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A self-commutated BTB (Back-To-Back) system is an ac-dc-ac power flow controller between two utility grids, and PLL (Phase-Locked-Loop) circuits are typically used for detecting their phase information. The performance of the BTB system during line faults is strongly affected by the dynamic behavior of the PLL circuit in terms of the ac-current fluctuation, the dc-voltage fluctuation, and the dc magnetic flux deviation in the converter-transformers. However, no paper or article has been discussed explicitly on their mutual relationship. The aim of this paper is to establish a design procedure of the PLL circuit which is suitable for the BTB system. This paper also deals with the dc magnetic flux deviation in the converter-transformers under double-line-to-ground (DLG) faults.Keywords -BTB (Back-to-Back) systems, power flow control, DLG fault, dc magnetic flux deviation, PLL (Phase-Locked-Loop) circuit.

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In this research, the zero-voltage switching (ZVS) of the GaN FETs-based high frequency three-port half-bridge converter (TPHBC), which is capable of interfacing a renewable energy source, an energy storage and a load is discussed. To achieve ZVS, which plays a key role in power loss reduction of the high switching frequency converters, not only the parasitic elements but also the dead-time between two switches in one converter arm must be taken into account. This research gives a detail analysis about the influence of the dead-time on the ZVS condition. Based on the analysis, a minimum dead-time which guarantees not only the ZVS but also the safe operation of the converter is obtained. Simulations in various load condition of the TPHBC are carried out to verify the validity and effectiveness of the proposed method.

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