Disturbance Rejection Ability Enhancement Using Repetitive Observer in Phase-locked Loop for More Electric Aircraft Mi Tang Power Electronics, Machine and Control (PEMC) group University of Nottingham Nottingham, UK mi.tang2@nottingham.ac.uk Andrea Formentini Dept. of Electrical and Electronic Engineering University of Nottingham Nottingham, UK andrea.formentini@nottingham.ac.uk Stefano Bifaretti Dept. of Industrial Engineering University of Rome Tor Vergata Rome, Italy stefano.bifaretti@uniroma2.it Shafiq Odhano School of Engineering University of Newcastle Newcastle, UK shafiq.odhano@newcastle.ac.uk Sabino Pipolo Power Electronics, Machine and Control (PEMC) group University of Nottingham Nottingham, UK sabino.pipolo@nottingham.ac.uk Pericle Zanchetta Dept. of Electrical and Electronic Engineering & Dept. of Electrical University of Nottingham & University of Pavia Nottingham, UK & Pavia, Italy pericle.zanchetta@nottingham.ac.uk Abstract—Under the concept of transportation electrification, more electric aircraft (MEA) involves more electrical energy to reduce emissions. Phase-looked loops (PLLs) have been well developed for synchronizing different power sources in a grid. Since MEA operates at variable frequency from 360 Hz to 800 Hz, a third-order model based steady-state linear Kalman filter PLL (SSLKF-PLL) has been proposed in literature to achieve fast tracking performance during such grid frequency variations. To suppress the potential disturbances due to harmonics in the grid, sensor scaling errors/unbalances and d.c offsets while maintaining low computational burden, this paper aims to enhance the disturbance rejection ability of SSLKF-PLL by adding a repetitive observer (RO). Simulation tests show that RO allows stable and effective suppression of disturbances from all above-mentioned sources during variable frequency operation. Keywords— repetitive control, phase-locked loops, more electric aircraft, power system harmonics, fault tolerant control I. INTRODUCTION The move towards more electric aircraft (MEA) under the concept of transportation electrification requires the use of more electrical power to partly or completely replace the mechanical, hydraulic and pneumatic power. Taking the military standards MIL-STD-704 for example, one of the main power buses should be a 115 Vac three-phase bus with its frequency varying from 360 Hz to 800 Hz. Accurate and fast tracking of phase and frequency of the three-phase voltages generated from multiple power sources is important for grid synchronization. A distinctive three-phase PLL is desired to achieve quick response to phase jumps with strong rejection ability against disturbance due to harmonics in grid, sensor scaling errors/unbalances, d.c. offsets in the grid. Besides, for MEA, the PLL also need to be adaptive to variable frequency. Vast majority of the existing PLLs in literature are proposed for constant-frequency grid with slight grid frequency variations allowed. If the operation frequency is too far from the design point, the performance of those constant- frequency PLLs will degrade. For MEA, according to the recent research in [1], the key shortcomings for the few available variable frequency PLLs techniques can be summarized as the follows: • Adaptive observer [2] and sliding mode observer [3] based method cannot handle the d.c. offsets. • Discrete Fourier transform (DFT) [4] based method has heavy computational burden. • Complex least mean square [5] based method cannot maintain its performance with grid unbalances. Therefore, authors in [1] has proposed an estimator to solve the d.c offsets issue. However, the harmonics in grid has not been considered in [1]. A third-order model based PLL, namely the steady state linear Kalman filter PLL (SSLKF-PLL), has been proposed in [6-9]. Fast tracking performance during grid frequency variation is achieved for MEA in [9] by the SSLKF-PLL because the Kalman filter not only observes the grid frequency, but also observes and compensates the acceleration of the grid frequency. It is also confirmed in [9] that SSLKF- PLL has faster dynamic than the DFT based method in [4]. Regarding the disturbance rejection ability, although SSLKF- PLL can reject the disturbance by reducing its control bandwidth, it cannot remove the disturbance completely. Therefore, this motivates the use of a repetitive observer (RO). RO is first proposed in [10] for motor drive applications. Benefiting from its disturbance-observer-like structure, it can be designed independently from the feedback controller of the system. Another big convenience of using RO is that no pre- knowledge of the target disturbance is required. It will self- learn and tackle disturbances at all harmonics within the Nyquist frequency. What is more, with the development of high-performance control platform [11], the extra execution time required for RO would be about 5 to 10 μs. Thus, RO is can be a promising add-on tool for the intended PLL. Hence, this paper proposes a novel dual-observer PLL (DO-PLL). In section II, the SSLKF-PLL will be reviewed. In section III, the proposed DO-PLL will be discussed. The case study section in IV will discuss how the parameters can be selected. Simulation tests in V will show that the proposed DO-PLL can achieve superior grid frequency and phase