Hybrid microgrids which consist of AC and DC subgrids interconnected by power electronic interfaces have attracted much attention in recent years. They not only can integrate the main benefits of both AC and DC configurations, but also can reduce the number of converters in connection of distributed generation sources, energy storage systems and loads to AC or DC buses. In this study, the structure of hybrid microgrids is discussed, and then a broad overview of the available protection devices and approaches for AC and DC subgrids is presented. After description, analysis and classification of the existing schemes, some research directions including communication infrastructures, combined control and protection schemes, and promising devices for the realisation of future hybrid AC/DC microgrids are pointed out.

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Challenges, advances and future directions in protection of hybrid AC/DC microgrids

In the event of a single line to ground (SLG) fault in a non-effectively grounded distribution network, the faulted current is weak (only a few amperes or less) and the existing devices cannot accurately judge the faulted feeder. In this paper, we proposed algorithms that combine complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and Hilbert transform to construct a multi-criteria comprehensive voting method. First, CEEMDAN algorithm is used to decompose the zero-sequence current to obtain the IMF1 (the first intrinsic mode function) component and the Hilbert transform is used to calculate the instantaneous amplitude and instantaneous phase. Then, according to the three largest instantaneous amplitudes information, we constructed the characteristic instantaneous phase, characteristic instantaneous energy relative entropy and characteristic instantaneous zero sequence current polarity criteria from the phase, energy and polarity, respectively. Finally, we proposed a comprehensive voting method, which is specifically shown as follows: when two or more criteria show that one feeder or the bus has an SLG fault, it is voted that the feeder or the bus has an SLG fault. In contrast, if the judgment results of the three criteria are inconsistent, then we would return to recalculation and then vote. Compared with existing method, simulation tests and field experiments show that the method proposed in this paper has higher accuracy and a faster calculation speed.

Single Line to Ground Fault Detection in a Non-Effectively Grounded Distribution Network

An interval type-2 fuzzy logic based strategy for microgrid protection

Protection of AC microgrid integrated with renewable energy sources – A research review and future trends

Coordination between protective device fault clearing times and distributed generation (DGs) stability is essential to avoid nuisance tripping of DG. In this paper, a multi-agent protection scheme based on a variable tripping time differential protection scheme (VTDPS) is proposed for micro-grid protection capable of operating in both grid-connected and islanded mode. Variable tripping time is used for coordination with laterals protective devices without the need for additional current measurement and data transmission. Each agent is composed of primary, backup, and bus protection layers, with their relevant VTDPS, that achieves main and backup protection for a microgrid. Moreover, the critical clearing time (CCT) curves of DGs and tripping time curve of VTDPS are analyzed in each feeder zone and a relevant formula is developed to check the CCT constraint for the proposed multi-agent VTDPS. The presented method is simulated on a modified CIGREE microgrid benchmark system with synchronous DGs. The obtained results prove the capability of the proposed protection scheme to prevent instability of DGs in both modes of operation.

Variable Tripping Time Differential Protection for Microgrids Considering DG Stability

Time-frequency-based differential scheme is proposed for microgrid protection using non-stationary signal analysis. The non-stationary signal processing algorithm Hilbert-Huang Transform (HHT) have been implemented for the protection objective and the comparative assessment with that of S-transform, differential current is carried out in order to demonstrate the reliability of the proposed protection scheme with different case studies. The series of simulation results demonstrates the efficacy of HHT, than that of S-transform in some critical cases such as high impedance fault where accurate detection is a challenge. The configuration of the microgrid test model has been developed in PSCAD with different distributed generation such as photovoltaic and wind generation system penetrated at different buses. Thus, it is found that the HHT scheme is quite reliable and efficient for fault detection objective.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2015.1563

Ashika Gururani

Papers

Microgrid protection using Hilbert–Huang transform based-differential scheme