Research On Photovoltaic DC Boost Collection System Based On DC Transformer

The matters including harmonic resonance and reactive power transmission in the AC system are aggravated because of increasing scale of photovoltaic(PV)power plants.To settle aforementioned problems,the DC boost technology is a feasible scheme.As the core equipment of DC step-up system,DC transformer has been studied broadly.However,the development of related technologies for PV DC step-up convergence is still in its infancy,especially in the research and implementation of large-capacity and high step-up ratio DC transformers,and there is a need for breakthroughs in engineering practice.This subject comes from the funding project "Research on the Key Control and Protection Technology of PV DC Boost Convergence System".Taking the DC transformer for PV DC boost collection system as the research object,the topology and control strategy of large capacity single type DC transformer in DC boost collection system are studied.Firstly,aiming at the problem of combined DC transformer,this paper proposes a single-type DC transformer.The DC transformer is composed of VSC and MMC connected by an AC transformer.The selection of the intermediate AC working frequency should not only consider reducing the volume and loss of the whole transformer and improving the power quality,but also coordinate the matching with the modulation frequency to achieve large capacity boost.For PV access,the traditional VSC can be used in the low voltage side;MMC is used in the high-voltage side to ensure the stability of the high-voltage side.Then,the operation principle and control strategy of DC transformer are studied.The low voltage side of the DC transformer is connected with the output port of boost circuit on PV side,and the high voltage side is connected with medium and high voltage DC bus.The low voltage side controls the input DC voltage,and the high voltage side controls the AC voltage amplitude and frequency of the intermediate frequency transformer side.The advantage of this boost collection scheme is that MPPT and DC boost are independent of each other,which can take into account the maximum power tracking efficiency and boost ratio,while achieving stable power transmission.The feasibility of DC transformer and its control strategy is verified by simulation.Finally,the application of the PV DC transformer in the DC set-up system is entirety studied.Among them,boost circuit at PV side realizes maximum power tracking,and the DC transformer is responsible for controlling the low-voltage side voltage stabilization and boosting,while the MMC-type grid-connected inverter works in the mode of constant DC bus and reactive power.It is verified by simulation that under this control strategy,the maximum power output and the stability of the DC bus voltage are guaranteed,and the power transmission and balance are also realized.