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The lack of relevant protection schemes and standards brings significant challenges to the promotion of direct current microgrid (DCMG) technologies. The existing line fault diagnosis methods usually require additional measuring devices to obtain fault signals. Although reducing the number of sensors can lower the system costs, it also increases the difficulty of diagnosis. Pole-to-pole (PP) and positive pole-to-ground (PPG) fault classification under unknown fault resistance, and negative-to-ground (NPG) fault detection under unobvious positive pole current characteristics are two major difficulties. To solve the above issues, a fuzzy granulation interval (FGI) theory-based line fault diagnosis method is proposed in this paper. Only locally measured bus-side voltage and positive pole current signals shared with the converter controller inputs are required. Firstly, the fault characteristics and diagnosis difficulties are analyzed. Then a new NPG fault index interval range (IR) is designed to extract the abnormal post-fault voltage fluctuations. Moreover, the PP/PPG fault classification strategy is designed according to the interval characteristics and capacitor discharge process. Finally, the performance of the proposed method has been evaluated through MATLAB/Simulink simulations and hardware-in-the-loop (HIL) tests. The results demonstrate that the proposed method can accurately discriminate different line faults within 1.5 ms under different fault conditions.

Constant power loads (CPLs) often lead to system instability and voltage collapse. This paper analyzes the existence and stability of equilibrium of meshed DC microgrids with multiple CPLs under distributed control, which aims to realize current sharing and voltage regulation. Firstly, the power-flow equation of the system is derived. To analyze the solvability of power-flow equation, we transform the problem of the multidimensional quadratic equation solvability into the existence of a fixed-point for a contraction mapping. Then, the sufficient solvability condition is derived based on Banach's fixed-point theorem. Secondly, we build the small-signal model to determine the system qualitative behavior around equilibrium. By analyzing the eigenvalue of system Jacobian matrix based on inertia theorem, the analytical stability conditions are obtained. Further, under the load uncertainty, a robust stability condition which only depends on the maximum load information instead of the real-time load information is derived. Simulation results verify the feasibility of the proposed theorems.

The onshore local grid is facing the challenge to avoid continuously increasing fault current caused by frequency regulation (FR) of broad-scale offshore wind farms (OWFs) integrated by voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) system. To tackle this challenge, an active fault current limitation (AFCL) method is proposed in four aspects: 1) a fault current contribution factor (FCCF) is introduced for grid side VSC (GSVSC) to quantify the incremental degree of fault current caused by FR of GSVSC under different fault types; 2) a current phase angle control (CPAC) strategy based on vector analysis is conducted for GSVSC to actively mitigate the fault current increment; 3) according to FCCF and CPAC, an active and reactive current quantification (ARCQ) strategy is developed for GSVSC to satisfy bilateral demands of AFCL and FR; 4) an adaptive frequency regulation (AFR) strategy, which considers FCCF and the operating conditions of the offshore system (e.g., rotor speed and DC voltage), is formulated for OWFs to quantify and generate the active power required for FR and AFCL. Numerous test results are analyzed to verify the effectiveness and performance of the proposed AFCL method in satisfying system fault current limitation during FR of OWFs.

Quadratic droop control is an innovative control strategy for power network, aiming to respect the inherently quadratic nature of reactive power flow. However, the system stability under quadratic droop control has not been addressed completely so far. This paper mainly focuses on the large-signal stability analysis of the AC network under quadratic droop control. We firstly analyze the stability of each equilibrium, it is concluded that the high-voltage equilibrium is stable as long as it exists while all low-voltage equilibria are unstable. Secondly, an analytical energy function for the system under quadratic droop control is established. It is proved that the high-voltage equilibrium is the local minimum of the proposed energy function while all the low-voltage equilibria are saddle points. Thirdly, to estimate the region of attraction (ROA) of the high-voltage equilibrium, an innovative method based on the analytical energy function is proposed. Comparing with the closest UEP method, the proposed ROA method significantly reduce the amount of computations. Simulation results verify the proposed theorems.

The present inaccurate fault current calculation, due to the incomplete consideration of whole process operation stages of fault ride-through (FRT) of DFIG-based wind farms (WFs), has a distinct impact on the safe operation of local power grid. To tackle this challenge, a fault current unified calculation (FCUC) method is proposed to achieve high-accuracy and high-speed mathematical analytical calculations of DFIG’s output fault current during FRT. The proposed FCUC method mainly includes: 1) Whole process unified state space model, which contains transition stage, crowbar protection stage, demagnetization stage, reactive current compensation stage, and dc-chopper protection stage, is established to thoroughly consider FRT’s different internal protection circuits and control strategies; 2) Stator output fault current is conducted by the unified state space model based on the stator residual voltage and the stator flux linkage; 3) Grid side converter (GSC) output fault current, which considers both injection mode and absorption mode, is developed based on the grid residual voltage and the unified state space model; 4) Fault current contribution mechanism, which is aggregated from both stator and grid side converter, is revealed based on Laplace transform from time-frequency-time domain, and is first expressed as an unified fault current expression. Extensive simulation/experimental/calculation results under different voltage sags have validated the correctness and effectiveness of the proposed FCUC method.