Browsing by Author "Townsend, Christopher David"
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Item Band-Limited Three-Level Modulation for Balancing Capacitor Voltages in Neutral-Point-Clamped Converters(2020-09-01) Beniwal, Neha; Townsend, Christopher David; Farivar, Glen Ghias; Pou, Josep; Ceballos, Salvador; Tafti, Hossein Dehghani; POWER ELECTRONICS AND SYSTEM EQUIPMENTIn this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.Item Feedforward Modulation for the Neutral-Point-Clamped Converter with Confined Capacitor Voltage Ripples and Reduced Switching Power Losses(2020-04) Beniwal, Neha; Pou, Josep; Ceballos, Salvador; Townsend, Christopher David; Konstantinou, Georgios; Tafti, Hossein Dehghani; Farivar, Glen Ghias; POWER ELECTRONICS AND SYSTEM EQUIPMENTThis article presents a new modulation technique with feedforward compensation for the three-phase three-level neutral-point-clamped converter. With the proposed technique, the capacitor voltage ripples are allowed to vary within certain limits. This enables an optimized design of the converter since the maximum capacitor voltages are predefined. Furthermore, the proposed modulation technique is able to reduce switching power losses compared to modulation techniques that eliminate capacitor voltage ripples completely. The proposed technique is therefore a tradeoff solution between aforementioned techniques and traditional modulation techniques where the capacitor voltage ripples are not limited. In the proposed technique, if the capacitor voltages are within the tolerable specified range, all the phases switch in two consecutive voltage levels in steady state. When the capacitor voltages go beyond the specified limits, one of the three phases is chosen to switch in three levels to inject an appropriate neutral point current for capacitor voltage balance. As the capacitor voltages are allowed to oscillate, the modulation technique is provided with feedforward compensation to avoid producing low-frequency distortion on the output voltages of the converter. Experimental results are presented operating under different loading conditions.