Research On Key Techniques Of Three-Level Photovoltaic Grid-connected Inverter Under Grid Faults

The shortage of traditional fossil energy and the constant increase of global energy consumption has greatly promoted the development of new energy with photovoltaic power generation as a typical representative.As a core interface device of photovoltaic power generation system and the large power grid,the three-level inverter is replacing the two-level inverter as the preferred device.The three-level inverter accesses to the grid in a transformerless way for higher efficiency,but leads for the current leakage and endanger personal safety.With the large-scale assessment of photovoltaic power generation,when the voltage sags of point of common coupling(PCC)caused by the grid transient fault,the photovoltaic grid-connected inverter must have the ability of low-voltage ride through(LVRT)to support the power grid recovery.However,the unbalanced voltage sags will leads different or even worse effects in the phase-locked loop(PLL),current control and midpoint potential balance modulation strategy of the three-level inverter grid-connected control,which would restrict the voltage support for the power grid.Therefore,in this paper,transformerless three-level photovoltaic grid-connected inverter is taken as the research object,and the key technologies such as three-phase phaselocked loop,leakage current suppression,neutral point potential balance modulation strategy and LVRT are studied.The main research contents and achievements are as follows:(1)For the three-phase phase-locked technology of photovoltaic grid-connected inverter,focusing on non-ideal grid voltage for unbalance,harmonic distortion,transitions of frequency and phase,and DC bias conditions,which caused by the grid faults,a prefiltering moving average filter PLL(PMAF-PLL)based on adaptive frequency is proposed with the moving average filter(MAF)as the prefilter.In the synchronous rotating coordinate system,the MAF window length is optimized,and the non-integer order MAF is adjusted on-line to adapt to the change of the grid frequency.The main 6n ± 1 harmonics of the three-phase power grid are rapidly filtered out,while positive and negative sequence components of the fundamental voltage are separated based on the general second order complex vector filter(GSO-CVF).Combing the characteristics of GSO-CVF,the DC bias suppression module is designed with the separated positive and negative sequence fundamental voltage only left.The correct acquisition of the amplitude,frequency and phase of the positive sequence voltage is realized by means of conventional PLL.Finally,a simple method is proposed for amplitude and phase compensation of negative sequence voltage.By the above step process on the non-ideal grid voltage,the positive and negative sequence components and phase information of the fundamental voltage are obtained quickly,which provides the possibility of high performance operation of the grid-connected inverter.(2)For the leakage current suppression and the neutral point potential balance of transformerless three-level photovoltaic grid-connected inverter,a hybrid modulation strategy which can achieve both is proposed.According to the analysis of the cause of the leakage current,combined with cost minimized hardware filtering,the leakage current can be reduced by suppressing the common mode voltage according to the modulation strategy,and the threelevel neutral point potential balance can be realized simultaneously.Taking reference from the already research,this paper proposes a modulation strategy based on dual virtual medium vector,which has the same effect as that of large middle zero vector modulation(LMZVM)in the common mode voltage amplitude while contrast to the neutral point potential.And it is combined with LMZVM to form a hybrid modulation strategy with the full range of the modulation index and all the power factor.The proposed strategy can effectively eliminate the DC offset of neutral point potential and make the AC low frequency fluctuation limited within the set value range,but also has better common mode voltage suppression ability,thus reducing the leakage current to meet the relevant technical specifications.(3)For string three-level photovoltaic grid-connected inverter with two-stage topology,which is more applied in small and medium power distributed power generation system,a simple and practical LVRT technology based on the inverter self-protection is proposed.With the minimum reactive power requirements,the adopted LVRT strategy switchs between the maximum power point tracking(MPPT)and non-MPPT control modes depending on the voltage sag level and the maximum output power of the photovoltaic array,which controls the DC bus voltage effectively so as to ensure the safe operation of the inverter.(4)Under the background of the large-scale photovoltaic power generation system,LVRT technology for central three-level photovoltaic grid-connected inverter in response to longdistance power grid fault is studied.A complete LVRT strategy is proposed to achieve flexible voltage support and imbalance mitigation for the three-phase grid,with maximum grid current limitation.The proposed strategy generates a new reactive current reference command for the inner current loop by the effective combination of positive and negative sequence currents to reduce the negative sequence voltage while increasing the positive sequence voltage of the PCC.The proposed voltage support outer control loop is designed with two PCC phase voltage amplitude reference instructions,adaptively adjusting the imbalance and calculating real-time online,which can properly recover the unbalanced PCC voltage back to normal operation range.Moreover,the maximum grid current limitation based on the priority injection of positive sequence reactive current can effectively avoid overcurrent for the voltage sag depth beyond the capacity of the photovoltaic power plant.In summary,this paper deeply studies some key techniques of transformerless three-level photovoltaic grid-connected inverter under grid faults,which provides a theoretical basis for the inverter to deal with grid faults.This paper provides a way for the inverter to achieve high performance operation and to enhance the flexible reactive power support for the grid.