Research On Power Pulsation Smoothing Technology For More Elecrtic Aircraft Based On Hybrid Energy Storage System

With the development of power system technology for more electric aircraft(MEA),the secondary energy in the aircraft system is gradually unified into electrical energy.The power levels of electrical equipment vary greatly,and there are more and more types,which result in system power showing pulsating characteristics.In order to meet the demand for pulsating power,the rated power design for generator needs to satisfy the peak value of the pulsating power,resulting in high power level,heavy weight,and large size for the generator.The working time of peak work for generator only occupies a small part of the flight cycle,and the generator works at low power for a long time,resulting in low efficiency.Meanwhile,the pulsating power can cause mechanical damage to the generator shaft and affect the reliability of the power system.Therefore,in order to reduce the influence of pulsating power on the generator,the energy storage system is selected to smooth the pulsating power,so that the generator provides the average power,which can also reduce the power level of the generator.It can also reduce the power level of the generator and improve the efficiency.Not only can the energy storage system absorb the feedback energy,provide transient power,reduce fuel consumption and exhaust emissions;it can also participate the response of power demand in the system to improve the reliability,stability and power quality.In this paper,the hybrid energy storage system(HESS)composed of batteries and super-capacitors is selected as the research object,and this thesis focuses on the high power density configuration,power distribution and system control for the HESS.First of all,aiming the issue that the configuration results of the energy storage system have a lot of redundancy when meeting the load power demand,the concept of equivalent time constant is proposed to quantify the characteristics of the energy storage device and the pulsating power.By comparing the equivalent time constant for pulsating power and the energy storage device,the appropriate energy storage system type is chosen.Due to the different types of cells in the same energy storage device,there are certain differences in parameters,resulting in different configuration results.In order to select a suitable energy storage cell to achieve the high power density configuration for the HESS,a vector synthesis method is presented based on the Ragone diagram of the energy storage device.Furthermore,the relationship between the pulsating power and the energy storage device is established.Then,under the constraints of power and energy,the configuration weight for the HESS is calculated by the complex plane method.As a result,cells of battery and super capacitor with the lightest configuration are chosen as the optimal combination for high power density.Secondly,based on the optimal combination cells of high power density configuration for the HESS,the pulsating power is allocated by taking full advantage of the energy storage cells as the power distribution target.Low-frequency power is allocated to battery to make full use of its energy advantages,and the high-frequency power is allocated to super-capacitor to give full play to its power advantages.In order to avoid the low frequency power containing high frequency components and the high frequency power containing low frequency components in the allocation results of arbitrary cut-off frequencies,the power characteristics of the energy storage cells,a selection method of cut-off frequency based on the frequency characteristics of energy storage device is proposed.In order to select the appropriate wavelet basis and decomposition levels in the power distribution for wavelet transform,the sum of the correlation coefficient is proposed,and the frequency characteristics for the energy storage device are used to select the decomposition levels.Comparing the power distribution results of low-pass filtering and wavelet transform,it can be found that the power distribution results of wavelet transform are more consistent with the characteristics of the energy storage cells,which can better exert the characteristics of the energy storage system.Then,the virtual impedance control is used to realize the frequency distribution for the load power.In order to eliminate the output voltage deviation caused by the virtual resistance droop control(VRDC),the secondary voltage compensation(SVC)for the output voltage in VRDC is proposed in this thesis.The controller parameters of SVC for VRDC are designed by simplifying the control block diagram.Due to terminal voltage over-limit for the super-capacitor caused by virtual capacitor droop control(VCDC),the SVC for terminal voltage in VCDC is put forward.Moreover,the transfer function is simplified by the dominant pole method,and the controller parameters of SVC for VCDC are designed on the premise that the settling time of transient power response is the same as the design value.The effectiveness of the proposed SVC is validated by the results of simulation experiment.Finally,multi-HESSs are paralleled to share the requirements of system power and improve system reliability.The internal power distribution of HESS can adopt virtual impedance control.In order to resolve the consensus problem of state variable among multi-HESSs,a distributed cooperative control based on consensus algorithm is presented.The consensus algorithm of VRDC can realize the inductance current sharing and output voltage deviation compensation for the battery converter.The consensus algorithm of VCDC can realize the dynamic response recovery of terminal voltage and the average voltage control for super-capacitor converter.By establishing the small signal model of the system,the stability of the multi-HESSs is proved.Meanwhile,distributed cooperative control that only exchanges information of state variables with the neighboring units can realize hot plug of HESS unit.