The phase angle-based principal component (PC) technique for islanding detection of distributed generations (DGs) is proposed. The phase angle between the positive-sequence components of voltage and current is derived at the DG terminal and used as an input feature vector for the PC technique to identify the islanding situation (IS). The change in phase angle is prominent for both ISs and non-ISs (NISs). By exploiting this change in phase angle, PCs are computed to discriminate between ISs and NISs. The proposed technique is evaluated using data simulated with a real-time digital simulator for IEEE 13-bus microgrids. Critical issues such as a perfect power match IS, different scenarios of microgrids having a synchronous generator, doubly fed induction wind generator, photovoltaic with various control strategies and battery energy storage systems are addressed during performance evaluation of the proposed technique. It is found that the technique identifies the IS under low active and reactive power mismatches and hence overcomes the non-detection zone problem. All NISs such as the fault type, DG tripping and feeder disconnection in the presence of multiple DGs are considered. It is noteworthy to mention that the proposed technique discriminates ISs and NISs within a cycle from the inception point.
The bidirectional power flow through the interlinking converter (IC), in ac/dc hybrid microgrids (HMGs) consisting of distributed generators (DGs) with droop controllers, plays an important role on the stability of such systems during islanding. This paper investigates the impact of the power flow direction on the small-signal stability of islanded droop-based HMGs. Firstly, a linearized state-space model of an HMG is developed. Secondly, eigenvalue analysis is carried out to realize the dominant modes, which are the rightmost eigenvalues. Thirdly, participation factor analysis is performed to identify the system and control parameters that effect stability the most. Lastly, sensitivity analysis is conducted to determine the critical droop gains and stability margin. It is observed from the eigenvalue and sensitivity analysis that the dominant modes of HMGs become more stable as more power flows from dc to ac subgrid. On the contrary, an increase in the power flow from ac to dc subgrid degrades the HMG stability. Additionally, the sensitivity of the dominant modes to changes in ac and dc droop gains is studied under bidirectional power flow through the IC to ascertain their impact on the stability margins. Finally, time-domain simulations, in MATLAB/Simulink, suggest that more generation on the dc subgrid would enhance the overall HMG stability margin during islanding.INDEX TERMS Bidirectional power flow, distributed generator, droop controller, ac/dc hybrid microgrid.
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