Control and Operation of islanded AC microgrid

There are basically two kinds of line faults that occur in the conventional power system. (i) Symmetrical and (ii) unsymmetrical faults. Out of the line faults, LLL (Line-Line-Line) fault is considered as symmetrical and the rest such as LG (Line-Ground), LL (Line-Line), and LLG (Line-Line-Ground) are considered as unsymmetrical faults. The percentage of occurrence of each fault in a power system transmission line is shown in Figure 3.
In islanded microgrid also, the micro-sources are connected to load through distribution lines (single/three phases). So, the occurrence of the fault on these lines might be a possibility. The occurrence of these faults may lead to frequency and voltage instability issues as there is no support from the utility grid. An early identification of these faults would help resolve such issues.
For each type of line fault, the load current was simulated before fault, during fault and post-fault. The plots are shown in Figure 4. A clear transition and magnitude variation can be observed during the fault duration. For some of the line faults, the magnitude is found to be more while for others it is less. This magnitude depends on the impedance (positive, negative and zero sequences) calculated from the corresponding sequence networks in each fault case. Here, the load current is observed to be more for LG and LLG faults, compared to LL and LLL faults. However, a clear distinction between the faults cannot be identified with just plotting the fault currents

传统电力系统中主要存在两类线路故障：(i) 对称故障，(ii) 非对称故障。在线路故障范畴内，三相短路故障（Line-Line-Line, LLL）属于对称故障，而单相接地故障（Line-Ground, LG）、两相短路故障（Line-Line, LL）以及两相接地短路故障（Line-Line-Ground, LLG）等其余故障类型均归类为非对称故障。各类故障在电力系统输电线路中的发生占比如图3所示。
在孤岛型微电网中，微型电源同样通过配电线路（单相/三相）与负载相连，因此此类线路存在发生故障的可能性。由于缺乏公用电网的支撑，这类故障的发生可能引发频率与电压失稳问题。及早识别此类故障，将有助于针对性解决上述问题。
针对每一类线路故障，本研究对故障前、故障期间以及故障后的负载电流开展了仿真实验，相关波形如图4所示。在故障持续阶段，可观测到清晰的波形跃迁与幅值变化。部分线路故障的电流幅值较高，其余则相对偏低，该幅值取决于各故障场景下由对应序网络计算得到的阻抗（正序、负序及零序）。相较于两相短路（LL）与三相短路（LLL）故障，单相接地（LG）及两相接地短路（LLG）故障的负载电流幅值更高。然而，仅通过绘制故障电流波形，无法实现不同故障类型间的明确区分。