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Description
A conventional rectifier based on a thyristor is widely used as a magnet power supply in the largest tokamak fusion device such as JT-60SA and ITER. The rectifier converts AC input power to DC output power supplied to the magnet bi-directionally. In order to energize and de-energize the magnet controlling power flow, the rectifier shifts phase angle of AC current with reference to input AC voltage. In the conventional rectifier, AC current always lags voltage due to the turn-on characteristics of the thyristor switch in forward bias state. The difference of phase angle causes the requirement of large reactive power and the unacceptable level of voltage drop in AC side. The Static Var Compensator and the motor-generator are utilized to compensate the lagging reactive power in ITER and JT-60SA respectively. However, the size and the capacity of these machines are quite large since the lagging reactive power is several times higher than the active power to energize the magnet.
In order to compensate the lagging reactive power, the rectifier with the series diodes and the commutating capacitors was proposed. In the configuration, the commutating capacitor provides forward bias voltage to the thyristor even at leading phase. Therefore, it can generate leading current in AC side. The capacitor is charged automatically during the commutation sequence.
This paper proposes the hybrid configuration composed of the conventional rectifier and the leading rectifier connected in series. The leading current cancels the lagging current generated by the conventional one. Consequently, the hybrid rectifier works with the unity power factor without any large compensation circuit. However, the hybrid rectifier generates voltage spikes in DC side at the current commutation and it may damage insulation inside the magnet. Feasibility study of the hybrid rectifier for superconducting magnets is carried out using resonance characteristics of them for JT-60SA.
