# Modelling, Analysis and Performance of a Low Inertia AC-DC Microgrid
###### Abstract
In [17], the sizing of the battery system required to emulate the virtual inertia is studied and a guideline was provided on how to match the VSM parameters with the energy storage system. High-power-density capacitors or ultracapacitors enable rapid charge and discharge. Because of the differences in the properties of batteries and capacitors [18], they may both be used in VSMs, where batteries counterbalance the long-term power needed by the droop control. Capacitors/ultracapacitors can support short-term power needed in inertia emulation; however, the emerging technologies of the ultracapacitors have yet to be commercialized [18]. Although the VSMs that adopt hybrid energy storage systems have been validated, they still need authorization to be utilized in the power grid. As a result, the VSM is an emerging rather than a tested and commercialized approach. Although employing synchronous condensers linked to the grid is a well adopted approach, it is only a second option for inertia augmentation owing to the high costs [19]. The DC grid poses a high potential for frequency support. DC grids can control and integrate distributed energy resources into power networks and provide services to AC systems, such as voltage and frequency support. This can be accomplished by regulating the interconnecting converters and the connected distributed energy resources. At the high and medium voltage levels [20], offshore wind farms are connected to the AC grids through high voltage DC (HVDC) and multiterminal DC (MTDC) networks based on VSC technology to increase operational flexibility and supply security [21]. On the other hand, adding more wind farms leads to a reduction in inertia. In [22], an adaptive droop control strategy is adopted such that the VSC-based (MTDC) method provides inertial support to the onshore AC microgrid system depending on the support from offshore wind turbines through an HVDC line. Numerous solutions were proposed in [22] for HVDC/MVDC systems to enable inertia emulation control algorithms, where the energy stored in the DC link capacitors was used to emulate the inertial response of a synchronous generator. Using the DC voltage of the MTDC grid, onshore converter stations can use wind farms' power for frequency support. A voltage-frequency droop was defined for offshore wind farm converter stations and an active power-frequency droop for doubly fed induction generator (DFIG) converters. A communication-less approach which was previously implemented is used in [23]. However, it is used with communication which depends on a fibre optic cable to quickly communicate the frequency errors to the offshore converters. In high voltage direct current (HVDC) transmission systems, the transients are often separated intentionally to contain instabilities within one system and prevent instabilities from affecting other connected systems. In [24], a novel approach is presented to utilize the capacitance of the HVDC lines to provide inertial support by allowing a permissible variation in the DC voltage. Another adaptive technique which did not need communication is presented in [22]. At the low voltage level, the DC-AC converter was controlled as a virtual synchronous machine, providing inertia during the AC and DC side disturbances and offering reserve to the grid [25]. Additionally, a decentralized droop control technique was employed for rapid active power regulation and increased system frequency support within the DC microgrid in [26]. Moreover, a virtual admittance approach was used to improve the voltage profile in the local DC microgrid during AC frequency DC fault events [26].Various studies focused on control techniques for DC grid characteristics such as power-sharing, local voltage stability and energy management within microgrid bounds. Wind turbines, photovoltaic (PV) and energy storage were previously explored to provide frequency support [27].