Fault Propagation Mechanism of Hybrid Microgrid with Neutral Grounding of DC Side Capacitor
Zhang Yi1, Zhang Baifu1, Han Xiaoqing1, Ren Chunguang1, Zhao Zhibo2
1. Shanxi Key Laboratory of Power System Operation and Control Taiyuan University of Technology Taiyuan 030001 China; 2. School of Resources and Geosciences China University of Mining and Technology Xuzhou 221000 China
Abstract：The AC-DC hybrid microgrid will become an important development form of the future power grid due to the efficient renewable energy consumption performance. However, the interconnection structure of AC-DC hybrid microgrid complicates the fault response and decreases the system robustness. In this paper, the typical structure of hybrid microgrid includes AC sub-grid, DC sub-grid, and a bidirectional power converter (BPC), and the response characteristics of non-fault sub-grid to fault are analyzed. The systematic and complete fault response characteristic analysis results can provide a theoretical basis for the design of hybrid microgrid cooperative control or cooperative protection strategy under fault conditions. The harmonic frequency change principle and the transient process of the fault loop caused by the propagation of fault components between sub-grids under typical fault conditions are analyzed in this paper. On this basis, the voltage and current expressions of the AC and DC sub-grids are deduced respectively with the consideration of the system interconnection structure. And then, the response results of the non-faulty sub-grid to the faulty sub-grid are obtained, which reveals the fault propagation mechanism of the hybrid microgrid. In the state of AC sub-grid fault, the harmonic transformation process caused by the propagation of negative sequence components and zero sequence components generated by symmetrical faults and asymmetric faults through BPC is deeply analyzed, and the general harmonic transformation expression is given. For the inter pole short-circuit fault and single pole grounding fault of DC sub-grid, this paper analyzes the transient process of short-circuit current loop in different stages, and deduces the expression of AC short-circuit current affected by DC sub-grid faults. The magnitude of the short-circuit current under a single-pole grounding fault is related to the BPC switch state. And the short-circuit current is characterized in the form of a piecewise function, by dividing the switch state of BPC, Furthermore, an AC-DC hybrid microgrid model is built and simulated for different fault types based on the Matlab/Simulink platform. And the following conclusions can be drawn： 1) DC bus voltage drop will be caused by three phase short-circuit fault. And the closer the faulted point is to BPC, the greater the drop amplitude is. In addition, (n+1)-th harmonics will be generated in DC sub-grid, when the n-th harmonic on the AC sub-grid is converted to DC sub-grid. And the (n+1)-th harmonics in DC sub-grid will react to the AC sub-grid and generate (n±1)-th harmonics. 2) AC sub-grid can be equivalent to the state of three-phase short-circuit fault, when a inter pole short-circuit occurs in DC sub-grid. In case of single pole grounding fault of DC sub-grid, the freewheeling diode works in the state of uncontrolled rectification, and the AC short-circuit current waveform no longer presents sinusoidal changes due to the unidirectional conduction. At the same time, the error between theoretical derivation and simulation results is less than 5%. Compared with the fault current of single-pole grounding, the fault current increases more rapidly in the inter pole short-circuit fault state by comparing the freewheeling diode current waveforms under two types of faults, which is more likely to damage the switching device.
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2023, Vol. 38 
