Liberalization of electricity markets has brought focus on the optimal use of generation and transmission infrastructure. In such a scenario, where the power transmission systems are being operated closer to their critical limits, Ancillary Services (AS) play an important role in ensuring secure and cost-effective operation of power systems. Emerging converter-based HVDC technologies and integration of renewable energy sources (RES) have changed the power system dynamics which are based on classical power plant operation and synchronous generator dynamics. Transmission system interconnections between different countries and integrated energy markets in Europe have led to a reduction in the use of energy from non-renewable fossil-based sources. This review paper gives an insight into ancillary services definitions and market practices for procurement and activation of these ancillary services in different control areas within the European Network of Transmission System Operators for Electricity (ENTSO-E). The focus lies particularly on ancillary services from HVDC systems. It is foreseen that DC elements will play an important role in the control and management of the future power system and in particular through ancillary services provision. Keeping this in view, the capability of HVDC systems to provide ancillary services is presented.

This paper proposes the Linear Swing Dynamicsbased Virtual Synchronous Generator (LSD-VSG) for HVDC systems. The aim is to define a new role and behavior for HVDC systems in their participation to AC system frequency support. This results in enhancing the frequency stability of HVDC connected-AC grids based on the LSD concept, whereas preserving a stable operation of HVDC system. Small signal stability and parametric sensitivity analysis have been performed to support the viability of the proposed LSD-VSG and study the sensitivity of the critical eigenvalues to the closed loop system states and corresponding stability region. Additionally, various time-domain simulations are performed and the results prove the effectiveness and features of the LSD-VSG.

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