Experimental study of the current commutation in hybrid DC-breakers

To interrupt a current in a DC system where the feeding voltage is higher than a few kilovolt is a challenge. One popular solution today is the hybrid DC-breaker where a semiconductor is bypassed by a mechanical switch in normal operation to decrease the on-state losses. This paper presents an experimental study of the commutation from the mechanical switch to a parallel diode and IGBT. Three sets of tests were performed with six different current levels between 50 and 450 A. The test circuit consists of a capacitor bank as an energy source, a current limiting inductor, and the breaker. As the pre-charged capacitor bank is discharged through the current limiting inductor, the current will rise linearly, similar to the fault current in a stiff DC system. It was seen that with a very low stray inductance in the commutation loop, the commutation occurs in some ten microseconds just after the contact separation. Due to the small contact separation, the contacts might regain metallic contact again, causing the commutation to stop until the contacts separate again and the arc reignites. Longer arcing times were studied by introducing a larger inductance in the commutation loop. For currents of some hundred amperes, the arc voltage is fairly independent of the current level, but increases with increasing contact separation resulting in an increasing current derivative and a faster commutation. Finally the commutation and interruption of a current in a hybrid DC-breaker with an IGBT was demonstrated.