The Commutation Booster, a New Concept to Aid Commutation in Hybrid DC-Breakers

SUMMARY The difficulty in interrupting DC currents lies in the lack of a natural current zero crossing. In an ACbreaker, the current is interrupted by separating the contacts to draw an electric arc. When the current naturally goes to zero, the arc is extinguished and the current is interrupted. For DC, the current zero has to be forced by the breaker. Hybrid solutions with mechanical switches and power semiconductors have been proposed to combine low conduction losses with the ability to force a current zero. This paper presents the commutation booster, a new concept to interrupt the current in a hybrid DCbreaker. In hybrid breakers, with parallel paths for the mechanical switch and semiconductor, the commutation from the mechanical switch to the semiconductor relies on the arc-voltage of the mechanical switch and a low stray inductance in the commutation loop. In a hybrid breaker with the commutation booster, the inductance is used to perform the commutation. The newly introduced advantage is that a fault current automatically leads to current zero crossings in the mechanical breaker, allowing it to interrupt the current in similar ways as an AC-breaker. By inductively coupling the primary branch containing the mechanical switch with the secondary branch containing the semiconductors, the rising fault current is used to enhance the commutation. As the inductance in the secondary branch is kept lower than in the primary branch, the current will rush through the semiconductors in case of a fault. The natural behaviour of an inductor to preserve the magnetic flux will decrease the current in the primary branch and ensure a fast commutation. The benefit of a hybrid breaker with the commutation booster is the lower demands on arc voltage. Since the commutation is ensured by the inductive coupling provided by the booster, the mechanical switch doesn't have to provide a high arc voltage and instead it can be optimized to interrupt the current in the same way as a conventional AC-breaker. The inductively driven current commutation will be faster the faster the rise of the fault current is. Hence, as long as the breaker is dimensioned to handle the commutation at nominal current, the operation will be faster with a lower fault impedance