| With the rapid decline of the global installation costs for solar photovoltaic(PV)in recent years,incorporating large-scale PV power plants(LPVPPs)into power systems is expected to be one of the main approaches to the energy transformation of the New Power Systems and achieve the “dual carbon” goals in China.Conventionally,LPVPPs are integrated into power systems with centralized structures,which adopt the global maximum power point tracking(GMPPT)technologies.When mismatch problems occur in the PV array,the energy production efficiency of centralized LPVPPs will be significantly reduced.To solve this problem,the distributed LPVPP and its distributed maximum power point tracking(DMPPT)technologies based on a DC optimizer(DCO)for each single PV panel have been widely concerned in recent years,which can effectively mitigate the solar energy loss of LPVPPs caused by array mismatch conditions.However,in the existing studies on the grid-connected operation of LPVPPs,the PV power station is usually equated as a single aggregated power source with a centralized control structure,which ignores the influence of the different internal structures of LPVPPs.In this way,the economy and reliability of grid-connected LPVPPs cannot be truly reflected,especially when the scale of PV plants is further expanded.This paper focuses on the economic evaluation and grid-connection stability analysis and control of LPVPPs equipped with DC optimizers.Based on the research ideas of multi-dimension and multi-time scale,the economic feasibility of the distributed structure of LPVPPs equipped with DCOs is firstly analyzed,and then the dynamic stability and frequency support control strategies of large-scale distributed PV grid-connected systems are studied.The main research work and achievements of this paper are as follows:1.For the long-term energy production characteristics analysis and operational economics evaluation of distributed LPVPPs equipped with panel-level DCOs,the static energy generation models of six several typical LPVPPs(including DCO distributed structures)are first established to study the annual energy generation characteristics with the hour-level accuracy based on historical hourly weather data.After that,the operational economics evaluation method is formulated based on the long-term energy production characteristics of LPVPPs,in which the Levelized cost of energy(LCOE)is used to analyze the economic benefits of six typical types of LPVPPs under centralized or distributed configurations.Finally,the distributed structure based on panel-level DCOs is proved to be the economically optimal structural design scheme of LPVPPs through simulation comparisons.2.A novel matrix-variables-based vector modeling method is proposed for the dynamic modeling of the large-scale distributed PV grid-connected system equipped with panel-level DCOs.Through the modular classification design and matrix block modeling ideas,the dynamic average-value model of the PV-DCO distributed system with any M×N scale can be transformed into a modular vector dynamic model with the 2×1 scale.The proposed model can be combined with the vector simulation function in Matlab/Simulink,to simulate the dynamic characteristics of thousands of PV generation units and their connected panel-level DCOs inside a megawatts-scale LPVPP.On the basis of ensuring high simulation accuracy,the proposed modeling methods can greatly relieve the serious calculation burden caused by the expansion of system scale during the dynamic simulation of the model.3.For the small-signal stability problems of large-scale PV grid-connected systems containing panel-level DCOs,the “super-high-dimensional” small-signal model corresponding to the PV system is first established.Then the key factors affecting the small-signal stability of the system are obtained through the eigenvalue analysis method and root locus method,in consideration of almost all the factors including environmental factors of the PV plants,aging conditions of PV panels,panel installation angle settings,parameters and topology structures of DCOs,controller parameters of the PV inverter,the scale of the LPVPP,the parameters of the DC line and DC link,and the strength of the AC system,and the analysis conclusions are verified by time-domain simulations.After that,the grid-connected suggestions and parameter design guidelines related to the safe and stable operation of distributed LPVPPs equipped with panel-level DCOs are further obtained.4.For the stability problems of distributed PV systems equipped with panel-level DCOs after large disturbance with severe mismatch conditions,the dynamic operating characteristics of the DCO-DMPPT PV array with different topologies under severe mismatch scenarios are firstly analyzed respectively.The forced equilibrium stability mechanism and the power output characteristics of the distributed PV system under this condition are revealed for the first time.Based on the dynamic characteristics analysis and operation laws of the distributed PV system,the improved design schemes for the DCO circuit topology and the DCO control structure are proposed to improve the transient stability of distributed PV plants when suffering severe mismatch disturbance,meanwhile,the reverse power consumption appearing in cascaded PV generation units(also named the “hot spot” effect)can also be avoided.Simulation results show the effectiveness of the proposed DCO improved scheme.5.For the active frequency support strategy provided by DCO-based LPVPPs to the power grid,a two-level decentralized frequency support control scheme based on DCOs is newly proposed.On the PV plant level,the frequency-superposition DC-link voltage control method is proposed to convert the grid frequency deviation into the changes of local voltages of DCOs,and meanwhile,achieve the inertia emulation of the DC-link capacitor;On the PV-DCO generation unit level,the decentralized virtual inertia control and droop control methods are developed based on DCO controllers to achieve the inertial response and primary frequency control ability for PV units.Simulation results show that the maximum use of the PV reserve power can be realized by the proposed frequency control scheme,without using energy storage devices and communication networks for distributed LPVPPs. |
