Research On Common Mode Voltage Suppression And Neutral Point Potential Balance Coordinated Control Of Non Isolated V2G

Under the background of building a strong smart grid,the V2 G technology which can transform electric vehicle into distributed energy storage unit has been developed greatly.V2 G technology is increasingly integrated benefit from the non isolated cascade structure,non isolated topology omitted the isolation transformer,with compact portable and high efficiency.However,the vehicle battery and power electrical contacts directly,the output side of the inverter will produce common mode voltage,causing great harm to the load and the surrounding electrical equipments,and even bring security risks;And the inherent neutral point potential of the three level system will seriously affect the effect of common mode voltage suppression.Therefore,in-depth study of the coordinated control of non isolated V2 G system,for the safe and effective operation of the system is of great significance.A control method has not been found to achieve the coordinated control of neutral point potential balance and common mode voltage suppression.Combined with the National Natural Science Fundings of Hebei(E2014502109)"A new type of V2 G bi directional converter topology and its control method",the corresponding control method for non isolated V2 G system is studied in this paper.By establishing the common mode voltage equivalent model of the non isolated V2 G system,the excitation source of the common mode voltage is determined,confirmed the effect of front bi-directional half bridge DC/DC converter on common mode voltage can be ignored;Meanwhile,reveals the three-phase three level V2 G neutral point potential unbalance will limit the common mode voltage suppression effect.Based on space voltage vector modulation(SVPWM)and pulse width modulation(PWM),the control strategy of the system is separately proposed.For the former,the control algorithm and topology improvement are selected to achieve collaborative control independently.The control algorithm approach,introduced virtual space vector modulation(NTV2),and the virtual small vector and the virtual medium vector are redefined to ensure the common mode voltage suppression effect.Under the new virtual vector space,to fix the switching frequency,adopt mixed modulation strategy based on different small regions,controlling the average neutral current is zero for neutral-point potential balancing.The topology improvement way,introduced H bridge balancing circuit(or equivalent single bridge arm balancing circuit)in the DC bus,according to the offset direction of neutral-point potential,choose the working mode of the balancing circuit to adjust the DC link capacitor capacity,thereby inhibiting offset of the neutral-point potential.Improved traditional SVM algorithm to effectively suppress the common mode voltage,and formed the five stage SVM algorithm as the matching control strategy.For the latter,respectively explore the possibility of implementing coordinated control of the in-phase and opposite phase disposition modulation,Then the three step method is proposed to assist the opposite phase disposition modulation to stabilize the fluctuation of neutral point potential,Furthermore,proposed a new modulation strategy of bipolar carrier modulation combined with NTV2,which based on Boole logic operations,established the mapping from a bipolar carrier modulation output state to a specific space vector,keeping the output of common-mode current be zero;Meanwhile,imported NTV2 to keep neutral-point potential balance in full range.Finally,the simulation results show that the coordinated control strategies can independently and effectively suppress common mode voltage and balance neutral point potential,at the same time to ensure the unit power factor and low harmonic V2 G system operation.