The project was aimed at an increase of power density (kW per kg) of a three-phase inverter for aerospace application (power range 10 … 50 kW) of at least 10% with a concomitant compliance with RTCA DO-160, chapter 21 CMC limits.
The increase in power density was achieved by implementing an innovative topology based upon:
- Transition of the H5 or HERIC topology from single phase to a three phase inverter implementation
- Application of the 3L-T-Type inverter which can be derived from a 2L-inverter with three additional, bidirectional switches which connect the AC terminals with the center-tapped DC link
- Applying of appropriate modulation techniques which make use of switching states with reduced CMV levels
The resulting solutions were compared with a state-of-the-art 2L-VSI power core based upon the following criteria:
- Produced CMV and CMC levels, given by a quality parameter based upon RTCA DO-160, chapter 21
- Total weight for a 10 kW and a 50 kW power core implementation taking into account all relevant parts incl. gate driver, passive components for filter and cooling. The 10 kW power will be air cooled (natural convection), the 50 kW power core liquid cooled
- The influence of the switching speed dv/dt will be taken into account, as today there exists a limit of approximately 5 kV/µs for the motor. Furthermore a too high dv/dt produces a significant amount of EMI
The CleanSky project ’Optimising Power Density of Aircraft Inverter Combing Topology and PWM Pattern’, with the acronym ’CoPoCo’ aimed to increase the power density of a DC/AC converter for aerospace application by 10 %. At the same time, the conducted disturbances shall be kept at least at the same or preferably lower level. This project was motivated through the advancements in the field of More Electric Aircraft (MEA). The More Electric Aircraft promised a reduction of emissions and decrease of fuel consumption.
Typically different combinations of energy, including electrical, hydraulic and pneumatic are used to drive the different systems in an aircraft. At least for the hydraulic and the pneumatic systems, power has to be provided constantly, to keep the pressure. For electrical systems, the power consumption is only proportional to the load neglecting a low power consumption for control electronics. A replacement of hydraulic and pneumatic systems through electrical systems could, therefore, save energy.
The increasing number of electric components leads to growing influence of their weight. This project aimed to increase the power density of power electronics for AC drives it tried to minimise this impact. To reach this goal several approaches were made in this project and analysed on two typical applications for today’s aircraft. The first application was a 10 kW DC/AC converter which could be used to drive flight controls situated in the wings. A second important application is maintaining the cabin pressure. For this example, a converter with 50 kW is analysed. To reach this goal different aspects of a converter are analysed. The first aspect was the reduction of conducted common mode noise since the used filters make up to 20% of the converters weight. Typically 2-Level converters with Space Vector Pulse Width Modulation (SVPWM) were used. The use of alternative topologies or different control schemes for the 2-Level converter can lead to common mode noise reduction.
Even though today's power electronics have efficiencies higher than 97 % the generated losses in this power classes are significant and have to be dissipated for a safe operation. The cooling System has also a significant influence on the converters weight. For example, the low power core shall be cooled through natural convection. This approach led to relatively big cooling to provide the necessary surface to transfer the heat to ambient air. One approach taken in this project to decrease the heatsink size was to use SiC-MOSFETs which promise lower switching losses through the higher switching speed compared to IGBTs. All possible solutions for weight reduction were analysed and compared to each other.
The optimal solution of reduced common mode emissions and generated losses will be described through Pareto optimisation.