Research On Cascaded H-brdige Photovoltaic Inverter And Its Virtual Synchronous Generator Strategy Based On Active Power Reserve

The cascade H-Bridge(CHB)photovoltaic(PV)inverter can realize the independent maximum power point tracking(MPPT)control of each PV module,and it has high efficiency and modularity feature,thus have become a research focus in the field of PV power generation in recent years.However,influenced by environmental factors such as irradiance intensity and temperature,the PV power of three-phase CHB PV inverter may be inconsistent,resulting in the asymmetry of three-phase grid current,thus reducing the current quality.On the other hand,the sudden change of load and other reasons will cause the change of grid frequency,but the conventional CHB PV inverter does not have the grid frequency supporting capacity,thus reducing the grid stability.The existing photovoltaic virtual synchronous generator(PV-VSG)technology with energy storage unit can make the PV inverter equipped with grid frequency supporting capacity,but the large volume and high cost of the energy storage unit make the system construction difficult.Aiming at the problem of three-phase power imbalance of CHB PV inverter,a zero-sequence component injection method is proposed in this paper to expand the stable operation range of the inverter under the condition of three-phase power imbalance and reduce the dc-side voltage fluctuation of H-Bridge module at the same time.Aiming at lack of grid frequency supporting capacity of the CHB PV inverter,a PV-VSG control strategy based on active power reserve for CHB PV inverter is proposed,without additional energy storage unit,only by changing the control algorithm makes the inverter simulating the inertia and damping characteristics of synchronous generator,thus equipped with the grid frequency supporting capability.The main works and innovation points of this paper are as follows:1)Aiming at the problem of grid current distortion caused by grid background harmonics,the grid-connected control method of the CHB PV inverter is analyzed.On this basis,two current harmonic suppression methods respectively based on low-pass filter and recursive discrete Fourier transformatin are proposed,ensures that the inverter still has good current quality under grid background harmonic conditions.2)Aiming at the problem of three-phase grid current asymmetry caused by the threephase power imbalance of CHB PV inverter,the existing zero sequence injection methods and their shortcomings are analyzed,and an optimized zero sequence injection method is proposed,including the fundamental frequency zero sequence component and the third harmonic component,which expands the stable operating range of the inverter under the three-phase power imbalance condition,and reduces the dc-side voltage fluctuation of the H-bridge module at the same time.3)Aiming at the problem that the existing CHB PV inverter without energy storage unit lacks the grid supporting capacity,the operation mechanism of the PV-VSG based on active power reserve is studied,and the CHB PV-VSG strategy based on integral power reserve and modular power reserve are proposed,which makes the inverter equipped with the grid frequency supporting capability even without energy storage unit.4)The experimental platform of CHB PV inverter is built,through which the effectiveness and feasibility of the proposed current harmonic suppression strategy,phase-phase power balance control strategy and PV-VSG strategy based on active power reserve are verified respectively.