| The photovoltaic(PV)grid-connected system based on the battery and super capacitor hybrid energy storage system(HESS),not only effectively solves the randomness and volatility of traditional PV power generation,but also overcomes the contradiction between the power and energy density of a single energy storage system.The PV grid-connected inverter,as the energy conversion interface between PV arrays/HESS and grid,plays an important role in making PV and HESS system safe,stable and high-quality operation.Compared with the neutral point clamped(NPC)three-level inverter,the T-type three-level inverter has some advantages,such as fewer passive components,lower output voltage harmonic distortion rate,and smaller switching device voltage stress.And these advantages can further improve the conversion efficiency and output power quality of the PV grid-connected system.Therefore,it is necessary to study the control strategy of the T-type three-phase three-level PV grid-connected system based on HESS to achieve stable operation and high-quality output power.A satisfactory control strategy of converter is fundamental for the PV and HESS grid-connected system to ensure reliable operation.Based on the idea of space vector pulse width modulation(SVPWM),an fast vector selection finite control set-model predictive control(FCS-MPC)is proposed in a two-phase stationary coordinate system to avoid huge on-line calculation of prediction model and cost function for the T-type three-level grid-connected inverter under traditional FCS-MPC.For the bidirectional DC/DC converter,a model prediction current control strategy is proposed to speed up its dynamic response and to prevent the hysteresis control caused by the control delay.Meanwhile,the reference current flag is introduced,the control of the circuit can be realized without model switching.For the whole PV and HESS grid-connected system,according to its structural characteristics,a coordinated control strategy combined grid-connected inverter output power control with DC bus voltage control is proposed.The control strategy realizes the control of the DC bus voltage by the HESS unit,the PV array power generation unit or grid connected inverter while considering the state of charge(SOC)of the battery and super capacitor in the HESS.The super capacitor absorbs or releases the high frequency component of the unbalanced power,and the battery is the opposite.Thereby impact caused by the reference grid connected power or the output of the PV array mutation on the DC bus voltage is suppressed,and the power of entire grid-connected system is balanced.The output power quality will be worse and the overcurrent will appear on the inverter side under unbalance grid voltage condition,thereby affecting the operation stability of the entire system.In order to restrain the doubling frequency pulsation of the current in theenergy storage converter and secondary pulsation in grid-connected power,and to improve the power quality,the harmonics and overcurrent problems of the grid current reference value are analyzed in the two-phase stationary coordinate system.And the current-limited model predictive direct power control(MPDPC)with power compensation is proposed.The double frequency notch filter is introduced into HESS,combining with the model predictive current control,the doubling frequency pulsation of the HESS is eliminated and the high power quality are realized.Finally,the system simulation model is built in MATLAB/Simulink,and the simulation is carried out under different working conditions for the proposed control strategy.The simulation results show that when the grid voltage is balanced,the proposed coordinated control strategy can suppress the power fluctuation of the system and adjust the grid connected active and reactive power effectively and quickly under the condition that the battery and super capacitor is not overcharge and over discharge.And the DC bus voltage can maintain stability.When the grid voltage is unbalanced,the proposed control strategy can stabilize the secondary ripple of the current in the energy storage converter and the output power of the inverter and at the same time can achieve the suppression of the fault overcurrent during the coordinated control of the active/reactive power and the negative sequence current under unbalanced grid voltage. |
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