Electrically powered vehicles, such as More Electric Aircraft (MEA) and electric vehicles (EV), consume less energy, have fewer emissions and maintenance, and cost less than conventional fossil-fueled vehicles. Combining electric drives with power electronic control strategies in MEAs and EVs improves overall system efficiency, reduces weight and costs, and meets reliability requirements [ 1 2 ].8,
In some commercial transport equipment, such as Boeing 787 and Airbus A380, the main engine generator is directly coupled to the jet engine through the gearbox, the AC voltage frequency of the aircraft is proportional to the engine speed, and the voltage frequency is between 360 Hz and 800 Hz [ 3 ]. The three-phase AC voltage is rectified by using an AC-DC converter before distributing to the aircraft DC load. Conventional aircraft use a transformer rectifier unit [ 4 ] or an autotransformer rectifier unit [ 5 ] to provide a DC side voltage with multiple pulses (12 or 18 pulses). These rectifying devices have met the requirements of the DO-160G for current harmonics but with enormous weight and volume [ 6 ]. PWM rectifiers are attracting much attention as they use fully controlled switching devices to control the AC-side current and DC-side voltage through switching choppers. Compared to autotransformer rectifiers, PWM rectifiers have a smaller size and weight and a wider control frequency. A suitable PWM control strategy can enhance the dynamic and static performance of the system, improve the stability of the DC-side voltage and reduce the total harmonic content of the AC current. Two-level, multi-level PWM rectifiers and Vienna rectifiers, for example, have power factor correction capability and can operate at a unit power factor. They also have the advantages of simple construction, a low number of semiconductor devices, and low total harmonic distortion of the input current [ 7 9 ]. Therefore, PWM active rectifiers have more advantages than conventional rectifiers in wideband power supplies.
Since direct power control does not require a current loop, the active power and reactive power of the system can be directly adjusted by checking the switch table to select the appropriate voltage vector. It is mostly used in motor speed control systems, active filters, etc. Direct power control require high dynamic performance of the rectifier. Due to its superior power control performance, it has been deeply studied by scholars.
Because a hysteresis comparator is used for power regulation in direct power control, the switching frequency changes, and high sampling frequency sensors are needed to obtain good control performance [ 10 ]. To solve the above problems, domestic and foreign scholars proposed a fixed frequency direct power control method. Reference [ 11 ] fixed frequency control is realized by using a PI regulator and SVPWM to direct the active and reactive power compensation, and the line voltage sensor was replaced by the virtual flux estimator. It has the characteristics of a simple algorithm, good dynamic response and constant switching frequency, especially when the supply voltage is not ideal; this scheme can reduce the total harmonic distortion. In reference [ 12 ], a control strategy combining direct power control and SVPWM was further established to compensate for voltage drop and harmonics under non-ideal conditions. Some of the literature improves the control performance of direct power control by using nonlinear controllers. In reference [ 13 ], sinusoidal input current with unit power factor can be generated using model predictive control without any current controller or modulator. The system has the characteristics of a fast dynamic response of directly controlling the input power. The literature [ 14 ] achieves a reduction in dc-side voltage bias under wide frequency conditions by using a double low-pass filter, but the problems of reactive power error and controller parameter mismatch are not investigated.
If the prediction model does not match the actual system parameters, the normal operation of the system will be affected, and the system will be unstable [ 15 ]. In order to solve the problem of parameter mismatch, scholars have proposed different solutions. In reference [ 15 ], a robust model predictive current controller with disturbance observer is proposed for three-phase voltage PWM rectifier. Luenberger observer is constructed for parameter mismatch and model uncertainty. The proposed method has fast dynamic response and good robustness. The literature [ 16 ] proposes a model predictive current control with fixed switching frequency and deadband compensation that has lower current THD compared to direct power control methods. In reference [ 17 ], robust current predictive control based on internal model disturbance observer is proposed to achieve accurate current loop control, which can achieve fast and accurate control when parameters are disturbed and there is model mismatch. This method significantly improves the robustness of the system. In reference [ 18 ], the relationship between parameter mismatch and current control effect is deduced on the basis of the discrete domain model of voltage type rectifier, and an improved model predictive control strategy for grid-connected parameter mismatch is proposed. The method is to correct the beat free control parameters in real time through on-line inductor identification. The simulation and experimental results show that the on-line inductance identification control strategy can accurately identify the inductance in static and dynamic processes, reduce the dependence of the deadbeat on the accuracy of model parameters, and improve the fault tolerance rate of the control system. According to the variation and uncertainty of input filter impedance, an adaptive model predictive control is proposed in reference [ 19 ], the proposed method can adaptively adjust the model parameters in the predictive equation and improve the stability of the model. The proposed modeling scheme of automatic adjustment does not require additional sensors while keeping the controller simple. Although the above predictive control schemes are effective, the fundamental frequency of the AC side of the rectifier is constant by default, and there is no measure to solve the error caused by frequency parameter mismatch.
American scholars proposed a deadbeat direct power control scheme combined with the variation of sampling frequency of instantaneous PLL, the proportional integral controller was used to adjust the voltage of the DC bus so that the total harmonic distortion was low and the AC side kept the unit power factor [ 20 ]. Since the expected value of the instantaneous reactive power is 0 and there is no control loop, the accumulation of reactive power error will occur under the condition of frequency and inductance resistance parameter mismatch, resulting in the steady-state error of the instantaneous reactive power, and the power factor will be affected by the steady-state error of the reactive power and thus reduced.
In this paper, the control strategy is improved to solve the problem of reactive power steady state error when the frequency changes and the model parameter mismatch is caused by the parameter changes. In the improved strategy, repeated control is added, and the error of instantaneous reactive power is repeatedly accumulated and iterated to reduce the steady-state error of reactive power. At the same time, a power compensation module is added to solve the extra instantaneous reactive power caused by the change of model parameters, which can quickly respond to the change of parameters and make the reactive power more stable than the traditional method under wide frequency conditions. The specific scheme is shown in Figure 1 . The whole system is divided into five modules: Rectifier Module, filter module, phase locked loop module, improved repetitive control module and PI control module. This paper mainly introduces the improved repetitive control modules.
The first section describes the modeling and lead control strategy of the three-phase AC-DC converter. In Section 2 , the deficiency of lead control and the improvement of repeated control method are explained. In Section 3 , experimental results are presented to demonstrate the performance of the proposed control scheme.
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