The Vienna rectifier in aircraft applications: current distortion mitigation methods

Abstract

Electrical power demand is steadily increasing in more electric aircraft (MEA) as conventional pneumatic, hydraulic and mechanical loads are being replaced by electrical loads. The main motivation behind this trend is to reduce emissions, fuel usage, noise, etc. The designers are increasing the voltage levels to cater the power demand with reduced ohmic losses and conductor size, resulting in weight reduction and improvement of overall system efficiency. This trend also demands high power density converters. This research is focused on identifying a suitable topology and improving the overall performance of the power converter to transfer power from a generator, to a dc distribution network in future MEA. The studied solution should provide high power density, efficiency, power quality, and reliability. After benchmarking with the widely used two-level converter, the Vienna rectifier topology has been selected.

The Vienna rectifier is based on a diode bridge rectifier and therefore it has unidirectional power flow. It can generate three voltage levels that can be operated with pulse-width modulation (PWM). Despite being able to generate three voltage levels, only the connection to the neutral-point is fully controllable using the bidirectional switches. When the neutral-point voltage is not imposed in a phase-leg, the polarity of the pulses generated depends on the diode that is conducting in that phase-leg, which is defined by the current direction. As a result, the voltage pulses generated can go in the opposite direction to what is demanded by the controller, and hence the current becomes distorted. The challenge of this research is to improve the performance of the Vienna rectifier under such operating conditions.

The Vienna rectifier is connected to a programmable ac source, where the frequency can be varied and the grid voltages can be measured. Initially, two operation modes based on a special zero sequence and reactive power injection have been proposed depending on the operating conditions of the converter. Then, a hybrid modulation method is proposed combining the above two methods, achieving a smooth transition between operating points with significant less reactive current injection. With the proposed methods, the Vienna rectifier can operate in a wide range of power factors without compromising the quality of the source currents and within the aerospace standards. The proposed solutions are validated in an experimental setup.