## 2 Overview of Available Countermeasures to Support Low Conventional System Inertia  
With each passing year, an increasing number of high-voltage direct current (HVDC) systems are being installed in the electrical power system. The increasing integration of HVDC systems into the electrical power transmission system is reducing CSIs, but efficient SI recovery technologies are not currently available in practice. Research has been ongoing for several years in an attempt to find a simple and effective solution. The articles describe many different methodologies used for the development of SI algorithms that are well established but difficult to apply in practice. Some offshore wind farms are connected to the electrical power system via HVDC systems; therefore, one of the solutions described in the literature uses the SI algorithm and exploits the capabilities of HVDC, which allows the required amount of power to be supplied to the electricity grid at the initial time, coordinating the HVDC system and offshore wind farms [14; 15; 16; 17]. In [15], the authors proposed a method based on a VSC (voltage source converter)-HVDC transmission link to replicate the primary frequency response behavior of a traditional synchronous generator connected to a common coupling point (PCC). In [17], the author presents a solution in which the kinetic energy (KE) stored in wind power plants connected via HVDC is used to maintain the frequency and simulate inertia. The algorithm adjusts the power of the HVDC system based on the grid frequency derivative feedback. Another SI method discussed in [18] is based on a frequency input signal. First, the HVDC system regulates the direct current (DC) voltage to control the power supply from the direct current capacitor, accounting for the frequency drop of the alternating current electrical power system. The instantaneous SI response is from the HVDC system, and after some time, the active power is supplied by the offshore wind farm. In [19], the authors suggested the use of a DC-link capacitor to obtain SI output from renewable energy sources. This method provides SI control logic, in which the power source does not provide continuously active power, but it contributes frequency control by injecting active power proportional to the \(\mathrm{d}\omega/\mathrm{d}t\). In the event of a failure in the electrical power system and a change in frequency, the operation of these methods, in which SI is derived from HVDC at the initial time, may affect the rotor speed and generation efficiency of the wind farm, and it is difficult to quickly restore the nominal rotor speed. To develop SI compilation methods, many models have been adapted to the SI control loop. The principles of fixed PID (proportional/integral/derived) application models are applied in SI control loops [20; 21]. The KE of wind turbines is used to adjust the electrical power set point of the converter [22]. This method allows for the frequency nadir and the rate of change of frequency (RoCoF) to be reduced [23]. In [24], SI is defined as the process that controls the active power of a generating source in proportion to the measured RoCoF at the terminal of that generating source. Other SI modeling principles are based on one-loop and two-loop SI modeling methods. The additional active power provided by the one-loop modeling method is only proportional to the measured RoCoF [25; 26; 27]. To use both RoCoF and frequency deviation measurements, it is necessary to use a two-loop modeling method [28]. One-loop or two-loop modeling methods enable wind farms to participate in frequency control, reducing the RoCoF of the electrical power system and thus preventing under-frequency load shedding in the electrical power system. The advantage of the two-loop modeling method is that it allows for the supply of additional power, which is proportional to the frequency deviation in the electrical power system.