Research On Feasible Operation Region Of Large Capacity Photovoltaic Inverter System Connected To Weak Grid

With the depletion of fossil fuels and the increasingly serious environment pollution, the demand for the utilization of renewable energy sources to generate electricity is increasing. Due to the advantages of decreasing cost and flexible installation, solar energy has become one of the dominating renewable energy sources. Solar energy can be utilized on a large scale through the integration into the existing power distribution infrastructure. However, restricted by the geographical conditions, most large-scale PV power plants are located in desert or semi-desert regions where the grid infrastructure is weak. Under this scenario, the network equivalent impedance of long transmission lines cannot be ignored. For systems with low PV penetration the connected grid can be regarded as a strong power grid because of the small capacity of the PV power plants. With the expansion of the PV power plants to a level comparable to the reference utility power generation capability, the connected grid should be treated as a weak grid.PV power generation systems are connected into the grid through PV inverters. PV inverters change the DC energy into AC to feed into the grid. Additional functionality of the grid-tie PV inverter includes operating the PV modules at the conditions where the maximum power is captured. When PV systems are connected to a grid which is relatively strong, the PV systems can be well controlled. With the expansion of PV inverter systems, the grid has the potential of being overcome by the PV systems. The inconsistency of power generation of PV systems, combined with its dominate position over a weak utility infrastructure could eventually bring the system into collapse. Therefore, it has important theoretical significance and practical value to evaluate the impacts of the high penetration of PV systems on a weak grid. New methods to improve the operation quality and maintain the safety margin of the system with high P V penetration need to be investigatedBased on a single-stage three-phase PV inverter system, the mathematical model including the PV array, LCL filter and DC/AC inverter is established. The output characteristic of the PV array is analyzed, the design method of main circuit parameters is given. A dual closed-loop control structure combined with maximum power point tracking (MPPT) and inverter power controls is built. Using the PSCAD/EMTDC platform, a PV inverter simulation system is created. This provides effective tools for the study of operation analysis and control of PV inverter systems, which are connected with the weak grid.Based on the field test of PV inverter systems connected with the grid, the index to evaluate the fragility of power grid and the definition of a weak grid are given. Then the concept of the feasible operation region of PV inverter system is established. Per unit value of the grid impedance on the boundary of feasible operation regions are used to measure the adaptability of PV inverter systems connected to the weak grid. The methods to analyze the boundary of the feasible region to meet the controller’s stability margin constraints and voltage regulation constraints are also presented.For the controller’s stability margin constraints the adverse effects of weak network on controller stability margin are analyzed. An improved control structure without using voltage feedforward is presented, which can significantly extend the feasible operation regions. The methods to optimize the controller parameters are presented.In the end with regard to the voltage regulation constraints on the access point, the impact of weak power grid on power transmission constraints is analyzed. Comparisons between the operation regions of PV inverter systems with different reactive power control methods show that it is necessary to improve the reactive power control strategy under weak grid conditions. Based on the simulation and field test results, the reactive adjustability of the single stage P V inverter is studied. The reactive control strategy to regulate the access point voltage is derived, which can be used to improve the voltage level of PV inverter systems to avoid the potential risk of voltage instability in the system.