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An impulse-commutated solid-state transfer switch (ICSTS) with a resonant commutation circuit is proposed. The thyristor-based solid-state transfer switch (STS) can utilize the dual power feeders to protect sensitive loads against voltage sags. Compared to gate-turn-off thyristors (GTOs), thyristors are favored for this application due to the lower conduction losses even though they can not self-commutate. For the conventional STS, the transfer process, which includes the detection of voltage sag and the thyristor commutation, may take up more than a quarter cycle. In this paper, the proposed ICSTS system can greatly reduce the transfer time and provide better protection against voltage sags.For the conventional STS, the thyristor commutation is dependent on the voltage difference between the two feeders and the power factor of the load. The total transfer time of the conventional STS may exceed 10 ms in some situations. Such long transfer time could interrupt the operation of sensitive loads, and can not satisfy the requirement of industrial standards like the SEMI F47 standards. The proposed ICSTS greatly reduces the transfer time by using an impulse commutation circuit which consists of inductor L r , capacitor C r and a auxiliary semiconductor switch as shown in Fig. 1.The impulse commutation circuits, which are placed across the thyristors on the primary feeder, are to expedite the commutation and to reduce the transfer time. Upon detection of voltage sags, the ICSTS controller will initiate the transfer process as follows: 1. Stop triggering the thyristors T1p (or T1n) on the primary feeder; 2. Start the LC resonance by triggering the auxiliary thyristors A11p (or A11n), and the current in the main thyristor will reduce and eventually reach cut-off; 3. Turn on the thyristors T2p (or T2n) on the alternate feeder to complete the load transfer. Fig. 2 shows the details of the commutation process. As the sag is detected, the commutation is immediately initiated. The main thyristor is cut-off in approximately 25us, which is much shorter Fig. 1. Schematic diagram of the ICSTS system than the detection time. Therefore the thyristor commutation time of ICSTS can be neglected. The thyristor of the alternate feeder is triggered 128us after the primary feeder thyristor is turned-off and the transfer process is completed.The proposed ICSTS employs the impulse commutation circuit on the thyristors for fast turn-off, thus it can complete the transfer immediately upon the detection of voltage sags or swells. The laboratory test results validate that the proposed ICSTS can accomplish load transfer within 4-5 ms under most voltage sag and voltage swell situations. The conventional STS may require more than a quarter cycle to complete the transfer because of the dependence on the point-on-wave of sag occurrence and the power factor of the load, and it may require even more transfer time in case of voltage swells. The transfer time of ICSTS can meet the requirement of industry standards like SEMI F47. Test results validate t...