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.

It is of great importance for Multi-terminal DC (MTDC) system to immediately isolate the DC fault line within several milliseconds after DC fault due to VSC or half-bridge based MMC converters with limited ability to control DC fault current during DC fault. Failing of the DC fault isolation during “Capacitor Discharging Stage” may lead to the overall MTDC system out of operation with the fast blocking of IGBTs. Therefore, initial DC fault current analysis is essential for system protection design, especially for fault detection. Accordingly, a transient simplified model is first proposed with only high-frequency components of the fault DC network reserved. As a result, the original complex DC network is largely reduced and the fault DC current with several milliseconds can be clearly expressed based on the proposed model. According to the distinct differences between the current of faulty cable and that of healthy cable, a non-unit DC fault detection method utilizing the transient average value of line current with low computational complexity and high-sensitivity is further proposed. Numerical simulations based on PSCAD/EMTDC have been carried out, which well-demonstrate the accuracy of the proposed transient simplified model for initial DC fault analysis and the effectiveness of the proposed transient average current based fault detection scheme. Compared with the traditional change of rate of current based protection method, the proposed one stands out for relatively low sampling frequency, high fault resistance tolerant ability, and robustness against outside noise and data missing.

Traditional permanent magnet synchronous generator (PMSG)‐based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG‐based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid‐side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC‐link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC‐link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small‐signal analysis of the proposed scheme indicates the DC‐link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non‐linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid‐forming control.

The requirement of fast fault isolation poses a great challenge to the safe operation of multi-terminal direct current (MTDC) systems. In order to make a better tradeoff between the speed and reliability of the protection scheme, it is imperative to mine more valuable information from fault transient signals. This paper puts forward a data-driven framework capable of digging out and synthesizing multi-dimensional features to achieve fast and reliable DC fault detection and classification in MTDC systems. Highly comparative time-series analysis (HCTSA) is first adopted to extract extensive features with clear physical interpretations from fault current waveforms, and a few features valuable to fault identification are then selected utilizing the greedy forward search. Based on the reduced features, a softmax regression classifier (SRC) is further proposed to calculate the probability of each fault category with a relatively minor on-line computational burden. Numerical simulations carried out in PSCAD/EMTDC have demonstrated the proposed approach is effective under different fault conditions, robust against noise corruptions as well as abnormal samplings, and replicable in various DC grids. In addition, comprehensive comparison studies with conventional derivative-based protection methods and some typical artificial intelligence based (AI-based) methods have been conducted. It is verified that the proposed method has the advantages of higher fault identification accuracy over conventional protections and shallow structure AI-based methods, better interpretability as well as lower on-line computing complexity over the deep architecture AI-based approaches.

Existing wind turbine (WT) control strategies realize damping support mainly by active power modulation, which may affect wind energy harvest. To improve energy efficiency and robustness, this article proposes a two-stage coordinated control of type-4 WT for damping support using the cost-effective WT resources, where the grid-forming control is applied. In the first stage, two reactive power-based dampers are designed using DC voltage synchronization control. The impacts of the dampers on stability are analytically evaluated. In this stage, the reactive power modulation, along with the partial DC capacitor energy, are jointly utilized for damping provision, while the WT is operated at maximum power point tracking (MPPT) mode. The second stage that seeks active power modulation is only activated when the DC voltage exceeds the threshold. An MPPT recovery process is further designed to eliminate the droop errors induced by the damper. The proposed scheme is validated under various contingencies compared with existing controls. Numerical results show it does not require a frequency estimator or the active power modulation for WT synchronization and grid-forming purposes; meanwhile is relatively energy-efficient and robust to provide strong damping support.