# Improved Control Strategy of MMC-HVDC to Improve Frequency Support of AC System
## 1 Introduction  
In recent years, with continuous attention focused on energy issues, the traditional power grid is facing a major challenge of renewable energy generation. In Korea, with the promulgation of the \"2030 renewable energy\" plan, the power generation of renewable sources will reach 20% of the total power generation in 2030, as proposed in the plan. The penetration level of renewable energy sources will increase to a large extent in the next 10 years, that is, it will penetrate the traditional power grid. In particular, maintaining the stability of the power system despite increase in the penetration level of offshore wind turbines is a significant challenge. Type 4 (full-scale converter wind turbines, or FSC-WTs) is a variable-speed wind turbine that is widely used for offshore wind-power generation. However, extensive type-4 penetration into the power system causes the onshore-power-grid inertia to decrease, debilitating the onshore AC power grid and making it difficult to maintain its frequency stability. As the type-4 wind turbine has a series connected to a back-to-back converter, in severe cases, it may cause power outages [1]. In a previous study [2], it was proposed that the type-4 wind turbine be integrated with a weaker grid because the phase-locked-loop parameters may cause low-frequency and synchronous-oscillation problems, as proven via dynamic modeling and eigenvalue analysis. In [3], it was recommended that the systeminertia be controlled by measuring the DC voltage of the converter instead of using the conventional method of measuring the system frequency. This method was verified using the hardware-in-loop method. The authors in [4] suggested that the inertial response constant of a single wind turbine was not the inertial response constant of the wind power system; this was verified using field measurements and simulations. In [5], the author explained in detail the calculation method of the inertia constant of the wind turbine with a formula. The authors in [6; 7; 8; 9] presented several different frequency control strategies for the entire power grid, which consisted of offshore wind power plants and line-commutated converter high-voltage direct-current transmission systems (LCC-HVDC). A control strategy that controls the grid frequency instead of reactive power control is used to control the firing angle of the HVDC rectifier or the DC-link current in [6] because of the reactive power controlled by the rotor-side converter and rectifier converter. A coordinated control strategy provides primary frequency control for the grid, which consists of the LCC-HVDC and offshore wind turbines [7]. The authors in [8] state that the modeling of the weak grid is highly sensitive to the effective DC inertia constant and effective short circuit ratio, which connect to the LCC-HVDC. The authors in [9] presented a full-order and four-state model to verify the frequency droop control strategy proposed in this paper, which can improve the frequency response speed of the weak grid. The voltage-source converter HVDC (VSC-HVDC) has more advantages than the traditional HVDC system. As is well-known, the LCC-HVDC is required to consider commutation failure, especially in the case of weak AC grids, and this may be more likely to cause grid failures. The short board of LCC-HVDC is mentioned in [10]. Compared with VSC-HVDC, LCC-HVDC has a high risk of the stability. The VSC-HVDC is not required to consider commutation failure and can compensate for a weak AC grid owing to its good active-power and reactive-power regulation ability. In [11], a virtual impedance control strategy is proposed to improve the dynamic adjustment ability. Furthermore, the VSC-HVDC can also transmit energy to the system via the passive power grid, especially when the wind turbine is subject to external interference. In a previous study [12], the stability of the power system was verified by the root-locus method. The system consists of hybrid offshore wind power plants and VSC-HVDC.