Short-Term Frequency Regulation and Inertia Emulation Using an MMC-Based MTDC System

The continuous displacement of conventional power plants by converter-interfaced generation systems reduces the system inertia and may lead to a degradation of the frequency stability. A supplementary control strategy for multi-terminal dc (MTDC) systems to improve the frequency regulation is presented in this paper. The scenario where the MTDC system cannot resort to the power reserves of either other ac systems or offshore wind farms is particularly studied. The proposed control is able to independently support both the primary frequency regulation and the system inertia by using the capacitive energy of the MTDC grid converters. On the other hand, the reactive power of the converter stations is coordinately controlled to improve the damping of low-frequency electromechanical oscillations. The simultaneous frequency support of the MTDC system and onshore wind farms is also studied and validated in a practical multi-machine power system. Transient and small-signal stability analyses are performed to evaluate and quantify the improvements and the control limitations of the proposed scheme. This enhanced frequency control can increase the system reliability and reduce the required amount of spinning reserve, diminishing the operation costs and allowing to increase the penetration of renewable energy sources.