## 1 Introduction  
In the proliferation of renewable energy, worldwide total wind-power installations in 2017 were about 52 GW, bringing the global total to nearly 540 GW. The off-shore segment showed an 87% increase on the 2016 market, recording 4334 MW in a year [1]. The electricity generation by renewable sources is expected to take up 40% of total electricity in the U.S. by 2030, and approximately half of it will be generated by wind turbine generators [2]. For South Korea, renewable energy capacity grew by as much as 13,846 MW in 2016, and the Korean government put forward the 3020 Project, which presents its objective for the portion of renewable energy source penetration from 7% to more than 20% of total generation by 2030. This means approximately 60 GW of renewable energy facility is required [3]. Multimegawatt wind farms are an attractive choice for renewable energy generation options. In addition, off-shore wind farms are preferable to on-shore solutions, because larger and more constant wind energy potential exists off-shore. To bring generated electric power on-shore, HVDC solutions are clearly taking their place [4]. Recently, an HVDC transmission system based on a modular multilevel converter (MMC) has been widely used given its improved controllability, advantage on smooth waveforms, and high voltage possibility by its modular topology. Therefore, it is expected that long-distance off-shore wind farms will be connected to the mainland by MMC HVDC [5]. For reliable operation, wind farms should not compromise the grid stability. System frequency or voltage stability must be sustained even though small disturbance occurs. That is, the wind farm should not have lost its generation by a small disturbance. The objective of grid codes is to improve voltage and frequency stability by mandating wind farms to provide ancillary services [6; 7]. The grid codes require wind-energy sources to stay connected in voltage sags according to a guided time-voltage profile, named a \"low-voltage ride-through\" (LVRT) profile. To satisfy the LVRT code, DC overvoltage and overcurrent and off-shore AC grid stability issues should be addressed. There have been numerous studies conducted to mitigate these problems [8; 9; 10]. The most commonly-used strategies are the crowbar and the DC chopper. The crowbar can dissipate surplus power when a fault occurs with the rotor-side converter [8; 9]. A DC chopper can expend excessive energy by using a chopper resistor implemented in a DC link [10]. However, they are not necessary if the kinetic energy controls studied by [PERSON] et al. [11] are implemented. In the study of [PERSON] et al [11], if surplus energy was stored in a rotor mass, even in the absence of the crowbar or DC chopper, a wind farm could ride through on-shore faults. There are also prior research works on using the stored kinetic energy of a rotor mass [12]. Deloading of a fully-rated converter wind turbine as an FRT strategy was introduced by [PERSON] et al. [13], and they employed a wind turbine-connected full-scale VSC converter DC link as an energy storage component. To broaden the versatility, a DC link can operate as a temporary energy-storage device, but needs fast and reliable communication. Frequency was used as a signal medium in [PERSON] et al. [14], who achieved communication-free control by changing the reference frequency of an off-shore grid-forming converter. However, it used DC choppers to dissipate excessive energy generated from wind turbines. In [PERSON] et al. [11], energy was stored in a wind turbine rotor, but it also needed fast and reliable communication. In this paper, the proposed coordinated control strategy stores excessive generated power in two energy storage mediums when an on-shore fault occurs in both a VSC converter DC link and a wind turbine rotor, while signaling control references in frequency droop. The proposed control has the following contributions:  
1. DC chopper, crowbar-free operation by energy control in a DC link and a wind turbine. 2.