This paper investigates two novel control strategies that enable system inertia supports by permanent magnet synchronous generator (PMSG) wind turbines during transient events. The first strategy seeks to provide inertia support to the system through simultaneous utilization of dc-link capacitor energy, and wind turbine (WT) rotor kinetic energy (KE). The second strategy supports system inertia through orderly exerting dc-link capacitor energy of WT and then WT rotor KE via a cascading control scheme. Both strategies can effectively provide system inertia support by fully utilizing WT's own potentials, while the second strategy distinguishes itself by minimizing its impacts on wind energy harvesting. Case studies of one synchronous generator connected with a PMSG-based WT considering sudden load variations have been studied to validate and compare the two proposed strategies on providing rapid inertia response for the system.

Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

Converters of variable speed wind turbines in a wind power plant may possess different levels of reactive power capacity under variable power generation of each WT due to wake effects. This letter proposes a variable droop gain control scheme that seeks to mitigate voltage fluctuations at point of common coupling (PCC) by fully utilizing the voltage regulation capability of each WT converter. Droop gain of voltage controller in each WT converter is adaptively adjusted based on its current maximum reactive power capacity so that WT converters at back rows that have higher reactive power capacity can contribute more to PCC voltage regulation.

It is of great significance for voltage source converter (VSC) based multi-terminal DC (MTDC) grid to isolate the DC fault lines within several milliseconds after DC fault. Existing protection schemes and fault analysis methods are mainly based on the numerical simulations, which are lack of the theoretical analysis. In this paper, a high-frequency equivalent model of VSC based MTDC grid that is utilize for initial DC fault current calculation is firstly proposed. In the proposed model, the parallel connected capacitors of VSCs are regarded as short-circuited, and the initial fault current calculation of the fault DC line and the healthy DC line can be reduced as a simplified RL and RLC circuit. Accordingly, a novel DC fault detection method for VSC based MTDC system is further proposed. In the proposed approach, the primary detection utilizes the transient average value of line current and the fault DC line can be identified quickly with only one-end information. In addition, the errors between the transient fault current calculation based on the distributed parameters line model and the lumped parameters line model are also evaluated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for initial DC fault analysis of VSC-MTDC system and the proposed protection scheme is effective under different fault locations and high fault resistances. Compared with the traditional protection schemes based on rate of change of current (ROCOC), the proposed one requires relatively low sampling frequency, low computation burden, and has high fault resistance tolerant ability, and high robustness with respect to the data missing.