Modeling Analysis And Control Of PV Grid-connected System Under Weak Power Grid

The contradiction between environmental protection and social development is increasingly prominent at present,an important way to eliminate this contradiction is to vigorously developing new energy power generation.Many efforts in academic and industrial research have been put into solar power generation,with its advantages of safety,reliability and free from geographical restrictions,etc.Solar power generation has been developed and utilized on a large scale.Nevertheless,as the grid-connected scale of distributed small photovoltaic power generation and large photovoltaic power stations increases,the "source" and "network" of the power grid gradually presents a "weak connection" state,which is defined as "weak power grid" in most literatures.For PV inverter parallel operation control,impedance of public power grid is therefore needed to be concerned at this time.Also,due to the influence of the power grid impedance,those background harmonics with different frequency inside the public power grid would be amplified at a public connected access point.This phenomenon ultimately affects the output of photovoltaic power generation system in power quality,even leads to instability of grid-connected power generation system.This can be severe for the safe operation of the power grid.In this thesis,the stability and system performance of grid-connected inverter under weak power grid condition are studied.This thesis focuses on the three-phase non-isolated photovoltaic grid-connected inverter with LCL filter working in weak power grid condition,and research works are organized as follows.(1)A detailed introduction of the photovoltaic power generation system is presented and the characteristics of the weak power grid condition is defined.On this basis,the model of single-machine and multi-machine multi-stage non-isolated photovoltaic grid-connected inverter under weak power grid condition is built,and its mathematical model is derived for brief analysis.(2)In order to analyze the performance of traditional control methods under weak power grid condition and the influence of PLL,the harmonic linearization method is used to deduce the grid-connected inverter positive and negative sequence output impedance model when considering PLL.A contrastive analysis is conducted with and without PLL,the positive and negative sequence impedance model of grid-connected inverter is calculated respectively under corresponding conditions,and the simulation is carried out to verify the results.The result shows a good performance which can't be achieved by traditional control method.(3)The discrete binary sequence(DIBS)signal is introduced as the disturbance signal of the injection system.In addition,the starting threshold criteria of the active measurement scheme is designed by referring to the power grid fault detection technology.The calculation result is optimized to improve the accuracy.The design of the whole network impedance online measurement scheme as well as relevant simulation verification is completed.(4)The traditional LCL grid-connected inverter parameter design and the problems caused by the traditional current control strategy of the inverter under the weak power grid condition are analyzed.Traditional design method,which is cumbersome process,uses traditional proportional grid voltage feedforward strategy where grid-connected current still be affected by the grid voltage distortion.On the basis of the sequence impedance analysis results,a series of solutions to the above two problems are proposed in this thesis.A parameter joint design method is proposed,and the coefficient simplified design is carried out according to the system characteristic parameters of the LCL grid-connected inverter.An improved voltage feedforward strategy is designed to improve the classical power control strategy.Capacitance current can be calculated by filter inductance and grid-connected current rather than using extra sensors.