| In photovoltaic microgrid,due to the randomness,intermittence and fluctuation of "source","load" and "storage",there are power quality problems caused by power fluctuation.In order to ensure the high-quality operation of photovoltaic microgrid,the key is to seek the control strategy of energy storage system to suppress power fluctuation effectively.Starting from the power fluctuation of photovoltaic microgrid,and studies the related control problems of energy storage system to suppress power fluctuation,which has theoretical significance and practical value.The main contents are as follows:Based on the structure of photovoltaic microgrid system,the causes of power fluctuation are analyzed.The charging and discharging characteristics of lead-acid battery and supercapacitor are studied deeply,and the operation characteristics of the energy storage system composed of their matching forms are studied.The mathematical models of lead-acid battery and supercapacitor are established respectively.In order to solve the problem of decomposition of fluctuating power in photovoltaic microgrid,aiming at the difference between lead-acid battery and super capacitor,a method of fluctuating power analysis based on wavelet packet decomposition is proposed.By comparing different wavelet basis functions,Daubechies wavelet function is used as the basis function of wavelet packet decomposition.According to the response time of lead-acid battery(1 ～60min)and super capacitor(less than 1min),and taking the minimum frequency of energy storage system as the standard,the wavelet packet decomposition layer number of 6 is determined to meet the decomposition demand of fluctuating power,and then the fluctuating power is decomposed into three parts: low frequency,sub high frequency and high frequency.After decomposition,the fluctuating power of different frequency bands is distributed reasonably,and the capacity of each energy storage unit is configured.The state of charge of lead-acid battery and supercapacitor in the example is simulated and analyzed to determine the accuracy of wavelet packet decomposition method in photovoltaic microgrid fluctuation power decomposition.In order to suppress the fluctuating power effectively,an improved fuzzy droop control strategy is proposed for the control of bidirectional DC / DC converter in energy storage system.On the basis of the traditional fuzzy droop control,the input of the fuzzy controller is improved.The fluctuating power and bus voltage stability in different frequency bands after wavelet packet decomposition are taken as the input of the fuzzy controller,and the corresponding fuzzy rule base is established,It solves the problem that the transition process of energy storage system is long due to the complicated process of traditional fuzzy droop control.By adjusting the droop coefficient quickly and changing the PWM duty cycle through voltage and current double closed-loop control,the dynamic control of the output of bidirectional DC / DC converter is realized,so as to achieve the goal of quickly and effectively suppressing the power fluctuation.The output of lead-acid battery and super capacitor in the example is simulated and analyzed,and the effectiveness of fuzzy droop control strategy in converter control is determined.The simulation model of photovoltaic microgrid is built in Matlab / Simulink.Compared with the traditional fuzzy droop control strategy,the simulation results show that the accuracy and speed of suppressing fluctuating power are improved.The feasibility and rapidity of the proposed improved fuzzy droop control strategy based on wavelet packet decomposition are verified. |
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