Research On LVRT Strategy And Fault Modeling Of PV System In Distribution Network Failure

The inverter interfaced distributed generation(IIDG)represented by the photovoltaic(PV)grid-connected power generation system can be flexibly connected to the grid to improve the flexibility of the power distribution and distribution of the distribution network,and is never exhausted and pollution-free.As solar power generation becomes larger and larger,it is required to have low voltage ride through(LVRT)capability.When a voltage drop occurs on the grid side,due to current limiting,there will be a problem that the input side of the inverter circuit is unstable due to the imbalance of input and output power at both ends.Measures are taken to ensure the stability of the DC side voltage,improve the LVRT capability of the PV system,and establish an equivalent model of the PV system when the distribution network fails.This is an important guarantee for the safe and stable operation of the distribution network.In this paper,the LVRT control strategy and its equivalent model of PV system are studied for the energy imbalance during LVRT of two-stage three-phase PV power generation system.Firstly,the mathematical model and output characteristics of the main structure of the PV system including PV cell array,Boost circuit and three-phase inverter are analyzed.The mathematical model and output characteristics of the PV cell are analyzed.The engineering mathematical model of the PV cell is deduced and the output model of the PV cell is obtained.During the fault of the distribution network,the output power of the inverter is fed forward to the proposed control strategy.The PV cell output power is rebalanced with the inverter output power.The effectiveness of the proposed active LVRT strategy is verified by simulation under different working conditions.Secondly,considering the output characteristics of PVs under shadow and the randomness of their output,a power rebalancing LVRT strategy considering the randomness of PV output is proposed.During the fault of the distribution network,through the switching control of the power-limiting strategy and the power-up strategy,and through the coordinated control of the inverter,the PV system is realized under complicated conditions such as shadow occlusion,light intensity change and voltage drop degree.The power on both sides of the inverter is rebalanced.The effectiveness of the proposed power rebalancing strategy is verified by simulation.Finally,considering the randomness of the PV system output,it is proposed that during the fault,when the maximum power output of the PV array cannot meet the power required by the inverter for LVRT,the inverter control structure is switched back to the voltage outer loop current inner loop.Under this control strategy,the output characteristics of PV system to deliver energy to the distribution network during faults in the distribution network are studied,and the current source controlled by the PV array output power and the grid-connected point voltage is established.The correctness is verified by simulation.