The black start capability is vital for microgrids, which can potentially improve the reliability of the power grid. This paper proposes a black start strategy for microgrids based on a parallel restoration strategy. Considering the characteristics of distributed generations (DGs), an evaluation model, which is used to assess the black start capability of DGs, is established by adopting the variation coefficient method. Thus, the DGs with good black start capability, which are selected by a diversity sequence method, are restored first in parallel under the constraints of DGs and network. During the selection process of recovery paths, line weight and node importance degree are proposed under the consideration of the node topological importance and the load importance as well as the backbone network restoration time. Therefore, the whole optimization of the reconstructed network is realized. Finally, the simulation results verify the feasibility and effectiveness of the strategy.

The fault characteristics of an inverter interfaced distributed generator (IIDG) depend on its control strategy and are closely related to whether it has a low voltage ride-through (LVRT) capability, which cannot be a simple equivalent to a constant power or current source. Thus, it is necessary to consider the specific control strategy of IIDGs and perform specific research on the fault detection method of a microgrid. Based on the fault models of a grid-connected PQ controlled DG (PQ-DG) with an LVRT capacity, this paper analyzed the fault component characteristics of a microgrid under different operating conditions, such as high-impedance faults (HIFs) and low-impedance faults (LIFs). The fault point is located using a new fault detection method, which is based on the phase differences between the positive-sequence fault component of the bus voltage and the positive-sequence fault components of the currents in the feeders. Finally, PSCAD/EMTDC is used in the simulation analysis, and the simulation results verified the validity of the fault detection method.

This paper presents the problem of parallel arc fault in the DC side of a photovoltaic (PV) system. First, an experimental platform for arc fault of a PV system was built to simulate the DC side parallel arc fault in the PV system. By extracting the current waveform at the exit of PV panels, a study was conducted on the characteristics of the current during parallel arc fault. The result of Fourier transform on the current suggests that the high-frequency component of the current mainly concentrates in the frequency range of 126–250 kHz. According to the analysis of arc fault characteristic in time domain and frequency domain, reversal current maximum, modulus maximum, and energy are chosen as the criteria to detect parallel arc fault. And finally, a mixed criterion is proposed to overcome the shortcomings of the above-mentioned criteria and its reliability is verified by experiments. It provides a theoretical basis for the detection of the parallel arc fault in the PV system .

Due to the inherent characteristics of inverter- interfaced distributed generators (IIDG), their fault responses are different from those of conventional synchronous generators. Investigating fault responses of IIDGs and establishing their fault models will benefit the fault calculation and protection scheme design for microgrids. This paper focuses on grid-forming IIDGs (GFGs) and hierarchically analyzes their fault characteristics under symmetrical and asymmetrical faults. A generic fault model of GFG, which covers both the transient- and steady-state, is proposed. Impacts of the transfer function of controllers, the current limiting module, and the fluctuation of DC-link voltage on GFG’s transient- and steady-state fault responses are respectively investigated. The influence of GFG’s topology is also considered, and both three-phase three-wire and three-phase four-wire GFGs are covered in the fault analysis. Model parameters calculation methods for all kinds of fault conditions are presented. It is found that the fault type, the fault severity, and the response speed of the DC power regulator all have significant impacts on GFG’s fault responses. The fault analysis and fault model are verified by simulation results in MATLAB/Simulink.

With the rapid development of distributed PV systems in distribution grids, unintentional islanding is one major concern which maybe cause safety hazards. The existing passive islanding detection methods are mainly based on the voltage and frequency deviations during islanding conditions. Passive methods have relatively large non-detection zone (NDZ) and susceptible to power system faults. Active islanding detection methods applied in inverters have relatively small non-detection zone, but they are usually harmful to power quality. This paper presents a new islanding detection method based on equivalent inter-connection line impedance, which is normally a small value. A measured impedance is calculated with the synchronous phasors from two lPMUs which are installed in a utility substation and a PV system. Under the grid-connected condition, the measured impedance is no more than the equivalent line impedance. While under the islanding condition, the measured impedance is much larger than the equivalent line impedance. So, an islanding condition can be effectively detected by the comparison of the measured impedance with the equivalent line impedance. The non-detection zone of this proposed method is nearly zero. The detection time is normally less than 250ms. This method is applicable for PV systems connected to the power system with a distribution transformer. It is also applicable for a distribution grid with multiple distributed generation systems.

Multi-microgrids have many new characteristics, such as bi-directional power flow, flexible operation and variable fault current consisting of the different control strategy of inverter interfaced distributed generations (IIDGs), which all present challenges in multi-microgrid protection. In this paper, the current and voltage characteristics of different feeders are analyzed considering faults at different locations of the multi-microgrid. Based on the voltage and current distribution characteristics of the line parameters, a new protection scheme for the internal faults of multi-microgrids is proposed, which takes the change of phase difference and amplitude of measured bus admittances as the criterion. This proposed scheme has high sensitivity and reliability, is based on a simple principle, and can be easily adjusted. Simulation results using PSCAD/EMTDC verify the correctness and effectiveness of the protection scheme.