Control Strategies for Minimizing the DC-Link Capacitance in Power Electronics Converters for Aircraft Electrical Power Systems



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In more electric aircraft (MEA), the electrical power system (EPS) consists of AC and DC power distribution systems to deliver power to various aircraft loads. In main power generation, variable frequency generators (VFGs) generate VF AC supply (380-800Hz 115/230V) to feed frequency insensitive loads. The VF AC is also converted into DC (+/-270V or 540V) by using transformers and power electronics converters (PECs) to feed DC loads. The DC is further converted into 115V 400Hz AC, and 28V DC using PECs to feed legacy loads. In MEA, the usage of PECs has increased significantly over the last few decades. These PECs have DC-link and use DC-link capacitors that are space-consuming, and heavy. In order to reduce the fuel consumption of aircraft, weight-saving and volume reduction are the critical challenges faced by engineers. Hence, reducing the DC-link capacitance value in the PECs is very crucial in MEA. In this dissertation, novel control algorithms and improvements are proposed for different configurations of VF brushless synchronous generator (BSG) based DC and AC power systems to minimize the DC-link capacitance in the PECs. First, a BSG based 270V DC system using a diode rectifier is investigated in which the DC-link voltage regulation is obtained through the excitation control of the exciter SG using a two-quadrant DC chopper. A steady-state feedforward duty ratio and output power-based feedforward exciter field current reference are proposed to improve the transient response of the DC system to meet the military standard (MIL-STD-704F) transient voltage specifications with minimized DC-link capacitance. Next, BSG based diode rectifier inverter (DRI) regulated 115V, 400Hz AC variable speed constant frequency system (VSCF) is investigated. In the VSCF system, the control improvement from the BSG based DC system isused along with a novel adaptive inverter voltage reference (AIVR) algorithm in the four-leg inverter side and to reduce the effects of inverter side fast dynamics. The combined control improvements help to meet the stringent MIL-STD-704F transient voltage specifications at the inverter output with minimum DC-link capacitance required in the VSCF system. Further, the BSG based DC system is investigated with an active rectifier in place of a diode rectifier. An improved DC-link voltage regulation based on field oriented control (FOC) of the BSG along with a feedforward excitation control for fast transient operation is proposed to further improve the transient response of the DC-link voltage and reduce the DC-link capacitance when compared to the diode rectifier based DC system. A steady-state rotor flux optimization algorithm based on BSG steady-state equations is proposed to maintain unity power factor (UPF) operation at the BSG terminals during steady-state operating conditions. Finally, the Electro-magnetic interference (EMI) filter design for the BSG based DC and AC systems are studied. The EMI filters are designed for different PECs such as two-quadrant DC chopper, diode rectifier, active rectifier, and four-leg inverter in the BSG based DC and AC systems. The effects of EMI filters in the control of BSG based DC and AC systems are investigated. All the proposed control algorithms are validated through extensive modeling and simulations in PLECS and the effectiveness of the control algorithms are verified through controller hardware in loop (C-HIL) testing results.



Aircraft power system, Brushless excitation of Synchronous Generator (SG), DC-link capacitance minimization, DC-link based Variable Speed Constant Frequency system, Four-leg inverter.


Portions of this document appear in: G. Selvaraj, K. R. Ramachandran Potti, S. R. P. Reddy, and K. Rajashekara, "Control Strategy for a Brushless Synchronous Generator based Active Rectifier regulated DC Power System with minimal DC-link capacitance for More Electric Aircraft," in Proc. IEEE COMPEL, Nov 2021; and in: G. Selvaraj, K. Rajashekara and K. R. R. Potti, "An Improved Feedforward Controller for Minimizing the DC-link Capacitance in a Brushless Synchronous Generator based Aircraft DC Power System," in Proc. IEEE ITEC, Jun 2021, pp. 658-664; and in: G. Selvaraj, K. Rajashekara and K. R. Ramachandran Potti, "An Enhanced Controller for Four Leg Inverter-fed Loads in an Aircraft Power System," in Proc. IEEE APEC, Jun 2021, pp. 1209-1214; and in: G. Selvaraj, K. Rajashekara and K. R. Ramachandran Potti, “Minimization of DC-link Capacitance for a DC-link Based Variable Speed Constant Frequency Aircraft Power System," in Proc. IEEE ECCE, Oct 2021.