| The output power of photovoltaic is influenced by light intensity, temperature andload and has a characteristic of fluctuation. Photovoltaic-energy storage system isproposed in which hybrid energy storage system composed of the super capacitor andbattery is connected to the PV system to make full use of the high power density of supercapacitor and high energy density of battery. Through the power delivery control, theimpact on grid and load is reduced and life of hybrid energy storage system is extended.This paper first introduced the structure that each energy storage unit connected tothe DC bus via bidirectional DC-DC converters. Three major energy managementstrategies of the photovoltaic-energy storage system were established. The operatingmode of each bidirectional DC-DC converter was then formulated, and control objectivesof the corresponding constant voltage, constant current were set.Secondly, as for the realization of MPPT, the incremental conductance method wasused to seek the voltage corresponding to the maximum power point, and PI controlmethod was used to track this voltage; As for the design for hybrid energy storage systemcontroller, based on the traditional small-signal model, the compensation circuits ofconstant voltage, constant current when bidirectional DC-DC converter operated indifferent modes were designed with the frequency domain compensation method.Then, the sliding mode variable structure MPPT controller was designed, reaching-law approach was adopted in reaching segment to shorten the reaching time; By theequivalent relationship between equivalent control and duty cycle, fixed frequency wasrealized. For the constant charge-discharge current control of battery, the single loopsliding mode variable structure control was researched based on hysteresis method. Forbidirectional DC-DC converter connected to the super capacitors, the paper proposed asliding mode variable structure controller combined with PI control, which obtained thecorresponding current reference value through PI controller. Thus the DC bus voltage waskept constant and life of hybrid energy storage system was extended.Finally the MATLAB/Simulink environment was used to set up the proposed systemand simulations were made. The simulation results show that sliding mode variablestructure control method works very well in steady state and dynamic performance, thus itis suitable to be used in photovoltaic-energy storage system. |
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