| Under the double pressure of environment pollution and fossil energy crisis,distributed generation on wind/PV is quickly developed,and its function in power supply and low carbon life is becoming more and more obvious.As wind power generation and photovoltaic power generation are influenced by external environment factors and have the characteristics of random and intermittent,direct connected-grid will seriously affect the power quality and reliability of power grid.So energy storage is needed as an energy buffer device and to ensure the safe and stable operation of system.Based on the limitations of a single energy storage,according to complementary of function and characteristics about super capacitor and storage battery,the paper puts forward to compose hybrid energy storage by super capacitor and storage battery,and applies it to micro-grid system on wind/PV.Firstly,the paper introduces working principle about photovoltaic power generation,wind power generation,storage battery and super capacitor,then establishes corresponding mathematical model.Based on this,their working characteristics are simulated and analysed in PSCAD.Secondly,the paper sets up the mathematical model of connected-grid inverter under dq rotating coordinate system,designs LC filter parameters and studies working principle of PQ control/droop control.In view of low voltage micro-grid with big line impedance ratio,it can't directly use the droop control of high voltage system,a improved droop control is put forward,namely to introduce into feedback perceptual impedance at the inductance current of voltage-current double loop control and make the equivalent output impedance of inverter for perception,to validate the rationality of the proposed method by simulating in PSCAD.Thirdly,the paper puts forward a kind active parallel structure of hybrid energy storage through comparing and analysing super capacitor and battery combination ways.In order to restrain wind/PV connected-grid wave power,ensure voltage and frequency stability of off-grid system and smooth switch micro-grid operation mode,the paper puts forward energy management and control strategy of hybrid energy storage.In the aspect of energy management,the paper follows the principle of super capacitor priority and chooses working ways of hybrid energy storage by judging super capacitor's voltage value.Super capacitor restrains high frequency part of wave power,and storage battery restrains low frequency fluctuation part,thus to reduce charging and discharging times of storage battery and prolong its service life.In the aspect of control strategy,the bidirectional DC/DC converter of storage battery adopts constant power control,and the bidirectional DC/DC converter of super capacitor adopts constant busbar voltage control,to ensure stability of DC bus voltage and achieve charging and discharging control of hybrid energy storage;besides,in view of micro-grid connected/off-grid operation mode switch,paper puts forward to switch between PQ control and improved droop control of bidirectional DC/AC converter of hybrid energy storage.Finally,in PSCAD,the paper builds micro-grid simulation mode containing wind power generation,photovoltaic power generation,hybrid energy storage power generation.The three conditions of connected-grid operation,off-grid operation,connected/off-grid switch operation are analyzed in simulation,the simulation results verify the correctness and effectiveness of proposed control strategy. |
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