differences in the response of the two control schemes to grid frequency transients are highlighted, demonstrating the inherent limitation of energy available for inertia support from the dc- voltage-controlled terminal without a coordinated response from the power-controlled terminal.