Impact of the switching frequency on the DC-side admittance in three-phase converter systems

Three-phase converter systems can be prone to negative impedance instability of the DC-bus. This paper studies the dynamic interactions between the DC- and AC-sides of a current-controlled three-phase system in order to reveal possible risks of instability. The system is modelled by using a small-signal method from a viewpoint of the DC-side. Time delays due to pre-calculation and discretization are considered. The stability analyses are performed for the small-signal model. The results of the stability analyses are compared with time-domain simulations. Comparisons demonstrate good agreement. In addition, a stabilizing controller is developed for the example system. According to simulations, the proposed controller can stabilize the system.