Research On Control Strategy Of Grid-connected Photovoltaic And Reactive Power Support

With the energy shortage and environmental problems becoming increasingly prominent,to speed up the adjustment of energy structure is imminent.In order to achieve sustainable development,it is the best choice to use the new energy instead of the traditional fossil energy.Due to the huge reserves,clean and pollution-free,wide range of distribution,without geographical restrictions and many other advantages,solar energy has become the most development potential of the new energy.However,with the increasing proportion of photovoltaic power generation capacity in power system in recent years,it will have some influence on the active-frequency characteristics,reactive-voltage characteristics,power-angle stability and power quality of power system.Firstly,the mathematical model of photovoltaic cells is deduced according to the working mechanism of photovoltaic cells.The influence of light intensity and ambient temperature on the outp.u.t characteristics of photovoltaic cells is analyzed by simulation.And,based on the Boost circuit,this paper introduces two kinds of maximum power point tracking control algorithms: Perturbation and Observation method and increment conductance.Secondly,the mathematical model of the PV inverters is deduced in detail,and the parameters of the LC filter and the PI controller are calculated according to the current ripple requirement and the closed loop transfer function.This paper presents the current decoupling control block diagram of the PV grid-connected power generation system in the dq coordinate frame.In this paper,the positive and negative sequence separation and grid synchronization schemes are designed based on the second order generalized integrator.This paper not only introduces the proportional resonance controller,which can simultaneously control the positive and negative sequence current in the ?? coordinate frame but also gives its parameters setting method.Five power control strategies were discussed in the end of the third chapter.For the existing power support strategies do not take into account the voltage lift range control,this paper proposes a reactive power support strategy,which can be applied to any voltage drop type.The strategy takes the three-phase voltage amplitude as the control target,and deduces the formula of the reactive power reference value required for the support voltage to ensure that the three-phase voltage amplitude is kep t within the range of the grid-connected standard.Meanwhile,in order to eliminate the active power oscillation,an improved current referrence generator is proposed based on the traditional positive/negative sequence control(PNSC).To limit the amplitud e of the grid current,a reactive power support strategy considering the current limit is proposed,and the reactive power reference value is calculated according to the maximum current allowed by the inverter.Finally,the simulation model is built in MATLAB/Simulink to verify the performance of PV power generation system and the feasibility of the proposed reactive control strategies.The results show that the reactive power support strategy with elimination of active oscillation function proposed in this paper could lift the three-phase voltagee to the range required by the grid-connected criterion,and realize the active power constant.And the reactive power support strategy with peak current limitation can effectively limit the grid current within the safe range.