# Improved Control Strategy of MMC-HVDC to Improve Frequency Support of AC System
## 1 Introduction
The authors in [13] proposed a method for maintaining the system frequency through coordinated control of the DC voltage. A similar power synchronization control is adopted to regulate the fault ride-through capability into a combined power grid of offshore wind power plants and VSC-HVDC [14; 15; 16]. But the proposed method increases the communication risk at both converter sides, which makes the control method more complicated. As mentioned in [17; 18], a new method called inertia emulation control is used to help the VSC-HVDC evaluate the inertia of the synchronous generator (SG) when faults occur and the load is changed. This method mainly uses DC link capacitors. Because of capacity limitations, compensation capabilities are limited. The authors in [19] proposed inertial and primary frequency support to control the frequency of the VSC-HVDC and wind turbines. The majority of modern modular multilevel converter high-voltage direct-current systems (MMC-HVDC) use half-bridge modeling with a DC breaker. The detailed MMC-HVDC model in [20; 21] uses average-value models, which can simplify the calculation. In [4], a high-level capacity of a type-4 wind turbine farm replaces the traditional SG and penetrates the system, resulting in a reduction in the available inertia of the system and oscillation of the system frequency stability. In this paper, an improved inertia control method is presented to evaluate the frequency stability of a hybrid system consisting of a full-scale-converter-wind-turbine (FSCWT) farm connected to an onshore AC system via an MMC-HVDC. The derivation of the improved control method is based on a synchronous-generator inertial mathematical model and hybrid AC/DC mathematical model. The inertia index is based on the inertia emulation control presented in [16; 17]. First, it uses the capacitor of the MMC-HVDC sub-modulation instead of the DC link capacitor to provide the virtual inertia constant without capacity limitation. The index is also calculated based on the effective short circuit ratio (ESCR) of the AC system. An improved method can aid the AC system in maintaining its frequency stability. The index is intended to provide a simpler and faster calculation method for the system inertia constant. As the strength of the onshore AC system is different, this index will provide the minimum inertia constant required for system frequency stability. This function eliminates the inconvenience caused by other auxiliary measurement methods. The rest of this paper is organized as follows. Section 2 presents the inertia of the power system. Section 3 describes the mathematical model of the MMC-HVDC, and the new index is calculated to evaluate the stability of the hybrid AC/DC system according to the ESCR of the AC system. Furthermore, in this section, the advantages of the proposed method are considered. The obtained results represent the stability of the entire system under AC faults or a small disturbance, and the verification of the correctness of the method is presented in Section 4. The conclusions of this study are presented in Section 5.